Chapter 9: Road drainage and the water environment

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The increase in storm runoff from the additional carriageway area will be attenuated to reduce the existing discharge to the river using attenuation ponds. All of the attenuation ponds are located outside the floodplain area and therefore will not impact on the floodplain. These measures will maintain the existing drainage into the River Avon and therefore the flood risk elsewhere will not be increased. The proposed level of the A14 and M1 slip roads is above the level of the floodplain and therefore the development itself is not at risk of flooding, access along the A14 and M1 will not be impaired during a flood event. 7.3 Summary It is considered that the proposed M1 Junction 19 improvement scheme is compliant with PPS25 and that it meets the requirements of the Sequential Test and the Exception Test. 30
8 Conclusion The M1 Junction 19 Improvements scheme is considered to be at risk of fluvial flooding as parts of the scheme are located within flood zone 3a, the 1 in 100 (1%) annual probability flood extents. The proposed development is classified as essential transport infrastructure and is only permitted in zone 3a if the Exceptions Test is satisfied. Due to the location of the existing junction it is not possible to relocate this development outside of the floodplain. The Exception Test (PPS 25) has been completed for the development. The development has been shown to have wider sustainability benefits by reducing the stress, risk and travel time associated with driving between the M1 and A14. The specific locations at fluvial risk are: • The A14 on the eastern part of the scheme, specifically the embankment shoulders, the highway deck is unaffected • Bridleway Crossing 1 north of the A14 over the River Avon will be totally submerged in the 1 in 100 year event • Bridleway Crossing 2 between the M1 and A14 (southeast quadrant) will be totally submerged in the 1 in 100 year event To mitigate against the risk of increasing flooding downstream flood compensation areas have been proposed. Dimensions of the compensation areas can be found in Appendix D. There will be a net total increase in impermeable area associated with the scheme and thus a commensurate increase in highway runoff. To mitigate against the effect of increased runoff in the River Avon, five attenuation ponds have been proposed. The design has been developed in consultation with the EA. Details of the attenuation ponds can be found in Appendix B. It is recommended signage is incorporated at the bridleway crossing to inform users of the risk of flooding The scheme will involve deep cuttings for the Swinford Road underpass and the A14-M1 link. This may result in groundwater seepage. To mitigate against this risk filter drains and slope drainage are being considered by the contractor. It is recommended that Severn Trent is pursued by the designers in respect of their assets (water supply) which appear to pass through the site. It is anticipated that the level of residual risk to the site, following the implementation of all identified mitigation measures, will fully satisfy the requirements of PSS 25. . 31
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M1 Junction 19 Improvement Environm
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M1 JUNCTION 19 IMPROVEMENT ENVIRONM
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Page 69 and 70: M1 JUNCTION 19 IMPROVEMENT ENVIRONM
Page 71 and 72: Appendix A - Pollution and Spillage
Page 73: Method D - Assessment of Pollution
Page 78 and 79: Groundwater: Hydrocarbons Hydrogeol
Page 83 and 84: Non Technical Summary Skanska Const
Page 85 and 86: Contents 1 Introduction 1 1.1 Overv
Page 87 and 88: 1 Introduction 1.1 Overview Jacobs
Page 89 and 90: 2 Policy Review 2.1 Planning Contex
Page 91 and 92: When the both the ‘Flood Zone’
Page 93 and 94: 3 Flood Risk to the Proposed Develo
Page 95 and 96: 3.2 Fluvial Flooding - Ordinary Wat
Page 97 and 98: Major earthworks affecting surface
Page 99 and 100: 3.7 Flooding from Infrastructure Fa
Page 101 and 102: 3.9 Summary of Risks to Site Table
Page 103 and 104: 4 Impacts of the Proposed Developme
Page 105 and 106: 4.6 Summary of Flood Impacts from D
Page 107 and 108: 5 Proposed Mitigation Measures 5.1
Page 109 and 110: The extents of this problem are cur
Page 111 and 112: 5.7 Summary of Recommended Mitigati
Page 113 and 114: 6 Residual Risks The level of flood
Page 115: 7 PPS25 Sequential and Exception Te
Page 119 and 120: Appendix A Existing Layout, Environ
Page 127 and 128: Appendix B Contractor’s Proposals
Page 129 and 130: Appendix C Hydrogeological Assessme
Page 131 and 132: Memorandum Fairbairn House, Ashton
Page 133 and 134: Memorandum (Continued) Page 3 of 6
Page 135 and 136: Memorandum (Continued) Page 5 of 6
Page 141 and 142: Appendix D Volume Distribution for
Page 143 and 144: Floodplain compensation area - elev
Page 145: Depth Bands (m) TOTAL (m³) Area Re
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Chapter 9: Road drainage and the water environment

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