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Drainage Strategy in Support of Commercial Development At Drainage Strategy In support of Commercial Development at 40 St. Thomas Street OXFORD OX1 1JP Tony Lee March 2021 Revision [0] Drainage Strategy 40 St. Thomas Street Contents Amendments ........................................................................................................................................... 2 References .............................................................................................................................................. 2 Introduction ............................................................................................................................................ 3 Existing Drainage ..................................................................................................................................... 3 Site Geology ............................................................................................................................................ 4 Ground Water ......................................................................................................................................... 4 Topography ............................................................................................................................................. 4 Design Objective ...................................................................................................................................... 4 Development Drainage Proposals ........................................................................................................... 5 Surface Water Drainage ...................................................................................................................... 5 Infiltration Feasibility ........................................................................................................................... 5 Sustainable Drainage Hierarchy .......................................................................................................... 5 Surface Water Drainage Strategy ........................................................................................................ 8 Foul Water Drainage ........................................................................................................................... 9 Residual Flood Risk .................................................................................................................................. 9 SuDS Maintenance ................................................................................................................................ 10 Appendices ............................................................................................................................................ 11 1 Drainage Strategy 40 St. Thomas Street Amendments Revision Date Description 0 19th March 2021 First Issue References Reference Title A CIRIA Publication 753 – The SUDS Manual (2015) B Sewers for Adoption 7th Edition (Wrc 2012) C Part H of the Building Regulations (2015) D Flood Risk Assessment (RSPD Ltd March 2021) 2 Drainage Strategy 40 St. Thomas Street Introduction This Drainage Strategy has been developed in support of a proposed planning application for a three storey office development. The site is located at 40 St. Thomas Street, OXFORD OX1 1JP Grid Reference: 450650, 206170 This document is to provide details of the methods of drainage to be used for the Surface and Foul Water Drainage. The aim of this Drainage Strategy is also to incorporate and adopt Best Management Practices (BMP’s) for Sustainable Urban Drainage Systems (SUD’s) in accordance with Site Specific Technical Reports and Published Documents referenced in Page (ii). It is proposed to demolish the existing children’s nursery and construct a 3 storey office building with associated paving and soft landscaping. The area of soft landscaping will reduce the impermeable area of the site by over 100m2 (25%). A Location Plan and layout of the proposed scheme can be found in the appendices of this document. Existing Drainage The Thames Water sewer records indicate an adopted foul sewer running within St Nicholas Road at a depth between 1.6 & 2.3m. There is an adopted surface water sewer within Hollybush Row at an approximately depth of 1.6m. There are gullies within St Thomas Street and chambers that are not indicated on Thames Water sewer records. It is presumed that there is a highway drain in St Thomas Street that outfalls to Thames Water Manhole 6103. The surface water runoff from the roof of the existing nursery building and the surrounding play areas drains to the private foul drainage network within the site. It is presumed that this outfalls to the adopted foul sewer in St Thomas Street. A copy of the Thames Water sewer records can be found in the appendices of this document. 3 Drainage Strategy 40 St. Thomas Street Site Geology Review of British Geological Society Borehole records show that a borehole was undertaken approximately 50m to the south east of the site for the Oxford Relief Road Scheme in 1966. This borehole records approximately 1.5m of sandy, silty clay overlying 3m of sands & gravel which are further underlain by Oxford Clay. • Clay – soft yellow brown organic calcareous sandy silty clay. • Sand and Gravel Member – Medium yellow brown clayey sand and fine to medium gravel. • Oxford Clay – stiff becoming hard calcareous clay A plan of the local geological formations can be found in the appendices of this report. Ground Water Records of local historical boreholes in the vicinity of this application site were found dating from 1969. Groundwater is not mentioned on the deeper of the three boreholes but the nearest borehole was drilled to 4.27m and water was encountered at 2.28m. All three boreholes were sunk in November 1969. Groundwater flooding is not considered an issue. Topography The site is currently a children’s day nursery. The nursery is a two-storey structure located towards the centre of the site. The nursery building is surrounded by impermeable play areas surfaces with either asphalt or rubber safety surfacing. The topography of the overall site is relatively flat with levels set at around the 57.30m AOD level. The site is approximately 430m2 in area and is 100% impermeable. Design Objective Sustainable Drainage techniques (SuDS) covers a range of approaches to manage surface water runoff so that- ‘Surface water arising from a developed site should, as far as is practicable, be managed in a sustainable manner to mimic the surface water flows arising from the site prior to the proposed development, while reducing the flood risk to the site itself and elsewhere, taking climate change into account.’ 4 Drainage Strategy 40 St. Thomas Street Drainage shall also be designed with the appropriate factor of safety, in accordance with the tables set out in CIRIA Publication 753 “The SUDS Manual”. The proposed sustainable drainage techniques for the development should accommodate the peak rainfall event for a 1 in 100 year storm event with an additional allowance for climate change. Table 5 of NPPG recommends for developments that have a life expectancy beyond 2085 that an additional factor of 40% is applied to the peak volume of runoff. Development Drainage Proposals Surface Water Drainage The development site area is approximately 430m2, of which 100% is impermeable. The existing runoff rates from the site are indicated below. Return period Runoff rate l/s 1 in 1 year 4.7 1 in 30 year 6.9 1 in 100 year 15.1 Infiltration Feasibility The thickness of the silty sandy clay underlying the site prevents the use of shallow infiltration techniques such as infiltration permeable paving. Building Regulations requires that any soakaways are position a minimum of 5m away from any structures. The size and shape of the site, and the orientation of the proposed development and the adjacent building means it is not possible to meet this requirement. Sustainable Drainage Hierarchy A hierarchical approach has been undertaken in consideration of the application of SuDS in relation to the development. This is in order to meet the design philosophy of ensuring that surface water run-off is managed as close to its source as possible and the existing situation is replicated as closely as possible. The following drainage hierarchy has been undertaken with reference to the procedures 5 Drainage Strategy 40 St. Thomas Street set out in the SuDS Manual (CIRIA C753, 2015) to assess the viability of the application of SuDS techniques to this scheme: 1. store rainwater for later use 2. use infiltration techniques, such as porous surfaces in permeable strata areas 3. Attenuate rainwater in ponds or open water features for gradual release to a watercourse. 4. attenuate rainwater by storing in tanks or sealed water features for gradual release to a watercourse, 5. discharge rainwater direct to a watercourse 6. discharge rainwater to a surface water drain 7. discharge rainwater to the combined sewer. The sustainable drainage hierarchy shown above is intended to ensure that all practical and reasonable measures are taken to manage surface water higher up the hierarchy (1 being the highest) and that the amount of surface water managed at the bottom of the hierarchy is minimised. Sustainable urban drainage systems have been considered for this development unless there are practical reasons for not doing so. Such reasons include the local ground conditions or density of development. The site-specific drainage hierarchy checklist considered for the drainage design for this development is detailed
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