Surface Water Guidance for Developers

Transcription

1 Surface Water Guidance for Developers Contents Overview of Sustainable Drainage Systems (SuDS)... 2 Information Required in a Flood Risk Assessment or Drainage Strategy... 3 Outline Planning Applications... 3 Full, Detailed, or Reserved Matters Applications... 5 Adoption and Maintenance of SuDS features Assessment of whether SuDS are inappropriate Further notes and points to consider for the detailed design of site drainage Recommended Information, Advice and Guidance: Contact Details

2 Overview of Sustainable Drainage Systems (SuDS) Surface water run-off should be controlled as near to its source as possible through a sustainable drainage approach to surface water management (SuDS). SuDS are an approach to managing surface water run-off which seeks to mimic natural drainage systems and retain water on or near the site as opposed to traditional drainage approaches which involve piping water off site as quickly as possible. SuDS involve a range of techniques including soakaways, infiltration trenches, permeable pavements, grassed swales, ponds and wetlands. SuDS offer significant advantages over conventional piped drainage systems in reducing flood risk by attenuating the rate and quantity of surface water run-off from a site, promoting groundwater recharge, and improving water quality and amenity. The variety of SuDS techniques available means that virtually any development should be able to include a scheme based around these principles. This should not be a piecemeal use of a few techniques. A fully integrated system is essential. Some SuDS options could require significant land take so it is essential that they are considered early on in the design process. SuDS solutions are also available for high density urban environments where space is at a minimum. It can be difficult to incorporate some options once the detailed development design is underway. Figure 1 shows the SuDS hierarchy with the most sustainable solutions at the top of the table. Figure 1 SuDS Hierarchy (Source: Environment Agency Thames Region, 2006, SuDS A Practical Guide) 2

3 Information Required in a Flood Risk Assessment or Drainage Strategy The following information should be provided for every flood risk assessment and drainage strategy submitted to Northamptonshire County Council for consideration. This has been split into the level of detail that would be expected as Outline and at Full/Detailed/Reserved Matters. Outline Planning Applications Non-technical summary of the proposed drainage strategy. Description of the type of development proposed and where it will be located. Include whether it is new development, an extension to existing development or change of use etc. We also require the area of the development site itself, how much of the site is currently hard standing, and the proposed area to be hard standing post-development. A location plan at an appropriate scale should be provided with the application, showing site outline and other adjacent land under the applicant s control. Assessment of all existing flooding risks to the site. This should include groundwater, overland surface water flows, sewer flooding, infrastructure flooding (from reservoirs/ponds/canals), watercourse flooding and the risk from the proposed development on site surface water drainage system. Northamptonshire County Council provide responses to data requests which include flood related data such as records of historic flooding, which can be requested through the forms at Explanation of how each of these flood risks will be mitigated. This may require modelling of some sources where significant flood risk is shown on high level datasets. It might mean applying the sequential approach to the site by avoiding building on one part of the site where there is known flooding and/or raising finished floor levels. Plans. Plans showing the existing site layout, its topography and how it currently drains will be required. Plans should also be provided of the proposed layout if available and demonstration that the proposed drainage system and other mitigation measures are achievable and that adequate space has been made for water. Explanation of how the drainage discharge hierarchy has been followed, providing evidence why any are inappropriate: - Firstly, to infiltration/soakaway - Secondly, to a watercourse or highway ditch (with permission) 3

4 - Thirdly, to a surface water sewer or highway drain (with permission) - Lastly, to a combined sewer (with permission) Evidence that the site has an agreed point of discharge. - If a significant portion of surface water is to be infiltrated on site, provide a BRE365 infiltration assessment to prove that this will effectively work. At outline stage it may be acceptable to base infiltration values on typical values for the local geology, as long as an alternative drainage design and agreed point of discharge is provided should infiltration rates prove to be unsuitable. - If discharge is to an ordinary watercourse, evidence will need to be provided to ensure that the system can accept the proposed flows to an acceptable downstream point without increasing risk to others. If the watercourse is not within the boundary of the site, evidence will be required that the developer has a right to cross 3 rd party land. - If discharge is to a surface water or combined sewer, or highways ditch or drain, permission in writing will need to be provided from the undertaker of the sewer/drain stating its capacity and the allowable peak discharge rates. - A site may already have a recognised right of discharge for its surface water, but there will still be a requirement to demonstrate that the development will not increase risk to others and will provide betterment by reducing risk where possible. Calculations of current runoff from site. For greenfield sites, existing runoff rates and volumes can be produced through the UK SuDS website If the site is previously developed and a piped drainage system already exists within the site, the existing capacity of these pipes will need to be estimated. Clearly state the existing impermeable area of the site for brownfield sites. Calculations of proposed runoff from site. Clearly state the proposed impermeable areas for the site and how this compares to existing values. It can be assumed that 5l/s/ha should be delivered for greenfield sites for the QBAR event. If the development is located within the Borough of Corby, this should be reduced to 2l/s/ha. Previously developed/brownfield sites should incorporate a minimum of a 20% reduction of the existing site runoff for commercial development, or a 30% reduction for residential development, to mitigate for the impacts of climate change. A calculation of storage volume will be required on site for the 1% (1in100) 1 plus climate change case, bearing in mind the controlled discharge rate. Both 1 Within the Upper Nene catchment this should be the 0.5% (1in200) plus climate change case 4

5 existing runoff rates and volumes and a rough estimation of storage can be produced through the UK SuDS website Plans showing a logical location of storage within the proposed development. It should be noted that attenuation storage areas will not work at the highest point on the site. Attenuation storage within areas at risk of flooding will not be acceptable. Explanation of likely forms of SuDS for the site and justification for their use. If no SuDS methods are proposed then evidence will need to be provided as to why they are not appropriate for the site. Explanation of who will maintain and fund the maintenance of the proposed system over the lifetime of the development and evidence that access will be physically possible to carry out that maintenance, without entering others land. Ideally, SuDS features should be located within public space. Phasing. An explanation of how the site will adequately consider flood risk at all stages of the development. Full, Detailed, or Reserved Matters Applications Non-technical summary of the proposed drainage strategy. Description of the type of development proposed and where it will be located. Include whether it is new development, an extension to existing development or change of use etc. We also require the area of the development site itself, how much of the site is currently hard standing, and the proposed area to be hard standing post-development. A location plan at an appropriate scale should be provided with the application, showing site outline and other adjacent land under the applicant s control. Assessment of all existing flooding risks to the site. At the detailed stage, a reasonable assessment should have been made of the actual risk to the site from all sources of flooding. As well as making reference to the local Strategic Flood Risk Assessment, the applicant should contact the Northamptonshire County Council for pre-application advice, which can be requested through the forms at - Surface water If the ufmfsw is felt to be accurate this can be used to assess the level of surface water flood risk to the site. If ufmfsw is disputed, or considered inaccurate, we would expect the developer to model the expected flows and use the results to determine the level of risk to the site. - Groundwater typically a geotechnical report to cover this. 5

6 - Canals Normally a letter from the Canal and River Trust stating that there is no risk, otherwise modelling of potential overtopping or breach. - Reservoirs Typically using the EA inundation maps to determine local level of risk. If inundation extent is disputed, may require modelling by developer. - Sewer Typically a letter or model report from the Water Company. - Fluvial (main river or ordinary watercourse) The EA have produced indicative floodplain modelling of most main rivers and some ordinary watercourses. If the developer wishes to obtain that modelling or dispute it, they will need to correspond with the EA in relation to this matter. The EA will make decisions on fluvial flooding issues. If only approximate modelling is available for an ordinary watercourse and it is felt to be inaccurate or is disputed, the developer will be required to model such flooding accurately to ensure their development is safe. Explanation of how each of these flood risks will be fully mitigated. This could require detailed modelling of some sources where significant risk is shown on high level datasets. It might mean applying the sequential approach by avoiding building on one part of the site where there is known flooding. Examples of mitigation measures (note: this list in not exhaustive): - Setting minimum floor levels of the development; - Utilising the sequential approach by locating more sensitive development out of the floodplain that affects the site; - Works to improve/divert infrastructure to eliminate risk; - Proposals to route flood flows through a development so they do not adversely affect the development; - Avoiding the use of below-ground development or basements adjacent to areas of flood risk unless they are designed for flood storage; - Setting residential development 150mm above the adjacent ground level. Detailed Drainage Plans for the proposed development. Showing the location of storage within the proposed development and how these relate to submitted calculations (including any manhole and pipe numbers that are referenced in Micro Drainage reports). It is likely that separate, more Detailed Engineering Plans will be needed for each of the SuDS and critical drainage elements. The methods of flow control must be detailed, as should nonconventional elements such as ponds, swales, permeable paving etc. Full explanation of the forms of SuDS used on the site. Including justification for their use, what flood mitigation, water quality, environmental and social benefits they might achieve. Modelling of the proposed SuDS 6

7 system for the site (typically Micro Drainage), showing the behaviour of the site for the main rainfall events as described above and ensuring: - Typical operation of the system for low rainfall and first-flush events, with indication of how treatment of surface water will be achieved. - No above ground flooding for any conventional element of the system for the 3.3% (1in30) event. - No flooding from the system to downstream property for the 1% (1in100) 2 plus climate change event. Explanation of how the drainage discharge hierarchy has been followed, providing evidence why any are inappropriate: - Firstly, to infiltration/soakaway - Secondly, to a watercourse or highway ditch (with permission) - Thirdly, to a surface water sewer or highway drain (with permission) - Lastly, to a combined sewer (with permission) Evidence that the site has an agreed point of discharge. At the detailed stage we would expect: - If surface water will be infiltrated on site, provide a BRE365 or CIRIA guidance R156 infiltration assessment and detailed calculations for the particular infiltration system. - If discharge is to an ordinary watercourse, evidence that the system can accept the proposed flows to an acceptable downstream point without increasing risk to others. Analysis of the effects on the drainage system if the outfall is likely to be surcharged during flooding events. Consent may be required for works that affect flows in the receiving watercourse. - A letter of confirmation from the Water Company or responsible body (stating their required discharge rate) if discharge is to a sewer or canal or highway drain. 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. Calculations of current runoff from site. Calculated runoff rates for the existing site for the following rainfall events: QBAR, 3.3% (1in30), 1% (1in100) 3 and, 1% (1in100) 3 plus climate change. A range of rainfall events 2 Within the Upper Nene catchment this should be the 1in200 event 7

8 should be assessed and the critical duration rainfall event selected for each case. For greenfield sites, the methodology in the EA/Defra document Preliminary Rainfall Runoff Management for Development (W5-074/A/TR1) should be used as the basis for calculations. For brownfield sites, clearly state the existing impermeable area and determine the capacity of any existing drainage system. Calculations of proposed discharge from site. Explanation of methodology and calculation. Clearly state the proposed impermeable area of the development and how this compares to the existing site. Use the calculation of current runoff to decide discharge rates on the following basis: - Greenfield sites should discharge at a maximum of the equivalent rate so that the site behaves like the original greenfield across the range of events. - Brownfield sites are strongly encouraged to discharge at the greenfield rate wherever possible. As a minimum, brownfield sites should reduce the discharge by 20% (for commercial development) or 30% (for residential development), to account for the impacts of climate change, from the existing site runoff OR from the original un-surcharged pipe-full capacity of the existing system, whichever is the lowest. - Developers have the option to install a complex discharge control which reflects the original discharge or run-off rates from the site across the range of storm events. E.g. QBAR, 3.3% (1in30), 1% (1in100) 3, 1% (1in100) 3 plus climate change; OR they should only limit discharge for all events to the flow predicted by the QBAR event. Using complex controls is more expensive but reduces the amount of attenuation storage required on the site and is probably worth doing on larger sites. Please note that for developments within the Borough of Corby, runoff from the site for all design events should be limited to 2l/s/ha. - Based on the existing and proposed discharge cases calculated as above, the applicant should now have detailed calculations of storage volume required on site for the 1% (1in100) 3 plus climate change case. Calculations of storage volume that will be required on site for the 1% (1in100) 3 plus climate change case, bearing in mind the controlled discharge rate. Plans should be provided clearly identifying where this storage will be provided, and the water level within each element for the design storm events. Storage elements should be designed to empty sufficiently within 24 hours to be able to accommodate 80% of the 10% (1in10) storm runoff. Infiltration design. Where any discharge to ground by infiltration is proposed, details of the infiltration system will be required. Full infiltration testing results 3 Within the Upper Nene catchment this should be the 1in200 event 8

9 are required along with a summary of the infiltration rate taken for each infiltration element. Infiltration elements should be designed to empty sufficiently within 24 hours to be able to accommodate 80% of the 10% (1in10) storm runoff. Residual Risk. As well as the consideration of the modelled events above, there should be a qualitative examination of what would happen if any part of the system fails, demonstrate that flood water will have flow routes through the site without endangering property and where possible maintaining emergency access/egress routes. Designing for exceedence. For events with a return-period in excess of 3.3% (1in30), surface flooding of open spaces such as landscaped areas or car parks is acceptable for short periods, but 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 (further guidance in CIRIA publication C635 Designing for exceedence in urban drainage - good practice). No flooding of property should occur as a result of a 1% (1in100) 4 storm event (including an appropriate allowance for climate change). In principle, a well-designed surface water drainage system should ensure that there is little or no residual risk of property flooding occurring during events well in excess of the return-period for which the sewer system itself is designed. This is called designing for exceedence. The CIRIA publication `Designing for exceedence in urban drainage-good practice' can be accessed via the following link: If the drainage system has been designed to allow flooding on site is during the 1% (1in100) 4 storm event (including an appropriate allowance for climate change), provide a plan clearly identifying where this flooding will occur. Explanation of who will maintain and fund the maintenance of the proposed system over the lifetime of the development and evidence that access will be physically possible to carry out that maintenance, without entering others land. Ideally, SuDS features should be located within public space and a maintenance manual be produced to pass to the future maintainer. Phasing. Explanation of how the site will adequately consider flood risk at all stages of the development. Avoiding interim developed phases that are unprotected. Phases can only progress if adequate flood mitigation measures are in place for that particular phase. This should avoid one small phase of the site being allowed to discharge at the calculated rate for a larger part of the entire development. Adequate flood risk measures for each individual phase should be able to stand alone, (until the entire site is completed), 4 Within the Upper Nene catchment this should be the 1in200 event 9

10 without themselves being at flood risk and without increasing flood risk for other parties. Adoption and Maintenance of SuDS features In order to discharge any condition imposed relating to the adoption or ownership and maintenance of SuDS features, we would expect the following: Clear statement of who will own or adopt each element of the drainage, along with signed agreements from all parties. Clear statement of who will undertake the maintenance of each element of the drainage, along with signed agreements from all parties, and confirmation that access will be possible for the maintainer of each drainage element. Schedule detailing how each element of the drainage should be maintained and how often this maintenance will need to be undertaken. Assessment of whether SuDS are inappropriate Under the revised planning guidance, SuDS must be included in any new development unless demonstrated inappropriate. If a traditional (below-ground piped) drainage system is proposed over a sustainable drainage system, the onus is on the applicant to provide evidence to demonstrate that SuDS would be inappropriate for the development. If we are not satisfied with the evidence provided, we may object to the proposed traditional drainage system. Further notes and points to consider for the detailed design of site drainage Consideration should be given to sub-catchments which may exist on the site. Calculations for greenfield runoff rates for individual watercourses should be based on the proposed area of impermeable land within its sub-catchment. It may be possible to divert water to a different sub-catchment only if the greenfield runoff rate for that watercourse is not exceeded. Discharge into any watercourse will need to consider whether the velocity of the discharge causes any erosion impacts on the channel. If any land raising is proposed on site, the surface water strategy should consider the effect this could have on runoff leaving the site and affecting third parties. Appropriate solution(s) to prevent this will need to be assessed. The critical storm duration for each element of the drainage network should be determined by considering a range of summer and winter storm durations from 15 minutes up to the minute (7 day) duration. FSR (Flood Studies Report) rainfall data should only be used for critical storm durations less than 1 hour and FEH (Flood Estimation Handbook) rainfall data should be used for storm durations greater than 1 hour. 10

11 Consideration should be given to the likelihood and impact of surcharged outfalls. The standard default setting of many surface water computer modelling programmes assumes a freely discharging outfall. Careful consideration is required and evidence provided to demonstrate that this assumption is correct. In many circumstances an outfall maybe surcharged affecting its hydraulic capacity and impacting on the surface water network. A surcharged outfall is likely to occur if discharging into a watercourse or surface water network near capacity. In these scenarios and with the absence of supporting information to the contrary it is expected the surface water calculations will assume a surcharged outfall. If computer modelling software has been used suitable model files must be submitted on a CD, or full printouts of model inputs and results provided, in order to accurately assess the modelled information. Recommended Information, Advice and Guidance: National Planning Policy Framework and associated guidance Planning Policy Statement 25: Development and Flood Risk and the accompanying Practice Guide Non-Statutory Technical Standards and associated Guidance CIRIA 523 (SuDS Best Practice Manual) CIRIA 609 (SuDS hydraulic, structural and water quality advice) CIRIA 697 (SuDS Manual) CIRIA R156 (Infiltration Drainage Manual of Good Practice) SusDrain Contact Details For enquiries relating to planning applications, contact can be made with the Surface Water Drainage Team at swdrainage@northamptonshire.gov.uk. For pre-application enquiries to request flood related data, such as records of historic flooding, or for formal pre-application advice, please use the forms available at It may be necessary to apply for a Flood Defence Consent for the construction of certain works relating to ordinary watercourses, therefore please review the following information: 11