sustainable drainage strategy

LF Architecture Ltd of 14

LF Architecture Ltd Architectural Consultants

Sustainable Drainage Strategy Grenville Cottages & Enfield Crematorium Winnington Road Enfield EN3 5RW This sustainable drainage report has been prepared to accompany the Planning Application for a Change of use from residential dwellings (C3) to cemetery use (Sui Generis) to provide additional burial space, involving the demolition of a pair of existing semi detached houses and erection of new Cemetery service yard and staff welfare facilities together with alterations/extension to existing parking areas at Grenville Cottages and Enfield Crematorium, Winnington Road, Enfield, EN3 5RW.

Surface Water Drainage and SUDS The following is a list of principal regulatory requirements and bodies relative to the design of the surface water drainage for this project. The Lead Local Flood Authority (LLFA) The LLFA is a statutory Consultee on Planning Applications with 10 or more residential units or over 1000m2 floor space for surface water flood risk management advice and sustainable drainage systems. LBoE as the LLFA promotes and encourages the uptake of sustainable drainage systems (SUDS) as a means to reduce flood risk. There is a requirement that SuDS be installed where appropriate to limit the amount of surface water runoff entering drainage systems and to return surface water into the ground to follow its natural drainage path.


The National Planning Policy Framework (NPPF) The NPPF require the effects of Climate change to be considered in the assessment of flood risk for developments and that when designing surface water drainage systems for developments an allowance for Climate change should be included. Building Regulations Part H3 of the Building Regulations 2002 sets out the surface water hierarchy for discharge (guidance & requirements for rainwater drainage) as follows: 1: Adequate soakaway or other infiltration system ie. Permeable paving, french drains, swales, infiltration basins, etc; 2: Watercourse – subject to consent for the Environment Agency or other applicable authority; 3: Sewer – approval only granted if capacity available. Proposed surface water drainage The application site is located to the North East of the Enfield Crematorium Site and includes Grenville Cottages comprising a pair of semi-detached houses with associated hard standing, driveways, a detached outbuilding and garden areas. In addition, part of the Enfield Crematorium site comprising tarmac access road, car park, gardener’s compound and some areas of lawn. From the existing Topographical survey the ground slopes down from South to North by approximately 600mm over the site to be developed. A ground investigation for the site was carried out by Clancy Consulting Ltd in January 2019 (See Appendix A) to confirm the suitability of the ground for a new burial site and to enable the design of the associated access roads & paths and drainage works. The report indicates that Made Ground was encountered in all exploratory locations to depths between 0.20m in WS4 and 2.10m in WS3 generally comprising gravelly sandy clay and gravelly slightly clayey sand with gravel of concrete, sandstone, brick, mudstone and occasional roots and rootlets. Made Ground was generally underlain by interbedded cohesive and granular deposits generally comprising firm and stiff sandy clay and medium dense to very dense clayey sand and sandy gravel, with gravel if sandstone, quartz and igneous lithologies. All boreholes were terminated in very dense gravelly sand or sandy gravel of sandstone and flint at depths between 2.0m in WS4 and 5.35m in WS6. Borehole logs indicate the presence of ground water between 4.0m to 5.0m below existing ground levels & test results indicate that infiltration solutions will be viable for the disposal of surface water run off. See the submitted Enfield SUDS Proforma Document & Greenfield Runoff Rates Document.


Surface Water Drainage Design Existing Site Area (see plan below) The total impermeable area of the existing site has been calculated at 1046.5m2 Roofs – 325.5m2 Access Road, driveways & non permeable pathways – 721m2 The existing car park & Service Yard have a rough aggregate surface finish with no surface or linear drainage and are therefore assumed to be permeable.

Proposed Site Area (see proposed drainage plan on next page) The total proposed impermeable area has been calculated at 1078m2 Roofs – 78m2 Access Road, driveways & non permeable pathways – 1000m2 There is a minimal proposed increase of 31.5m2 in impermeable areas on the site. The area of impermeable surfaces has been kept to a minimum by proposing permeable paving to all car parking bays and to the Gardeners Compound reserving the use of smooth hardwearing Tarmac surfaces to areas of increased pedestrian and vehicular traffic movement.


Drainage Design (see proposed drainage plan on previous page) The permeable paving areas have been designed using WinDes software and the results demonstrate that for a 1 in 30 year event there will be no flooding on site for the proposed surface water drainage system. ForWinDes results refer to the submitted Microdrainage Calculation Document. In addition, the drainage model has been tested for a 1 in 100 year event including 40% climate change. From the results it was concluded, if an extreme event such as I in 100 years was to occur, it would result in zero flooding. Maintenance Plan The proposed Maintenance Plan is included on the Design Drawing below. Conclusion An assessment of the practical use of sustainable drainage techniques has been carried out. As soil types will support the effective use of infiltration devices it is proposed that surface water is discharged into the ground via the permeable paving.


Appendix A - Ground investigation carried out by Clancy Consulting Ltd in January 2019

sustainable drainage strategy

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