Development at 2 St Albans Road, Kingston, London, KT25HQ

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1 Development at SuDS Report. Prepared for: Report prepared by: Dave Alonso Divine Ideas (UK) ltd Date: August 2015 Project/Issue number: Appendices Attached: 1510/01/SuDS A, B & C

2 Divine Ideas (UK) Ltd has prepared this report with all reasonable skill, care and diligence, and taking account of the Services and Terms agreed between Divine Ideas (UK) Ltd and the Client. This report is confidential, and Divine Ideas (UK) Ltd accepts no responsibility to third parties to whom this report or part thereof, is made known, unless formerly agreed with Divine Ideas (UK) Ltd beforehand. Any such party relies upon the report at their own risk Divine Ideas (UK) Ltd declaims any responsibility to the client and others in respect of any matters outside the agreed scope of services. Divine Ideas Ltd 1

3 Contents 1. Introduction 2. Existing Drainage 3. Ground Conditions and Groundwater 4. Surface Water Management Assessment of Runoff Rates and Volumes 5. Conclusion 6. Calculations Divine Ideas Ltd 2

4 1 introduction 1.1 Divine Ideas have been instructed to prepare a Surface Water Drainage Strategy in respect of a proposed residential development at 2 St Albans Road, Kingston, London KT25HQ (grid reference: E N). A location plan and aerial image are included in Appendix A. 1.2 The existing site comprises of a large house and associated hard standing. The site covers an area of approximately 660sq.m and is surrounded by residential properties on all sides. 1.3 Site levels vary from a high of 9.06metres Above Ordnance Datum (m AOD) to a low of 8.19m AOD. A topographical survey is included in Appendix B. 1.4 The proposed development is for the erection of 3 dwelling houses with 6 off-street car parking spaces, cycle storage and associated landscaping. A proposed development plan is included in Appendix C. 2. Existing drainage 2.1 The Thames Water combined sewer to St Albans Road with a current connection for the house. 2.2 It is assumed that surface water from the existing site discharges to the public sewer network. 3 Ground Conditions and Groundwater 3.1 The Cranfield University Soilscapes map shows that the site is underlain by Freely draining lime-rich loamy soils. (Appendix D) 3.2 The Environment Agency (EA) Nitrate Vulnerable Zones map (Appendix E) shows that the site is located within a vulnerable surface water zone. The EA Groundwater Vulnerability & Aquifer Zones map (Appendix F) shows that the site is outside a major aquifer and within no vulnerable groundwater zones. 4 Surface Water Management Assessment of Runoff Rates and Volumes 4.1 Pre- and post-development surface water runoff rates and volumes have been estimated in accordance with current best practice. There is likely to be a reduction in surface water runoff resulting from the reduction in impermeable area. SuDS and Water Quality 4.12 Sustainable Drainage Systems (SuDS) should be utilised to provide attenuation storage, pollution reduction and landscape and wildlife benefits. In accordance with The London Plan, the following SuDS hierarchy should be applied The first consideration for SuDS is rainwater harvesting. It is proposed to discharge surface water runoff from roof areas to a rainwater harvesting system Infiltration techniques such as porous surfaces will be used for parking areas and hardlandscaping Maintenance of the rainwater harvesting system includes the clearance of debris from filters (monthly) and inspection / removal of sediment from the tank (annual inspection, clearance as required). Divine Ideas Ltd 3

5 4.19 Maintenance of porous surfaces includes sweeping and regular brushing, ensuring any void spaces are kept clear. Summary 4.21 Surface water runoff from roof areas will be drained to a rainwater harvesting system. The overflow from the rainwater harvesting system will be directed to the public sewer at a restricted runoff rate of 5 l/s. A flow control device such as a hydro-brake will limit flows. Permeable surfaces will be used for all hardlandscaping. 5 Conclusion 5.1 The proposed development is for 3 dwellings with associated parking and landscaping. 5.2 All surface water disposal options have been considered. A connection to the 300mm public surface water sewer in St Albans Road appears to be the only feasible option for this site. 5.3 In accordance with the requirements of the London Plan, development sites should aim to achieve a greenfield runoff rate. Therefore discharge to the surface water sewer should be restricted to a rate of 5 l/s (minimum rate required to ensure self-cleansing of pipework). 5.4 In order to achieve a discharge rate of 5 l/s, rainwater harvesting system will be provided to accommodate runoff during all storm events up to and including the 1 in 100 year plus 30% climate change allowance event. 5.5 The proposed drainage system follows the SuDS hierarchy contained in The London Plan. Surface water runoff from roof areas will drain to a rainwater harvesting system of 440ltr. The overflow from the rainwater would connect to the mains drain. 5.6 Porous surfaces will be used for parking areas and hard-landscaping. 5.7 Maintenance of the surface water drainage system will be the responsibility of the site owner. 6. Calculations Existing site sq.m impermeable Proposed Site Building footprint 250sq.m Permeable paving and gravel path 255sq.m Garden lawn and planting 175sq.m The rainwater harvesting would be for internal use i.e. toilet flushing, washing machine use. An approximate tank size is 6m³ for the whole site based on a roof area of 250m² draining into the tank. This equates to a 3 m³ tank per property We estimate a total 5 occupants per dwelling. The demand therefore significantly exceeds the yield (3m³) and the tank would require mains top-up. The supplier / manufacturer of the rainwater harvesting system include for sizing the tank as part of their service. They would be able to provide a much more detailed tank size than our estimate. With the introduction of a rainwater harvesting system and the decrease of impermeable water-run off area we believe there is not a requirement for attenuation in terms of meeting the London Plan run-off rates. Divine Ideas Ltd 4