WARWICK FIRE STATIONS: LEA MARSTON DEPOT Drainage Strategy Curtins Ref: 071300-CUR-00-XX-RP-C-0002 Revision: V01 Issue Date: 15 September 2020 Client Name: Warwickshire County Council Site Address: Lea Marston Depot, Coton Road, Lea Marston Warwick B76 0DN Curtins 56 The Ropewalk, Nottingham, NG1 5DW Tel: 0115 941 5551 www.curtins.com STRUCTURES • TRANSPORT PLANNING • ENVIRONMENTAL • CIVILS & INFRASTRUCTURE • SUSTAINABILITY • STAKEHOLDER ENGAGEMENT Birmingham • Bristol • Cardiff • Douglas • Edinburgh • Kendal • Leeds • Liverpool • London • Manchester • Nottingham 071300-CUR-00-XX-RP-C-0002 WARWICK FIRE STATIONS: LEA MARSTON DEPOT Drainage Strategy Drainage Strategy Rev Description Issued by Checked Date V01 Planning Issue AH SW 15/09/20 This report has been prepared for the sole benefit, use, and information for the client. The liability of Curtins Consulting Limited with respect to the information contained in the report will not extend to any third party. Author Signature Date Aidan Hogan BEng (Hons) 15/09/20 Senior Engineer Reviewed Signature Date Stuart Williamson BEng (Hons) AMICE 15/09/20 Principle Engineer Rev V01 | Copyright © 2020 Curtins Consulting Ltd Page 1 071300-CUR-00-XX-RP-C-0002 WARWICK FIRE STATIONS: LEA MARSTON DEPOT Drainage Strategy Drainage Strategy Table of Contents 1.0 Surface Water .......................................................................................................................................... 3 1.1 National Guidelines .......................................................................................................................... 3 1.2 Proposed Surface Water Discharge ................................................................................................. 4 1.3 Proposed Surface Water Design ...................................................................................................... 5 2.0 Foul Water ............................................................................................................................................... 7 3.0 Appendices .............................................................................................................................................. 8 Rev V01 | Copyright © 2020 Curtins Consulting Ltd Page 2 071300-CUR-00-XX-RP-C-0002 WARWICK FIRE STATIONS: LEA MARSTON DEPOT Drainage Strategy Drainage Strategy 1.0 Surface Water 1.1 National Guidelines The aim of the surface water drainage proposals is to focus on the capture and management of all surface water within the site boundary, to prevent any likelihood of flooding to the proposed development, adjacent sites and infrastructure. General national guidance for the design of the surface water drainage systems include the following: • National Planning Policy Framework (NPPF) • Non-Statutory Technical Standards for Sustainable Drainage Systems, DEFRA, March 2015 • Written Ministerial Statement regarding Sustainable Drainage (HCWS161) • The SUDS Manual – C753, CIRIA Industry Best Practice Guidance • Flood Risk Planning Practice Guidance • Building Regulations Part H The NPPF recognises that flood risk and other environmental damage can be managed by minimising changes to the volume and rate of surface runoff from development sites and recommends that priority is given to the use of Sustainable Drainage Systems (SUDS) in new developments. Building Regulations 2010 Requirement H3 stipulates that rainwater from roofs and paved areas is carried away from the surface to discharge to one of the following, listed in order of hierarchy: I. Discharge into the ground; II. Discharge to a surface water body; III. Discharge to a surface water sewer; IV. Discharge to a combined sewer where there are absolutely no other options and only where agreed in advance with the relevant sewage undertaker. The site details are as follows: New Training Facility for Warwickshire Fire Service Lea Marston Depot Coton Road Lea Marston Warwick B76 0DN OS Grid Reference: 420756, 293782 The site covers a total area of 0.26ha and is currently undeveloped grassland. It is bound to the south east by a concrete access road then concrete bank to the River Tame / Lean Marston Lakes, to the south west by a concrete access road, and with the remaining boundaries finished as grassed land. The site is crossed by an overgrown historic access track and has no formal drainage features. A site location plan is included in Appendix A. Rev V01 | Copyright © 2020 Curtins Consulting Ltd Page 3 071300-CUR-00-XX-RP-C-0002 WARWICK FIRE STATIONS: LEA MARSTON DEPOT Drainage Strategy DrainageThe site Strategy is intended to be used as a training facility for Warwickshire Fire Service and the proposal for the site includes the construction of a new fire training house, welfare facilities and external hardstanding areas that will allow for the circulation of fire-trucks. The proposed site layout can be found in Appendix A. 1.2 Proposed Surface Water Discharge Discharge into the ground A Phase 1 Geo-Environmental Desk Study Report was undertaken by Wood Environment and Infrastructure Solutions UK Limited in November 2019 (Ref: 42234-WOOD-XX-XX-FG-OE- 0011_S0_P01.1). The report refers to the British Geological Survey records which state that the site is underlain by superficial alluvial deposits, which typically comprise clay, silt, sand and gravel. The bedrock geology under the site comprises the Sidmouth Mudstone Formation, which is classified as a secondary Aquifer of lower permeability layers. The study also found that made ground is likely to be present across the site. In addition the topographical survey shows that the site level is approximately 2.5m above the water level of the adjacent river at the time of the survey The combination of a likely high groundwater table, the impermeable strata and the possible contamination from made ground deems as an unsuitable form of surface water discharge. Extracts from the Phase 1 Geo-Environmental Desk Study can be found in Appendix B. Discharge into a surface water body The site is adjacent to the River Tame, just downstream of the Lea Masterton weir. The river at this location has a concrete bank with a concrete path running adjacent. The topographical survey has recorded the water level at a level of 66.66mAOD, approximately 2.5m below site level. Though the water level can change depending on the weir control, the level difference is deemed suitable to allow for a gravity connection from the site to the river without having a submerged outlet. This will require the construction of a new outfall headwall through the concrete embankment. The discharge arrangement will be subject to approval by the LLFA and the EA. On this basis discharge to a surface body is deemed suitable and is the proposed form of discharge for this development. Discharge into a surface water public sewer This option is deemed unfeasible for this site due to the lack of a nearby sewer. Discharge into a combined sewer This option is deemed unfeasible for this site due to the lack of a nearby sewer. Rev V01 | Copyright © 2020 Curtins Consulting Ltd Page 4 071300-CUR-00-XX-RP-C-0002 WARWICK FIRE STATIONS: LEA MARSTON DEPOT Drainage Strategy Drainage1.3 StrategyProposed Surface Water Design Water Quantity The proposed development is over an existing grassed area with no impervious surfaces. The proposed layout introduces approximately 0.212ha of new impermeable surface. It is proposed to limit the discharge of surface water to match the greenfield run-off rate. Using the HR Wallingford Greenfield Run-Off Rate Online Estimation Tool gives a rate of 1.4l/s. The greenfield run-off calculations can be found in Appendix D. As the greenfield run-off rate is very small a discharge rate of 2l/s is proposed to be used. This is in line with the Rainfall Runoff Management for Developments Report, reference CS030219 and the Flood Risk Management Pre-Application Advice Meeting Report, reference PRE000023 produced by Warwickshire County Council, which states that in an event of a low greenfield run-off rate a flow control with a minimum of 50mm opening should be used (noted under the advice for Kingsbury). It is proposed to provide a flow control device and surface water attenuation storage to allow for this discharge rate to be achieved. The detailed drainage design must present information to satisfy the conditions set out by the LLFA and must be provided prior to the completion of the development. This version of the report and supporting documents aims to satisfy these conditions. Further consultation will be required with the Environment Agency to confirm acceptance of the drainage design and attain the required permits. The LLFA planning conditions letter can be found in Appendix E. The Lead Local Flood Authority (LLFA) Warwickshire County Council (WCC) requires that the drainage design accommodates the 1 in 100 years storm event with a 30% allowance for climate change. The MicroDrainage Network Module has been used to establish the overall attenuation volume required for the 100 years plus 30% climate change storm event for the developed area. The total proposed impermeable area on site is approximately 0.212ha. The required attenuation volume has been calculated to be approximately 135.0m3 and is to be accommodated in an underground attenuation tank. Additional to the surface water run-off from site, the drainage strategy also accounts for the volume of water which is to be discharged on site as part of fire training drills. This water will become contaminated during the drills and is not to