kenneth pye associates ltd

British Energy Estuarine & Marine Studies

Technical Report Series 2010 no. 109 Ed 2

A consideration of ‘extreme events’at Hinkley Point

Production Date: January 2011Not Protectively Marked

Kenneth Pye Associates Ltd

Copyright © 2011 EDF Energy The text of this document may be reproduced free of charge in any format or medium providing that it is reproduced accurately and not in a misleading context. The material must be acknowledged as EDF copyright and the document title specified. Where third party material has been identified, permission from the respective copyright holder must be sought.

UK Protect; Commercial

Understanding the significance of extreme events is vital in order to effectively assess coastalgeo-hazards at Hinkley Point. This report was commissioned to provide an expert assessmentof extreme water levels at Hinkley Point and to consider the potential for, and nature of, futurecoastal change. The report considers the past geological evolution of the inner Bristol Channeland its current physical setting, including an assessment of past sea-level change, coastalchange and the evidence for past extreme water levels. There is a detailed analysis of methodsfor calculating future extreme water levels and historical datasets are used both as essentialcontext and as part of an analysis of potential future extreme water levels.

The report uses the best evidence available and has considered and explained well themethods of analysis reviewed and used. This second edition includes additional datasets, awider range of UKCP09 scenarios and a more considered treatment of wave effects. The reportis of very good quality and is supported with numerous explanatory diagrams and maps. Thesection dealing with methods for predictions of extreme events (Section 3) is important tounderstand the potential shortcomings of the various calculation methodologies. The reportclearly demonstrates the degree to which uncertainties related to extreme water levels in 2100are compounded by uncertainties associated with the UKCP09 climate projections.

Key points of relevance to BEEMS are:

• The calculation of extreme water return levels using statistical models should only beregarded as reliable if appropriate input data are available for a sufficiently long time period(ideally at least 25 years). Extrapolations beyond four times the period of record (e.g. 100years) should be treated with great caution. The water level record for Hinkley Point is lessthan 20 years and wave data are available only for the past 2 years. During this time therehave been no high magnitude events equivalent to the storm of 1607, or even to thesmaller, but damaging surge tide of 1981. Statistically-based estimates of ‘extreme’ waterlevels based on data for Hinkley Point data alone may therefore be underestimates.

• An alternative approach is to consider ‘worst case’ combinations of conditions. For thepresent day, the worst case would arise if the largest possible surge and waves coincidedwith the highest astronomical tide. Such a coincidence is considered extremely unlikely andwould, if it occurred, produce an extreme water level (>12 m) well in excess of the 1 in10,000 year event estimated by joint probability analysis.

• Using a joint probability approach the 1 in 10,000 year water level at current sea levels,resulting from combined tide plus surge plus waves, is estimated to be in the range 8.93 to9.24 m OD. Assuming a 1m rise in sea level rise and a 4% increase in surge height andwave height, the equivalent 1 in 10,000 year level in 2100 is estimated to be 10.02 to10.34 m OD.

• In terms of future climate change, the higher estimate for increase in mean sea levelidentified under the H++ emissions scenario (1.9 m), together with current UKCP09estimates of possible increases in skew surge and wave height, provide a possible near


pj03

Rectangle


‘worst case’ scenario for 2100. A combination of highest astronomical tide, surge of 1607magnitude and half the largest measured inshore significant wave height would give aresultant water level of 11.48 m OD under the SRES A1B1 medium emissions scenario and15.33 m OD under the H++ high estimate scenario. Higher estimates still would be obtainedif maximum wave heights were used.

• There is evidence that mean high water levels are likely to increase by more than MSL,potentially by as much as 40 - 50%, which would give a predicted increase in high waterlevel of 2.8 m in 2100 rather than the 1.9m identified under the H++ emissions scenario.

• No convincing sedimentological evidence has been provided to indicate that the inner BristolChannel coastal lowlands have been affected by a major tsunami or 'megastorm' ofhurricane-like proportions. The most devastating coastal flood in historical times occurred inJanuary 1607 and almost certainly represented a storm surge of around 2.0 to 2.3 msuperimposed on a relatively high (but not extreme) astronomical tide. An event of onlyslightly smaller proportions occurred in December 1981.

• Non-linear tide-surge interactions reduce the likelihood that maximum surges coincide withpredicted high water. Statistical modelling is unable to assess accurately the actual risk ofsuch coincidence, and for NNB planning purposes, a precautionary approach may be moreappropriate.

• The implications for coastal morphological change from single extreme events is consideredto be limited, but in the longer term, the greater wave energy incident at the coast wouldenhance the break up and erosion of the limestone intertidal platform which fronts theHinkley Point power station site and neighbouring cliffs, thereby increasing water depthsand acting to further increase the wave energy incident at the shoreline. However, this islikely to be a progressive rather than a catastrophic process.

• Several large-scale human interventions are currently being undertaken, or are beingconsidered, in the inner Bristol Channel and Severn Estuary which could have a significantimpact on the morphology and sedimentological character of the Hinkley Point coastalfrontage and adjacent areas, and on tide / surge levels. A full assessment of the potentialeffects of these proposed schemes on the Hinkley Point site was beyond the scope of thisreport.


pj03

Rectangle


Summary of the extreme water levels estimated in this study for years 2010 (black lines)and 2100 (red lines), at the present Hinkley Point site. Values represent theoreticalcombinations of tides, surges and significant wave heights, in 2010 and 2100, based onseveral UKCP09 scenarios.

Signed: ............................................................

Position: ..........................................................

Date: ................................................................


pj03

Rectangle

Kenneth Pye Associates LtdResearch, consultancy and investigations

A Consideration of "Extreme Events" at Hinkley Point, Somerset, With Particular

Reference to Coastal Flooding and Coastal Change

Kenneth Pye and Simon J. Blott

External Investigation Report No. EX1207

November 2010

A Consideration of "Extreme Events" at Hinkley Point, Somerset, With Particular Reference to Coastal Flooding and Coastal Change Kenneth Pye and Simon J. Blott External Investigation Report No. EX1207 This report was prepared by Professor Kenneth Pye ScD PhD (Cantab) MA (Oxon) CGeol FGS (Director) and Simon James Blott PhD (London) MRes BSc (Reading) FGS (Principal Consultant) as part of BEEMS 2009-10, Work Package 4 - Geomorphology).

Report history

Version 1.0 Draft issued for CEFAS comment

15 December 2009

Version 2.0 Final Version 3.0 Revised and Updated Final

24 March 2010 22 November 2010

Kenneth Pye Associates Ltd Research, Consultancy and Investigations Crowthorne Enterprise Centre Crowthorne Business Estate Old Wokingham Road Crowthorne Berkshire RG45 6AW United Kingdom Telephone / Fax + 44 (0)1344 751610 E-mail: [email protected] website: www.kpal.co.uk

Contents Page

List of Tables ............................................................................................................... iv List of Figures ............................................................................................................... v Summary ....................................................................................................................... 1 1.0 Introduction ....................................................................................................... 3 1.1 Report scope and purpose ..................................................................... 3 1.2 Methods and information sources ......................................................... 4 1.3 Report structure ..................................................................................... 4 2.0 Geological background and physical processes in the Hinkley Point area ...... 6 2.1 Location and setting .............................................................................. 6 2.2 Holocene stratigraphy and coastal evolution ...................................... 13 2.3 Geomorphological character of the coast ........................................... 22 2.4 Recent coastal change in the Bridgwater Bay area ............................. 30 2.5 Coastal processes ................................................................................ 38 2.5.1 Tides ...................................................................................... 38 2.5.2 Tidal currents ........................................................................ 52 2.5.3 Winds and waves ................................................................... 57 2.5.4 Suspended sediments and bed sediments .............................. 60 2.5.5 Storm surges ......................................................................... 64 2.5.6 Historical sea level rise ........................................................ 79 3.0 Review of methods for predicting extreme events ......................................... 89 3.1 The nature of extreme events, with particular reference to coastal water levels ............................................................................. 89 3.2 The Annual Maximum method ........................................................... 91 3.3 r-largest, GEV and Peaks over Threshold methods ............................ 93 3.4 Joint probability methods .................................................................... 94 3.5 Alternative / complementary methods ................................................ 99 4.0 Risk of coastal flooding and morphological change associated with extreme events in the Hinkley Point area ................................................................... 102 4.1 Previous studies ................................................................................ 102 4.2 New analysis of tide, surge and wave data ....................................... 105 4.3 Extreme water levels associated with tsunamis and other 'rare' events ........................................................................................ 111

4.4 Implications of future climate and sea level change ......................... 117 4.5 Effect of extreme water levels on coastal morphological change ..... 125

_________________________________________________________________________________________________________________Kenneth Pye Associates Ltd. Report EX1207

Hinkley Point Extremes_________________________________________________________________________________________________________________

- ii -

5.0 Conclusions ................................................................................................... 126 6.0 References ..................................................................................................... 133 Appendices Appendix 1: Aerial photographs of Bridgwater Bay Appendix 2: Digital surface models constructed from 2007-08 lidar data Appendix 3: Facsimiles of First edition County Series Six Inch maps Appendix 4: Ground photographs taken during field visits, September 2009 Appendix 5: Topographic profiles of the foreshore between Lilstock and Catsford Common, derived from 2007-08 lidar Appendix 6: Changes in position of MHW and MLW, based on comparison of Ordnance Survey maps and 2007-08 lidar data Appendix 7: Comparison of 2007-08 lidar data with ground topographic survey data Appendix 8: Water level and skew surge data for Class A tide gauge stations in the Bristol Channel Appendix 9 Future changes in sea level, skew surges and selected climate parameters at Hinkley Point, projected by UKCP09



- iii -

List of Tables Page Table 1 Elevations of the sea wall at Hinkley Point Power Station ..................... 28 Table 2 The positions of MHW and MLW shown on Ordnance Survey maps

surveyed in 1886, 1957 and 1976, and Environment Agency lidar survey data flown in 2007-8 .................................................................... 33 Table 3 Rates of (a) cliff recession on 20 profiles to the west of Hinkley Point Power Station, and (b) movement of the dune toe along Berrow and Brean dunes, between 1886 and 2008 ..................................................... 36 Table 4 Tidal levels at Standard and selected Secondary Ports in the Bristol

Channel, including Class A tide gauge stations ...................................... 40 Table 5 Tidal levels at Hinkley Point quoted by different sources ....................... 42 Table 6 Extreme astronomical high tides predicted at Hinkley Point 1990-2026 ................................................................................................ 55 Table 7 The 20 highest predicted tides at Hinkley Point in the period 1990-2008 ................................................................................................ 68 Table 8 The 20 highest observed tides at Hinkley Point in the period 1990-2008 ................................................................................................ 69 Table 9 The 20 largest high water skew surges recorded at Hinkley Point in the period 1990-2008 ............................................................................... 70 Table 10 Extreme annual water levels and surges observed at Hinkley Point 1990-2008 ................................................................................................ 72 Table 11 Frequency distributions of water levels and surges recorded and predicted at Hinkley Point in the period 1990-2008 ................................ 73 Table 12 Estimates of increases in MSL, MHW and MHWS recorded at Avonmouth between 1987 and 2008 over one lunar nodal cycle ............ 88 Table 13 Return periods for extreme high water levels (in m OD) predicted at Hinkley Point from previous studies ................................................. 103 Table 14 Extreme significant wave heights (Hs, in metres) predicted for Hinkley Point under 'present conditions' by HR Wallingford (2007) and Jacobs Babtie (2006) ........................................................... 106



- iv -

Table 15 Return periods of extremely high water levels (in m OD) at Hinkley Point, calculated using Extreme Value Theory (EVT) and Joint Probability Analysis (JPA) ..................................................... 109 Table 16 Return periods of extremely high water levels (in m OD) at Hinkley Point, calculated using Extreme Value Theory (EVT) and Joint Probability Analysis (JPA), assuming a tide equivalent to that which occurred in 1607 (8.50 m OD) had also occurred in the period 1991-2008 ............................................................................. 110 Table 17 Estimation of return periods high water of skew surges (SKHW, in metres) at Hinkley Point, using Extreme Value Theory (EVT) ............ 112 Table 18 Estimation of return periods of extremely high water levels (in m OD) at Hinkley Point, using Joint Probability Analysis of predicted tides, surges and waves .......................................................................... 113 Table 19 Estimation of return periods of extremely high water levels (in m OD) at Hinkley Point, using Joint Probability Analysis of predicted tides, surges and waves in the year 2100 ............................................... 124 Table 20 Summary of environmental extremes at Hinkley Point, based on

observations (O), estimates (E) and predictions (P) .............................. 127 Table 21 Summary of environmental extremes predicted at Hinkley Point in the year 2100, based on UKCP09 predictions ....................................... 128 Table 22 Projected still water levels, skew surge magnitude, and duration of high water above specified elevations, for different tide + surge conditions, under low, medium, high and H++ scenarios ....................... 131

List of Figures Figure 1 Location maps showing (a) the location of Hinkley Point Power Station in the context of the catchment of the River Parrett and the Bristol Channel, and (b) the main features within the Parrett Catchment .................................................................................................. 7 Figure 2 Generalised bathymetry of the Bristol Channel, based on Admiralty charts, and major net sediment transport pathways based on information from various sources .............................................................. 8 Figure 3 Simplified bathymetry map of Bridgwater Bay ....................................... 10 Figure 4 Seabed sediment distribution in the Bristol Channel and surrounding areas ......................................................................................................... 11



- v -

Figure 5 Simplified geological map of Bridgwater Bay ........................................ 12 Figure 6 The ‘standard’ postglacial geological sequence in the inner Bristol Channel and Severn Estuary .................................................................... 14 Figure 7 The extent of marine influence in the Somerset Levels at six dates during the Holocene ................................................................................. 18 Figure 8 Sections through the storm beach and marsh behind at Stolford ............ 19 Figure 9 Digital surface model of Hinkley Point Power Station and surrounding area, from lidar surveys flown 2007-2008 .......................... 24 Figure 10 Aerial photographs of the Hinkley Point frontage ................................... 25 Figure 11 Aerial photographs of the Bridgwater Bay frontage ................................ 27 Figure 12 Cross-section taken from lidar profile P20. Relevant observed and predicted extreme water levels are also shown ........................................ 29 Figure 13 Positions of MHW and MLW taken from Ordnance Survey County Series maps surveyed in 1886 and 1955-56 ............................................ 37 Figure 14 Annual mean tide, wind and wave parameters in the Bristol Channel .... 41 Figure 15 Observed trends in 15 minute water levels recorded at Hinkley Point,

Newport and Avonmouth during March 2002 ........................................ 43 Figure 16 Observed trends in 15 minute water levels recorded at Hinkley Point during 2002 .............................................................................................. 44 Figure 17 Locations of operative wave buoys and tide gauges ............................... 46 Figure 18 Frequency of observed water levels recorded at Hinkley Point in the period 1990-2008 ............................................................................... 47 Figure 19 Frequency of predicted water levels at Hinkley Point in the period 1990-2008 ................................................................................................ 47 Figure 20 Frequency of observed high waters recorded at Hinkley Point in the period 1990-2008 ..................................................................................... 48 Figure 21 Frequency of predicted high waters at Hinkley Point in the period 1990-2008 ................................................................................................ 48 Figure 22 Frequency of observed low waters recorded at Hinkley Point in the period 1990-2008 ..................................................................................... 49



- vi -

Figure 23 Frequency of predicted low waters at Hinkley Point in the period 1990-2008 ................................................................................................ 49 Figure 24 Maximum and minimum spring and autumn tides predicted at Hinkley Point ........................................................................................... 51 Figure 25 The 10 highest tides in each year observed at Hinkley Point in the period 1991-2008 ..................................................................................... 51 Figure 26 Observed and predicted trends in high water levels recorded at Hinkley Point, 1990 to 2008 .................................................................... 53 Figure 27 Observed and predicted trends in low water levels recorded at Hinkley Point, 1990 to 2002 .................................................................... 54 Figure 28 Fetch distances and bearings relative to Hinkley Point ........................... 58 Figure 29 Rose diagrams showing waves recorded at Hinkley Point (16/12/08 to 18/11/10), Scarweather (16/12/08 and 18/11/10), and Minehead (16/12/06 to 31/12/07) ............................................................................. 61 Figure 30 Frequency of significant wave heights recorded at Hinkley Point in the period 16/12/08 to 18/11/10 ............................................................... 62 Figure 31 Histogram showing frequency of significant wave height observed at the CEFAS wave buoy (16/11/08 to 18/11/10), and fitted Generalised Pareto Distribution calculated using the maximum likelihood method .. 63 Figure 32 Schematic diagram showing the main factors which control high water levels .............................................................................................. 66 Figure 33 Frequency of all non-tidal (surge) residuals recorded at Hinkley Point in the period 1990-2008 ........................................................................... 74 Figure 34 Frequency of all skew surges (SKHW and SKLW) recorded at Hinkley Point in the period 1990-2008 ................................................................. 74 Figure 35 Frequency of surge residuals at high water, recorded at Hinkley Point in the period 1990-2008 ........................................................................... 75 Figure 36 Frequency of skew surges at high water (SKHW), recorded at Hinkley Point in the period 1990-2008 ................................................................. 75 Figure 37 Frequency of surge residuals at low water, recorded at Hinkley Point in the period 1990-2008 ........................................................................... 76 Figure 38 Frequency of skew surges at low water (SKLW), recorded at Hinkley Point in the period 1990-2008 ................................................................. 76



- vii -

Figure 39 Cross-plot of predicted water level against positive surge residual >0.5 m at Hinkley Point (1990-2008) ...................................................... 77 Figure 40 Frequency histogram of a >0.5 m surge residual as a function of predicted water level at Hinkley Point (1990-2008) ............................... 77 Figure 41 Cross-plot of predicted water level against positive surge residual >1.0 m at Hinkley Point (1990-2008) ...................................................... 78 Figure 42 Frequency histogram of a >1.0 m surge residual as a function of predicted water level at Hinkley Point (1990-2008) ............................... 78 Figure 43 Sea level index points for the Bristol Channel, plotted as calibrated age against change in sea-level relative to the present (m), after Shennan and Horton (2002) ..................................................................... 80 Figure 44 Data completeness for POL Class A tide gauges at Newlyn and in the Bristol Channel .................................................................................. 82 Figure 45 Annual mean sea level recorded at seven tide gauge stations in the Bristol Channel ........................................................................................ 83 Figure 46 Trends in observed and predicted annual water levels at Newlyn .......... 84 Figure 47 Trends in observed and predicted annual water levels at Hinkley Point . 86 Figure 48 Trends in observed and predicted annual water levels at Avonmouth .... 87 Figure 49 Histogram showing frequency of observed water levels recorded at Hinkley Point (1991-2008) .................................................................... 108 Figure 50 Observed annual mean sea levels at Hinkley Point and future preditions based on an extrapolation of the 1992-2006 trendline, the DEFRA (2006) sea level rise allowances, and UKCP09 predictions .... 119 Figure 51 Future predictions of mean high water (MHW) at Hinkley Point based on UKCIP09 predictions for mean sea level ............................... 120 Figure 52 The long-term linear trend in skew surge at Hinkley Point from UKCP09 predictions .............................................................................. 121 Figure 53 Number of severe winter storms (October to March) per decade over the UK and Ireland between 1920 and 1999 (data from Allan et al., 2008) ...................................................................................................... 122 Figure 54 Extreme water levels estimated in this study in 2010 and 2100, overlain on lidar profile P20 .................................................................. 129



- viii -

Summary

This report considers "extreme events" relevant to the proposed new nuclear build site

at Hinkley Point, with particular reference to astronomical tides, surges and waves, and

to changes in coastal morphology. Following an introductory Section 1, a review of the

geological background, Holocene coastal evolution, present geomorphological character

and the physical processes is provided in Section 2. The Bristol Channel has a large

tidal range with strong tidal currents and moderate wave energy. The rocky shoreline at

Hinkley Point probably has not changed greatly over the past 5000 years, although

significant changes have occurred in the coastal lowlands to the east, and in the

intertidal and sub-tidal zones of Bridgwater Bay. Evidence from stratigraphic sequences

indicates rapid sea level rise between 9000 and 7500 years ago, after which time the rate

slowed, allowing alternating periods of marine regression and transgression to occur.

Tide gauge records for stations around the Bristol Channel suggest a possible

acceleration in sea level rise in the past two decades, although the longer record for

Newlyn in Cornwall provides no statistically significant evidence of recent acceleration.

The Blue Lias cliffs between Hinkley Point and Lilstock are relatively ‘soft’ but

erosion rates have been low since the 1880’s (< 0.2 m a-1) due to limited wave energy,

the existence of a limestone shore platform and the presence in many places of a

cobble upper beach which absorbs wave energy. East of Hinkley Point, longshore

drifting of sediment has resulted in foreshore lowering between Stolford and Catsford

Common. Given a lack of sediment supply and rising sea level, the sea defences in this

area are under increasing pressure.

Section 3 reviews the methods available to provide extreme water return level (RL)

estimates, including annual maxima, r-largest, GEV, peaks-over threshold and joint

probability methods. All statistical approaches are limited by the length and quality of

data which can be used as a basis for extrapolation. Section 4 presents the results of new

analysis of tide, surge and wave data for the area, and includes an assessment of the

potential implications of recent UKCP09 climate and sea level change projections.

Since the Hinkley Point Class A tide gauge record is short (c. 19 years), and nearshore

wave records are available only for the past 2 years, estimates of longer return period



- 1 -

levels based on these data may well be under-estimates. Uncertainties related to

extreme water levels in 2100 are compounded by uncertainties associated with the

UKCP09 climate projections.

Geological evidence suggests that sea level in the Inner Bristol Channel was 6 - 8 m

higher than at present during times in the Last Interglacial Period when global average

temperatures appear to have been 2ºC higher than at present. Rapid melting of the

Greenland and/ or parts of the West Antarctic ice sheet, leading to very rapid and

significant sea level rise, is considered unlikely in the next 100 to 200 years but cannot

be ruled out. It is therefore prudent to adopt a precautionary approach which takes

account of UKCP09 H++ sea level rise projections.

Using a joint probability approach applied to data for Hinkley Point, an estimate of the 1

in 10,000 year water level, resulting from combined tide plus surge plus waves, of 8.93

to 9.24 m OD has been obtained. Assuming a 1 m rise in sea level rise and a 4%

increase in surge height and wave height, the equivalent 1 in 10,000 year level in 2100

is estimated to be 10.02 to 10.34 m OD. An alternative approach is to consider ‘worst

case’ combinations of conditions. For the present day, the worst case would arise if the

largest possible surge and waves coincided with the highest astronomical tide. Such a

coincidence is extremely unlikely and would, if it occurred, produce an extreme water

level (>12 m) well in excess of the 1 in 10000 year event estimated by joint probability

analysis. A combination of HAT (7.12m OD), the estimated magnitude of the largest

surge previously recorded (c. 2m during the 1607 event) and half the largest measured

inshore significant wave height (1.41 m), would produce a resultant extreme water level

of 10.53 m.

In terms of future climate change, the higher estimate for increase in mean sea level

identified under the H++ emissions scenario (1.9 m), together with current UKCP09

estimates of possible increases in skew surge and wave height, provide a possible near-

‘worst case’ scenario for 2100. A combination of HAT, surge of 1607 magnitude and

half the largest measured inshore significant wave height would give a resultant water

level of 11.48 m OD under the SRES A1B medium emissions scenario (50% percentile

value) and 15.33 m OD under the H++ high estimate scenario.



- 2 -

A Consideration of "Extreme Events" at Hinkley Point,

Somerset, With Particular Reference to Coastal Flooding and

Coastal Change

1. 0 Introduction 1.1 Report scope and purpose

This report presents a consideration of "extreme events" relevant to the safety case for

the proposed new nuclear build (NNB) site and adjoining areas at Hinkley Point,

Somerset, with particular reference to coastal flooding and erosion. The nominated

NNB site is located just to the west of the existing Hinkley Point A power station, on

the southern shore of Bridgwater Bay and approximately 8 km to the west of the mouth

of the River Parrett (Figure 1a & b). Hinkley Point A is a twin-reactor Magnox power

station which operated from 1965 to 2000 and is now being de-commissioned. To the

east of Hinkley Point A is the Hinkley Point B power station, a twin reactor Advanced

Gas-Cooled Reactor (AGR) power station which began operation in 1976 and is

expected operate until at least 2016. The nominated site includes the area which

received planning consent in 1990 for a single Pressurised Water Reactor (PWR) power

station which was not constructed. The seaward boundary of the nominated site is

drawn at the high water mark, but it will also probably be necessary to build cooling

water intake and outfall structures and possible also coastal defences and marine off-

loading facilities to seaward of this boundary (EDF Energy, 2009).

In this study consideration has been given to the proposed new build site itself, the

adjoining coast between Lilstock and Brean Down, the Parrett Estuary, and areas of

adjoining low-lying land. A major objective of the work has been to provide a

formalised understanding of the environmental "extremes" which the site has

experienced, and might experience in the future, together with the methods which are

most commonly used to predict "extreme events", including statistical modelling and

expert assessment. The relevance of the concept of the "1 in 10,000 year event" is

considered in terms of the limitations of these methods and known facts about events



- 3 -

which have occurred in the geological past. The implications of future climate and sea

level change projections, as summarised in outputs from UKCP09, are also considered.

1.2 Methods and information sources

The study was mainly desk-based and did not involved the collection of new primary

field data, although a walk-over survey of much of the coast between Watchet and

Brean Down was undertaken in mid September 2009.

The following sources of information were used:

unpublished reports prepared as part of the Hinkley Point NNB

assessment, provided by CEFAS

published scientific papers and other unpublished reports relating to the

Bristol Channel, Severn estuary and Somerset Levels, and to climate and

sea level change more generally

published papers and unpublished reports dealing with methods of

statistical forecasting and expert assessment

primary hydrodynamic and geomorphological data, including records of

tides, waves, sea level, topographic and bathymetric survey data, lidar

data, and aerial photographs.

1.3 Report structure

The remainder of the report is divided into four further sections. Section 2 summarises

the geological and environmental setting of the Hinkley Point site, including the

Holocene coastal evolution of the area and the physical processes which impact on it.



- 4 -

Section 3 reviews the main methods available for the prediction of environmental

"extremes", with particular reference to extreme water levels and coastal flood risk.

Section 4 examines the nature of "extreme events" relevant to coastal flooding risk and

rapid coastal change in the Hinkley Point area, including the implications of future

climate and sea level change. Section 5 summarises the main conclusions and

implications arising from the study. Supplementary information is provided in the

appendices.



- 5 -

2. Geological background and physical processes in the Hinkley Point area

2.1 Location and setting

The Hinkley Point nuclear power station complex is located on the southern shore of the

inner Bristol Channel, close to the western limit of Bridgwater Bay. Following Posford

Duvivier and ABP Research (2000), the eastern limit of the Inner Bristol Channel is

defined in this study by a line drawn between Brean Down and Lavernock Point, while

the western limit is defined by a line drawn between Nash Point and Minehead Bluff.

To the east of the Brean Down - Lavernock Point line is the Severn estuary, which

extends upstream to the normal tidal limit at near Gloucester (Figure 1a). It should be

noted that different limits have been adopted in some previous studies (e.g. ABPmer,

2006).

Approximately 8 km to the east of Hinkley Point is the mouth of the River Parrett, a

significant river which drains approximately 1600 km2 of the Central Somerset Levels

and surrounding upland areas (Figure 1b). The rivers Brue and Huntspill also flow into

the Parrett estuary near Burnham-on-Sea. These rivers have been heavily engineered

since Roman times and the fluvial discharge is now highly regulated. Most of the

adjoining area is low lying and at high risk of river and coastal flooding; proposals for

further works to reduce the risk from fluvial and coastal flooding are currently under

consideration (Black & Veatch, 2006; Environment Agency, 2009).

The Inner Bristol Channel and Severn Estuary occupy a major submerged valley system

which links the alluvial plain of the River Severn (the Severn Vale) with the Outer

Bristol Channel and the Celtic Sea. The drowned river valley system is structurally

controlled but has been enlarged by fluvial incision into rocks of Carboniferous,

Jurassic and Triassic age. During several periods of the Pleistocene sea level was at

times up to 120 m lower than at present (Whittaker & Green, 1983; Hawkins, 1990). At

the present day, high tide water depths in the Outer Bristol Channel reach 55 to 65 m,

while those in the Inner Bristol Channel attain a maximum of 35 to 40 m (Figure 2).



- 6 -

B R I D G W A T E R B A Y

B R I S T O L C H A N N E L

ParrettEstuary

B L A C K D O W N

HI L

LS

D O R S E T H E I G H T S

P O L D E NH

I LL S

TAUNTON

BRIDGWATER

YEOVIL

BURNHAM-ON-SEA

River Cary

River Yeo

Axe Catchment

Brue Catchment

ParrettCatchment

OL

OL =NB =

Oath Lock (River Parrett)New Bridge (River Tone)

Q U A N T OC

K H

I L L S

BR

EN

DO

N H I L L S

HinkleyPoint

River Axe

River BrueRiver Huntspill

King’s S edgemoor Drain

River Parrett

Rive

r Isle

River Tone

NB

Normal Tidal Limits (NTL)

River Parrett

Swansea

Cardiff

Gloucester

Bristol

Yeovil

TauntonBarnstaple

Outer BristolChannel

Enlargement below

Figure 1 Location maps showing (a) the location of Hinkley Point Power Station in the context of the catchment of the River Parrett and the Bristol Channel, and (b) the main features within the Parrett catchment. The Catchments of the Parrett, Axe and Brue together form the Somerset Levels. Modified from Environment Agency (2009).

(a)

(b)

Inner BristolChannel

Central BristolChannel

LundyIsland

Severn

Estuary

WALES

ENGLAND

SO

ME

RS

ET

LE

VE

LS

St. Ann’sHead

WormsHead

HartlandPoint

BullPoint Minehead

Bluff

NashPoint

BreanDown

LavernockPoint

ParrettCatchment

Normal Tidal Limits atLlanthony Weir andMaisemore Weir

0

Scale

40 km302010

0

Scale

10 km862 4

HinkleyPoint



- 7 -

Figure 2 Generalised bathymetry of the Bristol Channel, based on Admiralty Charts, and major net sediment transport pathways based on information from various sources. Modified after Pye and Blott (2009).

MilfordHaven

Tenby

Carmarthen

Llanelli

Swansea

PortTalbot

Bridgend Cardiff

NewportChepstow

Gloucester

Bristol

Bath

Glastonbury

Bridgwater

Weston-Super-Mare

Barnstaple

St David's

Porthcawl

Barry

Burnham-on-Sea

MineheadIlfracombe

Gower

WestwardHo! Taunton

exposed bedrockBathymetry

(m CD)

-60

-40

-20

-10

coastline

0

Scale

20 km15105

HinkleyPointWatchetCeltic

Sea

WalesR. Severn

Avonmouth

South-WestPeninsula

(land)

Epney

Old Severn Bridge (M48)Second Crossing (M4)

_________________________________________________________________________________________________________________K

enneth Pye Associates Ltd. Report EX1207

Hinkley Point Extrem

es_________________________________________________________________________________________________________________

- 8 -

Bridgwater Bay has a maximum high tide depth of 20 to 25 m near its outer edge but is

generally shallow with wide inter-tidal flats exposed at low tide (Figure 3).

The Severn Estuary and Inner Bristol Channel broadly conform in shape and position to

the Severn Estuary Fault Zone which is known to have been active as a dextral strike-

slip system during the early Carboniferous period (Tappin et al., 1994). The area is

floored by a gently folded and faulted succession of Carboniferous to lower Jurassic

limestones, mudstones and siltstones. In the inner Bristol Channel the bedrock consists

mainly of folded Liassic shales and interbedded limestones. Between Brean Down and

Lavernock Point there is a zone of complex folding which expose inliers of

Carboniferous limestone (the islands of Flat Holm and Steep Holm), surrounded by

Triassic strata (Donovan et al., 1961; Donovan & Stride, 1961; Lloyd et al.1973).

Over much of the western part of the area, bedrock is exposed at the sea bed (BGS,

1986; Figure 4). Elsewhere, the superficial sediments (i.e. those above the bedrock)

include glacial till, post-glacial alluvial deposits, and recent (later Holocene) muds,

sands and localised gravels (Evans, 1982; Pantin, 1991; Figure 4). These sediments

partially infill an incised valley drainage system whose maximum axial depth increases

from c. -20 to -30 m OD in the east to c. -40 m north of Minehead.

The bedrock exposed in the coastal outcrops of western Bridgwater Bay area is

represented mainly by Lower Jurassic (Lias) strata which outcrop as low cliffs and a

wide inter-tidal shore platform at Hinkley Point. Similar Lias shore platforms backed by

soft cliffs extend along the coast westwards towards Watchet (May, 2007). To the east

of Hinkley Point is a broad coastal plain which consists almost entirely of Quaternary

sediments (Whittaker & Green, 1983; Figure 5). The headland at Brean Down which

forms the northern limit of the present study area is composed mainly of Carboniferous

limestone.

The coast of Bridgwater Bay consists of two main sandy barrier systems (Stolford to

Stert Point and Brean Down to Berrow, near Burnham), the combined Parrett-Brue-

Huntspill estuary, and an extensive area of estuary-margin and back barrier marshes

which were largely reclaimed in Roman, Saxon and Medieval times. The Stolford -

Stert barrier consists of mixed sand and gravel and is characterised by only low dune



- 9 -

%0

Scale

4 km1

Figure 3 Simplified bathymetry map of Bridgwater Bay, based on Admiralty Chart 1152 (published 1993, edition 2004, corrections to 2009, survey dates shown on inset map). The ‘coastline’ in this instance is taken from highest astronomical tide level from 2007-8 lidar surveys.

Watchet

Lilstock

WestQuantoxhead

HinkleyPoint Steart

Burnham-on-Sea

StertIsland

StertFlats

BerrowFlats

Brean

Berrow

BreanDown

WestonBay

Weston-Super-MareSteep

Holm

One Fathom Bank

Culver Sand

StokeSpit

BridgwaterBar

Gore SandNorth-West

Patches

CobberPatchKilve

Patch

GrahamBanks

SouthPatches

Stogursey

Bathymetry (m CD)

-15

-10

-5

0

coastline

+25 m ODcontour

-20

-30

2 3

(land)

a

a

a

a

1961-621965-711979-801989

bf

e

egd

dh

hc

a

abcd

199019911995-991999-2000

efgh

Survey Dates

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 10 -

GRAVEL

MUD SAND1:9 1:1 9:1

PERC

ENTA

GE

GRA

VEL

(not

to s

cale

)

80

30

5

SAND : MUDRATIO

G

mG msG sG

gM gmS gS

M sM mS S

M MudsM Sandy mudgM Gravelly mud (not present)S SandmS Muddy sandgmS Gravelly muddy sandgS Gravelly sandG GravelmG Muddy gravelmsG Muddy sandy gravelsG Sandy gravel

Licensed spoil dumping areas

No data

Licensed dredging areas

Outcrop of rock or till

Figure 4 Seabed sediment distribution in the Bristol Channel and surrounding areas. Modified from BGS (1986) and Pantin (1991).

Hinkley Point

Cardiff

Swansea

Bristol

Wales



- 11 -

Hangman Grits(Devonian)

Ilfracombe Slates(Devonian)

River terracedeposits

Burtle Beds

CarboniferousLimestone

Mercia Mudstone(Triassic)

Blue Lias(Jurassic)

Upper and MiddleLias (Jurassic)Blown sand

Peat TriassicSandstones

Gravel beachdeposits

SuperficialDeposits

Alluvium

Head

Bedrock

%0

Scale

2 km1

Figure 5 Simplified geological map of Bridgwater Bay, based on BGS 1:50,000 sheet numbers 279 and 295. Coastline (MHWS) and MLW taken from lidar surveys flown in 2007 and 2008.

MLW Weston-

super-Mare

BreanDown

BrentKnoll

Berrow

Brean

Lympsham

Burnham-on-Sea

Highbridge

Steart

HinkleyPoint

Stolford

Bridgwater

Woolavington

River Parrett

Stogursey

Lilstock

Watchet

WestQuantoxhead

Williton

NetherStowey

Stert Flats

BerrowFlats

StertIsland

BridgwaterBay

Gore Sand

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 12 -

development. The Brean - Berrow barrier system is predominantly sandy and is

characterised by dunes up to 12 m high. A third, smaller barrier system composed of

mixed sand and gravel, occurs between Hinkley Point and Stolford but has been heavily

modified by coastal engineering works. Behind this barrier is an area of low-lying

reclaimed grassland (Wick Moor) which extends between Hinkley Point power station

and Stolford village. Progressive loss of sediment from the barrier during the 19th and

20th centuries resulted in a series of coast protection and flood defence works, including

groynes, rock armour and a concrete sea wall.

The bedrock surfaces around the margins of the Inner Bristol Channel and Severn

estuary are overlain by patchy late Pleistocene deposits and up to 25 m of Post-glacial

(Holocene) alluvium. Glacial till occurs at a number of locations on the coast of North

Devon and Somerset (e.g. at Fremington and near Lilstock), but glacial sediments are

absent from much of the onshore area. There is evidence of early or middle Pleistocene

glaciation in the Bristol district, but not in Somerset. Ipswichian (last interglacial

period) and possibly earlier (Hoxnian) interglacial high sea level stands are represented

by sands, gravels and mud deposits in several parts of the area, including the 'Burtle

Beds' which cover areas of higher ground within the Somerset Levels (Figure 5). In

many places the surface of the Burtle Beds stands several metres above the level of the

adjoining Holocene estuarine deposits, suggesting a maximum sea level 6 - 9 metres

higher than present during one or more previous interglacial periods (Kidson et al.,

1978, 1981; Allen, 2002; Hunt, 2006).

2.2 Holocene stratigraphy and coastal evolution

In many places the bedrock is overlain by a soil or peat layer with in situ mature tree

stumps and fallen trunks (the "basal peat") which formed during the Last Glacial Period

(> 10,000 years ago). The basal soil / peat is in turn overlain by greenish coloured

estuarine clays, a further peat bed, and an upper sequence of more brownish coloured

estuarine clays which accumulated during the Holocene (Flandrian) period. On the

Severn Levels, similar bluish-green muddy sediments (termed the Wentlooge Formation

by Allen & Rae, 1987, 1988; Figure 6), form a sequence up to 12 m thick. The upper



- 13 -

Figure 6 The ‘standard’ postglacial geological sequence in the inner Bristol Channel and Severn Estuary. After Allen (1992).



- 14 -

part of the sequence contains peat beds which are locally overlain by a further 2 m of

Wentlooge sediments. The top of the uppermost Wentlooge peat on Caldicot Level gave

a date of 2660 +/- 100 yr BP (Godwin & Willis, 1964) while the uppermost 0.1 m of

peat at Rumney Great Wharf gave a date of 2180 +/- 50 yr BP (Allen & Fulford, 1986).

Large areas of the Wentlooge silts were embanked and reclaimed during the Roman

period (Allen & Fulford, 1986), leading to the formation of a palaeosol (the Wentlooge

Surface of Allen & Rae, 1987). Outside the Roman sea banks, (generally brownish)

estuarine sediments continued to accumulate. Erosion in the post-Roman period led to

localised set-back of the sea walls, notably during the medieval period, which allowed

renewed estuarine sedimentation on top of the Wentlooge Surface. The resulting

sediments, which are pinkish-brownish in colour, were termed the Rumney Formation

by Allen & Rae (1987). At Slimbridge Warth, near Berkeley in the inner estuary, 14th

century reclamations overlie at least 1.2 m of Rumney Formation sediments, suggesting

that the Rumney Formation began to accumulate in this area during early or high

medieval times. Elsewhere, it may have begun to form at an earlier date. Embanking

and reclamation of the Rumney Formation was extensive during the medieval and later

periods, leading to a further soil on top of the reclaimed Rumney Formation (the

Oldbury Surface of Allen & Rae, 1987). Outside the sea banks, vertical accretion of the

Rumney Formation has continued to the present except where further embanking and

reclamation occurred in the late medieval and post-medieval period, forming the

Oldbury Surface of Allen & Rae (1987). Episodic erosion at the seaward edge of the

Rumney Formation in some places led to the creation of saltmarsh cliffs, to seaward of

which in some places two further lithostatigraphic units (the Awre and Northwick

Formations of Allen & Rae, 1987; Figure 6) have accumulated in the past 400 years.

These deposits are mid to dark grey in colour. The Northwick Formation is often

separated from the Awre Formation by a small cliff and overlies an erosional surface cut

into the Awre and / or Rumney Formation sediments. Parts of the Awre Formation have

been embanked, creating the Awre Surface (Allen & Rae, 1987), but the Northwick

Formation has not been embanked.

On the English side of the estuary these formations can be traced downstream as far as

Middle Hope, although the younger formations become fragmentary and stratigraphic

relationships more obscure (Allen & Rae, 1987). On the Welsh side of the estuary



- 15 -

dominant erosion in the last few centuries has prevented the formation of an extensive

Awre Formation, small areas of which are generally restricted to the mouth of gullies

and tributary rivers. However, in several places a significant width / thickness of

Northwick Formation sediments has accumulated in front of cliffs cut into the

Wentlooge and Rumney Formations. Where erosion has prevented the accumulation /

preservation of the younger formations there are extensive intertidal and cliff exposures

of the Wentlooge and Rumney Formations (e.g. at Rumney Great Warth, near

Newport).

The age of the Awre Formation sediments is not well constrained, but based on

morphological relationships with construction features of known age, and the fact that

they contain moderate levels of industrial contaminants, Allen & Rae (1988) considered

an early to mid 19th century date for the onset of their accumulation to be most likely.

The erosional surface cut into the older sediments below the Northwick Formation

contains late 19th century to early 20th century pottery fragments, and the entire

sedimentary sequence contains high levels of industrial contaminants. This, combined

with historical map evidence, suggests a mid to late 20th century age for the Northwick

Formation.

Allen & Rae (1987 p 228) concluded that "Although incomplete, archaeological,

historical, radiocarbon dating and chemical evidence combine to make a prime facie

case for the view that each of these formations is broadly synchronous over the extent of

the estuary. With the exception of the (upper) Wentlooge Formation, each

lithostratigraphic unit overlies an erosion surface composed of a cliff and gently

shelving wave-cut platform. Each formation therefore seems to record an estuary-wide

retreat followed by advance of the shore. Within the limitations of available dating,

shoreline retreat (erosion) occurred during the Saxon to early mediaeval periods, and

again in the 19th and 19th-20th century".

In the Somerset Levels, Holocene sediments largely infill the former valleys of the

Rivers Axe, Brue and Parrett which are separated by areas of higher ground (Nyland

Hill, the Isle of Wedmore and the Polden Hills). Towards the landward margins of these



- 16 -

areas Holocene marine and estuarine silts give way to freshwater fen peats and raised

bog deposits of the Somerset Moors (Kidson & Heyworth, 1976). Three main Holocene

stratigraphic units, the Lower, Middle and Upper Somerset Levels Formations, have

been recognised on the basis of borehole data (Kidson & Heyworth, 1976; Haslett et al.,

2001; Long et al., 2002). The Lower and Upper Formations consist mainly of marine

silts, clays and sands, while the Middle Formation consists mainly of freshwater and

saltmarsh peats. Radiocarbon dates indicate that the Middle Somerset Levels Formation

began to form about 7000 years ago (7.0 ka cal yr BP) and was inundated by marine

transgression about 4000 years ago (4.0 ka cal yr BP). Further transgression in the post-

Roman period resulted in deposition of 0.5 to 1.0 m of sediment across much of the

coastal plain. The early to mid Holocene transgression apparently occurred under

conditions of relatively rapid sea level rise and limited sediment supply, while the mid

to later Holocene transgression occurred under conditions of lower sea level rise,

probably encouraged by the compaction of the early to mid Holocene peats (Haslett et

al., 1998a,b).

Based on borehole data and the results of radiocarbon dating of the peats, Kidson &

Heyworth (1976) produced a series of maps showing the varying inland extent of

marine influence at different times during the early to mid-Holocene (Figure 7). These

maps were modified by Long et al. (2002) but the differences are relatively minor.

Cross-sections showing the Holocene coastal stratigraphy between Stolford and Hinkley

Point are shown in Figure 8. On the inner part of Wick Moor there is essentially a

single, thick peat bed which is underlain and overlain by marine sediments, but near the

modern coast, and in the intertidal zone, a number of peat beds are present at different

depths, separated by marine silt layers. The modern coastal gravel barrier apparently

overlies an older gravel ridge, suggesting that the position of the shoreline in this area

probably has not changed dramatically in the last 6000 years.

During the period 5600 to 3000 BP a further peat sequence formed in the Central

Somerset Levels, although near the coast episodes of minerogenic sedimentation

occurred due to periodic marine incursions. These deposits are referred to as the Middle

Wentlooge Formation by Rippon (1993, 1997). Around 3000 to 2800 yr BP a more

persistent phase of marine transgression began which reached its greatest extent around



- 17 -

Figure 7

The extent of marine influence in the Somerset Levels (shaded areas) at six dates during the Holocene.Sources: (a) to (e) Kidson and Heyworth (1976); (f) Rippon (1997).

2000 B.P.

(a) (b) (c)

(d) (e) (f)

Sections through the storm beach and marsh behind at Stolford. Source: Kisdon and Heyworth (1976).Figure 8

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 19 -

2500 yr BP (Housley, 1988). Almost the entire area of the Gwent, Central Somerset and

North Somerset Levels was affected by this transgression. During the Roman period

extensive embanking and land reclamation was undertaken. A Roman settlement was

established at Combwich, which possibly acted as a port and/or fording point on the

River Parrett (Dewar, 1940; Pike & Langdon, 1981).

It remains unclear if sea level rise slowed or ceased during the early Roman period,

thereby facilitating land reclamation, and to what extent accelerated sea level rise

towards the end of Roman times contributed to the failure of many sea defences and

renewed marine sedimentation. Godwin (1943) referred to this period as the 'Late

Roman Marine Transgression', but the relative roles of sea level rise and cessation of

sea defence maintenance in contributing to the apparent transgression are uncertain. In

central Somerset the late Roman / early post-Roman transgression extended up to 6 km

inland (Rippon, 1997). The distribution of salt-production sites suggests that the area

presently occupied by the rivers Brue, Huntspill and Parrett were subject to regular tidal

influence at this time, and that a seabank may have existed between the sand dune

barrier south of Brean Down and the higher ground east of Brent Knoll (Rippon, 1997).

At this time the slightly elevated outcrops of the Burtle Beds at Huntspill and Pawlett

formed low islands which were sites of human occupation (Hawkins, 1973).

Sand dunes were present in the Brean Down area during the Bronze age (Bell, 1990),

and the low surface elevations (5.4 to 5.8 m OD) of the back-barrier marshes between

Brean and Berrow suggest that much of this area was protected from regular tidal

inundation from an early date (Rippon, 1997). A tidal creek (the 'Siger') formerly

flowed westwards to the sea just south of Brent Knoll, and the distribution of salt

workings suggests that in Romano-British times a seabank along its northern side

excluded tidal waters from the area further north (Rippon, 1997; Figure 7f). The first

documentary evidence of the existence of dunes further south at Berrow is in a charter

dated AD 973 (Costen, 1992), and it is possible that no dune barrier existed in this area

during Romano-British times.

Re-building of sea walls in the Central Somerset Levels had probably begun by the 9th

century, although evidence from the early Saxon period is extremely limited. Almost the



- 20 -

entire area (including the land south of the Siger) was apparently enclosed and settled

by the early 11th century (Rippon, 1997). Further sea defence and land drainage

improvements were undertaken between the 11th and 14th centuries. Major engineering

works undertaken in the high mediaeval period still dominate the modern drainage

system (Williams, 1970). In the 13th century the River Brue was diverted from its old

course, which joined the river Axe, to join the existing Westhill Rhyne tidal water

course and enter the sea at its present location between Burnham and Huntspill.

Reclamations in this period may have been favoured by relatively warm climatic

conditions and relatively few severe storms, compared with the early Saxon and late /

post medieval periods (Rippon, 1997).

The effects of the Black Death, rural depopulation and economic decline in the 14th

and 15th centuries contributed to widespread failure and non-repair of sea defences. The

14th and 15th centuries also saw a climatic deterioration and increased storminess

which may also have contributed to sea defence failures. Severe storms occurred in

1324, 1326, 1424, 1425, and 1485 and erosion was apparently widespread (Rippon,

1997 p.243). During the 15th century, the entire sea wall along the Gwent coast was set

back (Rippon, 1996). Erosion continued during the 16th century, leading to further set-

back. Contemporary reports also suggest that the Parrett estuary was badly affected by

storm flooding and erosion in the high medieval period, and the former causeway and

ford at Combwich was abandoned (Williams, 1970; Rippon, 1997).

Storm flooding and erosion continued during the 16th century, culminating in a

disastrous storm in 1607 which killed several thousand people and numerous cattle

around the Bristol Channel and Severn estuary (Horsburgh & Horritt, 2006). However,

it appears that the walls were rapidly repaired and there was no long-term, large-scale

abandonment of land to the sea. Following the 1607 flood there was a sustained period

of sea defence and inland drainage improvements which continued until 1660

(Williams, 1970). Further reclamations were made in the Central Somerset Levels at

Bleadon, Chislett, Dunball and Pawlett. Reclamation of riverside wharfs (marshes)

continued along the Parrett until the later 18th century, although there were also

periodic losses due to storm-induced channel-shifts (Dunning, 1992). These activities



- 21 -

were apparently driven by increasing population and returning economic prosperity

(Williams, 1970; Rippon, 1997).

2.3 Geomorphological character of the coast

A series of composite vertical aerial photographs, taken in September 2008 and which

show the general nature of the coast between St Audries' Bay and Brean Down, is

presented in Appendix 1. Composite digital surface model images of the same areas,

constructed using lidar data flown in 2007-08, are presented in Appendix 2, and the

corresponding First Edition Ordnance Survey Six Inch maps are shown in Appendix 3.

The positions of the High and Low Water Marks of ordinary Tides shown on the First

Edition Six Inch Ordnance Survey (OS) maps has been superimposed on the 2008 aerial

photographs and lidar images, while the level of the mean high water mark determined

from the 2007-08 lidar at Hinkley Point (4.63m OD) has been plotted on the First

Edition Six Inch maps. It should be noted that a single value for the level of MHW was

assumed for the entire coastal frontage between St Audrie's Bay and Brean Down; this

value strictly applies only to the Hinkley Point area and will be slight over-estimation of

the actual levels along the coast to the west and a slight under-estimation of the actual

levels in the eastern part of Bridgwater Bay.

Ground photographs of selected locations along the Bridgwater Bay frontage, taken in

late September 2009, are included in Appendix 4.

Hinkley Point itself forms a slight headland, the form of which is partly artificial.

During construction of the 'A' power station the high water mark was shifted seawards

by the creation of made ground and protecting seawall (see Map C in Appendix 3 and

Photographs 6 & 16 in Appendix 4).

The proposed NNB site lies at the eastern end of an unprotected cliff frontage,

extending between Hinkley Point A station and Lilstock, and formed in Lower Lias

(Jurassic) mudstones and limestones. The cliffs are c 25 m high at Lilstock but become

lower towards the east, reaching a minimum of c. 3 m near the proposed NNB site



- 22 -

(Figures 9 & 10; Photograph 7 in Appendix 4). Between Hinkley Point and the NNB

site the Lias strata form a low dome whose northern edge has been truncated by marine

erosion. The rocks show a slight dip of 8 to 10º towards the north and north-east.

Consequently the shore platform to seaward of the cliff toe consists largely of inclined

'ledges' formed by the more resistant limestone beds, separated by deeper troughs

formed in the more erodible mudstone beds (Photograph 18 in Appendix 4). Parts of the

cliff toe are protected by a cobble beach, but in places no sediment is present and the

cliff toe shows evidence of active undercutting (Photograph 12 in Appendix 4).

To the east of the existing power station lies a lowland area which extends eastwards to

the mouth of the River Parrett. Prior to land reclamation, the area consisted mainly of

saltmarshes and mudflats with localised mixed sand and gravel beach ridge deposits and

low sand dunes. Immediately east of Hinkley Point this extensive lowland is interrupted

by a ridge formed of head deposits which overlie Liassic rocks (Figure 9) and project

into the intertidal zone near Stolford. A former valley, now occupied by low-lying

grazing land (Wick Moor) runs inland along a SW-NE axis immediately east of the

Hinkley Point B power station site.

A 1 to 2 km wide intertidal rock platform is exposed at low tide immediately to the

north of the Hinkley Point A and B power stations. The headland and shore platform are

formed in Blue Lias rocks of Lower Jurassic age (Figure 5). Similar Lias shore

platforms and soft cliffs extend westwards towards Watchet (May, 2007). The Blue Lias

consists of alternating strata of grey-blue mudstones and pale-coloured limestone bands.

The mudstones become fissile on weathering and break down relatively rapidly. The

intervening limestone bands are much harder but break down on exposure into blocks

by enlargement of fracture planes formed during tectonic deformation of the rocks

(Photographs 7, 8 & 9 in Appendix 4).

Most of the existing power station infrastructure is located on ground with an elevation

of 8 to 12 m OD (Figure 9). The proposed NNB site lies just to the west of the existing

'A' station on ground which has a present general surface level of 12 to 15 m OD

(Figure 10). At the western margin of the site the ground level falls away towards a

small valley, but the site also contains two mounds of made ground. A number of cross



- 23 -

319000 319500 320000 320500 321000 321500 322000 322500 323000144500

145000

145500

146000

146500

147000

147500

0 200 400 600 800 1000

Scale (m)

Wick Moor

Stolford

Hinkley PointPower Station

BenholePoint

Figure 9 Digital surface model of Hinkley Point Power Station and surrounding area, from lidar surveys flown 2007‐2008. Overlain are MHW and MLW linestaken from Ordnance Survey County Series maps surveyed in 1886 and 1955‐56. MHW and MLW in 2008 taken from lidar surveys, assumingelevations of 4.63 and ‐3.84 m OD respectively. Original data source: Channel Coastal Observatory.

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (m OD)

proposedNNBsite

A B

shoreplatform

cooling wateroutflow channel

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 24 -

319000 319200 319400 319600 319800 320000 320200 320400 320600 320800 321000 321200 321400 321600 321800 322000146000

146200

146400

146600

146800

147000

Figure 10 Aerial photographs of the Hinkley Point frontage, taken 19/09/2008 (western half) and 31/05/2006 (eastern half). Also shown are the locations of profile linesused for topographic profiles and measurement of historical tide line movement. Source: Channel Coastal Observatory (west) and Google Earth (east).

BenholePoint

shore platform

proposedNNB site

Hinkley PointPower Station

cooling wateroutflow channel

P4P5

P6P7 P8 P9 P10 P11

P12 P13P14

P15P16

P17 P18 P19 P20 P21P22

P24P23

P25

P26

sections across the intertidal zone and immediate hind-shore area, corresponding to the

profile lines shown in Figure 11, are shown in Appendix 5.

The natural ground elevation near Benhole Point is > 20 m OD and the cliffs in this area

are c. 8 to 11 m high. Along the northeastern edge of the NNB site the ground level lies

at c. 14 m OD and the cliffs are c. 7 m high. The parapet of the present seawall in front

of the A and B power stations has a crest height of c. 8.43 to 8.83 m OD (Table 1 &

Figure 12), while the crest height of the gabion wall behind lies at c. 11.10 to 11.50 m

OD (Photograph 19 in Appendix 4). The gabions are attached to a wide embankment on

the north-eastern corner of the power station site, with a similar crest height (c. 11.50 m

OD, Photograph 24). The rock platform in front of the concrete seawall ranges in

elevation from c. 3.0 m OD to 0 m O.D. To the south and east of the power station the

ground level falls away and is c. 5.45 to 6.3 m OD across much of Wick Moor. The

maximum height of the most seaward rock armour protection along this frontage ranges

from 7.10 m OD near the power station to 9.10 m at the Stolford end (Photograph 34).

The secondary defence embankment near the power station has a crest height of 8.70 m

while that near Stolford has a crest height of 8.87 m OD. The foreshore in front of Wick

Moor consists of an upper gravel beach and a thin, patchy, sandy foreshore which

includes exposures of the Lias bedrock and early to mid Holocene peat beds (shown on

the older maps and charts as a 'submerged forest').

The eastern end of the Wick Moor embayment is relatively exposed to wave action on

account of its aspect and the fact that a NW-SE gap in the Lias shore platform allows

north-westerly waves from relatively deep water to reach the shore. Stolford hamlet is

protected by a concrete sea wall which has a crest (parapet) height of c. 9 m OD, fronted

by rock armour. To the east of Stolford, at Catsford Common and Wall Common, there

is a double sea defence consisting of an outer artificial sand and gravel embankment

(crest height 8.36 m OD) and an inner clay embankment (crest height 8.14 m OD).



- 26 -

310000 311000 312000 313000 314000 315000 316000 317000 318000 319000 320000 321000 322000 323000 324000 325000 326000 327000 328000 329000 330000 331000 332000 333000

141000

142000

143000

144000

145000

146000

147000

148000

149000

150000

151000

152000

153000

154000

155000

156000

157000

158000

159000

160000

Figure 11 Aerial photographs of the Bridgwater Bay frontage, taken 19 September 2008,locations of 64 profile lines used for topographic profiles and measurementof historical tide line movement, and areal coverage of available lidar surveys2007‐2008.

1

9-23

0 1000 2000 3000 4000

Scale (m)

Lidar Surveys

07/05/2008

24/02/2008

26/01/2008

26/10/2007

20/03/2007

17/03/2007

Ordnance Survey Description Ordnance SurveyBenchmark Elevation (m OD)ST 2068 4620 Rivet para sea defence wall 8.8263

21.7 m SW fence ang50.0 m E water outlet pipe

ST 2078 4626 NBM rivet sea defence wall 8.832441.28 m SW C outfall(Hinkley Point TG aux1)

ST 2094 4631 NBM bolt sea defence wall 8.836331.245 m SW end railings(Hinkley Point TG aux2)

ST 2104 4634 NBM bolt sea defence wall 8.83880.962 m NE of SE cnr steps(Hinkley Point TGBM)

ST 2123 4634 NBM steel bolt sea defence wall 8.849425.070 m E C outfall(TG aux3)

ST 2162 4622 NBM rivet para sea defence wall 8.827319.0 m SW ang wall

Topographic Elevation from lidar survey Elevation from lidar surveyProfile on 26/10/07 (m OD) on 26/01/08 (m OD)P15 8.30 8.60P18 8.48 8.68P20 8.53 8.66P22 8.43 nd

Table 1 Elevations of the sea wall at Hinkley Point Power Station, measured at Ordnance Survey bench marks and lidar surveys on 26/10/07 and 26/01/08 (profile locations shown on Figure 10).



- 28 -

Elevation (m OD)

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

Figure 12 Cross-section taken from lidar profile P20 (Figure 10). Relevant observed (black) and predicted (red) extreme water levels are also shown. Observed levels from 15 minute observations supplied by NTSLF. Estimated level of 1607 flood from Risk Management Solutions (2007). Return periods taken from HR Wallingford (2009) relate to predictions for 2080s.

Highest Astronomical Tide, 29/09/2015 (7.12 m OD)High tide 10/02/1997 (7.36 m OD)High tide 13/12/1981 (7.40 m OD)

Maximum predicted tide + maximum observed surge at high water (8.66 m OD)High tide 30/01/1607 (8.50 m OD, estimated by Risk Management Solutions, 2007)

Mean High Water (4.63 m OD)

Mean Spring High Water (5.93 m OD)

Maximum predicted tide + maximum observed surge residual (9.67 m OD)

1:10 year event (HR Wallingford, 2009) (8.20 m OD)



1:10,000 year event (HR Wallingford, 2009) (9.03 m OD)

1:100,000 year event (HR Wallingford, 2009) (9.39 m OD)

sea wall(8.83 m OD)

gabions(c. 11.10 m OD)

ground level(c. 8.60 m OD)

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 29 -

2.4 Recent coastal change in the Bridgwater Bay area

The positions of MLW, MHW, dune toe, cliff toe and cliff top are displayed with a

reasonable degree of accuracy on large scale Ordnance Survey Maps. The first detailed

survey (at a scale of 1:2500) of Somerset was conducted between 1882 and 1888

(termed the ‘first edition’), and published as ‘County Series’ maps at scales of 1:2500

and 1:10560 (six inches to one mile). The Hinkley Point area was surveyed between

1883 and 1887, and published between 1884 and 1887. A second edition was published

in 1904-1905 (survey revised 1902-1903). These maps display two tide lines, namely

‘High Water Mark of Ordinary Tides’ (HWMOT) and ‘Low Water Mark of Ordinary

Tides’ (LWMOT), and in addition show changes in ground cover and vegetation with

dashed lines and symbols. The seaward limit of dunes (assumed to be the dune toe) and

the toe of cliffs are therefore clearly defined, and may be assumed to equate to the level

of Highest Astronomical Tide (HAT). Close inspection of the coastline between West

Quantoxhead and the River Parrett indicated that tide lines and positions of cliffs and

dunes had not been revised on the second edition, presumably due to negligible

movement in the intervening 20 year period. A later ‘Provisional Edition’ of the County

Series was published in 1931-32 (revised 1928-1930), although this survey was not

complete and did not, for example, cover the coastline between West Quantoxhead and

the River Parrett.

Ordnance Survey began publishing maps based on National Grid squares in the 1950s,

and in Somerset these included a resurvey of the coastline between 1955 and 1957. The

first edition of these National Grid maps was published 1961 at the same scale at the old

County Series maps (six-inches to the mile) and used the same typeface and line styles.

From the 1970s, these maps were published on the metric scale of 1:10,000, with

revisions published in a rather piecemeal manner, with tide lines mapped largely from

aerial photographic surveys. Changes to lines styles and symbols meant that, although

MHW and MLW are shown with thick and thin lines, changes in vegetation cover were

not marked by solid or dashed lines, and therefore the dune or cliff toe is not clearly

defined.



- 30 -

Photocopies of the 1880s and 1950s Ordnance Survey maps were made on A3 sheets,

and scanned using an A4 scanner. The early maps contain no overlap between adjoining

sheets, so scans were cropped and stitched together using Adobe Illustrator in areas of

approximately 5 km2. The Golden Software Didger program was used to ortho-rectify

the maps. Approximately 20 to 30 calibration points were defined across each map.

Calibration points were chosen where there was a high confidence that there was no

change between the historical and modern maps, such as building corners, field

boundaries, and junctions between drainage ditches. The true co-ordinates of calibration

points were entered by digitising the same calibration points from modern digital raster

1:10,000 scale maps using British National Grid Coordinates. For later editions, which

are overlaid with national grid lines, calibration points were used at the intersections of

National Grid lines, providing a total of 36 calibration points over a 5 x 5 km area. A

Transverse Mercator map projection was used with standard British National Grid

settings:

Projection: Transverse Mercator Central Latitude: 49ºN Central Longitude: 2ºW Central Scale Factor: 0.9996012717 False Easting: 400000 m False Northing: -100000 m Datum: Ordnance Survey of Great Britain 1936 (OSGB36) Units used: metres

The RMS error in the position of calibration points was generally 2 to 3 m for Country

Series and National Grid maps. An exponential spline warping algorithm was then

applied to the map causing each calibration point to be adjusted to its true position, and

areas between calibrations points adjusted based on the distance to surrounding

calibration points. As a consequence, for the final ortho-rectified maps, areas between

calibration points may have a positional error of up to 2 to 3 m, whereas areas near to

calibration points will have errors approaching zero. Note that these error estimates

relate to errors caused by the scanning and digitisation process, not inaccurate mapping

of ground features by the Ordnance Survey at the time of survey.



- 31 -

Ortho-rectified maps were exported to the Golden Software Surfer program, and the tide

lines digitised by clicking along the lines at intervals of approximately 10 m, although

the frequency of data points varied depending on the complexity of the coastline. The

x,y files of tideline positions were saved in ASCII file format, to enable them to be

superimposed on ortho-rectified air photos or modern maps. The present positions of

tide lines, cliffs and dunes were taken from lidar topographic surveys flown in 2007 and

2008, with MHW and MLW elevations estimated from tidal records (+4.63 and -3.84 m

OD respectively).

Table 2 shows changes in the positions of MHW and MLW over the period 1883-7 to

2007-8, measured along 64 shore-normal profile lines (shown in Figure 11). The

analysis indicates rates of change of MHW have generally been very low along the

cliffed coast to the west of the power station, with no net change (-0.1 to +0.1 m y-1).

Along much of the power station frontage, apparent progradation of up to 112 m has

occurred due to creation of made ground during construction of the 'A' and 'B' power

stations. Only the NE corner (profiles 21 & 22) exhibited net recession (up to 8 m) over

the period.

Immediately to the east of the power station, the gravel barrier has experienced slight

net progradation (12 to 16 m), although the short section between North Ham and

Stolford, now protected by higher rock armour, saw recession of up to 22 m. The

situation at Catsford Common is mixed, with the northern part exhibiting up to 38 m of

recession, but the southern part exhibiting up to 64 m of accretion, due mainly to the

movement of the gravel barrier down the coast. A wide belt of marsh has developed

along the NW side of the Stert Peninsula since the 1880s, with MHW moving offshore

by up to 170 m. North of the River Parrett, MHW has moved seawards along the entire

frontage. The largest movement has been on the northern side of Burnham-on-Sea,

where MHW has moved up to 300 m to seaward.

Trends in the position of MLW are very different. On every profile (except P62 at Kilve

Pill) there has been net shoreward movement of MLW: up to 350 m at Lilstock, 50 to 60

m at Benhole Point, 287 m to the west of the existing power station, and 60 to 120 m in

front of the power station. To the east of the power station, the low water channel of the



- 32 -

Profile Location

1957 1976 2008 1886-2008 1957 1970 2008 1886-2008

annual rate annual rate

(m a-1

) (m a-1

)

P58 Perry Gully -2 13 17 0.14 11 15 -182 -1.49

P59 Blue Ben 0 0 9 0.07 25 97 -74 -0.61

P60 Quantock's Head -1 1 8 0.07 39 62 -7 -0.06

P61 Kilve Pill 0 -3 -4 -0.03 0 1 -25 -0.20

P62 Kilton 3 -2 8 0.07 71 87 59 0.48

P63 Park Farm -6 -5 -15 -0.13 -49 -71 -81 -0.66

P64 Lilstock -6 28 9 0.08 -151 -162 -352 -2.89

P1 Lilstock East 0 -6 -7 -0.06 -71 -135 -289 -2.37

P2 Lilstock East 3 -1 -6 -0.05 -119 -141 -251 -2.05

P3 Lilstock East -2 0 9 0.07 -55 -42 -103 -0.84

P4 Benhole Point -1 -12 -8 -0.06 -39 -11 -56 -0.46

P5 Benhole Point -1 -10 -8 -0.06 -16 -18 -49 -0.40

P6 Power Station west 4 6 4 0.04 -24 -34 -63 -0.52


P8 Power Station west -1 1 6 0.05 -95 -105 -128 -1.05






P14 Power Station site 1 16 21 0.17 -50 -33 -48 -0.39


P16 Power Station site -2 97 99 0.81 -33 -20 -76 -0.62





P21 Power Station site -34 -16 -4 -0.03 -112 -91 -123 -1.01

P22 Power Station site -24 -19 -8 -0.07 -100 -79 -122 -1.00




P26 Wick Moor 0 11 16 0.13 -222 -212 -301 -2.47

P27 Wick Moor 4 3 7 0.05 -483 -494 -577 -4.73

P28 Wick Moor 3 -3 -3 -0.03 -523 -549 -663 -5.43

P29 North Ham 0 -16 -22 -0.18 -579 -519 -742 -6.08

P30 North Ham 1 -3 -2 -0.02 -676 -593 -813 -6.67

P31 Great Arch 8 -5 -6 -0.05 -782 -701 -892 -7.31

P32 Stolford 4 11 12 0.10 -733 -636 -833 -6.83

P33 Stolford -1 4 -24 -0.20

P34 Catsford Common 0 -9 -38 -0.31

P35 Catsford Common -1 14 -28 -0.23

P36 Catsford Common -4 3 -9 -0.07

P37 Catsford Common -5 34 25 0.21

P38 Catsford Common 2 78 64 0.52

P39 Catsford Common 0 -17 -2 -0.01

P40 Catsford Common 0 62 21 0.17

in 1886

Mean High Water (MHW) Mean Low Water (MLW)

Measurements not possible

since profiles do not intersect

with the low water channel

The positions of MHW and MLW shown on Ordnance Survey maps surveyed in 1886, 1957 and 1976, and Environment Agency lidar survey data flown in 2007-8, relative to the positions in 1886, measured at 64 profile locations shown in Figure 11. Positive values indicate seaward movement, negative values indicate landward movement. MHW and MLW from lidar data are based on the averages of high and low water spring and neap levels for 2008-2026 quoted on NTSLF website, +4.63 m OD and -3.84 m OD respectively.

Table 2



- 33 -

Profile Location

1957 1976 2008 1886-2008 1957 1970 2008 1886-2008

annual rate annual rate

(m a-1

) (m a-1

)

P41 Wall Common 7 36 12 0.10 -969 -886 -1081 -8.86

P42 Steart 151 124 170 1.39 -864 -769 -958 -7.85

P43 Steart 134 175 110 0.90 -1300 -888 -1101 -9.03

P44 Stert Point 106 21 37 0.30 -385 -95 -254 -2.08

P45 West Huntspill 8 15 10 0.08 50 -9 -41 -0.34

P46 Brue Pill 3 7 43 0.35 -12 -17 -27 -0.22

P47 Burnham-on-Sea 8 6 8 0.06 -41 -43 -55 -0.45

P48 Burnham-on-Sea 21 47 49 0.40 -58 -25 -60 -0.49

P49 Berrow Dunes -6 31 74 0.60 -74 -67 -142 -1.17

P50 Berrow Dunes 82 335 300 2.46 -716 -856 -786 -6.44

P51 Berrow Dunes 94 281 222 1.82 -1610 111 -1015 -8.32

P52 Berrow Dunes 123 157 174 1.43 -860 -97 -962 -7.88

P53 Berrow Dunes 159 188 168 1.38 -649 -55 -932 -7.64

P54 Brean 100 166 164 1.34 -745 -131 -918 -7.53

P55 Brean 67 115 153 1.26 -861 -356 -1011 -8.28

P56 Brean 25 66 104 0.85 -514 -147 -626 -5.13

P57 Brean -3 13 34 0.28 -111 266 -195 -1.60

Mean High Water (MHW) Mean Low Water (MLW)

continued.Table 2



- 34 -

River Parrett has moved 300 to 900 m shorewards, although the mudflats remain

approximately 700 to 900 m wide along this section. MLW has fluctuated greatly along

the Burnham to Brean frontage, moving up to 1600 m landward between 1886 and

1957, moving seawards to approximately the 1886 position by 1976, and then moving

shoreward again by up to 1 km by 2008. The accuracy of the mapping of mean low

water mark on low gradient foreshores in the 19th century is open to question, but the

general trends indicated by successive map editions are considered likely to be valid.

Table 3a summarises the distances and rates of recession of the cliff-top edge between

West Quantoxhead and Hinkley Point. The greatest recession occurred between Park

Farm and Benhole Point (profiles P63 to P5) where the cliff-top moved shoreward by 10

to 22 m (representing an average rate of 0.08 to 0.18 m a-1). Elsewhere, the cliff-top has

generally receded less than 10 m in the 120 year period, representing < 0.08 m a-1. Table

3b shows changes in position of the dune toe between Burnham-on-Sea and Brean. A

section immediately to the north of Burnham (around profile P49) experienced dune

erosion of up to 47 m, caused mainly by the close proximity of the large bend on the

low water channel of the River Parrett, which moved shoreward during this period. To

the north, Berrow dunes prograded by up to 196 m, representing an average

progradation rate of the dune toe of 1.61 m a-1.

Figure 13 shows the large scale changes in the positions of MHW and MLW in plan

form. The most notable net change in MHW was seaward movement on the NW side of

the Steart Peninsula, the northward movement of Stert Point, and seaward movement of

MHW along the Berrow and southern Brean dunes frontage. Large fluctuations in the

width of Berrow Flats are evident from the varying position of MLW.

A variety of historical map and survey data indicate that the shoreline of the Stert

Peninsula experienced net erosion (landward movement) between c. 1800 and 1928

(Kidson, 1960, 1963; Ravensrodd Consultants, 1996; Long et al., 2002). In 1928 the

Somerset River Board built an artifical gravel bank along the shore near Steart village

and planted Spartina on the upper foreshore. Rapid seaward expansion of the Spartina

sward occurred between the 1930's and the mid 1960's (Ranwell, 1964; Carr, 1965),

after which time erosion of the seaward marsh edge set in. This process has continued to



- 35 -

Profile Location

Distance Annual Rate

(m) (m a-1

)

P58 Perry Gully 0 0.00

P59 Blue Ben -9 -0.07

P60 Quantock's Head 0 0.00

P61 Kilve Pill -3 -0.02

P62 Kilton -8 -0.06

P63 Park Farm -10 -0.08

P64 Lilstock -18 -0.14

P1 Lilstock East -22 -0.18



P4 Benhole Point -14 -0.12

P5 Benhole Point -14 -0.11

P6 Power Station west -8 -0.07

P7 Power Station west 0 0.00







Profile Location

Distance Annual Rate

(m) (m a-1

)

P49 Berrow Dunes -47 -0.38

P50 Berrow Dunes 196 1.61




P54 Brean 58 0.48

P55 Brean 21 0.17

Change (1886-2008)

Change (1886-2008)

Rates of (a) cliff recession on 20 profiles to the west of Hinkley Point Power Station, and (b) movement of the dune toe along Berrow and Brean dunes, between 1886 and 2008. Positions of the cliff-edge and dune toe determined from County Series Ordnance Survey maps surveyed in 1886, and Environment Agency lidar and aerial photography flown in 2007-8. Negative values indicate landward movement (cliff or dune recession), positive values indicate progradation.

Table 3

(a) Recession of cliff edge

(b) Movement of dune toe



- 36 -

310000 312000 314000 316000 318000 320000 322000 324000 326000 328000 330000 332000

144000

146000

148000

150000

152000

154000

156000

158000

160000

Figure 13

BridgwaterBay

HinkleyPoint

WestQuantoxhead

Burnham-on-Sea

BreanDown

Berrow

Brean

0 1000 2000 3000 4000

Scale (m)

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 37 -

the present day, although not at a constant rate (Long et al., 2002; Kirby & Kirby,

2008).

During the late 19th century the upper beach and dunes at Berrow and Burnham

experienced slow erosion, but around the turn of the century the regime changed to one

of net accretion. A wide saltmarsh progressively formed on the foreshore at Berrow,

followed by new dune ridges on the seaward side (Willis, 1990). However, slow erosion

continued along the northern part of the coastal frontage south of Brean Down, and the

low water mark showed a general landward movement throughout the period (Long et

al., 2002). The causes of these changes are uncertain, but may be related to changes in

the morphology of the Parrett ebb-tidal delta system, and/or to movement of the Culver

Sand offshore Bank. Changes in the position and morphology of both are known to have

occurred, based on examination of hydrographic charts, but no quantitative wave and/or

sediment transport modelling using successive historical bathymetries has been

undertaken. Changes in either nearshore or offshore banks could have increased the

degree of wave exposure between Berrow and Brean, especially in areas of wave

focusing, leading to the development of a steeper beach and encouraging the landward

movement of sediment towards the dunes. This hypothesis requires further testing.

The main low water channel of the River Parrett south of Berrow Flats moved

southwards by approximately 600 m over the period, with a pronounced spit (Gore

Sand) forming on the northern side by the 1950s. After this time the main channel

remained fairly static while a second, more northerly channel developed, breaking

through to the east of Gore Sand, leaving a sand bank between the two channels (Lee,

1990).

2.5 Coastal processes

2.5.1 Tides

The Bristol Channel / Severn estuary has one of the largest tidal ranges in the world.

The mean spring tidal range increases eastwards from c. 6.2 m in the Outer Bristol



- 38 -

Channel to 8.40 m at Ilfracombe, 10.87 m at Hinkley Point and 12.20 m at Avonmouth

(Table 4; Figure 14). The maximum tidal range occurs just above the old Severn Bridge

road crossing at Beachley and Sudbrooke (12.3 m). Mean neap tidal range increases

from c. 3.87 m at Ilfracombe to 5.34 m at Hinkley Point and 6.21 m at Avonmouth. The

maximum elevation (relative to Ordnance Datum, OD) of mean high water spring

(MHWS) and mean high water neap (MHWN) tides also increases up the Bristol

Channel and Severn estuary, with MHWS reaching a maximum elevation of 8.3 m OD

at Epney, just south of Gloucester (Table 2). Quoted tidal levels for Hinkley Point show

some variation, depending on data source and method of calculation (Tables 4 & 5).

The tidal regime is dominated by the M2 semi-diurnal lunar component (the ‘lunar day’

of 24.84 hours, producing high tides every 12.42 hours), and to a lesser extent, the S2

semi-diurnal solar component (with maxima every 12.00 hours), which when in phase

combine to produce alternating spring and neap tides every 14.77 days (Bennett, 1975;

Owen, 1980; Uncles, 1984; Figures 15 & 16).

Examples of daily spring and neap tidal curves for Hinkley Point, Newport and

Avonmouth are shown in Figure 15. At Hinkley Point, where the Bristol Channel is

wide and tidal currents are relatively unconfined, both spring and neap tidal stage curves

are almost symmetrical. Tidal flow rates on the rising and falling tides are therefore

similar, and the system is not markedly ebb or flood dominated. At Newport and

Avonmouth, where flows are more restricted by the estuarine morphology, tidal stage

curves are asymmetric, with water levels rising relatively quickly on the flood tide,

followed by a longer ebb (Uncles, 1981). This asymmetry is caused by a large pressure

gradient which develops as the rising tide is funnelled through the narrowing Severn

Estuary. On the ebb, flows are reduced as water flows with little restriction into the

much wider Bristol Channel.

Tidal levels were monitored at Hinkley Point over short periods in the 1970's and

1980's, but 'continuous' records in digital format are available only since 1990 for a

gauge located suspended from a steel pole attached to the cooling water intake tower at

Hinkley Point, located approximately 400 m offshore. Data for this gauge, and for

additional gauges at Avonmouth, Newport, Ilfracombe, Mumbles and Milford Haven



- 39 -

Station Elevations relative to Ordnance Datum (m)LAT MLWS MLWN MSL MHWN MHWS HAT MSTR MNTR

Milford Haven (1) -3.81 -3.01 -1.21 0.12 1.49 3.29 4.19 6.30 2.70Milford Haven (2) -3.79 -2.98 -1.26 nd 1.56 3.31 4.12 6.29 2.82Milford Haven (3) -3.75 -2.95 -1.24 nd 1.58 3.33 4.14 6.28 2.82Ilfracombe (1) -4.73 -3.80 -1.70 nd 2.20 4.50 5.65 8.30 3.90Ilfracombe (2) -4.91 -3.95 -1.70 nd 2.17 4.45 5.45 8.40 3.87Ilfracombe (3) -4.89 -3.94 -1.69 nd 2.19 4.47 5.46 8.41 3.88Mumbles (1) -5.08 -4.10 -1.90 nd 1.90 4.30 5.50 8.40 3.80Mumbles (2) -4.84 -3.89 -1.72 nd 2.32 4.55 5.54 8.44 4.04Mumbles (3) -4.86 -3.92 -1.75 nd 2.28 4.54 5.51 8.46 4.03Swansea (1) -5.00 -4.00 -1.90 nd 2.20 4.50 5.50 8.50 4.10Watchet (1) -5.80 -4.70 -1.90 0.07 2.50 5.50 6.82 10.20 4.40Hinkley Point (1) -6.20 -5.10 -2.30 0.10 2.50 5.60 6.96 10.70 4.80Hinkley Point (2) -6.10 -4.98 -2.33 nd 3.01 5.89 7.08 10.87 5.34Hinkley Point (3) -6.09 -4.98 -2.31 nd 3.01 5.93 7.12 10.91 5.32Hinkley Point (4) -6.10 -5.10 -2.30 0.10 2.50 5.64 7.12 10.74 4.80Burnham-on-Sea (1) nd -5.23 -2.73 nd 2.77 5.77 7.09 11.00 5.50Bridgwater (1) nd nd nd nd 3.20 6.10 7.37 nd ndCardiff (1) -6.36 -5.30 -2.60 0.29 3.00 5.90 7.18 11.20 5.60Weston-super-Mare (1) -6.06 -5.20 -3.00 0.10 2.80 6.00 7.42 11.20 5.80Clevedon (1) -6.68 -5.50 -2.50 0.30 3.10 6.30 7.71 11.80 5.60Newport (1) -6.37 -5.31 -2.61 nd 3.19 6.39 7.80 11.70 5.80Newport (2) -6.46 -5.35 -2.73 nd 3.16 6.27 7.49 11.62 5.89Newport (3) -6.40 -5.30 -2.69 nd 3.16 6.33 7.55 11.63 5.85Newport (4) -6.48 -5.41 -2.61 0.39 3.09 6.29 7.50 11.70 5.70Portishead (1) nd nd nd nd 3.20 6.60 8.10 nd ndAvonmouth (1) -6.60 -5.50 -2.70 0.46 3.30 6.70 8.20 12.20 6.00Avonmouth (2) -6.67 -5.41 -2.72 nd 3.49 6.79 8.11 12.20 6.21Avonmouth (3) -6.70 -5.44 -2.72 nd 3.49 6.83 8.15 12.27 6.21Avonmouth (4) -6.60 -5.50 -2.70 0.46 3.30 6.70 8.20 12.20 6.00Sudbrook (1) -6.42 -5.40 -2.80 0.36 3.40 6.90 8.44 12.30 6.20Beachley (1) -6.42 -5.40 -2.80 0.32 3.50 6.90 8.40 12.30 6.30Inward Rocks (1) -5.57 -4.78 -2.78 0.56 3.52 7.02 8.56 11.80 6.30Narlwood Rocks (1) -4.58 -4.07 -2.77 nd 3.53 7.03 8.57 11.10 6.30White House (1) -3.13 -3.05 -2.85 0.89 3.65 7.15 8.69 10.20 6.50Berkeley (1) -1.59 -1.63 -1.73 1.31 3.77 7.27 8.81 8.90 5.50Sharpness Dock (1) -1.19 -1.23 -1.33 nd 3.77 7.47 9.10 8.70 5.10Wellhouse Rock (1) -0.68 -0.72 -0.82 1.73 3.88 7.58 9.21 8.30 4.70Epney (1) nd nd nd nd nd 8.30 nd nd ndMinsterworth (1) nd nd nd nd nd 8.00 nd nd ndLlanthony (1) nd nd nd nd nd 7.80 nd nd nd

Notes on the calculation of tidal levels from 2009 Admiralty Tide Tables

- Ordnance datum for Hinkley Point is not stated in Admiralty Tide Tables. The value has been taken from the class A tide gauge data provided by Proudman Oceanographic Institute (NTSLF).- Values for HAT and LAT at Secondary Ports have been extrapolated from the values at the Standard Port of Avonmouth using the method described in the Admiralty Tide Tables.- Admiralty Tide Tables state that the tide does not normally fall below -5.23 m OD at Burnham-on-Sea. This has been taken as the level for MLWS, and LAT has not been calculated.

Tidal levels at Standard and selected Secondary Ports in the Bristol Channel, including Class A tide gauge stations. Data sources: (1) 2009 Admiralty Tide Tables; (2) National Tidal Sea Level Facility (NTSLF) predictions for the years 2005-2025; (3) NTSLF predictions for the years 2008-2026; (4) HR Wallingford (2009).

Table 4



- 40 -

Wind speed at 80 m (m/s) Wind power at 80 m (W/m2)

Spring peak tidal current (m/s)

Spring tidal range (m) Neap tidal range (m)

Wave height (m) Wave power (kW/m)

Neap peak tidal current (m/s)

Figure 14 Annual mean tide, wind and wave parameters in the Bristol Channel. Maps generated from gridded datasets compiled for the Atlas of UK Marine Renewable Energy Resources from average measurements of the Met Office UK Waters Wave Model for the period 01/06/2000 to 31/05/2007 (BERR, 2008).

(a) (b)

(c) (d)

(e) (f)

(g) (h)



- 41 -

Halcrow HR Wallingford 2009 Admiralty Tide NTSLF predictions NTSLF predictions NTSLF predictions Adopted in(1998) (2009) Tables (Hydrographic 1990-2008 (from 15 2005-2025 2008-2026 this study

Office, 2008) min. observations)HAT nd 7.12 (6.96) 7.09 7.08 7.12 7.12MHWS 5.7 5.64 5.6 5.87 5.89 5.93 5.93MHW nd nd (4.05) 4.63 (4.45) (4.47) 4.63MHWN 2.7 2.50 2.5 3.07 3.01 3.01 3.01MSL nd 0.10 0.1 0.33 (0.40) (0.41) 0.36MLWN -2.4 -2.30 -2.3 -2.39 -2.33 -2.31 -2.31MLW nd nd (3.7) -3.84 (-3.66) (-3.65) -3.84MLWS -5.0 -5.10 -5.1 -4.96 -4.98 -4.98 -4.98LAT nd -6.10 (-6.20) -6.14 -6.10 -6.09 -6.09CD -5.4 -5.9 nd -5.9 -5.9 -5.9 -5.9

Notes- Chart datum for Hinkley Point is not quoted in Admiralty Tide Tables; a value of -5.9 m OD has been assumed.- Chart datum is erroneously quoted as -5.4 m OD in Halcrow (1998).- MHW and MLW from Admiralty Tide Tables and NTSLF predictions (in brackets) are obtained by averaging the spring and neap levels.- MSL from NTSLF predictions (in brackets) are obtained by averaging the MHWS, MHWN, MLWN and MLWS levels.- HAT and LAT from Admiralty Tide Tables (in brackets) are obtained by extrapolating the trends between MHWS and MHWN, and MLWN and MLWS.- Values for 1990-2008 are averaged from 15 minute predictions made by NTSLF for calculating surge residuals. MHWS, MHWN, MLWN and MLWS have been calculated by finding the maximum spring or neap tide in each 14-day cycle, averaging with the following successive tide, and then averaging over all spring and neap tides over the period 1990-2008 (c. 450 tides)- HAT, MHWS, MHWN, MLWN, MLWS and LAT used in this study are taken from NTSLF predictions for 2008-2026.- MHW and MLW used in this study are calculated by averaging all high and low waters in the period 1990-2008 (from 15 minute observations).- MSL used in this study is calculated for 2009 from a linear trend since 1990 of 0.34 mm/yr, the rate based on an average for the period 1992-2006. This measure is considered more accurate than the average of MHWS, MHWN, MLWN and MLWS (0.41 m OD), due to the possible compounding of any errors in the four mean tidal levels.

Tidal levels at Hinkley Point quoted by different sources. Elevations are expressed in metres above Ordnance Datum.Table 5 _________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 42 -

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

00:0

0

02:0

0

04:0

0

06:0

0

08:0

0

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

00:0

0

Obs

erve

d w

ater

leve

ls (m

OD

)

Hinkley Point

Avonmouth

Newport

-3.50

-2.50

-1.50

-0.50

0.50

1.50

2.50

3.50

00:0

0

02:0

0

04:0

0

06:0

0

08:0

0

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

00:0

0

Obs

erve

d w

ater

leve

ls (m

OD

)

Hinkley Point

Avonmouth

Newport

Observed trends in 15 minute water levels recorded at Hinkley Point, Newport and Avonmouth during March 2002: (a) spring tides on 30/03/2002; (b) neap tides on 08/03/2002, plotted with reduced vertical scale to demonstrate differences in the shapes of the tidal curves; (c) neap tides on 08/03/2002, plotted with the same vertical scale as (a).Surge residuals were low (<20 cm) during both periods. Original data source: NTSLF.

Figure 15

(a)

(b)

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

00:0

0

02:0

0

04:0

0

06:0

0

08:0

0

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

00:0

0

Obs

erve

d w

ater

leve

ls (m

OD

)

Hinkley Point

Avonmouth

Newport

(c)

30/03/2002

08/03/2002

08/03/2002



- 43 -

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

01/0

3/2

002

03/0

3/2

002

05/0

3/2

002

07/0

3/2

002

09/0

3/2

002

11/0

3/2

002

13/0

3/2

002

15/0

3/2

002

17/0

3/2

002

19/0

3/2

002

21/0

3/2

002

23/0

3/2

002

25/0

3/2

002

27/0

3/2

002

29/0

3/2

002

31/0

3/2

002

Obs

erve

d w

ater

leve

ls (m

OD

)

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

01/0

1/2

002

31/0

1/2

002

02/0

3/2

002

02/0

4/2

002

02/0

5/2

002

02/0

6/2

002

02/0

7/2

002

01/0

8/2

002

01/0

9/2

002

01/1

0/2

002

01/1

1/2

002

01/1

2/2

002

31/1

2/2

002

Obs

erve

d w

ater

leve

ls (m

OD

)

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

00:0

0

02:0

0

04:0

0

06:0

0

08:0

0

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

00:0

0

Obs

erve

d w

ater

leve

ls (m

OD

)

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

00:0

0

02:0

0

04:0

0

06:0

0

08:0

0

10:0

0

12:0

0

14:0

0

16:0

0

18:0

0

20:0

0

22:0

0

00:0

0

Obs

erve

d w

ater

leve

ls (m

OD

)

Observed trends in 15 minute water levels recorded at Hinkley Point during 2002: (a) neap tides on 08/03/2002; (b) spring tides on 30/03/2002; (c) monthly spring-neap cycle in March 2002; (d) annual variation. Original data source: NTSLF.

Figure 16

High spring tide

Low spring tide

High spring tide

Low spring tide

High neap tide

Low neap tide

High neap tide

Low neap tide

Spring tidesSpring tidesSpring tides

Neap tides Neap tides

Spring Equinox

SummerSolstice

AutumnEquinox

WinterSolstice

(a) (b)

(c)

(d)



- 44 -

(Figure 17), are available from the National Tidal and Sea Level Facility (NTSLF)

website hosted by the Proudman Oceanographic Laboratory (POL). Chart Datum at the

Hinkley Point station is reported by NTSLF to be 5.9 m below OD, although no value is

given in Admiralty Tide Tables (UKHO, 2008).

The frequency distribution of water levels in the Inner Bristol Channel and Severn

estuary is bimodal, with a dominant mode around the level of MLWN and subsidiary

mode around the level of MHWN. Figures 18 & 19 show the frequency values for

Hinkley Point. Water level is changing relatively quickly when passing through the

mean level so this appears as a minimum. The slight skew in the distribution, with a

broader flatter mode at high water, indicates that the still stand at high water lasts

slightly longer than at low water, again because of slightly reduced ebb flows due to

shallow water effects in the Bristol Channel. The frequency distribution of high waters

is also bimodal, with the largest mode just below MHWS level and a smaller mode just

above MHWN level (Figures 20 & 21). The frequency distribution of low waters is

almost bimodal, with a dominant mode just above MLWS and a secondary 'shoulder'

just below MLWN (Figures 22 & 23).

Several lunar and solar cycles combine to produce higher and lower tides throughout the

year, and from year to year. In addition to the changing lunar phases which produce the

spring-neap cycle (‘synodic month’ of 29.5306 days), the moon’s orbit is distinctly

elliptical, varying from c. 364,397 km at closest approach (perigee) to c. 406,731 km at

its furthest distance (apogee). Tidal range is increased (and hence high tides are higher)

when the moon is closer to the earth. A complete cycle from perigee to perigee takes

27.5546 days (an ‘anomalistic month’), but unlike the synodic month (which has

maxima every half cycle), the anomalistic month has one maxima every cycle, with a

minima 13.78 days later. Therefore, when roughly in phase this cycle tends to suppress

successive spring tides, producing one large spring tide each month, followed by a

smaller one. This cycle is clearly demonstrated in the Hinkley Point records, as shown

in Figure 16c. When out of phase, successive spring and neap tides are roughly equal, as

shown around July 2002 in Figure 16d.



- 45 -

Turbot Bank/Pembroke

Scarweather

Minehead

HinkleyPoint

Newport

Avonmouth

Illfracombe

Chepstow

Cardiff

Tintern

Gloucester

Sharpness

Minsterworth

EpneyMilford Haven

Mumbles

Haverfordwest

Milton Tenby

Carmarthen

LlanelliPontarddulais

Swansea Neath

Blackpill

Kidwelly

Proudman Oceanographic Laboratory(BODC, NTSLF & PSMSL)

Environment Agency

Wave buoysTide gaugesCEFAS

UK Met Office

Channel Coastal Observatory

Braunton

AppledoreBridgewater

Bristol

CelticSea

Outer BristolChannel

Central BristolChannel

Inner Bristol Channel

Severn

Estuary

Enlarged areashown inFigure 3

Hindcast Waves

UK Met Office modelling point

Locations of operative wave buoys and tide gauges. Also shown are the limits of the Inner, Central and Outer Bristol Channel, after Posford Duvivier and ABP (2000).

Figure 17

CelticSea

Bathymetry(m CD)

-60

-40

-20

-10

MHW

0

Scale

20 km15105

%

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 46 -

0

1

2

3

4

5

6

7

-6.5

to -6

.0

-6.0

to -5

.5

-5.5

to -5

.0

-5.0

to -4

.5

-4.5

to -4

.0

-4.0

to -3

.5

-3.5

to -3

.0

-3.0

to -2

.5

-2.5

to -2

.0

-2.0

to -1

.5

-1.5

to -1

.0

-1.0

to -0

.5

-0.5

to 0

.0

0.0

to 0

.5

0.5

to 1

.0

1.0

to 1

.5

1.5

to 2

.0

2.0

to 2

.5

2.5

to 3

.0

3.0

to 3

.5

3.5

to 4

.0

4.0

to 4

.5

4.5

to 5

.0

5.0

to 5

.5

5.5

to 6

.0

6.0

to 6

.5

6.5

to 7

.0

7.0

to 7

.5

Freq

uenc

y (%

)

Water Level (OD)

Frequency of observed water levels recorded at Hinkley Point in the period 1990-2008 (total of 624837 fifteen minute observations). Original data source: NTSLF.

Figure 18

0

1

2

3

4

5

6

7

-6.5

to -6

.0

-6.0

to -5

.5

-5.5

to -5

.0

-5.0

to -4

.5

-4.5

to -4

.0

-4.0

to -3

.5

-3.5

to -3

.0

-3.0

to -2

.5

-2.5

to -2

.0

-2.0

to -1

.5

-1.5

to -1

.0

-1.0

to -0

.5

-0.5

to 0

.0

0.0

to 0

.5

0.5

to 1

.0

1.0

to 1

.5

1.5

to 2

.0

2.0

to 2

.5

2.5

to 3

.0

3.0

to 3

.5

3.5

to 4

.0

4.0

to 4

.5

4.5

to 5

.0

5.0

to 5

.5

5.5

to 6

.0

6.0

to 6

.5

6.5

to 7

.0

7.0

to 7

.5

Freq

uenc

y (%

)

Water Level (m OD)

Frequency of predicted water levels at Hinkley Point in the period 1990-2008 (total of 624837 fifteen minute observations). Original data source: NTSLF.

Figure 19



- 47 -

0

1

2

3

4

5

6

7

8

9

10

1.50

-1.7

5

1.75

-2.0

0

2.00

-2.2

5

2.25

-2.5

0

2.50

-2.7

5

2.75

-3.0

0

3.00

-3.2

5

3.25

-3.5

0

3.50

-3.7

5

3.75

-4.0

0

4.00

-4.2

5

4.25

-4.5

0

4.50

-4.7

5

4.75

-5.0

0

5.00

-5.2

5

5.25

-5.5

0

5.50

-5.7

5

5.75

-6.0

0

6.00

-6.2

5

6.25

-6.5

0

6.50

-6.7

5

6.75

-7.0

0

7.00

-7.2

5

7.25

-7.5

0

Freq

uenc

y (%

)

Water Level (OD)

Frequency of observed high waters recorded at Hinkley Point in the period 1990-2008(total of 12591 tides). Original data source: NTSLF.

Figure 20

0

1

2

3

4

5

6

7

8

9

10

1.50

-1.7

5

1.75

-2.0

0

2.00

-2.2

5

2.25

-2.5

0

2.50

-2.7

5

2.75

-3.0

0

3.00

-3.2

5

3.25

-3.5

0

3.50

-3.7

5

3.75

-4.0

0

4.00

-4.2

5

4.25

-4.5

0

4.50

-4.7

5

4.75

-5.0

0

5.00

-5.2

5

5.25

-5.5

0

5.50

-5.7

5

5.75

-6.0

0

6.00

-6.2

5

6.25

-6.5

0

6.50

-6.7

5

6.75

-7.0

0

7.00

-7.2

5

7.25

-7.5

0

Freq

uenc

y (%

)

Water Level (m OD)

Frequency of predicted high waters at Hinkley Point in the period 1990-2008 (total of 12591 tides). Original data source: NTSLF.

Figure 21



- 48 -

0

2

4

6

8

10

12

-6.5

0 to

-6.2

5

-6.2

5 to

-6.0

0

-6.0

0 to

-5.7

5

-5.7

5 to

-5.5

0

-5.5

0 to

-5.2

5

-5.2

5 to

-5.0

0

-5.0

0 to

-4.7

5

-4.7

5 to

-4.5

0

-4.5

0 to

-4.2

5

-4.2

5 to

-4.0

0

-4.0

0 to

-3.7

5

-3.7

5 to

-3.5

0

-3.5

0 to

-3.2

5

-3.2

5 to

-3.0

0

-3.0

0 to

-2.7

5

-2.7

5 to

-2.5

0

-2.5

0 to

-2.2

5

-2.2

5 to

-2.0

0

-2.0

0 to

-1.7

5

-1.7

5 to

-1.5

0

-1.5

0 to

-1.2

5

-1.2

5 to

-1.0

0

-1.0

0 to

-0.7

5

-0.7

5 to

-0.5

0

Freq

uenc

y (%

)

Water Level (OD)

Frequency of observed low waters recorded at Hinkley Point in the period 1990-2008(total of 12515 tides). Original data source: NTSLF.

Figure 22

Frequency of predicted low waters at Hinkley Point in the period 1990-2008 (total of 12515 tides). Original data source: NTSLF.

Figure 23

0

2

4

6

8

10

12

-6.5

0 to

-6.2

5

-6.2

5 to

-6.0

0

-6.0

0 to

-5.7

5

-5.7

5 to

-5.5

0

-5.5

0 to

-5.2

5

-5.2

5 to

-5.0

0

-5.0

0 to

-4.7

5

-4.7

5 to

-4.5

0

-4.5

0 to

-4.2

5

-4.2

5 to

-4.0

0

-4.0

0 to

-3.7

5

-3.7

5 to

-3.5

0

-3.5

0 to

-3.2

5

-3.2

5 to

-3.0

0

-3.0

0 to

-2.7

5

-2.7

5 to

-2.5

0

-2.5

0 to

-2.2

5

-2.2

5 to

-2.0

0

-2.0

0 to

-1.7

5

-1.7

5 to

-1.5

0

-1.5

0 to

-1.2

5

-1.2

5 to

-1.0

0

-1.0

0 to

-0.7

5

-0.7

5 to

-0.5

0

Freq

uenc

y (%

)

Water Level (OD)



- 49 -

The lunar orbit is also inclined to the plane of the ecliptic, by between 4.99º and 5.30º,

and when combined with the tilt of the earth, the lunar declination averages 23.5º. This

has a number of effects. Firstly, on a daily basis, there is an asymmetry between the two

high and two low waters for each earth rotation, producing a diurnal tidal component.

Although minor in the Bristol Channel, on certain parts of the planet this effect

counteracts the semi-diurnal lunar component to the extent that only a single tide is

observed each day. At Hinkley Point, the effect produces alternating slightly higher and

slightly lower successive tides. Secondly, the moon’s declination varies on a cycle of

27.2122 days (the ‘nodical’ or ‘draconitic month’), on average between +23.5º and -

23.5º, with maximum diurnal tidal ranges when the declination is zero, every 13.61

days. When zero declination coincides with perigee and spring tides, particularly high

tides are predicted to occur.

Similarly, the solar declination varies throughout the year (the ‘tropical year’, 365.2422

days), with minimum declination (and hence the highest high tides) occurring at the

spring and autumn equinoxes. Figure 16d shows that the highest tides observed at

Hinkley Point occur around the equinoxes, with reduced tidal ranges around the summer

and winter solstices (when the declination reaches 23.5º).

There are several other cycles which significantly affect the tidal range, which operate

on longer timescales. The orientation of the moon’s elliptical orbit is not fixed; the orbit

precesses over a cycle of 8.85 years, with the positions of perigee and apogee slowly

changing each anomalistic month. Every half cycle (4.4 years) the lunar perigee will

coincide with either the spring or autumn equinox, and higher than average spring tides

are predicted to occur in 2010, 2015, 2019 and 2024 (Figure 24).

Examination of the annual maximum tides recorded and predicted at Hinkley Point

(Figures 24 & 25) shows a significant correlation with this 4.4 year cycle. The

maximum tidal elevation recorded during the period was 7.36 m OD at 08.44 hrs on

10th February 1997, during a period of predicted peak high tides.

The moon’s angle of declination also varies considerably, between 18.3º and 28.6º in a

cycle lasting 18.61 years (the ‘regression of the moon’s nodes’, also termed the ‘lunar



- 50 -

6.0

6.1

6.2

6.3

6.4

6.5

6.6

6.7

6.8

6.9

7.0

7.1

7.2

7.3

7.4

7.5

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

Wat

er le

vel (

m O

D)

10 highest annual tides

Mean of 10 highest annual tides

Figure 25 The 10 highest tides in each year observed at Hinkley Point in the period 1991-2008. The mean value illustrates the 4.4 year cycle. Original data source: NTSLF.

-6.20

-6.00

-5.80

-5.60

-5.40

-5.20

-5.00

-4.80

-4.60

-4.40

5.40

5.60

5.80

6.00

6.20

6.40

6.60

6.80

7.00

7.20

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

2015

2016

2017

2018

2019

2020

2021

2022

2023

2024

2025

2026

2027

Pred

icte

d An

nual

Min

ima

(m O

D)

Pred

icte

d An

nual

Max

ima

(m O

D)

Predicted spring maxima 1991-2026 Predicted autumn maxima 1990-2026

Predicted spring minima 1991-2026 Predicted autumn minima 1990-2026

19972006

201520242019201020011992

Figure 24 Maximum and minimum spring and autumn tides predicted at Hinkley Point. Original data source: NTSLF.



- 51 -

nodal tidal cycle’). As mentioned above, maximum diurnal tidal ranges occur when the

declination is zero, and therefore average tidal ranges will be larger in years when the

declination is at a minimum. Figure 26c shows that at Hinkley Point high waters are on

average 30 cm higher when the declination is at a minimum, although the maximum

forcing will still only occur when the declination is zero (which occurs every 13.61

days), and therefore this cycle has little effect on annual maximum water levels. The last

lunar nodal minimum occurred in March 1997 and the next will occur in September

2015. Predicted and observed low waters show a similar influence (Figure 27).

Peak astronomical tides occur when all the lunar and solar tidal components

constructively coincide, when the earth, sun and moon are in line, at their closest

respective distances, and at zero declination. The condition of simultaneous perihelion

(closest approach to the sun, currently in January) and zero solar declination will next

occur in the year AD 6581. However, zero solar and lunar declination and lunar perigee

will next almost coincide on 29 September 2015, producing a predicted high tide of 7.12

m OD (Table 6). Other unusually high astronomical tides are predicted to occur in 2192,

2732, 2825 and 3002 (Cartright, 1974).

2.5.2 Tidal currents

Peak tidal current velocities increase up the Bristol Channel in parallel with the

increasing tidal range, with typical peak spring tidal currents of 0.6 to 1.2 m s-1 in the

Outer Bristol Channel and 2.1 to 2.4 m s-1 in the Middle and Inner Bristol Channel and

the outer Severn estuary (Figure 14c).

As noted above, at Hinkley Point the tidal curves show only slight asymmetry, but

upstream from Avonmouth asymmetry becomes increasingly pronounced, with a

prolonged, weaker ebb flow and relatively shorter, stronger flood flow which should

favour net landward movement of sediment up the Severn estuary. In the outer Severn

estuary and Bristol Channel there is ebb-dominant flow in the central main channel and

flood dominant flow on either side. Around headlands and within embayments,



- 52 -

6.20

6.40

6.60

6.80

7.00

7.20

7.40

7.60

01/0

1/1

990

01/0

1/1

991

01/0

1/1

992

31/1

2/1

992

01/0

1/1

994

01/0

1/1

995

01/0

1/1

996

31/1

2/1

996

01/0

1/1

998

01/0

1/1

999

01/0

1/2

000

31/1

2/2

000

01/0

1/2

002

01/0

1/2

003

01/0

1/2

004

31/1

2/2

004

01/0

1/2

006

01/0

1/2

007

01/0

1/2

008

31/1

2/2

008

Wat

er le

vel (

m O

D)

Observed annual high water maxima

Predicted annual high water maxima

4.45

4.50

4.55

4.60

4.65

4.70

4.75

4.80

4.85

01/0

1/1

990

01/0

1/1

991

01/0

1/1

992

31/1

2/1

992

01/0

1/1

994

01/0

1/1

995

01/0

1/1

996

31/1

2/1

996

01/0

1/1

998

01/0

1/1

999

01/0

1/2

000

31/1

2/2

000

01/0

1/2

002

01/0

1/2

003

01/0

1/2

004

31/1

2/2

004

01/0

1/2

006

01/0

1/2

007

01/0

1/2

008

31/1

2/2

008

Wat

er le

vel (

m O

D)

Observed annual mean high water

Predicted annual mean high water

Observed and predicted trends in high water levels recorded at Hinkley Point, 1990 to 2008: (a) all spring high waters; (b) annual high water maxima; (c) annual mean high waters. Note differing vertical scales. Original data source: NTSLF.

Figure 26

(a)

(b)

(c)

4.50

5.00

5.50

6.00

6.50

7.00

7.50

01/0

1/1

990

01/0

1/1

991

01/0

1/1

992

31/1

2/1

992

01/0

1/1

994

01/0

1/1

995

01/0

1/1

996

31/1

2/1

996

01/0

1/1

998

01/0

1/1

999

01/0

1/2

000

31/1

2/2

000

01/0

1/2

002

01/0

1/2

003

01/0

1/2

004

31/1

2/2

004

01/0

1/2

006

01/0

1/2

007

01/0

1/2

008

31/1

2/2

008

Wat

er le

vel (

m O

D)

Observed spring high waters

Predicted spring high waters



- 53 -

-6.50

-6.00

-5.50

-5.00

-4.50

-4.00

-3.50

01/0

1/1

990

01/0

1/1

991

01/0

1/1

992

31/1

2/1

992

01/0

1/1

994

01/0

1/1

995

01/0

1/1

996

31/1

2/1

996

01/0

1/1

998

01/0

1/1

999

01/0

1/2

000

31/1

2/2

000

01/0

1/2

002

01/0

1/2

003

01/0

1/2

004

31/1

2/2

004

01/0

1/2

006

01/0

1/2

007

01/0

1/2

008

31/1

2/2

008

Obs

erve

d w

ater

leve

ls (m

OD

)Observed spring low waters

Predicted spring low waters

-6.40

-6.30

-6.20

-6.10

-6.00

-5.90

-5.80

-5.70

-5.60

-5.50

-5.40

01/0

1/1

990

01/0

1/1

991

01/0

1/1

992

31/1

2/1

992

01/0

1/1

994

01/0

1/1

995

01/0

1/1

996

31/1

2/1

996

01/0

1/1

998

01/0

1/1

999

01/0

1/2

000

31/1

2/2

000

01/0

1/2

002

01/0

1/2

003

01/0

1/2

004

31/1

2/2

004

01/0

1/2

006

01/0

1/2

007

01/0

1/2

008

31/1

2/2

008

Obs

erve

d w

ater

leve

ls (m

OD

)

Observed annual low water maxima

Predicted annual low water maxima

-4.05

-4.00

-3.95

-3.90

-3.85

-3.80

-3.75

-3.70

-3.65

01/0

1/1

990

01/0

1/1

991

01/0

1/1

992

31/1

2/1

992

01/0

1/1

994

01/0

1/1

995

01/0

1/1

996

31/1

2/1

996

01/0

1/1

998

01/0

1/1

999

01/0

1/2

000

31/1

2/2

000

01/0

1/2

002

01/0

1/2

003

01/0

1/2

004

31/1

2/2

004

01/0

1/2

006

01/0

1/2

007

01/0

1/2

008

31/1

2/2

008

Obs

erve

d w

ater

leve

ls (m

OD

)

Observed annual mean low water

Predicted annual mean low water

Observed and predicted trends in low water levels recorded at Hinkley Point, 1990 to 2002: (a) all spring high waters; (b) annual high water maxima; (c) annual mean high waters. Note differing vertical scales. Original data source: NTSLF.

Figure 27

(a)

(b)

(c)



- 54 -

Table 6 Extreme astronomical high tides predicted at Hinkley Point 1990-2026.

Year

Spring Autumn Spring Autumn

tides tides tides tides

1990 5.95 6.38 -4.97 -5.20

1991 6.47 6.57 -5.54 -5.61

1992 6.91 6.84 -5.96 -5.83

1993 7.00 6.92 -6.07 -5.74

1994 6.79 6.71 -5.89 -5.50

1995 6.49 6.64 -5.59 -5.57

1996 6.85 6.95 -5.94 -5.85

1997 7.03 7.09 -6.14 -5.91

1998 6.99 6.98 -6.09 -5.76

1999 6.71 6.66 -5.80 -5.43

2000 6.51 6.62 -5.57 -5.61

2001 6.85 6.93 -5.93 -5.82

2002 6.93 6.96 -6.02 -5.73

2003 6.76 6.64 -5.83 -5.42

2004 6.34 6.44 -5.46 -5.44

2005 6.69 6.87 -5.84 -5.78

2006 6.98 6.95 -6.03 -5.83

2007 6.86 6.77 -5.90 -5.51

2008 6.43 6.32 -5.48 -5.26

2009 6.65 6.78 -5.68 -5.71

2010 6.98 6.98 -6.02 -5.83

2011 6.94 6.88 -5.99 -5.67

2012 6.68 6.61 -5.73 -5.39

2013 6.62 6.76 -5.63 -5.68

2014 6.99 7.06 -5.99 -5.91

2015 7.11 7.12 -6.09 -5.89

2016 6.97 6.96 -5.97 -5.68

2017 6.65 6.60 -5.66 -5.36

2018 6.68 6.81 -5.76 -5.70

2019 6.91 7.02 -6.02 -5.83

2020 6.96 6.92 -6.03 -5.71

2021 6.71 6.59 -5.79 -5.36

2022 6.41 6.64 -5.50 -5.56

2023 6.86 6.94 -5.91 -5.87

2024 7.03 7.00 -6.07 -5.81

2025 6.79 6.67 -5.79 -5.43

2026 6.33 6.41 -5.37 -5.37

1990-2026 7.11 7.12 -6.14 -5.91

Maxima Minima



- 55 -

including Bridgwater Bay, there is a more complex tidal circulation pattern, as shown

by tidal elipses and tidal residuals (Posford Duvivier & ABPmer, 2000).

Two CEFAS benthic Mini-Landers, equipped with Nortek acoustic wave and current

(AWAC) meters and a range of other instrumentation, were deployed sequentially on

the sea bed at the Gore Buoy, north of Hinkley Point, between December 2008 and

April 2009 (a period of 112 days). The first Mini-Lander was recovered in mid February

2009 and replaced by the second to provide an almost continuous data record. Both

AWAC instruments had an acoustic frequency of 1 MHz and 40 vertical (water depth

interval) bins with a bin size for current speed measurement of 0.5 m. The maximum

current speed recorded was 1.87 m s-1, 7.3 m above the sea bed (Foden, 2009).

Acoustic Doppler current profiler (ADCP) instrumentation was deployed from a vessel

off Hinkley Point for an approximate 8 hour period on 17 - 18 September 2009. The

results showed mean and maximum flood current speeds of 0.37 and 0.78 m s-1, while

the mean and maximum ebb current speeds were 0.66 and 1.21 m s-1, respectively (i.e.

ebb tidal velocity dominance).

Nortek AWACs were also deployed 1.5 m above the sea bed at three locations (H1, H5

and H6) off Hinkley Point between mid-August and late September 2008. These

instruments had 24, 26 and 26 bins of 0.5 m vertical bin size, respectively. H1 was

located approximately offshore from the proposed new build site, H5 approximately

offshore slightly to the northwest, and Site H6 approximately 6 km offshore in the

central part of Bridgwater Bay. Maximum recorded current speeds ranged from 1.55 m

s-1 (H1 11.4 - 11.8 m above the bed) to 2.22 m s-1 (H6 14.6 m above the bed).

Nortek acoustic current meters (Aquadopps) were also deployed at three intertidal

locations (H2, H3, H4) on the foreshore near Hinkley Point between mid-August and

the end of September 2008. Site H2 was located relatively close inshore opposite the

proposed NNB site, H3 was located at a greater distance from the shore opposite the

Hinkley Point A power station, and Site H4 was located at a similar distance from the

shore just to the east of the Hinkley Point B power station. The maximum recorded

current speed at Site H2 was 0.68 ms-1 (3.5 to 4.0 m above the bed), at H3 1.73 m s-1



- 56 -

(3.5 m above the bed) and at H4 1.63 m s-1 (1.3 m above the bed). The maximum tidal

elevations recorded at the three sites during the period were 6.55 m, 6.55 m and 5.88 m

OD, respectively (all at 20.00 hrs on 17 September 2008).

2.5.3 Winds and waves

The large width and westerly / south-westerly aspect of the Bristol Channel mean that

the area is relatively exposed to winds and ocean waves, although the degree of

exposure decreases north-eastwards and is significantly reduced in the Severn estuary

(Shuttler, 1982; Figure 28). Modelled mean wave height in the Outer Bristol Channel

exceeds 2 m in the Outer Bristol Channel but decreases eastwards to < 0.7 m in

Bridgwater Bay and the Outer Severn estuary (Figure 14g). Wave power, which varies

as the square of the wave height, shows a similar but exponential decrease up the Bristol

Channel towards the Severn estuary (Figure 14h).

Wave data collected using two Datawell directional wave rider (DWR) mark III buoys

were analysed by Foden (2009). The first DWRS (the 'Gore Buoy') was located at

51º13'.77N, 03º9'.66W in a nominal water depth of 10 m below Chart Datum,

approximately 3 km NW of Hinkley Point, between mid December 2008 and early July

2009 (202 days of data reported at 30 minute intervals). Parameters recorded were wave

height, water temperature, peak wave period, mean wave period, peak wave direction,

and peak wave directional spread. Data were sent by Orbcomm satellite telemetry to a

base station in Italy and then as e-mail attachments to CEFAS in Lowestoft. Data return

for the telemetry data was better than 99% for all parameters except peak wave period

(87.74%). The mean significant wave height (Hs) for the 202 day period 16th December

2008 to 29th July 2009 was determined to be 0.47 m, mean water temperature 9.7oC,

mean peak wave period (Tp) 6.2 s, mean wave period (Tm) 3.7 s., mean peak wave

direction 234º, and mean wave directional spread 27º. The maximum recorded values

for Hs, water temperature, Tp, Tm and wave directional spread were 2.32 m, 20.2ºC, 19.0

s, 9.0 s, and 79º, respectively.



- 57 -

370 km

125 kmPointClear

Dungarvan

75 km

22 km

35 k

m

Republicof Ireland

Wales

England

LinneyHead

ForelandPoint

Worms Head

Nas

h Po

int

Lavernock Point

BreanDown

Gol

dcli�

277°

290°

298°308°

357°

030°

0

Scale

100 km604020 80

Fetch distances and bearings relative to Hinkley Point. Fetch-limiting locations are also shown. The total spread of fetch is 113°, while the longest fetch of 370 km covers only 13° between W and WSW.

Figure 28

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 58 -

The second source of data was provided by the Scarweather DWR buoy, located in a

nominal water depth of 30 m approximately 15 km off Porthcawl. This buoy has

operated since May 2005, and data for the period to July 2009 were analysed by Foden

(2009). Telemetry data return for this period was better than 99% for all measured

parameters except peak wave period (92.75%) and mean wave period (98.34%). Post-

recovery data return for this buoy was better than 98% for all parameters. The mean

significant wave height for the 1055 day period July 2005 to May 2008 post recovery

data was determined to be 1.22 m, the mean water temperature 12.6ºC, the peak wave

period 8.5 s, the mean wave period 4.8 s, the peak wave direction 247º, and the wave

directional spread 23º. The respective maximum recorded values for the period July

2005 to May 2008 were reportedly Hs of 6.23, water temperature of 19.8ºC, Tp of 22.2

s, Tm of 12.1 s, and wave directional spread of 81º. Maximum values for the telemetry

data recorded between May 2008 and July 2009 were Hs of 4.24 m, water temperature

of 18.6ºC, Tp of 19.9s, Tm of 11.0 s, and wave directional spread of 79º (Foden, 2009).

The results obtained from the deployment of the CEFAS Mini-Landers off Hinkley

Point indicated a mean significant wave height for the period of 0.49 m, peak wave

period of 5.8 s., mean wave period of 2.8 s, peak wave direction of 242º and mean wave

direction of 249º. The maximum significant wave height recorded was 2.08 m, the

maximum wave height for a 30 minute record was 3.17 m, the peak wave period was

18.8 s and the mean wave period 6.5 s (Foden, 2009).

The mean significant wave heights recorded by the AWACS offshore from Hinkley

Point in 2008-09 ranged from 0.46 m (Site H1) to 0.77 m (Site H6), while maximum Hs

ranged from 2.23 m (H1) to 2.96 (H6). Mean peak wave period ranged from 5.0 s (H1)

to 5.8 s (H6) and average mean wave period from 2.9 (H5) to 3.8 (H1). Maximum Hs

values ranged from 2.23 m to 2.96 m, maximum Tp from 10.0 (H1) to 13.3 (H5),

maximum Tm from 5.2 (H5) to 6.8 (H1), maximum wave height for a 30 minute period

from 3.66 m (H5) to 4.34 m (H6). The data showed a strong positive correlation

between significant wave height (both mean and maximum) and water depth, but a

weaker correlation between wave period and water depth. The DWR buoy data

indicated a dominant wave approach direction from the WNW, with less frequent

slightly smaller waves from the W and a few small waves from the NNE (Larcombe &



- 59 -

Fernand, 2009). This is consistent with expectations based on measured wind data and

available fetch, which in this direction is approximately 370 km towards the southern

coast of Ireland (Figure 28).

A re-analysis of data from the CEFAS ‘Gore Buoy’ for the period 16/12/2008 to

18/11/2010 has confirmed dominant wave directions from the WNW at Hinkley Point

(Figure 29a). Further west on the Somerset coastline, at Minehead, waves approach the

coast from a more NW direction (Figure 29b), due to the protection afforded by the

headland of Minehead Bluff to the west. On the northern side of the Bristol Channel, at

Scarweather, the dominant waves approach from a W to WSW direction (Figure 29c).

Due to the greater exposure of this site to Atlantic swell waves, and the greater distance

from the shore, significant wave heights are much greater at Scarweather than at

Hinkley Point. Approximately 50% of all recorded waves at Scarweather are larger than

1 m, and 10% of waves are larger than 2 m. At the Gore buoy, 3 km NW of Hinkley

Point, 50% of recorded waves are higher than 0.42 m and less than 10% are higher than

1 m.

The frequency distribution of half-hourly significant wave heights at Hinkley Point for

the period 16/12/08 to 18/11/10 is shown in Figure 30. The distribution is highly

skewed with a modal Hs is 0.2 to 0.3 m. The frequency distribution of Hs values above a

threshold of 0.8 m is shown in Figure 31, with a Generalised Pareteo Distribution

(GPD) fitted using the Maximum Likelihood (ML) method.

2.5.4 Suspended sediments and bed sediments

The inner Bristol Channel and Severn estuary are characterised by high average levels

of turbidity, mainly due to high suspended sediment concentrations (Dyer, 1984). There

is a marked turbidity maximum in the area between Bridgwater Bay and Avonmouth, in

large part a reflection of the strong tidal current velocities which are responsible for the

rapid re-suspension of settled mud deposits. Suspended sediment concentrations are

generally higher on the English side of the inner Bristol Channel and outer Severn

estuary than on the Welsh side, and zones of turbid and relatively clear water are



- 60 -

0

45

90

135

180

225

270

315

0% 20% 40% 60%

<=0.5

>0.5 - 1

>1 - 1.5

>1.5 - 2

>2

Rose diagrams showing waves recorded at Hinkley Point (16/12/08 to 18/11/10),Scarweather (16/12/08 to 05/11/09) and Minehead (16/12/06 to 31/12/07).Original data sources: CEFAS Wavenet and Channel Coastal Observatory.

Figure 29

Significant waveheight (m)

0

45

90

135

180

225

270

315

0% 20% 40% 60%

(a) Hinkley Point

(c) Scarweather

0

45

90

135

180

225

270

315

0% 20% 40% 60%

(b) Minehead



- 61 -

0

2

4

6

8

10

12

14

16

18

0.0‐0.1

0.1‐0.2

0.2‐0.3

0.3‐0.4

0.4‐0.5

0.5‐0.6

0.6‐0.7

0.7‐0.8

0.8‐0.9

0.9‐1.0

1.0‐1.1

1.1‐1.2

1.2‐1.3

1.3‐1.4

1.4‐1.5

1.5‐1.6

1.6‐1.7

1.7‐1.8

1.8‐1.9

1.9‐2.0

2.0‐2.1

2.1‐2.2

2.2‐2.3

2.3‐2.4

2.4‐2.5

2.5‐2.6

2.6‐2.7

2.7‐2.8

2.8‐2.9

Frequency (%)

Significant wave height (m)

Frequency of significant wave heights recorded at Hinkley Point in the period 16/12/08 to 18/11/10 (total of 30938 half hourly observations). Directional waverider buoy located at at 51°13'.77N and 003°9'.66W in 10m of water, approximately 3 km NW of Hinkley Point Power Station. Original data source: CEFAS Wavenet.

Figure 30

n = 30938min = 0.02 m1% = 0.06 m10% = 0.13 m50% = 0.42 m90% = 0.94 m99% = 1.66 mmax = 2.82 m



- 62 -

0

5

10

15

20

25

0.80 1.00 1.20 1.40 1.60 1.80 2.00 2.20 2.40

Freq

uenc

y (%

)

Observed water level (m OD)

Observed data

Generalised Pareto Distribution

Figure 31 Histogram showing frequency of significant wave height observed at the CEFAS wave buoy (16/11/08 to 18/11/10), and fitted Generalised Pareto Distribution calculated using the maximum likelihood method. Original data source: CEFAS Wavenet.



- 63 -

separated by a front (Kirby & Parker, 1983a,b). Within Bridgwater Bay itself, very high

near bed concentrations of suspended fine-grained sediment persist for long periods, at

times creating fluid mud layers up to several metres thick (Kirby & Parker, 1980,

1983b).

Suspended particulate matter (SPM) was determined by optical sensor measurements of

water turbidity at two elevations above the sea bed (95 cm and 165 cm) at the CEFAS

Mini-Lander site off Hinkley Point, between December 2008 and April 2009. SPM

concentrations showed a strong correspondence with current velocity, rising to > 900 to

1000 FTU during periods of peak velocity and falling as low as 100 to 200 FTU during

periods of slack current. However, the absolute SPM values obtained should not be

regarded as accurate due to calibration issues associated with optical turbidity sensor

devices (Larcombe & Fernand, 2009). No information is currently available regarding

the particle size or composition of the material in suspension.

A bedload sediment transport parting zone has been suggested to exist between Barry

and Bridgwater Bay (BGS, 1986; Stride & Belderson, 1991). However, physical

evidence provided by bedforms (Harris & Collins, 1985, 1988) and the results of

residual current modelling suggest that the pattern of bedload transport is more

complex. Harris & Collins (1991) proposed a system of mutually evasive sediment

transport pathway, with ebb-dominated transport in the mid channel and flood-

dominated transport towards the margins. Local circulation eddies exist around the

larger sand banks (Helwick, Nash and Scarweather). On the Somerset shore there is

clear morphological and sedimentological evidence which indicates net west to east

transport of sediments in the littoral and nearshore zones (e.g. Kidson, 1960, 1963).

Sediment tracer studies have confirmed this process but suggest that transport rates are

relatively low (Kidson & Carr, 1961).

2.5.5 Storm surges

The Bristol Channel, like other parts of western Britain, is prone to relatively frequent

meteorologically-forced surges which can raise coastal water levels well above



- 64 -

predicted levels. The majority of surges are generated by North Atlantic low pressure

systems which can follow several different trajectories across the UK into continental

Europe (Lennon, 1963b). Very severe storms associated with depressions are known to

have occurred in 1483, 1607, 1703, 1910, 1947 and 1981, and others may have gone

unrecorded.

Several inter-dependent processes associated with passage of such low pressure systems

act to produce elevated coastal water levels (Figure 32). Lowered atmospheric pressure

alone has the effect of raising still water levels due to an 'inverted barometer effect',

whereby there is a theoretical increase in still water level of approximately 1cm for

every 1 mb reduction in pressure (Pugh, 2004, p136). Compared with a situation of

Standard Atmospheric Pressure of 1013 mb, a typical deep depression reaching 960 mb

would result in increase still water level of 53 cm. The lowest recorded pressures

associated with Atlantic depressions close to the UK are 916 mb in December 1986 and

January 1993 (Burt, 1993). By comparison, a 'world record' low pressure of 870 mb was

recorded in 1879 during Typhoon Tip (Dunnavan & Diercke, 1980).

Strong winds associated with low pressure centres also create drag on the sea surface

which both changes average sea level levels and creates waves. The drag increases

slightly more rapidly than the square of the wind speed, and has a greater effect in

shallow water (Pugh, 2004, pp 136-7). Where the water is driven onshore against the

coast, or into estuaries, the resistance created by the land boundary creates a sea surface

slope with higher levels adjacent to the coast (known as 'set-up'). For a Strong Gale

(average hourly wind speeds > 22 m s-1) blowing over 200 km of water with an average

water depth of 40 m, the increase in water levels at the coast would be about 0.85 m

With Storm Force winds (> 30 m s-1), the increase in level would be of the order of 1.60

m (Pugh, 2004, p 138).

In the northern hemisphere, the rotation of the earth causes any flow of water to be

deflected to the right; a phenomenon known as Ekman volume transport. The effect is

greater at the surface than at depth. If the wind blows parallel to a length of coast which

is located to the right, the flow of water is deflected to the right towards the coast, and

sea levels rise; if the coast lies to the left, the Ekman volume transport is away from the



- 65 -

predictedhigh tide level

resultant averagehigh water level

surgecurrents

currents

wave setup

resultantnearshore meanwater level

limit ofwave runup

ocean waveswind

Figure 32 Schematic diagram showing the main factors which control high water levels.



- 66 -

coast, and average levels fall. The Ekman volume transport gives rise to a locally

generated surge which is distinguished from externally generated surges which move

into an area and along the coast as progressive Kelvin waves. Externally generated

surges are highly important in the North Sea, but in the Bristol Channel locally

generated surges have greater importance. Depressions which move into the Southwest

Approaches and then across Wales and the English Midlands can produce significant

inverted barometric pressure effects and significant Ekman volume transport into the

Bristol Channel.

A further factor is that the Bristol Channel displays near-resonant behaviour to both

tidal and meteorological forcing. This produces intense but relatively short-lived surges

which decay during a single semi-diurnal tidal cycle, in contrast to the situation in the

North Sea where surges may persist over several tidal cycles. In March 1947 a surge of

3.54 m at Avonmouth was recorded on a falling tide but the normal tidal fall of the tide

was reversed only for a few hours (Pugh, 2004, p142).

The magnitude of a 'surge' can be defined in several different ways. Differences

between predicted and observed water levels at any time in the tidal cycle are normally

referred to as non-tidal residuals, and the magnitudes of the non-tidal residual at the

time of predicted and observed high water are frequently cited (e.g. Pye & Blott, 2008).

However, perhaps the most commonly used measure of a 'surge' is the elevation

difference between the predicted high tide and the observed high tide; this is referred to

in the IPCC Fourth Scientific Assessment (Solomon et al., 2007) and by UKCP09

(Lowe et al., 2009) as the skew surge. However, the height difference between predicted

and observed low waters can also be important (e.g. for navigation purposes or

dispersion of effluent from discharge pipes); for this reason, two types of skew surge are

recognised in this study, skew surge at high water (SkHW) and skew surge at low water

(SkLW).

Table 7 lists the highest predicted tides at Hinkley Point in the period 1990-2008 and

the recorded surges associated with them. Table 8 list the 20 highest recorded tidal

levels and associated surges in the same period, while Table 9 lists the 20 largest surges.

The largest recorded skew surge occurred on 24th February 1997 when a 1.54 m surge



- 67 -

Table 7 The 20 highest predicted tides at Hinkley Point in the period 1990-2008 (excluding BODC 'Improbable Values').

Time of observed Predicted high Observed high Surge residual at time of Surge residual at time of High water skew surge

high water water level (m OD) water level (m OD) observed high water (m) predicted high water (m) (SKHW) (m)

18/09/1997 20:00 7.09 7.08 -0.01 -0.01 -0.01

11/03/1997 08:15 7.03 6.91 -0.11 -0.16 -0.12

10/03/1997 07:45 7.02 6.99 -0.03 -0.03 -0.03

17/09/1997 19:15 7.01 7.03 0.03 0.03 0.03

10/03/1993 08:00 7.00 7.08 0.08 0.08 0.08

30/03/1998 08:15 6.99 7.22 0.23 0.23 0.23

10/02/1997 08:45 6.99 7.36 0.37 0.37 0.37

02/03/2006 08:30 6.98 7.06 0.08 0.08 0.08

07/10/1998 20:00 6.98 6.97 -0.01 -0.01 -0.01

29/03/1998 07:30 6.97 7.09 0.14 0.11 0.12

09/02/1997 08:00 6.96 6.98 0.02 0.02 0.02

07/10/2002 19:30 6.96 7.01 0.06 0.06 0.06

30/08/1996 20:00 6.95 6.82 -0.13 -0.13 -0.13

09/09/2006 20:00 6.95 7.07 0.12 0.12 0.12

01/03/1998 08:45 6.93 7.03 0.10 0.10 0.10

18/09/2001 19:30 6.93 6.76 -0.13 -0.17 -0.17

30/03/2002 08:00 6.93 6.94 0.01 0.01 0.01

17/10/1997 19:30 6.92 7.04 0.13 0.09 0.12

17/09/1993 19:30 6.92 7.08 0.18 0.16 0.16

20/08/1997 20:15 6.92 6.89 -0.03 -0.03 -0.03

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 68 -

Table 8 The 20 highest observed tides at Hinkley Point in the period 1990-2008 (excluding BODC 'Improbable Values').



10/02/1997 08:45 6.99 7.36 0.37 0.37 0.37

30/03/2006 07:15 6.80 7.26 0.46 0.46 0.46

30/03/1998 08:15 6.99 7.22 0.23 0.23 0.23

08/10/2006 19:30 6.85 7.22 0.36 0.36 0.36

04/12/1994 07:30 6.54 7.17 0.66 0.60 0.63

30/08/1992 08:00 6.71 7.14 0.43 0.43 0.43

29/03/1998 20:00 6.88 7.13 0.25 0.25 0.25

07/10/2006 19:00 6.82 7.11 0.29 0.29 0.29

10/03/2001 07:15 6.66 7.11 0.46 0.46 0.46

24/12/1999 20:00 6.34 7.10 0.77 0.77 0.77

11/03/2001 08:00 6.85 7.09 0.25 0.25 0.24

29/03/1998 07:45 6.97 7.09 0.14 0.11 0.12

29/08/1992 20:00 6.84 7.09 0.25 0.25 0.25

28/10/1996 19:45 6.19 7.08 0.90 0.90 0.90

17/09/1993 19:45 6.92 7.08 0.18 0.16 0.16

18/09/1997 20:00 7.09 7.08 -0.01 -0.01 -0.01

10/03/1993 08:00 7.00 7.08 0.08 0.08 0.08

09/09/2006 20:00 6.95 7.07 0.12 0.12 0.12

02/03/2006 08:30 6.98 7.06 0.08 0.08 0.08

05/11/1998 19:45 6.86 7.06 0.24 0.20 0.20

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 69 -

Table 9 The 20 largest high water skew surges recorded at Hinkley Point in the period 1990-2008 (excluding BODC 'Improbable Values'), ordered

in terms of high water skew surge (SKHW).



24/02/1997 20:15 5.50 7.04 1.54 1.54 1.54

03/12/2006 04:45 5.02 6.34 1.31 1.31 1.31

04/01/1998 10:45 5.29 6.55 1.27 1.18 1.26

16/02/1995 19:30 5.81 7.02 1.21 1.10 1.20

19/02/1997 17:45 4.28 5.28 1.17 0.84 1.01

08/01/2005 04:15 4.28 5.28 1.00 1.00 1.00

18/01/2007 05:45 4.84 5.83 1.02 0.98 0.99

25/12/1997 15:00 3.45 4.36 0.93 0.90 0.91

28/10/1996 19:45 6.19 7.08 0.90 0.90 0.90

20/10/2004 23:00 3.45 4.34 0.90 0.90 0.90

06/11/1996 02:45 2.83 3.72 0.91 0.84 0.89

11/02/1995 16:00 3.12 4.00 0.87 0.87 0.87

03/01/1999 07:15 6.18 7.02 0.84 0.84 0.84

08/12/2000 03:45 4.05 4.87 0.82 0.82 0.82

31/12/2006 16:00 4.15 4.96 0.82 0.79 0.81

31/03/1994 21:45 5.79 6.57 0.79 0.79 0.79

05/12/2006 06:30 5.62 6.40 0.79 0.79 0.79

25/11/2000 18:15 5.61 6.38 0.78 0.73 0.77

24/10/1998 20:30 5.03 5.80 0.82 0.76 0.77

24/12/1999 20:00 6.34 7.10 0.77 0.77 0.77

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 70 -

residual coincided exactly with a modest predicted high tide of 5.5 m OD. On 3rd

December 2006 a maximum surge residual of 1.31 m coincided with a predicted high

tide of 5.03 m OD. However, there were only five occasions in the period when the

skew surge exceeded 1 m (Table 9). A summary of the extreme annual high and low

water levels for each year in the period 1990 2008 is shown in Table 10. The highest

tidal level recorded was 7.36 m OD in February 1997, but maxima > 7.0 m OD were

recorded in 13 of the 19 years. The lowest recorded low tide was -6.33 m OD in 2007,

while tides lower than - 6.0 m OD were recorded in 9 of the 19 years.

The frequency distribution of all non-tidal residuals recorded at Hinkley Point in the

period 1990-2008, based on 15 minute observations, is shown in Figure 33 and

summarised in Table 11, while the frequency distribution of skew surges is shown in

Figure 34. The separate distributions for high and low waters are shown in Figures 35 to

38. All of the distributions are approximately normal but have a longer tail of higher

values.

An important question concerns the degree of inter-dependence between predicted water

levels and the magnitude of non-tidal (surge) residuals. Figure 39 shows the observed

relationship between surge residuals > 0.5 m and predicted water levels at Hinkley

Point. Figure 40 shows the respective frequency distribution for residuals > 0.5 m as a

function of predicted water level. Figures 41 and 42 show the same relationships for

surge residuals >1.0 m. In both cases the frequency distributions are markedly non-

normal, but they also differ significantly from the bimodal distributions of predicted and

observed tidal levels shown in Figures 18 and 19. The implication is that larger surges

are slightly more likely to occur in association with lower water levels than with high

water levels. This is generally attributed to shallow water effects which result in non-

linear tide-surge interactions. As seen in Figure 42, surge residuals larger than 1.0 m

only rarely coincide with predicted water levels higher than 5.5 m OD.



- 71 -

Table 10 Extreme annual water levels and surges observed at Hinkley Point 1990-2008. Note that values represent the annual extremes for each parameter,

each row representing a number of different individual extreme events

Year

Observed Observed Predicted Predicted Surge Surge Skew Observed Observed Predicted Predicted Surge Surge Skew

high tide low tide high tide low tide Residual3

Residual4

Surge1

high tide low tide high tide low tide Residual3

Residual4

Surge2

1990 6.43 -0.84 6.38 -1.30 1.34 1.64 0.59 1.74 -5.40 1.99 -5.20 -0.61 -0.67 -0.51

1991 6.62 -0.83 6.57 -1.31 1.64 1.64 0.75 1.73 -5.63 1.90 -5.61 -0.67 -0.67 -0.66

1992 7.14 -1.56 6.91 -1.51 1.17 1.17 0.76 1.82 -6.05 2.08 -5.96 -0.66 -0.66 -0.54

1993 7.08 -1.61 7.00 -1.84 1.37 1.59 0.75 2.22 -6.13 2.50 -6.07 -0.71 -0.76 -0.55

1994 7.17 -1.61 6.79 -1.70 1.87 1.88 0.79 2.06 -6.01 2.36 -5.89 -0.62 -0.78 -0.50

1995 7.02 -1.54 6.64 -1.59 1.83 1.94 1.20 1.92 -5.75 2.21 -5.59 -0.70 -0.70 -0.45

1996 7.08 -1.52 6.95 -1.67 1.05 1.05 0.90 2.31 -6.24 2.28 -5.94 -1.04 -0.81 -0.53

1997 7.36 -1.83 7.09 -1.82 1.69 1.76 1.54 2.38 -6.28 2.49 -6.14 -0.69 -0.57 -0.36

1998 7.22 -1.81 6.99 -1.91 2.21 2.55 1.26 2.39 -6.08 2.60 -6.09 -0.61 -0.67 -0.61

1999 7.10 -1.29 6.71 -1.54 1.34 1.43 0.84 1.99 -5.94 2.25 -5.80 -0.58 -0.66 -0.51

2000 6.83 -1.18 6.62 -1.39 1.99 2.12 0.82 1.89 -5.67 2.13 -5.61 -0.58 -0.57 -0.58

2001 7.11 -1.31 6.93 -1.39 1.12 1.29 0.57 2.05 -5.83 2.13 -5.93 -0.66 -0.62 -0.51

2002 7.03 -1.37 6.96 -1.52 1.52 1.68 0.63 2.16 -6.12 2.30 -6.02 -0.74 -0.62 -0.58

2003 6.77 -1.14 6.76 -1.35 1.24 1.24 0.64 1.79 -6.04 2.04 -5.83 -0.87 -0.87 -0.62

2004 6.53 -0.94 6.44 -1.15 1.38 1.38 0.90 1.50 -5.63 1.78 -5.46 -0.83 -0.83 -0.57

2005 6.84 -0.88 6.87 -1.25 1.34 1.34 1.00 1.56 -5.81 1.90 -5.84 -0.79 -0.79 -0.59

2006 7.26 -1.13 6.98 -1.41 1.31 1.32 1.31 2.09 -5.98 2.02 -6.03 -0.63 -0.63 -0.55

2007 7.01 -0.99 6.86 -1.40 1.27 1.24 0.99 1.76 -6.33 2.16 -5.90 -0.68 -0.68 -0.57

2008 7.02 -0.80 6.44 -1.25 1.12 1.12 0.70 1.68 -5.67 1.92 -5.48 -0.94 -0.94 -0.61

1990-2008 7.36 -0.80 7.09 -1.15 2.21 2.55 1.54 1.50 -6.33 1.78 -6.14 -1.04 -0.94 -0.66

1 Maximum positive skew surge (difference between observed and predicted high waters)

2 Maximum negative skew surge (difference between observed and predicted low waters)

3 Values extracted from the 15 minute archived datasets supplied by NTSLF

4 Extreme annual values taken from the monthly extreme tables supplied by NTSLF

Maxima Minima

_________________________________________________________________________________________________________________K


- 72 -

All At high At low All At high At low All At high At low All At high At low

water water water water water water water water

n 624837 12591 12515 624837 12591 12515 624817 12589 12515 25106 12591 12515

Mean 0.32 4.65 -3.88 0.33 4.63 -3.84 -0.01 0.02 -0.05 -0.01 0.01 -0.05

Standard deviation 3.04 1.10 1.01 3.03 1.08 0.98 0.20 0.18 0.20 0.19 0.18 0.20

Skewness 0.06 -0.17 0.27 0.06 -0.17 0.21 0.69 0.49 0.81 0.65 0.45 0.81

Minimum -6.33 1.50 -6.33 -6.14 1.78 -6.14 -1.04 -0.69 -0.66 -0.69 -0.69 -0.66

99% exceedance -5.07 2.25 -5.78 -5.02 2.34 -5.73 -0.42 -0.38 -0.46 -0.43 -0.39 -0.46

95% exceedance -4.27 2.79 -5.39 -4.25 2.82 -5.33 -0.30 -0.26 -0.34 -0.31 -0.26 -0.34

90% exceedance -3.66 3.12 -5.13 -3.66 3.11 -5.06 -0.24 -0.20 -0.28 -0.24 -0.20 -0.28

75% exceedance -2.32 3.80 -4.65 -2.30 3.78 -4.59 -0.13 -0.10 -0.18 -0.14 -0.10 -0.18

50% exceedance 0.24 4.74 -3.98 0.24 4.74 -3.95 -0.02 0.01 -0.06 -0.02 0.01 -0.06

25% exceedance 2.94 5.49 -3.12 2.93 5.47 -3.08 0.11 0.12 0.06 0.10 0.12 0.06

10% exceedance 4.44 6.05 -2.49 4.44 6.03 -2.47 0.24 0.24 0.21 0.23 0.23 0.21

5% exceedance 5.11 6.36 -2.16 5.11 6.31 -2.18 0.33 0.31 0.32 0.31 0.31 0.32

1% exceedance 6.01 6.77 -1.58 5.98 6.70 -1.72 0.56 0.50 0.56 0.53 0.49 0.56

Maximum 7.36 7.36 -0.80 7.09 7.09 -1.15 2.21 1.54 1.55 1.76 1.54 1.55

Skew surgesObserved Water levels Predicted water levels Surge residuals

Table 11 Frequency distributions of water levels and surges recorded and predicted at Hinkley Point in the period 1990-2008. Original data source: NTSLF.

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 73 -

0

5

10

15

20

25

-1.1

to -1

.0-1

.0 to

-0.9

-0.9

to -0

.8-0

.8 to

-0.7

-0.7

to -0

.6-0

.6 to

-0.5

-0.5

to -0

.4-0

.4 to

-0.3

-0.3

to -0

.2-0

.2 to

-0.1

-0.1

to 0

.00.

0 to

0.1

0.1

to 0

.20.

2 to

0.3

0.3

to 0

.40.

4 to

0.5

0.5

to 0

.60.

6 to

0.7

0.7

to 0

.80.

8 to

0.9

0.9

to 1

.01.

0 to

1.1

1.1

to 1

.21.

2 to

1.3

1.3

to 1

.41.

4 to

1.5

1.5

to 1

.61.

6 to

1.7

1.7

to 1

.81.

8 to

1.9

1.9

to 2

.02.

0 to

2.1

2.1

to 2

.22.

2 to

2.3

Freq

uenc

y (%

)

Surge Residual (m)

Frequency of all non-tidal (surge) residuals recorded at Hinkley Point in the period 1990-2008 (total of 624817 fifteen minute observations). Original data source: NTSLF.

Figure 33

0

5

10

15

20

25

-1.1

to -1

.0-1

.0 to

-0.9

-0.9

to -0

.8-0

.8 to

-0.7

-0.7

to -0

.6-0

.6 to

-0.5

-0.5

to -0

.4-0

.4 to

-0.3

-0.3

to -0

.2-0

.2 to

-0.1

-0.1

to 0

.00.

0 to

0.1

0.1

to 0

.20.

2 to

0.3

0.3

to 0

.40.

4 to

0.5

0.5

to 0

.60.

6 to

0.7

0.7

to 0

.80.

8 to

0.9

0.9

to 1

.01.

0 to

1.1

1.1

to 1

.21.

2 to

1.3

1.3

to 1

.41.

4 to

1.5

1.5

to 1

.61.

6 to

1.7

1.7

to 1

.81.

8 to

1.9

1.9

to 2

.02.

0 to

2.1

2.1

to 2

.22.

2 to

2.3

Freq

uenc

y (%

)

Skew Surge (m)

Frequency of all skew surges (SKHW and SKLW) recorded at Hinkley Point in the period 1990-2008 (total of 25106 high and low tides). Original data source: NTSLF.

Figure 34



- 74 -

0

5

10

15

20

25

-1.1

to -1

.0-1

.0 to

-0.9

-0.9

to -0

.8-0

.8 to

-0.7

-0.7

to -0

.6-0

.6 to

-0.5

-0.5

to -0

.4-0

.4 to

-0.3

-0.3

to -0

.2-0

.2 to

-0.1

-0.1

to 0

.00.

0 to

0.1

0.1

to 0

.20.

2 to

0.3

0.3

to 0

.40.

4 to

0.5

0.5

to 0

.60.

6 to

0.7

0.7

to 0

.80.

8 to

0.9

0.9

to 1

.01.

0 to

1.1

1.1

to 1

.21.

2 to

1.3

1.3

to 1

.41.

4 to

1.5

1.5

to 1

.61.

6 to

1.7

1.7

to 1

.81.

8 to

1.9

1.9

to 2

.02.

0 to

2.1

2.1

to 2

.22.

2 to

2.3

Freq

uenc

y (%

)

Surge Residual (m)

Frequency of surge residuals at high water, recorded at Hinkley Point in the period 1990-2008 (total of 12589 fifteen minute observations). Original data source: NTSLF.

Figure 35

0

5

10

15

20

25

-1.1

to -1

.0-1

.0 to

-0.9

-0.9

to -0

.8-0

.8 to

-0.7

-0.7

to -0

.6-0

.6 to

-0.5

-0.5

to -0

.4-0

.4 to

-0.3

-0.3

to -0

.2-0

.2 to

-0.1

-0.1

to 0

.00.

0 to

0.1

0.1

to 0

.20.

2 to

0.3

0.3

to 0

.40.

4 to

0.5

0.5

to 0

.60.

6 to

0.7

0.7

to 0

.80.

8 to

0.9

0.9

to 1

.01.

0 to

1.1

1.1

to 1

.21.

2 to

1.3

1.3

to 1

.41.

4 to

1.5

1.5

to 1

.61.

6 to

1.7

1.7

to 1

.81.

8 to

1.9

1.9

to 2

.02.

0 to

2.1

2.1

to 2

.22.

2 to

2.3

Freq

uenc

y (%

)

Skew Surge (m)

Frequency of skew surges at high water (SKHW), recorded at Hinkley Point in the period 1990-2008 (total of 12591 tides). Original data source: NTSLF.

Figure 36



- 75 -

0

5

10

15

20

25

-1.1

to -1

.0-1

.0 to

-0.9

-0.9

to -0

.8-0

.8 to

-0.7

-0.7

to -0

.6-0

.6 to

-0.5

-0.5

to -0

.4-0

.4 to

-0.3

-0.3

to -0

.2-0

.2 to

-0.1

-0.1

to 0

.00.

0 to

0.1

0.1

to 0

.20.

2 to

0.3

0.3

to 0

.40.

4 to

0.5

0.5

to 0

.60.

6 to

0.7

0.7

to 0

.80.

8 to

0.9

0.9

to 1

.01.

0 to

1.1

1.1

to 1

.21.

2 to

1.3

1.3

to 1

.41.

4 to

1.5

1.5

to 1

.61.

6 to

1.7

1.7

to 1

.81.

8 to

1.9

1.9

to 2

.02.

0 to

2.1

2.1

to 2

.22.

2 to

2.3

Freq

uenc

y (%

)

Surge Residual (m)

Frequency of surge residuals at low water, recorded at Hinkley Point in the period 1990-2008 (total of 12515 fifteen minute observations). Original data source: NTSLF.

Figure 37

0

5

10

15

20

25

-1.1

to -1

.0-1

.0 to

-0.9

-0.9

to -0

.8-0

.8 to

-0.7

-0.7

to -0

.6-0

.6 to

-0.5

-0.5

to -0

.4-0

.4 to

-0.3

-0.3

to -0

.2-0

.2 to

-0.1

-0.1

to 0

.00.

0 to

0.1

0.1

to 0

.20.

2 to

0.3

0.3

to 0

.40.

4 to

0.5

0.5

to 0

.60.

6 to

0.7

0.7

to 0

.80.

8 to

0.9

0.9

to 1

.01.

0 to

1.1

1.1

to 1

.21.

2 to

1.3

1.3

to 1

.41.

4 to

1.5

1.5

to 1

.61.

6 to

1.7

1.7

to 1

.81.

8 to

1.9

1.9

to 2

.02.

0 to

2.1

2.1

to 2

.22.

2 to

2.3

Freq

uenc

y (%

)

Skew Surge (m)

Frequency of skew surges at low water (SKLW), recorded at Hinkley Point in the period 1990-2008 (total of 12515 tides). Original data source: NTSLF.

Figure 38



- 76 -

0.50

0.70

0.90

1.10

1.30

1.50

1.70

1.90

2.10

2.30

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Surg

e re

sidu

al (m

)

Predicted water level (m OD)

Cross-plot of predicted water level against positive surge residual >0.5 m at Hinkley Point (1990-2008), based on 8764 instantaneous 15 minute predictions. Original data source: NTSLF.

Figure 39

0

1

2

3

4

5

6

7

8

-6.0

to -5

.5

-5.5

to -5

.0

-5.0

to -4

.5

-4.5

to -4

.0

-4.0

to -3

.5

-3.5

to -3

.0

-3.0

to -2

.5

-2.5

to -2

.0

-2.0

to -1

.5

-1.5

to -1

.0

-1.0

to -0

.5

-0.5

to 0

.0

0.0

to 0

.5

0.5

to 1

.0

1.0

to 1

.5

1.5

to 2

.0

2.0

to 2

.5

2.5

to 3

.0

3.0

to 3

.5

3.5

to 4

.0

4.0

to 4

.5

4.5

to 5

.0

5.0

to 5

.5

5.5

to 6

.0

6.0

to 6

.5

6.5

to 7

.0

Freq

uenc

y (%

)


Frequency histogram of a >0.5 m surge residual as a function of predicted water level at Hinkley Point (1990-2008), based on 8764 instantaneous 15 minute predictions (data shown in Figure 39 directly above). Original data source: NTSLF.

Figure 40

n = 8764min = -5.95 m OD1% = -5.14 m OD10% = -3.94 m OD50% = -0.83 m OD90% = +3.68 m OD99% = +5.69 m OD



- 77 -

1.00

1.20

1.40

1.60

1.80

2.00

2.20

2.40

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

Surg

e re

sidu

al (m

)


Cross-plot of predicted water level against positive surge residual >1.0 m at Hinkley Point (1990-2008), based on 429 instantaneous 15 minute predictions. Original data source: NTSLF.

Figure 41

0

2

4

6

8

10

12

-6.0

to -5

.5

-5.5

to -5

.0

-5.0

to -4

.5

-4.5

to -4

.0

-4.0

to -3

.5

-3.5

to -3

.0

-3.0

to -2

.5

-2.5

to -2

.0

-2.0

to -1

.5

-1.5

to -1

.0

-1.0

to -0

.5

-0.5

to 0

.0

0.0

to 0

.5

0.5

to 1

.0

1.0

to 1

.5

1.5

to 2

.0

2.0

to 2

.5

2.5

to 3

.0

3.0

to 3

.5

3.5

to 4

.0

4.0

to 4

.5

4.5

to 5

.0

5.0

to 5

.5

5.5

to 6

.0

6.0

to 6

.5

6.5

to 7

.0

Freq

uenc

y (%

)


Frequency histogram of a >1.0 m surge residual as a function of predicted water level at Hinkley Point (1990-2008), based on 429 instantaneous 15 minute predictions (data shown in Figure 41 directly above). Original data source: NTSLF.

Figure 42

n = 429min = -5.41 m OD1% = -5.09 m OD10% = -4.47 m OD50% = -1.91 m OD90% = +3.97 m OD99% = +5.47 m OD



- 78 -

2.5.6 Historical sea level rise

A composite Holocene sea level curve for the Bristol Channel, based on radiocarbon

dating of peats interpreted to provide sea level index points, mostly reported by Kidson

& Heyworth (1973, 1976) and Heyworth & Kidson (1982), is shown in Figure 43. Few

data are available for the period before 7600 years ago and for the period after 2500 BP.

However, rapid sea level rise from c. -30 m to c. -13 m is suggested between 9000 and

7500 years ago (average rate c. 11.33 mm a-1), followed by a period of slower rise

which apparently continued, at a diminishing rate, until c. 3000 years ago. Kidson &

Heyworth (1976) and Heyworth & Kidson (1982) concluded that sea level had reached

its present level in the inner Bristol Channel by 3000 BP, since which time there has

been little, if any, significant change. However, Haslett et al. (1998a) re-examined the

radiocarbon-dated peat evidence in the Somerset levels and concluded that, after

allowance is made for compaction, there is evidence for sea level rise of 0.41 - 0.82 mm

a-1 during the period 3700 to 1770.

Allen & Rae (1988) noted that there has been vertical accretion on mature saltmarshes

in the Severn estuary of 1.22 m since Roman Times, 1.05 m since the Medieval period

(c. AD 1250), 0.54 m since c. AD 1845 and 0.21 m since c. AD1945, and suggested

these data indicate continuing upward movement of relative sea level at a rate of a few

mm per year. Allen (1991) refined these data to suggest an acceleration of sea level rise

since the Roman period, with a minimum apparent average rate of rise of 0.4 mm a-1

between the late Roman Period and the Medieval Period, 0.79 mm a-1 from then until

the beginning of the Modern Period (c. 1845), 1.49 mm a-1 through the Modern Period

to c. 1945, and 4.65 mm a-1 since c. 1945. Allen (1991) also concluded that, depending

on the extent to which the Severn estuary has experienced marine transgression over the

period covered by the measurements, the relative rise of MHWS tide level to the present

could be up to 1.75 m. However, Haslett et al. (2001) concluded, on the basis of

foraminiferal tidal range indicators, that marsh sedimentation rates probably

overestimate rates of sea level rise, and it is certainly true that the surface elevations of

mature marshes can vary within the tidal frame over time due to changes in tidal range,

suspended sediment concentration, and wind/wave activity. Most mature marsh surfaces

attain a 'quasi-equilibrium' elevation within the tidal frame which is only weakly linked



- 79 -

Basal indexpoints

Intercalatedindex points

Best estimate oflate Holocene trend

Modelled relativesea level curve

0 5000 10000 150001250075002500-30

-25

-20

-15

-10

-5

0

5

Figure 43 Sea level index points for the Bristol Channel, plotted as calibrated age against change in sea-level relative to the present (m), after Shennan and Horton (2002). The solid line indicates the best estimate of the late Holocene relative sea level trend. The dashed line indicates the predicted relative sea level from model 4 described by Shennan et al. (2002).

Rel

ativ

e se

a le

vel (

m)

Years before present



- 80 -

to mean sea level. In the Severn estuary and inner Bristol Channel, 'mature' marsh

surface levels vary from c. MHWS level to up to 1.5 m above the level of MHWS, and

on some marshes there is a gradient from the seaward edge towards the landward

margin. Sea level rise estimates based on marsh accretion rates therefore provide only a

broad approximation.

Tide gauge records for stations in southwest England are in most instances too short

and/or incomplete to allow reliable sea trends to be identified. The length of data run

and data completeness of digital records available from NTSLF for Newlyn, Milford

Haven, Mumbles, Ilfracombe, Hinkley Point, Newport, Avonmouth is shown in Figure

44. Apparent trends in mean sea level for these stations indicated by data available from

PMSL are shown in Figure 45. Only Newlyn has a sufficiently long and complete

record to allow long-term linear trend to be identified. Based on our analysis of the

NTSLF data, an average rate of rise for the period 1916-2008 of +/- 1.76 +/- 0.1 mm a-1

has been calculated. This compares with an average rate of rise in MSL of 1.70 +/- 0.1

mm a-1 for the period 1916-2006 at Newlyn reported by Woodworth et al. 2009).

The apparent linear trend in annual mean sea level for the period 1991-2006 calculated

for Hinkley Point is 5.16 +/- 1.37 mm a-1 (using NTSLF data). However, if the low

value for 1991 is excluded, the apparent average trend is reduced to 3.32 +/- 0.96 mm a-

1. The average linear trend at Avonmouth for the period 1987-1998 is + 5.1+/- 0.2 mm

a-1 (PSMSL data). All of these figures should be treated with great caution due to the

short periods of record available.

Kidson & Heyworth (1976) suggested that the Bristol Channel area is essentially

tectonically stable, with no net crustal movement during the later Holocene. Shennan

(1989) suggested an average rate of crustal subsidence in the area of c. 0.29 mm a-1,

Based on a re-analysis of compaction-adjusted data for sea level index points, Haslett et

al. (1998) suggested a lower subsidence rate of 0.06 mm a-1. More recent estimates

based on geophysical models have suggested that a subsidence rate of c. 0.72 mm a-1 is

more likely (Bradley et al., 2008; Lowe et al., 2009).



- 81 -

0

10

20

30

40

50

60

70

80

90

100

1953

1956

1959

1962

1965

1968

1971

1974

1977

1980

1983

1986

1989

1992

1995

1998

2001

2004

2007

Dat

a co

mpl

eten

ess

(%)

(e) Hinkley Point

0

10

20

30

40

50

60

70

80

90

100

1953

1956

1959

1962

1965

1968

1971

1974

1977

1980

1983

1986

1989

1992

1995

1998

2001

2004

2007

Dat

a co

mpl

eten

ess

(%)

(g)Avonmouth

0

10

20

30

40

50

60

70

80

90

100

1953

1956

1959

1962

1965

1968

1971

1974

1977

1980

1983

1986

1989

1992

1995

1998

2001

2004

2007

Dat

a co

mpl

eten

ess

(%)

(f) Newport

0

10

20

30

40

50

60

70

80

90

100

1953

1956

1959

1962

1965

1968

1971

1974

1977

1980

1983

1986

1989

1992

1995

1998

2001

2004

2007

Dat

a co

mpl

eten

ess

(%)

(c) Mumbles

0

10

20

30

40

50

60

70

80

90

100

1953

1956

1959

1962

1965

1968

1971

1974

1977

1980

1983

1986

1989

1992

1995

1998

2001

2004

2007

Dat

a co

mpl

eten

ess

(%)

(d) Ilfracombe

0

10

20

30

40

50

60

70

80

90

100

1953

1956

1959

1962

1965

1968

1971

1974

1977

1980

1983

1986

1989

1992

1995

1998

2001

2004

2007

Dat

a co

mpl

eten

ess

(%)

(b) Milford Haven

Figure 44 Data completeness for POL Class A tide gauges at Newlyn and in the Bristol Channel, including and excluding values marked as 'improbable' by BODC quality control. Original data source: National Tidal Sea Level Facility.

0

10

20

30

40

50

60

70

80

90

100

1915

1918

1921

1924

1927

1930

1933

1936

1939

1942

1945

1948

1951

1954

1957

1960

1963

1966

1969

1972

1975

1978

1981

1984

1987

1990

1993

1996

1999

2002

2005

2008

Dat

a co

mpl

eten

ess

(%)

Including 'improbable' data

Excluding 'improbable' data

(a) Newlyn



- 82 -

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

Newton Noyes

Hakin

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

Swansea

Mumbles

(a) Newlyn

(g) Avonmouth

(c) Swansea & Mumbles

(b) Milford Haven

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

(f) Newport

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

(d) Ilfracombe

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

1915 1930 1945 1960 1975 1990 2005

Annual m

ean s

ea level (m

OD

)

(e) Hinkley Point

1916-2008 trend+1.8 mm/yr

1984-2008 trend+1.1 mm/yr

1992-2006 trend+3.4 mm/yr

1987-1998 trend+5.1 mm/yr

1994-2005 trend+2.8 mm/yr

1961-2006 trend+1.2 mm/yr

1987-2008 trend+7.2 mm/yr

Annual mean sea level recorded at seven tide gauge stations in the Bristol Channel.Original data source: PSMSL.

Figure 45



- 83 -

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al m

ean

wat

er le

vel (

m O

D)

Observed NTSLF values

Observed PSMSL values

linear trend1.8 mm yr-1

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

2.05

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al m

ean

high

wat

er (m

OD

)

Observed values

Predicted values


-1.90

-1.85

-1.80

-1.75

-1.70

-1.65

-1.60

-1.55

-1.50

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al m

ean

low

wat

er (m

OD

)

Observed values

Predicted values


-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Diff

eren

ce b

etw

een

obse

rved

and

pr

edic

ted

high

wat

ers

(m)


-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Diff

eren

ce b

etw

een

obse

rved

and

pr

edic

ted

low

wat

ers

(m)


3.30

3.35

3.40

3.45

3.50

3.55

3.60

3.65

3.70

3.75

3.80

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al m

ean

tidal

rang

e (m

)

Observed values

Predicted values


2.60

2.70

2.80

2.90

3.00

3.10

3.20

3.30

3.40

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al m

axim

um w

ater

leve

l (m

OD

)


0.00

0.20

0.40

0.60

0.80

1.00

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annu

al m

axim

um s

urge

resi

dual

(m)


Figure 46 Trends in observed and predicted annual water levels at Newlyn (excluding BODC 'Improbable Values'). Data sources: PSMSL and NTSLF.



- 84 -

Figure 46 shows a comparison of predicted and observed annual water levels at Newlyn.

The figure shows that there has been a steady increase in observed annual mean water

levels and an apparent reduction in the difference between predicted and observed

levels; however, the predicted levels are hind-cast and do not take account of mean sea

level rise over the period. The apparent rate of increase in annual mean high waters

(average rate = 2.29 mm a-1) is slightly higher than the apparent rate of increase in mean

annual low waters (average rate = 1.23 mm a-1), resulting in an apparent increase in

mean annual tidal range, although the trends overlap at the 95% confidence level. The

annual maximum observed water level shows an upward trend of 1.5 mm a-1 over the

period and the annual maximum surge residual shows a small upward trend of 0.5 mm

a-1.

Comparable changes in annual water levels at Hinkley Point are shown in Figure 47, but

the reliable record is too short to identify significant trends which exclude the effect of

the lunar nodal tidal cycle. The digital data record for Avonmouth is slightly longer, but

with significant data gaps (Figure 48). However, by taking years 18 and 19 years apart,

an approximate indication can be obtained of changes in MSL, MHW and MHWS over

this period (Table 12). From these data it appears that MHW and MHWS levels appear

to have risen approximately twice as fast as MSL over the period at this location.

However, as noted above, the apparent trends should be treated with great caution due

to the short record.



- 85 -

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

1990 1995 2000 2005 2010

Annu

al m

ean

wat

er le

vel (

m O

D)



Predicted values

4.40

4.45

4.50

4.55

4.60

4.65

4.70

4.75

4.80

4.85

4.90

1990 1995 2000 2005 2010

Annu

al m

ean

high

wat

er (m

OD

) Observed values

Predicted values

-4.10

-4.05

-4.00

-3.95

-3.90

-3.85

-3.80

-3.75

-3.70

-3.65

-3.60

1990 1995 2000 2005 2010

Annu

al m

ean

low

wat

er (m

OD

)

Observed values

Predicted values

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1990 1995 2000 2005 2010

Diff

eren

ce b

etw

een

obse

rved

and

pr

edic

ted

high

wat

ers

(m)

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1990 1995 2000 2005 2010

Diff

eren

ce b

etw

een

obse

rved

and

pr

edic

ted

low

wat

ers

(m)

8.10

8.20

8.30

8.40

8.50

8.60

8.70

8.80

8.90

1990 1995 2000 2005 2010

Annu

al m

ean

tidal

rang

e (m

)

Observed values

Predicted values

5.00

5.50

6.00

6.50

7.00

7.50

8.00

1990 1995 2000 2005 2010

Annu

al m

axim

um w

ater

leve

l (m

OD

)

0.00

0.50

1.00

1.50

2.00

2.50

1990 1995 2000 2005 2010

Annu

al m

axim

um s

urge

resi

dual

(m)

Figure 47 Trends in observed and predicted annual water levels at Hinkley Point (excluding BODC 'Improbable Values'). Data sources: PSMSL and NTSLF.



- 86 -

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

1972 1977 1982 1987 1992 1997 2002 2007

Annu

al m

ean

wat

er le

vel (

m O

D)



Predicted values

5.05

5.10

5.15

5.20

5.25

5.30

5.35

5.40

5.45

5.50

5.55

5.60

1972 1977 1982 1987 1992 1997 2002 2007

Annu

al m

ean

high

wat

er (m

OD

) Observed values

Predicted values

-4.70

-4.60

-4.50

-4.40

-4.30

-4.20

-4.10

-4.00

1972 1977 1982 1987 1992 1997 2002 2007

Annu

al m

ean

low

wat

er (m

OD

)

Observed values

Predicted values

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

1972 1977 1982 1987 1992 1997 2002 2007

Diff

eren

ce b

etw

een

obse

rved

and

pr

edic

ted

high

wat

ers

(m)

-0.24

-0.22

-0.20

-0.18

-0.16

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

1972 1977 1982 1987 1992 1997 2002 2007

Diff

eren

ce b

etw

een

obse

rved

and

pr

edic

ted

low

wat

ers

(m)

9.00

9.20

9.40

9.60

9.80

10.00

10.20

1972 1977 1982 1987 1992 1997 2002 2007

Annu

al m

ean

tidal

rang

e (m

)

Observed values

Predicted values

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

1972 1977 1982 1987 1992 1997 2002 2007

Annu

al m

axim

um w

ater

leve

l (m

OD

)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

1972 1977 1982 1987 1992 1997 2002 2007

Annu

al m

axim

um s

urge

resi

dual

(m)

Figure 48 Trends in observed and predicted annual water levels at Avonmouth (excluding BODC 'Improbable Values', and years with <80% data completeness). Data sources: PSMSL and NTSLF.



- 87 -

Years used Number of Data completeness MSL rise MHW rise MHWS rise MHW rise as MHWS rise asin the years (1st year / 2nd year) (n = c . 35040 (n = c . 705 (n = c . 24 a proportion a proportioncomparison separation each year) each year) each year) of MSL rise of MSL rise

(mm a-1) (mm a-1) (mm a-1)1987 and 2006 19 90% / 96% 5.79 8.00 7.32 138% 126%1988 and 2007 19 88% / 100% 2.68 9.42 10.9 351% 407%1989 and 2008 19 89% / 100% 4.16 9.21 8.91 221% 214%1988 and 2006 18 88% / 96% 4.11 7.50 5.22 182% 127%1989 and 2007 18 89% / 100% 3.33 9.17 10.67 275% 320%1990 and 2008 18 99% / 100% 3.61 6.06 6.76 168% 187%

Averages: 3.95 8.23 8.30 223% 230%

Table 12 Estimates of increases in MSL, MHW and MHWS recorded at Avonmouth between 1987 and 2008 over one lunar nodal cycle (18.6 years). Estimates are made by comparing annual average MSL, MHW and MHWS levels for two years 18 or 19 years apart. Original data source: NTSLF.

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 88 -

3.0 Review of methods for predicting extreme events

3.1. The nature of extreme events, with particular reference to coastal

water levels

'Extreme events' are sometimes defined as those which have an average return period of

more than 50 years; they may or may not be catastrophic in terms of their impact. There

are many different types of extreme events which could affect a coastal or estuarine

location in the UK, including high tides, large wave events, high river flows, tsunamis,

earthquakes, windstorms, high rainfall events, droughts, and periods of extreme high or

low temperature. For the purposes of this report, attention is focused on astronomical

tides, meteorological surges and waves which are the key factors of relevance to the

determination of flood risk, and the assessment of potential rapid coastal morphological

change, in open coast environments such as at Hinkley Point.

Coastal flood risk analysis essentially involves estimation of the frequency that some

specified level of the sea, z, will be exceeded in some specified time period. The

frequency, magnitude and duration of exceedance are all aspects of relevance to

engineering design and event impact assessment.

If relevant observational data exist, the first stage in any such analysis is to determine

the frequency distribution and cumulative frequency distribution of water level values

over the period of record. If the probability of a given level z being exceeded in one year

is Qz, that level can be said to have a return period which is the inverse of Q(z) in years.

Thus, if the probability of exceedance is 0.01 (or 1%), the equivalent return period is 1

in 100 years. Conversely, the level which has a probability of being exceeded once in a

hundred years is termed the 100-year return level (RL). The inversion of annual

exceedance probabilities to give return periods makes the assumption that the same

statistics are applicable for the whole period specified. In the case of very small

probabilities this may represent time periods of many hundreds or thousands of years,

and the assumption is very difficult to justify (Pugh, 2004, p181). In situations where no



- 89 -

or very few observational data exist, this type of analysis cannot be performed at all,

and alternative methods of assessment need to be employed (see below).

The value of Q(z) specified for coastal engineering and planning purposes varies with

the nature of the assets at risk and the level of flood risk which is considered acceptable.

For many flood and coastal erosion risk management schemes in the UK, water levels

with return periods ranging from 1 in 100 to 1 in 1000 years are used as the basis for

design. In the Netherlands, a level of protection of 1 in 10,000 years) was specified for

the developed coast following the destructive storm surge of 1953 (Deltacommissie,

1960; de Ronde, 1996), while in the case of high value / high risk assets such as nuclear

power stations even higher levels of 1 in 10,000 to 1 in a million years may be

specified.

For engineering design purposes, the risk may also be stated as the probability that a

particular extreme water level will be exceeded at least once during the specified

lifetime of the structure. This is often termed the design risk or encounter probability. In

year 1 the probability that the specified level (z) will not be exceeded is (1 - Q(z)). The

probability that the level z will not be exceeded in either of the first two years is given

by (1 - Q(z)) × (1 - Q(z)) = (1 - Q(z)2), and the overall design risk is given by 1 - (1 -

Q(z)TL, where TL is the design life of the structure.

Several factors may contribute to the actual water level encountered adjacent to a

coastal structure or development. These include:

astronomical tides

meteorological surges

tide, surge and/or wave-driven currents

wave set-up

individual waves

seiches



- 90 -

During extreme events, several of these factors may combine to give the resultant water

level, which may oscillate over time periods ranging from a few seconds to a few hours.

In small estuaries and tidal rivers, fluvial discharge may also be a significant

contributing factor. Short-term extreme water levels may also be generated by short-

term 'unexpected' events, such as earthquakes, landslides, sea floor gas escapes or

meteorite impacts. Such events often generate long-period waves, collectively referred

to as tsunami, which have very different characteristics to extreme still water levels and

short-period wind waves. Partly for this reason, and partly because of their very

infrequent, essentially random occurrence, these latter types of event are normally

considered separately from the interaction of regular tides, meteorological surges and

wind-generated waves.

3.2 The annual maximum method

Prior to the 1980's the most frequently used method of estimating extreme water levels

involved ranking the maximum level (annual maxima) observed in each of a number of

years (widely referred to as the Annual Maximum Method, AMM). The origins of this

method date back to the work of E.J. Gumbel and J.F. Jenkinson between the 1930's and

1950's, summarised by Gumbel (1958) and Jenkinson (1955). In these studies the

assumption was made that distribution function for the annual maximum is the extreme

value distribution. This assumption is not always correct, although in many instances it

provides a close approximation.

Lennon (1963a) and Suthons (1963) applied these ideas to estimate return levels for sea

levels around the coast of the UK. Lennon (1963a) examined annual sea level maxima

at a number of British west coast ports and considered the fit of a number of alternative

statistical frequency distributions, including that of Jenkinson (1955) to the data.

Suthons (1963) carried out a similar exercise for stations in southeast England but only

used Jenkinson's (1955) method since its general form was considered to provide an

adequate representation of each of the three types of frequency distribution found in the

extreme water level data for different ports. These distributions were found to conform

essentially to the Type 1, 2 and 3 distributions of Fisher & Tippett (1928). The Type 1



- 91 -

distribution represents the special Gumbel distribution, Type 2 is often referred to as

Frechet's distribution and Type 3 represents the Weibull distribution. All three

distributions form part of what is now widely referred to as the General Extreme Value

distribution (GEV) family (Coles, 2001, 47-48).

Blackman & Graff (1978) adopted Suthons' (1963) approach in a detailed analysis of

extreme levels at 10 south coast ports. Further work by Blackman & Graff (1979)

suggested that the frequency distributions of the annual maxima at a number of these

ports showed evidence of secular variation which showed dependence on the period

analysed. Graff (1981) also applied this approach as part of a study of variations in

annual maxima at 80 UK ports. The results showed that the data for different ports tend

towards different types of GEV family distribution. The frequency distribution curves

for ports in East Anglia were found to be of Fisher-Tippett Type 2, a few ports showed

the exact Gumbel (Type 1) fit, and most were of Type 3. Ports in estuaries, located only

a few kilometres apart, were in some instances found to have significantly different

distributions due to localised influences of coastal morphology and bathymetry on the

tidal regimes and/or surge behaviour. In the Bristol Channel, the frequency distribution

curves for Swansea, Cardiff, Newport and Avonmouth were all found to approximate a

Type 3 distribution, increasing in Type 3 tendency up-estuary.

Graff (1981) used the fitted and extrapolated frequency distribution curves to predict the

water levels associated with different return periods (20, 50, 100 and 250 years).

However, the maximum levels predicted by Graff for these stations were exceeded

during an event on 13 December 1981, when a depression moving along a track up the

Bristol Channel towards central England caused a surge of unexpected magnitude

(Proctor & Flather, 1989). A re-analysis by Blackman (1985), using additional data

(including the 1981 event) to supplement those by Graff (1981) for these stations,

produced considerably higher estimated return period levels; e.g., the estimated 100

year return level at Newport increased from 7.93 m to 8.40 m, while that for

Avonmouth increased and from 8.43 to 8.79 m, respectively). Blackman (1985)

concluded that the method is extremely sensitive to outliers and data quality, and the

curves should not be extrapolated to more than four times the data length. The observed

extreme value of 8.75 O.D. recorded at Avonmouth on 13 December 1981 was



- 92 -

estimated from the revised frequency distribution curve to have a return period of 75

years. This value was exceeded only once in the data record for Avonmouth; 8.83 m OD

in 1883. The 1981 event was the highest value ever recorded at Swansea, Cardiff and

Newport, and corresponded with estimated return periods of 1 in 40 years, 1 in 1500

years and 1 in 100 years, respectively. The return period for Cardiff was considered by

Blackman to be too long, reflecting the uncertainty / potential errors in extrapolating

from a small sample of data.

As noted by Pugh (2004 p185) at least 25 annual maxima values are needed for a

satisfactory analysis, and as a general rule extrapolations should be limited to return

periods not longer than four times the time period of available data.

3.3 r-largest, GEV and Peaks over Threshold methods

One of the weaknesses of prediction methods based on annual maximum values is that

they are wasteful of data (Pugh, 1987). This can be partly overcome by using the

highest several events in each year rather than only the annual maximum. The number

(r) of events for each year normally ranges from 3 to 10 (e.g. Smith, 1986). This method

is often referred to as the r largest order statistics approach, or simply 'r-largest'.

However, with some data sets a problem may arise in taking the same number of values

from each year of record because some years may have few or no high values, while

others with many high values may have some which are omitted from consideration.

In the past 30 years, preferred alternative approaches have been to fit a GEV

distribution model to the entire data set, or to fit a Generalised Pareto Distribution

(GPD) to the higher values above a prescribed threshold value. This latter method is

often referred to as the peaks over threshold (POT) method (e.g. Reeve et al., 2004).

As noted above, the GEV family of distributions includes the three limiting distributions

described by Fisher & Tippett (1928). The only requirement for application of GEV

distribution analysis is that the events are well separated in time and can be considered



- 93 -

as essentially independent. The GEV distribution (and also the GPD) is defined by three

parameters:

κ = shape parameter

σ - scale parameter (>0)

µ - location parameter

The most appropriate values for the distribution parameters are normally selected using

the maximum likelihood (ML) method, or less frequently the method of moments

(MM). These procedures also allow estimation of the standard errors associated with the

data and fitting procedures. The standard errors can be used to calculate confidence

intervals assuming a normal distribution for the estimated parameter values (Efron &

Hinkley, 1978). Confidence intervals can also be calculated using Bootstrap methods in

conjunction with the ML method (Efron, 1982). Confidence limits provide a useful aid

in assessing the likely reliability of extrapolated return values. However, all

extrapolations are made on the assumption that the 'best fit' distribution function

remains valid over the range of extrapolation. This assumption is often difficult to

justify and may be rendered obviously invalid where two or more event populations are

involved.

3.4 Joint probability methods

Joint probability analysis aims to predict the occurrence of events in which two or more

partially dependent variables coincide to produce extreme high or low values. Joint

probability of several different pairs of variables may be undertaken in relation to flood

risk, including:

astronomical tide and surge

wave height and combined water level (tide, or tide + surge)

wave height and surge

river flow and surge

river flow and combined water level (tide + surge)

precipitation and surge



- 94 -

wind waves and swell

More rarely, multivariate joint probability analysis, involving three or more variables

(e.g. combined tide + surge + waves + river flow) is undertaken (e.g. Hawkes, 2003,

2005).

Pugh & Vassie (1979, 1980) developed the first joint probability method (JPM) which

made use of full water level records (generally hourly values). Their method involved

separation of the astronomical tide and surge (non-tidal residual) components from tide

gauge records followed by consideration of the joint probability of the two components.

Using this method, combined probabilities of exceedance for various specified water

levels values can be estimated for a specified time period, T (e.g. 1 year or several

years). This joint probability method (JPM) can be applied to stations with short data

records (as short as one year, but preferably at least four so that significant storms are

included; Pugh, 2004, p187).

Between 1991 and 1997 MAFF funded a programme of research to develop joint

probability methods and to determine the probability of extreme sea-levels at every

point around the UK coastline. Much of the work was undertaken by Lancaster

University in collaboration with POL, building on earlier statistical modelling work by

Tawn & Vassie (1989), Coles & Tawn (1990) and Tawn (1992). The results of the

MAFF-funded work are described in Dixon and Tawn (1992, 1994, 1995, 1997); an

overview is given by Dixon (1997).

In order to overcome some of the limitations associated with the JPM described by Pugh

& Vassie (1979, 1980), Dixon & Tawn (1994) developed a Revised Joint Probabilities

Method (RJPM). The assumptions of the revised method are that:

the limiting generalised extreme value (GEV) distribution provides a good

approximation to the distribution of the annual maximum surge

limiting extreme value theory (EVT) results hold for temporal dependence



- 95 -

the interactions between tide and surge can be specified (treated either as

independent or partially dependent)

Dixon & Tawn (1994) concluded that, for short data sets (1-8 years in length) JPM is

the only viable option but is likely to underestimate return levels corresponding to long

return periods, being highly dependent on the observation period. Where long data

records are available, RJPM offers advantages and should be preferred.

Dixon & Tawn (1995) refined the RJPM method to make use of all types of available

data from the site (not just hourly values), spatially interpretable parameters, and

temporal trends consistent with those observed in mean sea levels. The two latter

aspects incorporated inputs from the POL/ Met Office models then in use of tides and

combined tides / surges on the Northwest European shelf (Flather, 1987). At the time

used by Dixon & Tawn (1994,1997), these models provided 39 years of synthetic tide

and surge levels for the period 1955 - 93 and for 2964 locations on a regular 36 x 36 km

grid (see also Flather et al., 1998). The tide levels were calculated on the basis of

physical laws (using a 12 km grid resolution) and the surge magnitudes predicted using

measured and modelled wind data. A sub-set of the data for 89 grid locations around the

UK coast was used by Dixon & Tawn (1995, 1997) and compared with measured data

for specific locations. Good general agreement was observed over relatively short time

periods, but the differences were large enough to conclude that the low resolution

tide/surge model alone cannot be used to obtain accurate return level estimates for all

coastal locations (Dixon, 1997). This further revised method, termed the Spatial Revised

Joint Probability Method (SRJPM), was initially applied by Dixon & Tawn (1995) to

the east coast; Dixon & Tawn (1997) subsequently applied it to stations around the rest

of UK. It was concluded that SRJPM using the 36 km grid model provided adequate

estimates of tidal characteristics and return levels on simple lengths of open coast, but

was inadequate on sections of morphologically complex coast and within estuaries.

Finer resolution models were therefore applied to the Bristol Channel and Severn

estuary (4 km and 3km grids, respectively). The models were run for the period 1981-

1985 and the output compared with the results of the annual maximum method (AMM).

The 100 year return period levels at Avonmouth, Newport, Cardiff and Swansea were

found to show good agreement. Since this work was carried out, the Northwest

European Shelf Local Area Model (LAM) and the Bristol Channel / Severn Estuary



- 96 -

Model have been further improved, and there are proposals to increase the grid

resolution in the latter area to 1 km in order to provide even better predictions.

Important aspects for successful application of JPM and RJPM methods are the accurate

estimation of the tail of the surge probability distributions, and accurate specification of

the degree or tide-surge interaction. As noted above, the selection of the most

appropriate distribution type within the GEV family can be assisted using the maximum

likelihood method or the method of moments. Specification of the degree of surge-tide

interaction varies from site to site and depends heavily on local bathymetry. This

requires examination of local observational records and may be problematical in areas

with only a short, or no, period of water level and/or bathymetric record. Comparisons

between short-term observed levels and values predicted using even fine-grid numerical

models suggest that significant discrepancies can arise due to inaccuracies in tidal

predictions and predicted surge behaviour in areas of complex morphology and

bathymetry. As a broad generalisation, the degree of dependence between surge

magnitude and tidal height increases in shallow water open coast areas and landwards

within estuaries. Where adequate data are available, the degree of dependence /

independence between tide and surge (or indeed other variables) can be estimated using

a correlation factor (CF) which is defined as the ratio of the actual frequency of a

particular joint exceedance event to its probability of occurrence if the two variables

were independent. However, values of CF depend both on return period and the number

of records per year (Hawkes & Svensson, 2003). Where available records are of short

duration, correlation analysis of surge-tide interaction offers a method of refining

predictive estimates made using the JPM (Pirazzoli & Tomasin, 2007; Tomasin &

Pirazzoli, 2008).

HR Wallingford (1994) described a JOINPROB analysis method for combined water

levels and waves. This was developed during the mid-1990's to produce the JOIN-SEA

methodology and software described by Owen et al. (1997), HR Wallingford &

Lancaster University (2000), and Hawkes et al. (2002). The JOIN-SEA methodology

was later extended to include extreme water levels in rivers and estuaries, taking into

account river flow, sea level and waves (Hawkes, 2003; Hawkes & Svensson, 2003),

but has not been widely used by organizations other than HR Wallingford.



- 97 -

On theoretical grounds, wave characteristics in shallow water, near-coastal areas may be

expected to be partially dependent on both tidal elevation and combined tide plus surge

water levels, since wave height increases with water depth. Wave heights are also likely

to show a degree of positive correlation with surge magnitude since winds play an

important role in the generation of both. The degree of dependence can be quantified by

correlation analysis similar to that described above for tides and surges. For a

correlation coefficient value of <0.11 waves and sea level can be considered

independent, whereas a value of > 0.70 can be considered to indicate 'super-correlation'.

At sites in the mid and inner Bristol Channel, values of the correlation coefficient

between sea level and waves from all directions range from 0.11 at Ilfracombe to 0.17 at

Avonmouth (classified as 'modestly correlated). Correlation coefficient values between

surge and wave height (all directions combined) was found to be 0.74 at Ilfracombe and

0.76 at Avonmouth (Hawkes & Svensson, 2003).

As noted by Pugh (2004, p188) and van den Brink et al. (2005), the reliability of all

extreme value methods is limited by the available data and by possible trends and

changes in the regional meteorology, oceanography and coastal morphology. If data

records are short, they are unlikely to capture larger, less frequent events. Measured

wave data, in particular, are often available only for short periods, frequently one year

or less. Modelled wave data may be available for a longer period based on wind

measurements, but frequently the estimation points are located well offshore and there

are considerable uncertainties and errors involved in translating modelled offshore wave

data to inshore areas. In some areas of the world, such as the east coast of the United

States, storms represent two distinct populations, one generated by 'normal'

cyclogenesis processes associated with regular seasonal variations in pressure gradients

and position of the jet stream, and another representing the essentially random

movement of external generated hurricanes. The latter type of storm is much less

frequent and difficult to predict in terms of location, severity and duration than the

typical mid-latitude westerly type of storm.

Events which have a very rare occurrence, such as tsunami, or which have an irregular

and essentially unpredictable spatial distribution, such as hurricanes, cannot be

adequately evaluated using joint probability methods since the necessary historical data



- 98 -

sets do not exist. Such events effectively can be considered to belong to a second, un-

sampled population which could result in higher return values than estimated from

standard extreme value analysis of available short-term records. As noted by van den

Brink et al. (2003, 2005) second populations are not detectable from records of less than

100 years in length, and extrapolations from 100 year records to 104 year return levels

are only likely to be valid if the extreme value distribution is single populated. For this

reason, the estimated 104 year values from 100 year records should always be

interpreted as a lower limit.

There have been significant cases where design basis flood protection levels based on

statistical modelling of extremes have been exceeded by actual events. A relevant

example was the flooding of EDF's Le Blayais nuclear power plant on the Gironde

estuary, southwest France, on the 27th December 1999 (Gorbatchev et al., 2000; IAEA,

2003). A combination of a high (but not exceptional) tide, 2.01 m storm surge and

relatively large waves (estimated Hs of 2 m) led to overtopping of defences and flooding

of the site, resulting in a loss of auxilliary power supplies and shutdown of three

operating units. The dykes surrounding the plant had been built using a Design Basis

Flood (DBF) level of 5.02 m, which was calculated as the water level resulting from the

maximum astronomical tide and the 1000 year storm surge. The maximum crest level of

the dyke was 5.2 m above NGF (French national datum), giving a DBF freeboard

allowance of 0.18 m. In addition to significant over-topping, some of the rock armour

protecting the dyke was washed away by wave action, although a full breach did not

develop. Flooding was most severe at the northwest corner of the site, where the water

depth reached approximately 0.3 m. The volume of water which entered the facilities

was estimated at c. 90,000 m3 (Gorbatchev et al., 2000). The occurrence of such events

underlines the importance of an adequate assessment of all circumstances which could

contribute to the risk of flooding, and the need to avoid undue reliance on the results of

extreme value statistical modelling.

3.5 Alternative / complementary methods

The evaluation of risk associated with such 'rare' events can only be assessed using

methods other than statistical modelling based on observational data. Several such



- 99 -

methods have been employed, including the use of synthetic extended time series (e.g.

van de Brink et al., 2005), Monte-Carlo simulation methods (Hawkes et al., 2002;

PhysE Ltd., 2009), Bayesian statistical approaches (e.g. van Gelder, 1996; Coles &

Tawn, 2005), evaluation of current geological and meteorological processes, historical

information about such processes provided by sedimentological and geomorphological

evidence and archival / documentary sources, mathematical and/or physical modelling

to quantify driver - response sensitivity, and 'expert assessment'. A combination of such

approaches is now widely used in 'catastrophe loss modelling' by the insurance industry

(e.g. Muir-Wood et al., 2005; Sanders, 2006).

The potentially more significant medium to long term changes which are likely to

produce long-term trends or cyclic changes which affect the frequency and magnitude

of extreme events include:

changes in mean sea level

changes in tidal range

changes in average weather patterns, which affect wind and wave

directions

changes in storminess which affect the frequency and magnitude

of storm surges, large waves, and related currents

changes in bathymetry and coastal morphology, which affect tide

and surge amplification, wave direction and wave height

Information about the likely nature and magnitude of future changes is these factors can

be obtained in several complementary ways, notably:

analysis of past changes recorded by sedimentary deposits and

geomorphological features

analysis of relatively long-term instrumental data sets (e.g. tide

gauge records, pressure records)

analysis of historical maps, charts and other survey data


Hinkley Point Extremes________________________________________________________________________________________________________________

- 100 -

computer modelling to assess system sensitivity to change (e.g.

global circulation models, regional wind-wave and tide-surge

models)

Information from these methods (and others) is usually combined, compared, and

evaluated in a process which is often referred to as "expert geomorphological analysis"

(Pye & van der Wal, 2000), or more generally as "expert analysis". The outputs of the

process may be expressed simply in qualitative terms as "expert opinion", or in semi-

quantitative terms as a rank score (e.g. perceived likelihood on a scale of 1 to 10). The

assessments can be performed by individual "experts", or by a group of several experts

for whom a measure of collective opinion may be expressed. In the evaluation process,

the likelihood of potential outcomes may be graded or expressed simply in terms such

as: extremely unlikely, highly unlikely, unlikely, equally unlikely / likely; likely; highly

likely; extremely likely. Descriptive assessments of this type have been extensively used

in reports by the Intergovernmental Panel on Climate Change (e.g. Solomon et al.,

2007).

________________________________________________________________________________________________________________Kenneth Pye Associates Ltd. Report EX1207


- 101 -

4.0 Risk of coastal flooding and morphological change associated

with extreme events in the Hinkley Point area

4.1 Previous studies

Several assessments of high and low water return levels (RLs) at Hinkley Point have

been made, using extreme value statistical modelling approaches (Table 13). As part of

the SMP I process, Halcrow (1998) provided estimates of extreme high water levels

under 'current' conditions with return periods ranging from 2 to 200 years (6.78 and 7.38

m OD, respectively). These values were adopted by Halcrow (2007a,b) in an assessment

of geological hazards relevant to proposed NNB sites in the UK. Jacobs Babtie (2006)

provided estimates of extreme high (still) water levels with return periods ranging from

2 to 10,000 years (7.24 and 7.65 m OD, respectively). The methodologies used to

produce these estimates were not described. HR Wallingford (2009) presented an

assessment of extreme water levels, extreme wave height, and the joint probability of

these parameters for return periods between 10 and 100,000 years, for the 2080's. The

results were based on analysis of observed tidal records for the Hinkley Point,

Avonmouth and Newport, combined with modelled wave data and the future sea level

rise allowances recommended by DEFRA (2006) which are based on the Third IPCC

Scientific Assessment (Houghton et al., 2001) and later Holocene isostatic rebound /

subsidence rates suggested by Shennan et al. (2002) and Shennan & Horton (2002).

Using these figures, the total sea level rise over the period 2008 to 2080 was assumed in

the HR Wallingford report to be 0.587 m, although a lower figure of 0.415 m was

recognised to be a more robust estimate based on the projections published in the Fourth

IPCC report (Solomon et al., 2007). High water extremes were calculated using the

Revised Joint Probabilities Method (RJPM) of Dixon & Tawn (1997). Low water

extremes were determined by fitting a Generalised Pareto Distribution (GPD) to the

observed low water levels. Results were presented with 70% confidence bands

To standardise records to the same date, HR de-trended the tidal data to the start of 2008

based on an assumed constant eustatic sea level rise over the period 1961 to 2008 of 1.8

mm a-1 (Solomon et al., 2007) and an isostatic rebound rate for the area of



- 102 -

Return

Period

(years) Jacobs Halcrow Lower Best Upper Lower Best Upper

Babtie (2006) (1998) Band Estimate Band Band Estimate Band

2 7.24 6.78 nd nd nd nd nd nd

5 nd 7.00 nd nd nd nd nd nd

10 7.37 7.11 7.34 7.38 7.42 7.93 7.97 8.01

20 nd 7.19 nd nd nd nd nd nd

50 7.48 7.29 7.55 7.61 7.67 8.14 8.20 8.26

100 7.51 7.33 7.70 7.76 7.83 8.28 8.35 8.42

200 7.54 7.38 nd nd nd nd nd nd

500 7.57 nd 7.89 7.98 8.08 8.48 8.57 8.66

1000 7.59 nd 7.98 8.09 8.20 8.57 8.68 8.79

5000 7.63 nd nd nd nd nd nd nd

10000 7.65 nd 8.27 8.44 8.62 8.85 9.03 9.21

100000 nd nd nd nd nd nd 9.39 nd

HR Wallingford (2009)HR Wallingford (2009)

Predictions for Predictions for 2008 Predictions for 2080s

date of study

Table 13 Return periods for extreme high water levels (in m OD) predicted at Hinkley Point from previous studies. HR Wallingford (2009) predictions for 2008 have been back-calculated, adjusting for a sea level rise of 0.587 m (assumed by HR Wallingford in their predictions for 2080s).



- 103 -

approximately -0.8 mm a-1 (Shennan et al., (2002). This was considered more robust

than de-trending the data series based on recorded values due to the relatively short run

of data available. High water values were determined by fitting a quadratic through the

three highest 15 minute levels recorded around high water. Where high water level data

for Hinkley Point were not available, values for Avonmouth and/or Newport were used.

This provided a total composite data run of 26 years which was used in the analysis.

The analysis provided estimates for the 1, 100 and 10,000 year return levels (relative to

2008) of 7.10 m, 7.76 m OD and 8.44 m OD, respectively (Table 13).

Taking into account assumed sea level rise of 0.587 m by 2080, best estimate levels for

the 1 in 100 year and 1 in 10,000 year events were calculated to be 8.35 and 9.03 m OD,

respectively. At the request of EDF, HR Wallingford (2009) extended the analysis to

include the 100,000 year event, while noting that existing statistical methods for events

of this magnitude may not be applicable. The best estimate extreme high water level for

the 100,000 year event was calculated to be 9.39 m OD.

HR Wallingford also used predicted wave heights generated by the Met Office

European Wave Model at prediction point 51.25oN 4.87oW (northwest of Lundy Island,

Figure 17) to drive a SWAN inshore wave model and obtain predicted inshore wave

heights close to Hinkley Point. Time series of modelled wave data for a period of 19.1

years were obtained for analysis in conjunction with the high water level records.

Selected events were chosen to correspond with conditions which would yield the

largest wave heights at the site of interest, and a restricted number of these were used in

joint wave - water level probability analysis. Wave heights were assumed not to change

with lower water levels and wave steepness was assumed to be constant for large wave

heights. Extreme estimates of wave heights at Hinkley Point were determined by fitting

a GPD to the modelled inshore wave data at six near-shore locations along the - 7 m OD

seabed contour. The parameters of the GPD were determined by the Maximum

Likelihood method. The results indicated that extreme wave heights at the site were

from an approximate 10º direction range of 290º to 300º (c. WNW). The wave steepness

associated with extreme wave activity was noted to be approximately constant, equal to

0.045. Best estimate extreme value estimates of nearshore Hs were determined to be



- 104 -

4.17 m for a 10 year event, 5.03 m for a 100 year event, and 6.48 m for a 10,000 year

event (Table 14). Equivalent best estimate values for mean wave period (Tm) were

calculated to be 7.71 s, 8.47 s and 9.61 s, respectively. As shown in Table 14, the best

estimate significant wave heights predicted by HR Wallingford are considerably higher

than those predicted by Jacobs Babtie (2006) for 'present' conditions, although the

methodology used in the latter study is not stated.

The joint probability of extreme water levels and extreme waves was considered by HR

Wallingford (2009) using the method of Hawkes et al. (2002). Analysis of the

significant wave height and water level records above set thresholds indicated that both

followed a single bivariate distribution. Fitting the time series to this distribution and

simulating 1,000,000 years of records allowed joint return periods to be estimated,

taking into account rises in sea level to 2080. The results indicated that for 2080 a water

level of 7.5 m OD associated with Hs of 6 m would represent a 1 in 10,000 year event.

No attempt was made in the HR study to compare the modelled inshore wave data with

observational data, and the reliability of the modelled estimates is uncertain. As pointed

out in the HR report, for most UK coastal defence studies long-term measured data

series are analysed in order to estimate extreme conditions with return periods only up

to about 200 years, and short-term measured data sets (<40 years) may be inadequate to

provide a realistic estimate of the likely return period of very rare events.

4.2 New analysis of tide, surge and wave data

As part of the present study, the frequency and magnitude of observed water levels and

waves at Hinkley Point have been examined. Analysis has also been performed on tidal

records for Avonmouth, Newport, Ilfracombe, Newlyn and Milford Haven, and on wave

records for Minehead and Scarweather.

The water level data for Hinkley Point are limited to c. 19 years of data, which is less

than the minimum of 25 years required for annual maximum analysis recommended by

Pugh (2004, p185). However, for the purposes of illustration, the available 18 annual

maximum values (shown in Table 10 and Figure 49a) have been compared with three



- 105 -

Return Period Jacobs This study(years) Lower Best Upper Babtie Generalised Pareto

Band Estimate Band (2006) Distribution (n=4611)Kolmogorov-Smirnov Test - reject? YesAnderson-Darling Test - reject? YesChi-Squared Test - reject? Yes1 nd nd nd nd 3.142 nd nd nd 2.70 3.355 nd nd nd nd 3.6110 3.73 4.17 4.70 3.21 3.8020 nd nd nd nd 3.9950 4.15 4.78 5.58 3.74 4.25100 4.31 5.03 5.96 3.98 4.44200 nd nd nd 4.22 4.62500 4.65 5.57 6.84 4.55 4.871000 4.78 5.80 7.21 4.80 5.065000 nd nd nd 5.41 5.4810000 5.15 6.48 8.46 5.68 5.66100000 nd 7.23 nd nd 6.26

HR Wallingford (2009)

Table 14 Extreme significant wave heights (Hs, in metres) predicted for Hinkley Point under 'present conditions' by HR Wallingford (2007) and Jacobs Babtie (2006). For comparison, return periods have been estimated in this study using wave data recorded at the Hinkley Point wave Buoy (16/12/08 to 18/11/10), with a GPD fitted to the recorded waves above 0.8 m significant wave height. Although return periods are quoted, the fitted distribution does not pass any of the statistical tests for goodness of fit. Original data source: CEFAS Wavenet.



- 106 -

types of distribution (GEV, Weibull and Gumbel) and tested for goodness of fit using

ExcelSTAT. The Gumbel and Weibull distributions passed using all three tests

(Anderson-Darling, Kolmogorov-Smirnov and Chi-Square), but the GEV distribution

failed on the basis of the Anderson Darling and the Chi-Square tests. The extrapolations

produced by the three models are very different: e.g., the 1 in 10,000 water level ranges

between 7.35 m OD using the GEV distribution to 8.47 m OD using the Gumbel

distribution (Table 15).

As previously noted, the Annual Maximum method is wasteful of data, and potentially

better extrapolations can be made using the full observational data set, or that part of the

data set above a prescribed threshold. The distribution of observed water level values

above 6.0 m OD has been plotted in Figure 49b and a Generalised Pareto Distribution

fitted using the Maximum Likelihood Method. This provides an estimate for the 10,000

event level of 7.27 m OD, a value which is only 8 cm higher than the 10 year RL and

only 21 cm higher than the 1 year event (Table 15).

For comparison, estimates based on the Joint Probabilities Method described by Pugh

(2004), which is similar to that described by Pugh & Vassie (1989, 1990), are also

shown in Table 15. Regardless of the definition of 'surge' which is used, the calculated

joint probabilities of tide plus surge are considerably higher than those indicated by all

the other methods except the Gumbel distribution (10,000 year RL values of 8.46 to

8.86 m OD). However, this method assumes that the surge and astronomical tidal

elevations are entirely independent, which, as discussed above, is not the case in the

Bristol Channel where significant non-linear tide - surge interaction reduces the

likelihood that a very large surge will coincide with a very high astronomical tide. The

JPM predicted values are therefore likely to be over-estimates.

The sensitivity of the predictions made using different models to the addition of a single

additional 'extreme' water level was investigated by including a value 8.50 m, which is

approximately equivalent to the level reached by the 1607 Bristol Channel flood,

indexed for sea level rise over the interim period (Horsburgh & Horritt, 2006). The

revised predicted return levels are shown in Table 16. The results show that this single

value results in a large increase in 1 in 10,000 year event level in the case of the GEV,



- 107 -

0

5

10

15

20

25

6.53 6.63 6.73 6.83 6.93 7.03 7.13 7.23 7.33

Freq

uenc

y (%

)


Observed data

Generalised Extreme Value

Weibull Distribution

Gumbel Distribution

(a)

0

5

10

15

20

25

6.53 6.63 6.73 6.83 6.93 7.03 7.13 7.23 7.33

Freq

uenc

y (%

)


Observed data

Generalised Pareto Distribution

Figure 49 Histogram showing frequency of observed water levels recorded at Hinkley Point (1991-2008) (a) annual maximum values (n=18) with fitted Generalised Extreme Values distributions, and (b) observed water levels above 6.0 m (n=1414) with fitted Generalised Pareto Distribution calculated using the maximum likelihood method. Original data source: NTSLF.

(b)



- 108 -

Return Period Elevations from 'Peaks Over Threshold'

(years) observed data Method, calculated from

1991-2008 Generalised Pareto

Generalised Weibull Gumbel Distribution Predicted high Predicted high Predicted high

Extreme (three (all tides >6.00 m OD) waters + surge waters + high water waters + surge

Value parameter) (1991-2008, n=1414) residuals occurring skew surges residuals at time of

at any time (SKHW) observed high water

(n = 624817) (n = 12591) (n = 12591)

No No No No n/a n/a n/a

Yes No No Yes n/a n/a n/a

nd No No nd n/a n/a n/a

1 7.07 nd nd nd 7.06 7.11 7.09 7.09

2 7.11 7.05 7.04 6.97 7.12 7.21 7.17 7.18

5 7.22 7.21 7.19 7.17 7.17 7.35 7.29 7.29

10 7.27 7.26 7.25 7.29 7.19 7.46 7.38 7.39

20 nd 7.29 7.30 7.42 7.21 7.59 7.48 7.50

50 nd 7.32 7.35 7.57 7.23 7.77 7.65 7.67

100 nd 7.33 7.38 7.69 7.24 7.93 7.79 7.80

200 nd 7.34 7.41 7.81 7.25 8.09 7.93 7.94

500 nd 7.34 7.44 7.96 7.26 8.31 8.10 8.11

1000 nd 7.35 7.46 8.08 7.26 8.46 8.21 8.21

5000 nd 7.35 7.50 8.35 7.27 8.76 8.39 8.39

10000 nd 7.35 7.51 8.47 7.27 8.86 8.46 8.46

100000 nd 7.35 7.56 8.86 7.27 9.10 8.58 8.58

calculated from annual maxima calculated from all tides in the period

Kolmogorov-Smirnov Test - reject?

Anderson-Darling Test - reject?

Chi-Squared Test - reject?

Extreme Value Theory (EVT) Method Joint Probabilities Method (Pugh, 2004),

1991-2008 (n=18) 1991-2008

Return periods of extremely high water levels (in m OD) at Hinkley Point, calculated using Extreme Value Theory (EVT) and Joint Probability Analysis (JPA). Generalised Extreme Value (GEV) distributions (using least squares, Weibull and Gumbel assumptions) and Generalised Pareto Distributions (GPD) have been fitted to annual maxima (n=18) and all high tides above a threshold of 6.00m OD (n=1414). Joint Probability Analysis calculates the occurrence probabilities of high water levels caused by the coincidence of predicted high tides and surge residuals, skew surges or surges at time of observed high water, using data recorded at Hinkley Point Tide Gauge in the period 1990-2008 (without EVT extrapolations). The goodness of fit is assessed by three statistical tests; a 'yes' indicates that the null hypothesis is rejected, and the data statistically do not fit a GEV or GPD.

Table 15 _________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 109 -




Generalised Weibull Gumbel Distribution Predicted high Predicted high Predicted high

Extreme (three (all tides >6.00 m OD) waters + surge waters + high water waters + surge

Value parameter) (1991-2008, n=1415) residuals occurring skew surges residuals at time of

at any time (SKHW) observed high water

(n = 624818) (n = 12592) (n = 12592)

No No No No n/a n/a n/a

No No No Yes n/a n/a n/a

No Yes Yes nd n/a n/a n/a

1 7.08 nd nd nd 7.08 7.11 7.09 7.09

2 7.11 7.02 7.02 7.02 7.14 7.21 7.17 7.18

5 7.22 7.32 7.39 7.38 7.19 7.35 7.29 7.30

10 7.36 7.54 7.61 7.61 7.22 7.46 7.39 7.40

20 nd 7.76 7.81 7.84 7.24 7.59 7.51 7.52

50 nd 8.06 8.05 8.13 7.27 7.77 7.69 7.71

100 nd 8.29 8.22 8.35 7.28 7.93 7.85 7.86

200 nd 8.54 8.38 8.57 7.29 8.09 8.00 8.01

500 nd 8.89 8.58 8.85 7.30 8.31 8.17 8.17

1000 nd 9.16 8.73 9.07 7.30 8.46 8.27 8.27

5000 nd 9.86 9.04 9.58 7.31 8.76 8.44 8.44

10000 nd 10.18 9.17 9.79 7.31 8.86 8.49 8.49

100000 nd 11.38 9.57 10.51 7.32 9.10 8.58 8.58




1991-2008 (n=19) 1991-2008

calculated from annual maxima calculated from all tides in the period

Extreme Value Theory (EVT) Method Joint Probabilities Method (Pugh, 2004),

Return periods of extremely high water levels (in m OD) at Hinkley Point, calculated using Extreme Value Theory (EVT) and Joint Probability Analysis (JPA), assuming a tide equivalent to that which occurred in 1607 (8.50 m OD) had also occurred in the period 1991-2008. See Table 15 for comparison, and an explanation of terms.

Table 16 _________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 110 -

Weibull and Gumbel models, but only a small change in the case of the Generalised

Pareto JPM models.

The frequency distribution of surge residuals and skew surges at Hinkley Point is

summarised in Table 11. Estimated return levels of the skew surges, calculated using

both annual maxima and peaks over threshold, are shown in Table 17, from which it can

be seen that the 10,000 year event values predicted by the GEV and GPD POT models

are considerably larger than those predicted by the Weibull, Gumbell and GPD AM

models.

The frequency distributions for waves measured at Hinkley Point between 16/12/08 and

18/11/10 are summarised in Figure 30. Large return period levels were calculated using

the GPD and recorded Hs values larger than 0.8 m (Table 14). Given the short observed

record, the extrapolated values beyond about 10 years need to be treated with caution,

since they are likely to be under-estimates.

Table 18 shows the joint probability of predicted tides and surges, and of tides, surges

and waves based on recorded values at Hinkley Point. Based on the available data sets,

the tri-variate joint probability level for the ten year event ranges from 7.87 m to 7.90 m

OD, depending on the definition of ‘surge’ which is used. The corresponding tri-variate

values for the 10,000 year event range from 8.93 m to 9.24 m OD.

4.3 Extreme water levels associated with tsunamis and other 'rare' events

Statistical analysis based on relatively short data records of observed events may

provide little or no information about the risk associated with infrequent extreme events

generated by perturbing factors other than those responsible for the main population of

observations. In the context of coastal flooding along the North West European coastal

margin, very extreme water levels can be associated with tsunamis and 'mega-storms'

which sometimes evolve from hurricanes and take unusual easterly trajectories across

the North Atlantic.



- 111 -




Generalised Weibull Gumbel Distribution

Extreme (three (all skew surges >0.35 m)

Value parameter) (1991-2008, n=514)

No No No No

No No No No

No No No Yes

1 0.78 nd nd nd 0.77

2 0.90 0.84 0.85 0.86 0.90

5 1.21 1.08 1.1 1.1 1.10

10 1.32 1.26 1.27 1.25 1.28

20 nd 1.45 1.43 1.4 1.48

50 nd 1.72 1.62 1.59 1.78

100 nd 1.94 1.76 1.74 2.05

200 nd 2.18 1.9 1.88 2.35

500 nd 2.54 2.07 2.07 2.81

1000 nd 2.84 2.2 2.21 3.22

5000 nd 3.63 2.48 2.55 4.39

10000 nd 4.03 2.6 2.69 5.01

100000 nd 5.63 2.98 3.16 7.83


1991-2008 (n=18)

Extreme Value Theory (EVT) Method

calculated from annual maxima



Estimation of return periods high water of skew surges (SKHW, in metres) at Hinkley Point, using Extreme Value Theory (EVT) . Generalised Extreme Value (GEV) distributions (using least squares, Weibull and Gumbel assumptions) and Generalised Pareto Distributions (GPD) have been fitted to annual maxima (n=18) and skew surges above a threshold of 0.35 m (n=415). The goodness of fit is assessed by three statistical tests; a 'yes' indicates that the null hypothesis is rejected, and the data statistically do not fit a GPD.

Table 17 _________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 112 -

Return Period(years)

Predicted tides Predicted tides Predicted tides Predicted tides Predicted tides Predicted tides+ surge + skew surges + surges at + surge + skew surges + surges at

residuals high water residuals + waves high water+ waves + waves

1 7.11 7.09 7.09 7.49 7.47 7.482 7.21 7.17 7.18 7.61 7.59 7.605 7.35 7.29 7.29 7.78 7.75 7.7510 7.46 7.38 7.39 7.90 7.86 7.8720 7.59 7.48 7.50 8.03 7.97 7.9850 7.77 7.65 7.67 8.19 8.12 8.13100 7.93 7.79 7.80 8.33 8.23 8.24200 8.09 7.93 7.94 8.47 8.34 8.35500 8.31 8.10 8.11 8.66 8.49 8.501000 8.46 8.21 8.21 8.81 8.59 8.605000 8.76 8.39 8.39 9.12 8.83 8.8410000 8.86 8.46 8.46 9.24 8.93 8.94100000 9.10 8.58 8.58 9.59 9.24 9.25

Probabilities of Tides, Surges and Waves(waves observed 16/12/08 to 18/11/10, n=4611)

Joint Probabilities of Tides and Surges1991-2008 (n=12591)

Estimation of return periods of extremely high water levels (in m OD) at Hinkley Point, using Joint Probability Analysis of predicted tides and surges (recorded at Hinkley Point Class A tide gauge) and waves (recorded at the CEFAS wave buoy, c. 3 km offshore from Hinkley Point, 16/12/08 to 18/11/10).

Table 18 _________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 113 -

The word 'tsunami' is derived from the Japanese meaning 'harbour wave'. They are fast

travelling long period waves which are generated by earthquakes, volcanic eruptions

and associated tectonic displacements of the sea floor, submarine slides, sub-aerial

landslides which enter the sea and, extremely rarely, asteroid impacts (Dawson &

Stewart, 2007; Bryant, 2008). Travel velocities can exceed 450 km hr-1, with wave

lengths of the order of hundreds of metres to tens of kilometres. Reductions in water

depth lead to a reduction in velocity and speed but an increase in wave height. Tsunamis

have high momentum and on passing the coast run-up may travel up to several

kilometres inland and reach several tens of metres in elevation. The extent of tsunami

run-up is dependent partly on the propagated waves and partly on the topography and

bathymetry of the coastal zone. Tsunami waves usually occur in groups which may

strike the coast over a period of several hours. The first wave is not always the largest;

smaller subsequent waves may interact with earlier reflected waves and become

amplified to create the largest wave in the series. Tsunamis are most frequent in

seismically active zones close to the margins of tectonic plates, but they are not

restricted to such locations. Similar, but more localised long-period waves can also be

generated by meteorological phenomena such as thunderstorms and squall lines; these

events have been referred to as "meteorological-tsunamis" or "meteo-tsunamis" (Haslett

& Bryant, 2009; Haslett et al., 2009). In restricted embayments, estuaries and lakes

meteorological forcing or other physical disturbances may also give rise to long-period

resonant waves (seiches) which typically raise water levels by 0.5 to 1 m but

exceptionally more, but such phenomena have not been recorded as being of major

significance in the Bristol Channel.

On 1st November 1755 an earthquake in the eastern Atlantic about 200km WSW of

Cape St Vincent in Portugal caused considerable destruction in Lisbon and other coastal

areas of Portugal. The earthquake generated major tsunami waves, estimated to be 5 to

13 m high, which travelled eastwards and impacted on the Atlantic coasts of Portugal

and Spain, the Algarve, Gulf of Cadiz in Spain, Morocco and Algeria, leading to the

deaths of 60,000 people in Portugal alone (Dawson & Stewart, 2007). Waves also

travelled westwards, reaching the Caribbean, and northwards towards southwest

England. William Borlase (1755) described the impact of multiple waves in Mounts

Bay, Cornwall, arising from the 1755 Lisbon earthquake, which raised the water level



- 114 -

by up to 3 m over a period of five and a half hours. Documentary and sedimentological

evidence from Cornwall and the Scilly Isles suggests that similar events have also

occurred on several other occasions (e.g. Edmonds, 1846, 1856, 1860; Foster et al.,

1991, 1993; Dawson et al., 2000).

Long-period tsunami waves are essentially extremely large swell waves which have low

steepness (height/length ratio) and tend to move sediment in a shorewards direction. If

mobile sediment is available in the nearshore zone, all sizes of clast, from mud to large

boulders, can be moved large distances inland from the shore. Large quantities of

sediment can be deposited near the incursion limit, often as landward-tapering sediment

wedges. The upper limit of sediment deposition is always less than the upper limit of

run-up, making it impossible to infer run-up levels from palaeo-tsunami deposits. In the

Algarve, sediment sheets produced by the 1755 earthquake extend up to 1 km inland

(Cunha et al., 2009).

Tsunamis are most frequently caused by earthquakes and associated vertical movements

of the sea floor. However, sub-marine or sub-aerial landslides can also be significant

triggers. A submarine slide off the coast of Norway (the Storegga Slide) approximately

8100 years ago caused a major tsunami which impacted on the coast of northeast

Scotland and other North Atlantic coastlines (Dawson & Smith, 2000; Smith et al.,

2004). This event led to the deposition of a sediment layer up to 0.6 m thick at various

locations in eastern Scotland and the Northern Isles. The maximum height of the

tsunami has been estimated at 21 m, although it is uncertain to what extent this reflects

the maximum elevation of wave run-up.

Although it has been argued that the devastating Bristol Channel floods of January 1607

could have been caused by a tsunami (Bryant & Haslett, 2003, 2007; Haslett & Bryant,

2004), and that high energy sedimentary deposits found along the seaboards of

southwest England, Ireland, Wales and Brittany may also have been formed by tsunamis

(Haslett & Bryant, 2007, 2008), the evidence in support of a tsunami origin for the

Bristol Channel 1607 flood is very limited and this event was most probably caused by

a storm surge. Horsburgh & Horritt (2006) suggested that a surge of up to 2.3 m

contributed to an exceptionally high resultant tide which reached 7.14 m OD at



- 115 -

Goldcliff (near Newport) and 7.74 m OD at Kingston Seymour, Somerset. RMS (2007)

concluded that, due to the effects of sea level rise since 1607, an equivalent event today

would achieve levels of c. 1 m above those marked on the churches around the outer

Bristol Channel after the 1607 event, and c. 1.5 m higher around the inner Bristol

Channel (i.e. the equivalent levels attained today would be c. 8.5 to 9.0 m OD).

However, the sea level rise estimates used by RMS may be too high, and equivalent

modern day values of 7.94 - 8.54 m OD are more likely.

Horsburgh & Horritt (2006) also modelled the 1755 tsunami which destroyed Lisbon

and concluded that flooding due to this event is likely to have been less severe in the

Bristol Channel than that associated with a storm surge. Telemac modelling by HR

Wallingford (2006), based on geological tsunami threats identified by Kerridge (2005),

also suggested that the likely size of a tsunami generated wave around Hinkley Point

would be less than 0.5 m. HR Wallingford (2009) conclude that to cause flooding

beyond a 1 in 10,000 year standard of protection at Hinkley Point would require a

tsunami of this magnitude to coincide with a high tide with return period in excess of 1

in 500 years. This would suggest that the 1607 event had a return period of between 1 in

150 and 1 in 3000 years, with a most likely value of c. 1 in 500 to 1 in 700 years (HR

Wallingford, 2009). Further modelling studies by Horsburgh et al. (2008) have

indicated that tsunami wave heights and period around the UK would be very sensitive

to the source event strength, location and orientation, and to bathymetric interaction

with the spreading tidal waves. Some model runs indicated waves up to 3.5 m on

exposed parts of the southern Cornish coast, but < 1 m in the Bristol Channel.

It is possible that higher tsunami waves could penetrate the Bristol Channel following a

more local severe seismic event, for example off southern Ireland or in the Bristol

Channel itself. During the period 1580-2005 eighteen earthquake events of magnitude

5.0 to 5.4 ML have been identified in Britain with epicentres onshore or just offshore,

and four events > 5.4 ML have been identified in the English Channel and North Sea

(Musson, 1994). Although larger events may have gone unrecorded, the likelihood of

local tsunami generation is considered to be low (Musson, 2008).



- 116 -

The possibility of a tsunami larger than that generated by the 1755 Lisbon earthquake

being generated by collapse of the Cumbre Vieja volcano on La Palma, Canary Islands

has been recognized, with the potential prospect of waves 10 m high reaching southwest

Britain; however, geologists remain divided regarding its stability and the likelihood of

collapse.

4.4 Implications of future climate and sea level change

Gallani (2007) reviewed the potential implications of climate change effects of medium

to long-term coastal risk at British energy sites, including Hinkley Point. The

assessment was based largely on outputs from the Met Office Regional Climate Model

using emissions scenarios used in the IPCC Third Scientific Assessment (Houghton et

al., 2001), assessments for the UK contained in UKCIP02 (Hulme et al., 2002), and

modelled surge predictions by Lowe et al. (2001). A net sea level increase in the range

0.13 to 1.12 m and a 1 in 50 year surge height change of 0.07 to 0.53 m by the 1980's

were suggested for the Hinkley Point area. Based partly on these figures, Halcrow

(2007b p4) suggested a worst case scenario of combined increase in 1:50 surge height

plus sea level rise to produce a resultant 1:50 surge height of 8.07 m OD at Hinkley

Point in the 2080's. However, the figure used for existing 1:50 year surge level (7.19 m)

was in error; using the figure of 7.29 m (Halcrow, 2007a) the resultant 2080's 1:50 surge

level would be 8.17 m OD. Halcrow also suggested a 14.47% increase in offshore Hs

from 4.56 to 5.22 m, based on a projected 7% increase in wind speed indicated by the

Met Office models.

The joint probability analysis by HR Wallingford (2009) took account of future sea

level change by adopting the allowances for net mean sea level rise in the south-west of

England recommended by DEFRA (2006): 3.5 mm a-1 for 1990-2025, 8.0 mm a-1 for

2025 -2055, 11.5 mm a-1 for 2055-85 and 14.5 mm a-1 for 2085-2115. These figures

were based on the extreme high estimates in the Third IPCC Assessment Report and

isostatic rebound figures for the UK taken from Shennan & Horton (2002). No

allowance was made for possible changes in tidal range (height of high waters), storm

surges, or wave heights.



- 117 -

Clarke & Hanson (2009) reviewed the implications of the Fourth IPCC Assessment and

the UKCP09 interpretations (Lowe et al., 2009; Murphy et al., 2009) for climate and sea

level in the Hinkley Point area, concluding that sea level is expected to increase by

between 19 and 82 cm using the UKCP09 5th and 95th percentiles for a range of

climate models, and using revised land movement estimates based on Bradley et al.

(2008). Changes in storminess and wave climate are highly uncertain, with UKCP09

results varying widely depending on the climate model chosen.

Further analysis of the implications of possible climate and sea level change has been

undertaken as part of the present study. Figure 50 shows a comparison of possible future

changes in annual mean sea level to 2100 based on extrapolation of the observed 1992-

2006 values at the Hinkley Point tide gauge, the DEFRA (2006) sea level allowances

for Southwest England, and UKCP09 predictions for Hinkley Point under low, medium

and high emissions scenarios. Figure 51 shows the projected increase in mean high

water at Hinkley Point to 2100 based on the UKCP09 predictions and the assumption

that MHW rises twice as fast as MSL (a rate suggested by records at Avonmouth

between 1987 and 2008, as indicated in Table 12). Figure 52 shows the long-term linear

trend in skew surge at Hinkley Point indicated by UKCP09 surge modelling, based on a

medium emissions scenario, and for 2, 10, 20 and 50 year return water levels. The best

estimate (50th percentile) projection is of a 9 cm increase in 1:50 skew surge magnitude

by 2100. Additional data for projected increases in mean sea level, skew surge and

various climate parameters are presented in Appendix 9.

The UKCP09 adoption of a linear trend in skew surge (and hence storminess) is open to

question. The frequency of severe winter storms is known to have varied significantly in

the past on a decadal timescale (e.g. Figure 53), in part correlated with fluctuations in

the North Atlantic Oscillation (Woodworth et al., 2007). Current modelling still

provides relatively poor prediction of observed storm surge levels around the UK, and

uncertainty in future predictions is high (Lowe & Gregory, 2005; Lowe et al., 2009).

Wave modelling using the third-generation spectral model WAM driven by winds from

a sub-set of the Met Office climate model ensemble members led to the conclusion that



- 118 -

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

Mea

nse

ale

vele

leva

tion

(mO

D)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

Mea

nse

ale

vele

leva

tion

(mO

D)

PSMSL annual means 1991-20061992-2006 trendline and extrapolation to 2100DEFRA (2006) allowancesUKCIP predictions (50 %ile)UKCIP predictions (5% to 95% error range)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

Mea

nse

ale

vele

leva

tion

(mO

D)

(a)

(b)

(c)

Figure 50 Observed annual mean sea levels at Hinkley Point (data from PSMSL) and future preditions basedon an extrapolation of the 1992‐2006 trendline, the DEFRA (2006) sea level rise allowances, andUKCIP09 predictions (grid cell 24092) based on: (a) low emissions (SRES B1) scenario; (b) mediumemissions (SRES A1B1 scenario); and (c) high emissions (SRES A1FI) scenario.


- 119 -

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

4.54.64.74.84.9

55.15.25.35.45.55.65.75.85.9

66.16.26.36.4

Mea

nhi

ghw

ater

elev

atio

n(m

OD

)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

4.54.64.74.84.9

55.15.25.35.45.55.65.75.85.9

66.16.26.36.4

Mea

nhi

ghw

ater

elev

atio

n(m

OD

)

Trend in MHW (50 %ile)Trend in MHW (5% to 95% error range)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

4.54.64.74.84.9

55.15.25.35.45.55.65.75.85.9

66.16.26.36.4

Mea

nhi

ghw

ater

elev

atio

n(m

OD

)

(a)

(b)

(c)

Figure 51 Future predictions of mean high water (MHW) at Hinkley Point based on UKCIP09 predictionsfor mean sea level (grid cell 24092) based on: (a) low emissions (SRES B1) scenario; (b) mediumemissions (SRES A1B1 scenario); and (c) high emissions (SRES A1FI) scenario. Mean high wateris assumed to rise at twice the rate of mean sea level, from 4.63 m OD in 2008.


- 120 -

Figure 52 The long‐term linear trend in skew surge (1951‐2099, re‐scaled to start in 1991) at Hinkley Point from UKCP09 predictions (grid cell 15803)based on a medium emissions (SRES A1B) scenario and return levels of (a) 2 years; (b) 10 years; (c) 20 years; and (d) 50 years.

Skew surge trend 50%ile5% to 95% error range

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

Incr

ease

insk

ewsu

rge

leve

l(m

)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

Incr

ease

insk

ewsu

rge

leve

l(m

)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

Incr

ease

insk

ewsu

rge

leve

l(m

)

1990 2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0.12

Incr

ease

insk

ewsu

rge

leve

l(m

)

(a) 2 year return level (b) 10 year return level

(c) 20 year return level (d) 50 year return level

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 121 -

0

2

4

6

8

10

12

14

16

18

20

1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s

Num

ber o

f sev

ere

stor

ms

per d

ecad

e

Figure 53 Number of severe winter storms (October to March) per decade over the UK and Ireland between 1920 and 1999 (data from Allan et al., 2008). Error bars shown are +/- one standard deviation.



- 122 -

seasonal mean and extreme waves are expected to increase slightly to the SW of the

UK, but the predicted pattern around the UK is highly variable and maps in Lowe et al.

(2009) suggest a slight decrease in the Bristol Channel area. The results are generally

subject to high uncertainty. Wave conditions in the northeast Atlantic and Southwest

Approaches are known from observations to show considerable variability on a decadal

timescale (e.g. Bacon & Carter, 1993; Wasa Group, 1998). Wave conditions within the

Bristol Channel and Severn Estuary are also likely to be influenced by changes in water

depth (sea level and surge magnitude). Geomorphological and sedimentological

evidence suggests that, during the warmest phase of the Last Interglacial period, when

sea level was 6 - 8 m higher than present, the Inner Bristol Channel and Severn estuary

experienced greater wave energy than now (Allen, 2002).

For the purposes of illustration, Table 19 shows return water levels calculated for 2100

based on joint probability estimates for astronomical tides and surges recorded at

Hinkley Point 1990-2008 and waves recorded off Hinkley Point in 2008-10. In this

analysis it has been assumed that: (a) high waters will increase by 1.0 m over the period

(approximately twice the 50th percentile increase in UKCP09 predicted MSL under the

medium emissions scenario, and slightly higher than the UKCP09 H++ scenario lower

estimate), (b) there will be an increase in skew surge height of 0.063 m (based on

UKCP09 modelling results for the medium emissions scenario), and (c) that significant

wave height will increase by c. 4%, equivalent to the projected increase in skew surge

magnitude. The results indicate that, under this assumed combination of conditions, the

1:10,000 year tide plus surge level could exceed 9.5 m OD and the tide plus surge plus

wave level could exceed 10 m OD. Even higher levels are indicated if the H++ higher

estimate value for mean sea level rise is used.

Table 20 provides a summary of the environmental extremes at Hinkley Point, including

observed and predicted values relevant to coastal flooding risk under different

combinations of conditions at the present day. For purposes of comparison, possible

resultant extreme water levels for 2100, under different climate change scenarios and

combinations of conditions, are summarised in Table 21. A diagrammatic representation

of potential future extreme water levels, relative to the current defended coastal profile

in front of Hinkley Point B, is shown in Figure 54.



- 123 -

Return Period(years)

Predicted tides Predicted tides Predicted tides Predicted tides Predicted tides Predicted tides+ surge + skew surges + surges at + surge + skew surges + surges at

residuals high water residuals + waves high water+ waves + waves

1 8.12 8.10 8.10 8.52 8.51 8.512 8.22 8.19 8.19 8.65 8.63 8.635 8.37 8.30 8.31 8.82 8.79 8.7910 8.49 8.40 8.41 8.95 8.91 8.9120 8.63 8.51 8.53 9.08 9.02 9.0350 8.82 8.69 8.71 9.26 9.18 9.19100 8.98 8.83 8.85 9.40 9.29 9.30200 9.16 8.98 8.99 9.54 9.41 9.42500 9.38 9.16 9.16 9.74 9.56 9.571000 9.53 9.27 9.27 9.90 9.67 9.685000 9.84 9.46 9.46 10.22 9.92 9.9310000 9.94 9.53 9.53 10.34 10.02 10.03100000 10.18 9.65 9.65 10.70 10.35 10.35

Probabilities of Tides, Surges and Waves Predicted for 2100

Joint Probabilities of Tides and SurgesPredicted for 2100

Estimation of return periods of extremely high water levels (in m OD) at Hinkley Point, using Joint Probability Analysis of predicted tides, surges and waves in the year 2100. Joint probablility is based on predicted tides and observed skew surges recorded at Hinkley Point Class A tide gauge (1990‐2008) and waves recorded at the CEFAS wave buoy, c. 3 km offshore from Hinkley Point (16/12/08 to 18/11/10). The following allowances are made for changes between 2008 and 2100: +1.00 m increase in astronomical high water (approximately equivalent to the H++ lower estimate of increase in mean sea level); 4% increase in skew surge (equivalent to a +0.063 m increase in 1 in 50 year skew surge projected by UKCP09); 4% increase in significant wave height (equivalent to the increase predicted for skew surges by UKCP09).

Table 19 _________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 124 -

4.5 Effect of extreme water levels on coastal morphological change

Occurrences of extreme water levels associated with the coincidence of high tides,

surges and even large waves are not necessarily of great importance from the viewpoint

of coastal morphological change (erosion or accretion). Catastrophic morphological

changes do sometimes occur in coastal environments, but are generally very rare. Most

morphological changes, such as changing patterns of coastal erosion / accretion or

changes in the pattern of estuarine banks and channels, are brought about by the

operation of coastal processes over periods of decades or longer.

The impact of storm waves at the shoreline during a single storm is dependent on

duration, as well of the elevation, of high water levels. The expenditure of wave energy

at various levels across the inter-tidal and supra-tidal zones is strongly dependent on the

time interval over which higher water levers are maintained. This factor can have major

significance in terms of short-term soft cliff, sand dune and saltmarsh erosion rates. In

areas of very large tidal range, such as Bridgwater Bay, the wave energy is spread over

a large vertical range during any single tide (contrasting, for example, with areas of

relatively small tidal range such as the central Suffolk coast where wave energy can be

concentrated near the high water mark for periods of many hours).

Sand beach and mudflat profiles are likely to change in response to variations in wave

energy conditions operating over periods of several years to decades (cf. Kirby, 2000).

An increase in average wave energy conditions, or the occurrence of several severe

storm surge events within a short period of time, is likely to result in lowered foreshore

levels and increase the potential for barrier breaching and over-wash. If sustained for a

significant period of time, greater wave energy may be expected to enhance the break-

up and erosion of the limestone intertidal platform which fronts the Hinkley Point

power station and neighbouring cliffs, thereby increasing water depths and acting to

further increase wave energy at the shoreline. However, this is likely to be a progressive

rather than catastrophic process.



- 125 -

5.0 Conclusions

The calculation of return levels for different time periods using statistical models should

only be regarded as reliable if appropriate data are available for a sufficiently long time

period (ideally at least 25 years). Extrapolations beyond four times the period of record

(e.g. 100 years) should be treated with great caution, even when the boundary

conditions (climate, sea level, coastal morphology, human intervention) are not

changing significantly. Even using the same data set, different statistical models yield

differing estimates of return periods and return levels. The water level record for

Hinkley Point is less than 20 years and wave data are available only for approximately

the past 2 years. During this time there have been no high-magnitude events equivalent

to the great storm of 1607, or even to the smaller, but damaging surge tide of 1981.

Statistically-based estimates of ‘extreme’ water levels based on data for Hinkley Point

data alone may therefore be underestimates. The estimates obtained in this study of

the 1 in 10,000 year event (0.0001 probability, or 0.01% chance of occurrence in any

one year), resulting from combined tide plus surge plus waves, range from 8.93 to 9.24

m OD, depending on the definition of ‘surge’ which is used (Table 18). Assuming a 1 m

sea level rise and a 4% increase in surge height and wave height by 2100, estimates of

the equivalent 1 in 10,000 year level range between 10.02 and 10.34 m OD (Table 19).

An alternative, ‘precautionary’, approach is to consider ‘worst case’ combinations of

conditions which might occur, based on knowledge about marine and atmospheric

processes and the physical constraints upon them. For the present day, a worst possible

case would arise if the largest possible surge and largest possible waves coincided with

high water of the highest astronomical tide. Such a combination is extremely unlikely

and would, if it occurred, produce an extreme water level (>12 m) which is well in

excess of the 1 in 10000 year event estimated by joint probability analysis. By

comparison, a combination of HAT (7.12 m OD), a surge equivalent to the largest on

record (c. 2m during the 1607 event) and half the largest measured inshore significant

wave height (1.41 m), would produce a resultant extreme water level of 10.53 m (Table

20).



- 126 -

Parameter Maximum Minimum Source Type

Mean sea level (during last 10,000 years) +0.36 m OD (present day) ‐35 m OD (start of the Holocene) (1),(2) (O),(E)

Mean sea level (during last 130,000 years) +6.0 to +9.0 m OD (Ipwichian maximum) ‐110 m OD (Devensian maximum) (3),(4) (E),(E)

Mean sea level rise (during last 10,000 years) +9.0 mm year‐1 (9500 to 7500 BP) +0.3 mm year‐1 (2000 BP to present) (2),(4) (E),(E)

Observed water level (1607 storm) +7.5 m OD (+8.5 m OD with sea level rise) nd (5),‐ (E),‐

Observed water level (1990‐2009) +7.36 m OD ‐6.33 m OD (1),(1) (O),(O)

Predicted astronomical tide (1990‐2026) +7.12 m OD ‐6.14 m OD (1),(1) (P),(P)

Positive surge residual (1990‐2009) +2.55 m 0.00 m (1),(1) (O),(O)

Negative surge residual (1990‐2009) ‐1.04 m 0.00 m (1),(1) (O),(O)

1 in 20 year positive skew surge (from 1990‐2009 data) +1.54 m 0.00 m (1),(1) (O),(O)

1 in 50 year positive skew surge (from 1990‐2009 data) +1.75 m 0.00 m (1),(1) (E),(E)

1607‐magnitude positive skew surge +2.0 m na (5),‐ (E),‐

Negative skew surge (1990‐2009) ‐0.66 m 0.00 m (1),(1) (O),(O)

Observed (1 in 2 year) significant wave height (Hs) (12/08 to 11/09) +2.82 m +0.02 m (6),(6) (O),(O)

Best estimate 1 in 500 year significant wave height (Hs) +4.80 m +0.00 m (6),(6) (E),(E)

Tsunami (1755 Lisbon earthquake, modelled by POL for Hinkley Point) +0.25 m nd (7),‐ (P),‐

Current speed (Gore Buoy Mini‐Lander site, 2008‐2009) 1.87 m s‐1 0.00 m s‐1 (8),(8) (O),(O)

Current speed (offshore AWAC station H6, 2008‐2009) 2.22 m s‐1 0.00 m s‐1 (8),(8) (O),(O)

Current speed (inshore AWAC station H3, 2008‐2009) 1.73 m s‐1 0.00 m s‐1 (8),(8) (O),(O)

Example theoretical combinationsHAT + 1 in 50 year skew surge +8.87 m OD ‐6.80 m OD (9),(9) (P),(P)

HAT + 1 in 50 year skew surge + 1 in 2 year Hs +10.28 m OD ‐6.80 m OD (9),(9) (P),(P)

HAT + 1 in 50 year skew surge + 1 in 500 year Hs +11.27 m OD ‐6.80 m OD (9),(9) (P),(P)

HAT + 1607‐magnitude skew surge +9.12 m OD nd (9),(9) (P),(P)

HAT + 1607‐magnitude skew surge + 1 in 2 year Hs +10.53 m OD nd (9),(9) (P),(P)

HAT + 1607‐magnitude skew surge + 1 in 500 year Hs +11.52 m OD nd (9),(9) (P),(P)

Summary of environmental extremes at Hinkley Point, based on observations (O), estimates (E) and predictions (P). Sources: (1) NTSLF data; (2) Shennan and Horton (2002); (3) Kidson et al. (1978); (4) Kidson and Heyworth (1976); (5) Risk Management Solutions (2007); (6) CEFAS Wavenet data; (7) HR Wallingford (2006); (8) Foden (2009); (9) this study.

Table 20

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 127 -

Parameter SRES B1 SRES A1B SRES A1FI H++ H++

Low Emission Medium Emission High Emission (lower (higher

5% value 50% value 95% value estimate) estimate)

Increase in mean sea level 2010‐2100 (m) 0.18 0.41 0.78 0.93 1.90

Mean sea level in 2100 (m OD) 0.54 0.77 1.14 1.29 2.26

HAT in 2100 (assuming same rate as mean sea level) (m OD) 7.30 7.53 7.90 8.05 9.02

HAT in 2100 (assuming double rate of mean sea level) (m OD) 7.47 7.95 8.68 8.98 10.92

Increase in 1 in 50 year skew surge 2010‐2100 (m) 0.02 0.07 0.11 0.13 0.63

1 in 50 year skew surge in 2100 (m) 1.77 1.82 1.86 1.88 2.38

1607‐magnitude skew surge in 2100 (m) 2.02 2.07 2.11 2.13 2.63

Increase in 1 in 2 year Hs (assuming same % rise as surge) (m) 0.03 0.11 0.17 0.20 0.75

1 in 2 year Hs in 2100 (assuming same % rise as surge) (m) 2.85 2.93 2.99 3.02 3.57

Best estimate increase in 1 in 500 year Hs (assuming same % rise as surge) (m) 0.05 0.19 0.28 0.33 1.27

Best estimate 1 in 500 year Hs in 2100 (assuming same % rise as surge) (m) 4.85 4.99 5.08 5.13 6.07

Example theoretical combinations for 2100, assuming HAT increases at the same rate as mean sea levelHAT + 1 in 50 year skew surge (m OD) 9.07 9.36 9.76 9.93 11.40

HAT + 1 in 50 year skew surge + 1 in 2 year Hs 10.49 10.82 11.25 11.44 13.18


HAT + 1607‐magnitude skew surge 9.32 9.61 10.01 10.18 11.65

HAT + 1607‐magnitude skew surge + 1 in 2 year Hs 10.74 11.07 11.50 11.69 13.43


Example theoretical combinations for 2100, assuming HAT increases at double the rate of mean sea levelHAT + 1 in 50 year skew surge (m OD) 9.24 9.77 10.54 10.86 13.30



HAT + 1607‐magnitude skew surge 9.49 10.02 10.79 11.11 13.55



Summary of environmental extremes predicted at Hinkley Point in the year 2100, based on values given in Table 20 and UKCP09 predictions for the period 2010‐2100 (UKCIP, 2009). The increase in skew surge under an H++ scenario has been estimated using the same percentage increase as that predicted by UKCP09 for a 3 m surge in the Thames Estuary, while the increase under low and high emission scenarios are interpolated from the values given for medium and H++ scenarios. The increase in wave heights is estimated using the same percentage increase as those calculated for skew surges.

Table 21

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 128 -

Elevation (m OD)

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

Figure 54 Extreme water levels estimated in this study in 2010 (black lines) and 2100 (red lines), overlain on lidar profile P20, in front of Hinkley Point ‘B’ Station. Values represent theoretical combinations of tides, surges and significant wave heights, in 2010 and 2100, based on several UKCP09 scenarios (see Tables 18-21 for more details).

13.0

14.0

15.0

2100 Highest Astronomical Tide, SRES A1B (7.53 m OD)

sea wall(8.83 m OD)

gabions(c. 11.10 m OD)

ground level(c. 8.60 m OD)

2100 Mean High Water, SRES A1B (5.04 m OD)

2010 Highest Astronomical Tide (7.12 m OD)

2010 Mean High Water (4.63 m OD)

2010 Tide + skew surge + wave joint probability 1 in 10,000 year event (8.93 m OD)

2010 Tide + skew surge + wave joint probability 1 in 200 year event (8.34 m OD)

2100 Tide + skew surge + wave joint probability 1 in 200 year event, H++ scenario (9.41 m OD)

2100 Tide + skew surge + wave joint probability 1 in 10,000 year event, H++ scenario (8.93 m OD)2010 HAT + 1 in 50 skew surge + 1 in 2 year Hs (10.28 m OD)

2010 HAT + 1607-magnitude skew surge + 1 in 2 year Hs (10.53 m OD)2100 HAT + 1 in 50 skew surge + 1 in 2 year Hs, SRES A1B scenario (10.82 m OD)2100 HAT + 1607-magnitude skew surge + 1 in 2 year Hs, SRES A1B scenario (11.07 m OD)

2100 HAT + 1607-magnitude skew surge + 1 in 2 year Hs, H++ lower estimate scenario (11.69 m OD)

2100 HAT + 1607-magnitude skew surge + 1 in 2 year Hs, H++ higher estimate scenario (13.43 m OD)

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 129 -

In terms of future climate change, the increase in mean sea level (1.9 m) postulated

under the higher H++ emissions scenario, together with UKCP09 estimates of possible

increases in skew surge and significant wave height (available only for 1 in 50 skew

surge under the medium emissions scenario), provide a near-worst case scenario for

2100. A combination of HAT, a surge of 1607 magnitude increased by 4%, and half the

largest measured inshore significant wave height increased by 4%, gives a resultant

water level of 11.48 m OD under the SRES A18 medium emissions scenario (50%

percentile value) and 15.33 m OD under the H++ high estimate scenario (Table 21).

There is no geological evidence to suggest that maximum mean sea level in the inner

Bristol Channel has been higher than present at any earlier time in the Holocene

(approximately the last 10,000 years). However, there is evidence that during the last

(Ipswichian) interglacial period, and possibly during the penultimate (Hoxnian)

interglacial period, the maximum local relative sea levels in the area were 6 to 8 m

higher than at present. This is consistent with studies in other parts of the world which

have suggested mean sea level up to 6 m higher than present during Marine Isotope

Stage 5c (part of the last interglacial period), when mean global surface temperatures

were at least 2ºC higher than at present (Cuffey & Marshall, 2000; Rohling et al., 2008).

No convincing sedimentological evidence has been provided to suggest that the inner

Bristol Channel coastal lowlands have been affected by a major tsunami or 'megastorm'

of hurricane-like proportions. The most devastating coastal flood in historical times

occurred in January 1607 and almost certainly represented a storm surge of the order of

2.0 to 2.3 m superimposed on a relatively high (but not extreme) astronomical tide. An

event of only slightly smaller proportions occurred in December 1981 and caused

extensive flooding of Burnham on Sea and the Central Somerset Levels adjacent to the

River Parrett estuary. This event reached c. 8.40 m OD at Avonmouth and 7.40 m OD at

Hinkley Point. A level of 7.36 m OD was also recorded at Hinkley Point on 10th

February 1997. Both of these events are well below the maximum still water level (8.66

m) which could result if the maximum currently predicted high tide (7.14 m) were to

coincide with the maximum observed skew surge of 1.54 m or the highest recorded

surge residual (Table 22).



- 130 -

Event Scenario Assumed Assumed Maximum still Skew Maximum

increase in increase in water level surge residual

sea level (m) surge (m) (m OD) (m) (m) 7.00 m OD 8.00 m OD 8.83 m OD 10.00 m OD 7.00 m OD 8.00 m OD 8.83 m OD 10.00 m OD

High tide recorded Observed 1997: 0 0 7.04 1.54 1.54 (at HW) 15 0 0 0 165 105 45 0

on 24/02/1997 SRES B1 estimate for 2100: 0.18 0.03 7.25 1.57 1.57 (at HW) 45 0 0 0 165 120 60 0

(the highest skew surge SRES A1B1 estimate for 2100: 0.41 0.07 7.53 1.61 1.61 (at HW) 75 0 0 0 180 135 75 0

recorded in the period SRES A1FI estimate for 2100: 0.78 0.10 7.92 1.64 1.64 (at HW) 105 0 0 0 195 165 105 0

1990‐2008) H++ higher estimate for 2100: 1.90 0.63 9.57 2.17 2.17 (at HW) 180 135 90 0 270 225 195 135

High tide recorded Observed 1997: 0 0 7.36 0.37 0.85 75 0 0 0 195 135 75 0

on 10/02/1997 SRES B1 estimate for 2100: 0.18 0.01 7.54 0.38 0.85 75 0 0 0 195 150 105 0

(the highest tide SRES A1B1 estimate for 2100: 0.41 0.02 7.79 0.39 0.86 105 0 0 0 210 165 105 0

recorded in the period SRES A1FI estimate for 2100: 0.78 0.02 8.16 0.39 0.87 135 45 0 0 225 180 135 45

1990‐2008) H++ higher estimate for 2100: 1.90 0.15 9.41 0.52 1.00 195 135 75 0 285 240 195 135

Hypothetical coincidence Coincidence in 2010: 0 0 8.63 1.54 1.54 (at HW) 135 75 0 0 210 165 135 75

of highest predicted tide SRES B1 estimate for 2100: 0.18 0.03 8.83 1.57 1.57 (at HW) 135 75 15 0 210 180 135 75

(18/09/1997, 7.08 m OD) SRES A1B1 estimate for 2100: 0.41 0.07 9.11 1.61 1.61 (at HW) 135 105 45 0 225 180 165 105

coinciding with highest SRES A1FI estimate for 2100: 0.78 0.10 9.51 1.64 1.64 (at HW) 165 120 75 0 240 195 165 120

skew surge (24/02/1997) H++ higher estimate for 2100: 1.90 0.63 11.16 2.17 2.17 (at HW) 225 195 165 105 300 270 225 195

Hypothetical coincidence Coincidence in 2010: 0 0 9.29 2.21 2.21 (at HW) 165 120 75 0 240 210 180 120

of highest predicted tide SRES B1 estimate for 2100: 0.18 0.03 9.50 2.24 2.24 (at HW) 180 135 75 0 255 210 180 135

(18/09/1997, 7.08 m OD) SRES A1B1 estimate for 2100: 0.41 0.07 9.78 2.28 2.28 (at HW) 195 150 105 0 255 225 195 150

coinciding with largest SRES A1FI estimate for 2100: 0.78 0.10 10.17 2.31 2.31 (at HW) 210 165 135 45 270 240 210 165

surge residual (04/01/1998) H++ higher estimate for 2100: 1.90 0.63 11.82 2.84 2.84 (at HW) 255 225 195 150 330 300 270 225

Time period still water level

above threshold (minutes)

Time period still water level plus 2 metre

wave contribution above threshold (minutes)

Table 22 The projected still water levels, skew surge magnitude, and duration of high water (to the nearest 15 minutes) above specified elevations, for different tide + surge conditions, under low emissions (50% SRES B1), medium emissions (50% SRES A1B1), high emissions (50% SRES A1FI) and H++ higher estimate sea level rise scenarios described by Lowe et al. (2009). The effect of a 2 metre wave contribution is also shown. Note that high tidal levels are assumed to increase at the same rate as mean sea level. Original data source: NTSLF and UKCP09.

_________________________________________________________________________________________________________________K



es_________________________________________________________________________________________________________________

- 131 -

While non-linear tide - surge interactions reduce the likelihood that maximum surges

coincide with predicted high water, there is no a priori reason why a very large surge

could not coincide with a very high astronomical tide. The actual risk of such

coincidence is difficult to assess accurately based on statistical modelling, and for NNB

planning purposes a precautionary approach may be more appropriate based on the

worst case scenario combination of credible conditions.

Predictions of future global climate and sea level change are subject to major

uncertainties. Under the extreme H++ scenario considered by UKCP09 a rise in MSL of

1.9 m is considered possible but unlikely. Mean high water levels are likely to increase

by more than MSL, potentially by as much as 40 to 50%, which would give a predicted

high water level c. 2.8 m higher than present under a H++ scenario. An increase of this

magnitude would require higher than expected rates of melting of land-based ice sheets

in Greenland and West Antarctica, producing rapid rates of sea level rise comparable

with those (c. 16 +/ - 8 mm a-1) inferred during Marine Isotope Stage 5e (Rohling et al.,

2008). Although catastrophic melting is currently considered unlikely by most scientists

(especially of the West Antarctic ice sheet), a precautionary approach to coastal flood

risk assessment would allow for such a possibility.

Several large-scale human interventions are currently being undertaken or are being

considered in the inner Bristol Channel - Severn estuary area which could potentially

have a significant impact on the morphology and sedimentological character of the

Hinkley Point coastal frontage and adjacent areas, and on tide and surge levels. These

include the construction of a tidal barrage and improved flood defences in the Parrett

estuary to protect Bridgwater, and large-scale managed realignment on the Stert

Peninsula (Environment Agency & Babtie, Brown & Root, 2002; Kirby & Shaw, 2005;

Black & Veatch, 2009; Environment Agency, 2009; Halcrow, 2009). Proposals for a

Severn tidal power scheme have recently been shelved but may be revived in the future.

A full assessment of the potential effects of these proposed schemes on the Hinkley

Point site is beyond the scope of the present report and warrants more specific

consideration.



- 132 -

6.0 References ABPmer (2006) Severn Estuary Coastal Habitat Management Plan (CHAMP). Environment Agency. Allan, R., Tett, S. & Alexander, L. (2008) Fluctuations in autumn - winter severe storms over the British Isles: 1920 to present. International Journal of Climatology doi: 10.1002/joc.1765. Allen, J.R.L. (1990) The Severn Estuary in Southwest Britain: its retreat under marine transgression and fine sediment regime. Sedimentary Geology 66, 13-28. Allen, J.R.L. (1991) Salt-marsh accretion and sea-level movement in the inner Severn Estuary, southwest Britain: the archaeological and historical contribution. Journal of the Geological Society, London 148, 485-494. Allen, J.R.L. (1992) Tidally influenced marshes in the Severn Estuary, southwest Britain. In Allen, J.R.L. & Pye, K. (eds.) Saltmarshes - Morphodynamics, Conservation and Engineering Significance. Cambridge University Press, Cambridge, 123-147. Allen, J.R.L. (2002) Interglacial high-tide coasts in the Bristol Channel and Severn Estuary, southwest Britain: a comparison for the Ipswichian and Holocene. Journal of Quaternary Science 17, 69-76. Allen, J.R.L. & Fulford, M.G. (1986) The Wentlooge Level: a Romano-British saltmarsh reclamation in southeast Wales. Britannia 17, 91-117. Allen, J.R.L. & Rae, J.E. (1987) Late Flandrian shoreline oscillations in the Severn Estuary: a geomorphological and stratigraphical reconnaissance. Philosophical Transactions of the Royal Society of London B 315, 185-230. Bacon, S. & Carter, D.J.T. (1993) A connection between mean wave height and atmospheric pressure gradient in the North Atlantic. International Journal of Climatology 13, 423-436. Beirlant, J., Goegebeur, Y., Segers, J. & Teugels, J.L (2005) Statistics of Extremes. Theory and Applications. Wiley, Chichester, 490pp. Bell, M. (1990) Brean Down Excavations 1983-87. English Heritage, London. Bennett,A.F. (1975) Tides in the Bristol Channel. The Geophysical Journal of the Royal Astronomical Society 40, 37-43. BERR (2008) Atlas of Renewable Energy Resources. Report for the Department of Business, Enterprise and Regulatory Affairs by ABPMer, Southampton, 181pp.



- 133 -

BGS (1986) Bristol Channel Sheet 51oN - 04oW, 1: 250,000 Series. Sea Bed Sediments and Quaternary Geology. British Geological Survey, Keyworth. Black and Veatch (2006) Bridgwater Bay to Burnham-on-Sea Flood Management Strategy. Environmental Report. Black & Veatch, Redhill. Blackman, D.L. (1985) New estimates of annual sea level maxima in the Bristol Channel. Estuarine Coastal and Shelf Science 20, 229-232. Blackman, D.L. & Graff, J. (1978) The analysis of annual extreme sea levels at certain ports in southern England. Proceedings of the Institution of Civil Engineers Part 2, 65, 339-357. Borlase, W. (1755) Letter to the Reverend Charles Lyttleton. Philosophical Transactions of the Royal Society of London 49, 373-378. Bradley, S.L., Milne, G.A., Teferle, F.N., Bingley, R.M. & Orliac, E.J. (2009) Glacial isostatic adjustment of the British Isles: new constraints from GPS measurements of crustal motion. Geophysical Journal International 178, 14-22. Bryant, E.A. (2008) Tsunami: the Underrated Hazard. Praxis, Chichester, 2nd edition. Bryant, E.A. & Haslett, S.K. (2003) Was the 1607 coastal flooding event in the Severn Estuary and Bristol Channel (UK) due to a tsunami? Archaeology in the Severn Estuary 13, 163-167. Bryant, E.A. & Haslett, S.K. (2007) Catastrophic wave erosion, Bristol Channel, UK- impact of tsunami? Journal of Geology 115, 253-269. Burt, S. (1993) Another new North Atlantic low pressure record. Weather 48, 98-103. Carr, A.P. (1965) Coastal changes at Bridgwater Bay, 1956-64. Proceedings of the Bristol Naturalist Society 31, 91-100. Cartright, D.E. (1974) Years of peak astronomical tides. Nature 248, 656-657. Clark, D. & Hanson, S.E. (2009) Summarising Existing Knowledge About Future Climate Change for British Energy (BE) Nuclear New Build (NBB) Priorities: Hinkley Point. Report to British Energy, CEFAS, 45pp. Coles, S.G. (2001) An Introduction to Statistical Modeling of Extreme Values. Springer Verlag, London, 208pp. Coles, S.G. & Tawn, J.A. (1990) Statistics of coastal flood protection. Philosophical Transactions of the Royal Society of London A 332, 457-476. Coles, S.G. & Tawn, J. (2005) Bayesian modelling of extreme surges on the UK east coast. Philosophical Transactions of the Royal Society A 363, 1387-1406.



- 134 -

Costen, M. (1992) The Origins of Somerset. Manchester University Press, Manchester. Cuffey, K.M. & Marshall, S.J. (2000) Substantial contribution to sea level rise during the last interglacial from the Greenland ice sheet. Nature 404, 591-594. Cunha, P.P.. Buylaert, J.P., Murray, A.S., Andrade, C., Freutas, M.C., Fatela, F., Munha, J.M., Martins, A.A. & Sugisaki, S. (2009) Optical dating of clastic deposits generated by an extreme marine coastal flood: the 1755 tsunami deposits in the Algarve (Portugal). Quaternary Geochronology doi: 10.1016/ j.quageo.2009.09.004 Dawson, A. & Stewart, I. (2007) Tsunami geoscience. Progress in Physical Geography 31, 575-590. Dawson, S. & Smith, D.E. (2000) The sedimentology of Middle Holocene tsunami facies in northern Sutherland, Scotland, UK. Marine Geology 170, 69-79. Dawson, A.G., Musson, R.M.W., Foster, I.D.L. & Brunsden, D. (2000) Abnormal historic sea-surface fluctuations, SW England. Marine Geology 170, 59-68. DEFRA (2006) Flood and Coastal Defence Appraisal Guidance. FCDPAG3 - Economic Appraisal. Supplementary Note to Operating Authorities - Climate Change Impacts. October 2006. DEFRA, London, 9pp. Deltacommissie (1960) Rapport Deltacommissie. SDU Uitgevers, Den Haag. de Ronde, J.G. (1996) Risk assessment and management of potential flooding in the Netherlands.International Workshop on Floodplain Risk Management 1996, Proceedings, 77-86. Dewar, H.S. (1940) A Romano-British settlement at Combwich. Somerset Archaeology and Natural History 86, 129-133. Dixon, M. (1997) Extreme Sea-Levels around the UK coastline. 32nd MAFF Conference of River and Coastal Engineers, Keele University, Wednesday 2 to Friday 4 July 1997 Proceedings, B.3.1 - B.3.10. Dixon, M.J. & Tawn, J.A. (1992) Trends in UK extreme sea levels: a spatial approach. Geophysical Journal International 111, 607-616. Dixon, M.J. & Tawn, J.A. (1994). Extreme Sea-Levels at the UK A-Class Sites: Optimal Site-by-Site Analyses. Proudman Oceanographic Laboratory, Bidston Observatory, Internal Document No. 65, 229pp. Dixon, M.J. & Tawn, J.A. (1995) Estimates of Extreme Sea Conditions. Final Report. Extreme Sea-Levels at the UK A-Class Sites: Optimal Site-by-Site Analyses and Spatial Analysis for the East Coast. Proudman Oceanographic Laboratory, Bidston Observatory, Internal Document No. 72, 300pp



- 135 -

Dixon, M.J. & Tawn, J.A. (1997) Estimates of Extreme Sea Conditions. Final Report. Spatial Analyses for the UK Coast. Proudman Oceanographic Laboratory, Bidston Observatory, Internal Document No. 112, 217pp. Donovan, D.T. & Stride, A.H. (1961) Erosion of a rock floor by tidal sand streams. Geological Magazine 98, 393-398. Donovan, D.T., Savage, R.J.G., Stride, A.H. & Stubbs, A.R. (1961) Geology of the floor of the Bristol Channel. Nature 189, 51-52. Dunnavan, G.M. & Diecke, J.W. (1980) An analysis of Supertyphoon Tip (October 1979). Monthly Weather Review 180, 1915-1923. Dunning, R.W. (1992) The Victoria County History of Somerset VI. London University Institute of Historical Research, London. Dyer, K.R. (1984) Sedimentation processes in the Bristol Channel / Severn Estuary. Marine Pollution Bulletin 15, 53-77. EDF Energy (2009) Strategic Siting Assessment for the Development of New Nuclear Power Stations in the UJ. Site Nomination Report for: Hinkley Point. Report SSA/HP/001, EDF Energy, London. Edmonds, R. (1846) An account of an extraordinary movement of the sea in Cornwall, in July 1843, with notes on similar movements in previous years, and also of earthquake which have occurred in Cornwall. British Association for the Advancement of Science 38, 112-121. Edmonds, R. (1856) An account of an earthquake shock on the 30th May and an extraordinary agitation of the sea on the 6th June 1855 in Penzance with observations on the cause of the latter. Edinburgh New Philosophical Journal 3, 280-285. Edmonds, R. (1860) An account of the extraordinary agitation of the sea in the west of England, on the 25th and 26th June, and on the 4th October; with notices of the earthquake shocks in Cornwall on the 11th of November, and the 21st October, 1859. Edinburgh New Philosophical Journal 12, 1-15. Edwards, R.A. (1999) The Minehead District - A Concise Account of the Geology. Memoir for 1:50 000 Geological Sheet 278 and Part of Sheet 294 (England and Wales). The Stationery Office, London, 128pp. Efron, B. (1982) The Jacknife, the Bootstrap and Other Re-sampling Plans. CBMS- NSF Regional Conference Series in Applied Mathematics, Monograph No. 38, Society for Industrial and Applied Mathematics, Philadelphia. Efron, B. & Hinkley, D.V. (1978) Assessing the accuracy of the maximum likelihood estimator: observed versus expected Fisher information (with discussion). Biometrica 65, 457-487.



- 136 -

Environment Agency (2007) The Parrett Catchment Flood Management Plan (CFMP) Consultation Draft. The Environment Agency South-west Region. Environment Agency (2009) Parrett Estuary Flood Risk Managment Strategy, Preferred Strategy Report - For Consultation. January 2009. Environment Agency South -West Region. Environment Agency & Babtie, Brown and Root (2002) Stolford to Combwich Coastal Defence Strategy Study Volume 1. Strategy Plan Summary. Report prepared on behalf of the Environment Agency, Babtie Brown & Root, Croydon. Evans, C.D.R. (1982) The geology and superficial sediments of the inner Bristol Channel and Severn Estuary. In Severn Barrage, Thomas Telford, London, 25- 42. Fisher, R.A. & Tippett, L.H.C. (1928) Limiting forms of the frequency distribution of the largest or smallest member of a sample. Proceedings of the Cambridge Philosophical Society 24, 180-190. Flather, R.A. (1987) Estimates of extreme conditions of tide and surge using a numerical model of the North-west European continental shelf. Coastal Estuarine and Shelf Science 24, 69-93. Flather, R.A., Smith, J.A., Richards, J.D., Bell, C. & Blackman, D.L. (1998) Direct estimates of extreme storm surge elevations from a 40 year numerical model simulations and from observations. The Global Atmosphere and Ocean System 6, 165-176. Foden, D. (2009) Hinkley Point Oceanographic Extremes Report. BEEMS Technical Report 086, Produced for British Energy by the Centre for Environment, Fisheries and Aquaculture Sciences (CEFAS), Lowestoft. Foster, I.D.L., Albon, A.J., Bardell, K.M., Fletcher, J.L., Jardine, T.C., Mothers, R.J., Pritchart, M.A. & Turner, S.E. (1991) High energy coastal sedimentary deposits: an evaluation of depositional processes in southwest England. Earth Surface Processes and Landforms 16, 341-356. Foster, I.D.L., Dawson, A.G., Dason, S., Lees, J. & Mansfield, L. (1993) Tsunami sedimentation sequences in the Scilly Isles, southwest England. Tsunami Hazards 11, 35-46. Gallani, M.L. (2007) Review of Medium to Long-Term Coastal Risks Associated With British Energy Sites: Climate Change Effects. Final Report. Report to British Energy, Met Office, Exeter, 114pp. Godwin, H. (1943) Coastal peat beds of the British Isles and North Sea. Journal of Ecology 31, 199-247.



- 137 -

Godwin, H. & Willis, E.H. (1964) Cambridge University natural radiocarbon measurements. III. Radiocarbon 6, 116-137. Gorbatchev, A., Mattel, J.M., Rebour, V. & Vial, E. (2000) Report on flooding of Le Blayais power plant on 27 December 1999. Paper presented at Eurosafe Seminar (www.eurosafe-forum.org/files/pe_297_24_1_sem1_1.pdf, accessed 24.11.09) Graff, J. (1981) An investigation of the frequency distribution of annual sea level maxima at ports around Great Britain. Estuarine Coastal and Shelf Science 12, 389-449. Gumbel, E.J. (1958) Statistics of Extremes. Columbia University Press, New York, 375pp. Halcrow (1998) Bridgwater Bay to Bideford Bay Shoreline Management Plan. Halcrow Group, Swindon. Halcrow (2007a) Review of Medium to Long-Term Geohazard Risks Associated With British Energy Sites. May 2007. Report to British Energy Generation Ltd., Halcrow Group Ltd., Birmingham, 194pp + appendices. Halcrow (2007b) Review of Medium to Long-Term Geohazard Risks Associated With British Energy Sites. September 2007. Report to British Energy Generation Ltd., Halcrow Group Ltd., Birmingham, 18pp. Halcrow (2009) North Devon & Somerset Shoreline Management Plan (SMP2 - Draft) Parts 1 -6 plus supporting appendices. Halcrow, Swindon (available at www.ndascag.org, accessed 15.11.09) Harris, P.T. & Collins, M.B. (1985) Bedform distributions and sediment transport paths in the Bristol Channel and Severn estuary, UK. Marine Geology 62, 153-166. Harris, P.T. & Collins, M.B. (1991) Sand transport in the Bristol Channel: bedload parting zone or mutually evasive transport pathways? Marine Geology 101, 209- 216. Haslett, S.K. & Bryant, E.A. (2004) The AD 1607 coastal flood in the Bristol Channel and Severn Estuary: historical records from Devon and Cornwall (UK) Archaeology in the Severn Estuary 15, 81-89. Haslett, S.K. & Bryant, E.A. (2007a) Evidence for historic coastal high energy wave impact (tsunami?) in North Wales, United Kingdom. Atlantic Geology 43, 137- 147. Haslett, S.K. & Bryant, E.A. (2007b) Reconnaissance of historic (post AD-1000) high- energy deposits along the Atlantic coasts of southwest Britain, Ireland and Brittany, France. Marine Geology 242, 207-220.



- 138 -

Haslett, S.K. & Bryant, E.A. (2008) Historic tsunami in Britain since AD 1000: a review. Natural Hazards Earth System Science 8, 587-601. Haslett, S.K. & Bryant, E.A. (2009) Meteorological tsunamis in southern Britain: an historical review. The Geographical Review 99, 146-163. Haslett, S.K., Davies, P., Curr, R.H.F., Davies, C.F.C. & Kennington, K., King, C.P. & Margetts, A.J. (1998a) Evaluating late-Holocene relative sea level change in the Somerset Levels, southwest Britain. The Holocene 8, 197-207. Haslett, S.K., Davies, P. & Strawbridge, F. (1998b) Reconstructing Holocene sea level change in the Severn Estuary and Somerset levels: the foraminifera connection. Archaeology in the Severn Estuary 8, 29-40. Haslett, S.K., Howard, K.L., Margetts, A.J. & Davies., P. (2001) Holocene stratigraphy of the northern coastal plain of the Somerset Levels. Proceedings of the Cotteswold Naturalists' Field Club 42, 78-88. Haslett, S.K., Mellor, H.E. & Bryant, E.A. (2009) Meteo-tsunami hazard associated with summer thunderstorms in the United Kingdom. Physics and Chemistry of the Earth doi: 10:1016/ j.pce.2009.10.005 Hawkes, P.J. (2003) Extreme Water Levels in Estuaries and Rivers. The Combined Influence of Tides, River Flows and Waves. DEFRA / Environment Agency Flood and Coastal Defence R & D Programme, R & D Technical Report FD0206/TR1, xii + 60pp. Hawkes, P.J. (2005) Use of Joint Probability Methods in Flood Management. A Guide to Good Practice. Defra / Environment Agency Flood and Coastal Defence R & D Programme, R & D Technical Report FD2308/TR2, Defra, London, 67pp. Hawkes, P.J. & Svensson, C. (2003) Joint Probability: Dependence Mapping and Best Practice. DEFRA / Environment Agency Flood and Coastal Defence R & D Programme, R & D Interim Technical Report FD2308/TR1, xiv + 106pp. Hawkes, P.J., Gouldby, B., Tawn, J. & Owen, M. (2002) The joint probability of waves and water levels in coastal engineering design. Journal of Hydraulic Research 40, 241-251. Hawkins, A.B. (1973) Sea-level changes around South-west England. In Blackman, D. (ed.) Marine Archaeology. Butterworths, London, 67-87. Hawkins, A.B. (1990) Geology of the Avon coast. Proceedings of the Bristol Naturalists' Society 50, 3-27. Heyworth, A. & Kidson, C. (1982) Sea-level changes in southwest England and Wales. Proceedings of the Geologists' Association 93, 91-111.



- 139 -

Horsburgh, K. & Horritt, M. (2006) The Bristol Channel floods of 1607 - reconstruction and analysis. Weather 61, 272-277. Horsburgh, K.J., Wilson, C., Bapotie, B.J., Cooper, A., Cresswell, D., Musson, R.M.W., Ottemoller, L., Richardson, S. & Sargeant, S.L. (2008) Impact of a Lisbon-type tsunami on the UK coastline and the implications for tsunami propagation over broad continental shelves. Journal of Geophysical Research 113, C04007, doi:10.1029/2007JC004425 Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden,P.J., Dai, X., Maskell, K. & Johnson, C.A. (2001) Climate Change 2001. The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, 881pp. Housley, R. (1988) The environment of Glastonbury Lake Village. Somerset Levels Papers 14, 63-83. HR Wallingford (1994) Validation of Joint probability Methods for Large Waves and High Water Levels. Report SR 347, HR Wallingford, Wallingford. HR Wallingford (2009) Hinkley Point: Extreme Sea Water Levels and Sedimentology Study. Joint Probability of Waves and Water Levels and Structure Response. Technical Note 02, HR Wallingford Ltd. Wallingford, 34pp. HR Wallingford & Lancaster University (2000) The Joint Probability of Waves and Water Levels: JOIN-SEA: A Rigorous But Practical New Approach. Report SR 537, HR Wallingford, Wallingford. HR Wallingford, British Geological Survey and Proudman Oceanographic Laboratory (2006) Tsunamis - Assessing the Hazard for the UK and Irish Coasts. Report to DEFRA, the Health and Safety Executive and The Geological Survey of Ireland. HR Wallingford Report EX5364, 91pp. Hulme, M., Jenkins, G.J., Lu, X., Turnpenny, J.R., Mitchell, T.D., Jones, R.G., Lowe, J., Murphy, J.M., Hassell, D., Boorman, P., McDonald, R. & Hill, S.(2002) Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia. Hunt, C.O. (2006) The Burtle Formation. In Hunt, C.O. & Haslett, S.K. (eds.) Quaternary of Somerset. Quaternary Research Association Field Guide, Quaternary Research Association, London, 173- 186. Hydrographer of the Navy (2004) Chart 1152. Bristol Channel Nash Point to Sand Point. Edition Number 3. Hydrographic Office, Taunton.



- 140 -

Jacobs Babtie (2006) Magnox OSD Coastal Evolution and Flood Risk. Briefing Note for Stratgeic Optioneering Workshop. Hinkley Point A. May 2006. Jacobs Babtie, Exeter, 10pp. Jenkins, G.J., Perry, M. & Prior, J. (2007) The Climate of the United Kingdom and Recent Trends. Revised edition published January 2009. UK Climate Impact Programme (UKCIP) Scientific Report, Met Office Hadley Centre, Exeter. Jenkinson, A.F. (1955) The frequency distribution of the annual maximum (or minimum) values of meteorological elements. Quarterly Journal of the Royal Meteorological Society 81, 158-172. Kerridge, D. (ed.) (2005) The Threat Posed by Tsunami to the UK. Report by British Geological Survey, commissioned by DEFRA, HMSO,London. Kidson, C. (1960) The shingle complexes of Bridgwater Bay. Transactions of the Institute of British Geographers 28, 75-87. Kidson, C. (1963) The growth of sand and shingle spits across estuaries. Zeitschrift fur Geomorphologie 7, 1-22. Kidson, C. & Carr, A.P. (1961) Beach drift experiments at Bridgwater Bay, Somerset. Proceedings of the Bristol Naturalists' Society 30, 163-180. Kidson, C. & Heyworth, A. (1973) The Flandrian sea level rise in the Bristol Channel. Proceedings of the Ussher Society 2, 565-584. Kidson, C. & Heyworth, A. (1976) The Quaternary deposits of the Somerset Levels. Quarterly Journal of Engineering Geology 9, 217-235. Kidson, C., Gilbertson, D.D. Haynes, J.R., Heyworth, C., Hughes, C.F. & Whatley, R.C. (1978) Interglacial marine deposits of the Somerset Levels, southwest England. Boreas 7, 215-228. Kidson, C., Beck, R.B. & Gilbertson, D.D. (1981) The Burtle Beds of Somerset: temporary sections at Penzoy Farm, near Bridgwater. Proceedings of the Geologists Association 92, 39-45. Kirby, J. R. & Kirby, R. (2008) Medium timescale stability of tidal mudflats in Bridgwater Bay, Bristol Channel, UK: influence of tides, waves and climate. Continental Shelf Research 28, 2615-2629. Kirby, R. (2000) Practical implications of tidal flat shape. Continental Shelf Research 20, 1061-1077. Kirby, R. & Parker, W.R. (1980) Settled Mud Deposits in Bridgwater Bay, Bristol Channel, UK. Institute of Oceanographic Sciences Report 107, 65pp.



- 141 -

Kirby, R. & Parker, W.R. (1983a) A suspended sediment front in the Severn Estuary. Nature 285, 396-399. Kirby, R. & Parker, W.R. (1983b) Distribution and behaviour of fine sediment in the Severn estuary and Inner Bristol Channel, UK Canadian Journal of Fisheries and Aquatic Science 40, (Supplement 1), 83-95. Kirby, R. & Shaw, T.L. (2005) Severn Barrage, UK - environmental appraisal. Proceedings of the Institution of Civil Engineers, Environmental Sustainability 158, 31-39. Larcombe, P. & Fernand, L. (2009) Hinkley Point - Hydrodynamics, Climatology, Sedimentology and Coastal Geomorphology - An Initial Assessment of Coastal Hazards Related to Potential New Nuclear Build - Interim Report. Lee, P.H. (1996) Recent changes in inter-tidal and near sub-tidal morphology in Bridgwater Bay. Proceedings of the Bristol Naturalists' Society 56, 67-79. Lennon, G.W. (1963a) A frequency investigation of abnormally high tidal levels at certain west coast ports. Proceedings of the Institution of Civil Engineers 25, 451-483. Lennon, G.W. (1963b) The identification of weather conditions associated with the generation of major storm surges along the west coast of the British Isles. Quarterly Journal of the Royal Meteorological Society 89, 381-394. Lloyd. A.J., Savage, R.T.J., Stride, A.H. & Donovan, J.T. (1973) The geology of the Bristol Channel floor. Philosophical Transactions of the Royal Society of London A, 274, 595-626. Long, A.J., Dix, J.K., Kirby, R., Lloyd-Jones, D., Roberts, D.H., Croudace, I.W., Cundy, A.B., Roberts, A. & Shennan, I. (2002) The Holocene and Recent Evolution of Bridgwater Bay and the Somerset Levels. Report to West Somerset District Council, 173pp. Lowe, J.A. & Gregory, J.M. (2005) The effects of climate change on storm surges around the United Kingdom. Philosophical Transactions of the Royal Society A, 363, 1313-1328. Lowe, J.A., Gregory, J.M. & Flather, R.A. (2001) Changes in the occurrence of storm surges around the United Kingdom under a future climate scenario using a dynamic storm surge model driven by the Hadley centre climate models. Climate Dynamics 18, 179-188. Lowe, J.A., Howard, T., Pardaens, A., Tinker, J., Holt, J., Wakelin, S., Milne, G., Leake, J., Wolf, J., Horsburgh, K., Reeder, T., Jenkins, G., Ridley, J., Dye, S., & Bradley, S. (2009) UK Climate Science Report. Marine and Coastal Projections. Met Office Hadley Centre, Exeter, 95pp.



- 142 -

May, V.J. (2007) Blue Anchor - Watchet - Lilstock, Somerset. In May, V.J. & Hansom, J.D. (eds.) Coastal Geomorphology of Great Britain. Geological Conservation Review Series No 28, Joint Nature Conservation Committee, Peterborough, 145-148. Muir-Wood, R., Drayton, M., Bergers, A. & Burgess, P. (2005) Catastrophe loss modelling of storm surge flood risk in eastern England. Philosophical Transactions of the Royal Society A 363, 1407-1422. Murphy , J., Sexton, D., Jenkins, G., Boorman, P., Booth, B., Brown, K., Clark, R., Collins, M., Harris, G. & Kendon, L. (2009) UK Climate Projections Science Report: Climate Change Projections. Met Office Hadley Centre, Exeter. Musson, R.M.W. (1994) A Catalogue of British Earthquakes. British Geological Survey Technical Report WL/94/04. Musson, R. (2008) The case for large (M >7) earthquakes felt in the UK in historical times. In Frechet, J., Meghraoul, M. & Stucchi, M. (eds.) Historical Seismology: Interdisciplinary Studies of Past and Recent Earthquakes. Springer, Dordrecht, 187-207. Owen, A. (1980) The tidal regime of the Bristol Channel: a numerical modelling approach. Geophysical Journal of the Royal Astronomical Society 62, 59-75. Owen, M., Hawkes, P., Tawn, J. & Bortot, P. (1997) The Joint probability of waves and water levels: a rigorous but practical new approach. Proceedings of the 32nd MAFF Conference of River and Coastal Engineers, Keele University, Wednesday 2 to Friday 4 July 1997, B4.1 - B4.10. Pantin, H. (1991) The Sea Bed Sediments Around the United Kingdom. Research Report SB/90/1, British Geological Survey, Keyworth, 47pp + maps. PhysE Limited (2009) A Monte Carlo Approach to Joint Probability of Wave, Tide and Surge in Extreme Water Level Calculations. Report prepared for the Health and Safety Executive. HSE Report RR740, 52pp. Picklands, J. (1975) Statistical inference using extreme order statistics. Annals of Statistics 3, 119-131. Pike, R.J. & Langdon, M. (1981) A Romano-British site at Combwich Passage. Bridgwater and District Archaeology Society Monograph 1. Pirazzoli, P.A. & Tomasin, A. (2007) Estimation of return periods for extreme sea levels: a simplified empirical correction of the joint probabilities method with examples from the French Atlantic coast and three ports in the southwest of the UK. Ocean Dynamics 57, 91-107. Posford Duvivier and ABP Research & Consultancy (2000) Bristol Channel Marine Aggregates: Resources and Constraints Research Project: Final Report. Posford



- 143 -

Duvivier, Peterborough, & ABP Research & Consultancy, Southampton, 2 volumes. Proctor, R. & Flather, R.A. (1989) Storm surge prediction in the Bristol Channel - the floods of 13 December 1981. Continental Shelf Research 9, 889-918. Pugh, D.T. (1987) Tides, Surges and Mean Sea-Level. John Wiley & Sons, Chichester, 472pp. Pugh, D.T. (2004) Changing Sea Levels. Effects of Tides, Weather and Climate. Cambridge University Press, Cambridge, 265pp. Pugh, D.T. & Vassie, J.M. (1979) Extreme sea levels from tide and surge probability. Proceedings of the 16th Coastal Engineering Conference, 1978, Hamburg. American Society of Civil Engineers, New York, 911-930. Pugh, D.T. & Vassie, J.M. (1980) Applications of the joint probability method for extreme sea level estimations. Proceedings of the Institution of Civil Engineers Part 2, 69, 959-975. Pye, K. & Blott, S.J. (2008) Decadal-scale variation in dune erosion and accretion rates: an investigation of the significance of changing storm tide frequency and magnitude on the Sefton coast, UK. Geomorphology 102, 652-666. Pye, K. & Blott, S.J. (2009) Coastal Processes and Shoreline Behaviours of Estuary-

Dominated Systems in Swansea Bay and Carmarthen Bay. Annex A1 in Halcrow (2010), Swansea Bay and Carmarthen Bay Shoreline Management Plan 2, Appendix C: Baseline Processes Understanding. Halcrow Group Ltd., Swindon.

Pye, K. & van der Wal, D. (2000) Expert Geomorphological Assessment (EGA) as a tool for long-term morphological prediction in estuaries. In Estuaries Research Programme Phase I., EMPHASYS Consortium, Modelling Estuary Morphology and Processes. Final Report, December 2000. Hydraulics Research Limited, Wallingford, 97-102. Ranwell, D.S. (1964) Rates of establishment, succession and nutrient supply at Bridgwater Bay, Somerset. Journal of Ecology 52, 95-105. Ravensrodd Consultant Ltd. (1996) Hartland Point to Brean Down. Summary of Existing Knowledge of Coastal Trends and Stability. Report to North Devon, Somerset and South Avon Coastal Group., Revensrodd Consultants Ltd, Taunton, 156pp. Reeve, D, Chadwick, A. & Fleming, C. (2004) Coastal Engineering. Processes, Theory and Design Practice. Spon, Press, London and New York, 461pp. Rippon, S. (1993) Landscape Evolution and Wetland Reclamation Around the Severn Estuary. PhD Thesis, University of Reading.



- 144 -

Rippon, S. (1997) The Severn Estuary - Landscape Evolution and Wetland Reclamation. Leicester University Press, London, xiii + 318pp. Rippon, S. (1998) Roman and Medieval settlement on the north Somerset Levels: survey and excavation at Barnwell and Buxton, 1996. Archaeology in the Severn Estuary 7, 39-52. RMS (2007) 1607 Bristol Channel Floods: 400 Year Retrospective. Risk Management Solutions Special Report, Risk Management Solution Ltd., London, 18pp. Rohling, E.J., Grant, K., Hemleben, C., Sidall, M., Hoogakker, B.A.A., Belshaw, M. & Kucera, M. (2008) High rates of sea level rise during the last interglacial period. Nature Geoscience 1, 28-42. Sanders, D.E.A. (2006) The modelling of extreme events. British Actuarial Journal 11, 519-557. Shennan, I. (1989) Holocene crustal movements and sea level changes in Great Britain. Journal of Quaternary Science 4, 77-89. Shennan, I. & Horton, B. (2002) Holocene land- and sea-level changes in Great Britain. Journal of Quaternary Science 17, 511-526. Shennan, I., Peltier, W.R., Drummond, R. & Horton, B.P. (2002) Global to local scale parameters determining relative sea level changes and the post-glacial isostatic adjustment of Great Britain. Quaternary Science Reviews 21, 397-403. Shuttler, R.M. (1982) The wave climate in the Severn Estuary. In Severn Barrage, Thomas Telford, London, 27-34. Smith, D.E.,Shi, S., Cullingford, R.A., Dawson, A., Dawson, S., Firth, C.R., Foster, I.D.L., Fretwell, P.T., Haggart, B.A., Holloway, L.K. & Long, D. (2004) The Holocene Storegga Slide tsunami in the United Kingdom. Quaternary Science Reviews 23, 2291-2321. Smith, R.L. (1986) Extreme value theory based on the r largest annual events. Journal of Hydrology 86, 27-43. Solomon, S., Qin, D., Manning, M., Marquis, M., Averyt, K., Tignor, M.M.B., Miller, H.L, & Chen, Z. (eds.) (2007) Climate Change 2007: The Physical Science Basis.Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, 996pp. Stride, A.H. & Belderson, R.H. (1991) Sand transport in the Bristol Channel east of Bull Point and Worms Head: a bed load parting model with some implications of mutually evasive sand transport paths. Marine Geology 101, 203-207.



- 145 -

Suthons, C.T. (1963) Frequency of occurrence at abnormally high sea levels on the east and south coasts of England. Proceedings of the Institution of Civil Engineers 25, 433-450. Tappin, D.R., Chadwick, R.A., Jackson, A.A., Wingfield, R.T.R. & Smith, N.J.P. (1994) The Geology of Cardigan Bay and the Bristol Channel. United Kingdom Offshore Regional Geology Report. HMSO, London, 107pp. Tawn, J.A. (1992) Estimating probabilities of extreme sea levels. Applied Statistics 41, 77-93. Tawn, J.A. & Vassie, J.M. (1989) Extreme sea levels: the joint probabilities method revisited and revised. Proceedings of the Institution of Civil Engineers, Part 2, 87, 429-442. Tomasin, A. & Pirazzoli, P.A. (2008) Extreme sea levels in the English Channel: calibration of the Joint Probability Method. Journal of Coastal Research 24, 1- 13. van den Brink, H.W., Konnen, G.P. & Opsteegh, J.D. (2003) The reliability of extreme surge levels, estimated from observational records of order hundred years. Journal of Coastal Research 19, 376-388. van den Brink, H.W., Konnen, G.P. & Opsteegh, J.D. (2005) Uncertainties in extreme surge level estimates from observational records. Philosophical Transactions of the Royal Society A 363, 1377-1386. van Gelder, P.H.A.J.M. (1996) A new statistical model for extreme water levels along the Dutch coast. Proceedings of the International Symposium on Stochastic Hydraulics. Balkema, McKay, 243-250. UKHO (2008) Admiralty Tide Tables United Kingdom and Ireland (including European Channel Ports. Vol 1, NP201-09, 2009. United Kingdom Hydrographic Office, Taunton, 366pp + appendices. Uncles, R.J. (1981) A note on tidal asymmetry in the Severn Estuary. Estuarine Coastal and Shelf Science 13, 419-432. Uncles, R.J. (1984) Hydrodynamics of the Bristol Channel. Marine Pollution Bulletin 15, 47-53. WASA Group (1998) Changing waves and storms in the Northeast Atlantic. Bulletin of the American Meteorological Society 79, 741-760. Whittaker, A. & Green, G.W. (1983) Geology of the Country Around Weston-Super- Mare. Geological Survey of Great Britain. HMSO, London, 147pp. Williams, M. (1970) The Draining of the Somerset Levels. Cambridge University Press, Cambridge, 288pp.



- 146 -

Willis, A.J. (1990) The development and vegetational history of Berrow salt marsh. Proceedings of the Bristol Naturalists' Society 50, 57-73. Woodworth, P.L., Flather, R.A., Williams, J.A., Wakelin, S.L. & Jevrejeva, S. (2007) The dependence of UK extreme sea levels and storm surges on the North Atlantic Oscillation. Continental Shelf Research 27, 935-946. Woodworth, P.L., Teferle, F.N., Bingley, R.M., Shennan, I. & Williams, S.D.P. (2009) Trends in UK mean sea level revisited. Geophysical Journal International 176, 19-30.



- 147 -

Appendix 1

Aerial photographs of Bridgwater Bay, taken 19th September 2008

Photographs are reproduced at a scale of 1:10,000, and overlain with the historical positions of MHW and MLW taken from the first edition County Series Ordnance Survey maps (Appendix 3). Original data source: Channel Coastal Observatory. Photographs of Hinkley Point Power Station have been inserted from Google Earth (surveyed 31st May 2006).

BridgwaterBay

HinkleyPoint

WestQuantoxhead

Burnham-on-Sea

BreanDown

Berrow

Brean

0 1 2 3 4 5

Scale (km)

A

B

310600 310800 311000 311200 311400 311600 311800 312000 312200 312400 312600 312800 313000 313200 313400 313600 313800 314000 314200 314400142200

142400

142600

142800

143000

143200

143400

143600

143800

144000

144200

144400

144600

Scale: 1:10,000

0 100 200 300 400 500

314400 314600 314800 315000 315200 315400 315600 315800 316000 316200 316400 316600 316800 317000 317200 317400 317600 317800 318000 318200

143600

143800

144000

144200

144400

144600

144800

145000

145200

145400

145600

145800

146000 Scale: 1:10,000

0 100 200 300 400 500

318200 318400 318600 318800 319000 319200 319400 319600 319800 320000 320200 320400 320600 320800 321000 321200 321400 321600 321800 322000144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

147200

Scale: 1:10,000

0 100 200 300 400 500

322000 322200 322400 322600 322800 323000 323200 323400 323600 323800 324000 324200 324400 324600 324800 325000 325200 325400 325600 325800144600

144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

CatsfordCommon

Stolford

NorthHam

325800 326000 326200 326400 326600 326800 327000 327200 327400 327600 327800 328000 328200 328400 328600 328800 329000 329200 329400 329600144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

147200

Scale: 1:10,000

0 100 200 300 400 500

WallCommon

325600 325800 326000 326200 326400 326600 326800 327000 327200 327400 327600 327800 328000 328200 328400 328600 328800 329000 329200 329400142200

142400

142600

142800

143000

143200

143400

143600

143800

144000

144200

144400

144600

144800Scale: 1:10,000

0 100 200 300 400 500

328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800 331000146800

147000

147200

147400

147600

147800

148000

148200

148400

148600

148800

149000

149200

149400

149600

149800

150000

150200

150400

150600

Scale: 1:10,000

0 100 200 300 400 500

328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800150600

150800

151000

151200

151400

151600

151800

152000

152200

152400

152600

152800

153000

153200

153400

153600

153800

154000

154200

154400

Scale: 1:10,000

0 100 200 300 400 500

328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800154400

154600

154800

155000

155200

155400

155600

155800

156000

156200

156400

156600

156800

157000

157200

157400

157600

157800

158000

158200

Scale: 1:10,000

0 100 200 300 400 500

327800 328000 328200 328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800 331000 331200 331400158200

158400

158600

158800

159000

159200

159400

159600

159800

160000

160200

160400

160600

Scale: 1:10,000

0 100 200 300 400 500

Appendix 2

Digital elevation models of Bridgwater Bay, constructed from 2007 and 2008 lidar data

Digital elevation data have been compiled from six separate surveys on 17/03/2007, 20/03/2007, 26/10/2007, 26/01/2008, 24/02/2008 and 07/05/2008. The areal coverage of each survey is shown on Figure 11 of the main report. Where available, the unfiltered ‘digital surface model’ has been shown. Where unavailable, for instance across many areas to the west of Hinkley Point, the filtered ‘digital terrain model’ has been shown. All images are reproduced at a scale of 1:10,000 and have the same elevation colour scale. Original data source: Channel Coastal Observatory.

BridgwaterBay

HinkleyPoint

WestQuantoxhead

Burnham-on-Sea

BreanDown

Berrow

Brean

0 1 2 3 4 5

Scale (km)

A

B

310600 310800 311000 311200 311400 311600 311800 312000 312200 312400 312600 312800 313000 313200 313400 313600 313800 314000 314200 314400142200

142400

142600

142800

143000

143200

143400

143600

143800

144000

144200

144400

144600

Scale: 1:10,000

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (metres)

314400 314600 314800 315000 315200 315400 315600 315800 316000 316200 316400 316600 316800 317000 317200 317400 317600 317800 318000 318200

143600

143800

144000

144200

144400

144600

144800

145000

145200

145400

145600

145800

146000 Scale: 1:10,000

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (metres)

0 100 200 300 400 500

Scale: 1:10,000

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (metres)

322000 322200 322400 322600 322800 323000 323200 323400 323600 323800 324000 324200 324400 324600 324800 325000 325200 325400 325600 325800144600

144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

CatsfordCommon

Stolford

NorthHam

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (metres)

325800 326000 326200 326400 326600 326800 327000 327200 327400 327600 327800 328000 328200 328400 328600 328800 329000 329200 329400 329600144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

147200

Scale: 1:10,000

WallCommon

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (metres)

Scale: 1:10,000

Elevation(metres)

0 100 200 300 400 500

328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800 331000146800

147000

147200

147400

147600

147800

148000

148200

148400

148600

148800

149000

149200

149400

149600

149800

150000

150200

150400

150600

Scale: 1:10,000

-8

-6

-4

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

Elevation(metres)

328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800150600

150800

151000

151200

151400

151600

151800

152000

152200

152400

152600

152800

153000

153200

153400

153600

153800

154000

154200

154400

Scale: 1:10,000

-8

-6

-4

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

Elevation(metres)

328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800154400

154600

154800

155000

155200

155400

155600

155800

156000

156200

156400

156600

156800

157000

157200

157400

157600

157800

158000

158200

Scale: 1:10,000

-8

-6

-4

-2

0

2

4

6

8

10

12

14

16

18

20

22

24

Elevation(metres)

327800 328000 328200 328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800 331000 331200 331400158200

158400

158600

158800

159000

159200

159400

159600

159800

160000

160200

160400

160600

Scale: 1:10,000

-8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20 22 24

Elevation (metres)

Appendix 3

Facsimilies of First Edition County Series Six Inch Ordnance Survey maps of the

coastline around Bridgwater Bay, reproduced at a scale of 1:10,000

The following maps have been reproduced, all from the County of Somerset:

Map Surveyed Published 36SE 1886 1887 37NW 1886 1886 37NE 1886 1886 37SW 1886 1886 37SE 1886 1886 38NW 1886 1886 38NE 1884-5 1886 38SW 1886 1886 25NW 1884 1884 25NE 1884 1884 25SW 1884 1885 25SE 1884 1884 16NW 1883 1884 16NE 1885 1886 16SW 1884 1884 16SE 1884 1884

BridgwaterBay

HinkleyPoint

WestQuantoxhead

Burnham-on-Sea

BreanDown

Berrow

Brean

0 1 2 3 4 5

Scale (km)

A

B

310600 310800 311000 311200 311400 311600 311800 312000 312200 312400 312600 312800 313000 313200 313400 313600 313800 314000 314200 314400142200

142400

142600

142800

143000

143200

143400

143600

143800

144000

144200

144400

1446000 100 200 300 400 500

Scale: 1:10,000

314400 314600 314800 315000 315200 315400 315600 315800 316000 316200 316400 316600 316800 317000 317200 317400 317600 317800 318000 318200

143600

143800

144000

144200

144400

144600

144800

145000

145200

145400

145600

145800

146000

0 100 200 300 400 500

Scale: 1:10,000

318200 318400 318600 318800 319000 319200 319400 319600 319800 320000 320200 320400 320600 320800 321000 321200 321400 321600 321800 322000144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

147200 0 100 200 300 400 500

Scale: 1:10,000

HinkleyPoint 'B'

Hinkley Point 'A'

322000 322200 322400 322600 322800 323000 323200 323400 323600 323800 324000 324200 324400 324600 324800 325000 325200 325400 325600 325800144600

144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000 0 100 200 300 400 500

Scale: 1:10,000

325800 326000 326200 326400 326600 326800 327000 327200 327400 327600 327800 328000 328200 328400 328600 328800 329000 329200 329400 329600144800

145000

145200

145400

145600

145800

146000

146200

146400

146600

146800

147000

147200 0 100 200 300 400 500

Scale: 1:10,000

325600 325800 326000 326200 326400 326600 326800 327000 327200 327400 327600 327800 328000 328200 328400 328600 328800 329000 329200 329400142200

142400

142600

142800

143000

143200

143400

143600

143800

144000

144200

144400

144600

144800

0 100 200 300 400 500

Scale: 1:10,000

328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800 331000146800

147000

147200

147400

147600

147800

148000

148200

148400

148600

148800

149000

149200

149400

149600

149800

150000

150200

150400

150600

0 100 200 300 400 500

Scale: 1:10,000

328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800150600

150800

151000

151200

151400

151600

151800

152000

152200

152400

152600

152800

153000

153200

153400

153600

153800

154000

154200

154400

0 100 200 300 400 500

Scale: 1:10,000

328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800154400

154600

154800

155000

155200

155400

155600

155800

156000

156200

156400

156600

156800

157000

157200

157400

157600

157800

158000

158200

0 100 200 300 400 500

Scale: 1:10,000

327800 328000 328200 328400 328600 328800 329000 329200 329400 329600 329800 330000 330200 330400 330600 330800 331000 331200 331400158200

158400

158600

158800

159000

159200

159400

159600

159800

160000

160200

160400

160600 0 100 200 300 400 500

Scale: 1:10,000

Appendix 4

Ground photographs taken during field visits, September 2009

Photo 1

Photo 2

View from Lilstock towards Hinkley Point.

Lilstock, view east towards Benhole Point and Hinkley Point.

Photo 3

Photo 4

Blue Lias cliffs and shore platform, east of Benhole Point.

Alternating shale and limestone beds exposed in coastal cliff, seaward side of proposed NNB site.

Photo 5

Photo 6

Historical cliff failure, central part of NNB site.

Cliff toe protected by gravel storm beach, western end of the proposed NNB site.

Photo 7

Photo 8

Exposed limestone shore platform, swept clear of sediment, in front of actively eroding Blue Lias cliffs, east central part of proposed NNB site.

Weathered joints in limestone shore platform fronting the proposed NNB site.

Photo 9

Photo 10

Angular loosened blocks on the shore platform fronting the east central part of the proposed NNB site.

Gabion protection in front of cliffs just west of Hinkley Point ‘A’ Station.

Photo 11

Photo 12

Fracture and potential cliff-fall in weathered Blue Lias shales, just west of Hinkley Point ‘A’ Station.

Undercutting of Blue Lias cliffs, just west of Hinkley Point ‘A’ Station.

Photo 13

Photo 14

View from cliff top just west of Hinkley Point ‘A’, looking west along proposed NNB frontage.

View inland across proposed NNB site, towards Hinkley Point ‘A’ Station.

Photo 15

Photo 16

View from sea wall in front of Hinkley Point ‘A’, looking westwards across proposed NNB frontage.

Western end of the sea wall in front of Hinkley Point ‘A’ Station: note projection of made ground seaward of the natural cliffline.

Photo 17

Photo 18

View along the sea wall at the western end of Hinkley Point ‘A’ Station; note absence of gabions as secondary defence.

View across the shore platform at the eastern end of Hinkley Point ‘A’ frontage, showing geological strike running obliquely away from the shore.

Photo 19

Photo 20

Sea wall in front of Hinkley Point ‘B’ Station, with line of gabions providing secondary defence.

View across the shore platfrom from Hinkley Point ‘B’, looking towards the combined ‘A’ and ‘B’ Station water intake.

Photo 21

Photo 22

Close-up view of the combined water intake structure on which the POL Class A tide gauge is mounted.

Cooling water outfall, in front of Hinkley Point ‘B’ Station.

Photo 23

Photo 24

Sea wall in front of the eastern part of Hinkley Point ‘B’ Station.

Hinkley Point ‘B’ Power Station, with secondary defence gabions and adjoining earth embankment.

Photo 25

Photo 26

Sea wall and secondary earth embankment in front of the eastern end of Hinkley Point ‘B’ Station.

End of the concrete sea wall and adjoining ad-hoc construction rubble ridge, eastern of the Hinkley Point ‘B’ Station frontage.

Photo 27

Photo 28

Shingle bank, re-inforced with armourstone, immediately east of the ‘B’ station.

Eastern end of the Hinkley Point Power Station property.

Photo 29

Photo 30

Armourstone revetment and concrete roadway, just east of the power station; Wick Moor is to the left of the picture.

View from the upper foreshore towards armourstone revetment and cobble upper beach, west of the ‘B’ station.

Photo 31

Photo 32

View eastwards towards Stolford, showing primary defence (armourstone revetment and concrete road) and secondary defence (earth bank to the right of picture)

View east along secondary defence embankment, Wick Moor on right of picture.

Photo 33

Photo 34

Eastern end of the Wick Moor sea defences (rock armour barrier and elevated road on embankment).

View westwards across the Wick Moor sea defences towards Hinkley Point.

Photo 35

Photo 36

Foreshore seawards of the Wick Moore sea defences, showing areas of exposed peat and in situ tree stump in foreground.

Exposed peat bed on the foreshore near Stolford.

Photo 37

Photo 38

View westwards towards Hinkley Point from the sea wall near Stolford.

Artificial shingle embankment, outer line of defence immediately east of Stolford.

Photo 39

Photo 40

Recent wave cliffing of the artificial shingle bank, immediately east of Stolford.

View towards Hinkley Point from the western end of the Steart Peninsula, with Phragmites and Spartina marsh in the foreground.

Photo 41

Photo 42

Shingle ridges on the central part of the Steart Peninsula; view looking west towards Hinkley Point.

Erosional mud mounds formed in former saltmarsh and high mudflat deposits, central Steart Peninsula.

Photo 43

Photo 44

Low angle aeolian sandsheet deposits, near Steart village; view looking north.

West bank of the River Parrett at Combwich.

Photo 45

Photo 46

Combwich jetty.

Recent foredune accretion near Brean.

Photo 47

Photo 48

View towards south-east, across the Brean foreshore, showing recent foredune accretion along the entire frontage.

The central part of the Brean frontage, showing ad-hoc sea defences and residential development along the former dune line.

Photo 49

Photo 50

The northern end of the Brean frontage, showing armourstone revetment and caravan parks behind.

Brean Down (Carboniferous limestone headland)

Appendix 5

Topographic profiles of the foreshore between Lilstock and Catsford Common, derived from lidar data flown 2007-2008

(profile locations shown in Figures 10 and 11)

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)

Distance along profile (m)

P1

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P2

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P3

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P4

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P5

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P6

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P7

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P8

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P9

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P10

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P11

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P12

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P13

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P14

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P15

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P16

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P17

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P18

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P19

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P20

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P21

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P22

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P23

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P24

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P25

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P26

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P27

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P28

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P29

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P30

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P31

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P32

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P33

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P34

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P35

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P36

-6

-3

0

3

6

9

12

15

18-1

00 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P37

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P38

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P39

-6

-3

0

3

6

9

12

15

18

-100 0

100

200

300

400

500

600

700

800

Elev

atio

n (m

OD

)


P40

5

6

7

8

9

10

11

12

13

-40

-30

-20

-10 0

10

20

Elev

atio

n (m

OD

)


P15

sea wall8.60 m OD

ground level11.20 m OD

5

6

7

8

9

10

11

12

13

-40

-30

-20

-10 0

10

20

Elev

atio

n (m

OD

)


P20

sea wall8.66 m OD

gabions11.07 m OD



5

6

7

8

9

10

11

12

13

-40

-30

-20

-10 0

10

20

Elev

atio

n (m

OD

)


P18

sea wall8.67 m OD

gabions11.49 m OD

building


5

6

7

8

9

10

11

12

13

-50

-40

-30

-20

-10 0

10

Elev

atio

n (m

OD

)


P22

sea wall8.43 m OD

embankment11.48 m OD


Enlargements of topographic profiles in the viscinity of Hinkley Point Power Station frontage, from lidar data flown 2007-2008, showing the elevations of sea defence structures.

5

6

7

8

9

10

11

12

13

-55

-45

-35

-25

-15 -5 5

Elev

atio

n (m

OD

)


P27

rock armour7.10 m OD

embankment8.70 m OD

marsh5.45 m OD

5

6

7

8

9

10

11

12

13

-50

-40

-30

-20

-10 0

10

Elev

atio

n (m

OD

)


P30

rock armour9.10 m ODembankment

8.87 m OD

marsh5.80 m OD


5

6

7

8

9

10

11

12

13

-220

-210

-200

-190

-180

-170

-160

Elev

atio

n (m

OD

)


P36 landward embankment

embankment8.14 m OD

ditch5.14 m OD

marsh6.30 m OD

5

6

7

8

9

10

11

12

13

-40

-30

-20

-10 0

10

20

Elev

atio

n (m

OD

)


P36 seaward gravel barrier

gravel barrier8.36 m OD

Appendix 6

Changes in the position of MHW and MLW between West Quantoxhead and Brean Down since 1886 shown on Ordnance Maps surveyed

in 1884-6, 1955-7 and 1960-76, and Environment Agency lidar survey data flown in 2007-8

(MHW and MLW taken as 4.63 and -3.84 m OD). Profile locations are shown in Figure 11.

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P1

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P2

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P3

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P4

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P5

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P6

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P7

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P8

Changes in the position of MHW since 1886 (Profiles 1 to 64)

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P9

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P10

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P11

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P12

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P13

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P14

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P15

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P16

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P17

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P18

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P19

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P20

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P21

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P22

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P23

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P24

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P25

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P26

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P27

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P28

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P29

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P30

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P31

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P32

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P33

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P34

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P35

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P36

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P37

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P38

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P39

-40.0

-20.0

0.0

20.0

40.0

60.0

80.0

100.0

120.0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P40

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P41

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P42

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P43

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P44

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P45

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

886

(m) P46

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P48

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P47

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P49

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P50

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P51

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P52

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P53

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P54

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P55

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P56

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P57

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P58

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P59

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P60

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P61

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P62

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P63

-40

0

40

80

120

160

200

240

280

320

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

HW

in 1

884

(m) P64

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P1-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P2

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P3-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P4

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P5-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P6

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P7-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P8

Changes in the position of MLW since 1886 (Profiles 1 to 64)

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P9-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P10

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P11-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P12

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P13-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P14

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P15-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P16

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P17-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P18

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P19-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P20

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P21-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P22

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P23-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P24

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P25-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P26

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P27-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P28

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P29-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P30

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P31-900

-800

-700

-600

-500

-400

-300

-200

-100

0

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

)

P32

NB: Measurements are not possible for Profiles 33-40 since profiles do not intersect with the position of the low water channel in 1886.

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

) P41

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

) P42

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

) P43

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

) P44

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

) P45

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

6 (m

) P46

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P47

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P48

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P49

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P50

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P51

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P52

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P53

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P54

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P55

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P56

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P57

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P58

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P59

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P60

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P61

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P62

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P63

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

200

1886 1906 1926 1946 1966 1986 2006

Dis

tanc

e fr

om M

LW in

188

4 (m

) P64

Appendix 7

Comparison of Environment Agency lidar data flown on 26/10/07 with nine ground topographic

profiles surveyed on 27/11/07 near Stolford. Data source: Channel Coastal Observatory.

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

100 130 160 190 220 250 280 310 340 370 400 430 460

Elev

atio

n (m

OD

)


Profile 7d01906

Topo survey 27/11/07

Lidar survey 26/10/07

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

0 30 60 90 120 150 180 210 240 270 300 330 360

Elev

atio

n (m

OD

)


Profile 7d01910



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

0 30 60 90 120 150 180 210 240 270 300 330 360

Elev

atio

n (m

OD

)


Profile 7d01915



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

0 30 60 90 120 150 180 210 240 270 300 330 360

Elev

atio

n (m

OD

)


Profile 7d01919



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

0 30 60 90 120 150 180 210 240 270 300 330 360

Elev

atio

n (m

OD

)


Profile 7d01923



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

0 30 60 90 120 150 180 210 240 270 300 330 360

Elev

atio

n (m

OD

)


Profile 7d01927



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

0 30 60 90 120 150 180 210 240 270 300 330 360

Elev

atio

n (m

OD

)


Profile 7d01931



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

70 100 130 160 190 220 250 280 310 340 370 400 430

Elev

atio

n (m

OD

)


Profile 7d01935



-2.00

0.00

2.00

4.00

6.00

8.00

10.00

70 100 130 160 190 220 250 280 310 340 370 400 430

Elev

atio

n (m

OD

)


Profile 7d01939



Appendix 8

Highest 20 recorded water levels, predicted high tides, and skew surges at five Class A Tide Stations in the Bristol Channel (Milford Haven,

Ilfracombe, Mumbles, Newport and Avonmouth). Values for Hinkley Point are shown in

Tables 7 to 10 of the main report.

Table A8.1 The 20 highest predicted tides at Milford Haven in the period 1953-2008 (excluding BODC 'Improbable Values').



18/09/1997 19:15 4.10 4.10 -0.01 -0.01 -0.01

07/10/2002 18:45 4.08 4.09 0.02 0.02 0.01

24/09/1953 19:00 4.06 4.35 0.29 0.29 0.29

17/09/1997 18:30 4.06 4.11 0.05 0.05 0.04

25/09/1980 19:00 4.05 3.91 -0.15 -0.15 -0.15

10/03/1993 07:30 4.04 4.01 -0.02 -0.04 -0.03

17/09/1993 19:00 4.04 4.07 0.03 0.03 0.03

07/09/1979 19:00 4.04 4.04 -0.01 -0.01 -0.01

18/03/1980 07:00 4.04 3.85 -0.19 -0.19 -0.19

09/09/2006 19:15 4.04 4.15 0.11 0.11 0.11

29/03/1998 07:00 4.04 4.10 0.11 0.05 0.06

09/02/1997 07:15 4.02 4.06 0.03 0.03 0.03

08/10/2006 19:00 4.02 4.40 0.40 0.38 0.38

17/10/1997 19:00 4.02 4.23 0.22 0.22 0.22

30/03/1998 07:45 4.02 4.20 0.20 0.17 0.18

30/03/2002 07:15 4.02 3.99 -0.03 -0.03 -0.03

18/09/2001 19:00 4.01 3.90 -0.11 -0.11 -0.11

10/02/1997 08:00 4.01 4.41 0.41 0.39 0.40

16/10/1997 18:15 4.01 4.20 0.19 0.19 0.19

29/08/1992 19:00 4.01 4.33 0.32 0.32 0.32

Table A8.2 The 20 highest observed tides at Milford Haven in the period 1953-2008 (excluding BODC 'Improbable Values').



10/03/2008 07:45 3.69 4.47 0.78 0.78 0.78

10/02/1997 08:15 4.01 4.41 0.41 0.39 0.40

08/10/2006 18:45 4.02 4.40 0.40 0.38 0.38

23/09/1953 18:00 3.95 4.38 0.43 0.43 0.43

30/03/2006 06:30 3.95 4.36 0.45 0.42 0.42

07/03/1962 07:00 3.96 4.35 0.39 0.39 0.39

24/09/1953 19:00 4.06 4.35 0.29 0.29 0.29

29/08/1992 19:00 4.01 4.33 0.32 0.32 0.32

24/10/1961 06:00 3.67 4.32 0.65 0.65 0.65

01/02/2002 08:30 3.67 4.31 0.65 0.60 0.64

13/12/1981 19:00 3.50 4.27 1.00 0.60 0.77

24/10/1995 18:15 3.61 4.27 0.67 0.67 0.67

07/03/1962 19:00 3.81 4.26 0.45 0.45 0.45

24/10/1961 18:00 3.74 4.26 0.52 0.52 0.52

06/03/1954 19:00 3.68 4.26 0.58 0.58 0.58

05/10/1967 19:00 3.99 4.25 0.26 0.26 0.26

01/02/1979 09:00 3.66 4.24 0.58 0.58 0.58

09/02/1974 08:00 3.92 4.24 0.32 0.32 0.32

08/02/1974 07:00 3.91 4.23 0.32 0.32 0.32

17/10/1997 19:00 4.02 4.23 0.22 0.22 0.22

Table A8.3 The 20 largest high water skew surges recorded at Milford Haven in the period 1953-2008 (excluding BODC 'Improbable Values'),

ordered in terms of high water skew surge (SKHW).



16/12/1989 21:00 2.72 3.92 1.20 1.20 1.20

04/01/1998 09:45 2.97 4.04 1.13 1.00 1.07

17/01/1969 18:00 2.87 3.91 1.04 1.04 1.04

03/12/2006 03:45 2.83 3.87 1.15 0.87 1.03

27/01/1974 21:00 2.69 3.68 0.99 0.99 0.99

16/01/1974 12:00 1.96 2.90 0.93 0.93 0.93

19/02/1997 16:45 2.26 3.16 0.90 0.90 0.90

15/03/1977 15:00 1.85 2.72 0.86 0.86 0.86

21/12/1989 00:00 1.57 2.43 0.85 0.85 0.85

17/01/1962 04:00 2.08 2.92 0.84 0.84 0.84

16/12/1989 08:00 3.04 3.88 0.84 0.84 0.84

24/03/1986 05:00 2.64 3.48 0.84 0.84 0.84

24/12/1997 14:15 1.65 2.48 0.85 0.83 0.84

16/09/1961 09:00 2.34 3.16 0.85 0.76 0.82

27/03/2006 04:30 2.77 3.59 0.82 0.82 0.82

15/01/1962 14:00 2.08 2.89 0.80 0.80 0.80

23/01/2002 13:00 1.54 2.32 0.78 0.78 0.78

10/03/2008 07:45 3.69 4.47 0.78 0.78 0.78

13/12/1981 19:00 3.50 4.27 1.00 0.60 0.77

12/12/1978 17:00 2.84 3.60 0.76 0.76 0.76

Table A8.4 The 20 highest predicted tides at Ilfracombe in the period 1968-2008 (excluding BODC 'Improbable Values').



18/09/1997 19:00 4.42 4.38 -0.04 -0.04 -0.04

07/10/1998 19:00 4.37 4.25 -0.12 -0.12 -0.12

17/09/1997 18:15 4.36 4.36 0.00 -0.01 0.00

10/03/1993 07:15 4.36 4.36 0.01 0.01 0.01

07/10/2002 18:30 4.36 4.32 -0.03 -0.03 -0.03

26/09/1984 19:00 4.36 4.51 0.15 0.15 0.15

29/03/1998 06:45 4.36 4.42 0.06 0.06 0.06

30/03/1998 07:30 4.35 4.50 0.16 0.16 0.16

17/09/1993 18:45 4.34 4.42 0.09 0.09 0.09

09/09/2006 19:15 4.33 4.38 0.05 0.04 0.04

17/10/1997 18:45 4.32 4.41 0.10 0.10 0.10

06/10/1998 18:15 4.32 4.23 -0.09 -0.09 -0.09

09/02/1997 07:00 4.31 4.29 -0.02 -0.03 -0.02

10/02/1997 07:45 4.31 4.61 0.31 0.29 0.31

09/03/1989 07:00 4.31 4.57 0.26 0.26 0.26

27/09/1988 19:00 4.31 4.46 0.15 0.15 0.15

16/10/1997 18:00 4.31 4.37 0.06 0.06 0.06

08/10/2006 18:45 4.30 4.57 0.27 0.27 0.27

29/08/1992 19:00 4.29 4.46 0.16 0.16 0.16

30/08/1996 19:15 4.29 4.17 -0.12 -0.12 -0.12

Table A8.5 The 20 highest observed tides at Ilfracombe in the period 1968-2008 (excluding BODC 'Improbable Values').



07/04/1985 07:00 4.21 4.65 0.44 0.44 0.44

10/02/1997 08:00 4.31 4.61 0.31 0.29 0.31

09/03/1989 07:00 4.31 4.57 0.26 0.26 0.26

08/10/2006 18:45 4.30 4.57 0.27 0.27 0.27

30/03/2006 06:30 4.23 4.55 0.32 0.32 0.32

26/09/1984 19:00 4.36 4.51 0.15 0.15 0.15

30/03/1998 07:30 4.35 4.50 0.16 0.16 0.16

10/03/2008 07:30 3.88 4.50 0.62 0.60 0.62

27/09/1988 19:00 4.31 4.46 0.15 0.15 0.15

23/11/1984 18:00 3.73 4.46 0.73 0.73 0.73

29/08/1992 19:00 4.29 4.46 0.16 0.16 0.16

29/03/1998 19:00 4.22 4.42 0.20 0.20 0.20

17/09/1993 18:45 4.34 4.42 0.09 0.09 0.09

07/10/1987 18:00 4.02 4.42 0.40 0.40 0.40

29/03/1998 06:45 4.36 4.42 0.06 0.06 0.06

17/10/1997 18:45 4.32 4.41 0.10 0.10 0.10

24/10/1984 18:00 4.12 4.41 0.29 0.29 0.29

07/10/2006 18:00 4.27 4.39 0.12 0.12 0.12

19/03/1988 07:00 4.24 4.39 0.15 0.15 0.15

18/09/1997 19:00 4.42 4.38 -0.04 -0.04 -0.04

Table A8.6 The 20 largest high water skew surges recorded at Ilfracombe in the period 1968-2008 (excluding BODC 'Improbable Values'),




17/01/1969 17:00 2.84 3.94 1.11 1.11 1.11

16/12/1989 21:00 2.65 3.63 0.98 0.98 0.98

24/03/1986 05:00 2.55 3.49 0.95 0.95 0.95

03/12/2006 03:45 2.81 3.73 0.96 0.90 0.92

16/12/1989 08:00 3.11 3.97 0.86 0.86 0.86

01/02/1988 05:00 2.21 3.00 0.79 0.79 0.79

11/02/1995 15:45 1.26 2.05 0.82 0.75 0.78

21/12/1989 00:00 1.19 1.95 0.75 0.75 0.75

23/11/1984 18:00 3.73 4.46 0.73 0.73 0.73

08/01/2005 03:30 2.20 2.93 0.73 0.73 0.73

06/01/1988 07:00 2.72 3.43 0.71 0.71 0.71

20/12/1989 11:00 1.58 2.28 0.70 0.70 0.70

17/12/1989 09:00 2.79 3.46 0.67 0.67 0.67

16/02/1995 18:30 3.37 4.04 0.74 0.64 0.67

01/01/1991 18:00 3.36 4.03 0.67 0.67 0.67

18/12/1968 04:00 2.76 3.43 0.67 0.67 0.67

09/11/1969 05:00 3.10 3.76 0.66 0.66 0.66

02/01/1984 18:00 2.59 3.24 0.65 0.65 0.65

11/04/1989 10:00 2.26 2.91 0.75 0.51 0.64

11/01/1996 21:15 2.27 2.91 0.63 0.63 0.63

Table A8.7 The 20 highest predicted tides at Mumbles in the period 1988-2008 (excluding BODC 'Improbable Values').



18/09/1997 19:30 4.72 4.64 -0.08 -0.08 -0.08

07/10/1998 19:15 4.68 4.53 -0.15 -0.15 -0.15

10/03/1993 07:30 4.67 4.63 -0.04 -0.04 -0.04

09/09/2006 19:30 4.66 4.59 -0.06 -0.06 -0.06

07/10/2002 18:45 4.65 4.57 -0.08 -0.09 -0.09

02/03/2006 08:00 4.65 4.55 -0.09 -0.11 -0.10

17/09/1997 18:45 4.63 4.62 -0.01 -0.01 -0.01

01/03/1998 08:00 4.63 4.60 -0.03 -0.03 -0.03

10/03/1989 08:00 4.63 4.58 -0.04 -0.04 -0.04

30/03/2002 07:15 4.62 4.50 -0.12 -0.12 -0.12

08/10/2002 19:30 4.61 4.59 -0.02 -0.02 -0.02

09/02/1993 07:45 4.61 4.44 -0.16 -0.16 -0.16

17/10/1997 19:00 4.60 4.66 0.06 0.06 0.06

08/10/2006 19:00 4.60 4.82 0.22 0.22 0.22

06/10/1998 18:30 4.59 4.47 -0.13 -0.13 -0.13

01/03/2002 07:45 4.59 4.54 -0.05 -0.05 -0.05

29/08/1992 19:00 4.58 4.82 0.23 0.23 0.23

18/09/2001 19:00 4.58 4.40 -0.19 -0.19 -0.19

16/10/1997 18:15 4.58 4.63 0.05 0.05 0.05

09/03/1989 07:00 4.57 4.79 0.21 0.21 0.21

Table A8.8 The 20 highest observed tides at Mumbles in the period 1988-2008 (excluding BODC 'Improbable Values').



08/10/2006 19:00 4.60 4.82 0.22 0.22 0.22

29/08/1992 19:00 4.58 4.82 0.23 0.23 0.23

30/03/2006 06:45 4.51 4.81 0.30 0.30 0.30

09/03/1989 07:00 4.57 4.79 0.21 0.21 0.21

10/03/2001 06:45 4.32 4.73 0.41 0.41 0.40

30/08/1992 20:00 4.53 4.72 0.18 0.18 0.18

01/02/2002 08:45 4.24 4.69 0.44 0.44 0.44

11/03/2001 07:30 4.48 4.68 0.20 0.20 0.20

08/09/1998 19:30 4.53 4.67 0.16 0.14 0.15

17/10/1997 19:00 4.60 4.66 0.06 0.06 0.06

18/09/1997 19:15 4.72 4.64 -0.08 -0.08 -0.08

16/10/1997 18:15 4.58 4.63 0.05 0.05 0.05

10/03/1993 07:30 4.67 4.63 -0.04 -0.04 -0.04

17/10/2001 18:30 4.48 4.62 0.14 0.14 0.14

17/09/1997 18:45 4.63 4.62 -0.01 -0.01 -0.01

07/10/2006 18:15 4.55 4.62 0.06 0.06 0.06

30/08/1992 08:00 4.24 4.61 0.38 0.38 0.38

05/11/1998 19:00 4.56 4.61 0.05 0.05 0.05

01/03/1998 08:00 4.63 4.60 -0.03 -0.03 -0.03

07/04/1989 07:00 4.50 4.60 0.10 0.10 0.10

Table A8.9 The 20 largest high water skew surges recorded at Mumbles in the period 1988-2008 (excluding BODC 'Improbable Values'),




04/01/1998 10:15 3.31 4.56 1.26 1.26 1.26

16/12/1989 21:00 2.97 4.03 1.06 1.06 1.06

16/12/1989 09:00 3.31 4.18 0.88 0.88 0.88

03/12/2006 04:00 3.20 4.05 0.92 0.61 0.86

08/12/2000 03:30 2.29 3.13 0.83 0.83 0.83

21/12/1989 00:00 1.52 2.30 0.77 0.77 0.77

01/01/1991 19:00 3.72 4.46 0.74 0.74 0.74

17/12/1989 09:00 3.05 3.79 0.74 0.74 0.74

08/01/2005 03:45 2.51 3.23 0.73 0.67 0.72

11/04/1989 10:00 2.60 3.29 0.69 0.69 0.69

20/12/1989 11:00 1.88 2.55 0.69 0.65 0.67

24/12/1997 14:00 1.60 2.26 0.66 0.66 0.66

25/11/2000 17:45 3.59 4.22 0.63 0.56 0.63

27/03/2006 04:45 3.03 3.64 0.63 0.61 0.62

23/01/2002 13:00 1.48 2.10 0.61 0.61 0.61

17/01/1993 01:00 1.93 2.53 0.60 0.56 0.60

06/12/2000 01:15 1.31 1.90 0.59 0.59 0.59

31/12/2000 21:45 2.24 2.82 0.59 0.57 0.58

29/11/2000 08:00 3.18 3.75 0.57 0.57 0.57

03/01/1999 06:45 3.96 4.53 0.57 0.57 0.57

Table A8.10 The 20 highest predicted tides at Newport in the period 1993-2008 (excluding BODC 'Improbable Values').



18/09/1997 20:15 7.51 7.53 0.06 0.02 0.03

11/03/1997 08:45 7.43 7.37 -0.07 -0.07 -0.07

17/09/1997 19:30 7.42 7.49 0.08 0.02 0.06

10/03/1997 08:00 7.41 7.44 0.03 0.03 0.03

07/10/1998 20:15 7.41 7.41 0.00 0.00 0.00

10/02/1997 09:00 7.40 7.83 0.43 0.43 0.43

30/03/1998 08:45 7.38 7.67 0.29 0.29 0.29

07/10/2002 19:45 7.36 7.36 0.00 0.00 0.00

30/08/1996 20:15 7.36 7.31 -0.05 -0.12 -0.05

01/03/1998 09:00 7.36 7.66 0.30 0.30 0.30

09/02/1997 08:15 7.36 7.45 0.09 0.09 0.09

29/03/1998 08:00 7.36 7.54 0.18 0.18 0.18

20/08/1997 20:45 7.35 7.36 0.00 0.00 0.00

17/09/1993 20:00 7.35 7.57 0.23 0.23 0.23

02/03/2006 08:45 7.34 7.55 0.21 0.21 0.21

17/10/1997 20:00 7.34 7.50 0.15 0.15 0.15

09/09/2006 20:15 7.34 7.63 0.29 0.29 0.29

06/10/1998 19:30 7.34 7.26 -0.08 -0.08 -0.08

18/09/2001 20:00 7.33 7.20 -0.14 -0.14 -0.14

30/03/2002 08:15 7.31 7.31 -0.01 -0.01 -0.01

Table A8.11 The 20 highest observed tides at Newport in the period 1993-2008 (excluding BODC 'Improbable Values').



10/02/1997 09:00 7.40 7.83 0.43 0.43 0.43

08/10/2006 20:00 7.25 7.81 0.56 0.56 0.56

30/03/2006 07:45 7.17 7.79 0.62 0.62 0.62

04/12/1994 07:45 6.93 7.78 0.85 0.85 0.85

24/12/1999 20:15 6.74 7.74 1.05 0.95 1.00

28/10/1996 20:15 6.64 7.71 1.08 1.08 1.08

03/01/1999 07:45 6.57 7.69 1.12 1.12 1.12

07/10/2006 19:15 7.23 7.68 0.45 0.45 0.45

16/02/1995 19:45 6.21 7.68 1.53 1.37 1.46

30/03/1998 08:45 7.38 7.67 0.29 0.29 0.29

01/03/1998 09:00 7.36 7.66 0.30 0.30 0.30

09/09/2006 20:15 7.34 7.63 0.29 0.29 0.29

31/03/2006 08:15 7.16 7.62 0.46 0.46 0.46

29/03/1998 20:15 7.28 7.61 0.33 0.33 0.33

24/02/1997 20:45 5.86 7.61 1.78 1.70 1.75

17/09/1993 20:00 7.35 7.57 0.23 0.23 0.23

02/03/2006 08:45 7.34 7.55 0.21 0.21 0.21

30/03/2006 20:00 7.00 7.55 0.55 0.55 0.55

29/03/1998 08:00 7.36 7.54 0.18 0.18 0.18

18/09/1997 20:30 7.51 7.53 0.06 0.02 0.03

Table A8.12 The 20 largest high water skew surges recorded at Newport in the period 1993-2008 (excluding BODC 'Improbable Values'),




24/02/1997 20:45 5.86 7.61 1.78 1.70 1.75

03/12/2006 05:15 5.35 6.89 1.54 1.54 1.54

04/01/1998 11:15 5.68 7.15 1.54 1.40 1.47

16/02/1995 19:45 6.21 7.68 1.53 1.37 1.46

08/01/2005 04:45 4.61 5.79 1.18 1.18 1.18

20/10/2004 23:30 3.63 4.81 1.18 1.18 1.18

03/01/1999 07:45 6.57 7.69 1.12 1.12 1.12

19/02/1997 18:00 4.52 5.61 1.20 0.86 1.09

28/10/1996 20:15 6.64 7.71 1.08 1.08 1.08

25/12/1997 15:30 3.64 4.71 1.07 1.07 1.07

06/11/1996 03:00 2.90 3.95 1.05 1.00 1.05

24/12/1999 20:15 6.74 7.74 1.05 0.95 1.00

04/12/2006 18:30 5.91 6.87 0.96 0.96 0.96

31/03/1994 22:00 6.18 7.11 0.92 0.92 0.92

11/02/1995 16:15 3.25 4.15 0.91 0.91 0.91

24/10/1998 20:45 5.36 6.27 1.03 0.84 0.91

10/12/1993 04:00 4.61 5.48 0.94 0.87 0.87

04/12/1994 07:45 6.93 7.78 0.85 0.85 0.85

05/12/2006 19:15 5.98 6.83 0.84 0.84 0.84

25/11/2000 18:45 6.00 6.84 0.84 0.84 0.84

Table A8.13 The 20 highest predicted tides at Avonmouth in the period 1961-2008 (excluding BODC 'Improbable Values').



18/09/1997 20:30 8.10 8.10 0.00 0.00 0.00

11/03/1997 08:45 8.08 7.92 -0.10 -0.20 -0.15

30/03/1998 08:45 8.03 8.28 0.25 0.25 0.25

10/03/1997 08:00 8.02 8.01 0.01 -0.07 0.00

10/02/1997 09:15 8.00 8.43 0.43 0.43 0.43

07/10/1998 20:15 7.99 7.98 0.00 -0.06 -0.02

29/03/1998 08:00 7.99 8.08 0.15 0.07 0.08

17/09/1997 19:45 7.99 8.03 0.04 0.04 0.04

01/03/1998 09:00 7.97 8.07 0.13 0.08 0.10

25/09/1961 20:00 7.96 7.95 -0.01 -0.01 -0.01

09/02/1997 08:30 7.96 8.01 0.05 0.05 0.05

28/02/1975 09:00 7.95 7.87 -0.08 -0.08 -0.08

30/08/1996 20:30 7.95 7.81 -0.02 -0.14 -0.14

30/03/2002 08:15 7.94 7.84 -0.07 -0.11 -0.11

27/02/1975 08:00 7.94 7.83 -0.11 -0.11 -0.11

17/10/1997 20:00 7.94 8.04 0.15 0.06 0.10

09/09/2006 20:30 7.93 8.14 0.21 0.21 0.21

28/08/1961 21:00 7.93 7.98 0.06 0.06 0.06

09/02/1993 08:45 7.92 7.83 -0.05 -0.13 -0.09

10/03/1997 20:30 7.92 7.75 -0.17 -0.17 -0.17

Table A8.14 The 20 highest observed tides at Avonmouth in the period 1961-2008 (excluding BODC 'Improbable Values').



13/12/1981 21:00 7.14 8.93 1.79 1.79 1.79

26/02/1990 08:00 7.12 8.50 1.38 1.38 1.38

04/12/1994 08:00 7.49 8.47 1.05 0.87 0.99

10/02/1997 09:15 8.00 8.43 0.43 0.43 0.43

30/03/2006 07:45 7.77 8.35 0.58 0.58 0.58

28/10/1996 20:15 7.19 8.34 1.15 1.05 1.15

08/10/2006 20:00 7.84 8.30 0.46 0.46 0.46

24/02/1997 20:45 6.34 8.29 1.95 1.95 1.95

30/03/1998 08:45 8.03 8.28 0.25 0.25 0.25

30/01/1975 09:00 7.87 8.23 0.36 0.36 0.36

09/02/1974 09:00 7.78 8.22 0.44 0.44 0.44

29/08/1992 20:00 7.86 8.22 0.36 0.36 0.36

07/04/1962 09:00 7.82 8.20 0.38 0.38 0.38

07/10/2006 19:30 7.75 8.20 0.44 0.44 0.44

30/08/1992 21:00 7.84 8.17 0.33 0.33 0.33

08/02/1974 08:00 7.69 8.17 0.48 0.48 0.48

29/03/1998 20:30 7.91 8.17 0.26 0.26 0.26

03/01/1999 07:45 7.10 8.16 1.06 1.06 1.06

09/09/2006 20:30 7.93 8.14 0.21 0.21 0.21

05/11/1998 20:00 7.87 8.14 0.27 0.27 0.27

Table A8.15 The 20 largest high water skew surges recorded at Avonmouth in the period 1961-2008 (excluding BODC 'Improbable Values'),




24/02/1997 20:45 6.34 8.29 1.95 1.95 1.95

13/12/1981 21:00 7.14 8.93 1.79 1.79 1.79

03/12/2006 05:15 5.75 7.36 1.61 1.61 1.61

06/01/1988 08:00 5.42 6.94 1.53 1.16 1.52

04/01/1998 11:15 6.10 7.58 1.50 1.41 1.49

16/09/1961 11:00 4.77 6.16 1.39 1.39 1.39

26/02/1990 08:00 7.12 8.50 1.38 1.38 1.38

20/10/2004 23:30 3.99 5.36 1.39 1.30 1.37

18/01/2007 06:15 5.59 6.94 1.38 1.28 1.35

07/02/1990 05:00 4.46 5.80 1.34 1.34 1.34

25/12/1997 15:45 3.96 5.18 1.22 1.22 1.22

06/11/1996 03:15 3.20 4.38 1.22 1.13 1.18

28/10/1996 20:15 7.19 8.34 1.15 1.05 1.15

06/02/1990 16:00 3.87 5.02 1.15 1.15 1.15

19/02/1997 18:15 4.99 6.13 1.40 0.89 1.15

16/12/1989 09:00 6.02 7.15 1.14 1.14 1.14

28/02/2007 17:15 4.12 5.25 1.13 1.09 1.13

15/12/1979 04:00 3.80 4.90 1.10 1.10 1.10

07/02/1990 17:00 4.74 5.82 1.34 0.81 1.08

31/03/1994 22:15 6.65 7.71 1.06 1.06 1.06

Appendix 9

Future changes in sea level, skew surges and selected climate parameters at Hinkley Point,

projected by UKCP09

Absolute sea level rise for Relative sea level rise forthe UK since 1990 (cm) Hinkley Point since 1990 (cm)5% 50% 95% 5% 50% 95%

Low emissionsscenario (SRES B1)2008 1.3 3.4 5.5 2.7 4.8 6.92020 2.4 6.1 9.8 4.6 8.4 12.12050 5.5 14.1 22.8 10.0 18.7 27.32080 9.4 24.1 38.8 16.2 30.9 45.62100 12.4 31.8 51.3 20.7 40.1 59.5

Medium emissionsscenario (SRES A1B1)2008 1.5 4.2 7.0 2.9 5.6 8.32020 2.7 7.6 12.4 4.9 9.8 14.72050 6.2 17.5 28.8 10.7 22.0 33.32080 10.6 29.8 49.1 17.4 36.6 55.92100 14.0 39.4 64.8 22.2 47.7 73.1

High emissionsscenario (SRES A1FI)2008 1.8 5.2 8.7 3.1 6.6 10.12020 3.1 9.3 15.5 5.4 11.6 17.82050 7.3 21.6 36.0 11.8 26.1 40.52080 12.4 36.9 61.3 19.2 43.6 68.12100 16.4 48.7 81.0 24.7 57.0 89.2

Projected future changes in absolute sea level around the coast of the UK, and changes in relative sea level at Hinkley Point (Grid Cell 24092), which assumes a local land subsidence rate of c. -0.76 mm/year. Data source: UK Climate Impacts Programme (UKCP09)

Table A9.1

Long-term linear trend in skew surge(1951-2099) (mm/yr)5% 50% 95%

Medium emissionsscenario (SRES A1B1)2 year return level 0.249 0.345 0.44210 year return level 0.424 0.589 0.75420 year return level 0.486 0.679 0.87150 year return level 0.565 0.793 1.021

Projections of future trends in skew surges at Hinkley Point. Data source: UK Climate Impacts Programme (UKCP09)

Table A9.2

5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95%

1961-1990 baseline: 9.77 4.68 8.38 15.35 10.58

Low emissions scenario (SRES B1)2010-2039 0.63 1.42 2.35 0.29 1.19 2.19 0.49 1.25 2.17 0.45 1.62 3.04 0.67 1.61 2.622020-2049 0.76 1.70 2.82 0.37 1.46 2.68 0.59 1.44 2.50 0.48 1.94 3.68 0.83 1.89 3.062030-2059 0.95 1.98 3.23 0.60 1.67 2.93 0.72 1.65 2.87 0.62 2.26 4.26 0.99 2.16 3.472040-2069 1.03 2.15 3.57 0.58 1.81 3.25 0.79 1.75 3.08 0.71 2.46 4.67 1.09 2.33 3.752050-2079 1.16 2.41 3.99 0.75 2.02 3.55 1.06 2.05 3.49 0.83 2.69 5.10 1.19 2.53 4.112060-2089 1.24 2.60 4.32 0.88 2.21 3.85 1.16 2.21 3.78 0.84 2.84 5.45 1.28 2.69 4.372070-2099 1.37 2.76 4.58 1.12 2.44 4.12 1.23 2.35 4.03 0.93 2.94 5.66 1.36 2.86 4.66

Medium emissions scenario (SRES A1B1)2010-2039 0.56 1.40 2.40 0.39 1.27 2.26 0.52 1.22 2.12 0.25 1.56 3.10 0.54 1.55 2.632020-2049 0.76 1.74 2.94 0.42 1.49 2.71 0.71 1.48 2.56 0.42 1.98 3.87 0.87 1.92 3.112030-2059 0.98 2.08 3.48 0.70 1.78 3.08 0.87 1.77 3.07 0.59 2.30 4.48 1.15 2.30 3.652040-2069 1.28 2.47 4.06 0.90 2.07 3.54 1.04 2.08 3.59 0.92 2.75 5.20 1.39 2.70 4.262050-2079 1.46 2.81 4.65 1.09 2.37 4.01 1.17 2.37 4.10 1.12 3.09 5.84 1.59 3.07 4.852060-2089 1.67 3.14 5.17 1.24 2.62 4.43 1.33 2.63 4.56 1.42 3.50 6.52 1.76 3.35 5.322070-2099 1.85 3.47 5.71 1.31 2.82 4.81 1.51 2.93 5.04 1.66 3.93 7.26 1.98 3.68 5.82

High emissions scenario (SRES A1FI)2010-2039 0.60 1.39 2.33 0.24 1.22 2.29 0.47 1.23 2.15 0.25 1.48 2.95 0.54 1.60 2.732020-2049 0.87 1.80 2.95 0.48 1.51 2.70 0.65 1.56 2.72 0.55 1.99 3.79 0.89 2.02 3.282030-2059 1.14 2.22 3.61 0.80 1.93 3.30 0.97 1.95 3.30 0.73 2.41 4.59 1.08 2.36 3.832040-2069 1.44 2.75 4.45 1.08 2.29 3.86 1.19 2.36 4.01 1.00 3.08 5.76 1.47 2.88 4.562050-2079 1.76 3.27 5.28 1.34 2.68 4.51 1.46 2.76 4.69 1.31 3.65 6.77 1.84 3.48 5.462060-2089 2.05 3.80 6.15 1.49 2.99 5.08 1.75 3.21 5.44 1.68 4.30 7.88 2.18 4.07 6.372070-2099 2.43 4.33 6.96 1.76 3.39 5.73 2.01 3.67 6.20 2.16 4.97 8.96 2.54 4.59 7.14

Annual mean Winter mean (DJF) Spring mean (MAM) Summer mean (JJA) Autumn mean (SON)

Predictions of future changes in mean air temperature (at 1.5 m in °C) at Hinkley Point (Grid Cell 1619), relative to the 1961-1990 baseline. Data source: UK Climate Impacts Programme (UKCP09)

Table A9.3

5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95%

1961-1990 baseline (mm/day): 2.41 3.04 2.07 1.90 2.66

Low emissions scenario (SRES B1) (% change)2010-2039 -4.74 0.91 6.98 -5.73 4.17 15.68 -8.68 0.95 11.65 -31.3 -7.5 21.5 -8.1 4.4 18.82020-2049 -4.70 0.97 7.07 -4.67 5.48 17.73 -8.20 1.73 12.83 -35.5 -9.0 24.3 -8.8 3.7 18.32030-2059 -4.63 0.98 7.02 -4.38 6.35 19.77 -8.26 1.78 13.04 -39.6 -11.3 24.7 -8.7 3.7 18.02040-2069 -5.45 0.27 6.38 -4.12 7.37 22.18 -9.42 0.68 11.95 -46.9 -15.2 29.1 -10.9 2.7 18.62050-2079 -5.15 0.50 6.55 -3.99 8.55 25.06 -11.34 0.06 12.94 -49.2 -17.2 27.3 -11.8 2.3 19.02060-2089 -4.44 0.95 6.74 -3.64 8.89 26.03 -11.30 0.05 12.88 -52.3 -18.4 31.0 -12.4 2.3 19.82070-2099 -4.23 1.45 7.63 -3.28 9.61 27.69 -12.49 0.31 15.02 -49.2 -16.8 24.7 -11.3 2.2 18.2

Medium emissions scenario (SRES A1B1) (% change)2010-2039 -5.79 0.19 6.60 -10.73 -0.58 10.68 -10.73 -0.58 10.68 -33.6 -8.4 23.1 -7.4 2.4 13.42020-2049 -5.39 0.40 6.63 -10.99 -0.82 10.47 -10.99 -0.82 10.47 -37.9 -10.8 23.2 -6.8 3.2 14.42030-2059 -5.68 0.21 6.54 -11.55 -1.29 10.09 -11.55 -1.29 10.09 -43.4 -14.7 21.7 -7.2 2.4 13.02040-2069 -6.30 0.07 6.96 -10.07 -0.41 10.27 -10.07 -0.41 10.27 -50.3 -20.9 17.3 -7.4 2.6 13.72050-2079 -5.85 0.52 7.46 -9.37 0.72 11.97 -9.37 0.72 11.97 -54.8 -23.2 20.0 -6.3 3.5 14.42060-2089 -6.06 0.32 7.29 -9.40 0.97 12.61 -9.40 0.97 12.61 -56.5 -24.7 16.9 -6.6 3.8 15.52070-2099 -6.06 0.46 7.64 -11.29 0.12 13.01 -11.29 0.12 13.01 -58.7 -25.5 18.4 -5.9 4.6 16.5

High emissions scenario (SRES A1FI) (% change)2010-2039 -5.89 0.31 6.98 -4.69 4.88 16.07 -9.94 0.04 11.14 -31.0 -5.0 27.4 -9.4 1.8 14.62020-2049 -6.42 0.12 7.17 -4.79 5.92 18.94 -10.99 -0.07 12.19 -38.8 -11.9 21.0 -9.2 2.6 16.22030-2059 -6.19 0.38 7.54 -3.47 8.47 23.68 -11.27 -0.18 12.30 -43.1 -14.4 19.1 -7.7 3.1 15.32040-2069 -6.83 -0.03 7.39 -3.92 10.00 28.47 -10.24 -0.38 10.56 -53.0 -21.4 19.9 -7.1 4.4 17.52050-2079 -7.17 0.07 8.04 -2.54 12.55 33.78 -9.95 0.21 11.53 -59.0 -24.8 23.5 -6.3 4.7 17.42060-2089 -7.00 0.52 8.94 -3.76 13.51 38.66 -9.47 0.50 11.61 -63.5 -28.0 22.6 -7.4 5.2 19.72070-2099 -8.16 0.56 10.44 -4.91 14.85 44.36 -10.54 0.67 13.33 -67.5 -31.8 17.7 -6.9 4.9 18.6


Predictions of future changes in precipitation (in % change from 1961-1990 baseline values in mm/day) at Hinkley Point (Grid Cell 1619). Data source: UK Climate Impacts Programme (UKCP09)

Table A9.4

5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95%

1961-1990 baseline: 880 1108 757 692 970

Low emissions scenario (SRES B1)2010-2039 -42 8 61 -63 46 174 -66 7 88 -217 -52 149 -79 42 1822020-2049 -41 9 62 -52 61 196 -62 13 97 -246 -62 168 -86 36 1772030-2059 -41 9 62 -49 70 219 -62 13 99 -274 -78 171 -84 35 1752040-2069 -48 2 56 -46 82 246 -71 5 90 -324 -106 202 -106 26 1812050-2079 -45 4 58 -44 95 278 -86 0 98 -340 -119 189 -114 22 1842060-2089 -39 8 59 -40 99 288 -86 0 97 -362 -128 215 -120 22 1922070-2099 -37 13 67 -36 106 307 -95 2 114 -340 -117 171 -109 21 177

Medium emissions scenario (SRES A1B1)2010-2039 -51 2 58 -119 -6 118 -81 -4 81 -233 -58 160 -72 23 1292020-2049 -47 4 58 -122 -9 116 -83 -6 79 -263 -75 161 -66 31 1402030-2059 -50 2 58 -128 -14 112 -87 -10 76 -300 -102 150 -70 23 1262040-2069 -55 1 61 -112 -5 114 -76 -3 78 -348 -145 120 -72 25 1332050-2079 -51 5 66 -104 8 133 -71 5 91 -379 -161 138 -61 34 1402060-2089 -53 3 64 -104 11 140 -71 7 95 -391 -171 117 -64 37 1502070-2099 -53 4 67 -125 1 144 -85 1 98 -406 -176 128 -57 45 160

High emissions scenario (SRES A1FI)2010-2039 -52 3 61 -52 54 178 -75 0 84 -215 -34 190 -91 18 1412020-2049 -56 1 63 -53 66 210 -83 -1 92 -268 -82 145 -90 25 1572030-2059 -55 3 66 -38 94 262 -85 -1 93 -298 -99 132 -74 30 1482040-2069 -60 0 65 -43 111 315 -78 -3 80 -367 -148 138 -69 43 1702050-2079 -63 1 71 -28 139 374 -75 2 87 -409 -171 163 -62 46 1682060-2089 -62 5 79 -42 150 428 -72 4 88 -440 -194 157 -72 50 1912070-2099 -72 5 92 -54 165 492 -80 5 101 -467 -220 123 -67 48 180


Predictions of future changes in precipitation (in mm/year) at Hinkley Point (Grid Cell 1619), relative to the 1961-2008 baseline. Data source: UK Climate Impacts Programme (UKCP09)

Table A9.5

5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95%

1961-1990 baseline:

Low emissions scenario (SRES B1)2010-2039 -3.31 -1.21 0.64 -0.75 -0.07 0.58 -3.56 -1.02 1.34 -8.59 -2.93 2.51 -2.81 -0.84 1.062020-2049 -3.90 -1.44 0.69 -0.78 -0.07 0.61 -4.13 -1.27 1.34 -9.78 -3.24 3.00 -3.17 -0.90 1.272030-2059 -4.56 -1.74 0.66 -0.81 -0.07 0.64 -4.79 -1.37 1.80 -11.55 -3.94 3.39 -3.54 -1.03 1.342040-2069 -5.10 -1.99 0.59 -0.83 -0.05 0.69 -5.06 -1.58 1.46 -12.94 -4.53 3.54 -3.65 -1.09 1.262050-2079 -5.59 -2.23 0.51 -0.85 -0.03 0.73 -5.76 -1.68 2.03 -14.06 -5.15 3.31 -4.18 -1.39 1.162060-2089 -5.90 -2.27 0.68 -0.86 -0.01 0.77 -6.21 -1.85 2.10 -14.84 -5.20 4.00 -4.26 -1.46 0.992070-2099 -6.09 -2.30 0.71 -0.88 -0.01 0.77 -6.53 -1.82 2.49 -14.70 -4.99 3.93 -4.58 -1.60 1.02

Medium emissions scenario (SRES A1B1)2010-2039 -3.47 -1.19 0.82 -0.72 -0.05 0.60 -3.90 -1.15 1.48 -9.64 -3.05 3.50 -2.69 -0.55 1.532020-2049 -4.04 -1.46 0.70 -0.77 -0.04 0.66 -4.61 -1.46 1.49 -11.10 -3.78 3.33 -2.77 -0.51 1.642030-2059 -4.83 -1.83 0.60 -0.84 -0.06 0.66 -5.29 -1.70 1.62 -12.39 -4.27 3.43 -3.34 -0.84 1.482040-2069 -5.86 -2.39 0.42 -0.89 -0.07 0.67 -6.01 -1.95 1.76 -14.43 -5.46 2.94 -4.14 -1.35 1.172050-2079 -6.46 -2.64 0.32 -0.96 -0.07 0.71 -6.67 -2.06 2.17 -15.74 -5.94 3.09 -4.66 -1.74 0.802060-2089 -7.02 -2.85 0.29 -0.98 -0.03 0.80 -7.36 -2.20 2.57 -16.78 -6.51 2.59 -5.02 -1.87 0.812070-2099 -7.66 -3.08 0.38 -0.99 0.03 0.91 -8.13 -2.48 2.78 -18.44 -7.28 2.69 -5.25 -1.91 0.84

High emissions scenario (SRES A1FI)2010-2039 -3.44 -1.16 0.87 -0.71 -0.04 0.62 -4.40 -1.34 1.62 -8.73 -2.30 4.06 -2.85 -0.83 1.132020-2049 -4.43 -1.71 0.60 -0.78 -0.06 0.63 -5.34 -1.73 1.73 -11.23 -3.78 3.48 -3.26 -1.10 0.902030-2059 -5.23 -2.09 0.42 -0.87 -0.07 0.67 -6.23 -2.07 1.86 -12.76 -4.43 3.40 -3.65 -1.21 0.952040-2069 -6.52 -2.74 0.24 -0.95 -0.07 0.73 -7.14 -2.59 1.55 -15.93 -6.06 3.31 -4.53 -1.64 0.902050-2079 -7.62 -3.17 0.32 -1.04 -0.06 0.80 -8.33 -2.84 2.23 -18.00 -6.84 3.67 -5.37 -2.03 0.862060-2089 -8.70 -3.58 0.45 -1.09 -0.01 0.92 -9.06 -3.01 2.48 -20.27 -7.87 3.66 -6.09 -2.32 0.852070-2099 -9.76 -4.02 0.49 -1.17 0.02 1.03 -10.03 -3.11 3.28 -22.66 -9.12 3.42 -6.76 -2.50 1.08


82.16 86.32 79.27 78.57 84.59

Predictions of future changes in relative humidity (% at 1.5 m) at Hinkley Point (Grid Cell 1619), relative to the 1961-2008 baseline. Data source: UK Climate Impacts Programme (UKCP09)

Table A9.6

5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95% 5% 50% 95%

1961-1990 baseline:

Low emissions scenario (SRES B1)2010-2039 -0.75 -0.07 0.60 -1.30 0.15 1.61 -1.28 -0.02 1.24 -0.73 -0.04 0.66 -2.09 -0.37 1.342020-2049 -0.79 -0.06 0.67 -1.65 0.01 1.66 -1.22 0.04 1.30 -0.83 -0.09 0.65 -1.93 -0.17 1.592030-2059 -0.70 -0.01 0.68 -1.73 -0.01 1.71 -1.21 0.11 1.43 -0.93 -0.09 0.75 -1.51 -0.05 1.422040-2069 -0.65 0.09 0.82 -2.13 -0.13 1.87 -0.93 0.34 1.59 -0.74 0.02 0.78 -1.06 0.12 1.282050-2079 -0.62 0.10 0.82 -2.34 -0.28 1.79 -1.03 0.45 1.93 -0.77 0.03 0.84 -0.88 0.18 1.252060-2089 -0.60 0.08 0.77 -2.09 -0.27 1.55 -0.91 0.49 1.89 -0.81 0.08 0.98 -1.20 0.02 1.242070-2099 -0.62 0.07 0.75 -1.90 -0.33 1.24 -1.11 0.52 2.15 -0.86 0.10 1.07 -1.26 -0.03 1.20

Medium emissions scenario (SRES A1B1)2010-2039 -0.75 0.06 0.87 -1.79 0.55 2.93 -1.37 0.35 2.07 -0.52 0.20 0.99 -1.32 -0.15 1.022020-2049 -0.79 0.03 0.86 -2.14 0.43 3.05 -1.20 0.54 2.29 -0.51 0.29 1.20 -1.45 -0.25 0.952030-2059 -0.61 0.11 0.84 -2.31 0.40 3.19 -0.92 0.68 2.28 -0.50 0.43 1.49 -1.07 -0.01 1.062040-2069 -0.52 0.18 0.88 -2.53 0.49 3.63 -0.55 0.72 2.00 -0.46 0.64 1.92 -1.06 0.08 1.222050-2079 -0.56 0.16 0.88 -2.99 0.52 4.15 -0.57 0.62 1.82 -0.55 0.74 2.23 -0.85 0.17 1.192060-2089 -0.66 0.16 0.98 -3.41 0.69 4.93 -0.62 0.69 1.99 -0.68 0.80 2.49 -1.02 0.06 1.142070-2099 -0.92 0.09 1.10 -3.79 0.60 5.15 -0.66 0.80 2.26 -0.83 0.78 2.63 -1.08 -0.03 1.01

High emissions scenario (SRES A1FI)2010-2039 -0.71 -0.02 0.67 -1.76 0.43 2.66 -1.31 0.26 1.83 -0.80 0.12 1.09 -1.84 -0.19 1.452020-2049 -0.94 -0.03 0.88 -2.23 0.45 3.18 -1.54 0.36 2.25 -0.78 0.23 1.31 -1.99 -0.27 1.442030-2059 -0.99 -0.07 0.86 -2.80 0.21 3.31 -1.25 0.48 2.22 -0.80 0.35 1.61 -1.75 -0.23 1.282040-2069 -0.86 0.04 0.94 -3.41 0.34 4.19 -0.71 0.67 2.05 -0.73 0.52 1.95 -1.55 -0.04 1.462050-2079 -0.68 0.12 0.91 -3.41 0.46 4.50 -0.62 0.74 2.11 -0.78 0.68 2.36 -0.98 0.18 1.342060-2089 -0.60 0.14 0.88 -3.56 0.72 5.22 -0.62 0.79 2.20 -0.92 0.78 2.74 -0.99 0.15 1.282070-2099 -0.66 0.12 0.90 -3.95 0.88 5.98 -0.56 0.84 2.25 -1.15 0.84 3.11 -0.83 0.11 1.04

1015.57 1014.80 1015.00 1017.08 1015.38


Predictions of future changes in sea level pressure (in hPa) at Hinkley Point (Grid Cell 1619), relative to the 1961-2008 baseline. Data source: UK Climate Impacts Programme (UKCP09)

Table A9.7

British Energy Estuarine and Marine Studies(BEEMS) is a research programme funded byEDF Energy to provide scientific backgroundfor the marine issues surrounding newnuclear build.

kenneth pye associates ltd

Documents