Chapter 9: Road drainage and the water environment

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3.6 Flooding from Artificial Drainage Systems Artificial drainage systems are those which have been installed into an area during development to manage runoff or effluent discharges. They include pipes, land drains, sewers and drainage channels. Sites currently or previously used for agricultural purposes may additionally have systems of land drains as well as open ditches and watercourses. The main risk from this source of flooding occurs during periods of heavy rain when the capacity of the system is exceeded. Debris and sediment can often get trapped in these systems causing a reduction in the capacity of the network. 3.6.1 Assessment of highways runoff and land drains There are no known issues with regards to the highway drainage system currently. The existing drains are maintained regularly and thus the likeliness of the highways flooding due to highway runoff is thought to be low. The proposed development will increase the total impermeable area contributing to highway runoff. Without a commensurate improvement in the proposed drainage system the flood risk from highway runoff will increase. The consequence of flooding on the highway would be significant as it would present a significant risk to vehicles and drivers. Mitigation measures will therefore be required to maintain the existing low level of flood risk. Flood risk presented by highway runoff to the proposed development (vehicles and drivers) is considered to be high. There are a number of culverts (see Figure 4 in Section 5.4) which convey small unnamed watercourses within the limits of the scheme. Where embankments are being widened these culverts will be lengthened. Lengthening the culverts will have the effect of increasing friction (marginally for the lengths being considered) and thus reducing the conveyance of the culverts. There are no known issues of under capacity, blockage or flooding from the existing culverts. Mitigation measures will be required to maintain or reduce the level of flood risk. Flood risks presented by the culverts to the proposed development is considered to be low. 12
3.7 Flooding from Infrastructure Failure This type of flooding occurs as result of the failure of infrastructure which exists to retain, transmit or control the flow of water. The failure can be caused by structural, hydraulic, mechanical, geotechnical or operation problems. The risk of flooding comes from infrastructure such as canals, dams, water mains, flood defences, pumping station and blockages of pipes. Investigations of the Severn Trent infrastructure suggest that there are water supply pipes that pass directly through the site. Inspection of the OS background indicates that there is a reservoir upstream of the site. (a) Reservoir Failure From Ordnance Survey data it is observed that Stanford reservoir lies approximately 4km upstream of the site and feeds the River Avon at the upstream limit. Severn Trent has a comprehensive and ongoing mandatory inspection and maintenance regime of their reservoirs under the Reservoirs Act. Any structural problems with this reservoir would be identified by this regime and rectified. A sudden failure of the reservoir is considered to be a very low probability event. A dam break analysis has been performed for Stanford Reservoir, and an inundation map was produced as part of this. Severn Trent’s reservoir supervising engineer 3 has stated that the M1/M6 junction was part of the detailed analysis and the report concludes that "The M1/M6 motorway junction and wide bridge opening across the valley between the dam and the disused railway embankment, at 4.8 km downstream, would have a negligible effect on the flood wave". The remainder of the dam break information is considered a confidential nature at present. A request for the inundation map has been made but has not at the time of writing been made available. (b) Water Supply and Sewage Infrastructure Failure The existing data obtained from Severn Trent does not indicate the size of the pipes, depth of the pipes or what they convey. The designers should ensure that the water supplier (Severn Trent) is contacted and details of the pipes obtained. Where necessary an application for diversions will have to be made. The failure of any of these pipes during construction may lead to overland flows towards the low spots such as Swinford Road crossing and the A14-M1 link. The water supply pipes are unlikely to represent a serious risk to the scheme or highway users. The flow rates are likely to be low and the water would be intercepted by the drainage network. The risk of flooding from identified infrastructure failure is considered to be low. 3 Email (13/07/09) from Mr. Francis Simba Chigara, Reservoir Supervising Engineer, Severn Trent. email: francis.chigara@severntrent.co.uk 13
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M1 Junction 19 Improvement Environm
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M1 JUNCTION 19 IMPROVEMENT ENVIRONM
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Page 48 and 49: 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: Major earthworks affecting surface
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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