Advice for the provision of surface water drainage systems for new developments

Transcription

1 1 Advice for the provision of surface water drainage systems for new developments Central Bedfordshire Council, April 2015 This guidance note if for anyone wishing to submit a planning application with surface water drainage implications. CONTENTS GUIDANCE FOR APPLICANTS... 2 WHAT IS A SURFACE WATER DRIANAGE STRATEGY... 2 VALIDATION REQUIREMENTS FOR MAJOR APPLICATIONS IN CENTRAL BEDFORDSHIRE... 2 COMPLIANCE WITH NATIONAL POLICY FOR SUDS... 3 OTHER GUIDANCE DOCUMENTS... 4 SATISFYING THE HIERARCHY OF DISCHARGE DESTINATIONS FOR SURFACE WATER... 4 PROVIDING EXSITING SITE INFORMATION... 4 WHAT IS ALLOWABLE DISCHARGE... 5 CONTROLLING DISCHARGE RATES... 5 CONTROLLING DISCHARGE VOLUMES & LONG TERM STORAGE... 5 TAKING ACCOUNT OF CLIMATE CHANGE AND URBAN CREEP... 6 WATER QUALITY... 7 DISPOSAL OF SURFACE WATER TO A PUBLIC SEWER... 7 DISPOSAL OF SURFACE WATER TO AN INTERNAL DRAINAGE BOARD (IDB) SYSTEM... 7 LAND DRAINAGE CONSENT FOR DISCHARGING TO A WATERCOURSE... 7 DESIGNING FOR SYSTEM EXCEEDENCE... 8 CONSTRUCTABILITY AND MAINTAINABILITY... 8 ACKNOWLEDGEMENT OF A VESTED DRAINAGE AUTHORITY... 8 LIKELY RECOMMENDATIONS FROM THE LLFA FOR PROPOSED SURFACE WATER SYSTEMS... 9 FURTHER INFORMATION AND GUIDANCE... 9 APPENDIX 1. REQUIREMENTS FOR OUTLINE MAJOR APPLICATIONS APPENDIX 2. REWUIRMEENTS FOR DETAILED OR FULL MAJOR APPLICATIONS 12 APPENDIX 3. REQUIREMENTS FOR SURFACE WATER MANAGEMENT ON MINOR APPLICATIONS ONLY... 14

2 2 GUIDANCE FOR APPLICANTS We expect all development will be sustainable and where appropriate contribute to the creation of infrastructure and communities that are safe from flooding for their intended lifetime through the use of Sustainable Drainage Systems (SuDS). It is essential that the type of sustainable drainage system for a proposed site, along with details of its extent/position, function and future management arrangements are identified at the design stage of a proposed development. This information will be required by the Local Planning Authority in a clearly labelled Surface Water Drainage Strategy at the time that an application is made. The following document provides a general overview of what a surface water drainage strategy should consider, this is not an exhaustive list and it intended as guidance only at the preliminary design stage. WHAT IS A SURFACE WATER DRIANAGE STRATEGY A Surface Water Drainage Strategy should demonstrates planning, design, construction and maintenance considerations for surface water management systems. This applies to both greenfield and previously developed sites and is in addition to a Flood Risk Assessment (FRA) (where this applies), the two documents will include similar details and should inform one another. A Surface Water Drainage Strategy may form an appendix to the FRA however for validation purposes should be submitted separately and clearly identified. Failure to do so may result in an application not being made valid. A strategy should be appropriate to the scale, nature and location of the development that is proposed. Detailed guidance on of what this should include are set out in the Appendix. CENTRAL BEDFORDSHIRE COUNCIL S VALIDATION REQUIREMENTS In order to be made valid, any major application submitted after the 15 April 2015 must include an appropriately detailed Surface Water Drainage Strategy.

3 3 For Central Bedfordshire Council purposes, a large scale major application can be defined as any of the following: A residential development of 200 or more houses A residential development on a site of at least 4.0 hectares Creation or change of use of a commercial development, where the floor space exceeds 10,000 square metres, or where the site area is 2.0 hectares or more in size. Minor developments (up to a maximum of 10 dwellings or 1,000m² of non-residential property) should still provide sufficient detail on surface water management and the use of SuDS must be prioritised. This should be based on the scale and nature of development, see Appendix 3 for more information. COMPLIANCE WITH NATIONAL PLANNING POLICY FOR SUDS Government policy set out in paragraph 103 of the National Planning Policy Framework (NPPF) expects Local Planning Authorities to give priority to the use of SuDS in determining planning applications. Where SuDS are used, it must be established that these options are feasible, can be adopted and properly maintained and would not lead to any other environmental problems. This is a material planning consideration for all major applications as of the 6 April The use, design and layout of SuDS should satisfy strategic requirements as set out in the Central Bedfordshire Sustainable Drainage Guidance (adopted April 2014). The document emphasises that surface water should be managed as close to source (where it falls) and on the surface wherever possible, and that the principles of the SuDS management train be applied. Wherever possible the passage of water between individual parts of the train should be considered through the use of natural conveyance systems (e.g. swales and filter trenches) in place of conventional pipework. This document should be read in conjunction with the National Planning Policy Framework, Planning practice guidance, Non-statutory technical standards for the design, maintenance and operation of sustainable drainage, and CIRIA s SuDS manual (CIRIA C697) for the detailed specification of features (see CIRIA s website for more information on this),

4 4 OTHER GUIDANCE DOCUMENTS The Environment Agency s Rainfall runoff management for developments Report SC when completing a drainage strategy for submission. Planning for SuDS making it happen (CIRIA C687) to guide the planning of a site. The CIWEM publication Planning advice for integrated water management. SATISFYING THE HIERARCHY OF DISCHARGE DESTINATIONS FOR SURFACE WATER Details of how the proposed development is going to dispose of its surface water will be needed (in accordance with the Rainfall runoff management for developments report SC030219). This should demonstrate compliance with the hierarchy of discharge destinations in the Building Regulations (Part H) and a viable solution to discharging surface water should be presented. Surface run off should be disposed of as high up the hierarchy as is reasonably practicable: 1. Into the ground (infiltration) and re-use, or then 2. to a surface water body, or then 3. to a surface water sewer, highway drain, or another drainage system, or then 4. to a combined sewer. This should be applied even where infiltration can only account for a proportion of the runoff from the design event. For this purpose the suitability of the subsurface for the installation of infiltration sustainable drainage systems (SuDS) should be fully assessed along with any other potential site constraints or hazards. Site specific tests should be provided for detailed proposals to demonstrate permeability and the suitability of either infiltration, combined or attenuation systems (in accordance with Building Research Establishment Digest BRE 365). PROVIDING EXSITING SITE INFORMATION Applicants should find out how the site currently drains and if there are any site constraints or hazards which will affect the proposed development and management of surface water.

5 5 Evidence of this type of assessment should be given in the Surface Water Drainage Strategy. The Council s Lead Local Flood Authority (LLFA) may be able to provide drainage information to assist with the preliminary assessment of a site, see our website for details. Water Authorities, Local Authority s drainage or highways team may also have some information. CONTROLLING DISCHARGE RATES Existing Discharge Rates and Volumes will need to be provided within a Surface Water Drainage Strategy as a comparison to the proposed. The allowable discharge from the positively drained impermeable areas of a site should be based on the greenfield run off rate for the developable area only. Otherwise the run-off from permeable areas that will continue to drain naturally will be double counted, which in turn will result in an overall increase in run-off and therefore an increase in flood risk. To mitigate for climate change the volume discharged from the site should be no higher than the 1 in 100 storm event for the pre-developed site. To meet greenfield discharge rates, water needs to be stored/attenuated so it can be released slowly over the same time period as before the site was developed. Even when hard standing area stays the same, more storage will be needed because of the effects of climate change. Remember, the Attenuation Volume (attenuates rates) is different from the Long Term Storage Volume (attenuates volumes) and both need to be calculated. Attenuation storage aims to limit the rate of runoff into the receiving system to that which takes place before the site is developed or redeveloped. A practicable minimum limit on a discharge rate from an attenuation device will often be a compromise between attenuating to a satisfactorily low flow rate while keeping the risk of blockage to an acceptable level. CONTROLLING DISCHARGE VOLUMES & LONG TERM STORAGE Discharge volumes on developed sites can be up to 10 times greater than when the site was undeveloped. To meet existing discharge volumes, the difference between existing and proposed volumes of water should not be discharged off site. This volume is referred to as the Long Term Storage Volume.

6 6 Controlling discharge volumes is important because: On developed sites, more of the volume is released more quickly and Even if the volume was released over the original, longer time period, downstream watercourses have limited storage. More water will mean more flooding. Even where rates are maintained, the peak rate lasts longer due to increased volumes leaving the site. Even when hard standing area stays the same, the volume of surface water will increase because of the effects of climate change. Options for dealing with the Long Term Storage Volume include: Providing infiltration to deal with the extra volume (where ground conditions allow), or Limiting the extra volume to 2l/s/ha (i.e. trickle discharge) and provide a storage area, or Limiting the extra volume to 2l/s/ha (i.e. trickle discharge) and provide a storage area. TAKING ACCOUNT OF CLIMATE CHANGE AND URBAN CREEP The NPPF provides advice on the impact of climate change. Paragraph 68, Part 4, of the Planning Practice Guidance indicates the following increases in peak rainfall intensity should be applied: Development still in existence by 2085 (residential) 30%. Developments with a life expectancy which ends prior to 2085 (commercial) 20%. Urban creep is now an acknowledged issue which results in an increase in runoff over time. An allowance should be made by factoring the impermeability percentage by 1.1 (10% increase) unless a more precautionary requirement is specified by the Local Planning Authority.

7 7 WATER QUALITY SuDS units should be used to achieve water quality improvements and amenity benefits as well as achieving compliance to these hydraulic criteria. Best practice in achieving water quality protection should be used and care taken to ensure SuDS will not pollute groundwater or mobilise contaminated material. This should be shown to link to the delivery of wider environmental and social objectives of the development site. DISPOSAL OF SURFACE WATER TO A PUBLIC SEWER Before disposal of surface water to the public sewer is considered all other options set out in Approved Document Part H of the Building Regulations 2010 should be exhausted. When no other practicable alternative exists to dispose of surface water other than the public sewer, the Water Company or its agents should confirm that there is adequate spare capacity in the existing system taking future development requirements into account. The Water Company will be the responsible body for reviewing and approving the surface water drainage plan in this instance. DISPOSAL OF SURFACE WATER TO AN INTERNAL DRAINAGE BOARD (IDB) SYSTEM The Bedford Group of Internal Drainage Boards will need to review and approve the surface water drainage plan and proposed discharge rates in this instance, their local drainage byelaws may apply. LAND DRAINAGE CONSENT FOR DISCHARGING TO A WATERCOURSE Where the design or construction works are likely to affect a watercourse Land Drainage consent may be needed (e.g. to carry out work in, over, or adjacent to a watercourse, or which will likely affect the current flow of water in an ordinary watercourse). For a main river this should be checked with the Environment Agency. For a non-main river (an ordinary watercourse ) please check with the Bedford Group of Internal Drainage Boards as to what may be needed. Where an existing sewer will be affected please contact the appropriate Water and Sewerage Undertaker.

8 8 Please note that an application to culvert a watercourse will not normally be acceptable unless proven that there is no reasonably practicable alternative, or if the detrimental effects of culverting would be so minor that they would not justify a more costly alternative. DESIGNING FOR SYSTEM EXCEEDENCE No flooding of property should occur as a result of a 1 in 100 year storm event (including an appropriate allowance for climate change) on the proposed development, For rainfall events with a return-period in excess of 30 years, surface flooding of open spaces such as landscaped areas or car parks is acceptable for short periods, the layout and landscaping of the site should aim to route water away from any vulnerable property, and avoid creating hazards to access and egress routes. Measures to convey and store exceedance flow should be demonstrated, making best use of the existing urban area through minor topographical changes, for example those made to the profile or a highway, footpath or kerb. Further guidance can be found in the CIRIA publication (Designing for exceedance in urban drainage - good practice (C635). CONSTRUCTABILITY AND MAINTAINABILITY In the early stages of the site design, consideration should be given to how the drainage system will be adopted and maintained in the future. This will likely influence the design just as much as the technical considerations. The final submitted design to manage surface water will need to take account of the construction, operation and maintenance requirements of both surface and subsurface components of the system. This should allow access by personnel, vehicle or machinery as required to undertake this work. ACKNOWLEDGEMENT OF A VESTED DRAINAGE AUTHORITY Where necessary we ask that details be provided from: The sewerage undertaker where a connection with a public sewer is proposed. The Environment Agency if the drainage system directly/indirectly involves the discharge of water into a main river.

9 9 The highway authority for an affected road or highway drainage. The Canal and River Trust if the drainage system directly/indirectly involves the discharge of water into or under a waterway managed by them. The Internal Drainage Board, if the drainage system directly or indirectly involves the discharge of water into an ordinary watercourse (within the meaning of section 72 of the Land Drainage Act 1991) or an Internal Drainage Board s main drain. LIKELY RECOMMENDATIONS FROM THE LLFA FOR PROPOSED SURFACE WATER SYSTEMS No development should take place until details of the design, implementation, maintenance and management of a surface water drainage scheme based on sustainable drainage principles and an assessment of the hydrological and hydrogeological context of the development, have been submitted to and approved in writing by the Local Planning Authority. Clear details of the maintenance and/or adoption proposals of all element of the proposed drainage system must be provided prior to the commencement of the development/granting of planning permission. The surface water drainage scheme should subsequently be implemented in accordance with the approved as submitted to and agreed in writing by the Local Planning Authority before the development is completed. FURTHER INFORMATION AND GUIDANCE Applicants are strongly advised to discuss their proposals with the ouncil at the preapplication stage to ensure that an acceptable SuDS scheme is submitted. You should contact Central Bedfordshire Council directly for details of the pre-application process.

10 10 APPENDIX 1. REQUIREMENTS FOR OUTLINE MAJOR APPLICATIONS At outline the following should be addressed by the Surface Water Drainage Strategy at a minimum, this is not an exhaustive list and additional detail may be required depending on the scale and nature of development. 1. Site information relating to the proposed development and the hydrological and hydrogeological context, details of the developable area and existing/proposed impermeable areas. 2. Details of the existing and proposed runoff destination and discharge points. Justification of a drainage method should be given in line with the hierarchy set out in Part H of the building regulations (March 2015), this might also include consideration for any consent needed from a drainage body. 3. Details of the existing and proposed peak flow rate & discharge rates. Include details of the difference between the existing and proposed impermeable areas, including estimated surface water run-off from the site before and after development. This is usually supported by calculations for various critical storm seasons and durations (i.e. QBAR, 1 in 1 yr, 1 in 30 yr, 1 in 100 yr, 1 in 100 yr, and 1 in allowance for climate change). 4. Details of the existing and proposed discharge volumes. This should be shown for critical storm seasons and durations used above and a method given for mitigating any increase in discharge volumes. 5. Overview of the water quality hazard created by the proposed development and proposed mitigation, with any measures or works required off-site. 6. Construction and maintainability of SuDS. Including the management and maintenance of the SuDS so it continues to meet the requirements (currently in the draft National Standards) for the lifetime of the development and the arrangements for adoption by any public authority or statutory undertaker and any other arrangements to secure the operation of the scheme throughout its lifetime.

11 A (non-technical) summary of the SuDS features recommended, including a layout showing where SuDS infrastructure will be locate on the site. 8. Ensure that plans and drawings submitted with the application show the location of the chosen SuDS. Please note it is likely that an outline planning permission will have a condition(s) attached requiring the submission of more detailed drainage information which must be approved before the development can commence.

12 12 APPENDIX 2. REQUIREMENTS FOR DETAILED OR FULL MAJOR APPLICATIONS In addition to the requirements for an outline application, the Surface Water Drainage Strategy at a minimum should address the following, this is not an exhaustive list and additional detail may be required depending on the scale and nature of development. 1. Detailed information relating to the site and site investigation results. Each site should be evaluated on its own merits by undertaking comprehensive assessment in line with the code of practice for site investigations (BS 5930: 1999) i.e. to identify topography, site levels and flwo paths; hydrological context including rainfall, surface water drainage network, flood risks, local water features, aquifers, source protection zones (SPZ), and groundwater levels; underlying geology, soil types, permeability and infirltration rates; proximity to a sewer network, Internal Drainage Board area, conservation area or other designated site. 2. Confirmation of the existing and proposed impermeable areas of the site and final SuDS design with calculations. These should be based upon the national SuDS guidance to demonstrate: Conformity with the design criteria for the site for peak flow, volume control and greenfield runoff, and/or brownfield runoff where appropriate. pre-development (greenfield or brownfield as relevant) and post-development runoff rates, Confirmation of final storage volumes and flow control rates. Critical storm duration and associated storage estimates to determine the scale (and associated land take) of conveyance and storage structures. management for exceedance including flow routes both on and off site in the event of system exceedance or failure. 3. Design principles and final details for propriety products and flow controls. i.e. what measures are to be used and how they work in sequence, including any proposed attenuation and flow control measures. Operational characteristics of any mechanical features, including maintenance and energy requirements, should be given with justification of any pumping needed for the operation of the system. Consider

13 13 provision for community engagement and integration of the drainage system with public space and contribution to the over all ecological value of the site. 4. Finalised management details. i.e. reasonable operating and maintenance requirements of the drainage system and arrangements for adoption or other arrangements to secure the operation of the scheme throughout its lifetime details of the of the system. 5. Finalised plans/detailed drawings and labels to demonstrating the proposed surface water drainage system with appropriate labels. i.e. including location, levels, gradients, dimensions, and pipe reference numbers, and long sections and cross sections for the proposed drainage system, flow paths and flooded areas for exceedance measures, existing and proposed site sections and site levels, proposed split of the surface water management systems between private (i.e. within curtilage) and public (i.e. in public open space and/or highway). 6. Details of the construction of the drainage system (and phasing plan where needed). i.e. with details of any pollution prevention measures to be used during the construction phase of any development, temporary drainage during construction, protection of SuDS against construction impacts such as compaction including any diversions, erosion control, etc., workmanship or materials, planting & landscaping (if proposing vegetated SuDS), refurbishment of existing culverts and headwalls or removal of unused culverts where relevant. Please note as built drawings of the entirety of the drainage system will be requested to be provided upon completion of the site.

14 14 APPENDIX 3. REQUIREMENTS FOR SURFACE WATER MANAGEMENT ON MINOR APPLICATIONS ONLY Small developments (up to a maximum of 10 dwellings or 1,000m² of non-residential property) should provide sufficient detail, based on the scale and nature of development, including but not limited to: Site constraints which affect the proposed development and management of surface water, including and existing means of drainage etc. Clear descriptions of any changes to permeable and impermeable areas. Estimated surface water run-off from the site before and after the proposed development. The increase of surface water run-off as a result of any new impermeable surfaces associated with the development (this will include roofs that are not green or brown roofs, and hard surface at the ground level) and the volume of attenuation storage required (m 3 ). Different options and recommendation for SuDS, in relation to the proposed site layout and topography, which can be implemented to reduce both the volume and the speed of water run-off to the drainage system. Provide details on the function, operation and long term approach to maintenance of SuDS solutions chosen for the development. Details of any consultation undertaken with regulators where appropriate Ensure that plans and drawings submitted with the application show the location of the chosen SuDS. This information must be submitted along with the planning application. SuDS solutions that are chosen will need to be reflected in the drawings submitted with the planning application. A solution should be designed by a suitably qualified professional. The following types of development may be exempt but should include a statement with the application explaining why, these are: If the development will not lead to an increase in impermeable surfaces and surface water run-off, then SuDS are not required.

15 15 Basements: which do not extend beyond the above ground structure into the garden and there is less than 1m of permeable ground cover (soil) above the basement there is not less than 1m of permeable ground cover (soil) above the basement, or when the ground above the basement is not altered from permeable to impermeable. Please note additional details may be required by the Local Planning Authority. Contact us by telephone: by on the web: Write to Central Bedfordshire Council, Priory House, Monks Walk, Chicksands, Shefford, Bedfordshire SG17 5TQ