«LONDONWEST MIDLANDS ENVIRONMENTAL STATEMENT Volume 2 | Community Forum Area report CFA7 | Colne Valley November 2013 ES 184.108.40.206 VOL LondonWest Midlands ...»
potential impacts on the risk of river flooding at the crossings of the floodplain of the River Colne and Newyears Green Bourne; and potential impacts on the risk of surface water flooding at the dry valley close to Old Shire Lane.
Scope, assumptions and limitations 13.2 13.2.1 The assessment scope, key assumptions and limitations for the water resources and flood risk assessment are set out in Volume 1, and in the SMR and its addendum (see Volume 5: Appendix CT-001-000/1 and Appendix CT-001-000/2). This report follows the standard assessment methodology.
13.2.2 The spatial scope of the assessment was based upon the identification of surface water and groundwater features within 1km of the centre line of the route, except where there is clearly no hydraulic connectivity. For surface water features in urban areas, the extent was reduced to 500m. Outside of these distances it is unlikely that direct impacts upon the water environment will be attributable to the Proposed Scheme. Where works extend more than 200m from the centre line, for example at stations and depots, professional judgement has been used in selecting the appropriate limit to the extension in spatial scope required. For the purposes of this assessment this spatial scope is defined as the study area.
Water Framework Directive – Directive 2000/60/EC of the European Parliament and of the Council of 23 October 2000 establishing a framework for Community action in the field of water policy, Strasbourg, European Parliament and European Council Affinity Water Limited
Environment Agency (2009) River Basin Management Plan, Thames River Basin District CFA Report – Colne Valley/No7 | Water resources and flood risk assessment study area are assessed within Volume 5: Appendix WR-002-007. Table 21 includes features potentially affected by the Proposed Scheme.
13.3.3 The route will cross the Mid Colne Valley SSSI (see Map EC-01-011 and Map EC-01-012 (Volume 5, CFA7 Ecology Map Book). This is a ‘very high’ value receptor due to its national SSSI status, the water related aspects of which includes the River Colne, local wetlands and wildlife dependent on the water environment.
Table 21: Surface water features potentially affected by the Proposed Scheme
Water-feature classifications: Section 113 of the Water Resources Act 1991 defines a main river as a watercourse that is shown as such on a main river map. Section 72 of the Land Drainage Act 1991 defines an ordinary watercourse as ‘a watercourse that is not part of a main river’. Section 221 of the Water Resources Act 1991 defines a watercourse as including ‘all rivers and streams, ditches, drains, cuts, culverts, dikes, sluices, sewers (other than public sewers) and passages through which water flows’. Main rivers are larger rivers and streams designated by Defra on the main river map and are regulated by the Environment Agency.
For examples of receptor value see Table 43 in the SMR addendum (Volume 5: Appendix CT-0001-000/2).
Existing baseline – groundwater resources Geology and hydrogeology 13.3.7 The geological formations within this area are described further, with a schematic geological cross-section, Volume 5: Appendix WR-002-007.
13.3.8 The location of private abstractions, geological formations and indicative groundwater levels are shown on Map WR-02-007 (Volume 5, Water resources and flood risk assessment Map Book).
13.3.9 A summary of the superficial and bedrock geology and hydrogeology is presented in Table 22. Unless otherwise stated, the geological groups listed are all crossed by the route.
Table 22: Summary of geology and hydrogeology in the study area
Superficial deposits 13.3.10 Superficial deposits are present over the majority of the study area (as illustrated in Map WR-02-007, Volume 5, Water Resources and Flood Risk Assessment Map Book).
These consist of River Alluvium, mainly clay, peat, silt, sand and gravel associated with the River Colne and Shepperton Gravel exposed by workings to the north and south of the Proposed Scheme and in the central section of the study area around Broadwater
Lake. Other gravel deposits in the area include:
the Winter Hill Gravel on an elevated ridge above Tilehouse Lane and which are not expected to contain any significant groundwater;
Bedrock aquifers 13.3.11 The first 150m at the southern end of the route in the study area is underlain by the clay, silt and sand deposits of the Lambeth Group.
13.3.12 The majority of the Proposed Scheme in this study area is underlain by the Cretaceous White Chalk Subgroup. The White Chalk Subgroup is classified as a Principal aquifer.
The uppermost formation within the White Chalk Subgroup in this study area is the Seaford Chalk Formation.
13.3.13 The regional hydrogeological map data and available Environment Agency borehole monitoring data indicate that Chalk groundwater levels close to the Colne valley are from 30-37m AOD in the south-east, rising to 40-60m AOD to the north-west. The map indicates the regional direction of groundwater flow in the vicinity of the route to be towards the southeast (i.e. crossing the valley floor in this area). Peak groundwater levels are below the lowest elevation of tunnels and cuttings in the study area but are above the foundations of the viaduct. Other groundwater level data, such as the groundwater levels from South West Chilterns Groundwater Model122 and data provided by Affinity Water support this conclusion.
13.3.14 Groundwater level data show that the superficial deposits, where present, are generally in a degree of hydraulic connection with the underlying Chalk.
13.3.15 Groundwater flow in the Chalk is usually dominated by flow in fissures. Desk studies suggest that the depths of major fissures bands under the floor of the valley include a zone around 20 to 30m below ground level (m bgl) and another zone around 45m bgl.
Further details are given, Volume 5: Appendix WR-002-007.
LBH report123 that there is an area of groundwater contamination in the Chalk aquifer 13.3.16 associated with a closed landfill north of the route near Ickenham (see Map LQ-01F1 to F4, Volume 5, Land Quality Map Book). Monitoring data for this site will be taken into account in the detailed design of the Proposed Scheme. There is another area of historic groundwater contamination known to exist in the area of Denham Studios (see Map LQ-01-011, B7 to C8, Volume 5, Land Quality Map Book).
13.3.17 There is a former sewage works at Denham Green adjacent to the River Colne (Map LQ-001-011 area 7-11, Volume 5, Land Quality Map Book). Sewage sludge was previously deposited in this area and there is potential for organic and inorganic contamination to exist in soils overlying the Chalk bedrock. Further information is provided in Section 8, Land quality, of this report.
Atkins (2007) South West Chilterns Phase 1 Conceptual Model Final Report. February 2007.
London Borough of Hillingdon (2011) Environmental Protection Act 1990, Part 2A – Section 78B,Record of Determination of the Land at the Former Landfill Site at Newyears Green Lane, Harefield, Middlesex.
CFA Report – Colne Valley/No7 | Water resources and flood risk assessment
Water Framework Directive status 13.3.18 No WFD classification has been given by the Environment Agency to the superficial deposits or the Lambeth Group.
13.3.19 The Environment Agency has classified the overall WFD status of the Mid Chilterns Chalk groundwater body as Poor Status with an objective to achieve Good Status by 2027.
Abstractions and permitted discharges 13.3.20 There are three groundwater abstractions for PWS protected by SPZ in the study
area. Further details of the SPZ protecting these PWS are provided, Volume 5:
Appendix WR-002-007 and shown on Map WR-02-007 (Volume 5, Water Resources and Flood Risk Assessment Map Book). All the SPZ protecting the PWS are located to the north of the route, with their designated SPZ1 and SPZ2 catchments crossed by the route. The location of the PWS sources ranges from less than 50m up to almost 1km from the route. The route also passes through the SPZ for a source in the adjacent study area (CFA6) and one source in the CFA8 study area. These are discussed in more detail, Volume 5: Appendix WR-002-007.
13.3.21 The Environment Agency reports that there are ten private licensed abstractions and
one unlicensed private abstraction within the study area as set out, Volume 5:
Appendix WR-002-007. There is the potential for further unlicensed abstractions to exist, as a licence is not required for abstraction volumes below 20m³ per day.
13.3.22 The Environment Agency reports that there are five consented discharges to ground/groundwater within 1km of the Proposed Scheme in the study area (details in Map WR-02-007, Volume 5, Water Resources and Flood Risk Assessment Map Book).
Surface water/groundwater interaction 13.3.23 Gravel deposits form a shallow aquifer across the valley floor and the lakes occur where these gravels have been excavated. Groundwater levels in the gravels mirror those in the lakes. Any potential wide scale changes to the groundwater levels and quality in the gravels may therefore impact surface water quality and levels and vice versa.
13.3.24 Vertical groundwater flow is generally restricted by a layer of weathered Chalk at the surface of the Chalk and some thin layers of finer material in the superficial deposits.
However, the lower permeability layers are not consistent across the valley either in thickness or presence. Therefore in places the Chalk aquifer is vulnerable to contamination from the gravels and lakes due to the potential hydraulic continuity that is present.
Water dependent habitats 13.3.25 The River Colne, adjacent flooded gravel pits (including Savay Lake which is an important fishery) and the Grand Union Canal each occupy parts of the valley floor.
13.3.26 The Mid Colne Valley SSSI (see Map Series CT-10, Volume 2, CFA7 Map Book) consists of four large lakes between the River Colne and Grand Union Canal and part of the River Colne itself (refer to Volume 5: Appendix WR-002-007 for details). The water
related aspects contributing to the SSSI status include the River Colne, local wetlands and associated bird life as discussed in Section 7 of this report.
Existing baseline – flood risk River flooding 13.3.27 The agreed data set for river flooding is the Environment Agency Flood Zone Mapping, as shown on Map WR-01-008 (Volume 5, Water resources and flood risk assessment Map Book).
13.3.28 The route will be on viaduct starting near to the Newyears Green Bourne close to HOAC before it crosses five lakes on the valley floor, the Grand Union Canal, and then the River Colne to the north of Denham Green (see Map WR-01-008, reference SWCCFA7-02).
13.3.29 The Newyears Green Bourne has an upstream catchment size of approximately 5km² at this crossing (see Map WR-01-008, reference SWC-CFA7-02). According to Environment Agency mapping, the route will cross approximately 250m of Flood Zone 3 on viaduct. No records of historical flooding have been found for the Newyears Green Bourne. The land use within the floodplain in the vicinity of the Proposed Scheme is largely made up of arable farm land and pasture (moderate value receptor), with the exception of the residential dwelling at Dew’s Farm (this would be a high value receptor however under the Proposed Scheme it will be demolished and thus will not be assessed further) and leisure uses associated with HOAC (moderate value receptor).
13.3.30 The Environment Agency flood zone mapping shows an area of fluvial floodplain along a natural valley to the north of Dew’s Farm. The flood zone maps show the area of flooding ending 500m downstream of Harvil Road, however LiDAR information suggests that any flooding will continue to flow overland and discharge to Harefield No.2 Lake, a further 200m downstream and outside the study area (see Map WR-01G5).
13.3.31 The River Colne has a much larger upstream catchment of approximately 725km² at the crossing (see Map WR-01-008, Water resources and flood risk assessment Map Book, reference SWC-CFA7-03). The floodplain of the River Colne is dominated by the presence of historical gravel pit workings that are now flooded and form the lakes in the area. The route will cross approximately 1.1km of Flood Zone 3.
13.3.32 There are a number of receptors that will have the potential to be affected by the Proposed Scheme across the Colne valley including residential properties at Savay Lane, Savay Farm, Widewater Lock and Weybeards Cottages, the Horse and Barge Public House (all high value receptors), electricity substations (very high value receptors), leisure facilities (moderate value receptors) and pumping stations (low value receptors).
Sewer flooding 13.3.37 The agreed datasets for sewer flooding are Thames Water records in the Buckinghamshire, Hertfordshire127 and LBH PFRA reports.
13.3.38 The LBH PFRA reports that there have been five historical records of sewer flooding within the study area (as at June 2010). The Buckinghamshire PFRA reports that there have been no recorded incidents of sewer flooding within the study area. There is not sufficient localised information for sewer flooding in the Hertfordshire PFRA to be able to identify historical records within the study area.
13.3.39 The route will not pass through any significantly urbanised areas within the study area. Consequently, there is currently a low risk of flooding from sewers.
Hertfordshire County Council (2011), Hertfordshire County Council Preliminary Flood Risk Assessment.
Buckinghamshire County Council (2011) Buckinghamshire Preliminary Flood Risk Assessment. Jacobs London Borough of Hillingdon (2011) London Borough of Hillingdon Preliminary Flood Risk Assessment Hertfordshire County Council (2011) Hertfordshire Preliminary Flood Risk Assessment
London Borough of Hillingdon (2008) London Borough of Hillingdon Strategic Flood Risk Assessment.
Department for Transport (2013) Design Manual for Roads and Bridges: Volume 4, Geotechnics and drainage, Section 2.
Murname, E., Heap, A. and Swain, A. (2006) C648 Control of Water Pollution from Linear Construction Sites, CIRIA, London, UK.
CFA Report – Colne Valley/No7 | Water resources and flood risk assessment 13.4.10 The following measures will reduce potential impacts to flood risk that could arise from construction.
13.4.11 The Proposed Scheme will be raised on viaduct over the Newyears Green Bourne and the River Colne, along with their associated floodplains. This will reduce the footprint of the Proposed Scheme in the floodplain and the potential to increase flood risk elsewhere. At the Newyears Green Bourne crossing, mitigation will include the addition of two meandering bends (approximately 10m either side from the current channel) over a distance of approximately 140m.
13.4.12 At the River Colne crossing the length of the realignment has been limited to approximately 170m to maximise the retention of the existing channel. The realignment enables the channel to pass between two of the piers within the viaduct.
This avoids an increase in flood risk which would have arisen had a pier been placed in the channel. Due to the angle of the river as it passes under the viaduct there may be a short section of narrowed channel. To address any potential increase in flood risk the Proposed Scheme includes replacement floodplain storage and channel improvement works to offset this impact.
13.4.13 Replacement floodplain storage areas will be provided at the edge of the modelled floodplain of the Newyears Green Bourne and the River Colne to mitigate loss of floodplain storage resulting from permanent structures in the floodplain such as piers for the Newyears Green Bourne and Colne Valley viaducts and the Ickenham National Grid feeder station, as shown on Map CT-06-019 to CT-06-020 (Volume 2, CFA7 Map Book). The replacement floodplain storage will mitigate for temporary loss of floodplain storage resulting from the construction works in the floodplain.