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Flood Risk Interactions St Denys Southampton

July 2014

Prepared for: Southampton City Council

UNITED KINGDOM & IRELAND

Flood Risk Interactions, St Denys Southampton

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Rev Date Details Prepared by Checked by Approved by

1 25/04/14 Report Draft for Comment

David Clarke Hydrogeologist Baoxing Wang Coastal Modeller

Jonathan Short Coastal Engineer Jane Sladen Technical Director

Jane Sladen Technical Director

2 20/06/14 Update report following consultation

Adrian Wright Coastal Modeller David Clarke Hydrogeologist

Jonathan Short Coastal Engineer


Final July 2014 Final Jane Sladen Technical Director

Jonathan Short Coastal Engineer


URS Infrastructure & Environment UK Limited

Scott House

Alençon Link

Basingstoke

Hampshire

RG21 7PP

Telephone: +44(0)1256 310200


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Limitations

URS Infrastructure & Environment UK Limited (“URS”) has prepared this Report for the sole use of Southampton City Council (“Client”) in accordance with the Agreement under which our services were performed (Contract No. EC09/01/1783 dated October 2013). No other warranty, expressed or implied, is made as to the professional advice included in this Report or any other services provided by URS. This Report is confidential and may not be disclosed by the Client nor relied upon by any other party without the prior and express written agreement of URS.

The conclusions and recommendations contained in this Report are based upon information provided by others and upon the assumption that all relevant information has been provided by those parties from whom it has been requested and that such information is accurate. Information obtained by URS has not been independently verified by URS, unless otherwise stated in the Report.

The methodology adopted and the sources of information used by URS in providing its services are outlined in this Report. The work described in this Report was undertaken between October 2013 and June 2014 and is based on the conditions encountered and the information available during the said period of time. The scope of this Report and the services are accordingly factually limited by these circumstances.

Where assessments of works or costs identified in this Report are made, such assessments are based upon the information available at the time and where appropriate are subject to further investigations or information which may become available.

URS disclaim any undertaking or obligation to advise any person of any change in any matter affecting the Report, which may come or be brought to URS’ attention after the date of the Report.

Certain statements made in the Report that are not historical facts may constitute estimates, projections or other forward-looking statements and even though they are based on reasonable assumptions as of the date of the Report, such forward-looking statements by their nature involve risks and uncertainties that could cause actual results to differ materially from the results predicted. URS specifically does not guarantee or warrant any estimate or projections contained in this Report.

Unless otherwise stated in this Report, the assessments made assume that the sites and facilities will continue to be used for their current purpose without significant changes.

Where field investigations are carried out, these have been restricted to a level of detail required to meet the stated objectives of the services. The results of any measurements taken may vary spatially or with time and further confirmatory measurements should be made after any significant delay in issuing this Report.

Copyright

© This Report is the copyright of URS Infrastructure & Environment UK Limited. Any unauthorised reproduction or usage by any person other than the addressee is strictly prohibited.


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TABLE OF CONTENTS 1 INTRODUCTION ................................................................ 1

1.1 Background ........................................................................ 11.2 Scope of Works .................................................................. 11.3 Report Layout ..................................................................... 2

2 SITE SETTING ................................................................... 3

2.1 General Description ........................................................... 32.2 Hydrology .......................................................................... 32.3 Geology .............................................................................. 42.4 Hydrogeology ..................................................................... 62.5 Historical Flooding .............................................................. 7

3 GROUND INVESTIGATION AND MONITORING ............. 9

3.1 Overview of Site Activities .................................................. 93.2 Ground Investigation ........................................................ 103.3 Monitoring ........................................................................ 103.3.1 Rainfall ............................................................................. 103.3.2 Water Levels .................................................................... 103.4 CCTV Survey ................................................................... 10

4 DATA INTERPRETATION ............................................... 11

4.1 Foul and Surface Water Sewer Survey ............................ 114.2 Groundwater Interaction with Tidal Levels ....................... 124.3 Landward Extent of Tidal Influence .................................. 16

5 TIDAL ANALYSIS ............................................................ 18

5.1 Approach .......................................................................... 185.2 Tidal Analysis Methodology ............................................. 18

6 JOINT PROBABILITY ANALYSIS ................................... 24

6.1 Introduction ...................................................................... 246.2 Approach .......................................................................... 246.3 Analysis Results ............................................................... 246.4 Identification of Historic Extreme Events ......................... 26

7 TIDAL FLOODING SIMULATION .................................... 27

7.1 Modelling Introduction ...................................................... 277.2 Model setup ...................................................................... 277.3 Boundary Conditions ........................................................ 277.4 Model Calibration ............................................................. 277.5 Model Results .................................................................. 287.6 Tidal Modelling Summary................................................. 29

8 SIMULATING THE INTERACTIONS BETWEEN WATER LEVEL AND RAINFALL ................................................... 31

9 VISUAL PRESENTATION ............................................... 33


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9.1 Multi source interaction – visualisation ............................ 34

10 CONCLUSIONS AND RECOMMENDATIONS................ 35

11 REFERENCES ................................................................. 38

APPENDICES

APPENDIX A: GROUND INVESTIGATION FACTUAL REPORT

APPENDIX B: CCTV SEWER SURVEY

APPENDIX C: TOPOGRAPHIC SURVEY

APPENDIX D: HYDROGEOLOGY

APPENDIX E: WATER LEVEL VARIATIONS RIVER ITCHEN

APPENDIX F: ST DENYS FLOOD MODEL

APPENDIX G: JOINT PROBABILITY ANALYSIS


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TABLE OF FIGURES FIGURE 5-1: WATER LEVELS AT WOODMILL AND DOCK HEAD (PEAK-TO-PEAK) .............................................................................. 19

FIGURE 5-2: - WATER LEVELS AT ST DENYS AND DOCK HEAD (PEAK-TO-PEAK) .............................................................................. 20

FIGURE 5-3: COMPARISON BETWEEN WATER LEVEL INCREASE AND DISCHARGE (GREEN CIRCLE = LARGEST INCREASE IN WATER LEVEL RELATIVE TO DOCK HEAD) ........ 21

FIGURE 5-4: COMPARISON BETWEEN WATER LEVEL INCREASE AND RAINFALL (GREEN CIRCLE = LARGEST INCREASE IN WATER LEVEL RELATIVE TO DOCK HEAD) ........ 22

FIGURE 5-5: RELATION BETWEEN WATER LEVEL INCREASE AND AIR PRESSURE (GREEN CIRCLE = LARGEST INCREASE IN WATER LEVEL RELATIVE TO DOCK HEAD) ............................ 22

FIGURE 5-6: RELATION BETWEEN WATER LEVEL INCREASE AND WIND SPEED (GREEN CIRCLE = LARGEST INCREASE IN WATER LEVEL RELATIVE TO DOCK HEAD) ................................. 23

FIGURE 5-7: RELATIONSHIP BETWEEN WATER LEVEL INCREASE AND POSITIVE SURGE RELATIVE TO DOCK HEAD . 23

FIGURE 7-1 MODELLED TIDAL INUNDATION FOR THE FEB 2014 EXTREME CONDITION (2.94M AOD, 5.68 MCD) ................... 30

FIGURE 8-1: DIFFERENCE (INCREASE) IN WATER DEPTH (METRES) WITH RAINFALL. ............................................................ 32

FIGURE 9-1: SCREENSHOT OF ONE OF THE ANIMATIONS SHOWING A LARGE TIDAL FLOOD EVENT IN ST DENYS. THIS WAS USED AT THE COMMUNITY WORKSHOP AND EXHIBITIONS. .................................................................................... 33

ADDITIONAL FIGURES ARE INCLUDED IN APPENDIX D


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1 INTRODUCTION

1.1 Background

Southampton City Council (SCC) was awarded funding through the Defra Community Flood Resilience Pathfinder scheme to work with residents within the significant flood risk area in St Denys, Southampton, in order to develop a property and community level flood risk management and mitigation scheme. The aim of the scheme is to improve household and community flood resilience using an integrated approach to managing all sources of flood risk which are present in this area.

The option to implement property level resistance measures was identified in the Southampton Coastal Flood & Erosion Risk Management Strategy (URS, 2012) as the preferred approach to manage tidal flood risk within the St. Denys area. This option was preferred to a frontline flood defence scheme as such a scheme is not desired by the residents and at present is unlikely to attract sufficient Flood Defence Grant in Aid funding for the next 50 years.

Engagement with residents within the community at significant risk identified that flood risk in this area may not be limited to purely a tidal source as there is a reported interaction between the tide level and both groundwater and surface water. Hence, there is a need to improve the understanding of the interactions between the different sources, and URS was commissioned by SCC to carry out surveys and technical analysis of the issues.

1.2 Scope of Works

The purpose of this project is to undertake investigations to establish the interaction between groundwater, tidal and surface water flood risk in the St. Denys and then evaluate how this affects overall flood risk within the area. This will assist with increasing awareness and subsequent development of appropriate individual and community resilience to this risk. The outputs of the study may also inform emergency planners and the development of a flood warning system should this be required.

The following specific project requirements were identified in order to improve the understanding of flood risk interactions:

• Interrogation of existing borehole data on groundwater levels;

• Borehole investigations in the local area to monitor the level of groundwater and its interaction with tidal level and surface water;

• Monitoring and analysis of the tidal levels near Priory Hard and at Woodmill to establish the difference in tide levels in these areas relative to Dock Head;

• Consideration of tide monitoring against groundwater levels;

• Establishing the extent, landwards, of the tidal influence on the groundwater level;

• A CCTV survey of the surface water sewer network in this location (approximate length 1km) to establish the condition of the existing infrastructure and to identify any defects;

• Undertaking a joint probability analysis to determine the risk caused by rainfall events coinciding with high tide;

• Undertaking a localised detailed modelling exercise using appropriate modelling software to illustrate the identified interactions; and


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• Providing a visual tool to illustrate the interactions between different sources of flooding and the potential impacts on the overall risk.

1.3 Report Layout

The report is set out in sections covering these requirements. It starts with a general description of the site setting (including hydrological conditions and flood history overview), followed by a summary of the ground investigation, monitoring and foul sewer and surface water sewer survey works that have been carried out as part of this project. This summary is supported by a series of technical appendices which provide more detail.

The data interpretation sections cover:

• The findings of the CCTV foul sewer and surface water sewer survey;

• the interaction of groundwater and tidal levels;

• the relationship between tidal levels at Dock Head, St Denys and Woodmill;

• joint probability analysis;

• modelling of tidal flooding;

• simulation of the interactions between water levels and rainfall; and

• suggestions for visual tools to illustrate the interactions.


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2 SITE SETTING

2.1 General Description

The site is located at St Denys, Southampton. The study area is bound by the railway which forms a crescent from the west to the east of site. Contained within this boundary are the high flood risk flood areas of Priory Road and Adelaide Road. The south side of the study area is confined by the River Itchen, as shown on Figure 1 below and in Appendix D.

Note: Figures are included in the text to assist the reader. For the larger version see Appendix D.

The study area is comprised of fairly flat low lying land, typically ranging between 3 maOD and 6 maOD (metres above Ordnance Datum Newlyn) or 5.74 mCD to 8.74 mCD (Chart Datum which is 2.74 m above Ordnance Datum Newlyn) with the topography gently rising inland from the river frontage. Priory Hard, to the south west of the study area, comprises historically reclaimed land. The study area includes many residential properties, a railway and railway station, several public houses and some converted commercial properties.

The area is served by a network of surface water sewers and foul sewers which are managed by Southern Water Services.

2.2 Hydrology

The River Itchen is located adjacent to, and upstream of the study area. It originates south of the village of Cheriton, Hampshire and is groundwater fed (Chalk) (Capita, 2010). It enters the Southampton District boundary adjacent to Mansbridge Road flowing south, south west before it reaches the Woodmill Activity Centre, located approximately 1.5km upstream of the study area (see Figure 4 in Appendix D).


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At the Woodmill Activity Centre the river is culverted and passes through two sluice gates. The sluice gates segregate tidal influence upon the River; to the south the River is tidally influenced and to the north the River is mainly absent of tidal influence and is predominantly fresh water. South of the Woodmill Activity Centre the river flows in a southerly direction with the river meanders amplifying down gradient to the sea approximately 3 km south of the St Denys study area.

2.3 Geology

British Geological Survey (BGS) mapping shows the site is underlain by River Terrace Deposits, with a small section to the southwest underlain by Tidal Flat Deposits. These superficial deposits are underlain by more than 20 m of London Clay Formation.

The superficial and solid geology of the study area are shown on Figure 2 and Figure 3 respectively in Appendix D.


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Six boreholes were drilled at St Denys during the flood risk study. A Ground Investigation report was produced and is provided in Appendix A.

Table 2-1 provides a summary of the geology encountered at the six boreholes. A BGS log (Geoindex) from a borehole on Adelaide Road was also referred to and is provided in Appendix A.

Table 2-1: Borehole Geology

Borehole Number

Ground Level (maOD)

Made Ground Thickness (m)

Backfilled Material Thickness (m)

Top of River Terrace Gravel Deposits (maOD)

River Terrace Gravel Deposits Thickness (m)

Top of London Clay Formation (maOD)

1 3.97 0.65 - 3.32 2.1 1.22

2 2.62 0.6 2 uncertain uncertain -0.63

3 4.06 1.25 (0.15 - grass and topsoil)

- 2.81 1.55 1.26

4 3.83 0.95 - 2.88 0.85 2.03

5 3.28 0.9 (0.2 - turf and topsoil)

- 2.38 1.6 0.78

6 4 0.95 (0.15 – grass and topsoil)

- 3.05 1.05 2


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Borehole Number

Ground Level (maOD)

Made Ground Thickness (m)

Backfilled Material Thickness (m)

Top of River Terrace Gravel Deposits (maOD)

River Terrace Gravel Deposits Thickness (m)

Top of London Clay Formation (maOD)

SU41SW194

6.14 0.9 - 5.24 2.3 2.94

All six boreholes encountered River Terrace Deposits ranging in thickness from 0.85 m to 2.1 m. The BGS borehole recorded a thickness of 2.3 m. These deposits consist of sandy clayey gravel with some sandy clay present in parts. Borehole 2 closest to the River Itchen encountered 2 metres of sandy silty gravelly clay which is believed to be backfill material overlying River Terrace Deposits. All boreholes encountered London Clay, a firm grey silty clay, at depths of between 1.8 m and 4.0 m. The elevation of the top of the London Clay falls from around 2 maOD furthest inland at Borehole 4 to around -0.6 maOD at Borehole 2 nearer to the river.


2.4 Hydrogeology

Of interest to the flood risk study is the presence of permeable deposits at St Denys which could transmit water and cause groundwater flooding.

The River Terrace Deposits are described by the Environment Agency as a Secondary A Aquifer, defined as “permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers.


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These are generally aquifers formerly classified as minor aquifers”. There are no Groundwater Source Protection Zones in the vicinity of site.

The ground investigation at St Denys found variable thickness and distribution of River Terrace Deposits which does not therefore appear to be a continuous aquifer layer capable of supporting local water supplies. Groundwater levels were recorded at between 1.2 and 2.5 m below ground level after drilling. Levels were measured again after the boreholes were cleared by bailing. The rate of recovery of water levels after bailing gives an indication of the permeability of the formation. Borehole 1 and Borehole 5 recovered faster than the other boreholes.

The underlying London Clay Formation is described the Environment Agency as an aquiclude. An aquiclude is a geological formation of which there is virtually no permeability/water movement.

In summary, the River Terrace Deposits are water bearing with variable thickness and permeability and containing some less permeable silty and clayey horizons. They are underlain by London Clay, a low permeability formation which does not readily transmit water. The potential for flood risk from groundwater will depend on groundwater levels in the River Terrace Deposits and interconnections with the River Itchen and drainage. This is discussed further in Section 5.

2.5 Historical Flooding

The St Denys is area is vulnerable to tidal flooding and, based on the findings of the Coastal Strategy (URS, 2012), approximately 70 residential properties are at risk of a present day 1 in 50 year event (2% chance of flooding in any year). The majority of these properties are located in the area near to Priory Hard, and several properties were flooded to a depth of 15cm in March 2008 and 14th February 2014 from approximately a 1 in 20 year tidal flood event (2.86 maOD, ~5.6 mCD still water level at Dock Head) (URS, 2012).

The 14th February 2014 flood event (Plate 1) fell within the time window of this study. Details were recorded and analysed in the joint probability assessment section later in this report.


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Plate 1: Tidal flooding associated with a storm surge near Priory Hard, St Denys (22.50 on 14th February 2014)


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3 GROUND INVESTIGATION AND MONITORING

3.1 Overview of Site Activities

A monitoring network was set up for the study, consisting of water level recorders in the six boreholes drilled in the St Denys area and tidal level recorders near Priory Hard and at Woodmill on the River Itchen. The locations are shown on Figure 1 and Figure 4 in Appendix D.


An initial walkover site visit by Southampton City Council and URS was conducted on 18 October 2013 to assess potential borehole and stilling well locations as well as to assess the condition of the existing Woodmill Activity Centre stilling well.

Woodmill Activity Centre stilling well clearance and St Denys (84 Priory Road) stilling well installation took place on 3 December 2013. Both stilling wells were equipped with water level and barometric data loggers set at 5 minute intervals.

Six boreholes were drilled and developed between 28 and 29 January 2014. The elevation survey and groundwater level logger installation took place on 31 January 2014. The first full round of monitoring took place on 13 February 2014 to check the installations before the predicted flood event on 14 February.

A foul sewer and surface water sewer CCTV survey of Priory Road and other adjoining roads, was undertaken between 17 and 19 March 2014.


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3.2 Ground Investigation

The borehole location selection process involved a number of investigation stages. Firstly, aerial photography was used to identify those areas across site where boreholes could be located i.e. on grass verges, road sides, etc. These areas were prioritised with a preference for locations which could triangulate groundwater levels to determine the direction of groundwater flow.

Council maps were used to identify land ownership and access.

The site walk over survey undertaken on 18 October 2013 was used to determine drilling rig accessibility and to agree proposed borehole locations with the Council. Each borehole location was inspected, narrowing the list of proposed boreholes to twelve. Six final borehole locations were chosen and drilled and their locations are shown on Figure 1 in Appendix D. They were drilled using the cable percussion technique and soil samples were taken from the holes for inspection and testing. Slotted casing was installed where the River Terrace Deposits were present and the Made Ground was screened out. The ground investigation factual report is provided as Appendix A.

3.3 Monitoring

3.3.1 Rainfall

Daily rainfall data from Portswood was obtained from the Environment Agency. The data covers a period of 26 years from 1986 to 2013. Long term daily precipitation from Otterbourne was also used for the joint probability analysis and is available for the past 102 years. The study period, December 2013 through to March 2014, was wet with rainfall well above average during these months.

3.3.2 Water Levels

Water levels were recorded in the 6 boreholes from 31 January 2014. River levels at St Denys and Woodmill were recorded from 3 December 2013. At the time of writing (June 2014) the water level recorders were still in place.

Corresponding water level information at Dock Head was downloaded for the survey period from the SOTONMET Weather Reports (http://www.sotonmet.co.uk/) web site. Tidal water level data for Woolston was also obtained from the Environment Agency towards the end of the project in order to try to fill in missing data from the Dock Head tide gauge, but given the timing of the data receipt, full analysis of this data was not undertaken in this study.

All tidal water level information is presented in metres relative to Ordnance Datum Newlyn (OD) and also to Chart Datum (CD).

3.4 CCTV Survey

A foul sewer and surface water sewer CCTV survey was undertaken between 17 and 19 March 2014. The location of the survey is shown on the location map Figure 1, Appendix D. The survey was carried out by InSewer Surveys with permission from Southern Water Services. The aim of the survey was to investigate the drainage in the areas most prone to flooding. Manhole covers were removed on these roads and a camera was lowered in to trace the sewer network. Depth, pipe size, construction and condition detail was recorded each time. The full survey report is in Appendix B. Maps of the foul sewers and surface water sewers and the location of the surveys are provided in the CCTV report in Appendix B (Figures 5a, b, c).


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4 DATA INTERPRETATION

4.1 Foul and Surface Water Sewer Survey

The CCTV survey findings are summarised in Appendix B.

From the survey data it is evident that there are some problems with the tidal flaps on the surface water sewers and blockages in the surface water sewers and foul sewers from debris and encrustations. Several fractures and cracks were also noticed in some areas. The western most tidal flap near Priory Hard was found to be faulty, whilst conducting a survey on 17 March 2014 (survey section 2 in Appendix B). At 47m down the surface water sewer, tidal water was beginning to back up as the tide was coming in. This would indicate a faulty tidal flap. The surface water sewer was re-surveyed on 19 March 2014 (survey section 21) and again water was encountered in the sewer downstream.

The condition of the central tidal flap, approximately 110m east of the Priory Hard tidal flap, could not be surveyed. The surface water manhole (SU43132652 in Appendix B) was flooded when access was attempted. The water level within the access manhole was measured at 1.8 m below ground level (bgl) whereas the surface water sewer inlet level has been documented at 2.7mbgl. Dewatering would be required to survey this surface water sewer.

The survey company recommended clearance of the sewer near to Adelaide Road (manhole SU43131603).

Sewer water levels are shown on Figure 5 in Appendix D and are seen to flow towards the outfalls except when tidal water backs up the surface water sewers.


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4.2 Groundwater Interaction with Tidal Levels

Tidal water level measurements were recorded during the study along the River Itchen at:

• St Denys; and

• Woodmill.

The locations are shown on Figure 4 in Appendix D and the data is presented in Figure 8 of Appendix D.


The flood event on the night of 14 February 2014 can be seen as the highest recorded water level at both Woodmill Activity Centre and 84 Priory Road. The rainfall recorded in the area on 14 February was 24.2 mm. It should be noted that this was not the highest rainfall recorded in a day. Higher rainfall was recorded in the area on 15 and 23 December 2013, 31 January 2014 and 6 February 2014, however these coincided with lower tidal peaks.

Groundwater levels are presented in Figure 9 Appendix D and also cover the flood event of 14 February 2014. It can be seen that the highest groundwater levels are at Borehole 4, which is the furthest away from the River Itchen. There are fluctuations at all boreholes and a rise in groundwater levels on 14 February 2014 during the flood, although these were not the highest recorded levels except at Borehole 5.

At the three more inland boreholes (BH4, BH6 and BH1) tidal levels remained below groundwater levels for most of the monitoring period. At the boreholes nearer the estuary, tidal


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levels were higher than groundwater levels during high tides. The groundwater response to tides can be seen in the borehole hydrographs at Borehole 5 and Borehole 2 as short (tidal) duration fluctuations superimposed on the more sustained rise and fall due to rainfall (Figure 11 in Appendix D).



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Figure 5 in Appendix D shows the groundwater level contours on 14 February 2014. The flow direction is towards the River Itchen. The preliminary conclusion drawn from the hydrographs is that groundwater levels are responding primarily to rainfall rather than tidal flood events. The exception is Borehole 5 which is close to the river and encountered more permeable deposits and appears to have a tidal response during high tides.

To better understand the interaction of groundwater levels, foul sewer and surface water sewer levels and tidal levels, cross sections were prepared. Figure 5 in Appendix D shows the locations of these cross sections. Figure 6 and Figure 7 in Appendix D present schematic cross sections; one to the west from Adelaide Road to Priory Hard and one further east from Priory Road to the park near Borehole 5.

The western cross section shows that during the St Denys flood event at around 23:00 on 14 February 2014 the groundwater level at Borehole 2 had only increased by 26 cm compared to during low tide and it was still 86 cm below ground level even though Priory Road had flooded and the flood level was 24 cm above ground level at Borehole 2. The conclusion drawn from these levels is that groundwater levels were not contributing to above ground level flooding.


The eastern cross section shows a similar response with groundwater at Borehole 5 only increasing by 23 cm and the level still 1.15 m below ground level. In this area, because the ground is higher, the tidal level was also below ground level during the flood event.


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The cross sections and comparison of groundwater, tidal and sewer levels also indicate that the surface water sewers and foul sewers are below groundwater level in the Priory Road area and were not therefore contributing to above ground flooding during the flood event in February 2014.

There is however potential for water to pass up through the surface water sewers if tidal flaps are not operating properly. The CCTV survey in March 2014 indicated that some flaps may not be functioning correctly and therefore it is concluded that the surface water sewers may be contributing to above ground flooding in places. This would occur when tidal levels are higher than ground level along the length of the surface water sewer and at the manhole. Levels of manholes, surveyed during the study, are shown in Appendix C. The lowest manhole cover level is 2.54 maOD (5.58 mCD) at MH12 (2652 in the CCTV survey) just to the east of the Adelaide Street/Priory Road junction. This surface water sewer has an outfall and tidal flap near to Priory Hard. If the flap is not working and when tidal levels exceed the manhole level (2.54 maOD - 5.58 mCD) water could discharge out of the manhole.

The manholes and surface water sewers further east are less vulnerable to tidal flooding. There are three more outfalls shown in Appendix C. The associated inland manholes have levels of 3.07 maOD (5.81 mCD) at MH4 at the Ivy Road/Priory Road junction and 3.47 maOD (6.21 mCD) at MH14. The most eastern manhole was not surveyed but is expected to be higher. If tidal levels exceed these manhole levels, flooding could occur but is less likely than at MH12 given the water level of a 1 in 20 year event of around 2.94 maOD (5.68 mCD).

Where there are cracks there is the potential for leakage of groundwater into the surface water sewers and foul sewers which may be exerting some control on groundwater levels, keeping them lower than they may otherwise be.


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4.3 Landward Extent of Tidal Influence

Groundwater levels at Borehole 2 and Borehole 5 are shown in Figure 10 in Appendix D together with the tidal level at 84 Priory Road. It can be seen that tidal levels rise above groundwater levels only around high tide at certain boreholes.


This connection between tidal levels and groundwater levels was examined by looking at the tidal efficiency. Tidal efficiency can be calculated using the equation (Smith & Hick, 2001):

Where Fa is the amplitude of forcing at the tidal boundary, Ra is the amplitude of the response at a point in the aquifer and k denotes the tidal constituent or frequency (Smith & Hick, 2001).

Groundwater levels can be seen increasing and decreasing with the high tide at Borehole 5, whereas Borehole 2 only increases and decreases slightly. Both these boreholes are similar distances from the River Itchen, but react differently to the tide. The difference is likely to be due to the restored ground at Borehole 2. The deposits are more clayey and less permeable than the River Terrace Deposits found at Borehole 5 meaning the tide will infiltrate these deposits more slowly.


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Tidal efficiency was estimated for Borehole 5 as 0.07 i.e. a change in tidal level of 1 m would result in a change in groundwater level of 0.07 m. This effect only occurs during the higher portion of the tide when tidal levels rise to the height of the groundwater level in the River Terrace Deposits aquifer. Below that height the presence of the London Clay effectively isolates groundwater in the River Terrace Deposits from the river.

There is also a lag time between the high tide and the peak of the groundwater fluctuation. This is not readily calculated because of the double peak in the tides at St Denys.

The landward extent of groundwater level response to the tide can be examined using the following equation (Smith & Hick, 2001):

x – Distance from the tidal boundary

TE – Tidal efficiency

T (Transmissivity), S (Aquifer storage) and Pk (Time period of tide) are constant assuming the aquifer is one dimensional, semi-infinite and homogeneous with uniform transmissivity.

The equation above can be used to determine the distance from the River Itchen where the tidal efficiency is small enough (say 0.01) to have little effect on groundwater levels. The tidal efficiency at Borehole 5 was used. The distance of Borehole 5 from the tidal boundary is approximately 32 m. This gives an approximate distance of 50 m from the tidal boundary where the tidal efficiency is 0.01. Beyond this distance there would be little predicted effect on groundwater as a result of tidal levels.

The distance is an approximate value and the tidal efficiency may still vary across the site because the aquifer does not appear to behave as a homogeneous body due to the varying permeability and thickness of the River Terrace Deposits.


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5 TIDAL ANALYSIS

5.1 Approach

Tidal water level data was obtained from three locations (Dock Head, St Denys and Woodmill) along the Itchen for the survey period December 2013 to March 2014. The water level analysis is contained in Appendix E.

Figure 2 in Appendix E shows the comparison of water level data between the Dock Head tidal station and the St Denys gauge station. Figure 3, Appendix E, shows the comparison of water level data between the Dock Head tidal station and the Woodmill gauge station. As shown in the figures, data gaps exist within the Dock Head gauge measurements of up to several days. In order to consider the significance of the missing periods, data was made available from the EA tide gauge at Woolston and was substituted in place of the Dock Head data.

The data gaps represent a loss of around 17% of the overall data over the duration of the tide gauge deployment within the River Itchen. An analysis of these data gaps show that of the top 26 tidal events, which are the most significant in terms of flood risk analysis, only a small number of events (about 5) occurred when data was not available from the Dock Head tide gauge. Furthermore, of the top 10 events recorded over the measurement period, all were available at Dock Head.

Therefore, although data was not available at Dock head for around a fifth of the data measurement period, typically, the timing of the data loss did not coincide with the largest 10 tidal events and were only partially missing in the top 26 events suggesting that the majority of the relevant information is present within the measured time-series data.

5.2 Tidal Analysis Methodology

The analysis of water levels was based on time series data interpolated to a 1 minute interval for all sites. This high frequency data analysis was undertaken to ensure that the small difference in tidal phase (time of high water) could be accurately captured. The analysis of water levels shows a phase lag between high water of typically less than 5 minutes between Dock Head and Woodmill.

Two approaches have been used to investigate the differences in water level along the River Itchen:

1. Peak-to-peak; and

2. Simultaneous comparison.

The peak-to-peak method provides the difference between the maximum water levels over individual single tidal (12.5 hr) events. Importantly, the peak to peak method does not consider the influence of time, for example the peak water level at Dock Head may be several minutes earlier than at Woodmill. The peak to peak method identifies the absolute maximum values for Dock Head, St Denys and Woodmill regardless of the interval in time before these events occurred. This approach provides an indication of maximum water levels along the river although does not provide an accurate indication of the slope in water surface at any given time.

For the simultaneous comparison approach, peak water levels were identified at St Denys or Woodmill, and then compared to the corresponding water level at Dock Head for the same


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instant in time. This approach provides a measure of the water level ‘slope’ along the River Itchen and considers how for example, the largest slope that may occur when the tide is not at its peak, i.e. when the tide is ebbing at the entrance to the river, but still just flooding at the tidal limit of the river due to the lag in the timing of peak water levels.

Table 5-1: Summary of high tide water level analysis

Items St Denys – Dock Head Woodmill – Dock Head

Peak-to-Peak Simultaneous Peak-to-Peak Simultaneous

Mean Difference (cm)

6 7 8 9

Max. Difference (cm)

15 23 24 27

Based on the peak-to-peak approach, the peak water levels rise 6cm and 8cm on average at St Denys and Woodmill respectively. The maximum increases are 15cm and 24cm respectively (Figure 5-1 and Figure 5-2).

Figure 5-1: Water levels at Woodmill and Dock Head (Peak-to-Peak)

Surf

ace

Ele

vatio

n m

OD

N


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Figure 5-2: - Water levels at St Denys and Dock Head (Peak-to-Peak)

As shown in Figure 5-1, short term oscillations or ‘noise’ in the measured water levels are evident in the time-series with increasing amplitude upstream within the River Itchen. A review of the occurrence of these oscillations shows no significant correlation to individual meteorological events or river discharge. Possible explanations could include the point measurement approach adopted (rather than time averaged), local wave effects or a ‘sieche’ effect within all or part of the River Itchen. A sieche is defined as a local increase in water levels resulting from the complex interactions of the tidal wave with local bathymetry and meteorological impacts. It is possible that these complex interactions will at certain points along the river come together to form localised changes in water levels, such as those shown in Figure 5-1. However, the actual cause of these apparent oscillations is outside the scope of this current investigation.

As shown in Figure 5-2, peak water levels observed at St Denys contain fewer and smaller water level oscillations, suggesting that the findings from the data measured from the St Denys gauge has less uncertainty. Since this gauge is located adjacent to the study frontage and shows less oscillations this was used as the primary reference station.

Within this study, the following key meteorological parameters were investigated.

• Air Pressure;

• Wind Speed;

• River Flow; and

• Rainfall.

Time series data of air pressure, wind speed, rainfall and river flow were also collected over the 3-month water level measurement period to investigate the correlation between high water levels and these parameters.

As indicated in Figure 5-3 to Figure 5-6, the comparison of peak water level increases measured at St Denys relative to Dock Head water level values show no or at best a weak correlation to the individual meteorological parameters considered, suggesting that over the

Surf

ace

Ele

vatio

n m

OD

N


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monitoring period at least, these parameters had little or no influence on increasing peak water levels within the River Itchen compared to the measured water levels at Dock Head.

The measured data at St Denys and Dock Head contains the meteorological influence of all the parameters considered. To demonstrate the influence of the combination of these individual elements the largest increase in water level measured at St Denys relative to Dock Head is highlighted in Figure 5-3 to Figure 5-6 (Green Circle). Again, this demonstrates that of all the parameters considered a week correlation exists between air pressure and increases in water levels within the River Itchen compared to Dock Head.

Further analyses showing the relationship between positive tidal surge and increases in water levels along the Itchen relative to the tide Gauge at Dock Head are shown in Figure 5-7. Again, no strong correlation between tidal surge and increasing water level relative to Dock Head is evident from the information recorded during the measurement period.

Woodmill St Denys

Figure 5-3: Comparison between water level increase and discharge (Green Circle = largest Increase in Water Level Relative to Dock Head)


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Woodmill St Denys

Figure 5-4: Comparison between water level increase and rainfall (Green Circle = largest Increase in Water Level Relative to Dock Head)

Woodmill St Denys

Figure 5-5: Relation between water level increase and air pressure (Green Circle = largest Increase in Water Level Relative to Dock Head)


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Woodmill St Denys

Figure 5-6: Relation between water level increase and wind speed (Green Circle = largest Increase in Water Level Relative to Dock Head)

Woodmill St Denys

Figure 5-7: Relationship between water level increase and positive surge relative to Dock Head

It is recommended that to quantify and reduce the uncertainty in the correlation analysis, further studies should be undertaken using a longer -term time series data. As a rule of thumb the longer the data set the lower the uncertainty in the results presented, therefore, a dataset length as long as possible (i.e. several years) should be used.

However, it should be noted that during the 3-month sampling period for this study analysis, a number of significant storm and tidal surge events occurred which provide some assurance that the findings from this limited period provide a reasonable representation of the correlation between the key meteorological parameters considered.


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6 JOINT PROBABILITY ANALYSIS

6.1 Introduction

Among the parameters investigated to consider their significance in influencing (increasing) peak water levels along the River Itchen (air pressure; wind speed; river flow; and rainfall), coincident high rainfall and peak tide levels is considered to present a significant risk.

Furthermore, consultation feedback received from local residents during development of the Southampton Coastal Strategy (URS, 2012), coupled with our understanding of processes operating in this area suggest that flooding is likely to be experienced from a variety of means, but predominantly as a result of tidal locking of the local surface water sewers and interaction of tidal levels and rainfall preventing surface water draining away.

Therefore, a joint probability analysis has been undertaken to establish the likelihood of occurrence from such a combined event. Details are contained in Appendix G.

6.2 Approach

Joint probability refers to the chance of two or more conditions occurring at the same time. It provides the probability of the relevant variables taking high values simultaneously. A joint probability assessment was undertaken in the study by using the calculated dependence relationship between the two variables (water level and rainfall) from the available data.

The analysis has been conducted in accordance with the guidance provided by the Defra and Environment Agency joint publication: Use of Joint Probability Methods in Flood Management: A Guide to Best Practice – R&D Technical Report FD2308/TR2, 2005. The guide includes a summary of the desk study approach to joint probability analysis, and a software tool for its application. The analysis has been undertaken adopting the following approaches:

a. Obtaining the marginal distributions for extreme water levels and extreme rainfall. b. Estimation of dependence between water levels and rainfall based on the published EA

dependence mapping of water levels and rainfall. c. Estimation of dependence of water levels and daily rainfall using JOIN-SEA software. d. Calculating the joint exceedance return periods.

6.3 Analysis Results

Extreme sea levels occur as a result of the combination of tides with storm surges associated with weather systems. Therefore in order to assess the chances of two extreme parameters (rain and elevated water levels) combining, measured rainfall and water level data was obtained. Daily rainfall data from Portswood and Otterbourne was collected from the Environment Agency. Measured water levels at Dock Head were obtained for the period 1991 to 2013.

The rainfall data at Portswood covers a period of 26 years from 1986 to 2013. Long term daily precipitation at Otterbourne in the past 102 years have also been collected and analysed to complement the statistics results.

Extreme water levels were analysed for Dock Head as a part of Southampton Coastal Flood and Erosion Risk Management Strategy, URS (2012), listed in Table 8:1. Extreme rainfall


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totals were evaluated separately as part of this study and then combined with extreme water levels to estimate the joint probability return periods.

Analysis of rainfall data was used to determine the extreme statistics as shown Table 6-1. To accommodate the broad scale of the study, a statistical analysis of the rainfall data at Otterbourne was conducted based on 102 years annual extreme daily rainfall data. The results from this analysis show that the results between the rainfall information collected at Portswood and Otterbourne are comparable Table 6-1.

Table 6-1: Extreme Water level and Rainfall

Return Periods (yrs)

Water Levels

(maOD)

Water Levels

(mCD)*

Rainfall at Portswood

(mm)

Rainfall at Otterbourne (mm)

1 2.45 5.19 26.4 27.4

2 2.56 5.30 33.1 34.3

5 2.67 5.41 41.9 43.4

10 2.76 5.50 48.6 50.2

20 2.84 5.58 55.0 59.3

50 2.94 5.68 64.1 66.2

100 3.02 5.76 70.7 73.1

200 3.09 5.83 77.4 79.9 *A correction of +2.74m from aOD to CD

The method described in Environment Agency practitioners guidance (FD2308/TR2) uses a correlation coefficient ‘’ or ‘correlation factor’ (CF) to quantify inter-dependence of the two parameters of interest. The dependence map for two-hourly rainfall and sea level in England and Wales is produced in FD2308/TR2 (Page 24, Figure 3). It suggests that on the south and west coasts of England and Wales, the dependence is consistently characterised as ‘modest’, where the correlation coefficient in the Southampton area is =0.32. This indicates a low meteorological correlation between hourly rainfall and sea level.

For the joint return period(s) of interest, the appropriate tables for the relevant strength of dependence from the published best practice guidance have been applied to convert the listed joint exceedance extremes from marginal return periods to actual values.

Adopting the ‘modest’ dependence coefficient between water levels and rainfall gives the joint probability results presented in Table 6-2. Based on the analysis, there would be a benefit to further investigating the potential impact for coincident storm tide and rainfall events.


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Table 6-2: Joint exceedance extreme water level and rainfall combinations

6.4 Identification of Historic Extreme Events

The joint probability study provides an opportunity to identify past extreme events. The highest 7 combined events were selected from the analysis of the available water levels and rainfall data spanning a 20 year record between 1991 and 2014 (Table 6-3). Note these combined events exclude extreme water level events during periods of low rainfall. The largest combined event from the water level records and rainfall occurred on the 14/02/2014 with an estimated joint exceedance return period is approximately 1 in 200 years.

Table 6-3: Extreme water levels and rainfalls

No. Date WL (maOD) at

Dock Head WL (mCD)*

at Dock Head Rainfall (mm) at

Portswood

Joint Probability Return Period

(yrs)

1 14/02/2014 2.84 5.58 24.2 1 in 200

2 31/01/2013 2.37 5.11 32.8 1 in 25

3 23/12/2013 2.08 4.82 43.0 1 in 10

4 10/03/2008 2.86 5.60 7.0 1 in 50

5 25/12/1999 2.72 5.46 7.6 1 in 20

6 26/11/1995 2.25 4.99 32.4 1 in 10

7 02/01/1995 2.96 5.70 0.4 1 in 60

*A correction of +2.74m from aOD to CD

1 2 5 10 20 50 100 200

1.87 26.4 33.1 41.9 48.6 55.0 64.1 70.7 77.4

1.95 26.3 33.1 41.9 48.6 55.0 64.1 70.7 77.4

2.05 17.6 25.8 36.7 45.0 53.1 64.1 70.7 77.4

2.16 10.8 19.3 30.0 38.3 46.6 57.4 65.9 74.2

2.25 4.8 12.5 23.5 31.7 40.0 50.9 59.1 67.5

2.36 4.4 14.7 23.0 31.2 42.1 50.4 58.6

2.45 8.2 16.3 24.6 35.5 43.8 51.9

2.55 2.2 9.7 18.0 28.8 37.1 45.4

2.67 1.8 9.3 20.2 28.3 36.6

2.76 3.3 13.4 21.8 29.9

2.84 7.1 15.1 23.4

2.94 6.7 14.6

3.02 0.7 8.1

3.09 2.1

Joint exceedence return period (years)

Water

Level

(m,AOD)

Daily Rainfall (mm)


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7 TIDAL FLOODING SIMULATION

7.1 Modelling Introduction

The aim of the numerical modelling study was to understand the flood risk interactions a St Denys and improve the management of flood risk in this area. URS has made use of the state of the art MIKE 21 Flexible Mesh hydrodynamic modelling software developed by the Danish Hydraulics Institute (DHI). The Flexible Mesh (MIKEFM) model is based upon triangular and quadrangular mesh elements which are able to provide enhanced resolution covering important features such as natural bathymetric features and buildings.

The following sections summarise the key elements and findings of the study. Further detail is provided in Appendix F.

7.2 Model setup

The flood model domain was configured to cover the residential area along the River Itchen between St Denys and Woodmill (Figure 1, Appendix F). The bathymetry has been created from a number of LiDAR survey datasets with a 1 m resolution. The high-resolution LiDAR data provide a detailed description of the buildings and roads in the St Denys residential area. Some supplementary data from the URS existing model of the River Itchen (URS, 2012) has been used in areas where the survey data did not provide coverage.

Within the model, the river area is defined by triangular elements, whereas the potential flood zone (on land) is defined by quadrangular elements. This element arrangement provided the most computationally efficient model design. Figure 2 in Appendix F shows the 5m coarser resolution over the River Itchen and the higher 2m model resolution for the residential sites.

7.3 Boundary Conditions

The model boundary has been defined along the length of the River Itchen using a uniform level. This approach includes the established (Section 5) increase in water level at the St Denys area. The model was run for a single tidal event (12.5 hrs) for various extreme conditions and the corresponding overland tidal flooding assed. Figure 3 in Appendix F shows an example of the boundary conditions applied along the boundary of the model in order to simulate the various extreme conditions.

7.4 Model Calibration

A tidal surge occurred on 14/02/2014, with the maximum water level at Dock Head measured at 2.84 maOD (5.58 mCD), equivalent to a 1 in 20 year event. The corresponding maximum water level at St Denys was 2.94 maOD (5.68 mCD). The impacts of this tidal surge event within the St Denys area have been recorded this dataset provides an invaluable opportunity to demonstrate the level of model calibration against an actual event.

The primary focus of the model calibration has been the adjustment of the bed roughness within the model to reflect the roughness of bed materials (concrete, road surfaces) within the model domain. Figure 5 in Appendix F shows the flooding within the St Denys area predicted by the flood model, also shown is a photograph taken at approximately the same time showing a similar extent and level of tidal inundation within the St Denys region.


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7.5 Model Results

Water levels at Dock Head were analysed as a part of the Southampton Coastal Flood and Erosion Risk Management Strategy (URS, 2012). In this study extreme water levels were estimated for the period 1924 to 2009. The results from this extremes analysis are given in Table 7-1.

Table 7-1: Extreme water levels (2010).

Return Period (yrs) Water Levels (maOD) Water Levels (mCD)*

1 2.45 5.19

2 2.56 5.30

5 2.67 5.41

10 2.76 5.50

20 2.84 5.58

50 2.94 5.68

100 3.02 5.76

200 3.09 5.83

500 3.18 5.92

*A correction of +2.74m from aOD to CD

Extreme water levels at St Denys and Woodmill were investigated in this study. The study provides the increase of water level over a single tidal cycle at St Denys and Woodmill relative to the water levels at Dock Head (see Section 5.2).

The hydrodynamic model was run for a wide range of extreme water level conditions from 2.94 to 3.45 m (Figure 7-1 Modelled tidal inundation for the Feb 2014 extreme condition (2.94m AOD, 5.68 mCD). Peak water levels increase up the river and the corresponding return periods will also vary. Based on the maximum difference in peak water levels measured at the St Denys and Woodmill gauges, the corresponding return periods for these two sites have been identified in Table 7-2


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Table 7-2: Model Runs

Model

Runs

Modelled

Water Level

(maOD)

St Denys Woodmill

Mean (0.06 m

increase

estimated)

Max (0.15 m

increase

estimated)

Mean (0.08 m

increase

estimated)

Max (0.24 m

increase

estimated)

1 3.00 1:50 n/a* n/a n/a

2 3.05 n/a n/a 1:50 n/a

3 3.10 1:100 1:50 1:100 n/a

4 3.15 1:200 n/a n/a n/a

5 3.20 n/a 1:100 1:200 1:50

6 3.25 1:500 1:200 1:500 1:100

7 3.35 n/a 1:500 n/a 1:200

8 3.45 n/a n/a n/a 1:500

*This combination has not been identified

7.6 Tidal Modelling Summary

A detailed overland flood model was developed for the St Denys area to investigate the impact of tidal inundation under a range of extreme water level conditions. The model includes the influence of large structures (e.g. buildings) in order to aid our understanding of potential flood pathways, water depths and extents for various extreme conditions.

The model was calibrated and validated against actual events and demonstrates that the modelled extent of flooding (areas in which tidal inundation in predicted) agrees with the in situ observations.


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Figure 7-1 Modelled tidal inundation for the Feb 2014 extreme condition (2.94m AOD, 5.68 mCD)


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8 SIMULATING THE INTERACTIONS BETWEEN WATER LEVEL AND RAINFALL

A modelling exercise was carried out to illustrate the potential interactions between extreme water levels and rainfall. The MIKEFM model (See Section 9) was used to simulate and illustrate the processes leading to flooding under a combination of extreme water levels and rainfall.

The consultation feedback received from local residents during development of the Southampton Coastal Strategy, coupled with our understanding of processes operating in this area suggest that flooding is likely to be experienced from a variety of sources, but predominantly as a result of tidal locking or back flow along local surface water sewers and interaction of tidal levels preventing surface water draining away.

The results of the joint probability assessment were used to estimate the ‘worst case’ combinations of rainfall and high tides for a return period of 1 in 200 year combined event on 14/02/2014. For the model simulation, the water level is 2.84m at Dock Head (2.94m at St Denys) and daily rainfall is 24.2mm. To illustrate the influence of rainfall on flooding in the area, we configured two models including rainfall and excluding rainfall. The models were used to simulate the events which will allow comparison between the two scenarios.

To illustrate situations where flooding may occur, Figure 8-1 shows the difference in water depth between two model runs. The figure shows the additional contribution to inundation from rainfall. As no drainage network has been included, the model may overestimate the impact of rainfall, however, the modelling does provide an inundation area for the worst case (i.e. if the surface water sewers are fully surcharged by the tide, or are tide locked).

A detailed investigation of impact, time series data for water surface elevation and water depth have been considered at 5 Locations BH2, E, B, C and D (Figure 8-1). At Locations BH2, E, B and C, the maximum water depths have been increased by a modest 2.4 cm. However, a larger increase of 0.50m has been found at Location D, this is due to an accumulation of water in a low lying area under the assumption of no ground drainage.

Again, for this high level assessment, further studies with drainage included may be needed to fully address the flooding issues which include a significant rainfall event.


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Figure 8-1: Difference (increase) in water depth (metres) with rainfall.


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9 VISUAL PRESENTATION

Various visual tools can be used to demonstrate the flooding process, such as a 2-dimensional animation and cross sections. These animations were presented at community events and are briefly described below.

In order to demonstrate the flooding processes operating in the study area, the key findings were illustrated using a 2D animation and displayed to the community during exhibitions (Figure 9-1).

Figure 9-1: Screenshot of one of the animations showing a large tidal flood event in St Denys. This was used at the community workshop and exhibitions.


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This method of illustrating the impact and processes leading to a flood can often highlight the speed of travel of the flood wave and identify the flow paths which would otherwise be lost by a static 2D map.

9.1 Multi source interaction – visualisation

An illustrative visualisation was developed to illustrate the interactions between different sources of flooding and the potential impacts on the overall risk.

Indicative cross sections have been developed to illustrate the groundwater, tidal, and surface water interactions in the upper Itchen study area.


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10 CONCLUSIONS AND RECOMMENDATIONS

The purpose of the flood risk study was to undertake investigations to establish the interaction between groundwater, tidal and surface water flood risk in the St. Denys and then evaluate how this affects overall flood risk within the area. The following summarises the outcome of the study and makes recommendations for further work.

Borehole Investigations and Monitoring

A monitoring network was set up for the study, consisting of water level recorders in six new boreholes drilled in the St Denys area and tidal level recorders near Priory Hard and at Woodmill on the River Itchen.

The ground investigation confirmed the geological mapping of the area and shows the St Denys area to be underlain by River Terrace Deposits with variable thickness and permeability and containing some less permeable silty and clayey horizons. These superficial deposits are underlain by London Clay, a low permeability formation which does not readily transmit water. The potential for flood risk from groundwater will depend on groundwater levels in the River Terrace Deposits and interconnections with the River Itchen and drainage.

Interaction of Groundwater with Tidal Level and Surface Water

The conclusion drawn from groundwater and tidal level monitoring is that groundwater levels respond primarily to rainfall rather than tidal fluctuations.

Groundwater is several metres below ground level and does not appear to be contributing to above ground level flooding in the Priory Road area, based on monitoring during the flood event on 14 February 2014.

Cross sections can be used to compare groundwater, tidal and sewer levels. These show that groundwater levels are mostly higher than tidal levels at St Denys with some reversal so that tidal levels are above groundwater levels at high tide. Surface water sewers and foul sewers were below groundwater level in the Priory Road area and are not therefore contributing to groundwater flooding potential. Conversely, where there are cracks there is the potential for leakage of groundwater into the surface water sewers and foul sewers which may be exerting some control on groundwater levels, keeping them lower than they may otherwise be.

Tidal Influence on Groundwater Levels

Tidal effects on groundwater only occur during the higher portion of the tide when tidal levels rise to above the height of the groundwater level in the River Terrace Deposits aquifer. Below that height the presence of the London Clay effectively isolates groundwater in the River Terrace Deposits from the river.

Tidal efficiency was estimated for Borehole 5 as 0.07 i.e. a change in tidal level of 1 m would result in a change in groundwater level of 0.07 m during high tide.

The extent inland of tidal influence was estimated to be approximately 50 m from the tidal efficiency at a single borehole (BH5). This means that further inland than 50 m tidal effects on groundwater are unlikely. This value may change depending on local permeability and hydraulic connection and should only be used as an indicative value.


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Drainage CCTV Survey

A CCTV survey and level survey of surface water sewers and foul sewers was undertaken. From the survey data it is evident that there are some problems with the tidal flaps on the surface water sewers and blockages in the surface water sewers and foul sewers from debris and encrustations. Several fractures and cracks were also noticed in some areas.

There is potential for water to pass up through the surface water sewers if tidal flaps are not operating properly. The CCTV survey in March 2014 indicated that some flaps may not be functioning correctly and therefore it is concluded that the surface water sewers may be contributing to above ground flooding in places. This would occur when tidal levels are higher than ground level along the length of the surface water sewer and at the manhole.

Levels of manholes surveyed during the study show the lowest manhole cover level is 2.54 m AOD (5.58 m CD) just to the east of the Adelaide Street/Priory Road junction. This surface water sewer has an outfall and tidal flap near to Priory Hard and is vulnerable to flooding when tidal levels exceed the manhole level (2.54 m AOD - 5.28 m CD).

The manholes and surface water sewers further east with connections to outfalls are less vulnerable to tidal flooding; the next most vulnerable is at the Ivy Road/Priory Road junction where the manhole level is 3.07 maOD (5.81 mCD).

Tidal Levels along River Itchen

Two approaches have been used to investigate the differences in water level along the River Itchen:

1. Peak-to-peak; and

2. Simultaneous comparison.

Based on the peak-to-peak approach, the peak water levels rise 6cm and 8cm on average at St Denys and Woodmill respectively. The maximum increases are 15cm and 24cm respectively.

At Woodmill, the local influence of the river discharge is significant and may not, therefore, provide an accurate representation of the slope in water levels along the whole river section down to Dock Head. This is evident from the oscillations in the measured water levels. The data from Woodmill should be considered with a high degree of uncertainty when used to interpret longitudinal slopes in water surface.

The peak in water levels (occurring during times of high river discharge) observed at St Denys contains less short period oscillations in water levels suggesting that the findings from the data measured from the St Denys gauge has less uncertainty.

Factors Affecting Water Levels

Multiple factors can lead to variations in water levels along the River Itchen. Within this study, the following key parameters were investigated.

• Air Pressure;

• Wind Speed;

• River Flow; and

• Rainfall.


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The comparison of peak water level increases measured at St Denys relative to Dock Head water level values shows no, or at best a weak, correlation to the individual meteorological parameters considered. Of all the parameters considered, a weak correlation exists between air pressure and increases in water levels within the River Itchen compared to Dock Head.

There was also no strong correlation between tidal surge and increasing water level relative to Dock Head from the information recorded during the measurement period.

Joint Probability Analysis

The joint probability study provides an opportunity to identify past extreme events. The highest 7 combined events were selected from the analysis of the available water levels and rainfall data spanning a 20 year record between 1991 and 2014 (excluding extreme water level events during periods of low rainfall). The largest combined event from the water level records and rainfall occurred on the 14 February 2014 with an estimated joint exceedance return period of approximately 1 in 200 years.

Flood Modelling

A detailed overland flood model was developed for the St Denys area to investigate the impact of tidal inundation under a range of extreme water level conditions. The model includes the influence of large structures (e.g. buildings) in order to aid the understanding of potential flood pathways, water depths and extents for various extreme conditions.

The model was calibrated and validated against actual events and demonstrates that the modelled extent of flooding (areas in which tidal inundation in predicted) agrees with the in situ observations. The model can be used with caution to predict the effect of future extreme events.

A modelling exercise was carried out to illustrate the potential interactions between extreme water levels and rainfall. The MIKEFM model was used to simulate and illustrate the processes leading to flooding under a combination of extreme water levels and rainfall.

Flooding is likely to be experienced from a variety of sources at St Denys, but predominantly as a result of tidal locking or back flow along local surface water sewers and interaction of tidal levels preventing surface water draining away.

Recommendations

There is the potential for flooding via outfalls and surface water sewers and manholes where tidal flaps are not working correctly. The surface water sewer between Priory Hard and the connected manhole to the east of the Adelaide Street/Priory Road junction is particularly vulnerable. The tidal flap at Priory Hard and those further east should be maintained and checked regularly to ensure that they are in good working order.

The CCTV survey company recommended clearance of the sewer near to Adelaide Road which is blocked with silt. It is recommended that the survey results are passed on to Southern Water Services.

The study has evaluated the risk of flooding above ground. However there may be underground structures, foundations and basements and it is recommended that the level of these is determined so that potential risks can be assessed.


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Water level monitoring should continue over the summer months to determine whether there are significant changes in the relationship between groundwater levels, surface water sewers and tidal levels. Some of the more inland boreholes are unlikely to provide useful water level information with respect to tidal interaction but may be useful in future if groundwater level data is required.

It is recommended that to quantify and reduce the uncertainty in the correlation analysis, further studies should be undertaken using a longer -term time series data. As a rule of thumb the longer the data set the lower the uncertainty in the results presented, therefore, a dataset length as long as possible (i.e. several years) should be used.

Visual tools including animations, cross sections and CCTV footage are recommended to illustrate the groundwater, tidal, and surface water interactions in the upper Itchen study area.

11 REFERENCES

Capita (2010) Strategic Flood Risk Assessment Level 2 Volume 3 CSL

Defra (2005a) Joint Probability: Dependence Mapping and Best Practice: Technical report on dependence mapping. FD2308/TR1.

Defra (2005b) Use of Joint Probability Methods in Flood Management: A guide to best practice. FD2308/TR2

HR Wallingford (2000a). The joint probability of waves and water levels: JOIN-SEA: A rigorous but practical new approach. HR Report SR 537.

HR Wallingford (2000b). The joint probability of waves and water levels: JOIN-SEA- Version 1.0, User Manual.

Smith, A.J. & Hick, W.P, February 2001. Hydrogeology and Aquifer Tidal Propagation in Cockburn Sound, Western Australia. CSIRO Land and Water, Technical Report 6/01.

URS (2012) Southampton Coastal Flood and Erosion Risk Management Strategy - Appendix 1A: Conceptual Understanding Report.

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