Guidance for New Developments Surface Water Drainage Assessments

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1 Guidance for New Developments Surface Water Drainage Assessments Updated: May 2014 The Planning, Design and Adoption of Sustainable Drainage Systems (SuDS) Under Schedule 3 of the Flood and Water Management Act 2010, yet to be implemented, it is to be the responsibility of the Lead Local Flood Authority to establish a SuDS Approval Body (SAB). This will mean that all new SuDS, serving two or more properties will need to gain approval by Devon County Council as the SAB. In the interim, it is essential that due regard is made for the provision of SuDS therefore this guidance note is intended to inform developers and planners of the standards that would be expected. New developments must have an effective and robust surface water drainage system which should not increase flood risk to adjacent properties or land and should be undertaken in accordance with SuDS principles. It is vital that applicants consult with the Local Planning Authority at the Pre-Application stage. Any new development should consider the benefits to: 1. Flood Risk 2. Water Quality 3. Amenity and Biodiversity Sustainable Drainage Systems (SuDS) are to be designed in accordance with CIRIA Report C697 SuDS Manual (2007) and if using infiltration then BRE 365 Soakaway Design (2003) or Part H of the Building Regulations (2000). When designing the drainage scheme for a site it is essential to: Ensure that people and property on the site are protected against flooding, and to Ensure that the impact of the development does not exacerbate flood risk at any other point (either upstream or downstream) in the catchment of a receiving watercourse/sewer.

2 The drainage design for the site should provide on-site flood protection from: Flooding from a watercourse. The drainage design should demonstrate that the development, including the SuDS system, is outside the functional floodplain and, wherever possible, outside the 1 in 100 year fluvial (or 1 in 200 year tidal) flood risk zone. Flooding from the drainage system. The drainage design should demonstrate that the SuDS will cater for the full range of storms up to the 100 year event(with a 30% allowance for climate change for all storms), without causing any significant unplanned flooding. The design should demonstrate that any blockage in the drainage system will be accommodated by suitable overflows, and exceptional rainfall by exceedance flow routing. When SuDS are overwhelmed by exceptional rainfall then exceedance routes protect people and property by providing unobstructed overland flow routes through the development and should be considered for all drainage schemes. Flooding from overland flows. The drainage design should demonstrate how exceptional flows generated within or from outside the site will be managed including overland flow routes, protection of buildings to prevent entry of water, and protection of major access routes including access to institutional buildings. The SuDS design standard SuDS design is to be based on a range of storm duration events with an annual probability of 1% (1 in 100 year storm event) with an allowance of 30% increase in predicted rainfall volume (due to climate change) for Southwest region (Ref. UKCIP). Under certain circumstances an alternative storm event may need to be catered for and other statutory authorities may need to be consulted. (Environment Agency, South West Water, etc.). The drainage design must ensure that the frequency and rate of discharge from the new development is, wherever possible, equal to the frequency and rate of discharge that would be discharged under equivalent greenfield conditions. Designs that consist solely of underground storage and an orifice plate/hydrobrake, are not in the truest sense a SuDS system as they do not mimic greenfield conditions. It must confirm the agreed greenfield surface water runoff rate and will take into account, where appropriate, the difference between the 1 in 1 year event, the 1 in 30 year event, and the 1 in 100 year event with a 30% allowance for climate change. The drainage design must ensure that the frequency and volume of surface water runoff from the new development is, wherever possible, equal to or less than the frequency and volume that would be discharged under equivalent greenfield conditions.(i.e. mimic greenfield conditions). It will confirm the storage volumes for the site taking into account: Interception storage Attenuation storage Long term storage.

3 Water Quality An appropriate management train of SuDS components should be implemented to effectively mitigate the pollution risks associated with different site users/activities. To remove the major proportion of pollution from surface water runoff it is necessary to: Capture and treat the surface water runoff from frequent, small events, and Capture and treat a proportion of the initial surface water runoff (the first flush) from larger and rarer events. Maintenance The design of each SuDS feature must take into account the day to day maintenance requirements, occasional tasks and long term design life elements of each structure. SuDS maintenance requirements for a development should be set out in a SuDS management plan that includes:- A SuDS overview A management statement to describe the SuDS scheme and set out the management aims for the site. It should consider how the SuDS will perform and develop over time anticipating any additional maintenance tasks to ensure the system continues to perform as designed. Specification notes that describe how work is to be undertaken and the materials to be used. A maintenance schedule describing what work is to be done and when it is to be done using frequency and performance requirements as appropriate. A site plan showing maintenance areas, control points and outfalls. Supporting documents. General Guidance: SuDS must be readily accessible via easements or sited within publically owned land. Siting of SuDS must take into account future maintenance and any foreseeable future developments which may affect or be affected by the SuDS. Where SuDS are to be sited on sloping ground the developer/engineer will have to prove that the outflowing water will not interfere with structures downslope or discharge at ground level and that the slope itself will not be affected. The ground slope downstream of the infiltration device should be no greater than 1:10. SuDS are not to be used in loose made ground/fill or where the introduction of water will adversely affect the soil structure (e.g. collapse, solution, heave etc.) Ground water quality must not be adversely affected by the use of SuDS. The EA should be contacted for further information regarding sensitivity of the site.

4 Adoption of SuDS When Schedule 3 of the Flood and Water Management Act 2010 is fully implemented, then the SAB will be required to adopt and maintain the approved SuDS that serve more than one property provided these Standards for SuDS are met. However, in the interim, there are no plans to adopt any SuDS. There are also no plans for the retrospective adoption of SuDS designed and built prior to the SAB being setup; however there is the possibility of these being considered on a site by site basis, dependant on the level of information provided. Full Application or Reserved Matters Application Requirements. The following lists give an idea of the types of information required to accompany a drainage application. The list is not exhaustive but gives a general indication of the information required. This will obviously vary on a site by site basis. Effectiveness Appraisal: 1. Flood Risk Assessment. 2. A description of the current surface water drainage arrangements for the site. 3. A location plan. 4. The proposed development layout drawing. 5. A description of the proposed surface water drainage system. 6. A brief summary of the SuDS incorporated with treatment stages highlighted. 7. The soil WRAP classification for the site. 8. Results of soil permeability testing. Impact Appraisal: 1. Calculations showing the pre and post development peak run off rate for the agreed critical rainfall event(s) using the ADAS Reference Book 345 Appendix 5, Institute of Hydrology Report 124, or R&D Technical Report W5-074-A Greenfield Runoff Revision D. 2. Design Storage calculations for the agreed control rates, and preliminary sizes of attenuation required on the site s drainage plans. 3. A survey of the conditions of agreed sections of existing downstream watercourses or culverts. CCTV information may be requested. Design Appraisal: 1. Design for Exceedance (flood routing) is required for all sites. Design must address the management of all excess water above the 1 in 100 year event plus 30% for climate change. The flood routing must take into account any existing or foreseeable future developments which may affect or be affected by the SuDS. 2. For design purposes the drained surface area must include all highway areas including road, footpath and any other adoptable impermeable surface which could contribute to water entering the SuDS.

5 Amenity Appraisal: 1. An assessment of the impact of the surface water proposals on habitats 2. The benefits for amenity from the proposed system Construction Appraisal: 1. Details of surface water runoff prevention measures during the construction phase. To the standards in C Details of protection of proposed SuDS during construction. 3. Provision of as-built drawings upon completion to the SAB. Maintenance Agreement: The Maintenance Agreement should include: 1. Confirmation of the maintenance responsibility for each of the drainage elements including who is responsible. 2. Lifespan, maintenance schedule and costs (annual and whole life costing) of SuDS. 3. Risk Assessments for each of the maintenance activities. 4. Method Statements for undergoing the routine maintenance.

6 Requirements for specific SuDS elements Rainwater Harvesting. Note: must not count towards attenuation volumes 1. Full design and supporting calculations. 2. Specification. 3. Pipework design layout and confirmation of system overflow arrangements. 4. Maintenance who has responsibility & the total life cost Infiltration Systems Infiltration systems should be designed in accordance with BRE Digest 365, Building Regulation 2000 Part H or BS EN752 Part 4 and Ciria C697 SuDS Manual. Where the CIRIA method is used for the calculation of the soakaway design the appropriate factor of safety based on the location of the SUDS should be used. The following will be required to accompany an application. The list is not exhaustive. 1. Full design, specification, details of management train/ treatment stages and supporting calculations including flood routing is to be supplied prior to approval. 2. Drawings indicating the proposed location of the infiltration system, conformation of the type of construction used and sectional drawings for embankments, pipework, flow control etc. 3. Plans showing topographical survey of the site, finished levels etc. Site Testing Regime 1. Percolation testing to determine soil infiltration rate to be carried out in accordance with BRE 365 Soakaway Design (2003) and CIRIA Report 156 Infiltration drainage manual of good practice (1996). Method of test must be relevant to proposed SuDS. 2. Testing must be carried out at or as near as possible to the proposed soakaway location (No greater than 25m from proposed soakaway for uniform subsoil conditions. For nonuniform subsoil conditions testing must be carried out at the location of the soakaway). 3. Testing must be carried out at the appropriate depth for proposed SuDS (e.g. invert level, base level of soakaway etc.) relative to existing ground levels. 4. Where more than one ring soakaway is required then testing must be carried out at each proposed soakaway location if they are sited more than 25m apart (10m intervals for non-uniform soils). Where a trench soakaway is used and is greater than 25m in length, testing must be carried out at a minimum interval of 25m (10m intervals for non-uniform soils). 5. Three percolation tests are to be performed at each trial pit location to determine the infiltration rate, where possible. Where slower infiltration rates are experienced, testing must be carried out over a minimum period of 24 hours (longer if 25% Effective depth is not reached). 25% Effective depth must be reached. Extrapolated test data will not be accepted.

7 General guidance for infiltration systems Maximum groundwater levels must be at least 1m below the base level of the proposed SuDS where infiltration is required. Maximum groundwater levels to be monitored and ascertained over a 12 month period prior to development. Where necessary the Environment Agency is to be consulted with regards to sensitivity of receiving aquifer. The location of trial holes/percolation tests must be carried out at, or as near to, the proposed soakaway location for the development. It should also be at the same depth/level as the proposed infiltration device. For any covered (below ground) infiltration device (including permeable paving) adequate monitoring devices must be provided to allow inspection with regards to functioning and performance. The outer edge of adjacent soakaways should be at least 10m apart to prevent them from interacting. Soakaways to be located 5m from any structure/ highway etc. Soakaways or similar infiltration devices are not to be used within the floodplain. Trench/ring soakaways or similar infiltration devices are not to be sited below the highway. The maximum depth of covered/ below ground infiltration devices will be 2-3m below the proposed ground level. Depths below 3m will be considered subject to maintenance issues. Permeable paving will be assessed on an individual site basis and will require additional special considerations including subgrade conditions, type/length of road, maximum gradient of 1 in 26 etc. At present Devon County Council would only consider small residential roads (such as cul-de-sacs) of approximately 50m length for permeable paving. Porous Asphalt pavements are not permitted at present.

8 Attenuation Systems. The following will be required to accompany an application. The list is not exhaustive and will obviously vary on a site by site basis. 1. Confirmation of the maximum discharge rate (litres/ second). 2. Full design, specification, details of the management train, treatment stages and supporting calculations including flood routing. 3. Plans showing topographical survey of the site, finished levels etc. 4. Drawing(s) indicating the proposed location of the attenuation system(s), confirmation of the type of construction used and sectional drawings for embankments, pipework, flow control etc. General guidance for attenuation For any covered (below ground) attenuation device (including permeable pavements) adequate monitoring devices must be provided to allow inspection with regards to functioning and performance. Where polypropylene cellular units (crates) are to be used as the storage device, the developer must provide information to prove the long term life of the units and that they are suitable for use for the site taking into account such matters as depth of soakaway etc. Such systems, where approved, must have BBA certification.

9 Further Guidance The SuDS manual (C697) Free: Follow this link Site handbook for the construction of SuDS (C698) Free: Follow this link Designing for exceedance in urban drainage: Good practice (C635) Free: Follow this link Interim Code of Practice for SuDS Free: Follow this link