16. hydrology and water resources 16-0_hydrology and water...environmental statement hydrology and...

Dunsfold Park Chapter 16 Environmental Statement Hydrology and Water Resources

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16. HYDROLOGY AND WATER RESOURCES

INTRODUCTION 16.1. This chapter considers the impact of the proposed scheme in relation to the

hydrological environment at and around the site.

16.2. At first glance the impact of the proposed scheme on the hydrological regime of the area may appear to be limited due to the lack of obvious ‘on site’ receptors. A number of receptors are however located at the site periphery or boundary, and linkages do exist via groundwater and storm water mechanisms.

16.3. The aim of this chapter is as follows:

• To highlight key legislation and drivers relating to the water environment.

• To provide a brief over view of the current baseline condition.

• To provide an indication of the key likely impacts and ongoing assessments being undertaken.

LEGISLATIVE, REGULATORY AND POLICY FRAMEWORK 16.4. The main drivers underpinning regulation of the water environment include the 2003

Water Act, the 1991 Water Resource Act, the 1991 Land Drainage Act and the 1995 Environmental Protection Act. These, and other Acts, also enable a series of EU Directives which are mostly implemented through Statutory Instruments related to these Acts. This situation is soon to be significantly modified as the Water Framework Directive (WFD) was enacted into English and Welsh Law in December 2003. The practical applications of this are currently under formulation. This requires that all controlled waters achieve ‘good status’ by 2015. As such, rigorous controls are typically enforced by both the Environment Agency and Local Authorities to ensure that no deterioration occurs or programmes (i.e. improvement to sewage treatment works) are implemented where improvements are deemed necessary.

16.5. If any properties within the proposed scheme are to be sourced from non mains water supplies then all such proposals will need to comply with the Private Water Supplies Regulations 1991 (PWSR).

16.6. In addition to this there are a number of planning policy documents published by the Department for Communities and Local Government that will need to be considered when assessing impacts to the water environment. These include:

• Planning Policy Statement 1: Delivering Sustainable Development.

• Planning Policy Statement 23: Planning and Pollution Control.

• Planning Policy Statement 25: Development and Flood Risk.


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16.7. The Pollution Prevention Guidelines (PPGs) are published jointly by the Scottish Environmental Protection agency (SEPA) and the Environment Agency (EA). These are the principal documents used for guidance on preventing contamination of surface water from construction activities. Those relevant to the proposed scheme include:

• PPG1: General guide to the prevention of pollution (EA).

• PPG2: Above ground oil storage tanks (EA).

• PPG5: Works in, near or liable to affect watercourses (EA).

• PPG6: Working at construction and demolition sites (EA).

• PPG8: Storage and disposal of used oils (EA).

• PPG26: Storage and handling of drums & intermediate bulk containers (EA).

ASSESSMENT METHODOLOGY 16.8. The assessment of the potential impact posed to the hydrological environment within

this report has been undertaken based on a source pathway receptor model. As such, receptors of potential impacts are identified based on the existence of realistic pathways for impact and not just on proximity to the site.

Desk Study 16.9. The baseline condition for all identified receptors has been described and from this

their sensitivity has been assessed based on issues covering ecological importance, water resource value, vulnerability to flooding, risks posed to human health and risks to buildings and services. The overall significance of impacts takes in to account both receptor sensitivity and magnitude of impact.

16.10. The Environment Agency and Thames Water have both been consulted with regards to this proposed scheme and the information they have provided and the concerns that they have raised are reflected (where relevant) within this chapter.

16.11. A full desk study was undertaken for the site and as part of this data has been reviewed from a wide range of sources. This includes:

• Ordnance Survey mapping.

• British Geological Survey (BGS) maps.

• BGS site report1.

• Site Survey.

• National Rivers Authority (NRA) groundwater vulnerability maps.

1 BGS (2005), Water Borehole Prognosis, BGS Report No: GR036179_1, British Geological Survey, UK


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• Environment Agency website (www.environment-agency.gov.uk).

• Natural England website (www.naturalengland.org.uk).

Field Survey 16.12. A walkover survey was carried out by Enviros on the 2 May 2006 during which key

potential pathways and receptors were identified.

16.13. Between the 12th and 16th of June 2006 Enviros conducted an intrusive investigation of the site for the purposes of defining the shallow geology and hydrogeology at the site. This investigation involved the following;

• Excavation of 20 window sample holes across the main body of the site.

• Installation of 10 shallow wells.

• Excavation of 6 trial pits.

16.14. Following on from this the following monitoring was undertaken:

• Infiltration testing in the trial pits on the 12th July.

• Recording of groundwater levels at the site were monitored on the 6th, 11th and 28th of July at each of the installed boreholes.

• Water sampling and chemical analysis of samples collected on the 6th July.

• Slug testing (aquifer permeability) at each of the installed boreholes on the 11th July.

16.15. Figure 16-1: Details of Previous Intrusive Investigations, shows the locations of the field testing undertaken across the site.

Approach to the Assessment of Impacts 16.16. A formal impact assessment has been undertaken using a risk based approach in line

with DEFRA (previously DETR)2 and IEMA3 guidance. Formal definition of the constituent parts of this assessment and a detailed description of the process is contained in Appendix 16-1: Assessment Criteria Hydrology and Hydrogeology. It should be noted that this assessment methodology is not the same as that detailed in the scoping report in that it considers the importance / sensitivity of receptors more explicitly.

16.17. The sensitivity of all receptors identified will be assessed based on both their vulnerability to changes in the hydrological regime and their importance as defined by local national and international designations. This assessment will then be combined with an assessment of the magnitude of hydrological effect to derive the assessed level of impact significance.

2 DETR (1998),:Guidance on the New Approach to Appraisal, Department for Transport and the Regions, UK 3 IEMA (2004): Guidelines for Environmental Impact Assessment. Institute of Environmental Management and Assessment, Lincoln


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16.18. The significance of the assessed impact will be assessed using a seven point scale as listed below:

• Minor (positive and negative).

• Moderate (positive and negative).

• Major (positive and negative).

• Neutral.

Flood Risk Assessment 16.19. The methodology used to assess flood risk is necessarily different to that used for

environmental impacts due to the fact that it falls under PPS25. This requires that a risk based approach is followed. As such the probability of impact is more formally considered.

16.20. Full details of the assessment undertaken and the assessment methodology used are contained in the flood risk assessment4. Within the text of this chapter summary statements concerning flood risk are included, reference should however be made to the flood risk assessment for full details.

BASELINE

Topography 16.21. A site survey, including topography, was undertaken in February 2004 by Premier

Surveys Limited. This shows that site levels range between 56m aOD and 37m aOD.

16.22. The highest area of the site is along the northern boundary, from here the ground slopes down to the main central section of the site, including the entire runway, set between 50m aOD and 52m aOD. This central section generally slopes down to the south eastern boundary and the Wey and Arun canal which is at an elevation of approximately 47m aOD.

16.23. Ground levels also slope down from the central area to the west where a watercourse called Springfield Rew is situated. The level of this watercourse is approximately 37m aOD.

Land Cover and Land Use 16.24. As an airfield a large proportion of the site away from the roads and runways is

managed grassland. The major exception to this is the industrial area to the north of the site where is almost all covered by impermeable surfaces.

Meteorology 16.25. The closest location where detailed meteorological data is readily available is Wisley

located approximately 20km to the north of the site near Woking. At this location

4 Enviros (2008): Dunsfold Park Flood Risk Assessment, Dunsfold Park application document 4, Enviros Consulting Ltd, London


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the average annual rainfall (1970-2000) is 647.1mm. Monthly averages indicate that the majority of rainfall falls between September and January.

Geology / Hydrogeology 16.26. Broadly speaking the site is underlain by the Weald Clay formation, with a band of

superficial deposits located above. These superficial deposits are restricted to a strip across the site running from east to west.

16.27. Infiltration testing and slug tests conducted in these superficial deposits indicate that the formation generally has a low permeability, with isolated bands of higher permeability. These are however discontinuous and therefore poorly connected.

16.28. Both the infiltration testing and observations of water ponding at the surface to the east of the site indicate that vertical movement of water at the site is slow. Although the observed flows from the site drainage system do indicate that natural recharge does occur.

16.29. The Environment Agency (EA) groundwater vulnerability map5 shows that the majority the site is located on a non aquifer of negligible permeability, the exception being for the north western and eastern boundaries where deposits are classified as minor aquifers. The minor aquifers are associated with more permeable superficial deposits.

16.30. Measurable groundwater discharge was noted at five locations. Groundwater flow was observed at the two interceptor tanks, located to the west and east of the site. Three discharge points were also noted along the Wey and Arun Canal in close proximity to Compasses Bridge.

16.31. The groundwater drainage pattern is likely to be affected by the presence of land drains but in general groundwater in the central and eastern areas of the site is likely to drain to the east into the Wey and Arun canal. Groundwater in the west of the site is likely to drain towards the west and the topographic low. Water levels in the canal are likely to be an expression of the water table.

16.32. The groundwater monitoring undertaken showed that groundwater typically occurs between one and two metres below the site. The monitoring appears to confirm the general flow pattern described above, but variations in local permeability tend to mask this in places.

16.33. The Weald Clay is not considered an aquifer. However, the limestone and sandy beds in the Weald Clay may yield small quantities of groundwater. The permeable bands in the superficial strata are however discontinuous and any flow therefore likely to be limited.

Surface Hydrology 16.34. Figure 16-2: Hydrological Features shows graphically the local surface water

features and a schematic of the site drainage system.

5 NRA, Groundwater Vulnerability Map, Sheet 45, Sussex and Surrey, National Rivers Authority


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16.35. Given that this site is an existing airfield, surface water features are not prominent at the site. The surface water features that do exist are primarily the product of the site drainage system or otherwise occur at the periphery of the site.

16.36. The site does not lie within a known floodplain and the entire site is shown on the Environment Agency flood map to be classified as Flood Zone 1, the lowest risk category. To the east of the site there is an area classified as Flood Zone 2 along the Wey and Arun Canal with the Canal itself classified as Flood Zone 3. Full descriptions relating to flood risk are contained within the separate flood risk assessment6 prepared to accompany the planning application.

Springfield Rew

16.37. A watercourse called Springfield Rew flows in a southerly direction to the west of the site boundary. It is estimated7 that this watercourse has a catchment area of approximately 2km2, comprised predominantly of the fields and woodland to the west and north of the Dunsfold Park site. The catchment of this watercourse also includes most of the village of Dunsfold.

16.38. The watercourse runs along the site boundary. The profile of this drain along this reach appeared natural with the stream generally meandering within a narrow floodplain at the base of a small steep valley.

16.39. This stream forms a small pond just beyond the very western tip of the site before passing beneath Alfold Road and flowing to the southwest. This watercourse eventually flows into Loxwood Stream and on to the River Arun, which is used as a resource for the canal at locations to the south of the site.

16.40. Loxwood Stream is monitored by the Environment Agency. On the basis of this they have classified the watercourse to have a biological quality indicator class of A (excellent) and a chemical quality indicator (GQA) class of C (fair) upstream and D (poor) downstream of the confluence with Springfield Rew. This is the closest monitored reach to the site.

Wey and Arun Canal

16.41. The Wey and Arun canal runs along the southern and eastern boundary of the site. Along this reach the canal is disused although some stretches do still hold some water. Generally, flow within the canal is minimal.

16.42. During the site walkover the condition of the canal was observed to be markedly different either side of the Compasses Bridge Entrance to the site. To the south of this the canal was effectively silted up with very shallow depths of water remaining whilst to the north water depths are more appreciable in the canal, although it is understood that this reach is not navigable and that inflows are currently not sufficient to enable restoration.

6 Enviros (2008): Dunsfold Park Flood Risk Assessment, Dunsfold Park application document 4, Enviros Consulting Ltd, London 7 CEH (2006), Flood Estimation Handbook CD Rom, V2.0, Centre for Ecology and Hydrology


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16.43. It should be noted that water levels observed either side of the bridge were the same and are estimated to be approximately 1.5m below the typical level across the site. Flow between these two reaches of the canal is possible through a small culvert at Compasses Bridge.

16.44. The summit of the Wey and Arun canal is situated towards the southern corner of the site. As such water would have originally been fed into the system at this point and would then drain down in either direction. The majority of discharge from the site into the canal would therefore flow in a north easterly direction towards the River Wey. The River Wey then flows in a northerly direction to the Thames Valley. Any flow discharging in the opposite direction would discharge to the River Arun.

Benbow Rew

16.45. This small drain flows from east to west across the very northern most part of the site. The channel continues to flow in an easterly direction discharging into the canal a distance downstream via a small holding pond that it is believed was constructed as a source for the canal.

16.46. Whilst this feature is located on the site it is situated to the north of the proposed scheme area and is elevated above the remainder of the site receiving drainage from the fields further north. In addition to this both natural and altered groundwater flows at the site are believed to be away from this feature. As such no hydrological pathway of impact to this feature has been identified and it will not be considered further.

Site Drainage System

16.47. Across the main body of the site there are no significant surface water features, but it is known that under drainage exists that directs the majority of site storm flows as well as shallow groundwater flow. This system is extensive and complex as it has been constructed over a number of years as proposed scheme across the site has occurred.

16.48. The system comprises a series of major storm drains that receive storm water either directly via surface drains within impermeable areas of the site, or via an extensive field drain system.

16.49. The field drain system exists to prevent water logging across the central grassed areas of the site. No plans of this field drainage system have been identified, but it is known from intrusive investigation that it is both dense and extensive. Base flows from the field drain system within the site are at times considerable and remain strong even during prolonged dry periods and as such may form an important component of baseflow.

16.50. Six significant outfalls from the site area have been identified and these are all shown on Figure 16-2: Hydrological Features, along with catchment areas estimated from the site survey. The majority of the site outfalls towards the Wey and Arun canal, with a sizable proportion also discharging to Springfield Rew. Both of these receptors receive runoff from areas of the industrial estate, the runway and the


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greenfield areas of the site. As already discussed the area of the site discharging to Benbow Rew is negligible and falls entirely outside of the proposed scheme area.

16.51. Of the discharge to Springfield Rew approximately 80% discharges via an interceptor into a small channel that then runs down to Springfield Rew at the western site boundary. This channel is deeply incised and is continuing to cause erosion to the side of the system. This indicates that the channel was created (when the airfield was first constructed) as most natural channels in this area would be expected to have reached some kind of equilibrium.

16.52. Approximately 60% of the discharge towards the Wey and Arun canal flows via a large interceptor located at the site boundary to the east of the site. Flows from here are routed into a culvert then an open channel down towards the canal discharging at the south-eastern site boundary.

16.53. Whilst a few other small oil interceptors exist, the majority of the remainder of the site discharges into the receiving water bodies without any form of treatment. This includes discharge from areas of the runway, from the industrial areas to the south of the site and the fire training area.

Utilities

Foul Drainage

16.54. The existing flows from the Dunsfold Park site discharge to sewer and are then pumped to Cranleigh Waste Water Treatment Works operated by Thames Water.

16.55. The Environment Agency confirmed that this works is currently under stress and will need to be upgraded and extended to be able to accept more flows. Thames Water has confirmed that the works is under stress but have indicated that an upgrade is planned at some stage between 2010 and 2015.

Water Supply

16.56. Potable supplies within the existing Dunsfold Park site are provided by Thames Water via several small sized pipes around the site. The supply to development could be a 250mm supply pipe which is 2km away from the site.

Designated Sites 16.57. As previously detailed this assessment is based on a source pathway receptor model

and as such the following designated sites are identified based on both proximity and the existence of a hydrological pathway of impact.

16.58. On the far side of Wey and Arun Canal is situated Chiddingfold Forest Site of Special Scientific Interest (SSSI). This area is designated primarily as a result of the variety of tree species although there are also important faunal communities in a small watercourse draining from this area. Whilst flows from the site flow past and at one location through the designated area, no pathway for hydrological impact to the designated feature exists. As a result this designated site is not considered further within this assessment.


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16.59. As detailed above Springfield Rew is a tributary of the River Arun. A number of hydrologically sensitive sites exist along the River Arun. Those identified are:

Upper Arun SSSI

16.60. This site is a 14km reach of the River Arun designated for the dragonfly populations found along it. A direct pathway of impact from Dunsfold Park to this site exists.

Arun Valley Ramsar site and Special Protection Area (SPA) / Pulborough Brooks SSSI

16.61. This site comprises a series of wetland meadows that exist in the floodplain of the River Arun and has been designated due to the diverse floral and faunal populations. A direct path of impact from the Dunsfold Park site to designated features at this location would only exist during flood conditions.

Arun Banks SSSI

16.62. This site is an old tidal meander of the River Arun that still receives a limited amount of through flow from the main river. The site has been designated due to its rich and varied floral population. A limited potential pathway from Dunsfold Park to this site exists.

16.63. Flows entering the Wey and Arun Canal will eventually discharge towards the River Wey. Two fluvially dependant designated sites were identified along this watercourse. These are:

Wey Valley Meadows SSSI

16.64. This site is situated in the floodplain of the River Wey and consists of grassland areas. The site has been designated due to the rich plant communities and undisturbed grassland areas. A pathway from Dunsfold Park to the designated features at this site would only exist during flood conditions.

Papercourt SSSI

16.65. The site consists of wetland habitat and a flooded gravel pit adjacent to the River Wey. The area has been designated for the range of flora plus breeding and wintering bird populations.

Receptor Sensitivity 16.66. Table 16-1 lists the potential hydrological receptors, possible pathways of impact and

their assessed sensitivities. It should be noted that only where viable pathways for impact are identified are receptors carried forward to the impact assessment stage.


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Table 16-1: Receptor Sensitivity

Receptor Pathways of hydrological impact Sensitivity / Importance

Site drainage network Direct activity, surface runoff, shallow groundwater Very Low

Springfield Rew Surface runoff, shallow groundwater Low

Wey and Arun Canal Surface runoff, shallow groundwater Very Low

Benbow Rew None -

River Arun (including designated sites)

Flow via Springfield Rew and the Wey and Arun Canal High

River Wey (including designated sites) Flow via the Wey and Arun Canal High

Drift Cover Infiltration Low

Weald Clay Infiltration Very Low

Chiddingfold Forest None -

Water Supply Network Higher potable demand Medium

Foul Sewerage Network Higher foul flows Medium

Water / Sewage Treatment Works High potable demand / foul flows High

Fallback Position 16.67. It is assumed that should planning permission not be granted, operation of the site

would continue as currently, but temporary permissions would eventually expire and not be renewed. As such additional pressure on local water supply and sewerage infrastructure would be minimal, but existing high rates of site storm runoff to Springfield Rew and the Wey and Arun Canal would remain.

16.68. The ongoing operation of the site could be associated with a notable potential impact to the local hydrological environment from spillage of oils and from sediments washed off areas of hardstanding.

ASSESSMENT OF IMPACTS 16.69. This section provides an assessment of the magnitude of likely impacts arising from

the scheme, focusing on those which are potentially significant and negative. As detailed in the assessment methodology, when this measure is combined with the receptor sensitivity / importance through the matrix contained in Appendix 16-1: Assessment Criteria Hydrology and Hydrogeology the significance of the potential impacts can be determined. These final results, considered prior to any mitigation are detailed within Table 16-2: Construction Phase: Predicted impacts prior to mitigation, and Table 16-3: Operation Phase: Predicted impacts prior to mitigation.


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Construction 16.70. The section below defines construction impacts related to the proposals prior to

mitigation. In combination with receptor sensitivity overall significance is determined, as given in Table 16-2: Construction Phase: Predicted Impacts Prior to Mitigation, using the methodology set out in Appendix 16-1: Assessment Criteria Hydrology and Hydrogeology.

16.71. Assessing construction impacts is complicated by the phased nature of the proposed scheme. For any such phased proposed scheme the precise receptors affected by impact will likely alter through the course of proposed scheme as the proposed scheme process moves and additional drainage infrastructure is constructed to serve that proposed scheme area. It should however be noted that the construction phase impacts are all generally short term and most will only occur while proposed scheme is actually progressing.

Water Resources

Runoff from Compaction / Hardstanding

16.72. The process of development construction will require the operation of heavy machinery and plant around the site both within and adjacent to the proposed scheme areas. This is likely to result in the compaction of soils within the site thus reducing infiltration to the drift cover and increasing runoff rates to Springfield Rew and the Wey and Arun Canal. Both this and the construction of any temporary areas of hardstanding are likely to lead to higher rates of site runoff.

16.73. Whilst this could result in changes in the nature of the flows discharging to local surface water receptors the assessed magnitude of this potential effect is Minor or less for all of the identified receptors.

Damage to Existing Site Drainage Network

16.74. The existing drainage network across the site could be damaged during the construction process in a number of ways including accidental damage by plant and inappropriate early decommissioning of site services to prepare future proposed scheme areas. If this occurred it is possible that uncontrolled runoff / discharge could result changing the distribution of flows around the site and bypassing existing pollution control measures.

16.75. Whilst the environmental impact of such changes in flow distribution is assessed to be Negligible, the resulting potential effects relating to the creation of new pathways for pollution are assessed to be Moderate in magnitude for all immediate receptors of site runoff, but Minor for receptors further downstream.

De-watering

16.76. Construction at the site will require the excavation for foundation of buildings. If these excavations are deeper than one metre there is a potential for groundwater to start seeping into these pits. In order to allow construction to continue dewatering of the drift cover would therefore potentially be required in the immediate vicinity of the excavation.


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16.77. Testing at the site has shown that the permeability of the shallow deposits beneath the site is low. As a result of this the rate of inflow into the excavation is likely to be extremely slow and the radius of influence (i.e. the area potentially impacted) very limited. Given this the magnitude of the potential impact is assessed to be Negligible. It should be noted that if dewatering is required, under the 2003 Water Act, permission needs to be sought from the Environment Agency. Given the site situation approval for this should be a formality, but permission would be sought well in advance of construction commencing.

Pollution Control

Mobilisation of Sediments

16.78. Construction sites typically result in an increase in the mobilisation of sediments in surface runoff from the proposed scheme areas. This results from the removal of vegetation, open soil surfaces, stock piles of soil and other construction materials and the erosion caused by the movement of heavy plant and other traffic on untarmaced surfaces.

16.79. Higher sediment loads could get washed downstream into the canal and Springfield Rew. If the sediment load was sufficiently high this would start to smother vegetation, alter the bed profile / conveyance of the feature and / or lead to the blockage of culverts or other structures through which the water flowed. The magnitude of this potential effect is assessed to be High.

16.80. Further downstream the sediment would start to settle out, particularly within ponds or lakes, such as exist along Springfield Rew downstream of the site. As a result the potential effect would increasingly reduce progressing downstream from the site.

Pollution from Hydrocarbons and Chemicals

16.81. Construction at the site will require the use of and storage of a wide range of hydrocarbons and other chemicals. Spillage or uncontrolled disposal of these could rapidly lead to pollution of the shallow groundwater on site or surface water features at the periphery of the site. Additional pollution pathways exist whereby contaminated groundwater could flow into the field drain system or directly towards the surface water bodies.

16.82. If the amount of pollution was significant or if the nature of the pollutant was toxic such spillages could result in a significant deterioration in the water quality of runoff from the site. This in turn would impact the receptors of site runoff and have the potential to cause significant adverse ecological impact. The magnitude of this potential effect is assessed to be High on immediate receptors including the shallow geology, Springfield Rew and the Canal. The magnitude of the effect would however be Moderate for receptors further downstream.

Operation 16.83. The section below defines operational impacts related to the proposals prior to

mitigation. In combination with receptor sensitivity overall significance is determined, as given in Table 16-3: Operation Phase: Predicted impacts prior to


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mitigation, using the methodology set out in Appendix 16-1: Assessment Criteria Hydrology and Hydrogeology.

Water Resource

Changes in Runoff Distribution

16.84. The drainage strategy proposed for the site is designed to reflect the current distribution of flow and as such there should be little change in the total discharge to either the canal or Springfield Rew. The development of the site will however significantly increase the coverage of hard standing within the site which will result in less infiltration into the shallow cover across the site and storm water generally flowing from the site more rapidly.

16.85. The primary impact of this, as discussed within the flood risk assessment, will be to exacerbate flood risk downstream. Environmental impacts, as discussed below are, however, also likely.

16.86. A reduction in infiltration will lower recharge a High magnitude potential effect on the minor aquifer beneath the site is predicted. This in turn will reduce base flows to the canal and Springfield Rew and potentially exacerbate the severity of drought conditions. Given the nature of the systems, this is considered to be a High magnitude potential effect for Springfield Rew and a Moderate magnitude potential effect for the Wey and Arun Canal. The magnitude of effect will reduce for receptors further downstream.

16.87. Any significant increase in the rate of flow from the site could lead to rapid water level changes in the receiving watercourses. Resulting regular flooding will alter the nature of the riparian habitat and lead to an adverse impact on some ecological receptors. This is considered to be a Moderate magnitude potential effect on Springfield Rew and a Minor magnitude potential effect on the Wey and Arun canal and all receptors further downstream.

16.88. Higher flow velocities from the site could potentially alter sediment transport / channel geomorphology in the receiving water bodies through erosion or sedimentation. Blockage of or scouring around channel structures could potentially result. This is considered to be a Moderate magnitude potential effect on Springfield Rew and a Minor magnitude potential effect on the Wey and Arun canal and all receptors further downstream.

Potable Water Supply

16.89. The water requirements for the completed proposed scheme will be significant due to the scale of the proposed scheme and without mitigation would be of the order of 2Ml/day. This is in excess of the requirements of the existing operations on the site which are sourced via two four inch pipes, but the Water Strategy (Para. 2.4.2, P.14) has indicated that this supply network is sufficient to supply the first Phase.

16.90. The existing infrastructure does not have sufficient capacity to supply the predicted peak water demand for the completed proposed scheme. Thames Water has stated during the consultation process that modelling of the local network is required to


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ascertain whether sufficient capacity exists locally and what upgrades might be required to supply the site. The Thames Water investment programme for 2010-15 plans already include proposed enhancements to the network, which will be able to accommodate the total development by 2015.

16.91. As insufficient local network exists it is not currently possible to construct the proposed scheme to the scale proposed. If proposed scheme proceeded without mitigation water supply and water pressure to the properties on site would be intermittent or such problems would impact other Thames Water customers in the areas served by the same local systems. The Magnitude of this potential effect prior to consideration of Thames Water Plans and site specific mitigation is assessed as High.

16.92. Concerns also exist regarding the security of supply in this area and whether Thames Water has sufficient additional water resource to supply the proposed scheme irrespective of network capacity. Recent discussions have however indicated that given recent improvement in network leakage rates sufficient resource probably does exist. Whilst discussions concerning this are ongoing the magnitude of the effect on water treatment works and water resource supply in general is therefore indicatively set at Negligible.

16.93. It should be noted that the potential impact of any additional abstractions which may be needed by Thames Water to supply the proposed scheme have not been considered as part of this assessment, but may constitute a potential cumulative impact if other large proposed schemes are also being brought forward in the area.

Foul Water Disposal

16.94. The requirement for foul water disposal will increase markedly as a result of the proposed scheme. Current systems are not sufficient to receive this flow. In particular the capacity of the receiving sewage treatment works in Cranleigh is insufficient for the likely additional burden. The additional burden posed by the proposed scheme would therefore be considered as a High potential effect. Further detail on foul water disposal is contained within the Water Strategy Report which accompanies this application.

16.95. The proposed scheme cannot occur unless a suitable foul water disposal system is available as discharge into an undersized system would potentially result in backing up of flows or flooding of foul sewage. The Magnitude of this potential effect on the local sewerage network prior to the consideration of likely system upgrades is therefore assessed to be High.

16.96. It should be noted that the potential impact of any additional discharges from the receiving sewage treatment works in Cranleigh have not been considered as part of this assessment. However, the Cranleigh Works will be upgraded in the 2010 – 2015 Thames Water development programme which will decrease the effect of this risk.


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Pollution Control

Increase in Sediment Load

16.97. The increase in built surface can lead to flushing effects, whereby site runoff mobilises sediment accumulated over time. This sediment could then be discharged into the Canal or Springfield Rew and could lead to sedimentation or smothering with the system.

16.98. The effect of an increase in sedimentation over time would be to reduce the size of channels, block structures and cause an adverse ecological impact. The magnitude of this potential effect is assessed to be Moderate for immediate surface water receptors but Minor / Negligible for receptors further downstream including both the River Wey and the River Arun.

Pollution of Site Runoff by Oils and Hydrocarbons

16.99. Routine use of cars and other vehicles across the site and on access routes to the site, and spills from the industrial areas of the site all have the potential to contaminate runoff from the site with hydrocarbons or other chemicals.

16.100.Oils and fuels spilt on areas of hardstanding typically get washed into site drainage networks during the first part of a large storm event and get washed through resulting in pollution of the receptor of the site drainage network. The magnitude of this potential effect is assessed to be Moderate for immediate receptor of the site drainage network, but Minor for receptors further downstream.

16.101.In residential areas it is not uncommon for residents to dispose of oils and other chemical by pouring them into the surface water drainage system. From here they discharge to the receptors of the system. It is assessed that the magnitude of this potential effect is Moderate for immediate receptors of the site drainage system, but Minor for features further downstream.

16.102.Where spills occur in permeable areas of the site (including both landscaped and permeably paved areas) such contaminants can infiltrate into the shallow geology. From here pathways of impact exist for deeper geological units and surface water features that receive baseflow from the drift cover. The magnitude of this potential effect is therefore assessed as Moderate for the shallow geology but Minor for the deeper geology or surface water features receiving baseflow from the drift cover due to the potential for natural attenuation of the pollutant in the groundwater.

Pollution from Operation of CHP Plant and Waste Management Facility

16.103.The CHP plant and the Waste Management Facility, both located to the south east of the proposed scheme area, are potential sources of pollution that could impact the local hydrological environment. The main potential pathways of impact would result from a poor control of storm water resulting in contaminated water either infiltrating into the drift cover or entering the storm water system and then discharging into the Wey and Arun Canal.

16.104.The magnitude of the potential effect of such contamination would be linked to the toxicity and leachability of the source material. For both the CHP plant this will


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consist of wood chips and for the Waste Management Facility this will mainly be recyclable inert material although it is likely that compostable waste will also be handled at this location. Given this the magnitude of the potential effect is assessed to be Moderate for both the drift cover and the Wey and Arun Canal and Minor for receptors further downstream such as the River Arun.

Pollution of Site Runoff by Gritting

16.105.In many area roads are gritted with salt during winter periods to prevent ice forming on the road surfaces and during subsequent melts or rainfall this becomes entrained and dissolved in site runoff potentially polluting the receptors of site runoff.

16.106.If such contamination occurred it could lead to a degradation of the water quality in the downstream receptors and ecological deterioration. The periods of potential contamination are however limited to the winter months. The magnitude of this potential effect is therefore assessed as Minor on immediate receptors of site runoff and Negligible for receptors further downstream.

Direct Physical Changes

Crossing of the Wey and Arun Canal

16.107.The Masterplan includes a new crossing of the Wey and Arun Canal to the north west of the site to improve access routes onto and off of the site. This crossing would be a free span bridge across the canal to ensure that the integrity of the tow path was maintained.

16.108.Crossings of water ways can result in a fragmentation of riparian habitat. The choice of a free span bridge will however largely prevent this and therefore the magnitude of this potential effect is therefore assessed as Negligible.

16.109.No other potential environmental impacts of this crossing on local hydrological features has been identified.

Operation of Canal Basin

16.110.The proposed canal basin to be constructed to the south east of the site will be in direct hydraulic connection with the Arun and Wey Canal. The canal basin will not however convey any significant quantities of flow and will be lined to prevent infiltration. As such the impact of the canal basin on water resources within the canal will be Negligible.

16.111.It is proposed that the canal basin be used for a small number of permanent houseboats and if the canal is restored, moorings for visiting barges may also be available. No foul waste from these would be allowed to discharge into the canal or canal basin and therefore the only water quality impacts are likely to be derived from fuel and engine running. These impacts are minimal and assessed to be appropriate to the canal environment. The magnitude of this potential effect is therefore assessed to be Minor.


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Change in Outfall Location

16.112.It is proposed that the numerous outfalls to the Wey and Arun canal are replaced by one single outfall situated at the summit of the canal. This is proposed to aid future possible efforts to restore the canal resulting in a Beneficially Moderate impact magnitude.

16.113.This one single outfall will necessarily be larger than existing outfalls and as such hydraulic impacts such as scouring (localised intense erosion) around this discharge points could result. Whilst possible, such an impact is considered unlikely due to the size of the receiving water body and the engineered characteristics of the canal including solid built banks and the natural clay lining. The magnitude of this potential effect is therefore assessed as Minor.

Flood Risk 16.114.As detailed in the flood risk assessment8, the risk of flooding occurring within the

proposed scheme site from all identified external sources is assessed to be Low through both the construction and operation phases of the proposed scheme.

16.115.The risk posed to the proposed scheme by rising groundwater levels associated with the removal of the site field drain system was assessed to be Moderate.

16.116.During the construction phase there is a High risk of flooding from the site drainage system. This could occur where;

• The existing drainage network has been constructed to a lower standard.

• Where construction processes have damaged either old or new drainage features.

• Where proposed scheme occurs in advance of the drainage infrastructure thus placing additional burden on existing systems.

16.117.Following the completion of the proposed scheme this risk would however reduce to a Low level due to the new drainage network being constructed in line with current design standards.

16.118.A potential risk during both the construction and operation phases was identified where one of the new site access routes passes over an area defined as flood zone 3 and then through an area defined as flood zone 2 to the east of the site along the Wey and Arun Canal. This potential risk has however been assessed to be Low due to:

• The vulnerability classification of a road (less sensitive proposed scheme).

• The relatively low probability of flooding (annual probability less than 1%).

• The nature of any potential flooding (limited in both depth and duration).

8 Enviros (2008): Dunsfold Park Flood Risk Assessment, Dunsfold Park application document 4, Enviros Consulting Ltd, London


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• The availability of alternate access routes onto the site.

16.119.This access route could also result in a reduction of floodplain storage within this part of the floodplain. This would potentially result in flood risk downstream being very slightly increased. This is therefore considered as a High potential risk,

16.120.The risk of flooding posed to properties downstream of the site along the receptors of site runoff will increase following completion of the proposed scheme. This will occur due to an increase in impermeable coverage across the site resulting in higher peak discharges. Any quantifiable increase in flood risk to off site built development is unacceptable and is therefore assessed as a High risk.

Dunsfold Park Chapter 16Environmental Statement Hydrology and Water Resources

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Table 16-2: Construction Phase: Predicted Impacts Prior to Mitigation

Source Pathway Receptor Sensitivity Magnitude Significance Adverse /Beneficial

On site water features Low Minor Negligible AdverseSpringfield Rew Low Minor Negligible Adverse

Wey and Arun Canal Very Low Minor Negligible AdverseDownstream receptors High Negligible Negligible -

Drift Cover Low Minor Negligible Adverse

Reduction in infiltration and subsequent reduction tobaseflows

Weald Clay Very Low Negligible Negligible -On site water features Low Minor Negligible Adverse

Springfield Rew Low Minor Negligible AdverseWey and Arun Canal Very Low Minor Negligible Adverse

Larger and more rapid water level response to rainfall

Downstream receptors High Negligible Negligible -On site water features Low Minor Negligible Adverse


Change in runoffdistribution

Erosion / geomorphological changes within channelresulting from increased peak flows

Downstream receptors High Negligible Negligible -On site water features Low Negligible Negligible -

Springfield Rew Low Negligible Negligible -Wey and Arun Canal Very Low Negligible Negligible -

Change flow distribution

Downstream receptors Medium Negligible Negligible -On site water features Low Moderate Minor Adverse

Springfield Rew Low Moderate Minor AdverseWey and Arun Canal Very Low Moderate Negligible Adverse

Damage / removal ofexisting site drainage

network

Creation of new pollution pathways

Downstream receptors High Minor Moderate AdverseDrift Cover Low Negligible Negligible -Pumped excavations Groundwater drawdown and subsequent altering of sub

surface flows Weald Clay Very Low Negligible Negligible -On site water features Low High Moderate Adverse

Springfield Rew Low High Moderate AdverseWey and Arun Canal Very Low High Minor Adverse

Sediment from spoilheaps and other exposed

earth

Vegetation stripping and movement of plant incombination with heavy rainfall

Downstream receptors High Moderate Moderate AdverseOn site water features Low Moderate Minor Adverse

Springfield Rew Low Moderate Minor AdverseWey and Arun Canal Very Low Moderate Minor Adverse

Downstream receptors High Minor Moderate Adverse

Hydrocarbons and otherchemicals from vehicles /fuel and chemical storage

areas

Leakages and spill from poor maintenance or accidentsand resultant infiltration to the ground.

Drift Cover Low High Moderate Adverse


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Weald Clay Very Low Moderate Minor AdverseOn site water features Low High Moderate Adverse

Springfield Rew Low High Moderate AdverseWey and Arun Canal Very Low High Minor Adverse

Leakages and spill from poor maintenance or accidentsand resultant flow into the drainage systems

Downstream receptors High Moderate Moderate Adverse

Table 16-3: Operation Phase: Predicted Impacts Prior to Mitigation


On site water features Low High Moderate AdverseSpringfield Rew Low High Moderate Adverse

Wey and Arun Canal Very Low Moderate Negligible AdverseDownstream receptors High Moderate Moderate Adverse

Drift Cover Low High Moderate Adverse

Reduction in infiltration and subsequent reduction tobaseflows

Weald Clay Very Low Negligible Negligible -Springfield Rew Low Moderate Minor Adverse

Wey and Arun Canal Very Low Minor Negligible -Larger and more rapid water level response to rainfallDownstream receptors High Minor Moderate Adverse

Springfield Rew Low Moderate Minor AdverseWey and Arun Canal Very Low Minor Negligible Adverse

Increased coverage ofhardstanding

Erosion / geomorphological changes within channelresulting from increased peak flows

Downstream receptors High Minor Moderate AdversePotable water supply network Medium High Moderate AdverseIncreased potable water demand

Water treatment works High Negligible Negligible -Foul drainage network Medium High Moderate Adverse

Higher site usage /proposed scheme density

Increased foul site flowsSewage Treatment works High High Major Adverse

Springfield Rew Low Moderate Minor AdverseWey and Arun Canal Very Low Moderate Negligible Adverse

Sediment deposited onareas of hardstanding Mobilisation in the first flush of storm

Downstream receptors High Minor Moderate AdverseSpringfield Rew Low Moderate Minor Adverse

Wey and Arun Canal Very Low Moderate Negligible AdverseLeakage / spill on hardstanding and mobilisation duringfirst flush of storm

Downstream receptors High Minor Moderate AdverseSpringfield Rew Low Moderate Minor Adverse

Oils / fuels routinely usedacross the site

Disposal into storm water networkWey and Arun Canal Very Low Moderate Negligible -


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Downstream receptors High Minor Moderate AdverseOn site water features Low Minor Negligible Adverse


Downstream receptors High Negligible Negligible -Drift Cover Low Moderate Minor Adverse

Leakage or spillage and infiltration into shallow geology

Weald Clay Very Low Minor Negligible AdverseWey and Arun Canal Very Low Moderate Negligible AdverseContaminated storm runoff entering site drainage

system Downstream receptors High Minor Moderate AdverseWey and Arun Canal Very Low Minor Negligible Adverse

Downstream receptors High Negligible Negligible -Drift Cover Low Moderate Minor Adverse

Operation of CHP plantand Waste Management

Facility Infiltration of contaminated runoff into shallow geology

Weald Clay Very Low Minor Negligible AdverseSpringfield Rew Low Minor Negligible Adverse

Wey and Arun Canal Very Low Minor Negligible AdverseSalts used for gritting of

roads within the site Mobilisation and dissolution into storm waterDownstream receptors High Negligible Negligible -

Crossing of Wey andArun Canal Fragmentation of riparian habitat Wey and Arun Canal Very Low Negligible Negligible -

Loss of water through infiltration Wey and Arun Canal Very Low Negligible Negligible -Operation of Canal BasinPollution from house boats Wey and Arun Canal Very Low Negligible Negligible -Alteration of distribution of flow within canal Wey and Arun Canal Very Low Moderate Minor BeneficialChange in outfall locationScouring at outfall Wey and Arun Canal Very Low Minor Negligible Adverse


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MITIGATION AND MONITORING 16.121.Much of the mitigation of hydrological impact revolves around the implementation of

a sustainable drainage system. The details of this system are set out in Sustainable Drainage Strategy (an appendix to the Flood Risk Assessment, Volume 4 in the schedule of application documentation) 9. In summary this system is designed, where possible, to mimic flows of water across and through a natural site through encouraging infiltration and slowing the flow of water from the site.

Construction 16.122.The section below defines the likely construction impacts arising from the proposals

incorporating mitigation. In combination with receptor sensitivity overall significance is determined, as given in Table 16-4: Construction Phase: Impacts Following Mitigation, using the methodology set out in Appendix 16-1: Assessment Criteria – Hydrology and Hydrogeology.

16.123. If possible works should be avoided, or sensibly managed, in accordance with adverse ground conditions and/or weather conditions occurring.

16.124.A construction environmental management plan will be prepared by the building contractors prior to work commencing on the site. This will contain detailed instruction regarding the techniques and methods that will be used to limit the environmental impact of the construction phase on the water environment. This document will be in line with the pollution prevention guidance published by the Environment Agency outline and any additional supplementary guidance provided within this document. Further details concerning this document are contained in the outline construction environmental management plan (refer to Chapter 5: Description of Site and Proposals).

16.125.The Method Statement will be submitted to the Environment Agency for approval prior to work commencing.

Mobilisation of Sediments

16.126.Specific guidance in dealing with silt arising from construction activities is contained within PPG5 (Ref 3). A number of measures could be put in place in order to minimise the potential for transport of silt into surface water features including:

• Use of sediment traps and installing settlement ponds or tanks where practical.

• Directing infiltration into designated vegetated ground to reduce silt and suspended solids.

• Use of designed drainage filters.

• Provision of track or road side drainage channels with small dams to assist with silt retention.

9 Biwater (2008): Water Strategy, Dunsfold Park application document 15, Farrer Consulting Ltd, London


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16.127.The proposed phasing of the landscaping has been devised to ensure that the major downstream features of the proposed Sustainable Drainage Strategy will be constructed in advance of the proposed scheme programme. This will help to allow sediment derived from proposed scheme activities to settle out and filtered prior to discharge from the site. These features would then be restored following the completion of the proposed scheme programme to ensure the ongoing effectiveness of the drainage system.

16.128.Given appropriate mitigation in line with the guidance provided above, the magnitude of the effect of pollution from sediment derived from construction activities is assessed to be at most Minor.

Contamination of Shallow Groundwater/ Surface Water

16.129.The storage of polluting materials should be kept to a minimum and where less hazardous or inert materials are available these should be specified. For example, construction materials containing sulphides which could potentially alter the pH of runoff should be avoided and the use of biodegradable hydraulic oils could be considered for construction plant. In addition, absorbent mats/pads, absorbent granules and sand should be made available and site operatives trained in their use, to deal with any spillages.

16.130.Further measures to be adopted include locating of mobile plant, batching plant, materials storage, top soil storage, and waste disposal facilities at least 50m from water features and positioning of fuel storage tanks and other potentially polluting materials and maintenance facilities on areas of hard standing with dedicated drainage systems. Where possible, bunded areas would be protected from direct rainfall by organic mulch or a temporary sward and stored materials on site will be checked regularly for containment integrity (both primary and secondary), quantity stored and security of storage.

16.131.The early phased implementation of the sustainable drainage strategy (appended to the Flood Risk Assessment which accompanies this application) in advance of the proposed scheme programme will help to remove oils and other contaminants from the surface water flows within the site; thus providing addition protection to the surface water receptors.

16.132.Given appropriate mitigation as detailed above the magnitude of the potential effects of pollution from oils, fuels and other chemical substances during the construction phase is assessed to be Minor or less for all receptors when considering surface pathways. The assessed magnitude of the effect on the shallow groundwater is however still assessed to be Moderate due to the potential for infiltration.

Operation 16.133.The section below defines likely operational impacts arising from the proposals

incorporating mitigation. In combination with receptor sensitivity overall significance is determined, as given in Table 16-5: Operation Phase: Impacts Following Mitigation, using the methodology set out in Appendix 16-1: Assessment Criteria – Hydrology and Hydrogeology.


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Increase in Hard-standing

16.134.As stated, the sustainable drainage strategy proposed for the site aims to mimic the flow of water expected for a natural site. As such the purpose of the system is to counteract the changes in runoff that occur through an increase in hard-standing.

16.135.Wherever possible areas of hard-standing will be constructed using permeably paved materials to encourage infiltration into the drift cover. In addition to this runoff from the site will be directed to naturalised surface attenuation features, again all located in areas underlain by drift cover. Infiltration within these areas will again be allowed thus promoting recharge and maintaining baseflows to the surface water features within and around the site. Subsequently this effect following mitigation is assessed to be Negligible.

16.136.The sustainable drainage system will be designed such that peak flows from the site will be controlled at or below greenfiled runoff rates. Given that currently significant portions of the site are developed and little or no attenuation of flow from these areas occurs, this will be a Beneficial effect assessed to be Moderate in magnitude relating to both water level variations and geomorphological impacts.

High Site Usage / Proposed Scheme Density

16.137.The current mains water and foul sewer infrastructure is not sufficient for the proposed scheme. As a result a detailed water strategy report has been undertaken to consider options for the use and disposal of water at the Dunsfold Park site. This is included within the Water Strategy report which accompanies this application.

16.138.The proposed water supply strategy intends to reduce the demand for potable water within the proposed scheme from the UK average of 150 litres / person / day down to 80 litres / person / day. This will primarily be undertaken through the installation of water efficient devices. In addition 30% of this demand will be sourced from non potable sources (i.e. rainfall harvesting) reducing average potable usage down to 56 litres / person / day. These measures will ensure compliance with Grade 6 under the Code for Sustainable Homes. Further detail is contained within the Water Strategy Report which accompanies this application.

16.139.Even with such stringent measures additional potable supply to the site will be required and with regards to this negotiations with Thames Water are currently in progress. This application is submitted on the assumption that the local supply capacity will be upgraded by Thames Water subject to an appropriate developer contribution. When such an agreement is reached the network upgrade required is likely to result in a net improvement in the local system with increased capacity, reduced leakage and improved reliability. The net effect with respect to the water supply network would therefore be Minor, but Beneficial.

16.140.As with the local mains water network, the existing foul sewerage network does not have sufficient capacity for the proposed scheme. Negotiations with Thames Water are currently in progress. This application is however submitted on the assumption that the local foul drainage capacity will be upgraded by Thames Water subject to an appropriate developer contribution. When such an agreement is reached the major renewal works required are likely to result in a net improvement in the local system


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with increased capacity, reduced leakage and improved reliability. The net effect with respect to the local foul sewerage network would therefore be Minor, but Beneficial.

16.141.Whilst the local Cranleigh Sewage Treatment Works does not currently have sufficient capacity to receive the additional foul water that would be created within the whole developed site, future expansion of the works will rectify this problem. Under the Asset Management Plan (AMP) programme, Thames Water are already planning on upgrading the works to provide capacity for new proposed schemes. Evidently ongoing discussions with Thames Water will be necessary, particularly concerning the phasing of the proposed scheme; however in the light of this, this effect is therefore considered to be Negligible.

Increase in Sediment Load

16.142.The sustainable drainage strategy to be implemented across the site will have the effect of allowing any sediment entrained in runoff from the site to either settle or be filtered out, thereby significantly improving water quality.

16.143.The proposed tiered drainage system will slow the flow of water through an urban system of canals and filter drains. Flows will then be directed via naturalised areas including ponds and wetlands. It is envisaged that this system will result in sediment loading within site runoff equivalent to a greenfield site. This will constitute an improvement on the current situation and therefore this effect is assessed to be Moderate and Beneficial.

Pollution of Site Runoff by Oils and Hydrocarbons

16.144.The implementation of the sustainable drainage strategy will help to prevent pollution of surface water receptors via the drainage system. Any area with significant potential for pollution including the retained industrial area and fuel tanks or waste storage areas will be drained via oil interceptors. All storm flows will then flow through the tiered drainage system that will provide significant potential for the removal of oils and fuels particularly in the downstream wetland areas.

16.145.Given the mitigation measures proposed the potential for pollution of surface water receptors with hydrocarbons via the site drainage system will be reduced. This effect is therefore assessed to be Moderate and Beneficial.

16.146.The system design within the site will be extremely visible and this should act to discourage people from polluting it by inappropriate disposal of oils. In addition to this storm drainage points across the proposed scheme will be clearly labelled and oil collection disposal / recycling facilities will be provided within the site. The effect of oil disposal within the storm water drainage system would therefore be reduced to Minor.

16.147.The sustainable drainage strategy will encourage infiltration both on a local scale, through use of permeably paved surfacing, and within the naturalised surface water features to be created. As a result the potential for contamination of the shallow groundwater with hydrocarbons will remain, although some mitigation will occur through source control measures introduced such as dedicated drainage around potential pollution sources including the fuel tanks and waste management facilities.


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The magnitude of the potential effect on the shallow groundwater following mitigation is therefore assessed to be Moderate with the effect on subsequent receptors being Minor.

Pollution from the Operation of the CHP Plant and Waste Management Facility

16.148.These elements of the proposed scheme will be designed to minimise the potential for pollution to occur. Measures that will be included include:

• The siting of all storage areas within buildings to prevent contact with rainwater occurring,

• Siting of all storage areas on solid hardstanding to prevent infiltration; and

• The drainage of any potentially contaminative external areas to the foul drainage system.

16.149.Through the implementation of these measure the magnitude of the potential effect on the all identified hydrological receptors will be reduced to Negligible.

Flood Risk 16.150.The new site drainage system will include below ground features such as filter drains

and traditional storm water sewers. These will act to control groundwater levels within the proposed scheme. In addition to this finished flood levels will be constructed a minimum of 300mm above surrounding ground levels. Through these actions the risk posed to the proposed scheme from the groundwater will be reduced to Low.

16.151.Any possible loss of flood storage volume associated with the construction of the site access route through the floodplain will be minimised through good design. Any residual loss of floodplain storage will be more than compensated for by the construction of the canal basin. The mitigated risk level is therefore assessed to be Very Low.

16.152.The risk posed to the site from site drainage system during the construction phase will be mitigated by the phased implementation of the sustainable drainage strategy in advance of the development programme. The strategy has been designed such that it can be implemented in a phased approach.

16.153.In addition to this prior to the commencement of each phase of work a detailed drainage assessment will be undertaken. As well as providing detailed drainage design, this assessment will assess, in detail, the integrity of the strategy for that phase throughout the remainder of the proposed scheme process. If identified as necessary additional interim measures, such as perimeters drains will be introduced to isolate the new proposed scheme from potential risk sources in the undeveloped areas of the site.

16.154.Consequently the mitigated flood risk posed to the proposed scheme by site drainage network is assessed to be Low.


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16.155.The sustainable drainage strategy has been design to ensure that peak site runoff from the proposed scheme areas will be controlled at or below peak runoff from an equivalent greenfield site for all interim stages as well as the completed proposed scheme. This will constitute a significant reduction in site runoff and as such the additional risk posed to off site built proposed scheme is assessed to be Negligible and, in fact, the outcome could be regarded very slightly Beneficial compared to the status quo.


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Table 16-4: Hydrology and Water Resources – Summary of Impacts from Construction

Source Pathway Receptor Mitigation Sensitivity Magnitude Significance Adverse /Beneficial



Drift Cover Low Minor Negligible Adverse

Reduction in infiltration andsubsequent reduction tobaseflows

Weald Clay

None required

Very Low Negligible Negligible -On site water features Low Minor Negligible Adverse


Larger and more rapid waterlevel response to rainfall

Downstream receptors

None required, but ConstructionMethod Statement & conformancewith good practice should mean the

impact significance is even less High Negligible Negligible -On site water features Low Minor Negligible Adverse


Change inrunoff

distribution

Erosion / geomorphologicalchanges within channelresulting from increased peakflows Downstream receptors


impact significance is even less High Negligible Negligible -On site water features Low Negligible Negligible -

Springfield Rew Low Negligible Negligible -Wey and Arun Canal Very Low Negligible Negligible -

Change flow distribution



impact significance is even less High Negligible Negligible -On site water features Low Minor Negligible Adverse


Damage /removal ofexisting site

drainagenetwork Creation of new pollution

pathwaysDownstream receptors

Phased implementation of SuDS

High Negligible Negligible -Drift Cover Low Negligible Negligible -

Pumpedexcavations

Groundwater drawdown andsubsequent altering of subsurface flows Weald Clay

None required, but any significantactivity will be subject to licensingunder the 2003 Water ResourceAct Very Low Negligible Negligible -


Wey and Arun Canal Very Low Minor Negligible Adverse

Sediment fromspoil heaps andother exposed

earth

Vegetation stripping andmovement of plant incombination with heavyrainfall Downstream receptors

Phased implementation of SuDSand use of good construction

methodology to be agreed withthe EA prior to proposed scheme High Negligible Negligible -

On site water features Low Minor Negligible AdverseHydrocarbonsand other

Leakages and spill from poormaintenance or accidents and Springfield Rew

Minimising storage and maintainingwithin bunded and lined Low Minor Negligible Adverse


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Drift Cover Low Moderate Minor Adverse

resultant infiltration to theground.

Weald Clay

compound when not in use. Useof good construction methodologyto be agreed with the EA prior to

proposed scheme Very Low Minor Negligible -On site water features Low Minor Negligible Adverse


chemicals fromvehicles / fueland chemicalstorage areas

Leakages and spill from poormaintenance or accidents andresultant flow into thedrainage systems Downstream receptors

Phased implementation of SuDSand use of good construction

methodology to be agreed withthe EA prior to proposed scheme High Negligible Negligible -


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Table 16-5: Hydrology and Water Resources – Summary of Impacts from Operation


On site water features Low Negligible Negligible -Springfield Rew Low Negligible Negligible -

Wey and Arun Canal Very Low Negligible Negligible -Downstream receptors High Negligible Negligible -

Drift Cover Low Negligible Negligible -

Reduction in infiltration andsubsequent reduction tobaseflows

Weald Clay

Sustainable drainage strategyincluding permeably paved surfaces

and naturalised surface waterfeatures from which infiltration

would be encouragedVery Low Negligible Negligible -

Springfield Rew Low Moderate Minor BeneficialWey and Arun Canal Very Low Moderate Negligible Beneficial

Larger and more rapid waterlevel response to rainfall


Sustainable drainage strategy toensure peak runoff controlled at

greenfield rates High Negligible Moderate BeneficialSpringfield Rew Low Moderate Minor Beneficial

Wey and Arun Canal Very Low Moderate Negligible Beneficial

Increasedcoverage ofhardstanding

Erosion / geomorphologicalchanges within channelresulting from increased peakflows Downstream receptors

Sustainable Drainage Strategy toensure peak runoff controlled at

greenfield rates High Negligible Moderate Beneficial

Potable water supplynetwork Medium Minor Minor BeneficialIncreased potable water

demandWater treatment works

Water efficiency measure, rainfallharvesting and Thames Water

improvements to supply network High Negligible Negligible -Foul drainage network Medium Minor Minor Beneficial

Higher siteusage /

proposedscheme density Increased foul site flows Sewage Treatment

works

Thames Water improvements todrainage network and expansion of

STWs High Negligible Negligible -


Sedimentdeposited on

areas ofhardstanding

Mobilisation in the first flush ofstorm


Sediment removal withinsustainable drainage strategy.

High Minor Moderate Beneficial


Leakage / spill on hardstandingand mobilisation during firstflush of storm Downstream receptors

Oil interceptors in vulnerable areasand pollution removal withinsustainable drainage strategy High Minor Moderate Beneficial

Springfield Rew Low Minor Negligible AdverseWey and Arun Canal Very Low Minor Negligible AdverseDisposal into storm water

networkDownstream receptors

Labelling of drains, provision of oildisposal / recycling facilities and

pollution removal withinsustainable drainage strategy High Negligible Negligible -



Oils / fuelsroutinely usedacross the site

Leakage or spillage andinfiltration into shallow geology

Drift Cover

Isolation and formal drainage withinvulnerable areas, though outcomes

will no result in a step change inoutcome

Low Moderate Minor Adverse


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Weald Clay Very Low Minor Negligible AdverseWey and Arun Canal Very Low Negligible Negligible -Contaminated storm runoff

entering site drainage system Downstream receptors

Storage areas sited within buildingand contaminative external areas

drained to foul system High Negligible Negligible -

Wey and Arun Canal Low Negligible Negligible -Downstream receptors Very Low Negligible Negligible -

Drift Cover Very Low Negligible Negligible -

Operation ofCHP plant and

WasteManagement

FacilityInfiltration of contaminatedrunoff into shallow geology

Weald Clay

Siting of hardstanding

High Negligible Negligible -Springfield Rew Low Minor Negligible Adverse

Wey and Arun Canal Very Low Minor Negligible AdverseSalts used for

gritting of roadswithin the site

Mobilisation and dissolutioninto storm water

Downstream receptorsNone required

High Negligible Negligible AdverseCrossing of

Wey and ArunCanal

Fragmentation of riparianhabitat Wey and Arun Canal None required Very Low Negligible Negligible -

Loss of water throughinfiltration Wey and Arun Canal None required Very Low Negligible Negligible -Construction of

Canal BasinPollution from house boats Wey and Arun Canal None Required Very Low Negligible Negligible -Alteration of distribution offlow within canal Wey and Arun Canal None required Very Low Moderate Minor BeneficialChange in

outfall locationScouring at outfall Wey and Arun Canal None Required Very Low Minor Negligible Adverse


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RESIDUAL IMPACTS 16.156.This section sets out the impacts to the hydrological environment which remain once

the mitigation measures have been put in place.

Construction 16.157.The phased implementation of the sustainable drainage strategy in advance of the

construction process along with good construction methodology will reduce most of the potential impacts within the construction phase down to Negligible. The only remaining assessed adverse impact is the potential for the shallow ground water to be polluted by hydrocarbons and other chemicals in use across the site through the construction process.

16.158.This adverse impact was assessed to be Minor and any off site impact would be Negligible due to dilution and attenuation effects.

Operation 16.159.The change in land use at the site combined with the implementation of a tiered

sustainable drainage strategy will remove most of the potential adverse impacts identified and will result in significant beneficial impacts to the local hydrological environment.

16.160.A potential adverse impact to the shallow groundwater was identified. This arises as storm water (with any entrained oils etc) will be encouraged to infiltrate, to reduce storm peaks and surface pollution pathways. This impact was assessed to be Minor and as detailed above any off site impact would be Negligible due to dilution and attenuation effects.

Flood Risk 16.161.There will be no significant residual flood risks posed to the proposed scheme and

the proposed scheme will not result in an increase in flood risk off site.

SUMMARY AND CONCLUSIONS 16.162.Flows from the Dunsfold Park Site currently discharge to either the Wey and Arun

Canal or Springfield Rew. Other significant surface water receptors on site are limited to drains discharging to these features and a pond in the southern corner of the site. None of the immediate receptors of site runoff are therefore considered particularly important or sensitive to environmental impact.

16.163.The site is located entirely in flood zone 1 and no significant potential for flooding has been identified other than the site drainage network, particularly during the construction phase. This will be mitigated through the phased implementation of the sustainable drainage strategy and detailed drainage assessments to be undertaken in advance of each phase that will seek to identify and as necessary isolate new proposed scheme from flood risk sources along the existing site drainage network.


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16.164.Construction processes typically result in adverse impacts to the hydrological environment through generation of sediment laden runoff and the potential for pollution from oils and other chemicals associated with use of plant or construction materials. It is proposed that these potential impacts are mitigated through the implementation of good construction methodology and the phased implementation of the sustainable drainage strategy within the construction programme.

16.165.Following construction the potential for pollution from the site is reduced, and residual impact will be largely managed through the sustainable drainage strategy. In addition to this the drainage strategy will also regulate flows from the site ensuring that infiltration is encouraged and peaks flows are reduced down to that expected from an equivalent greenfield site. This will help reduce flood risk downstream of the site; improve baseflows entering local water features and ameliorate any potential environmental problems such as geomorphological changes.

16.166.The sustainable drainage strategy will result in the creation of quite significant hydrological features within the site including, canals, ponds and wetlands. Whilst not directly considered within this chapter these will provide significant new habitat areas within the site and consequently result in aesthetic and ecological improvements over the current situations. These, and the other benefits detailed within this chapter, notably exceed the very limited adverse potential impacts that are assessed to exist once mitigation has been fully considered.

16. hydrology and water resources 16-0_hydrology and water...environmental statement hydrology and...

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