Surface Water Management Plan. Intermediate Assessment of Groundwater Flooding Susceptibility. Tier 2 March Prepared for

Transcription

1 Intermediate Assessment of Groundwater Flooding Susceptibility Tier 2 March 2011 Prepared for

2 Revision Schedule 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 This document has been prepared in accordance with the scope of Scott Wilson's appointment with its client and is subject to the terms of that appointment. It is addressed to and for the sole and confidential use and reliance of Scott Wilson's client. Scott Wilson accepts no liability for any use of this document other than by its client and only for the purposes for which it was prepared and provided. No person other than the client may copy (in whole or in part) use or rely on the contents of this document, without the prior written permission of the Company Secretary of Scott Wilson Ltd. Any advice, opinions, or recommendations within this document should be read and relied upon only in the context of the document as a whole. The contents of this document do not provide legal or tax advice or opinion. Scott Wilson Scott House Alencon Link Basingstoke RG21 7PP Tel Scott Wilson Ltd 2011

3 Table of Contents Abbreviations... 1 Glossary Introduction Groundwater Flooding The Current Report Topography, Geology and Hydrogeology Topography and Hydrology Geology Hydrogeology Assessment of Groundwater Flooding Susceptibility Groundwater Flooding Mechanisms Evidence of Groundwater Flooding Potential for Elevated Groundwater Data Sets Summary of Potential for Elevated Groundwater Importance of Long Term Groundwater Level Monitoring Water Framework Directive and Infiltration SUDS Conclusions and Recommendations Conclusions Recommendations References List of Tables Table 1 Table 2 Table 3 River Terrace Deposit Units and Nomenclature Geological Units in the Study Area and Hydrogeological Significance 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

4 Abbreviations ACRONYM BGS DEFRA EA LiDAR SUDS SWMP DEFINITION British Geological Survey Department for Environment, Fisheries and Rural Affairs Environment Agency Light Detection and Ranging Sustainable Drainage Systems 1

5 Glossary TERM Aquiclude Aquifer Aquitard Climate Change Flood defence Floods and Water Management Act Fluvial flooding Groundwater Nomeculture Lithology Pluvial Flooding Risk Sewer flooding Sustainable Drainage Systems DEFINITION Formations that may be sufficiently porous to hold water, but do not allow water to move through them. 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. Formations that permit water to move through them, but at much lower rates than through the adjoining aquifers. Long term variations in global temperature and weather patterns, caused by natural and human actions. Infrastructure used to protect an area against floods, such as floodwalls and embankments; they are designed to a specific standard of protection (design standard). 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. Flooding by a river or a watercourse. Water that is underground. For the purposes of this study, it refers to water in the saturated zone below the water table. Is a term given to a list of sub identified lithologies and their names within the main named geological unit. The gross physical character, description of a rock or rock formation 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. The product of the probability and consequence of the occurrence of an event. Flooding caused by a blockage, undercapacity or overflowing of a sewer or urban drainage system. 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. 2

6 1 Introduction 1.1 Groundwater Flooding 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 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 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 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. 3

7 1.2 The Current Report The Greater London Authority (GLA) has commissioned Scott Wilson to complete tier 2 of the Richmond Borough Council (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) 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 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. 4

8 2 Topography, Geology and Hydrogeology 2.1 Topography and Hydrology 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 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 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 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 maod. Close to the River Thames, ground elevations are generally below 10 maod. 2.2 Geology 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 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 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 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 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 5

9 investigation data, although the London Clay Formation on the down thrown side of the faults is expected to be thicker. Superficial Geology 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 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 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 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 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 River Terrace Deposits 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 Black Park Gravel Member 3 Sand and gravel with no lenses of clay and silt (* Thicknesses derived from BGS, Lexicon. 2011) 6

10 2.3 Hydrogeology 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 Superficial Deposits Bedrock Geology 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 Kempton Park Gravel Formation Taplow Gravel Formation Hackney Gravel Member Lynch Hill Gravel Member Boyn Hill Gravel Member Black Park Gravel Member Very Low - Low High - Very High Variable (but probably an aquitard). Unproductive strata. 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 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 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 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 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. 7

11 Superficial Hydrogeology 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 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 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 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 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 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 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 There may be some hydraulic continuity between the Black Park Gravel Member and the underlying Claygate Member in the Richmond Park area. 8

12 Surface Water / Groundwater Interactions 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 In the 19 th Century groundwater water supplies in London were obtained from the shallow superficial and bedrock deposits. In the early 20 th 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 There may be some smaller private abstractions from the superficial deposits and this information will be held by the Environment Agency. Artificial Groundwater Recharge 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 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. 9

13 3 Assessment of Groundwater Flooding Susceptibility 3.1 Groundwater Flooding Mechanisms 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. 10

14 3.2 Evidence of Groundwater Flooding 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* Taplow Gravel Fn Taplow Gravel Fn Alluvium Taplow Gravel Fn Location Richmond London Hampton Court Teddington NGR Incident N o ** Reported Incident Year 1 Flooded Cellar Landowner has been informed there is shallow groundwater under his property & that he is at risk of groundwater flooding Flow from bank for 22/23yrs Basement flooding 2003 Taplow Gravel Fn Teddington Rising WL under home 2000 Kempton Park Gravel Fn Teddington Water in air raid shelter in garden 2001 Kempton Park Gravel Fn GW Flooding enquiry 2007 Kempton Park Gravel Fn Kingston-on Thames Water in Cellar 2001 Kempton Park Gravel Fn Hampton Wick Water in cellar 2001 Kempton Park Gravel Fn Hampton Wick Wet Basement 2000 London Clay Edge of Alluvium Twickenham Formation Boggy Garden 2000 Edge of Langley Silt Fn Richmond Waterlogged patch of ground Edge of Langley Silt Fn Twickenham Installed sump and pump 2000 Kempton Park Gravel Fn Recent flooding through ground floor 2007 Kempton Park Gravel Fn Kew Occasional water seepage in basement 2007 Edge of Taplow Gravel East Sheen Fn and Head SW Standing water in garden 2000 Head SW Waterlogged Garden 2000 Head SW Flooded basement 2003 Taplow Gravel Fn Richmond Flooded Cellar 2010 Kempton Park Gravel Fn SW Buying property -info on flooding 2001 Alluvium Water in cellar after heavy rain 2008 Taplow Gravel Formation TW 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 11

15 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 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 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 o o o British Geological Survey (BGS). Groundwater Flood Susceptibility maps; Environment Agency (EA). Thames Estuary, 2100 groundwater hazard maps; DEFRA. Groundwater emergence maps; and JBA. Groundwater flood maps However, only the BGS groundwater flooding susceptibility and EA Thames Estuary data sets are relevant to the Richmond BC area 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 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 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 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 12

16 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 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 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 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 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 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. 13

17 3.5 Importance of Long Term Groundwater Level Monitoring 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 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 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 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 14

18 4 Water Framework Directive and Infiltration SUDS 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 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 Environment Agency guidance on infiltration SUDS is available on their website at: 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) 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 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) 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 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. 15

19 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 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 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 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) 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 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. 16

20 5 Conclusions and Recommendations 5.1 Conclusions 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 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. 17

21 5.2 Recommendations 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. 18

22 6 References DEFRA, March Technical Guidance. Environment Agency, December 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., 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 Geology of London. Memoir of the British Geological Survey, Sheets 256, 257, 270 and 271. Jacobs, August Richmond Upon Thames and Royal Borough of Kingston First Edition. Draft Final report. Wade, S, Hossell, J, Hough, M and Fenn, C The impacts of Climate Change in the South East: Technical Report, WS Atkins, Epsom. 19

23 THIS DRAWING MAY BE USED ONLY FOR THE PURPOSE INTENDED Legend Richmond Borough Council NORTH 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 19 New Pit Chalk Formation 13 Notes London Borough Richmond Filepath: D Drain London_Tier 2_Group 8\700 Technical\711 GIS\01 Layout\Richmond\Bedrock Geology.mxd 2 Crown Copyright. All rights reserved. GLA (LA ) 2011 Covers 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 A3 1:50, Date 22/03/2011 Drawn by C.Woolhouse Approved by S.Cox 3 Bedrock Geology Consultants URS / Scott Wilson 6-8 Greencoat Place London SW1P 1PL Drain London Programme Board Members Kilometres FIGURE 1

24 THIS DRAWING MAY BE USED ONLY FOR THE PURPOSE INTENDED Legend Richmond Borough Council NORTH 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 17 Black Park Gravel Member Bedrock Geology Bagshot Beds 19 Claygate Member London Clay Lambeth Group 13 Harwich Formation Thanet Sand Formation Upper Chalk (Undifferentiated) Middle Chalk(Undifferentiated) Filepath: D Drain London_Tier 2_Group 8\700 Technical\711 GIS\01 Layout\Richmond\Bedrock and Superficial Geology.mxd Lewes Nodular Chalk Formation 12 New Pit Chalk Formation London Borough Richmond Crown Copyright. All rights reserved. GLA (LA ) 2011 Covers 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 A3 1:50, Date 22/03/2011 Drawn by C.Woolhouse Approved by S.Cox 3 Bedrock and Superficial Geology Consultants URS / Scott Wilson 6-8 Greencoat Place London SW1P 1PL Drain London Programme Board Members Kilometres FIGURE 2

25 THIS DRAWING MAY BE USED ONLY FOR THE PURPOSE INTENDED Legend Richmond Borough Council NORTH Groundwater Flood Incident (EA Records) Main Rivers Artificial (Undivided) Increased Potential for Elevated Groundwater in Permeable Superficial Deposits Consolidated Aquifers Notes 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; and Overflowing of sewers and drains 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 elevated groundwater 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 , partially derived from data supplied by the Environment Agency Filepath: D Drain London_Tier 2_Group 8\700 Technical\711 GIS\01 Layout\Richmond\Increased Potential for Elevated GW.mxd The increased Potential for Elevated Groundwater map shows those areas within the London Boroughs where there is an increased 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: 12 London Borough Richmond Crown Copyright. All rights reserved. GLA (LA ) 2011 Covers 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 A3 1:50, Date 22/03/2011 Drawn by C.Woolhouse Approved by S.Cox Increased Potential For Elevated Groundwater Consultants URS / Scott Wilson 6-8 Greencoat Place London SW1P 1PL Drain London Programme Board Members Kilometres FIGURE 3