Godstow Road Car Park Surface Water Drainage Strategy On behalf of Oxford Direct Services Project Ref: 332110212-2001-R003 | Rev: - | Date: March 2021 Registered Office: Buckingham Court Kingsmead Business Park, London Road, High Wycombe, Buckinghamshire, HP11 1JU Office Address: 1 Cambridge Terrace, Oxford, OX1 1RR T: 01865 410000 E: [email protected] Surface Water Drainage Strategy Godstow Road Car Park Document Control Sheet Project Name: Godstow Road Car Park Project Ref: 332110212 Report Title: Surface Water Drainage Strategy Doc Ref: 2001-R003 Date: March 2021 Name Position Signature Date Prepared by: Jasper Syms Assistant Engineer JS March 2021 Reviewed by: James Lambeth Apprentice Engineer JL March 2021 Approved by: Simon Hudson Associate SH March 2021 For and on behalf of Stantec UK Limited Revision Date Description Prepared Reviewed Approved This report has been prepared by Stantec UK Limited (‘Stantec’) on behalf of its client to whom this report is addressed (‘Client’) in connection with the project described in this report and takes into account the Client's particular instructions and requirements. This report was prepared in accordance with the professional services appointment under which Stantec was appointed by its Client. This report is not intended for and should not be relied on by any third party (i.e. parties other than the Client). Stantec accepts no duty or responsibility (including in negligence) to any party other than the Client and disclaims all liability of any nature whatsoever to any such party in respect of this report. ii Surface Water Drainage Strategy Godstow Road Car Park Contents 1 Introduction ........................................................................................................................... 1 1.1 Appointment and Brief ................................................................................................ 1 1.2 Site Proposals ............................................................................................................ 1 1.3 Reference Documents ................................................................................................ 1 2 Site Context ........................................................................................................................... 2 2.1 Site Description and Location ..................................................................................... 2 2.2 Flood Risk................................................................................................................... 2 2.3 Site Topography ......................................................................................................... 4 2.4 Geology and Hydrology .............................................................................................. 4 2.5 Existing Drainage Regime .......................................................................................... 4 3 Surface Water Drainage ........................................................................................................ 5 3.1 Proposed Surface Water Drainage ............................................................................. 5 3.2 Further Drainage Design Notes .................................................................................. 6 3.3 Maintenance and Management of Surface Water Drainage Features ......................... 6 4 Summary ................................................................................................................................ 9 Figures Figure 2.1: Site Location and Flood Zones (Environment Agency Product 4 data) ............................... 3 Figure 2.2: Extract of Surface Water flood map (gov.uk) ...................................................................... 3 Figure 2.3: BGS Surface Geology Mapping.......................................................................................... 4 Figure 3.1: Extract of Planning Policy Guidance on Flood Risk and Coastal Change. .......................... 5 Tables Table 3.1: Proposed SuDS Features Maintenance Responsibility ........................................................ 6 Table 3.2: Maintenance Requirements for Gullies and Pipework.......................................................... 7 Table 3.3: Maintenance Requirements for Soft Landscaping receiving run-off from car park ............... 8 Appendices Appendix A Site Location Plan Appendix B General Arrangement Drawing Appendix C Drainage Strategy Drawing Appendix D Flood Exceedance Plans iii Surface Water Drainage Strategy Godstow Road Car Park iv Surface Water Drainage Strategy Godstow Road Car Park 1 Introduction 1.1 Appointment and Brief 1.1.1 Stantec UK Ltd (Stantec) have been commissioned by Oxford Direct Services (ODS) to prepare a surface water drainage strategy to support a planning application for the resurfacing of a car park owned by Oxford City Council. The car park is located off Godstow Road, Wolvercote, Oxford, and from here on is referred to as the Site. 1.1.2 The purpose of this report is to identify the existing site drainage, with consideration to ground conditions, geology and ground water across the site and describe any alterations with the resurfacing of the car park. 1.2 Site Proposals 1.2.1 The car park off Godstow Road, Oxford, will be resurfaced by Oxford Direct Services. The current car park surface is a deteriorated asphalt surfacing. The resurfacing works will replace this surface with a new asphalt surface course. 1.2.2 The car park primarily serves those using Port Meadow and the river, being adjacent to Wolvercote Mill Stream and Port Meadow. 1.3 Reference Documents 1.3.1 This surface water drainage strategy has been designed and prepared with reference to documents and information sources provided and/or published by the following bodies: a) CIRIA C753 – The SUDs Manual and SUDs Hierarchy; b) National Planning Policy Framework (NPPF) and Planning Practice Guidance (PPG); c) Oxfordshire County Council, the Lead Local Flood Authority (LLFA) - Local Standards and Guidance for Surface Water Drainage on Major Development in Oxfordshire (November 2018); d) Oxford City Council, the Local Planning Authority – SuDS Design Guide (2018); e) Environment Agency (EA) Standing Advice; f) Thames Water; g) Sewers for Adoption where applicable; h) Relevant Building Regulations where applicable; and i) Other Industry best practice. 1 Surface Water Drainage Strategy Godstow Road Car Park 2 Site Context 2.1 Site Description and Location 2.1.1 The Site is located off Godstow Road, Wolvercote, Oxford, at National Grid Reference 448700E 209430N. 2.1.2 The site is bounded to the south and east by Port Meadow, a SSSI and open common land used for grazing of livestock. To the north of the site is Godstow Road. To the west of the site is Wolvercote Mill Stream; a secondary channel of the River Thames diverging above Kings Lock and re-joining 400m south of the site downstream of Godstow Lock and weir. Figure 2.1, below, shows the site boundary, adjacent to Wolvercote Mill Stream. 2.1.3 The Site is comprised of an impermeable asphalt-surfaced car park and limited areas of associated soft landscaping. Thus, the Site is classified as a Brownfield Site, with an impermeable area of approximately 0.23ha. 2.1.4 A Site Location Plan can be found in Appendix A, drawing no. 332110212-2001-004, with a General Arrangement drawing included in Appendix B, drawing no. 332110212-2001-005. 2.2 Flood Risk 2.2.1 As the Site is within Flood Zone 3 a site-specific Flood Risk Assessment (FRA) is required as set out within Environment Agency (EA) standing advice, updated October 2015. Please see Stantec’s report no. 332110212-2001-R002 for the site-specific detailed FRA. 2.2.2 The Site is located almost entirely within Flood Zone 3 and therefore has a high probability of flooding from the nearby River Thames, with the majority of the Site determined to flood during the 1 in 20 year flood event. The Wolvercote Mill Stream is located on the western boundary of the site. Figure 2.1 (overleaf) shows the flood extents in the area, as taken from the Environment Agency’s Product 4 data provided for the site. 2 Surface Water Drainage Strategy Godstow Road Car Park Figure 2.1: Site Location and Flood Zones (Environment Agency Product 4 data) 2.2.3 In addition, Figure 2.2, below, is an extract from the Environment Agency’s GIS mapping portal, which shows the surface water flooding risk within the Site and surrounding area. The mapping shows the site is not at risk of surface water flooding. Figure 2.2: Extract of Surface Water flood map (gov.uk) 2.2.4 The resurfacing works will not alter the existing drainage infrastructure and hence there should be no change in the surface water flood risk from this site. 3 Surface Water Drainage Strategy Godstow Road Car Park 2.3 Site Topography 2.3.1 The site elevation ranges from 57.72mAOD to 58.15mAOD, with some high points along Godstow Road within soft landscaping of 59.00mAOD. The site grades from higher levels at the northern boundary along Godstow Road downwards to the south and the boundary with Port Meadow. Immediately west of the site the land drops off as the bank of Wolvercote Mill Stream. 2.4 Geology and Hydrology 2.4.1 The geological record data from the British Geological Society (BGS) indicate that the bedrock geology of the Site is the Oxford Clay Formation and West Walton Formation (Mudstone). The site lies on superficial deposits of Alluvium (clays, silts, sands and gravels). See Figure 2.3, below, for British Geological Society