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«LONDONWEST MIDLANDS ENVIRONMENTAL STATEMENT Volume 2 | Community Forum Area report CFA7 | Colne Valley November 2013 ES 3.2.1.7 VOL LondonWest Midlands ...»

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13.4.14 Culverts for minor watercourses and land drains, such as at Old Shire Lane, have been designed to convey the 1 in 100 years return period (1% annual probability) flood flows including an allowance for climate change. This has ensured continued conveyance (flow) and no increase in downstream flood risk.

13.4.15 The draft CoCP sets out the measures and standards of work that will be applied to the construction of the Proposed Scheme (see Volume 5: Appendix CT-003-000/1).

These will provide effective management and control of the impacts during the construction period.

13.4.16 The following examples illustrate how measures in the draft CoCP will reduce potentially adverse effects arising during construction on water resources and flood risk.

13.4.17 With regard to surface water, Section 16 of the draft CoCP stipulates that works in or near the watercourses at the crossings and realignments of the River Colne and Newyears Green Bourne will be designed in consultation with the Environment Agency, so that sediment mobilisation is managed, the potential for contamination from fuel spills is minimised and the works are timed to minimise the impact on water quality and water dependent habitats and species.

13.4.18 Pro-active management practices will ensure that, should a pollution incident occur, the impact is minimised, controlled and reported to relevant parties and remediated in accordance with Section 5 of the draft CoCP.

CFA Report – Colne Valley/No7 | Water resources and flood risk assessment 13.4.19 With regard to groundwater, the Chiltern tunnel southern portal area will be associated with a major construction compound and there will be extensive handling and temporary stockpiling of material excavated from the tunnel and from cuttings.

An area of sustainable placement will be to the east of Harvil Road and south-east of Harefield. The solid material placed at areas and temporary material stockpiles will be tested and the handling and storage adapted to the conditions required over an SPZ1.

Fluids will be reused or disposed of appropriately to avoid risks to surface water or groundwater.

13.4.20 Section 11 of the draft CoCP states that the CL:AIRE Definition of Waste:

Development Industry Code of Practice132 will be followed which states that the material must be suitable for use and will not create an unacceptable risk of pollution to the water environment. The CL:AIRE Code of Practice also states that an appropriate Materials Management Plan, that must be reviewed by a ‘Qualified Person’, must be prepared for the project that describes how the material will be used and verifies that it is suitable for reuse 13.4.21 The Tilehouse Lane cutting will be excavated through unsaturated Chalk and will not affect groundwater levels. Despite this, the naturally fractured nature of the Chalk may provide preferential pathways to the groundwater table for any spillages. There may, therefore be issues relating to groundwater quality resulting from turbidity or release of fluids from construction equipment. The draft CoCP Section 16 requires selection, management and monitoring of material and construction practices to follow best practice, including spillage management. Further detailed discussion is provided, Volume 5: Appendix WR-002-007.

13.4.22 Specific monitoring to determine the potential impact to PWS (Affinity Water) and private abstractions will be undertaken. The monitoring schedule (to be agreed with the Environment Agency and in consultation with Affinity Water) will include monitoring before, during and after construction until the groundwater quality has stabilised within acceptable limits. The monitoring data will be assessed and used to define appropriate mitigation, should it be required.

Vertical migration of poorer quality surface water or poor quality porewater133 in soils 13.4.23 into the Chalk aquifer will be minimised by selection of piling methods which will provide an appropriate seal in the superficial deposits or surface soil layers. If contamination is encountered this will be remediated before piling is undertaken in that location. Application of measures within the draft CoCP (see section 16) will ensure suitable installation techniques for the foundations are applied. The risk assessment and design measures will be included with the method statement for the temporary works. Options include minimising the penetration into the gravels or cutting off the piles above the lake bed and leaving them in situ after construction 13.4.24 Section 16 of the draft CoCP sets out the requirements for dewatering of shallow groundwater for excavation works to ensure that and changes to local groundwater Contaminated Land: Applications in Real Environments (2011) The Definition of Waste: Development Industry Code of Practice (Version 2, March 2011).

Porewater is the water within the soil or rock matrix above the water table. It represents a small volume of water that does not drain under gravity as the water is held in place by surface tension or adsorption by other forces onto soil particles.

deemed to be negligible which for this high value receptor would be a neutral effect and therefore not significant.

13.4.32 Any migration of turbid groundwater to surface water is likely to be a slow process allowing natural attenuation within the chalk and dilution, to reduce turbidity to levels that are unlikely to significantly affect surface water quality. Therefore, the impact of any change in groundwater quality in the wider groundwater body on surface water and water dependent habitats will be negligible. Surface water features and associated water dependent habitats in the area are of high value leading to a neutral effect.





13.4.33 Although effects on wider water body receptors are considered to be neutral, if fissures connect the working area of the Proposed Scheme directly to very high value receptors such as PWS, the impact of even low levels of turbidity could cause the closure of a source due to the high quality required to be met for potable use. This risk is especially the case where the Colne Valley viaduct piers are sited within the areas designated SPZ1 TH177 and SPZ1 TH174 and where the SPZ1 TH027 (see Map WR-02Volume 5, Water resources and flood risk assessment Map Book) will be intercepted by the retaining walls for the Tilehouse Lane cutting. If a PWS was forced to shut down this would be a major impact and will therefore result in a significant adverse effect.

13.4.34 In addition, there is potential to impact groundwater quality at high value receptors such as PWS in this study area that may result from construction of the Proposed Scheme in the neighbouring CFA8, such as the Chiltern tunnel. This is because the direction of groundwater flow is from west to east and south-east from CFA8 into this area. As such, there is a risk that there could be an adverse effect on the PWS in this study area resulting from tunnelling activities in CFA8.

13.4.35 The sources protected by SPZ TH027 and SPZ TH177 (see Map WR-02-007, Volume 5, Water resources and flood risk assessment Map Book) will be a substantial distance down hydraulic gradient from the Proposed Scheme (Chiltern tunnel) in the neighbouring CFA8. As a result, natural attenuation is likely to make any impact from the tunnelling on these SPZ negligible, resulting in a neutral and not significant effect on these PWS.

13.4.36 The source protected by SPZ TH171 (see Map WR-02-007, Volume 5, Water resources and flood risk assessment Map Book) is much closer to and directly down gradient of the Proposed Scheme (Chiltern tunnel) in CFA8 than TH027 and TH177. As a result of this proximity the risk of turbid water entering this abstraction point is greater than for those protected by SPZ TH027 and TH177 and hence would result in a major impact that would be a significant effect.

Flood risk 13.4.37 A temporary jetty will be constructed across the River Colne and floodplain for construction of the viaduct. In the vicinity of these works, the 1 in 100 years return period (1% annual probability) flood water level including an allowance for climate change varies between 37.73m AOD and 37.49m AOD. The top of the bank and adjacent towpath is at approximately 37m AOD along this reach of the watercourse,

CFA Report – Colne Valley/No7 | Water resources and flood risk assessment

which equates to a flood depth of approximately 500mm – 750mm above the towpath level. The deck and supporting structure of the jetty will be designed to take account of the potential for increased flood risk through measures to be incorporated within a site-specific flood risk management plan, as described in ‘Other mitigation’. There remains the potential for the jetty to obstruct some flood flows temporarily during the construction works resulting in moderate impacts on flood risk to very high value receptors with a resulting large and significant adverse effect.

13.4.38 The assessment has identified no significant increase in flood risks from all other sources during the construction process and therefore no other significant temporary effects.

13.4.39 More detailed information is contained in the Flood Risk Assessment (Volume 5:

Appendix WR-003-007).

Cumulative effects 13.4.40 There are no committed developments that have been identified which will result in significant cumulative effects.

–  –  –

Groundwater 13.4.42 One private licensed borehole used for groundwater abstraction at Tilehouse Lane lies within the land required for the construction of the Proposed Scheme The borehole will be decommissioned and backfilled. An alternative supply or compensation will be provided. The impact on overall water resources will be negligible and not significant.

13.4.43 The Colne Valley viaduct will be built on foundation piling that will penetrate the Newhaven and Seaford Chalk Principal aquifer, with some shallower footings for temporary supports. The groundwater table at this location is close to surface. The foundation piling is likely to disrupt groundwater flow. If significant flow horizons within the Chalk are obstructed this could lead to a reduction in flow to PWS abstractions that are particularly close to the route. The source protected by TH177 (see Map WR-02-007, Volume 5, Water resources and flood risk assessment Map Book) is located approximately 25m north-east of the route. It is predicted that the drawdown of groundwater levels at the source is likely to increase or there could be a reduction in yield by the same proportion. This potential additional drawdown or decline in yield could give rise to a major impact on the operation of this very high value receptor, particularly during times of drought. This would then be a very large and significant effect. Further details are provided, Volume 5: Appendix WR-002-007.

–  –  –

Cumulative effects 13.4.45 There are no committed developments that have been identified which will result in significant cumulative effects.

Other mitigation measures 13.4.46 Due to the potential impacts on water quality in the lakes during construction, mitigation will be required where the temporary jetty and coffer dams are constructed around each pier footing. This is likely to include the use of floating booms to trap any hydrocarbons and floating detritus. Silt fences will be installed if there is a risk of construction giving rise to suspended solids due to disturbance of the silts in the lake bed or other causes. With these measures in place no significant residual effects will occur.

13.4.47 The Proposed Scheme could give rise to a significant adverse effect to groundwater quality and flow and thereby on water supplies that depend on the groundwater. As a result, the programme of monitoring to be undertaken in the study area, prior to, during and following completion of the construction works, will be integrated with monitoring undertaken by the owners to address these receptors. The programme will be structured taking into account all the construction processes that could have an impact on the quantity and quality of surface water and groundwater resources, and the interaction between the water resources and water supplies. The monitoring programme scope and duration will be developed and agreed with the Environment Agency in consultation with Affinity Water.

13.4.48 In respect of PWS, HS2 Ltd will agree a management strategy with the Environment Agency in consultation with Affinity Water that will cover timing of any physical mitigation, the scale and nature of monitoring and the thresholds at which actions are invoked (in terms of both quality and flow) the nature of other intervention measures and the responsibilities for ensuring agreed actions occur. These mitigation options

could include:

 minimising construction durations in areas of risk for ground water impacts from turbidity;

 treatment of water at abstractions affected by turbidity; reduced amounts, or suspension, of abstraction at specific periods of construction. Reduction or suspension of abstraction will result in groundwater rebound occurring around the source in question but since this is permitted under the existing abstraction licence, the rebound will have negligible impact;

 temporarily importing water from another source such as those in the Colne Valley that are not affected by the Proposed Scheme and those in neighbouring areas for example, CFA8. Since these other sources would operate within their abstraction licence limits, there would be negligible impacts to groundwater at these other sources;

 use of scavenger wells to intercept poor quality groundwater between the works and the PWS abstraction points. This would require discharge of water arising from the scavenger wells, however, since higher levels of turbidity are 13.4.54 Piling for the viaduct piers could disturb the groundwater flow regime to the Affinity Water groundwater abstraction protected by source protections zones referenced as TH177. Flow horizons to the abstraction are likely to be penetrated and obstructed and as a result there could be a permanent reduction in yield at the source, resulting in a very large and significant effect which could occur during construction works.

13.4.55 Until a management strategy is agreed with the Environment Agency in consultation with Affinity Water, one potentially significant temporary residual effect and one potentially permanent adverse effect on the Affinity Water groundwater abstractions remain.

13.4.56 Until design of the temporary jetty is complete and the site specific flood risk management plan is agreed with the Environment Agency, a potentially significant temporary residual effect on the risk of fluvial flooding remains. During construction works flood conveyance capacity will be reduced by the presence of a temporary jetty across the River Colne resulting in a moderate impact on very high value receptors and a large and significant effect.

Effects arising from operation 13.5 Avoidance and mitigation measures 13.5.1 Generic examples of design measures that will mitigate impacts so that there will be no significant adverse effects on the quality and flow characteristics of surface water courses and groundwater bodies during operation and management of the Proposed Scheme are described in Volume 1, Section 8.

13.5.2 Site specific examples of design measures that will mitigate impact include the drainage arrangements for the Proposed Scheme in the study area. This comprises a number of balancing ponds for either railway or highway drainage and land drainage areas. These ponds and their associated access tracks are shown on Maps CT-06-019 to CT-06-023 (Volume 2, CFA7 Map Book).

13.5.3 Generic examples of management measures during operation and management of the Proposed Scheme that will mitigate impacts so that there are no significant adverse effects on the quality and flow characteristics of surface water courses and groundwater bodies are described in Volume 1, Section 9 and in the draft operation and maintenance plan for water resources and flood risk included, Volume 5 Appendix WR-001-000.

13.5.4 For protection of sensitive areas such as the Mid Colne Valley SSSI and the SPZ associated with PWS measures will be developed to reduce impacts from track runoff draining directly to the surface water receptors. These will include measures such as controls on painting, track maintenance and the application of de-icing fluids and track grease.



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