Surface Water Management Plan
Transcript of Surface Water Management Plan
Prepared for
Surface Water Management Plan
Intermediate Assessment of Groundwater Flooding Susceptibility
Tier 2
March 2011
Richmond Borough Council
Surface Water Management Plan
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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
Richmond Borough Council
Surface Water Management Plan
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
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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
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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.
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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.
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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.
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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
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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)
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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.
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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.
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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.
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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.
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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
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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
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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.
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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
Richmond Borough Council
Surface Water Management Plan
Intermediate Assessment of Groundwater Flooding Susceptibility March 2011 15
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.
Richmond Borough Council
Surface Water Management Plan
Intermediate Assessment of Groundwater Flooding Susceptibility March 2011 16
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.
Richmond Borough Council
Surface Water Management Plan
Intermediate Assessment of Groundwater Flooding Susceptibility March 2011 17
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.
Richmond Borough Council
Surface Water Management Plan
Intermediate Assessment of Groundwater Flooding Susceptibility March 2011 18
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.
Richmond Borough Council
Surface Water Management Plan
Intermediate Assessment of Groundwater Flooding Susceptibility March 2011 19
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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FIGURE 1
ConsultantsURS / Scott Wilson6 - 8 Greencoat PlaceLondonSW1P 1PL
Surface Water Management Plan
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
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© 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
Scale at A31:50,000
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Bedrock and Superficial Geology
Drain London Programme Board Members
FIGURE 2
ConsultantsURS / Scott Wilson6 - 8 Greencoat PlaceLondonSW1P 1PL
Surface Water Management Plan
London Borough Richmond
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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© 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
Scale at A31:50,000
Approved byS.Cox
Date22/03/2011
Drawn byC.Woolhouse
Drain London Programme Board Members
FIGURE 3
ConsultantsURS / Scott Wilson6 - 8 Greencoat PlaceLondonSW1P 1PL
Surface Water Management Plan
London Borough Richmond
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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© 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
Scale at A31:50,000
Approved byS.Cox
Date22/03/2011
Drawn byC.Woolhouse
Drain London Programme Board Members
FIGURE 4
ConsultantsURS / Scott Wilson6 - 8 Greencoat PlaceLondonSW1P 1PL
Surface Water Management Plan
London Borough Richmond
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.