surface water management plan

Prepared for

Surface Water Management Plan

Intermediate Assessment of Groundwater Flooding Susceptibility

Tier 2

March 2011

Richmond Borough Council


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Revision Schedule Surface Water Management Plan – Intermediate Assessment of Groundwater Flooding Susceptibility March 2011

Rev Date Details Prepared by Reviewed by Approved by

01 31/03/11 Draft Report Christopher Woolhouse Hydrogeologist

Steve Cox Senior Hydrogeologist

Jane Sladen Technical Director



Table of Contents

Abbreviations ............................................................................................. 1

Glossary ..................................................................................................... 2

1 Introduction ..................................................................................... 3

1.1 Groundwater Flooding .................................................................................................... 3

1.2 The Current Report......................................................................................................... 4

2 Topography, Geology and Hydrogeology ..................................... 5

2.1 Topography and Hydrology............................................................................................. 5

2.2 Geology .......................................................................................................................... 5

2.3 Hydrogeology ................................................................................................................. 7

3 Assessment of Groundwater Flooding Susceptibility................ 10

3.1 Groundwater Flooding Mechanisms ............................................................................. 10

3.2 Evidence of Groundwater Flooding............................................................................... 11

3.3 Potential for Elevated Groundwater Data Sets.............................................................. 12

3.4 Summary of Potential for Elevated Groundwater .......................................................... 12

3.5 Importance of Long Term Groundwater Level Monitoring ............................................. 14

4 Water Framework Directive and Infiltration SUDS...................... 15

5 Conclusions and Recommendations........................................... 17

5.1 Conclusions .................................................................................................................. 17

5.2 Recommendations........................................................................................................ 18

6 References..................................................................................... 19

List of Tables Table 1 River Terrace Deposit Units and Nomenclature Table 2 Geological Units in the Study Area and Hydrogeological Significance Table 3 Available Flooding Records List of Figures Figure 1 Solid Geology Map Figure 2 Solid and Superficial Geology Map Figure 3 Increased Potential For Elevated Groundwater Figure 4 Infiltration SUDS Suitability



Intermediate Assessment of Groundwater Flooding Susceptibility March 2011 1

Abbreviations

ACRONYM DEFINITION

BGS British Geological Survey

DEFRA Department for Environment, Fisheries and Rural Affairs

EA Environment Agency

LiDAR Light Detection and Ranging

SUDS Sustainable Drainage Systems

SWMP Surface Water Management Plan




Glossary

TERM DEFINITION

Aquiclude Formations that may be sufficiently porous to hold water, but do not allow water to move through them.

Aquifer Layers of rock sufficiently porous to hold water and permeable enough to allow water to flow through them in quantities that are suitable for water supply.

Aquitard Formations that permit water to move through them, but at much lower rates than through the adjoining aquifers.

Climate Change Long term variations in global temperature and weather patterns, caused by natural and human actions.

Flood defence Infrastructure used to protect an area against floods, such as floodwalls and embankments; they are designed to a specific standard of protection (design standard).

Floods and Water Management Act

Legislation constituting part of the UK Government’s response to Sir Michael Pitt’s Report on the Summer 2007 floods, the aim of which is to help protect ourselves better from flooding, to manage water more sustainably and to improve services to the public.

Fluvial flooding Flooding by a river or a watercourse.

Groundwater Water that is underground. For the purposes of this study, it refers to water in the saturated zone below the water table.

Nomeculture Is a term given to a list of sub identified lithologies and their names within the main named geological unit.

Lithology The gross physical character, description of a rock or rock formation

Pluvial Flooding Flooding as a result of high intensity rainfall when water is ponding or flowing over the ground surface before it enters the underground drainage network or watercourse, or cannot enter it because the network is full to capacity.

Risk The product of the probability and consequence of the occurrence of an event.

Sewer flooding Flooding caused by a blockage, undercapacity or overflowing of a sewer or urban drainage system.

Sustainable Drainage Systems

Methods of management practices and control structures that are designed to drain surface water in a more sustainable manner than some conventional techniques. The current study refers to the ‘infiltration’ category of sustainable drainage systems e.g. soakaways, permeable paving.




1 Introduction

1.1 Groundwater Flooding

1.1.1 Groundwater flooding occurs as a result of water rising up from the underlying aquifer or from

water flowing from springs. This tends to occur after long periods of sustained high rainfall, and

the areas at most risk are often low-lying where the water table is more likely to be at shallow

depth. Groundwater flooding is known to occur in areas underlain by principal aquifers,

although increasingly it is also being associated with more localised floodplain sands and

gravels.

1.1.2 Groundwater flooding tends to occur sporadically in both location and time, and tends to last

longer than fluvial, pluvial or sewer flooding. Basements and tunnels can flood, buried services

may be damaged, and storm sewers may become ineffective, exacerbating the risk of surface

water flooding. Groundwater flooding can also lead to the inundation of farmland, roads,

commercial, residential and amenity areas.

1.1.3 It is also important to consider the impact of groundwater level conditions on other types of

flooding e.g. fluvial, pluvial and sewer. High groundwater level conditions may not lead to

widespread groundwater flooding. However, they have the potential to exacerbate the risk of

pluvial and fluvial flooding by reducing rainfall infiltration capacity, and to increase the risk of

sewer flooding through sewer / groundwater interactions.

1.1.4 The need to improve the management of groundwater flood risk in the UK was identified

through DEFRA’s Making Space for Water strategy. The review of the July 2007 floods

undertaken by Sir Michael Pitt highlighted that at the time no organisation had responsibility for

groundwater flooding. The Flood and Water Management Act identified new statutory

responsibilities for managing groundwater flood risk, in addition to other sources of flooding and

has a significant component which addresses groundwater flooding.




1.2 The Current Report

1.2.1 The Greater London Authority (GLA) has commissioned Scott Wilson to complete tier 2 of the

Richmond Borough Council Surface Water Management Plan (SWMP). A SWMP is a plan

which outlines the preferred surface water management strategy in a given location. In this

context surface water flooding describes flooding from sewers, drains, groundwater, and run-off

from land, small water courses and ditches that occurs as a result of heavy rainfall (DEFRA,

March 2010).

1.2.2 The current report provides an intermediate assessment of groundwater flooding susceptibility

as part of the SWMP tier 2 and provides recommendations for a more detailed assessment.

1.2.3 The following sections outline the geology and hydrogeology in the Richmond Borough Council

(BC) administrative area. From this analysis:

• Potential groundwater flooding mechanisms are identified;

• Evidence for groundwater flooding is discussed;

• Areas with Increased Potential For Elevated Groundwater are identified;

• Suitability for Infiltration SUDS is discussed; and

• Recommendations are provided for further investigation.




2 Topography, Geology and Hydrogeology

2.1 Topography and Hydrology

2.1.1 The study area is defined by the administrative area of Richmond BC. A large proportion of the

borough is situated in close proximity to the River Thames and its tributaries (River Crane and

Beverley Brook) (Jacobs, 2009). These are key features of the borough and are described

further below.

2.1.2 The River Thames flows north to south for approximately 5 km through the centre of the

borough; between Teddington and Isleworth. The River Crane flows west to east across the

north west of the borough, with the confluence with the River Thames just to the east of

Isleworth.

2.1.3 The Beverley Brook runs close to the eastern boundary of the borough in the area of Richmond

Park and west of Putney, before discharging into the River Thames at Barn Elms crossing.

2.1.4 Within the Richmond BC area, the highest ground level is about 60 maOD at Richmond Park in

the east. In the interfluvial areas, such as west of Teddington, elevations generally range

between 10 - 20 maOD. Close to the River Thames, ground elevations are generally below

10 maOD.

2.2 Geology

2.2.1 Geological information for Richmond and the surrounding area is presented in Figures 1 and 2,

reproduced from the British Geological Survey (BGS) 1:50,000 scale geological series and

from the BGS Geology of London memoir.

Bedrock Geology

2.2.2 The bedrock geology of the borough is comprised of the Upper Chalk, which is overlain by the

Thanet Sand Formation, Lambeth Group, London Clay Formation and Claygate Member.

However, many of these units are at considerable depth below ground level. The London Clay

Formation underlies much of the borough, although in the higher topographic area of Richmond

Park to the east, the Claygate Member conformably rests on top of the London Clay Formation.

2.2.3 The full thickness of the London Clay Formation is known only where the formation is capped

by the Claygate Member and is approximately 80 to 140 m. The thickness of the overlying

Claygate Member is known to range from 2 m to 28 m across the London Basin, although BGS

geological logs would be required to confirm the thickness in Richmond.

2.2.4 The Claygate Member is the youngest deposits of the London Clay Formation and is comprised

of orange sands interbedded with pale clays. The London Clay Formation is a mixture of

brown, grey silt and fine sand.

2.2.5 Three normal faults are identified within Richmond to the south west of Richmond Park. Two of

the faults are trending north west to south east and the third north east to south west. The first

two faults down throw the London Clay Formation to the south west and the third to the south

east. The exact displacement of London Clay Formation is not known without further site




investigation data, although the London Clay Formation on the down thrown side of the faults is

expected to be thicker.

Superficial Geology

2.2.6 The superficial geology of the area consists of Head, Peat, Alluvium, Langley Silt Member and

River Terrace Deposits (Kempton Park Gravel Formation, Taplow Gravel Formation, Hackney

Gravel Member, Lynch Hill Gravel Member, Boyn Hill Gravel Member and the Black Park

Gravel Member). These superficial deposits blanket the bedrock geology across much of the

Richmond BC area, with the main exception being Richmond Park.

2.2.7 Head deposits are only found to outcrop in the vicinity of the Beverley Brook and Richmond

Park. The deposits are clay dominated, derived from the London Clay Formation, and are

found to be less than 2 m thick across London.

2.2.8 The Alluvium deposits consist mainly of sand, silt and clay, with Peat interbedded in some

areas. The surface outcrop marks the path of the River Thames, River Crane and Beverley

Brook, all associated sources of deposition. The thickness of Alluvium adjacent to the River

Thames is found to vary between 10 and 20 m, but will be significantly less thick (perhaps up to

a few metres) along the River Crane and Beverley Brook.

2.2.9 The Langley Silt Member, formerly referred to as ‘Brickearth’, consists of very fine grained sand

and clayey silt up to 3 m in thickness. It is found outcropping only in two small patches either

side of the River Thames, to the east of Twickenham.

2.2.10 The River Terrace Deposits form the largest superficial deposit outcrop across the Richmond

BC area. Further details on each of the separate River Terrace Deposit units are provided in

Table 1.

Table 1 River Terrace Deposit Units and Nomenclature

Geological Unit Nomenclature Average Thickness (m)*

Lithological Description

Kempton Park Gravel Formation 6 Sand and gravel with clay, silt lenses

Taplow Gravel Formation 5 Sand and gravel with clay, silt lenses

Hackney Gravel Member 6 Sand and gravel with clay, silt lenses

Lynch Hill Gravel Member 7 Sand and gravel with clay, silt lenses

Boyn Hill Gravel Member 5 Sand and gravel with clay, silt lenses

River Terrace Deposits

Black Park Gravel Member 3 Sand and gravel with no lenses of clay and silt

(* Thicknesses derived from BGS, Lexicon. 2011)




2.3 Hydrogeology

2.3.1 The hydrogeological significance of the various geological units within the study area is

provided in Table 2. The range of permeability likely to be encountered for each geological unit

is also incorporated in Table 2, based on BGS permeability data.

Table 2 Geological Units in the Study Area and their Hydrogeological Significance

Geological Unit Permeability Hydrogeological Significance

Head Very Low - High

Variable (probably an aquitard but sand or gravel horizons may locally form an aquifer). Secondary Aquifer (undifferentiated)

Alluvium Very Low - High Secondary Aquifer (undifferentiated)

Langley Silt Formation Very Low - Low Variable (but probably an aquitard). Unproductive strata.

Kempton Park Gravel Formation

Taplow Gravel Formation

Hackney Gravel Member

Lynch Hill Gravel Member

Boyn Hill Gravel Member

Superficial Deposits

Black Park Gravel Member

High - Very High

Secondary Aquifer (A) on the eastern side of the River Thames. Principal Aquifer on the western side of the River Thames.

Claygate Member Low - High Secondary Aquifer (A)

London Clay Formation Very Low - Low Aquiclude. Unproductive Strata

Bedrock Geology

Upper Chalk High – Very High Principal Aquifer

‘Principal Aquifer’ - layers that have high permeability. They may support water supply and/or river base flow on a strategic

scale.

‘Secondary Aquifer (A)’ - 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.

‘Aquitard’ - allows some groundwater movement (see glossary)

‘Aquiclude’ - does not allow groundwater movement (see glossary)

Bedrock Hydrogeology

2.3.2 The London Clay Formation, which underlies the majority of the Richmond BC area, is an

aquiclude and does not permit groundwater flow. It is classed by the Environment Agency as

unproductive strata.

2.3.3 In a small area in the east of Richmond BC, in Richmond Park, the Claygate Member is found

outcropping, though laterally limited. The Claygate Member permits groundwater flow but can

significantly vary in permeability due to the presence of clay horizons. Groundwater tables may

exist within the sandy horizons of the Claygate Member, perched over the more clayey

horizons or the London Clay Formation aquiclude.

2.3.4 The Upper Chalk, Thanet Sands and Lambeth Group, underlying the London Clay Formation,

are classified as principal or secondary (A) aquifers. However, the significant thickness of the

London Clay Formation in the Richmond area confines these aquifers, and therefore they are

not pertinent to the current study.




Superficial Hydrogeology

2.3.5 Head deposits are generally expected to behave as aquitards, although sand and gravel

horizons may locally form a secondary aquifer depending on their lateral extent and thickness.

The Langley Silt Member is also expected to behave as an aquitard and is classed by the

Environment Agency as unproductive strata.

2.3.6 Alluvium in the River Thames valley is classified by the Environment Agency as a Secondary

Aquifer (undifferentiated), with inconsistent hydraulic conductivity due to the variability of clay

content. The presence of interbedded peat may further reduce the hydraulic conductivity.

2.3.7 The River Terrace Deposits are expected to behave as a Secondary Aquifer (A) to the east of

the River Thames and a Principal Aquifer to the west, due to the dominance of sand and

gravels. The presence of clay lenses could lead to locally variable perched groundwater,

depending on the horizontal extent of the clay.

2.3.8 The Black Park Gravel Member is possibly in hydraulic continuity with the underlying Claygate

Member in the area of Richmond Park, both forming a perched aquifer(s), overlying the London

Clay Formation aquiclude.

Groundwater Levels

Bedrock Geology

2.3.9 Chalk, Thanet Sand and Lambeth Group groundwater levels have not been considered as part

of this study due to the significant thickness of London Clay Formation, which confines these

aquifers in the Richmond BC area i.e. water levels will not cause groundwater flooding.

2.3.10 The Claygate Member in the Richmond Park area may contain perched water tables. However,

the Environment Agency do not monitor groundwater levels in this bedrock unit, probably owing

to its limited lateral extent and thickness.

Superficial Geology

The Alluvium, Langley Silt Member and River Terrace Deposits form a perched aquifer over

the London Clay Formation aquiclude. The Environment Agency does not monitor

groundwater levels in any of these superficial aquifers. However, borehole logs are available

from the British Geological Survey and these often provide details of water strikes, providing

an indication of depth to groundwater. It is recommended that under Tier 3 of Drain London

borehole logs are obtained.

Hydraulic Relationships

2.3.11 The London Clay Formation overlies the Chalk, Thanet Sands and Lambeth Group aquifers in

the Richmond BC area and hydraulically separates them from the ground surface.

2.3.12 There may be some hydraulic continuity between the Black Park Gravel Member and the

underlying Claygate Member in the Richmond Park area.




Surface Water / Groundwater Interactions

2.3.13 Perched water tables are expected to exist within the Alluvium, Head and, in particular, the

River Terrace Deposits, and some hydraulic continuity is expected between these deposits and

surface water courses (River Thames, River Crane and Beverley Brook). However, these

interactions will be significantly reduced where the surface water courses have been artificially

modified i.e. where they flow within a lined or partially lined channel. An improved

understanding of flood risk could be gained by undertaking monitoring of groundwater levels /

river stage.

Water Supply Abstractions

2.3.14 In the 19th Century groundwater water supplies in London were obtained from the shallow

superficial and bedrock deposits. In the early 20th Century this was abandoned in favour of

deeper boreholes and wells into the Chalk. (Jones et al. 2000) Due to the significant thickness

of the London Clay Formation in the borough there is no hydraulic connection between the

Chalk and superficial aquifers in the borough. Therefore abstractions from the Chalk are not

pertinent to this study as they will not have an impact on groundwater flooding susceptibility.

2.3.15 There may be some smaller private abstractions from the superficial deposits and this

information will be held by the Environment Agency.

Artificial Groundwater Recharge

2.3.16 Water mains leakage data for the Richmond BC administrative area were not provided for this

study. It should be noted that due to the limited thickness of superficial deposits in some areas

and the presence of perched groundwater, additional recharge through leaking mains could

lead to a rise locally in groundwater levels. These rises might not prove significant under dry

conditions, but could exacerbate the risk of groundwater flooding following periods of heavy

rainfall.

2.3.17 The drainage/sewer network can act as a further source of artificial recharge. When pipes are

installed within principle or secondary aquifers, the groundwater and drainage network can

become hydraulically connected due to leakage. In dry pipe conditions groundwater can leak

into the drainage network with water flowing in through cracks and porous walls, draining the

aquifer and reducing groundwater levels. During heavy rainfall when pipes are full, flows are

reversed with drains acting as recharge points, artificially recharging the groundwater table and

subsequently increasing groundwater levels.




3 Assessment of Groundwater Flooding Susceptibility

3.1 Groundwater Flooding Mechanisms

3.1.1 Based on the hydrogeological conceptual understanding of the study area, the potential

groundwater flooding mechanisms that may exist are:

• Claygate Member outcrop area in Richmond Park: Water levels within the outcropping

Claygate Member (and overlying Black Park Gravel Member) will be perched on top of the

London Clay Formation aquiclude. This means that basements / cellars and other

underground structures in this area may be at risk from groundwater flooding following

periods of prolonged rainfall, increased utilisation of infiltration SUDs and / or artificial

recharge from leaking pipes.

• Superficial aquifers along the River Thames, River Crane and Beverley Brook:

groundwater flooding may be associated with the Alluvium, Head and, in particular, River

Terrace Deposits, where they are in hydraulic continuity with surface water courses. Stream

levels may rise following high rainfall events but still remain “in-bank”, and this can trigger a

rise in groundwater levels in the associated superficial deposits. The properties at risk from

this type of groundwater flooding are probably limited to those with basements / cellars,

which have been constructed within the superficial deposits.

• Superficial aquifers in various locations: a third mechanism for groundwater flooding is

also associated with the Head and River Terrace Deposits (gravel and sand) where they are

not hydraulically connected to surface water courses. Perched groundwater tables can exist

within these deposits, developed through a combination of natural rainfall recharge and

artificial recharge e.g. leaking water mains. The properties at risk from this type of

groundwater flooding are probably limited to those with basements / cellars.

• Impermeable (silt and clay) areas down slope of superficial aquifers in various

locations: a forth mechanism for groundwater flooding may occur where groundwater

springs / seepages form minor flows and pond over impermeable strata where there is poor

drainage (artificial or natural).

• Artificial ground in various locations: a final mechanism for groundwater flooding may

occur where the ground has been artificially modified to a significant degree. If this artificial

ground is of substantial thickness and permeability, then a shallow perched water table may

exist. This could potentially result in groundwater flooding at properties with basements, or

may equally be considered a drainage issue. Areas mapped by the BGS as containing

artificial ground are shown in Figures 1 and 2. It is noted that the artificial deposits are

mostly over the River Terrace Deposits and may either form a continuous aquifer with these

superficial deposits, or provide a low permeability cap, depending on the composition of the

artificial ground.




3.2 Evidence of Groundwater Flooding

3.2.1 Figures 1 and 2 show the location of a number of groundwater flooding incidents between 2000

and 2010 within the study area that have been reported to the Environment Agency. Further

details are presented in Table 3. It should be noted that there has not been a statutory

obligation to record incidences of groundwater flooding in the past. It is therefore likely that this

list of groundwater flooding incidents is not exhaustive.

Table 3 Available Groundwater Flooding Records

Bedrock Geological

Unit*

Overlying Superficial Deposits*

Location NGR Incident

No**

Reported Incident Year

Taplow Gravel Fn Richmond 513594 169622

1 Flooded Cellar 2003

Taplow Gravel Fn London 513021 170670

2 Landowner has been informed there is shallow groundwater under his property & that he is at risk of groundwater flooding.

2005

Alluvium Hampton Court 514600 169200

3 Flow from bank for 22/23yrs 2001

Taplow Gravel Fn Teddington 514768 171311

4 Basement flooding 2003

Taplow Gravel Fn Teddington 514900 171300

5 Rising WL under home 2000

Kempton Park Gravel Fn Teddington 516480 170470

6 Water in air raid shelter in garden 2001

Kempton Park Gravel Fn - 516519 170452

7 GW Flooding enquiry 2007

Kempton Park Gravel Fn Kingston-on-Thames

517200 169800

8 Water in Cellar 2001

Kempton Park Gravel Fn Hampton Wick 517200 169700

9 Water in cellar 2001

Kempton Park Gravel Fn Hampton Wick 517300 169700

10 Wet Basement 2000

Edge of Alluvium Twickenham 516000 172200

11 Boggy Garden 2000

Edge of Langley Silt Fn Richmond 517749 173031

12 Waterlogged patch of ground. 2004

Edge of Langley Silt Fn Twickenham 517200 174100

13 Installed sump and pump 2000

Kempton Park Gravel Fn - 518485 175405

14 Recent flooding through ground floor 2007

Kempton Park Gravel Fn Kew 519062 176193

15 Occasional water seepage in basement 2007

Edge of Taplow Gravel Fn and Head

East Sheen SW14

520100 175000

16 Standing water in garden 2000

Head SW14 520200 175300

17 Waterlogged Garden 2000

Head SW14 520286 175147

18 Flooded basement 2003

Taplow Gravel Fn Richmond 520411 174638

19 Flooded Cellar 2010

Kempton Park Gravel Fn SW14 521178 175888

20 Buying property -info on flooding 2001

Alluvium - 521988 176110

21 Water in cellar after heavy rain 2008

London Clay Formation

Taplow Gravel Formation TW2 514212 174535

22 Water under floorboards 2003

Note: * Geology of incident based on plotted location (Figures 1, 2 and 3) and Environment Agency record ** Incident reference number as shown on Figures 1, 2 and 3. Fn = Formation




3.2.2 Table 3 shows that the many of the reported incidents occurred during late 2000 / early 2001; a

particularly wet period that resulted in both surface and groundwater flooding incidents in a

number of locations across the country.

3.2.3 All of the flood incidents are located where permeable superficial deposits overlie the London

Clay Formation aquiclude. A perched groundwater table is expected to exist within these

superficial deposits and so it is likely the flood incidents are true groundwater flooding

incidents.

3.3 Potential for Elevated Groundwater Data Sets

3.3.1 The areas in the borough where there is an increased potential for groundwater levels to rise

within 2 m of the ground surface during periods of higher than average recharge are shown in

Figure 3. These are separated into permeable superficial deposits and bedrock (consolidated)

aquifers. The data set was produced for the whole of the Drain London project area, derived

from four individual data sources:

o British Geological Survey (BGS). Groundwater Flood Susceptibility maps;

o Environment Agency (EA). Thames Estuary, 2100 groundwater hazard maps;

o DEFRA. Groundwater emergence maps; and

o JBA. Groundwater flood maps.

3.3.2 However, only the BGS groundwater flooding susceptibility and EA Thames Estuary data sets

are relevant to the Richmond BC area.

3.3.3 Figure 3 shows that areas in Richmond BC where there is an increased potential for elevated

groundwater are associated with permeable superficial deposits; North Twickenham, north

Richmond and west Teddington have been defined as having the most potential for elevated

groundwater levels.

3.3.4 In general, the areas identified by the data set as having an increased potential for elevated

groundwater are sensible and show a good correlation with recorded groundwater flood

incidents. However, there are a number of discrepancies; incidents 1 to 5, 19 and 20 are

located outside of the areas with increased potential for elevated groundwater. It is possible

that the BGS data set may need to be refined at these locations.

3.4 Summary of Potential for Elevated Groundwater

3.4.1 Due to the significant thickness of underlying London Clay Formation in the Richmond BC

area, the susceptibility from groundwater flooding from rising groundwater levels in the Chalk

and ‘Basal Sands’ is considered to be negligible. Therefore, the key groundwater flooding

mechanisms are associated with permeable superficial deposits.

Claygate Member in the Richmond Park Area

3.4.2 The Claygate Member and overlying Black Park Gravel Member are thought to be water

bearing. There are no groundwater level data to confirm the depth to water and therefore site




investigation will be important for any proposed development sites, particularly those

considering basements / underground structures such as soakaways.

Locations where the London Clay Formation is overlain by superficial deposits

3.4.3 Figure 3 indicates that the superficial deposits (primarily River Terrace Deposits) in the borough

are water bearing and have an increased potential for elevated groundwater. Whilst no

groundwater level data are available for the superficial deposits, where groundwater tables

exist they are expected to be close to or at ground level, and may fluctuate with river stage.

Therefore basements and cellars may be at risk from groundwater flooding and use of

structures such as sheet piling may exacerbate the problem if they intercept the water table. It

should be noted that only part of the superficial deposit outcrop is defined as having an

increased potential for elevated groundwater. This is probably due to variations in the thickness

and elevation of the deposits.

Locations where London Clay Formation outcrops at surface in the Richmond and

Richmond Park area

3.4.4 The London Clay Formation is an aquiclude and does not permit groundwater flow. Therefore

in areas where there are no overlying superficial deposits and the London Clay Formation is of

an appreciable thickness, the potential for elevated groundwater levels is considered to be

negligible. However, where the London Clay Formation has been removed and replaced with

more permeable artificial ground, there may be increased potential of elevated groundwater as

groundwater becomes trapped in these deposits.

3.4.5 Finally, it is possible that groundwater springs could emerge from permeable superficial

deposits and flow over the London Clay Formation, resulting in groundwater flooding. It is

recommended that rolling ball analysis is undertaken as part of a more detailed assessment.

Future Susceptibility

3.4.6 Susceptibility to groundwater flooding in the Richmond BC area may change as a result of

climate change, or changes to flood management. One of the climate change predictions

includes an increase of high rainfall events. This could lead to further groundwater flooding in

the Richmond BC area due to increased perched groundwater levels and associated spring

flows. It is also noted that a shift in drainage policy, with increased infiltration SUDS, may also

lead to increased incidents of groundwater flooding.

3.4.7 Finally, the areas with increased potential for elevated groundwater may also change owing to

future trends in river stage and changes to / increased flood defences. The Thames Estuary

2100 project is considering a number of options to manage the anticipated future increase in

tidal and fluvial flood risk along the River Thames Estuary. The impact of these options should

be considered further as part of a more detailed assessment.




3.5 Importance of Long Term Groundwater Level Monitoring

3.5.1 Groundwater flow direction, depth to groundwater, topography and the degree of artificial

influence in the subsurface (e.g. leaking water mains or groundwater abstractions) play an

important role when considering the susceptibly of an area to groundwater flooding. Without

long term (and continuous) groundwater monitoring, it is not possible to derive groundwater

level contours or likely maximum seasonal fluctuations. Therefore it is not possible to provide a

detailed assessment of groundwater flood risk or provide detailed advice on suitability for

infiltration SUDS.

3.5.2 It is probably not sufficient to rely on the work undertaken by developers through the planning

application process, unless longer term (and continuous) monitoring is included as a condition

attached to planning approval. Groundwater levels are often only measured once, or, at most,

for a number of weeks. It would be advisable for Richmond BC, in combination with the

Environment Agency, to begin long term monitoring of superficial aquifer groundwater levels.

3.5.3 It is also important to understand how changing policies relating to infiltration SUDS can impact

groundwater levels. For example, historically, drainage from existing sites with artificial

impermeable surfaces may have been directed to surface water courses, leading to a potential

lowering of groundwater levels. The introduction of infiltration SUDS (e.g. soakaways) to these

sites may slowly reverse this process, leading to increased groundwater recharge and a

subsequent rise in groundwater levels. This could prevent soakaways from operating and the

reduction in unsaturated zone thickness may not be acceptable to the Environment Agency

owing to its responsibilities under the Water Framework Directive. It may also cause

groundwater flooding of infrastructure, basements / cellars etc that were designed and

constructed during the period of reduced groundwater recharge.

3.5.4 Long term groundwater level monitoring is required to support decision making with respect to

future land development and future co-ordinated investments to reduce the risk flooding and

inform the assessment of suitability for infiltration SUDS. Once sufficient data has been

collected, it may be suitable to develop a groundwater level warning system using the

observation borehole network. Finally, the data may also be used to calibrate a numerical

groundwater model, which could provide an improved understanding of groundwater conditions

and the testing of water management options.

Schematic demonstrating the importance of long term groundwater level monitoring




4 Water Framework Directive and Infiltration SUDS

4.1.1 The Water Framework Directive approach to implementing its various environmental objectives

is based on River Basin Management Plans (RBMP). These documents were published by the

Environment Agency in December 2009 and they outline measures that are required by all

sectors impacting the water environment. The Thames River Basin District is considered within

the current study since, infiltration Sustainable Drainage Systems (SUDS) have the potential to

impact the water quality and water quantity status of aquifers.

4.1.2 Improper use of infiltration SUDS could lead to contamination of the superficial deposit or

bedrock aquifers, leading to deterioration in aquifer quality status or groundwater flooding /

drainage issues. However, correct use of infiltration SUDS is likely to help improve aquifer

quality status and reduce overall flood risk.

4.1.3 Environment Agency guidance on infiltration SUDS is available on their website at:

http://www.environment-agency.gov.uk/business/sectors/36998.aspx. This should be

considered by developers and their contractors, and by Kingston Borough Council when

approving or rejecting planning applications.

Key Water Level Considerations (Figure 3)

4.1.4 The areas that may be suitable for infiltration SUDS exist where there is a combination of high

ground and permeable geology. However, consideration should be given to the impact of

increased infiltration SUDS on properties further down gradient. An increase in infiltration /

groundwater recharge will lead to an increase in groundwater levels, thereby increasing the

susceptibility to groundwater flooding at a down gradient location. This type of analysis is

beyond the scope of the current report.

4.1.5 It is important to be aware of groundwater level conditions at a potential development site. The

maximum likely groundwater levels should be assessed, to confirm that soakaways will

continue to function even during prolonged wet conditions. The areas where there is increased

potential for elevated groundwater are shown on Figure 3.

Key Geological Considerations (Figure 4)

4.1.6 The infiltration SUDS suitability assessment shown on Figure 4 is based on minimum

permeability data obtained from the BGS. There also exist maximum permeability data,

however, only the minimum permeability is used, as this is understood to be more

representative of the bulk permeability.

4.1.7 Three permeability zones have been identified:

1) Infiltration SUDS potentially suitable: Minimum permeability is high or very high for

bedrock (and superficial deposits if they exist).

2) Infiltration SUDS potentially unsuitable: Minimum permeability is low or very low for

bedrock (and superficial deposits if they exist).

3) Infiltration SUDS suitability uncertain: Minimum permeability is low or very low for

bedrock and high or very high for superficial deposits OR minimum permeability is low or

very low for superficial deposits and high or very high for bedrock.




4.1.8 The third category is required because the thickness of superficial deposits is uncertain. If they

are thick and impermeable, shallow soakaways may not intercept underlying higher

permeability bedrock. If they are thin and permeable, but perched over impermeable bedrock,

they may not have the capacity to receive the additional recharge from infiltration SUDS. Under

the third category, it is particularly important that the developer undertakes an appropriate site

investigation to determine infiltration SUDS suitability.

4.1.9 Figure 4 shows that there are no areas identified as potentially suitable for infiltration SUDS

across the Richmond BC area. The majority of the borough has been identified as having an

uncertain suitability for infiltration SUDS with a need for enhanced site investigation. These

areas are associated with River Terrace Deposits overlying the London Clay Formation

aquiclude. Site investigations will be required to identify the thickness of deposits and

demonstrate that they are able to accept the additional recharge.

4.1.10 Areas of Richmond BC that have been defined as potentially unsuitable for infiltration SUDS

are those where the London Clay Formation or Head deposits outcrop at surface, and along

the low lying valley areas next to the River Thames, River Crane and Beverley Brook, where

Alluvium is found.

4.1.11 It is stressed that this is a high level assessment and only forms an approximate guide to

infiltration SUDS suitability; a site investigation is required to confirm local conditions.

Key Water Quality Considerations (Figure 4)

4.1.12 Infiltration SUDS should be located away from areas of historic landfill (as identified in

Figure 4) and areas of known contamination or risk of contamination, where possible, to

ensure that the drainage does not re-mobilise latent contamination or exacerbate the risk to

groundwater quality and possible down gradient groundwater receptors, such as abstractors,

springs and rivers. A preliminary groundwater risk assessment should be included with the

planning application.

4.1.13 Restrictions on the use of infiltration SUDS apply to those areas within Source Protection

Zones (SPZ). However, no SPZs have been identified by the Environment Agency in the

Richmond BC area.




5 Conclusions and Recommendations

5.1 Conclusions

5.1.1 The following conclusions can be drawn from the current study:

• The significant thickness of London Clay Formation hydraulically separates the underlying

Chalk principal aquifer from overlying Claygate Member and superficial deposits. Therefore,

the Chalk aquifer is not pertinent to the current study.

• The superficial deposits, particularly the River Terrace Deposits, are expected to form a

significant perched aquifer over the London Clay Formation aquiclude, particularly at lower

elevations. However, the Environment Agency / Richmond BC do not currently monitor

groundwater levels in the superficial deposits.

• A number of potential groundwater flooding mechanisms have been identified. Of

significance are those flooding mechanisms associated with the superficial aquifers and

their hydraulic continuity with surface water courses. Underground structures including

basements and cellars are at most risk from groundwater flooding.

• A data set showing the increased potential for elevated groundwater has been provided,

which is primarily based on the BGS groundwater flooding susceptibility data set for the

Richmond BC area. The map indicates that there is no increased potential for elevated

groundwater within the consolidated (bedrock) aquifers. The permeable superficial deposits

that have been identified as having an increased potential for elevated groundwater are

Head, Alluvium, and in particular, River Terrace Deposits, where they overlie the London

Clay Formation, ground elevations are low and they are near to surface water courses.

• Groundwater flooding incidents provided by the Environment Agency have been assessed

and a fairly good correlation was found with the increased potential for elevated

groundwater data set. There are a small number of discrepancies between these data sets,

which suggests that the BGS data set may need to be refined. The majority of the

groundwater flooding incidents are thought to be related to perched water tables within

superficial deposits, particularly the River Terrace deposits.

5.1.2 Without long term (and continuous) groundwater monitoring, it is not possible to derive

groundwater level contours or understand maximum seasonal fluctuations and potential climate

change impacts. Therefore, at this stage, it is not possible to provide a detailed assessment of

groundwater flood risk or provide detailed advice on suitability for infiltration SUDS.




5.2 Recommendations

5.2.1 The following recommendations are made based on the current study. These will allow for a

more detailed assessment of increased potential for elevated groundwater and suitability for

infiltration SUDS:

• The areas identified as having increased potential for elevated groundwater should be

compared with those areas identified as being susceptible to other sources of flooding e.g.

fluvial, pluvial and sewer. An integrated understanding of flood risk will be gained through

this exercise;

• Acquisition of 1:10,000 scale geological mapping, if it exists, for the areas identified as

having increased potential for elevated groundwater;

• Information on mains leakage, foul sewer leakage and groundwater infiltration should be

obtained from Thames Water, if available;

• Data identifying properties with basements / cellars should be used to improve the

understanding of susceptibility to groundwater flooding;

• Site investigation reports for historic development sites could be reviewed to obtain

additional groundwater level information, to improve the conceptual understanding of the

area. Water level information on BGS borehole logs will be another source of information;

• The impact of infiltration SUDS on groundwater levels (and therefore groundwater flooding

susceptibility) should be considered further. This may require the construction of a local

groundwater model;

• Monitoring boreholes should be installed in the River Terrace Deposits, fitted with automatic

level recording equipment for a minimum period of one year and water quality sampling

undertaken. At this point a review of the monitoring network should be undertaken and an

update on potential for elevated groundwater analysis and infiltration SUDS guidance

provided.

• The proposed monitoring boreholes should assist the Environment Agency with water

quality and quantity assessments for the next River Basin Management Plan. Therefore, site

selection should be agreed with the Environment Agency and the necessity for water quality

monitoring agreed;

• Construction of a numerical groundwater model for the River Terrace Deposits should be

considered when at least 3 years of monitoring has been undertaken. The model could then

be used as a tool for assessing the impact of infiltration SUDS, other water management

options and climate change on the aquifers; and

• The Thames Estuary 2100 project is considering a number of options to manage the

anticipated future increase in tidal and fluvial flood risk along the River Thames Estuary. The

impact of these options on the potential for elevated groundwater should be considered

further as part of a more detailed assessment.




6 References

• DEFRA, March 2010. Surface Water Management Plan Technical Guidance.

• Environment Agency, December 2009. River Basin Management Plan. Thames River Basin

District.

• Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald,

A M, Coleby, L M, Talbot, J C, McKenzie, A A, Bird, M J, Cunningham, J, and Robinson, V

K., 2000. The physical properties of minor aquifers in England and Wales. British Geological

Survey Technical Report, WD/00/4. 39pp. Environment Agency R&D Publication 68.

• Ellison, R A, Woods, M,A, Allen, D, J, Forster, A, Pharoah, T, C and King , C. 2004.

Geology of London. Memoir of the British Geological Survey, Sheets 256, 257, 270 and 271.

• Jacobs, August 2009. Richmond Upon Thames and Royal Borough of Kingston First Edition

Surface Water Management Plan. Draft Final report.

• Wade, S, Hossell, J, Hough, M and Fenn, C. 1999. The impacts of Climate Change in the

South East: Technical Report, WS Atkins, Epsom.

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THIS DRAWING MAY BE USED ONLY FORTHE PURPOSE INTENDED

© Crown Copyright. All rights reserved. GLA (LA100032379) 2011Covers all data that has been supplied and distributed under license for the Drain London project.Digital geological data reproduced from British Geological Survey(c) NERC Licence No 2011/053A

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Bedrock Geology

Drain London Programme Board Members

FIGURE 1

ConsultantsURS / Scott Wilson6 - 8 Greencoat PlaceLondonSW1P 1PL


London Borough Richmond

Legend

NORTH Richmond Borough Council

" Groundwater Flood Incident (EA Records)

Main Rivers

Faults

Artificial (Undivided)

Bedrock Geology

Bagshot Beds

Claygate Member

London Clay

Lambeth Group

Harwich Formation

Thanet Sand Formation

Upper Chalk (Undifferentiated)

Middle Chalk(Undifferentiated)

Lewes Nodular Chalk Formation

New Pit Chalk Formation

Notes

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Bedrock and Superficial Geology


FIGURE 2




Legend

NORTH Richmond Borough Council

" Groundwater Flood Incident (EA Records)

Main Rivers

Faults

Artificial (Undivided)

Superficial Geology

Head

Peat

Alluvium

Langley Silt Member

River Terrace Deposits (Undifferentiated)

Kempton Park Gravel Formation

Taplow Gravel Formation

Hackney Gravel Member

Lynch Hill Gravel Member

Boyn Hill Gravel Member

Black Park Gravel Member

Bedrock Geology

Bagshot Beds

Claygate Member

London Clay

Lambeth Group

Harwich Formation

Thanet Sand Formation

Upper Chalk (Undifferentiated)

Middle Chalk(Undifferentiated)

Lewes Nodular Chalk Formation

New Pit Chalk Formation

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FIGURE 3




Legend

NORTH Richmond Borough Council" Groundwater Flood Incident (EA Records)

Main RiversArtificial (Undivided)

Increased Potential for Elevated Groundwater inPermeable Superficial DepositsConsolidated Aquifers

Increased Potential ForElevated Groundwater

1.The increased Potential for Elevated Groundwater map shows those areas within the London Boroughs where there is anincreased potential for groundwater to rise sufficiently to interact with the ground surface or be within 2m of the groundsurface. Such groundwater rise could lead to the following:

-Flooding of basements of buildings below ground level;-Flooding of buried services or other assets below ground level;-Inundation of farmland, roads, commercial, residental and amenity areas;-Flooding of ground floors of buildings above ground level; andOverflowing of sewers and drains

2.Incident records shown are generally unconfirmed and may include issues such as water main bursts or non-groundwater related problems.3.Areas not shown to have increased potential for elevatedgroundwater should be considered to have a low potential for elevated groundwater - Lack of information does not imply 'no potential' of elevated groundwater in that area.4.Includes groundwater flood mapping provided by JBA consulting, Copyright. Jeremy Benn Associates Limited 2008-2011, partially derived from data supplied by the Environment Agency.

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FIGURE 4




Legend

NORTH Richmond Borough CouncilEA Groundwater Source Protection Zone

Inner ZoneOuter ZoneHistoric Landfill Site

Infiltration SUDS SuitabilityInfiltration SUDS potentially suitableInfiltration SUDS potentially unsuitableInfiltration SUDS Suitability Uncertain -Site investigation required

Infiltration SUDS Suitability Map

NotesThis map forms an approximate guide to Infiltration SUDS Suitability. However, for all new developments, site investigation is required to confirm local geology, depth to groundwater and infiltration rates.

surface water management plan

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