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15 ROAD DRAINAGE AND THE WATER ENVIRONMENT
15.1 Introduction
15.1.1 This chapter assesses the impacts of the Scheme on road drainage and the water environment during construction and operation, focussing on the effects of highway drainage on the quality and hydrology of receiving waters. In view of the long design-life of the Scheme (30 years for new gantries, 40 years for new carriageway construction, and 120 years for new bridges), the decommissioning phase of the Scheme has not been considered in this chapter because its effects are not predicted to be worse than the effects assessed during the construction and operational phases. The chapter assesses four principal impacts: a) effects of routine runoff on surface water bodies; b) effects of routine runoff on groundwater; c) pollution impacts from spillages; and d) flood impacts. 15.1.2 Although Interim Advice Note (”IAN”) 161/13 ‘Managed Motorways, All lane running’ (Ref 15-1) has scoped out the assessment of ‘Road Drainage and the Water Environment’ for smart motorway schemes, the assessment is required to ensure the protection of the water environment, to prevent its degradation, and ensure adequate mitigation measures are in place to prevent any adverse impacts. 15.1.3 The road drainage and water environment assessment for the Scheme has been undertaken in accordance with standard industry practice and statutory guidance. 15.1.4 This chapter details the methodology followed for the assessment, and summarises the regulatory and policy framework relating to road drainage and the water environment. It then discusses Scheme-wide considerations. Next, the Scheme-wide baseline, mitigation, and residual effects are discussed. Location-specific considerations and effects are then described for each link of the Scheme from west (junction 12) to east (junction 3) to assess specific residual effects between each link. Cumulative effects are then discussed, followed by an analysis of the limitations of the assessment. Finally, a summary table is provided.
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15.1.5 All drawings referenced within this chapter are presented in Document Reference 6.2 (ES Figures), and all appendices referenced in this chapter are presented in Document Reference 6.3 (ES Appendices).
15.2 Methodology
General approach 15.2.1 The method applied follows the approach outlined in Design Manual for Roads and Bridges (“DMRB”) Volume 11, Section 3, Part 10 (HD 45/09) (Ref 15-2) and Volume 4, Section 2, Part 3 (HD 33/06) (Ref 15-3). HD 45/09 gives guidance on assessing the impacts on the water environment (water quantity and quality), whilst HD 33/06 provides guidance on the drainage of trunk roads including motorways. 15.2.2 Baseline data have been collated from secondary sources but no field surveys have been undertaken. An assessment of the potential for environmental impacts on the surface water environment has been undertaken for the construction and operation phases following a desk study approach. 15.2.3 The following documents have been prepared and form part of the information in respect of the Scheme: a) Drainage Strategy, focused on the management of surface water runoff (Document Reference 7.5); and b) Flood Risk Assessment (“FRA”), to identify potential sources of flood risk in relation to the Scheme (Document Reference 5.3). Drainage strategy methodology 15.2.4 Using design information shown on the drawings appended to the Drainage Strategy (Document Reference 7.5), Google maps and the LIDAR data 1 of the existing carriageway, the total increase in impermeable area as a result of widening of the carriageway and creation of Emergency Refuge Areas (“ERAs”) has been estimated. The Drainage Strategy outlines suitable mitigation measures to manage the additional runoff from the increased impermeable area. These are described in the Drainage Strategy (Document Reference 7.5) and are summarised below:
1 Light Detection and Ranging ("LIDAR") is an airborne mapping technique, which uses a laser to measure the distance between the aircraft and the ground.
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a) new drainage systems will be required in the central reserve and the verge. Proposed drainage collection and carrier pipe systems will be designed in accordance with the HD 33/06 for a 1 in 5 year storm event with a 20% allowance for climate change on new paved areas only; b) additional volumes of surface water runoff generated from new paved areas are to be facilitated by increased attenuation and use of overflow outfalls to systems such as soakaways; c) discharge rates at outfalls will not exceed existing discharge rates. So far as practicable discharge rates will be restricted to less than 5 l/s by flow control devices. To reduce the risk of blockages within the system, flows will not be restricted by small drainage system outfalls; d) where proposed impermeable areas increase by more than 10% relative to existing impermeable areas from localised carriageway widening, oversized pipes or manhole chambers are to be used to provide attenuated of flow rates that are to be restricted to existing flow rates at outfalls. Attenuation and flow restrictions to existing rates at outfalls would minimise the impact of flood risk following development of the Scheme; e) the discharges from the Scheme will utilise the existing surface water discharges from the M4. There will be no new discharges or outfalls and the M4 will continue to discharge as it does now; and f) the Drainage Strategy includes measures that will augment existing pollution control measures. The drainage systems are either active or passive in operation. Active systems (requiring operator intervention) comprise penstocks, valves and notched weirs. Passive systems comprise swales, ponds, wetlands, ditches, basins, silt traps, filter drains, soakaways and oil separators.
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15.2.5 Where proposed areas of non-coplanar hard shoulder are to change to coplanar 2 , the drainage system has been modelled to identify the requirement for any upgrades to the existing drainage system. The Drainage Strategy examines the need for providing new drainage systems in the central reserve and the verge. Flood risk methodology Development classification 15.2.6 The FRA (Document Reference 5.3) has been produced in accordance with the Technical Guidance to the National Planning Policy Framework (“NPPF”) (Ref 15-4). 15.2.7 The National Networks National Policy Statement (“NN NPS”) (Ref 15-5), particularly paragraphs 5.92 to 5.97, has been used to inform the FRA. One objective of the NN NPS is for schemes to contribute towards reducing the risk of flooding in the hinterland. Paragraph 5.95 of the NN NPS notes that further guidance can be found in the NPPF Technical Guidance. The NPPF Technical Guidance states that considerations during design should be areas of hard standing, design standards for drainage systems, interactions of national networks with flood plains and watercourses and maintenance standards. 15.2.8 The FRA uses flood risk vulnerability to classify the Scheme. This is important as all developments should take into account the flood risk vulnerability of land uses to ensure development is steered towards appropriate locations. Under this classification, roads fall within the category of ‘essential infrastructure’. 15.2.9 Flood Zones are defined in the NPPF Technical Guidance and refer to the probability of river and sea flooding, ignoring the presence of existing defences (Table 15.1).
2 Coplanar surfaces are surfaces that are in alignment with each other i.e. surfaces in hard shoulder areas slope in the same direction as the main carriageway. Non-coplanar surfaces are where hard shoulder areas are slope in the opposite direction to the main carriageway.
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Table 15.1 Definition of flood zones
Annual Probability of Corresponding Annual Flood Zone Flooding (%) Chance of Flooding (1 in x) *
Flood Zone 1 Fluvial <0.1% >1,000 Low Probability
Flood Zone 2 Fluvial 0.1-1.0% 1,000-100 Medium Probability
Flood Zone 3a Fluvial >1.0% <100 High Probability
Flood Zone 3b Fluvial >5.0%** The Functional <20
Floodplain
* Replace x with annual chance of flooding in column
** Starting point for consideration. Local Planning Authorities should identify the Functional Floodplain, which should not be defined solely by rigid probability parameters. Sequential and exception tests 15.2.10 The NPPF requires that the new development should pass the Sequential and Exception tests. The Sequential Test aims to steer new development to areas at the lowest probability of flooding (Flood Zone 1). For the Exception Test to be passed, it must be demonstrated that the development provides wider sustainability benefits to the community that outweigh flood risk. Further details on the Sequential and Exception tests are contained within the FRA. Assessment of groundwater conditions 15.2.11 Groundwater conditions have been assessed by identifying the existence of aquifers and Source Protection Zones (“SPZ”) in the study area, and then assessing the impacts upon them from the Scheme. The Environment Agency (“EA”) has identified different types of aquifer, which are defined as underground layers of water-bearing permeable rock or drift deposits from which groundwater can be extracted.
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15.2.12 The types of aquifers are defined below. a) Principal Aquifers - layers of rock or drift deposits that have high intergranular and/or fracture permeability (meaning they usually provide a high level of water storage). They may support water supply and/or river base flow on a strategic scale; b) Secondary A - permeable layers capable of supporting water supplies at a local rather than strategic scale, and in some cases forming an important source of base flow to rivers. These are generally aquifers formerly classified as minor aquifers; and c) Secondary B - predominantly lower permeability layers which may store and yield limited amounts of groundwater due to localised features such as fissures, thin permeable horizons and weathering. These are generally the water-bearing parts of the former non- aquifers. 15.2.13 SPZs are used to identify those areas close to drinking water sources where the risk associated with groundwater contamination is greatest. The three main SPZs are defined below. a) Inner zone (Zone 1) - Defined as the 50 day travel time from any point below the water table to the source. This zone has a minimum radius of 50m; b) Outer zone (Zone 2) - Defined by a 400 day travel time from a point below the water table. This zone has a minimum radius of 250 or 500m around the source, depending on the size of the abstraction; and c) Total catchment (Zone 3) - Defined as the area around a source within which all groundwater recharge is presumed to be discharged at the source. Consultation 15.2.14 Consultation undertaken has included relevant consultee groups between junction 12 and junction 3. In particular, this includes consultation with the EA and the Lead Local Flood Authorities whose jurisdiction is crossed by the Scheme to discuss the approach to be taken to the assessment of flood risk and the management of routine rainfall runoff and spillage risk. A scoping opinion was requested from the EA and Lead Local Authorities. These have formed part of the wider consultations for the Scheme
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proposals as set out in the Consultation Report (Document Reference 5.1). Study area 15.2.15 The study area has been defined having regard to the area most likely to be directly affected by the Scheme. This encompasses: a) the area within the Order limits where the Scheme could be expected to have an effect on drainage and water resources during construction and after opening; b) a 100m buffer either side of the existing M4, including slip roads and bridges; c) watercourses or water-dependent nature conservation sites up to 1km downstream of any outfalls that discharge highway drainage; and d) a 100m buffer around the temporary construction compounds associated with the Scheme. 15.2.16 The study area for the Flood Risk Assessment (“FRA”) (Document Reference 5.3) includes all proposed localised carriageway widening within existing highway verges and all alterations to overbridges and ERAs that are potentially at risk of flooding from heavy rain, river, artificial, groundwater and sewer sources between junctions 12 and 3. Defining the baseline 15.2.17 The baseline conditions have been established in a desk study through the collation and review of existing data and documents listed below: a) Ordnance Survey (“OS”) mapping; b) topographic survey; c) site plans; d) EA online data sets for water quality and pollution incidents; and e) the Multi-Agency Geographic Information for the Countryside (“MAGIC”) interactive mapping (geographic information about the natural environment at http://www.natureonthemap.naturalengland.org.uk/MagicMap.aspx). 15.2.18 No Scheme-specific surveys or monitoring have been undertaken, as sufficient information is readily available via the sources listed above.
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15.2.19 The drainage and water receptors have been assigned a value/importance based upon criteria derived from DMRB Volume 11, Section 3, Part 10 (HD 45/09) (Ref 15-2) and are presented in Table 15.2.
Table 15.2 Receptor Value/Importance
Typical Value/importance Typical example descriptors
Very High Attribute Surface EC Designated Salmonid/Cyprinid fishery has a high waters River Quality Objective (“RQO”) River quality and Ecosystem Class RE1* rarity on a regional or Site protected under EU or UK wildlife national legislation (SAC, SPA, SSSI, Ramsar site) scale. Groundwater Major aquifer providing a regionally important resource or supporting site protected under wildlife legislation; Source Protection Zone (“SPZ”) I
Flood risk Flood plain or defence protecting more than 100 residential properties from flooding
High Attribute Surface RQO River Ecosystem Class RE2* has a high Waters Major Cyprinid Fishery quality and rarity on a Species protected under EU or UK wildlife local scale. legislation
Groundwater Major aquifer providing locally important resourced or supporting river ecosystem; SPZ II
Flood risk Flood plain or defence protecting between 1 and 100 residential properties or industrial premises from flooding.
Medium Attribute Surface RQO River Ecosystem Class RE3 or has a waters RE4* medium quality and Groundwater Aquifer providing water for agricultural or rarity on a industrial use with limited connection to local scale. surface water SPZ
Flood risk Flood plain or defence protecting 10 or fewer industrial properties from flooding
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Typical Value/importance Typical example descriptors
Low Attribute Surface RQO River Ecosystem Class RE5* has a low waters quality and rarity on a Groundwater Non-aquifer local scale. Flood risk Flood plain with limited constraints and low probability of flooding of residential and industrial properties.
Source: (DMRB Volume 11 Section 3 Part 10 (HD 45/09)) *Note- included as an explanation in DMRB however the RQOs are not included under the WFD. Mitigation 15.2.20 Appropriate mitigation measures will be put in place to manage the level of risk during both the construction and operation phases. Appropriate mitigation options have been identified based on a review of guidance provided in DMRB Volume 11, Section 3, Part 10 (HD 45/09) and are based upon professional judgement. Assessing impacts 15.2.21 The approach to assessing the impacts of the Scheme on the water environment involves an assessment of the relative significance of the impacts. The methodology assesses the likely impacts of the Scheme on the water environment; identifies the need for mitigation through improved design and environmental management during construction and operation; and assesses the residual effects of the mitigated Scheme. The methodology for this approach is listed below: a) assessment of baseline environmental importance - an assessment of baseline importance has been undertaken by drawing together and reviewing available data and identify existing environmental conditions (i.e. the environment without the Scheme). The drawings appended to the Drainage Strategy (Document Reference 7.5) have been used to assess the existing drainage system. Online flood map data from the EA, contained within the FRA (Document Reference 5.3), has been used to assess the flood risk within the study area;
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b) assessment of impact magnitude - the magnitude of impact is described with reference to the Scheme-specific effect descriptors presented in Table 15.3. The Scheme has been assessed to ensure that all effects on road drainage and the water environment are identified and that any environmental issues associated with the underlying groundwater are also considered; c) assessment of impact significance - the significance of an environmental effect is typically a function of the value or importance of the receptor and the magnitude or scale of the impact. Combining the environmental value of the resource or receptor with the magnitude of impact produces a significance of effect category; and d) assessment of residual effects after mitigation - effects that remain after mitigation are referred to as "residual effects". Therefore, the key outcome of the assessment is the significance of the residual effects after mitigation or enhancement. 15.2.22 The magnitude of impacts of the Scheme on receptors is assessed considering the scale, extent of change, nature and duration of the impact as described in chapter 5 Environmental Impact Assessment Methodology. The magnitude of the impact may be major, moderate or minor, adverse or beneficial, or negligible in accordance with DMRB (Ref 15-2). These terms are defined in Table 15.3.
Table 15.3 Definitions of magnitude of impact
Magnitude of Impact Typical Criteria Descriptors
Total loss or large scale damage to existing character or distinctive features and elements, and/or the addition of new but uncharacteristic conspicuous features and elements . Surface Water : Failure of both soluble and sediment- bound pollutants in Highways Agency Water Risk Major adverse Assessment Tool ("HAWRAT") and compliance failure with Environmental Quality Standard ("EQS") values. Groundwater : Loss of, or extensive change to, an aquifer. Potential high risk of pollution to groundwater from routine runoff. Flood Risk : Increase in peak flood level (1% annual probability) >100 mm
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Magnitude of Impact Typical Criteria Descriptors
Partial loss or noticeable damage to existing character or distinctive features and elements, and/or the addition of new but uncharacteristic noticeable features and elements. Surface Water : Failure of both soluble and sediment- bound pollutants in HAWRAT but compliance with EQS Moderate adverse values. Groundwater : Partial loss or change to an aquifer. Potential medium risk of pollution to groundwater from routine runoff. Flood Risk : Increase in peak flood level (1% annual probability) >50 mm.
Slight loss or damage to existing character or features and elements, and/or the addition of new but uncharacteristic features and elements. Surface Water : Failure of either soluble or sediment- bound pollutants in HAWRAT. Calculated risk of pollution Minor adverse from spillages >0.5% annually and <1% annually. Groundwater : Potential low risk of pollution to groundwater from routine runoff. Flood Risk : Increase in peak flood level (1% annual probability) >10mm.
Barely noticeable loss or damage to existing character or features and elements, and/or the addition of new but uncharacteristic features and elements. Surface Water : No risk identified by HAWRAT (Pass both Negligible adverse soluble and sediment-bound pollutants). Groundwater : No measurable impact upon an aquifer and risk of pollution from spillages. Flood Risk : Negligible change in peak flood level (1% annual probability) <+/- 10 mm.
No noticeable loss, damage or alteration to character or No change features or elements.
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Magnitude of Impact Typical Criteria Descriptors
Barely noticeable improvement of character by the restoration of existing features and elements, and/or the removal of uncharacteristic features and elements, or by the addition of new characteristic elements. Surface Water : No risk identified by HAWRAT (Pass both Negligible beneficial soluble and sediment-bound pollutants). Groundwater : No measurable impact upon an aquifer and risk of pollution from spillages. Flood Risk : Negligible change in peak flood level (1% annual probability) <+/- 10 mm.
Slight improvement of character by the restoration of existing features and elements, and/or the removal of uncharacteristic features and elements, or by the addition of new characteristic elements. Surface Water : HAWRAT assessment of either soluble or Minor beneficial sediment-bound pollutants becomes Pass from an existing site where the baseline was a Fail condition. Groundwater : Calculated reduction in existing spillage risk by 50% or more to an aquifer. Flood Risk : Reduction in peak flood level (1% annual probability) >10 mm.
Partial or noticeable improvement of character by the restoration of existing features and elements, and/or the removal of uncharacteristic and noticeable features and elements, or by the addition of new characteristic features. Surface Water : HAWRAT assessment of both soluble and Moderate beneficial sediment-bound pollutants becomes Pass from an existing site where the baseline was a Fail condition. Groundwater : Calculated reduction in existing spillage risk by 50% or more. Flood Risk : Reduction in peak flood level (1% annual probability) >50 mm.
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Magnitude of Impact Typical Criteria Descriptors
Large scale improvement of character by the restoration of features and elements, and/or the removal of uncharacteristic and conspicuous features and elements, or by the addition of new distinctive features. Surface Water : Removal of existing polluting discharge, or removing the likelihood of polluting discharges occurring to Major beneficial a watercourse. Groundwater : Removal of existing polluting discharge to an aquifer or removing the likelihood of polluting discharges occurring. Recharge of an aquifer. Flood Risk : Reduction in peak flood level (1% annual probability) >100 mm.
15.2.23 The significance of the effects of the Scheme on receptors is based on a combination of the value of the resource and the magnitude of the impact, as shown in Table 15.4. This matrix table is derived from chapter 5 Environmental Impact Assessment Methodology and follows the criteria set out in DMRB (Ref 15-2). Professional judgement has been used when selecting the significance of effect where there is a choice, e.g. slight/moderate.
Table 15.4 Significance matrix table
Magnitude of impact Value/importance of receptor Negligible Minor Moderate Major
Very high Neutral Moderate/large Large/very large Very large
High Neutral Slight/moderate Moderate/large Large/very large
Medium Neutral Slight Moderate Large
Low Neutral Neutral Slight Slight/moderate
Source: (DMRB Volume 11 Section 3 Part 10 (HD 45/09)) 15.3 Regulatory/policy framework
15.3.1 A summary of relevant legislation and policies that have been considered as part of this assessment is provided in Table 15.5. Local plans and policies are summarised in Appendix 15.1.
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Table 15.5 Water and drainage regulatory and policy framework
Policy/Legislation Summary of requirements Scheme response
European and National
The Water Framework The Directive provides a The Scheme will aim Directive (“WFD”) framework for the protection of to attain the highest (2000/60/EEC) surface (fresh) water, estuaries, achievable level of (Ref 15-6) coastal water and groundwater. water quality The objectives of the Directive standards. This will are to enhance the status, and be achieved through prevent further deterioration, of the drainage design aquatic ecosystems, promote the which will aim to sustainable use of water, reduce improve the quality of pollution of water (especially by the runoff from the ‘priority’ and ‘priority hazardous’ Scheme. substances) and ensure Measures will be progressive reduction of implemented during groundwater pollution. Among construction and the main features of the Directive operation to mitigate are that all inland and coastal potential impacts to waters within defined river basin hydromorphological, districts must reach at least good biological and status by 2015. physico-chemical elements of the water environment.
The Flood and Water The Flood and Water Through the Management Act 2010 Management Act 2010 provides preparation of a FRA (Ref 15-7) comprehensive management of (Document flood risk for people, homes and Reference 5.3) and businesses. the Drainage Strategy (Document Reference 7.5), mitigation measures have been incorporated to ensure the Scheme will not be exposed to an unacceptable degree of flood risk or increase the flood risk to third parties.
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Policy/Legislation Summary of requirements Scheme response
National Planning Policy The NPPF sets out Government An FRA for the Framework ("NPPF") policy on development and flood Scheme has been (Ref 15-4) risk. Its aims are to ensure that carried out in and Planning Policy flood risk is taken into account at accordance with Statement ("PPS") 25 all stages in the planning process NPPF. Practice Guide to avoid inappropriate (Ref 15-8) development in areas at risk of flooding, and to direct development away from areas of highest risk. Where new development is, exceptionally, necessary in such areas, the policy aims to make it safe, without increasing flood risk elsewhere, and, where possible, reducing flood risk overall. NPPF is supported by the PPS25 Practice Guide, which is still used to supplement the NPPF.
National Networks The NN NPS sets out the need An FRA has been National Policy for, and Government’s policies to prepared to identify Statement (“NN NPS”) deliver, development of and assess the risks (Ref 15-5) nationally significant of all forms of infrastructure projects on the flooding to and from national road networks in the Scheme and England. The development of the demonstrate how national road network should to these flood risks will be sustainable and designed to be managed, taking minimise environmental impacts, climate change into this includes improving water account. quality and reducing flood risk. The Secretary of State expects that reasonable steps have been taken to avoid, limit and reduce the risk of flooding to the proposed infrastructure and others. Where linear infrastructure has been proposed in a flood risk area, it is expected that reasonable mitigation measures will be made, to ensure that the infrastructure remains functional in the event of predicted flooding.
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15.4 Scheme-wide considerations
Baseline features and value (importance) of resource Surface watercourses/water features 15.4.1 There are over 60 watercourses which pass under the M4 between junctions 12 and 3 (Drawing 15.1). Those watercourses which are monitored under the Thames River Basin Management Plan (“RBMP”), have defined WFD classifications on the EA’s website, or which are named on OS maps, are listed below in Table 15.6.
Table 15.6 Summary of watercourses crossing the M4 within the study area
Watercourse Crossing location
Kennet & Holy River Crosses the M4 once near junction 12
Kennet & Avon Canal Crosses the M4 once near junction 12
Crosses the M4 once to the west of the services Clayhill Brook between junction 11 and 12
Crosses the M4 once between junction 11 and Kennet & Foudry Brook 12
Crosses M4 once between junction 10 and 11 Barkham Brook adjacent to River Loddon
Crosses the M4 once between junction 10 and River Loddon 11
Emm Brook Crosses the M4 once at junction 10
Crosses the M4 three times, once between The Cut junction 7 and 8/9 and twice close to junction 8/9.
River Thames Crosses the M4 once west of junction 7
Crosses the M4 once (west of junction 7), then Roundmoor Ditch follows the direction of the M4 until it joins the Jubilee River
Crosses the M4 once (west of junction 7), then Jubilee River flows to the east, in close proximity to the M4
Salthill Stream Crosses the M4 once near junction 6
Chalvey Ditch Crosses the M4 once near junction 6
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Watercourse Crossing location
Datchet Common Brook Crosses the M4 once near junction 5
Horton Brook (River Colne) Crosses the M4 once at junction 4b
Colne Brook Crosses the M4 once at junction 4b
River Colne and Grand Union Crosses the M4 at junction 4b with three Canal separate channels
River Crane Crosses the M4 once at junction 3
15.4.2 There are unnamed surface water drains and ditches crossing the M4 which are likely to receive local drainage and form tributaries of the watercourses listed above. Using professional judgement, the unnamed watercourses have been assessed as part of the catchment wide assessment as a single receptor because each can reasonably be expected to perform in a broadly similar fashion. With the mitigation measures discussed in paragraph 15.4.40 below, it is considered that appropriate measures will be in place to ensure no deterioration of the waterbodies as a result of the Scheme. 15.4.3 From OS maps there are several lakes, ponds and reservoirs within 100m of the Scheme. These include the following identified in Table 15.7.
Table 15.7 Additional water features within 100m of the Scheme
Surface water feature Location
Located to the south of junction 12, approximately 35m from the south side of M4. Lake at Arlington Business The lake discharges to an unnamed drain Park which receives highway drainage (upstream of the outfall location). The unnamed drain discharges to the River Kennet.
Two small ponds located approximately 44m from the M4 south of junction 12 between the Unnamed ponds railway and Holy Brook/River Kennet on the south side of the M4.
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Surface water feature Location
Located west of junction 11 (NGR 466152, 170700), approximately 20m from the M4. Comprises several lakes between the River Ameys Lakes or Theale Kennet and Reading Motorway Service Area Lakes ("MSA"). The lake on the south side is connected to the lake to the north of the M4. The lake on the north side drains into Clayhill Brook downstream of M4.
Located in-between junction 12 and 11. Two are located to the south of M4 adjacent to Unnamed lakes near Foudry Foudry Brook at Pingewood House and three Brook are located to the north of the M4. All features are located within 30m of the M4.
Located approximately 70m from the M4 Bray Lake and two between junction 7 and 8/9 adjacent to The additional (unnamed) ponds Cut (NGR 490851, 178871).
Located approximately 100m from the M4 near The Queen Mother junction 5 (NGR 500646, 177340) and Reservoir adjacent to Datchet Common Brook and Horton Brook.
Adjacent to Colne Brook at junction 4b. Old Slade Lake Located approximately 25m south of the M4.
Adjacent to the River Colne at junction 4b. Saxon Lake Located approximately 60m south of the M4.
Existing flood risk 15.4.4 The potential sources of flooding for the Scheme are heavy rain, rivers, surface water, groundwater, sewers and artificial/man-made sources (e.g. reservoirs and canals). Tidal flooding has been discounted from the assessment due to the Scheme being upstream of the tidal limit of the River Thames, and at no risk of flooding from this source. Fluvial flood risk 15.4.5 The Scheme crosses a number of floodplains classified as Flood Zone 2 and Flood Zone 3, which are summarised in Table 15.8 and detailed on Drawing 15.1.
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Table 15.8 List of Fluvial Flood Zones crossing Scheme between junctions 12 and 3
Flood Component Water Body Source of Information Zone Affected
Kennet & Holy River 3a, 3b Junctions 12 - 11 West Berkshire Strategic Flood Risk Assessment ("SFRA"), 2008
Kennet & Avon 3a, 3b Junctions 12 - 11 West Berkshire SFRA, 2008 Canal
Kennet and Foundry 2, 3a, 3b Junctions 12 - 11 West Berkshire SFRA, 2008 Brook
Clayhill 2, 3a Junctions 12 - 11 Wokingham Borough Council Brook/Foundry SFRA, 2007 Brook
Barkham Brook 2, 3 Junctions 11 - 10 Wokingham Borough Council SFRA, 2007
River Loddon 2, 3a, 3b Junctions 11 - 10 Wokingham Borough Council SFRA, 2007
Emm Brook 2, 3a, 3b Junctions 11 - 10 Wokingham Borough Council SFRA, 2007
The Cut 2, 3 Junctions 10 - 8/9 Royal Borough of Windsor & Construction Maidenhead SFRA, 2009 compound 5
River Thames 2, 3 Junctions 8/9 - 7 Royal Borough of Windsor & Maidenhead SFRA, 2009
Jubilee River 2 Junctions 8/9 - 7 South Bucks District Council SFRA, 2008
Roundmoor Ditch 2 Junctions 8/9 - 7 South Bucks District Council SFRA, 2008
Chalvey Ditch 2, 3 Junctions 7- 6 Slough Local Development Framework SFRA, 2007
Salthill Stream 2, 3 Junctions 6 – 5 Slough Local Development Construction Framework SFRA, 2007 compound 6
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-19
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Flood Component Water Body Source of Information Zone Affected
Datchet Common 2, 3 Junctions 6 - 5 Slough Local Development Brook Framework SFRA, 2007
Horton Brook (River 2, 3 Junctions 5 - 4b Slough Local Development Colne) Framework SFRA, 2007
Colne Brook 2, 3 Junctions 4b - 4 Slough Local Development Framework SFRA, 2007
River Colne and 2, 3a, 3b Junctions 4b - 4 London Borough of Hillingdon Grand Union Canal SFRA, 2008
River Crane 2, 3a Junctions 4 - 3 London Borough of Hillingdon SFRA, 2008
15.4.6 Widening works are required within the floodplain (Flood Zone 2 and 3) at the following sites: a) junctions 10 to 8/9 - addition of two ERAs and three localised carriageway widenings; b) junctions 8/9 to 7 - addition of two ERAs and a localised carriageway widening; c) junctions 7 to 6 - addition of three ERAs and seven localised carriageway widenings; d) junctions 6 to 5 - one carriageway localised widening; and e) junctions 5 to 4b - one carriageway localised widening. 15.4.7 Works are required to overbridges within the floodplain (Flood Zone 2 and 3) at the following overbridge sites: a) junction 8/9 - Ascot Road (The Bourne), Monkey Island Lane (River Thames) and Marsh Lane (Jubilee River/Thames); b) junctions 7 to 6 - Wood Lane (Jubilee River); c) junctions 6 to 5 - Riding Court Road (River Thames); and d) junctions 5 to 4b - Old Slade Lane (Colne Brook). 15.4.8 The proposed works at Ascot Road have been assessed against EA flood modelling and show a small part of the proposed side road alignment is within the 1% Climate Change (“CC”) Annual Exceedance Probability (“AEP”) flood extent.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.9 The proposed works at Monkey Island Lane have been assessed against EA flood modelling and show that the re-alignment of the bridge is located within the 1% and 1% CC AEP flood extent. 15.4.10 The proposed works at Marsh Lane have been assessed against EA flood modelling and show there are no significant changes between existing and proposed road alignment extent. For that reason, the works at Marsh Lane are unlikely to have a significant impact on the 1% CC AEP flood extent. 15.4.11 The proposed works at Wood Lane have been assessed against EA flood modelling and show that the re-alignment would affect the 1% and 1% CC AEP flood extent. 15.4.12 The proposed works at Riding Court Road have been assessed against EA flood modelling and show that the re-alignment would affect the 1% CC AEP flood extent. 15.4.13 The proposed works at Old Slade Lane have been assessed against EA flood modelling and show there is no significant change between existing and proposed road alignment extent. For that reason, the works at Old Slade Lane are considered unlikely to have a significant impact on the 1% CC AEP flood extent. 15.4.14 Works are required adjacent to and within the channel of watercourses at the following sites: a) junction 8/9 - Thames at Bray (River Thames) ; b) junctions 7 to 6 - Chalvey culvert (Chalvey Ditch); and c) junctions 6 to 5 - Ashleys Arch culvert (Datchet Common Brook). 15.4.15 In channel works are proposed to the Thames Bray underbridge over the River Thames. These works will include provision of new river-level supports. In channel works are also proposed to extend the existing Chalvey culvert on the Chalvey Ditch and the Ashley’s Arch culvert on Datchet Common Brook. Flood risk from artificial sources 15.4.16 In addition to flooding from rivers, the following small sections of the Scheme are identified on the EA online mapping facility (“What’s in your backyard?”) (Ref 15-9) as being at risk from reservoir flooding: a) a section between junction 11 and junction 10 associated with Bearwood Lake;
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
b) a section immediately west of junction 10 associated with Queensmere Reservoir; and c) the majority of areas between junction 6 and junction 4b associated with a number of reservoirs located to the south of the M4 and three areas between junction 4b and junction 4 associated with the Ruislip Lido Reservoir. 15.4.17 Widening works are required within the following sites identified as being at risk of flooding from reservoirs: a) junctions 6 to 5 - one carriageway localised widening; and b) junctions 5 to 4b - one carriageway localised widening. Surface water and sewer flood risk 15.4.18 The data within the Highways Agency Drainage Data Management System ("HADDMS") have been used to assess the flood risk from surface water and the surface water drainage system (sewers). According to the register, all historic flood events listed below occurred due to blocked drains and ponding of surface water on the carriageway. Further details of specific events are provided in the HADDMS records contained within the FRA (Document Reference 5.3). The following locations have recorded flood events: a) junctions 12 to 11 - two recorded flood events on the eastbound carriageway; b) junctions 11 to 10 - four recorded flood events; c) junctions 10 to 8/9 - four recorded flood events; d) junctions 8/9 to 7 – 22 recorded flood events; e) junction 7 - one recorded flood event on the westbound carriageway; and f) junctions 6 to 5 - 12 recorded flood events. 15.4.19 Widening works are required within the following sites identified as being at risk of flooding from reservoirs: a) junctions 8/9 to 7 - addition of two ERAs and a localised carriageway widening; b) junction 7 - two localised carriageway widenings; and c) junction 6 – one carriageway localised widening.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Groundwater flood risk 15.4.20 According to the SFRAs which cover the study area, there are no known incidences of groundwater flooding. Figure 1 of the West Berkshire SFRA covers junctions 12 to 11. The Wokingham Borough Council SFRA covers junctions 11 to 10. Figure B of the Royal Borough of Windsor and Maidenhead SFRA covers junction 10 to 7. The Slough Borough Council SFRA covers junctions 7 to 3. 15.4.21 There have been no other known incidents of flooding due to high groundwater levels within the study area. Therefore, the risk of flooding from groundwater is considered to be low. Surface water quality 15.4.22 Table 15.9 summarises the water quality categories of surface water resources in the study area. According to the EA, all of the surface watercourses and lakes classified under the WFD along this Scheme are identified as being “at risk” of failing to meet “Good” status by 2015, mostly due to their heavily modified (engineered) nature and the poor to moderate ecological potential of the waterbodies. Of all the waterbodies classified under the WFD, only the Kennet and Avon Canal and the Ameys Lake or Theale Lakes are considered to have current ecological quality of Good Potential. The Colne and Grand Union Canal and River Loddon fail in terms of current chemical status. 15.4.23 All of the rivers crossing the Scheme are categorised as ‘cyprinid’ rivers, meaning that they support carp, minnows and related species, with a compliance status of guideline fail and/or imperative pass. The compliance status reflects the compliance with the wider WFD objectives relating to chemical and biological quality. In addition, the navigable section of the Kennet and Avon Canal is a designated ‘salmonid’ river, meaning that it supports species in the salmonidae family. Species in the salmonidae family include salmon, trout, chars, freshwater whitefishes and grayling. None of the watercourses identified above have any statutory nature conservation designations within 1km of the Scheme. 15.4.24 There will be no new discharges or outfalls as a result of the Scheme and the M4 will continue to discharge as it does now. The discharges from the Scheme will utilise the existing surface water discharges from the M4.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.25 A WFD Compliance Assessment has been completed for the Scheme (Document Reference 7.6). In the assessment the components of the Scheme that were found to be relevant to the WFD objectives were Thames Bray underbridge, Ashley Arch culvert and Chalvey Ditch culvert. The review of these components against WFD elements found no indication that further assessment was required. Scheme mitigation proposals in place will ensure no deterioration in water quality. 15.4.26 The WFD Compliance Assessment concludes that the Scheme is compliant with the objectives of the WFD, provided appropriate mitigation measures are implemented prior to and during construction. On this basis, no further assessment is expected.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Table 15.9 Summary of surface water quality status between junctions 12 and 3
Water Body Name Typology Designated Status Current Current Protected Area Description Ecological Chemical Quality Quality
Ameys Lake or High Artificial Good potential Does not require Yes - Nitrates Directive Theale Lakes alkalinity, assessment very shallow
Kennet and Holy Low, large, Heavily modified Moderate Good Yes - Nitrates Directive Brook calcareous potential
Kennet & Avon Canal Artificial Good Potential Does not require Yes - Nitrates Directive Canal, Copse Lock assessment to Reading (River Kennet and canal sections)
Kennet and Foudry Low, small, Heavily modified Moderate Good Yes - Drinking Water Brook and Clayhill calcareous potential Protected Area, Nitrates Brook in Reading Directive
Barkham Brook Low, small, Not Designated Moderate Good Yes - Nitrates Directive calcareous Status
Loddon Low, Not designated Moderate Fail Yes - Drinking Water (Swallowfield to medium, potential Protected Area, Nitrates River Thames calcareous Directive confluence)
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Water Body Name Typology Designated Status Current Current Protected Area Description Ecological Chemical Quality Quality
Emm Brook Low, small, Heavily modified Moderate Does not require Yes - Nitrates Directive siliceous potential assessment
Cut (Binfield to Low, Heavily modified Poor potential Good Yes - Nitrates Directive, River Thames medium, Urban Waste Water confluence) and calcareous Treatment Directive Maidenhead Ditch
Thames (Cookham Low, large, Heavily modified Moderate Good Yes- Drinking Water to Egham) calcareous potential Protected Area, Nitrates Directive, Urban Waste Water Treatment Directive
Roundmoor Ditch Low, small, Heavily modified Moderate Good Yes - Nitrates Directive, and Boveney Ditch siliceous potential Urban Waste Water Treatment Directive
Chalvey Ditches at Low, small, Heavily modified Moderate Does not require Yes - Nitrates Directive Slough siliceous potential assessment
Salthill Stream Low, small, Heavily modified Poor potential Does not require Not designated siliceous assessment
The Queen Mother High Artificial Poor potential Does not require Yes - Drinking Water Reservoir alkalinity, assessment Protected Area deep
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Water Body Name Typology Designated Status Current Current Protected Area Description Ecological Chemical Quality Quality
Datchet Common Low, small, Heavily modified Moderate Does not require Brook calcareous potential assessment
Horton Brook Low, small, Not designated Moderate Does not require Yes - Natura 2000 calcareous potential assessment
Colne Brook (from Low, small, Heavily modified Moderate Does not require Yes –Nitrates Directive confluence with calcareous potential assessment Alderbourne to confluence with Horton Brook)
Colne and Grand Low, Heavily modified Moderate Fail Yes –Nitrates Directive Union Canal (from medium, potential confluence with calcareous Chess to Ash)
Crane (including Low, small, Heavily modified Poor potential Good Yes –Nitrates Directive part of the Yeading calcareous Brook)
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Groundwater 15.4.27 The EA aquifer designation map (Ref 15-10) indicates the following aquifer types along the Scheme between junctions 12 and 3: a) localised Chalk Principal aquifer at junction 12; b) widespread Secondary A aquifer and localised Secondary B aquifer comprising river terrace deposits and alluvium between junctions 12 and 10; c) very localised areas of Secondary A aquifer comprising superficial deposits between junctions 10 and 8/9, as well Secondary A aquifer comprising bedrock in the area of junction 8/9; d) Principal and Secondary A aquifer types comprising superficial deposits between junctions 8/9 and 6, as well as some areas of Secondary A aquifer comprising bedrock (Reading Beds); and e) localised areas of Principal aquifer comprising river terrace deposits and alluvium between junctions 5 and 3 and localised Secondary A aquifer near junction 5. 15.4.28 Water levels within the river terrace deposits, forming part of the Principal aquifers, could be in hydraulic continuity with surface waters. These deposits contain little fine sand and clay and therefore tend to have high permeability values and potential for rapid groundwater movement. 15.4.29 The EA’s SPZ mapping published on its website shows the following around the Scheme between junctions 12 and 3. a) an SPZ between junctions 12 and 11, with the M4 carriageway lying within the outer zone and total catchment of the SPZ; b) a localised SPZ with inner and outer zones at junction 10. The carriageway lies within the inner zone; c) an SPZ with inner, outer and total catchment zones between junctions 10 and 8/9. Both carriageways lie within the inner zone in two locations; d) an SPZ with inner, outer and total catchment zones between junctions 8/9 and 7; e) an SPZ between junctions 6 and 5, with the inner and outer zones around junction 6 and to the east of it and the total catchment zone reaching junction 5 to the east (both the east and westbound carriageways lie within the inner zone); and
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
f) a localised SPZ with inner and outer zones between junctions 5 and 4b - part of the carriageway lies within the inner zone. 15.4.30 Between junctions 12 and 4 the Scheme is located within a Surface Water Safeguard Zone. These safeguard zones are designated areas in which the use of certain substances must be carefully managed to prevent the pollution of raw water sources which are used to provide drinking water. The zones are safeguarded for pesticides. The Scheme is not located within a Groundwater Water Safeguard Zone or a Water Protection Zone. 15.4.31 Similar to the surface waterbodies, the Thames RBMP (Ref 15-11) classifies groundwater bodies within the Thames River Basin District ("RBD"). Each groundwater body has quantitative and chemical components representing its overall status. The study area overlies three groundwater bodies; the Berkshire Downs Chalk, Thatcham Tertiaries, and Twyford Tertiaries. All the groundwater bodies are considered to be at risk of failing their environmental objectives for groundwater quality and all the groundwater bodies have poor quantitative quality (Table 15.10).
Table 15.10 Status of groundwater quality along the M4 corridor between junctions 12 and 3
Water Body ID GB40601G6009 GB40602G601600 GB40602G602700
Water body name Berkshire Downs Thatcham Twyford Tertiaries Chalk Tertiaries
River basin district Thames Thames Thames
Current quantitative Poor Poor Poor quality
Current chemical Poor Poor Good quality
Upward chemical Yes No No trend
2015 Predicted Poor Poor Poor quantitative quality
2015 Predicted Poor Poor Good chemical quality
Overall risk At risk At risk At risk
Protected area Yes - Drinking Yes - Drinking Yes - Drinking
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-29
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Water Body ID GB40601G6009 GB40602G601600 GB40602G602700 Water Protected Water Protected Water Protected Area, Nitrates Area, Nitrates Area, Nitrates Directive Directive Directive
15.4.32 The Envirocheck Reports (Ref 15-12) show that there are 62 water abstraction points located between junctions 12 and 3 within 500m of the Scheme. There could potentially be significant effects from the Scheme on the abstractions points (in terms of water quantity and quality), so an assessment of the Scheme's effect on abstraction points is required. The Envirocheck Reports show that there are 12 water discharge consents located between junctions 12 and 3 within 500m of the Scheme. Discharges are only likely to be impacted by the Scheme if the associated infrastructure is affected. Existing drainage system 15.4.33 The existing highway drainage along the M4 between junctions 12 and 3 mainly comprises kerbs and gullies which divert surface water runoff into the surface water drainage system. The central reserve drainage consists mainly of a filter drain system. The majority of highway runoff is discharged via outfalls to watercourses. However, in some areas there are also soakaways to groundwater. 15.4.34 For the purposes of this assessment, it is assumed that the existing drainage system is sufficient for the existing M4. Whereas surface water flooding is reported to have occurred from blocked drainage, it is assumed that maintenance/repair or substitution/replacement of the systems is capable of providing a satisfactorily functioning system. As such, since there is no reason to believe that the existing system cannot be rendered wholly effective, this provides a reasonable worst case baseline scenario. Future conditions 15.4.35 The south-east region as a whole and the M4 are expected to experience increased flood risk when climate change is taken into account. The regional flood risk appraisal for the south-east concludes that climate change will be the major cause of increased flood risk. As a result, the volumes of fluvial and surface water are likely to increase and in the future watercourses may not be able to contain the water within river channels or drainage systems.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.36 The Scheme design takes climate change and the potential for increased flood risk into account, as a 20% allowance for climate change has been applied for the new surface water drainage designs. 15.4.37 Legislative drivers, such as the WFD are expected to yield improvements in future surface and groundwater quality. Value (importance) of resource 15.4.38 The key water receptors and their importance/value are identified in Table 15.11.
Table 15.11 Key receptors and their importance/value
Receptor – Typical Explanation of potentially directly Importance descriptors importance assigned affected
Flood plain or defence River Thames (at protecting more than 100 Very High Bray) Attribute has a residential properties high quality and from flooding. rarity at the regional or EC Designated Salmonid national scale. and Cyprinid fishery Kennet & Avon Canal Very High (navigable section of canal)
Groundwater - Bedrock - Secondary High A aquifer Aquifer providing locally Groundwater - important resources or Bedrock - Secondary High supporting river A aquifer ecosystems
Groundwater Attribute has a High abstractions high quality and rarity at a local Kennet & Holy Brook High scale. Flood plain or defence protecting more than 100 residential properties Jubilee River High from flooding.
Roundmoor Ditch High Flood plain or defence protecting more than 100 residential properties River Loddon High from flooding.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Receptor – Typical Explanation of potentially directly Importance descriptors importance assigned affected
Datchet Common RQO River Ecosystem Medium Brook Class RE3*
RQO River Ecosystem River Crane Medium Class RE3*
River Colne and RQO River Ecosystem Grand Union Canal Class RE4* (including the Duke of Medium Northumberland’s River)
Attribute has a RQO River Ecosystem Colne Brook Medium medium quality Class RE4* and rarity at a Horton Brook (River local scale. RQO River Ecosystem Medium Colne) Class RE3*
Chalvey Ditch Medium RQO River Ecosystem Class RE3* Emm Brook Medium
Barkham Brook Medium
Kennet & Foudry Medium Brook
Clayhill Brook Medium
The Cut Low Flood plain with limited constraints and low Salthill Stream Low Attribute has a probability of flooding of low quality and residential and industrial Frogs Ditch Low rarity on a local properties. scale. Unnamed ditches and Low drains
*Note- included as an explanation in DMRB however the RQOs are not included under the WFD.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Design and mitigation Construction Water resources 15.4.39 To ensure the quality of the water environment does not deteriorate during construction, a Construction Environmental Management Plan ("CEMP") will document all construction phase mitigation measures. These will include a pollution control plan, standard best practices and relevant Pollution Prevention Guidelines ("PPGs"). Refer to chapter 4 Scheme Description and to the Outline CEMP (Appendix 4.2) for further information. 15.4.40 Pursuant to the CEMP, method statements and management plans will be prepared by the successful Contractor(s) detailing their approach to construction. These will include the control of site activities and the prevention of pollution by introducing measures: a) providing a suitable construction site drainage system including cut- off ditches or drains and Sustainable Drainage Systems (“SuDS”), or equivalent, with suitably sized treatment facilities such as settlement or detention basins; b) ensuring that flow attenuation ponds, pollution control ponds, swales and oil interceptors required for the permanent works will be completed before the start of earthwork operations. Obtaining the necessary consents for any soakaway or filtration systems or to enable discharge of surface water runoff from the construction site to watercourses or foul sewers or disposal offsite; c) putting in place appropriate measures such as use of bunds of non- erodible material or silt or sediment fences adjacent to watercourses; d) implementing a surface water or groundwater monitoring plan, particularly in relation to works that could affect aquifers or drilling works; and e) adopting measures to comply with relevant PPGs: temporary construction methods and CIRIA publications (including ‘Control of water pollution from construction sites. Guidance for consultants and contractors’ (C532), ‘Control of water pollution from linear construction projects. Technical guidance’ (C648), ‘Control of water
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pollution from linear construction projects. Site guide’ (C649) and ‘Site handbook for construction of SuDS’ (C698)). 15.4.41 The Contractor will comply with BS 6031 ‘Code of Practice for earthworks’ regarding the general control of site drainage including, for example, all washings, dewatering, abstractions and surface water runoff, unless otherwise agreed by the employer’s representative. 15.4.42 If any water abstraction is required as part of the construction process, the EA will be contacted and the appropriate licenses will be obtained. Any abstraction practices will be in accordance with the guidelines and requirements of these licences. 15.4.43 There are 62 water abstraction points located between junctions 12 and 3 within 500m of the Scheme. Impacts could occur on the local water resources and associated infrastructure from plant and access to the works. Method statements will be prepared for works in sensitive locations, such as adjacent to abstractions. These will include details of the particular environmental protection measures to be implemented. 15.4.44 Between junction 12 and 4 the Scheme is located within a Surface Water Safeguard Zone safeguarded for pesticides. Workers within the Safeguard Zone will take care if pesticides are used, as their use could cause pollution of the raw water which may result in the need for additional, costly treatment. Through the CEMP, the Contractor will be required to monitor the use of any pesticides and ensure site procedures are in place that promote best construction practice. 15.4.45 The Contractor will comply with The Control of Pollution (Oil Storage) (England) Regulations 2001 that apply in relation to storage of any oil- based materials including petrol, diesel, waste and vegetable and plant oil, but excluding uncut bitumen. Above ground fuel and oil storage tanks will also comply with PPG2 which sets out requirements including those relating to positioning, specification, capacity, secondary containment and ancillary equipment for storage tanks. Stationary plant used by the Contractor will be fitted with measures such as drip trays to retain any leakage of oil or fuel. The Contractor will empty trays at regular intervals to prevent overflow. 15.4.46 Particular attention will be given to areas where dewatering will occur. This could include: a) estimating surrounding area potentially impacted by dewatering (e.g. use anticipated pump rates, rate of recharge etc.);
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b) identifying all receptors susceptible to groundwater level changes (public and private); c) minimising dewatering as far as practicable; d) containing poor quality discharge water and treat prior to disposal; and e) undertaking measurements in boreholes (subject to permission). 15.4.47 Method statements for works in sensitive locations, such as over or adjacent to watercourses, will also be produced. These will include details of the environmental protection measures to be implemented. 15.4.48 The CEMP will include an emergency preparedness and response plan. This will provide a full list of protocols and communication channels with the EA in the event of a pollution incident. 15.4.49 Any contractor will be required to demonstrate that all site managers, supervisors, foremen and operatives, together with security staff, will be provided with the relevant environmental training and awareness of site procedures and best construction practice. 15.4.50 As with many major construction project, working areas are likely to affect existing, minor drainage channels and ditches. The mitigation for such effects is well understood (and described in PPG 5 and PPG6). It includes: a) ensuring appropriate bunding for construction areas and compounds, in particular compounds located within the floodplain (construction compounds 5 and 6) and construction areas associated with works to the overbridges located in the floodplain at Ascot Road, Datchet Road, Monkey Island Lane, Riding Court Road and Wood Lane. For any bunding, the base and bund walls should be impermeable to the material stored and able to contain at least 110% of the volume stored. Spill kits should be located near drains; b) the routes of any temporary traffic diversions should be planned and the routes will be covered by drainage Incident Plans; c) procedures for concreting, cement mixing and washing areas should be sited at least 10m away from any watercourse or surface water drain to minimise the risk of runoff entering a watercourse. Wash waters from any concrete and cement works should never be discharged in to the water environment;
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d) in any wash down areas ensuring that plant and wheel washing is carried out in a designated area of hard standing at least 10m from any watercourse or surface water drain; and e) the disposal of surface water runoff from excavations which may be contaminated with silt, consideration of the use of silt fences at the toe of slopes, made from geotextiles, to reduce silt transport. Minimising the amount of exposed ground and soil stockpiles from which the water drains and the period of time such water drains. Flood risk 15.4.51 Consultation with the EA and Lead Local Flood Authorities is required where any of the Scheme lies within 20m of a stated designated main river or ordinary watercourse listed below. Such watercourses are ordinarily subject to byelaw control and consent would also be required under the Water Resources Act 1991 (Ref 15-13) for works on, over or within the main river channel, including temporary works required for construction purposes and the construction of surface water outfalls. For the Scheme, main rivers and ordinary watercourses are listed below together with the contact to be consulted (in brackets): a) Kennet & Holy River- Main watercourse (Environment Agency); b) Kennet & Avon Canal- Main watercourse (Environment Agency); c) Kennet & Foudry Brook- Ordinary watercourse (West Berkshire County Council); d) Clayhill Brook- Main watercourse (Environment Agency); e) Barkham Brook- Main watercourse (Environment Agency); f) River Loddon- Main watercourse (Environment Agency); g) Emm Brook- Main watercourse (Environment Agency); h) The Cut- Main watercourse (Environment Agency); i) River Thames- Main watercourse (Environment Agency); j) Roundmoor Ditch- Main watercourse (Environment Agency); k) Jubilee River- Main watercourse (Environment Agency); l) Salthill Stream- Main watercourse (Environment Agency); m) Chalvey Ditch- Main watercourse (Environment Agency); n) Datchet Common Brook- Main watercourse (Environment Agency);
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o) Horton Brook (River Colne)- Ordinary watercourse (Slough County Council); p) Colne Brook- Main watercourse (Environment Agency); q) River Colne and Grand Union Canal- Main watercourse (Environment Agency); and r) River Crane- Main watercourse (Environment Agency) 15.4.52 An Emergency Flood Response Plan is required by the CEMP. This plan sets out: a) what to do before a flood (to include the locations at risk of flooding and the sources of flooding); b) what to do during a flood (to include information about local volunteers/flood wardens, important telephone numbers and arrangements between authorities); and c) what to do after a flood (review the list of companies and reputable contractors whose help could be required after a flood). 15.4.53 During construction the Contractor(s) will be required to monitor EA Flood Warnings, and take action to minimise the effect of flooding. Construction compounds 5 and 6 are located in the floodplain. Therefore, if flood warnings are given, action will include the removal of plant and materials from those compounds prior to flood events. If movement is not practical, protective measures will be undertaken to safeguard the construction compounds. So far as practicable, the storage of material within the floodplain will be prohibited. 15.4.54 During the construction phase, there could be an increased mobilisation of silt in the surface water runoff. However, with good site management practice, the construction works are anticipated to generate only minor changes from the existing situation. Additional siltation or any blocked drains or areas where ponding occurs are likely to be identified earlier and remedial action will be taken to prevent flood events occurring. Operation Water resources 15.4.55 The Drainage Strategy (Document Reference 7.5) is based on the principle of not polluting the water environment. The Highways Agency (the "Agency”) has a proprietary approach to drainage design as detailed in DMRB Volume 4, Section 2, Part 9 (HA 119/06) which has been used to
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
inform the Drainage Strategy for the Scheme. In accordance with HA 119/06, IAN 161/13 and the NN NPS, the Drainage Strategy has incorporated SuDS within the drainage design and it is proposed to use soakaways as part of the attenuation for the Scheme. 15.4.56 When considering road runoff, relevant pollutants and their limiting concentrations need to be identified. Discharges from roads must not lead to a deterioration in the classification status of the receiving surface water body as determined in the relevant RBMP. The final appraisal of Spillage Risk will be carried out during the detailed design phase of the Scheme. Where there is a significant modification of the drainage system a Highways Agency Water Risk Assessment Tool (“HAWRAT”) appraisal will be undertaken for all outfalls affected by the Scheme. This will confirm the environmental risk as a result of the Scheme which will then be dealt with through the design to ensure the water quality will not deteriorate compared to the existing situation. 15.4.57 The Scheme will include measures to augment or replace the existing drainage system in the M4. The new measures will be specified during the HAWRAT appraisal, the deployment of this tool will ensure mitigation measures are included to ensure water quality will not deteriorate compared to the existing situation. Mitigation measures likely to be included will be interceptors and sluices. 15.4.58 The Scheme will include measures that will augment existing pollution control measures. Drainage systems may be either active or passive in operation. Active systems (requiring operators) comprise penstocks, valves and notched weirs. Passive systems comprise swales, ponds, wetlands, ditches, basins, silt traps, filter drains, soakaways and oil separators. Flood risk 15.4.59 Carriageway widening will take place by using the existing hard shoulder and central reserve width to provide an extra running lane. The Scheme widening works do not change the route or levels of the existing carriageway in the Flood Zone. It is therefore considered that the risk of fluvial flooding to or from the Scheme will not increase more than the existing situation as a result of the widening works.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.60 There will be a relatively small increase in impermeable area (due to the ERAs), so there will be no change to the operation of the vast majority of the drainage system. The drainage plans available at this stage of the Scheme’s design (contained within Drainage Strategy (Document Reference 7.5)) indicate that the existing drainage system is fit for purpose and in a reasonable condition, and that there are no identifiable risks to the water environment (whether through pollution or flood risk). This will be further verified by an additional survey. The survey will allow the drainage and design assumptions to be verified. In addition, the survey will identify any defects in the existing drainage system and identify any remedial measures that need to be implemented by the highway maintainer before commencement of works ensuring the safe operation of the Scheme. This will enable repairs to be carried out so that the reasonable worst case assumptions set out at paragraph 15.4.35 above can properly be assumed. 15.4.61 Operational mitigation for the Scheme has been included in the design of the drainage arrangements during the preliminary design phase (see drawings appended to the Drainage Strategy (Document Reference 7.5)). The mitigation measures to be included in the final design and management of runoff will ensure that there will be no significant changes as a result of the existing drainage network and outfalls. The Drainage Strategy includes the measures listed below to ensure storm water discharges from the Scheme are restricted to existing rates through attenuation in oversized pipes within the drainage network. For the purposes of this assessment (subject to paragraph 15.4.60 above), it is assumed that the existing drainage system is sufficient for the existing M4: a) the risk of flooding from the carriageway will be mitigated by augmenting the existing drainage system and by implementing a maintenance plan; b) new drainage systems will be required in the central reserve and the verge. Drainage in the central reserve will be replaced or upgraded where appropriate; c) the existing kerb and gully system in the verge will be replaced with linear drains and combined kerb and drainage systems where appropriate;
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-39
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
d) at ERAs, attenuation of runoff from the additional carriageway areas will be in the form of oversized kerb units; pipes and/or manhole chambers with flow and spillages control devices prior to the outfalls. This will maintain the existing established discharge rates; e) the proposed drainage collection system will be designed for a 1 in 5 year storm event with a 20% allowance for climate change in accordance with the guidance in Section 6.2 of HD 33/06. Existing carrier pipes would be checked for 1 in 5 year storm event with 20% allowance for climate change against new impermeable areas only; f) where proposed impermeable areas increase by more than 10% relative to existing impermeable areas from localised carriageway widening, oversized pipes or manhole chambers are to be used to provide attenuated of flow rates that are to be restricted to existing flow rates at outfalls. Attenuation and flow restrictions to existing rates at outfalls would minimize the impact of flood risk following development of the Scheme; g) additional volumes of surface water runoff generated from new paved areas are to be facilitated by increased attenuation and use of overflow outfalls to systems such as soakaways; and h) discharge rates at outfalls will not exceed existing discharge rates. So far as practicable discharge rates will be restricted to less than 5 l/s by flow control devices. To reduce the risk of blockages within the system flows will not be restricted by small drainage system outfalls. 15.4.62 Currently the maintenance regime is to rectify the causes of flood events that impact on the M4. These maintenance requirements will continue and are assessed in order to ensure that the risk of flooding from the Scheme will remain low. When any failure of the drainage system occurs that results in flood events, this will be repaired or replaced as required, to maintain the integrity of the drainage system, thereby managing the risk of flooding over the lifetime of the Scheme. 15.4.63 Works are required within the floodplain (Flood Zone 2 and 3) at the following overbridge sites: a) junction 8/9 - Ascot Road (The Bourne), Monkey Island Lane (River Thames) and Marsh Lane (Jubilee River/Thames); b) junctions 7 to 6 - Wood Lane (Jubilee River);
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-40
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
c) junctions 6 to 5 - Riding Court Road (River Thames); and d) junctions 5 to 4b - Old Slade Lane (Colne Brook). 15.4.64 The proposed works to Ascot Road and Ascot Road overbridge include a new earthwork embankment within the 1% climate change AEP flood extent. Current designs indicate that 1400m² of works to the new embankment are located within the floodplain. Mitigation to compensate for any loss of floodplain as a result of these works can be provided by removal of part of the existing Ascot Road embankment to create floodplain compensation. The indicative flood storage area available by removal of the existing embankment is 3800m². Consequently, sufficient land is available adjacent to the proposed works to provide flood compensation. The precise arrangement of any required floodplain compensation will be confirmed following further assessment during the detailed design phase. The 1% CC AEP and the area of land available for flood storage are shown on the Ascot Road AEP Flood Extent Assessment Map in Annex C of the FRA (Document Reference 5.3). 15.4.65 The proposed works to Monkey Island Lane and Monkey Island Lane overbridge include works within the 1% climate change AEP flood extents. These works include a new earthwork embankment for Monkey Island Lane and the associated realignment of the flood relief channels running parallel to the M4. Current designs indicate that 2000m² of works to the new embankment are located within the floodplain. Mitigation to compensate for any loss of floodplain as a result of these works can be provided by removal of part of the existing Monkey Island Lane embankment to create floodplain compensation. The indicative flood storage area available by removal of the existing embankment is 3300m². Consequently, sufficient land is available adjacent to the proposed works to provide flood compensation. The precise arrangement of any required floodplain compensation will be confirmed following further assessment during the detailed design phase. The 1% CC AEP and the area of land available for flood storage are shown on the Monkey Island Lane AEP Flood Extent Assessment Map in Annex C of the FRA (Document Reference 5.3). 15.4.66 The proposed works at Marsh Lane will not result in a significant change between the existing and proposed road alignment extent. Therefore, Marsh Lane is unlikely to have a significant impact on the 1% CC AEP flood extent and mitigation measures will not be required.
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-41
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.67 The proposed works to Wood Lane and Wood Lane overbridge include a new earthwork embankment within the 1% climate change AEP flood extents. Current designs indicate that 2100m² of works to the new embankment are located within the floodplain. Mitigation to compensate for any loss of floodplain as a result of these works can be provided by removal of part of the existing Wood Lane embankment to create floodplain compensation. The indicative flood storage area available by removal of the existing embankment is 4000m². Consequently, sufficient land is available adjacent to the proposed works to provide flood compensation. The precise arrangement of any required floodplain compensation will be confirmed following further assessment during the detailed design phase. The 1% CC AEP and the area of land available for flood storage are shown on the Wood Lane AEP Flood Extent Assessment Map in Annex C of the FRA (Document Reference 5.3). 15.4.68 The proposed works to Riding Court Road and Riding Court Road overbridge include a new earthwork embankment within the 1% climate change AEP flood extent. Current designs indicate that 800m² of works to the new embankment are located within the floodplain. Mitigation to compensate for any loss of floodplain as a result of these works can be provided by removal of part of the existing Riding Court Road embankment to create floodplain compensation. The indicative flood storage area available by removal of the existing embankment is 1200m². Consequently, sufficient land is available adjacent to the proposed works to provide flood compensation. The precise arrangement of any required floodplain compensation will be confirmed following further assessment during the detailed design phase. The 1% CC AEP and the area of land available for flood storage are shown on the Riding Court Road AEP Flood Extent Assessment Map in Annex C of the FRA (Document Reference 5.3). 15.4.69 The proposed works to the overbridge at Old Slade Lane will not result in a significant change between existing and proposed road alignment extents and mitigation measures will not be required. 15.4.70 Works are required adjacent to and within the channel of watercourses at the following sites: a) junction 8/9 - Thames at Bray (River Thames); b) junctions 7 to 6 - Chalvey culvert (Chalvey Ditch); and c) junctions 6 to 5 - Ashleys Arch culvert (Datchet Common Brook).
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.71 Works are proposed to the Thames Bray underbridge over the River Thames. Widening of the Thames Bray underbridge would result in a total river bed displacement of 50m². The impact of this displacement is considered to be negligible. However, to minimise the impact on any hydraulic conveyance capacity of the river, if required, mitigation measures such as floodplain compensation may be provided. The need for such floodplain compensation will be confirmed during the detailed design phase. Any displacement effect can be accommodated within the flood storage areas adjacent to the new Monkey Island Lane embankment. Works are proposed to extend the Ashley’s Arch culvert on Datchet Common Brook and Chalvey culvert on the Chalvey Ditch. As the works involve extending existing culverts, the impact on the surrounding floodplain will be negligible and mitigation measures will not be required. 15.4.72 The design will include the mitigation measures listed below to ensure flood risk to the Scheme or third parties is not increased as a result of altering the course of existing ditches or creating new channels/ditches. The details of the measures in question will be specified as part of the detailed design of the Scheme. These include: a) realignment of ditches, which will only be considered if other options are not possible; b) existing ditches that are affected by the Scheme widening are to be re-aligned or hydraulically connected to upstream and downstream ditches using pipe culverts where required; c) careful consideration of the design of the new river channel will be undertaken to ensure that it is geomorphically stable (i.e. the design does not result in increased erosion or deposition); and d) any channel realignments would be designed to account for the following: i. reduction/increase in the channel length due to alteration of channel planform, potentially impacting channel gradient and consequentially flow and sediment dynamics; and ii. incorporation of geomorphological and ecological features. 15.4.73 As a result of the ERA between junctions 4 and 3, works will be required to Frogs Ditch. The existing ditch can be made hydraulically contiguous with upstream and downstream ditches by installing pipe culverts where
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
required. The design will ensure that the ditch is geomorphically stable and maintains the current hydraulic capacity. Enhancement measures 15.4.74 The current drainage network within and serving the M4 is designed for a 1 in 2 year storm event and has not been designed with an allowance for climate change. The risk of flooding from the Scheme itself will be alleviated by the provision for road drainage as part of the Scheme described at paragraph 15.4.61 above. For the existing M4, improving the existing drainage system and by implementing a routine maintenance plan will have benefits. The proposed drainage within the ERAs will be designed for a 1 in 5 year storm event, with a 20% allowance for climate change, in accordance with the guidance in Section 6.2 of HD 33/06, Volume 4, Section 2 (Ref 15-3). This enhances flood attenuation. Assessment of residual effects Construction 15.4.75 Any effects are expected to be short-term during the construction of the Scheme. This is due to the scale of the construction works associated with smart motorways and all lane running. It is considered that the effects during construction will be short-term and there will be no major effect in the long-term. Water resources 15.4.76 Water resource effects are considered in terms of effects on water quality, and their consequent habitat effect. The potential effects, without mitigation, on water quality during the construction phase will arise from normal construction activity and include surface water runoff and pollutant contamination. 15.4.77 Construction works will occur near to and within watercourses, abstraction point and surface water features and will also involve works to the drainage network. These have the potential to provide a pathway for pollutants to reach watercourses and surface water features. Sources of pollutants could include storage and management of fuels and oils, use of cement-based products and the potential release of sediment. Additional hazards arising from construction activities include accidental release of floatable material, such as plastic and plastic film, and loss of material during storm events from surface water runoff. In addition, there is also a higher risk of entraining fine sediment in runoff, which could increase siltation in the receiving watercourse. Once appropriate mitigation
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-44
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
measures (as listed in paragraph 15.4.40 above) are in place, it is considered that there would be a negligible adverse magnitude of impact to the characteristics of the receptors listed in Table 15.11. 15.4.78 Another potential effect is the mobilisation of contamination and migration into controlled waters. Sections of the Scheme are located within a Secondary A Aquifer, which has been classified as having high importance. Once appropriate mitigation measures (as listed in paragraphs 15.4.39 and 15.4.50 above) are in place and with good site management practices it is considered that this will result in a negligible adverse magnitude of impact. Flood risk 15.4.79 The risk of flooding occurring during construction is most likely to arise from heavy rainfall. During high intensity rainstorms, runoff may be unable to drain away quickly and may pond on the surface of the carriageway, potentially resulting in flooding of working areas, excavations and the carriageway. During the initial earthworks, topsoil and subsoil will be exposed. Water-logging and ponding may occur more frequently. Additionally, there is a higher risk of entrained sediment in runoff, leading to blockages of structures such as drains and gullies. With good site management practices, as listed in the Outline CEMP (Appendix 4.2), any blocked drains or areas where ponding occurs are likely to be identified early and remedial action will be taken to prevent flood events occurring. It is considered that this will result in a negligible adverse magnitude of impact. Operation (opening year, 2022) 15.4.80 Any effects during operation are expected to be longer term with the potential for effects to occur throughout the operational phase. Water resources 15.4.81 During the operational phase, a potential effect is the mobilisation of contamination into controlled waters from vehicles using the Scheme. Road drainage could be contaminated by spills and leaks of oil and fuel, and by other materials deposited onto the drained surfaces, and contaminated runoff could be released into the surface water environment via this route.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.82 During the operational phase, there is a pollution risk associated with storm water runoff. Road runoff can contain hydrocarbons, heavy metals, chemicals and de-icing agents. These pollutants when combined with rainfall can runoff into the drainage system and have an adverse effect on the receiving watercourses. The pollutants may also find their way into groundwater through filter drains and overland flows during heavy rainfall events. It is considered that this will result in a negligible magnitude of impact. 15.4.83 During operation, there is a risk that polluting materials may be spilt onto the road surface as a result of a road accident. These pollutants have the potential to enter surface and ground waters. As a result of the Scheme, congestion and the number of accidents are anticipated to be reduced, which in turn would reduce the risk of pollution incidents as a result of road accidents. On the assumption that the number of accidents decreases during operation, it is considered that the magnitude of impact to accidental spillage risk compared to the existing situation will be negligible . Flood risk 15.4.84 The drainage design has taken the increased impermeable surface area, which at 12.8ha represents an increase of approximately 2.34%, into consideration. Attenuation will be provided in the form of underground tanks and over-sized pipes to maintain the existing rate of road drainage to outfalls. Therefore, on opening of the Scheme there will be no additional mitigation measures required and there will be no change in the flood risk resulting from surface water. The risk of flooding caused by the runoff from the Scheme will be alleviated by improving the existing drainage within the ERAs (increasing the standard of protection to a 1 in 5 year storm event, with a 20% allowance for climate change). Therefore, it is considered that this will result in a negligible magnitude of impact. 15.4.85 Works are required within the floodplain to extend the following overbridge sites: a) junction 8/9 - Ascot Road (The Bourne) and Monkey Island Lane (River Thames); b) junctions 7 to 6 - Wood Lane (Jubilee River); and c) junctions 6 to 5 - Riding Court Road (River Thames).
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.4.86 Floodplain compensation will be provided at the overbridge sites listed above to compensate for any works within the floodplain to ensure that flood risk is not increased to third parties. This will result in a negligible magnitude of impact upon flood risk to third parties. 15.4.87 In channel works are required at the Thames Bray underbridge and to the flood relief channels in the vicinity of the Monkey Island Lane overbridge. This will involve works within the channel and may impact upon the physical nature of the channels and the biological quality of the watercourse. It is considered that with suitable mitigation during construction and operation, including flood storage areas adjacent to the new Monkey Island Lane embankment and the reinstatement or rebuilding of affected flood relief channels, this will result in a negligible magnitude of impact upon flood risk to third parties. 15.4.88 As a result of the ERA between junctions 4 and 3, works will be required to Frogs Ditch. The existing ditch can be made hydraulically contiguous with upstream and downstream ditches by installing pipe culverts where required. With mitigation included in the design to ensure hydraulic capacity is maintained, this will result in a negligible magnitude of impact upon flood risk to third parties.
15.5 Junction 12 to junction 11
15.5.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 12 to 11 are summarised in Table 15.12.
Table 15.12 Summary of residual effects between junctions 12 and 11
Mitigation Significance Potential Receptors Magnitude Importance of residual likely effects affected of impact effect
Construction
Water Kennet & To ensure the Negligible Very high Neutral Quality Avon Canal quality of the adverse Construction water works will Groundwater environment occur near Bedrock does not (Secondary A Negligible and within High deteriorate Neutral watercourses, aquifer), during adverse surface water Kennet & construction, a features Holy Brook CEMP will detail
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Mitigation Significance Potential Receptors Magnitude Importance of residual likely effects affected of impact effect
which could Abstractors all construction provide a phase mitigation pathway for Kennet & measures. pollutants to Foudry Negligible Medium Neutral reach Brook, adverse watercourses Clayhill Brook and surface water Unnamed Negligible features. ditches and Low Neutral adverse drains
Flood Risk Floodplain of An Emergency Negligible the Kennet & High Flood Response Neutral The increase adverse in the Holy Brook Plan will reduce impermeable the impact of Floodplain of area and flooding. the Kennet & therefore Good site Negligible Foudry Medium Neutral increased management adverse Brook, surface water practices will Clayhill Brook runoff could reduce the pose flood likelihood of risk to the siltation and Scheme and blockages. to third Floodplain of The storage of parties the unnamed materials within Negligible Low Neutral located within ditches and construction adverse
the floodplain. drains. compounds during flood warnings will be avoided.
Operation (opening year, 2022)
Water Kennet & As a result of Moderate Very High Negligible Quality Avon Canal the Scheme, beneficial Surface water congestion and falling on the Groundwater the number of Scheme Bedrock accidents are (Secondary A could be High anticipated to be Negligible Neutral contaminated aquifer) and reduced, which by spills Kennet & in turn would deposited on Holy Brook reduce the risk the drained of pollution surfaces and Kennet & Medium incidents as a Negligible Neutral
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-48
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Mitigation Significance Potential Receptors Magnitude Importance of residual likely effects affected of impact effect contaminated Foudry result of road runoff could Brook, accidents. be released Clayhill Brook into the surface water Unnamed environment ditches and Low Negligible Neutral via this route. drains.
Flood Risk Floodplain of There are two The increase the Kennet & High recorded Negligible Neutral in the Holy Brook incidents of impermeable surface water Floodplain of area and flooding due to the Kennet & therefore blockages in the Foudry Medium Negligible Neutral increased drainage Brook, surface water system. Clayhill Brook runoff could Mitigation pose flood includes regular risk to the maintenance of Scheme. The the drainage receptor systems to includes the reduce the river and the likelihood of surrounding blockages. area (i.e. The drainage floodplain). design will provide attenuation to Floodplain of maintain the the unnamed Low existing rate of Negligible Neutral ditches and road drainage to drains. outfalls. The standard of protection will be improved for the drainage system within the ERAs. Therefore, there will be no change in the flood risk.
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-49
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.6 Junction 11 to junction 10
15.6.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 11 to 10 are summarised in Table 15.13.
Table 15.13 Summary of residual effects between junctions 11 and 10
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Construction
Water Groundwater To ensure Quality Bedrock the quality of Negligible Construction (Secondary A the water Neutral adverse works will and B aquifer), High environment occur near River Lodden does not and within deteriorate watercourses, Abstractors during surface water construction, Barkham Brook Negligible features Medium a CEMP will Neutral which could and Emm Brook document all adverse provide a construction pathway for phase pollutants to mitigation reach Unnamed ditches measures Negligible Low Neutral watercourses and drains adverse and surface water features
Flood Risk Floodplain of the An Negligible High Neutral The increase River Lodden Emergency adverse in the Flood Floodplain of the impermeable Response Negligible Barkham Brook, Medium Neutral area and Plan will adverse Emm Brook therefore reduce the increased impact of surface water flooding. runoff could Good site pose flood Floodplain of the management Negligible risk to the unnamed ditches Low practices will Neutral adverse Scheme and and drains reduce the to third likelihood of parties siltation and located within blockages.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect the floodplain The storage of materials within construction compounds during flood warnings will be avoided.
Operation (opening year, 2022)
Water Groundwater As a result of Quality Bedrock the Scheme, Surface water (Secondary A High congestion Negligible Neutral falling on the and B aquifer) and the Scheme and River Lodden number of could be accidents Barkham Brook contaminated Medium are Negligible Neutral by spills and Emm Brook anticipated deposited on to be the drained reduced, surfaces and which in turn contaminated would runoff could Unnamed reduce the be released ditches and Low risk of Negligible Neutral into the drains pollution surface water incidents as environment a result of via this route road accidents.
Flood Risk Floodplain of the The High Negligible Neutral The increase River Lodden drainage in the design will Floodplain of the impermeable provide Barkham Brook Medium Negligible Neutral area and attenuation and Emm Brook therefore to maintain increased the existing surface water rate of road runoff could Floodplain of the drainage to pose flood unnamed ditches Low outfalls. Negligible Neutral risk to the and drains The Scheme. The standard of receptor protection
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect includes the will be river and the improved for surrounding the drainage area (i.e. system floodplain). within the ERAs. Therefore, there will be no change in the flood risk.
15.7 Junction 10 to junction 8/9
15.7.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 10 to 8/9 are summarised in Table 15.14.
Table 15.14 Summary of residual effects between junctions 10 and 8/9
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Construction
Water Groundwater To ensure the Quality Bedrock quality of the Construction (Secondary A water Negligible High Neutral works will aquifer) environment adverse occur near does not and within Abstractors deteriorate watercourses, during surface water construction, a features CEMP will which could document all provide a The Cut and construction pathway for unnamed phase mitigation Negligible Low Neutral pollutants to ditches and measures adverse reach drains watercourses and surface water features
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-52
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Flood Risk An Emergency The increase Flood Response in the Plan will reduce impermeable the impact of area and flooding. therefore Good site increased management surface water practices will runoff could reduce the pose flood likelihood of risk to the siltation and Scheme and Floodplain of blockages. to third the Cut and The storage of Negligible parties unnamed Low Neutral materials within adverse located within ditches and construction the floodplain. drains compounds Construction during flood compound 5 warnings will be is located avoided. within an EA Flood Warning Area. from the compounds prior to flood events
Operation (opening year, 2022)
Water Groundwater As a result of Quality Bedrock the Scheme, High Negligible Neutral Surface water (Secondary A congestion and falling on the aquifer) the number of Scheme accidents are could be anticipated to be contaminated reduced, which The Cut and by spills in turn would unnamed reduce the risk deposited on Low Negligible Neutral ditches and the drained of pollution drains surfaces and incidents as a contaminated result of road runoff could accidents.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect be released into the surface water environment via this route
Flood Risk There are four The increase recorded in the incidents of impermeable surface water area and flooding due to therefore blockages in the increased drainage surface water system. runoff could Mitigation pose flood includes regular risk to the maintenance of Scheme. The the drainage receptor systems to includes the reduce the river and the likelihood of Floodplain of surrounding blockages. the Cut and area (i.e. The drainage unnamed Low Negligible Neutral floodplain). design will ditches and provide drains attenuation to maintain the existing rate of road drainage to outfalls. The standard of protection will be improved for the drainage system within the ERAs. Therefore, there will be no change in the flood risk.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
15.8 Junction 8/9 to junction 7
15.8.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 8/9 to 7 are summarised in Table 15.15.
Table 15.15 Summary of residual effects between junctions 8/9 and 7
Potential Receptors Magnitude Significance of Importance Mitigation likely effects affected of impact residual effect
Construction
Water River To ensure the Negligible Quality Thames at Very high quality of the Neutral adverse Construction Bray water works will environment occur near Jubilee does not River, The and within deteriorate Negligible Bourne and High Neutral watercourses, during adverse surface water Roundmoor construction, a features Ditch CEMP will which could document all provide a construction pathway for phase pollutants to The Cut and mitigation unnamed Negligible reach Low measures Neutral watercourses ditches and adverse and surface drains water features
Flood Risk Floodplain of An Emergency the River Flood The increase Very high Negligible Neutral in the Thames at Response impermeable Bray Plan will area and reduce the Floodplain of therefore impact of the Jubilee increased flooding. River, The surface water High Good site Negligible Neutral Bourne and runoff could management Roundmoor pose flood practices will Ditch risk to the reduce the Scheme and likelihood of to third Floodplain of siltation and parties the Cut and Low blockages. Negligible Neutral located within unnamed The storage of the floodplain ditches and materials
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Potential Receptors Magnitude Significance of Importance Mitigation likely effects affected of impact residual effect drains within construction compounds during flood warnings will be avoided.
Flood Risk Mitigation Displacement measures (i.e. of flood water temporary within the flood storage) floodplain and will be a localised included to increase in ensure that the flood levels. proposed works do not Works on the increase flood banks of the risk to third River Thames parties. If the and Floodplain of course of the associated the River channel is flood relief Thames at Very High altered the Negligible Neutral channels at Bray existing ditch Monkey will be Island Lane hydraulically overbridge connected to upstream and downstream ditches using pipe culverts where required. The design will to ensure it is geomorphically stable.
Operation (opening year, 2022)
Water River As a result of Moderate Quality Thames at Very High the Scheme, Negligible beneficial Surface water Bray congestion falling on the and the Scheme Jubilee number of could be River, The High accidents are Negligible Neutral Bourne and
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Potential Receptors Magnitude Significance of Importance Mitigation likely effects affected of impact residual effect contaminated Roundmoor anticipated to by spills Ditch be reduced, deposited on which in turn the drained would reduce surfaces and the risk of contaminated pollution The Cut and runoff could incidents as a unnamed be released Low result of road Negligible Neutral ditches and into the accidents. drains surface water environment via this route.
Flood Risk Floodplain of There are 22 the River recorded The increase Very High Negligible Neutral in the Thames at incidents of impermeable Bray surface water area and flooding due to Floodplain of therefore blockages in the Jubilee increased the drainage River, The surface water High system. Negligible Neutral Bourne and runoff could Mitigation Roundmoor pose flood includes Ditch risk to the regular Scheme. The maintenance receptor of the drainage includes the system to river and the reduce the surrounding likelihood of area (i.e. blockages. floodplain). The drainage design Floodplain of provides the Cut and attenuation to unnamed Low maintain the Negligible Neutral ditches and existing rate of drains road drainage to outfalls. The standard of protection will be improved for the drainage system within the ERAs.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Potential Receptors Magnitude Significance of Importance Mitigation likely effects affected of impact residual effect Therefore, there will be no change in the flood risk.
Mitigation measures (i.e. floodplain compensation) will be Flood Risk included to ensure that the Displacement proposed of flood water works do not within the increase flood floodplain and risk to third a localised parties. increase in flood levels. Widening of the Thames Works on the Bray banks of the underbridge River Thames Floodplain of results in a and the River Very High total river bed Negligible Neutral associated Thames at displacement flood relief Bray of 50m². The channels at impact of this Monkey displacement Island Lane is considered overbridge to be negligible. However, this is to be Works are managed required to though the Ascot mitigation Road and measures Wood Lane such as overbridges floodplain within the compensation, floodplain if required.
Floodplain of Displacement the Jubilee of flood water High Negligible Neutral River and within the The Bourne floodplain will
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-58
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Potential Receptors Magnitude Significance of Importance Mitigation likely effects affected of impact residual effect require floodplain compensation to be provided to minimise effects to third parties located within the floodplain.
15.9 Junction 7 to junction 6
15.9.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 7 to 6 are summarised in Table 15.16.
Table 15.16 Summary of residual effects between junctions 7 and 6
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Construction
Water Groundwater To ensure Quality Bedrock the quality of Construction (Secondary A the water Negligible High Neutral works will aquifer) environment adverse occur near does not and within Abstractors deteriorate watercourses, during surface water construction, Negligible Chalvey Ditch Medium Neutral features a CEMP will adverse which could document all provide a construction pathway for phase pollutants to mitigation reach Unnamed ditches measures Negligible Low Neutral watercourses and drains adverse and surface water features
Flood Risk Floodplain of the An Medium Negligible Neutral Emergency The increase Chalvey Ditch
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-59
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect in the Flood impermeable Response area and Plan will therefore reduce the increased impact of surface water flooding. runoff could Good site pose flood management risk to the practices will Scheme and reduce the to third Floodplain of the likelihood of parties unnamed ditches Low Negligible Neutral siltation and located within and drains blockages. the floodplain The storage of materials within construction compounds during flood warnings will be avoided.
Operation (opening year, 2022)
Water Groundwater As a result of Quality Bedrock the Scheme, High Negligible Neutral Surface water (Secondary A congestion falling on the aquifer) and the Scheme number of Chalvey Ditch Medium Negligible Neutral could be accidents are contaminated anticipated to by spills be reduced, deposited on which in turn the drained would reduce surfaces and the risk of pollution contaminated Unnamed ditches Low incidents as Negligible Neutral runoff could and drains be released a result of into the road surface water accidents. environment via this route
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-60
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Flood Risk Floodplain of the The drainage Medium Negligible Neutral The increase Chalvey Ditch design will in the provide impermeable attenuation to area and maintain the therefore existing rate increased of road surface water drainage to runoff could outfalls. pose flood The standard risk to the Floodplain of the of protection Scheme. The unnamed ditches Low will be Negligible Neutral receptor and drains improved for includes the the drainage river and the system within surrounding the ERAs. area (i.e. Therefore, floodplain). there will be no change in the flood risk.
15.10 Junction 6 to junction 5
15.10.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 6 to 5 are summarised in Table 15.17.
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-61
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Table 15.17 Summary of residual effects between junctions 6 and 5
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Construction
Water Quality Groundwater To ensure the Construction Bedrock quality of the works will (Secondary A water Negligible High Neutral occur near and aquifer) environment adverse within does not watercourses, Abstractors deteriorate surface water during Datchet Negligible features which Medium construction, Neutral could provide a Common Brook a CEMP will adverse pathway for document all pollutants to Salthill Stream construction reach and unnamed phase Negligible Low Neutral watercourses ditches and mitigation adverse and surface drains measures water features
Flood Risk Floodplain of An The increase in the Datchet Medium Emergency Negligible Neutral the Common Brook Flood impermeable Response area and Plan will therefore reduce the increased impact of surface water flooding. runoff could Good site pose flood risk management to the Scheme Floodplain of practices will and to third the Salthill reduce the parties (i.e. Stream and likelihood of Low Negligible Neutral floodplain). unnamed siltation and ditches and blockages. drains The storage of materials within construction compounds 7 and 8 (if used) during flood warnings will
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-62
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect be avoided.
Operation (opening year, 2022)
Water Quality Groundwater As a result of Bedrock the Scheme, Surface water High Negligible Neutral falling on the (Secondary A congestion Scheme could aquifer) and the be number of Datchet contaminated Medium accidents are Negligible Neutral Common Brook by spills anticipated to deposited on be reduced, the drained which in turn surfaces and would reduce contaminated the risk of Salthill Stream pollution runoff could be and unnamed released into Low incidents as a Negligible Neutral ditches and result of road the surface drains water accidents. environment via this route
Flood Risk Floodplain of There are 12 The increase in the Datchet Medium recorded Negligible Neutral the Common Brook incidents of impermeable surface water area and flooding due therefore to blockages increased in the surface water drainage runoff could system. pose flood risk Mitigation Floodplain of to the Scheme. includes the Salthill The receptor regular Stream and maintenance includes the Low Negligible Neutral unnamed river and the of the ditches and surrounding drainage drains area (i.e. systems to floodplain). reduce the likelihood of blockages. The drainage design will provide
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-63
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect attenuation to maintain the existing rate of road drainage to outfalls. The standard of protection will be improved for the drainage system within the ERAs. Therefore, there will be no change in the flood risk.
15.11 Junction 5 to junction 4b
15.11.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 5 to 4b are summarised in Table 15.18.
Table 15.18 Summary of residual effects between junctions 5 and 4b
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Construction
Water Groundwater To ensure the Negligible Quality Bedrock quality of the adverse Construction (Principal aquifer) High water Neutral works will environment occur near Abstractors does not and within deteriorate watercourses, Colne Brook and during Negligible surface water Horton Brook Medium construction, adverse Neutral features (River Colne) a CEMP will which could document all Unnamed ditches Negligible provide a Low construction Neutral pathway for and drains phase adverse
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-64
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect pollutants to mitigation reach measures watercourses and surface water features
Flood Risk Floodplain of the An Negligible The increase Colne Brook and Emergency adverse Medium Neutral in the Horton Brook Flood impermeable (River Colne) Response area and Plan will therefore reduce the increased impact of surface water flooding. runoff could Good site pose flood management risk to the practices will Scheme and reduce the to third likelihood of parties (i.e. Floodplain of the siltation and Negligible floodplain). unnamed ditches Low Neutral blockages. adverse and drains The storage of materials within construction compounds during flood warnings will be avoided.
Operation (opening year, 2022)
Water Groundwater As a result of Negligible Quality Bedrock High the Scheme, Neutral Surface water (Principal aquifer) congestion falling on the and the Scheme Colne Brook and number of Negligible could be Horton Brook Medium accidents are Neutral contaminated (River Colne) anticipated to by spills be reduced, deposited on Unnamed ditches which in turn Negligible Low Neutral the drained and drains would reduce surfaces and the risk of
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-65
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect contaminated pollution runoff could incidents as a be released result of road into the accidents. surface water environment via this route
Flood Risk Floodplain of the The drainage Negligible The increase Colne Brook and design will Medium Neutral in the Horton Brook provide impermeable (River Colne) attenuation to area and maintain the therefore existing rate increased of road surface water drainage to runoff could outfalls. pose flood The standard risk to the of protection Scheme. The Floodplain of the will be receptor unnamed ditches Low improved for Negligible Neutral includes the and drains the drainage river and the system within surrounding the ERAs. area (i.e. Therefore, floodplain). there will be no change in the flood risk.
15.12 Junction 4b to junction 4
15.12.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 4b to 4 are summarised in Table 15.19.
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-66
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Table 15.19 Summary of residual effects between junctions 4b and 4
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact Effect
Construction
Water Quality Groundwater - Construction Bedrock – Negligible works will Principal aquifer High To ensure the Neutral adverse occur near and quality of the water within Abstractors environment does watercourses, not deteriorate surface water River Colne and during features which Grand Union construction, a Canal (including Negligible could provide Medium CEMP will Neutral a pathway for the Duke of document all adverse pollutants to Northumberland’s construction phase reach River) mitigation watercourses measures Unnamed ditches Negligible and surface Low Neutral water features and drains adverse
Flood Risk Floodplain of the An Emergency The increase River Colne and Flood Response Grand Union Plan will reduce in the Negligible Canal (including Medium the impact of Neutral impermeable adverse area and the Duke of flooding. therefore Northumberland’s Good site increased River) management surface water practices will runoff could reduce the pose flood risk likelihood of to the Scheme siltation and and to third blockages. Floodplain of the parties (i.e. Negligible unnamed ditches Low The storage of Neutral floodplain). adverse and drains materials within construction compounds during flood warnings will be avoided.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact Effect
Operation (opening year, 2022)
Water Quality Groundwater - Surface water Bedrock – High Negligible Neutral falling on the Principal aquifer As a result of the Scheme could Scheme, be River Colne and congestion and the contaminated Grand Union number of by spills Canal (including Medium accidents are Negligible Neutral deposited on the Duke of anticipated to be the drained Northumberland’s reduced, which in surfaces and River) turn would reduce contaminated the risk of pollution runoff could be incidents as a released into result of road the surface Unnamed ditches Low accidents. Negligible Neutral water and drains environment via this route
Flood Risk Floodplain of the The drainage River Colne and design will provide The increase Medium Negligible Neutral in the Grand Union attenuation to impermeable Canal maintain the area and existing rate of therefore road drainage to increased outfalls. surface water The standard of runoff could protection will be pose flood risk Floodplain of the improved for the to the Scheme. unnamed ditches Low drainage system Negligible Neutral The receptor and drains within the ERAs. includes the Therefore, there river and the will be no change surrounding in the flood risk. area (i.e. floodplain).
15.13 Junction 4 to junction 3
15.13.1 Receptors potentially affected, mitigation, residual magnitude of impact and residual effects during construction and operation between junctions 4 to 3 are summarised in Table 15.20.
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-68
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Table 15.20 Summary of residual effects between junctions 4 and 3
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Construction
Water Quality Groundwater To ensure the Construction - Bedrock - quality of the works will Principal water Negligible High Neutral occur near and aquifer environment adverse within does not watercourses, Abstractors deteriorate surface water during Negligible features which River Crane Medium construction, a Neutral could provide a CEMP will adverse pathway for document all Negligible pollutants to Frogs Ditch Low construction Neutral reach phase mitigation adverse watercourses measures Unnamed and surface Negligible ditches and Low Neutral water features adverse drains
Flood Risk Floodplain of An Emergency Negligible the River Medium Flood Response Neutral The increase in adverse the Crane Plan will reduce impermeable the impact of area and flooding. therefore Good site increased management surface water practices will runoff could reduce the pose flood risk likelihood of Floodplain of to the Scheme siltation and the unnamed Negligible and to third Low blockages. Neutral ditches and adverse parties (i.e. drains The storage of floodplain). materials within construction compounds during flood warnings will be avoided.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect
Operation (opening year, 2022)
Water Quality Groundwater Surface water - Bedrock - High Negligible Neutral falling on the Principal As a result of Scheme could aquifer the Scheme, be congestion and contaminated River Crane Medium the number of Negligible Neutral by spills accidents are deposited on anticipated to be the drained reduced, which surfaces and in turn would contaminated Unnamed reduce the risk runoff could be ditches and Low of pollution Negligible Neutral released into drains incidents as a the surface result of road water accidents. environment via this route
Flood Risk Floodplain of The drainage The increase in the River Medium design will Negligible Neutral the Crane provide impermeable attenuation to area and maintain the therefore existing rate of increased road drainage to surface water outfalls. runoff could The standard of pose flood risk Floodplain of protection will the unnamed to the Scheme. Low be improved for Negligible Neutral The receptor ditches and the drainage includes the drains system within river and the the ERAs. surrounding Therefore, there area (i.e. will be no floodplain). change in the flood risk.
Flood Risk The existing Flood risk to ditch will be the Scheme or Frogs Ditch Low hydraulically Negligible Neutral third parties contiguous to could be upstream and
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-70
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Significance Potential Receptors Magnitude Importance Mitigation of residual likely effects affected of impact effect increased as a downstream result of ditches using altering the pipe culverts course of where required. existing The design will ditches. be confirmed at detailed design and will ensure it is geomorphically stable and maintains the current hydraulic capacity.
15.14 Cumulative effects
15.14.1 The following schemes are located within 1km of the Scheme. The type of developments proposed are of a scale and nature that the developments could impact on the Scheme (discussed further below): a) land at Church Lane (residential development 276 units); b) land at The Manor (residential development 126 units); c) land at the Berkshire Retail Park Pincents Lane (IKEA development covering 4ha); d) Bearwood Park (sports facilities and 26 residential units); e) London Air Traffic Control Centre (mixed use development); and f) the two highway schemes Cutbush Lane Arborfield Road and Shinfield Road Shinfield Reading. 15.14.2 The cumulative effects of the developments listed above could include a potential increase in flood risk associated with the increase in the impermeable area of the developments within the wider Thames catchment. However, in line with EA policies, new developments are required to restrict runoff to greenfield rates. This means that surface runoff on committed developments cannot exceed the runoff from the undeveloped sites, in order that the developments do not exacerbate flooding. In line with current best practice, mitigation measures will be
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
required to manage runoff and as appropriate mitigation measures will be required to be implemented on these developments, and as such the cumulative effects are assessed as being not significant. 15.14.3 There is the potential for the Cutbush Lane, Arborfield Road and Shinfield Road developments to displace water in the floodplain through the regrading of embankments, landscaping and provision of utilities. However, appropriate mitigation measures will be included in the design, in accordance with the NPPF, to mitigate this risk.
15.15 Limitations encountered in compiling the ES
15.15.1 To offset works in the floodplain it has been confirmed that floodplain compensation will be required at the overbridge works at Ascot Road, Monkey Island Lane and Wood Lane. It has been confirmed that there are indicative areas of storage land adjacent to the proposed works to provide floodplain compensation; however the exact volume of compensatory storage has not been confirmed at this stage. This exact mitigation measures will be confirmed before the detailed design phase. 15.15.2 The assessment does not benefit from a detailed survey of the existing M4 highway drainage network, although some flooding events are known to have occurred. Therefore, the assessment has assumed that the existing system is sufficient for the purposes of draining the current M4 and can be repaired and/or maintained so as to do so. This produces a reasonable worst case for assessment purposes.
15.16 Summary
15.16.1 The residual effects that could arise from construction and operation of the Scheme are set out in Table 15.21.
ENVIRONMENTAL STATEMENT CHAPTER 15 ROAD DRAI NAGE AND THE WATER ENVIRONMENT MARCH 2015 PAGE 15-72
HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Table 15.21 Road drainage and the water environment impact summary table
Duration of Significance of Impact description Receptor(s) affected Mitigation Impact residual effect
During construction, the use of River/surface Slight adverse potentially polluting material on site watercourses such as fuels, oils, and concrete could pose a risk to the existing The Contractor will water quality. prepare and implement a CEMP setting out The Scheme is located on Principal Groundwater Slight adverse measures to protect the water supply aquifers (SPZ). During Temporary environment. construction there is the potential Effects for the creation of pathways for contamination into gravel causing pollution of groundwater.
During construction, there is a risk River/surface The CEMP will include Slight adverse of increased surface water flooding watercourses a Flood Management through earthworks and changes to Plan. drainage patterns.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Duration of Significance of Impact description Receptor(s) affected Mitigation Impact residual effect
The M4 corridor between junctions River/Surface The Contractor will Slight adverse 12 and 3 crosses Flood Zones 2 watercourses apply for a Flood and 3 at a number of locations. Defence Consent which Construction of ERAs or bridge will include a method works within the floodplains could statement to
potentially impact upon the existing demonstrate how the flood levels and flood volumes. works will be undertaken without affecting local flood levels.
During operation, potential increase River/Surface The Drainage Strategy Neutral in traffic flows could increase the watercourses includes mitigation to dust, sediments and hydrocarbons ensure that water settling on the carriageway which quality will not are washed off during storms and deteriorate from the discharged to receiving waters. existing level following Accidental spillages could pose a construction of the Permanent risk to the existing water quality. Scheme. Effects During operation, the new areas of Surface watercourses The increased drainage Neutral hard-standing for the ERAs will area will be offset by increase runoff into surface increased storm water drainage ditches. storage in over-sized drains resulting in no change in maximum discharge rates.
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Duration of Significance of Impact description Receptor(s) affected Mitigation Impact residual effect
The Scheme corridor is located on Groundwater The reduction in Neutral Principal water supply aquifers congestion should within SPZs. During operation, there result in a negligible is the potential for the creation of reduction in the risk of pathways for contamination into accidents. gravel following accidental spillages.
Works to alter the overbridge The Bourne and the Mitigation, such as alignments are located within the Jubilee River floodplain floodplain. Flood risk could be compensation to increased to third parties as flood compensate for any levels could increase as a result of loss of floodplain as a the works. result of the proposed side road alignment, will be provided. There is sufficient land available Neutral within the Agency land to provide suitable flood compensation. In addition slopes of the road embankment may be adjusted to further reduce the impact on flood levels
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HIGHWAYS AGENCY – M4 JUNCTIONS 3 TO 12 SMART MOTORWAY
Duration of Significance of Impact description Receptor(s) affected Mitigation Impact residual effect
Works at the River Thames River Thames For in channel works overbridge and the associated flood the Contractor will be relief channels may lead to physical required to obtain a changes of the watercourse and Flood Defence Consent river channel. which will include Neutral construction/operation method statements and environmental mitigation measures.
Works to extend the existing Datchet Common Brook As the works involve Neutral Ashley’s Arch culvert (Datchet and Chalvey Ditch. extending existing Common Brook) and Chalvey culverts the impact on culvert (Chalvey Ditch). the surrounding floodplain will be negligible and as such mitigation measures will not be required.
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