Chapter 9: Road drainage and the water environment

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two locations of interest. The results of groundwater monitoring are summarised below: Location Average Ground water level (mAOD) Lowest Proposed road level (mAOD) Difference (m) Swinford Road Cutting 111.5 108.1 3.4 A14-M6 link 109.6 107 2.6 Table 4: Ground water levels and proposed finish road level for Swinford Road Cutting and A14-M6 link The likeliness of groundwater flooding at these locations is high (probable). However the hydraulic conductivity of the ground is relatively low thus seepage volumes are likely to be low. Any seepage would be intercepted by the proposed highway drainage system and may therefore not present a risk to vehicles using this stretch of the scheme. It may be precautionary, however, to consider mitigation measures so that these seepage volumes do not surface (see Section 5). Flood risk presented by groundwater to the scheme therefore is considered to be low. 3.5 Surface Water Runoff Surface water runoff is defined as water flowing over the ground that has not yet entered a drainage channel or artificial drainage system. It usually occurs as a result of an intense period of rainfall, which exceeds the capacity of the ground to soak up the water. Typically, runoff occurs on sloping land and where the ground surface is relatively impermeable. The ground can be impermeable, either naturally through the soil type/geology or due to development which places a large area of impervious material over the ground surface (i.e. paving and roads). The flow path taken by surface water runoff is strongly influenced by the local topography and the built form. Runoff will gravitate towards the lowest areas. The places at greatest risk from surface water runoff are usually situated in topographical low spots where water will pond or within the flow path for the runoff. 3.5.1 Initial assessment of surface water run-off A review of the topographical data provided on the 1:50,000 scale Ordnance Survey maps allows surface water pathways to be inferred. These are shown below: 10
Major earthworks affecting surface water pathways Surface water flow path Figure 3: Surface water pathways inferred from contours According to the hydrogeology assessment the drift geology is River Terrace deposits and alluvium. The alluvium is described as soft sand clay with some organic material, therefore, rainfall falling onto the site may generate significant volumes of surface water runoff. Surface water runoff is thought to find is way into the many existing land drains and ditches in place to assist in draining surrounding farmland. Surface water runoff is not considered to present a risk to the scheme due to the elevated nature of the highways. In the proposed scheme the raised embankment shown in Figure 3 has the potential to change the surface water runoff flow path. However, this water will flow from the earthworks and the highways into the proposed drainage system. Flood risk presented by surface water to the scheme therefore is considered to be low. 11
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M1 Junction 19 Improvement Environm
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M1 JUNCTION 19 IMPROVEMENT ENVIRONM
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Page 46 and 47: 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: 3.2 Fluvial Flooding - Ordinary Wat
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 and 116: 7 PPS25 Sequential and Exception Te
Page 117 and 118: 8 Conclusion The M1 Junction 19 Imp
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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