SUSTAINABLE SURFACE WATER DRAINAGE. Supplementary Planning Guidance No.17

Transcription

1 SUSTAINABLE SURFACE WATER DRAINAGE Supplementary Planning Guidance No.17

2 LONDON BOROUGH OF CROYDON

3 Prepared by the Planning and Transportation Department, Croydon Borough Council. For a large print version of this document or any other enquiries please direct to the Policy & Strategy Group, Printed 14 July 2004 This guidance is non-statutory planning guidance, which supports various policies in the Second Deposit Draft Replacement Unitary Development Plan- Revised Draft Croydon Plan (2003). Only the policies in the Development Plan can have the special status afforded by section 54A of the Town and Country Planning Act 1990 (as amended), in deciding planning applications. However, the Government advises that supplementary planning guidance (SPG) may be taken into account as a material consideration, the weight accorded to it being increased if it has been prepared in consultation with the public and has been the subject of a Council Resolution.

4 This SPG has undergone public consultation and was adopted by the UDP Cabinet Sub- Committee on 12 th July 2004.

5 Sustainable Surface Water Drainage Supplementary Planning Guidance No.17 Contents 1. Introduction AIMS OF THIS SUPPLEMENTARY PLANNING GUIDANCE (SPG) TOWARDS SUSTAINABILITY: ENVIRONMENTAL CONTEXT PLANNING POLICY FRAMEWORK Planning Policy Guidance Note 25: Development and Flood Risk (PPG25: 2001) Regional Planning Guidance for the South East (RPG9: March 2001) The London Plan (February 2004) Building Regulations CROYDON UNITARY DEVELOPMENT PLAN (UDP) Sustainable Drainage WHAT IS SUSTAINABLE DRAINAGE? BENEFITS OF SUSTAINABLE DRAINAGE SYSTEMS (SUDS) Application Process PERMITTED DEVELOPMENT IMPLICATIONS FOR DEVELOPMENTS WHAT WILL COUNCIL WANT TO KNOW ABOUT? RESPONSIBILITIES OF OWNERS Sustainable Drainage Methods NATURAL PROCESSES Natural Drainage and Waterways The Urban and Engineered Environment WHERE SUDS MAY BE INAPPROPRIATE? SUSTAINABLE DRAINAGE TECHNIQUES Prevention Green Roofs Soakaways Permeable Footpaths and Car Parks Swales and Vegetation Filter Strips Infiltration Trenches Retention Ponds and Basins Further Information: REGULATIONS USEFUL CONTACTS...24

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7 1. Introduction 1.1 AIMS OF THIS SUPPLEMENTARY PLANNING GUIDANCE (SPG) This SPG is one of a series prepared by the Council to update and supplement the UDP policies and provide detailed practical guidance on their implementation. As SPG, these guidelines become a material consideration when determining planning applications and are taken into account when deciding whether to grant planning permission. The objectives of this SPG are: To contribute to the current and future use of sustainable drainage within Croydon Borough. To encourage and increase the widespread use of sustainable drainage systems in all developments as an alternative to traditional piped drainage systems. To promote the benefits of using sustainable drainage systems. 1.2 TOWARDS SUSTAINABILITY: ENVIRONMENTAL CONTEXT The expansion and development of urban areas in Croydon and continued reliance on traditional piped drainage systems for collecting surface water runoff from hard surfaces (e.g. roofing and hard paving) is placing additional pressure on natural water systems, and in particular, water absorption into the ground This impairment of the process of natural water systems has resulted in the following adverse effects: The reduction of water absorption affects the recharge of groundwater reserves and those feeding into waterways. Impermeable surfaces increase the volume of surface water collected, and which must then be drained away through the piped system. This in turn, increases the reliance on piped drainage and the overall risk of flooding. Water released from piped drainage systems increases the volume and rate of flow in waterways, especially at points of source discharge. This accelerates erosion and causes siltation of waterways. Pollutants and rubbish make their way into local waterways through piped drainage systems 1

8 Impermeable surfaces collect and concentrate a range of contaminants such as oil, heavy metals and organic matter. The cumulative discharge of these materials to piped systems particularly after heavy rain and in turn groundwater and rivers can result in poor water quality affecting their overall ecological function. The realisation of the damaging effects of surface water runoff has lead to the search for alternative management solutions, and sustainable drainage methodologies have been identified through research as a practical solution with many surprising benefits. This Supplementary Planning Guide (SPG) for Sustainable Surface Water Drainage, provides the national and local policy context which has guided it s formation, explains what sustainable drainage is, the implications for developers and/or landowners, and gives examples of specific applications of sustainable drainage methods and techniques, particularly their integration into new development. 1.3 PLANNING POLICY FRAMEWORK There has been an increasing commitment demonstrated in national and local government policy towards achieving sustainable development to the benefit of all. The following documents highlight key provisions that Council is encouraged to take account of when planning for the management of surface water drainage Planning Policy Guidance Note 25: Development and Flood Risk (PPG25: 2001) National planning guidance in PPG25 (Appendix E) outlines the potential for sustainable drainage systems as a tool to effectively help control surface water runoff. PPG25 advises that: new development should not increase runoff from the undeveloped situation and that redevelopment should reduce runoff. To meet this objective local planning authorities should work closely with the Environment Agency, sewerage undertakers, highway authorities, and potential developers in considering the suitability of sustainable drainage systems (SuDS) for managing runoff implication of proposed development and redevelopment." Regional Planning Guidance for the South East (RPG9: March 2001) RPG9 lends additional support to the application of sustainable drainage principles with provision for supplementary guidance to take the lead in their implementation. Policy INF1 (a): Development plans should: (iv) encourage the adoption of sustainable urban drainage practices 2

9 Policy INF2 (b): Development plans should: (iii) promote the introduction of water conservation measures and sustainable drainage solution. Detailed supplementary planning guidance or site specific development briefs can help to facilitate the adoption of these measures The London Plan (February 2004) The London Plan provides a strategic framework for UDP development, dealing with matters of strategic importance to Greater London In it, the Mayor adopts a proactive approach to sustainable methods of development, including the use of sustainable drainage to reduce the amount and intensity of urban runoff and pollution. Policy 4C.8 sets out the general approach to the management of surface water runoff through Sustainable Drainage: boroughs should, seek to ensure that surface water runoff is managed as close to its source as possible. The use of sustainable urban drainage systems should be promoted for development unless there are practical reasons for not doing so. Such reasons may include the local ground conditions or density of development. In such cases, the developer should seek to manage as much runoff as possible on site and explore sustainable methods of managing the remainder as close as possible to the site. Sustainable drainage techniques will be one of the keys to ensuring that longterm flood risk is managed. Many of these techniques also have benefits for biodiversity by creating habitat and some can help reduce the demand for supplied water Building Regulations A series of approved documents have been prepared at a national level to provide practical guidance to the Building Regulations Part H (April, 2002) sets out the legal requirements for Drainage and Waste Disposal, and within this Part H3 encourages the use of infiltration systems as a means to manage rainwater drainage. Part H3 (1) Adequate provision shall be made for rainwater to be carried from the roof of the building. (2) Paved areas around the building shall be so constructed as to be adequately drained. Rainwater from a system provided pursuant to sub-paragraphs (1) or (2) shall discharge to one of the following, listed in order of priority - 3

10 - an adequate soakaway or some other adequate infiltration system; or, where that is not reasonably practicable, - a watercourse; or, where that is not reasonably practicable, a sewer. 1.4 CROYDON UNITARY DEVELOPMENT PLAN (UDP) The Council s UDP sets out the policies for land use and development in the Borough employed in the determination of planning applications. Through the Second Deposit of the revised Draft Croydon Plan (2003), policies have emerged requiring greater consideration of sustainability issues. Policies particularly relevant to the sustainable management of rainwater drainage include urban design policy (UD1), and environmental protection policies (EP3) and (EP12). UD1- Design and Environmental Performance Statements- To ensure the achievement of quality design in all new development, the Council will require Design Statements for all development proposals within the Borough. In addition, in order to demonstrate how sustainability issues have been addressed in the design process, environmental performance statements will be required to accompany proposals for: a development of 10 or more dwellinghouses or a residential development of 0.5 hectare or more; a development of a building or buildings of 1,000sq metres or more; or a development on a site of 1 hectare or more. EP3- Pollution of Water- In order to prevent harm to underground water, open water and watercourses, the Council will use conditions or, where appropriate, seek to enter into agreements (see Policy IMP1) to alleviate environmental damage that might otherwise arise. Permission will be refused if the risk cannot be satisfactorily alleviated. EP12-Water- Flooding, Drainage and Conservation- The Council will encourage the use of sustainable drainage systems for all developments that require drainage especially those likely to lead to a significant increase in surface water runoff from the site into a watercourse prone to flooding. Large developments should incorporate sustainable drainage system wherever practical. The Council will, where appropriate, seek to enter into agreements where this is necessary to provide the drainage system and safeguard its long term use (see Policy IMP1) Conventional drainage can cause flooding because rainwater is piped to watercourses before it has a chance to soak into the ground, this results in surges of water entering streams and rivers. It can also cause pollution because silt, oils and other pollutants are carried straight to

11 streams and rivers before they can be trapped, removed and broken down naturally Sustainable drainage systems allow collection and storage of runoff to allow natural cleaning to occur prior to infiltration or controlled release to watercourses. They can also enhance visual amenity and provide new habitats. Given that surface water is a scarce feature in Croydon the provision of sustainable drainage systems should be considered wherever practicable. Green roof can also be used to absorb rainfall and therefore reduce surface water runoff (see also paragraph 8.16a) 9.29 It is important that sustainable drainage systems are managed and maintained correctly. It may therefore be necessary to use Agreements to achieve this. The Council will publish Supplementary Planning Guidance to provide further advice on the use of sustainable drainage systems. 2. Sustainable Drainage 2.1 WHAT IS SUSTAINABLE DRAINAGE? The revised Draft Croydon Plan defines sustainable drainage as: an alternative approach to conventional 'engineered drainage systems that replicates natural drainage and deals with runoff where it occurs. Types of sustainable drainage include swales, filter strips, detention/retention ponds, wetlands and the use of porous materials. An open drainage area providing for natural infiltration The principle of sustainable drainage is to adopt more environmentally friendly/sensitive engineering solutions as an alternative to traditional piped drainage systems as a means of reducing flooding, reducing pollution discharge to waterway systems conserving water resources and creating habitat. 2.2 BENEFITS OF SUSTAINABLE DRAINAGE SYSTEMS (SuDS) Sustainable development is to strive for a balance between the demands of present generations for a better quality of life without compromising the ability of future generations to meet their own needs. Crucial to this balance is ensuring that a healthy natural environment, on which all life depends, is maintained. The implementation of sustainable drainage systems (SuDS) as an 5

12 alternative to conventional drainage systems can achieve significant direct and indirect long-term environmental benefits. They include: Reduction in overall flood risk by reducing surface water runoff to the watercourses either permanently or after peak flow periods in the system. Providing opportunity for infiltration of surface water into soil, to replenish groundwater and help to maintain base flows in rivers. Promoting a healthier waterway flow regime and reducing the impact of bank erosion and habitat damage caused by the increase and flow rate of additional surface water runoff. Reducing the amount of pollutants reaching waterways and infiltrating the ground. A drainage retention pond in Woodside, Croydon provides ecological value to the area Habitat creation and enhancement of the amenity of an area. This applies predominantly to open drainage options, especially where wet ponds or wetlands are implemented. The provision of wetland, marsh and pond environments not only helps to increase amenity but also wildlife and biodiversity in these areas. Potential reduction in development costs, by reducing costs for the provision of surface water drainage on site. SuDS do involve some costs and require some maintenance, however, depending on the system chosen, they present an opportunity to reduce the capacity size of remaining services and the provision of all surface water piping infrastructure and associated costs. Refund of the annual surface water drainage fee that is usually charged for the provision of surface water drainage services on the property. If a SuDS manages all of a site s surface water and there is an agreement ensuring no surface water connection will be made to foul sewers, the service provider (e.g. Thames Water) will annually refund this portion of the service rate. 6

13 If adopted as a philosophy rather than simply an engineering solution and integrated at the outset of a design layout, SuDS can offer significant infrastructure savings and add amenity and value to a project. 3. Application Process 3.1 PERMITTED DEVELOPMENT Open drainage areas can offer a variety of uses and ecological value The construction of SuDS within the curtilage of an existing dwelling/house may be considered permitted development and therefore permission will not be required from Croydon Council. However, it is recommended that the principles within this document be closely followed and applied. In the majority of instances, all other use classes apart from residential will require planning permission. If in doubt as to whether your proposed SuDS development is permitted please discuss with a Council officer in the Planning Control Team who will be able to advise on your situation (refer Section 6.2 for contact details). 3.2 IMPLICATIONS FOR DEVELOPMENTS Sustainable drainage should be the first option for all developments requiring drainage. It can only be excluded for implementation where sufficient justification is demonstrated on the grounds of impracticality or otherwise. All developments seeking planning permission are required to submit a design statement with their planning application in accordance with the Revised Draft Croydon Plan (Policy UD1). The content and information requirements are outlined in the Preparation of Design and Sustainability Statements SPG (at the time of printing this SPG is currently in preparation). As part of that process large or major developments (see definition in UD1) must also include an environmental performance statement demonstrating how sustainability issues have been addressed during the design process. These statements are likely to consider water conservation through permeable stormwater systems. The Revised Draft Croydon Plan (Policy EP12) requires large developments to incorporate SuDS wherever practical. 3.3 WHAT WILL COUNCIL WANT TO KNOW ABOUT? 7 When seeking planning permission, certain information must be provided in applications with specific reference to sustainable drainage so that Council s Planning Officers can ensure there will be no significant adverse effects from a development. Required information is detailed below together with possible

14 sources from which this information can be obtained. Figure 1 summarises the various steps in this information gathering process. 1. A description of the site s soil structure and type, including drainage history. Soil structure determines the volume and rate at which water infiltrates into the ground. Soil structure details for a site can be obtained from soil maps from your local library, or the Croydon Council Drainage Team. In larger developments it is likely that a ground investigation survey will be required, and where infiltration systems are to be used, proposals should be supported by infiltation/soakage tests. 2. Approximate levels of the water table on the site year round. Water levels affect the potential infiltration of water into the ground especially in areas where the top of the water table is close to the surface. The height of the water table can be obtained through the Groundwater and Contaminated Land Department of the Environment Agency. 3. Location of site in regards to sensitive aquifers. Where there are sensitive aquifers close to the surface it may be inappropriate to have SuDS especially for sites involving polluted surfaces. The location of aquifers can be obtained from the Groundwater and Contaiminated Land Department of the Environment Agency 1. The comprehensiveness of the following information requirements vary according to the complexity or scale of a SuDS application. Smaller, simple applications could be completed by a homeowner or such other who can provide concise information to an acceptable standard. Larger scale and more complex projects would require the assistance of a professional designer or engineer, who could provide the expected level and detail of information. If in doubt, a Council Officer will be able to advise you on the detail required in order to process your application. 4. If any of the runoff from the site is polluted and to what degree? This involves a simple, clear and concise description of the activity that will occur on site and the identification of possible pollution sources such as cars, with the potential to spill or leak oil and diesel. More intensive uses with higher pollution production would be expected to have more detailed information Proposed SuDS technique/s? A brief but concise summary of what SuDS technique/s will be used on site, why that type of SuDS was chosen and how it will function. 1 See also the Environment Agency s guide Policy and Practice for the Protection of Groundwater (2 nd edition), 1998.

15 6. Size of the impermeable area generating runoff? Development plans (scaled) must include estimates of the total impermeable surface area for which the SuDS will be applied (including roof areas, impermeable driveways and carparks etc). This helps to calculate and determine the amount of water that would be generated and managed by a SuDS in a set sized rainfall event. 7. Building specifications of any SuDS infrastructure (complying with building regulations and those in BRE Digest 365). A scaled design with the specifications of any SuDS to be used, meeting requirements of the existing building regulations and those of relevance under the BRE Digest Outline of the design and any visual implications. A design brief should be prepared providing an overview of any perceived design of landscape issues and remedy or mitigation measures taking into account any relevant landscape or design supplementary guidance. 9. Contingency/allocation for storage capacity failure - if necessary. This is a requirement in predominately larger SuDS applications, where large volumes of water are being managed and there is a danger of the SuDS failing. A design for an appropriate overflow mechanism should be included together with a detailed explaination of how it is to function and where the overflow will direct excess water. This may involve a second SuDS in conjunction with some other overland flood routing system, or a pipe-to-surface water drainage connection. Where there is any uncertainty as to the infiltration of soil it may be necessary for the developer to undertake further tests on site to determine the viability of SuDS. Pre-Application Gather information on matters 1,2 and 3 above, and prepare a brief outline of the proposed SuDS development and intentions, for the Council Planning Officer. 9 Submission of Application Arrange an initial meeting with a Council Planning Officer to discuss your SuDS development accompanied by the above information. Seek Officer s advice on the preparation of the design statement and guidance on any further requirements that may be required for the design statement or in order to meet other pro isions of the UDP Submit all material and details on sustainable surface water drainage for assessment as part of a design statement.

16 Figure 1: Requirements and various stages in an application for the construction of SuDS (where planning permission is required). 10

17 3.4 RESPONSIBILITIES OF OWNERS The success of SuDS as a tool for environmental improvement is dependant on good communication between all relevant parties (the Council, the developer, private landowners, and infrastructure service providers) and each being clear as to their responsibilities for the operation and ongoing maintenance and repair of SuDS. These responsibilities are summarised below: The Council acting as Local Planning Authority ensuring that the proposed SuDS is appropriate for the development, as part of a planning application. Entering into a Section 106 Agreement, where necessary (see below). The Council acting as Building Control Inspector - responsible for approving the location and design components. Landowners- responsible for the ownership and ongoing maintenance of SuDS located on private property as for any other approved private structure. The exception is when an agreement is established with Council. Agreements- 1. Where a SuDS is used for all surface water on site and subsequently pipe size is reduced in a development, the landowner is required to enter into an agreement under section 106 of the Town and Country Planning Act The agreement ensures the SuDS is satisfactorily maintained, that there is no connection of surface water to foul sewage pipes and thus no future exceeding of capacity leading to flooding. The agreement becomes a condition of the title of Ownership, so that if the property is sold, subsequent owners are aware of the attached condition. 2. If a developer or landowner is unwilling to agree to this condition they will be required to provide full capacity pipes as determined by the sewerage undertaker, despite having a SuDS application. 3. Where a SuDS structure is designed for a public space or an area that will become public space and impacts on infrastructure, there may have to be an agreement between parties involved ensuring that the SuDS will be maintained during the course of its useful life. This type of agreement will apply predominantly towards larger scale developments and other authorities and service providers that undertake public development projects. 4. The Council may be prepared to adopt SuDS areas and associated structures under a section 106 agreement. However, this would be subject to a commuted sum since the degree of maintenance will be higher than that of open land. 11

18 4. Sustainable Drainage Methods 4.1 NATURAL PROCESSES Natural Drainage and Waterways The processes by which water is stored and moved throughout the natural environment are described by the hydrological or water cycle (Figure 2). Drainage and waterways are integral parts of the cycle, with water quality dependant on what feeds into it. There are three main processes that have implications for sustainable drainage management. Condensation Interception Evaporation Precipitation Runoff Infiltration Lake/Pond Temporary Storage Runoff Sea, Storage To vegetation, soil, and storage Infiltration Figure 2: The Hydrological Cycle Interception vegetation (trees or other plants) has a positive impact in relation to surface water as it intercepts rainwater, slowing and delaying its passage to the soil, enabling water to be absorbed at a more gradual rate. Root systems also ameliorate the impact of precipitation by increasing the permeability of the soil, enabling better absorption. Plants also use water directly for essential processes such as transpiration. Infiltration once rainwater hits the ground it is absorbed and percolates down through the subsoil to be stored in aquifers or flowing onwards to recharge the earth s waterways. Runoff is the phenomenon that occurs when either the entire area below the ground is saturated or through heavy rain, where water falls at a faster rate than can be absorbed into the ground. Water is forced to remain on the surface running off the ground surface. Gravity taking effect, water naturally follows the lowest and easiest route, along gullies and low-lying terrain to where it will be stored or infiltrated. 12

19 4.1.2 The Urban and Engineered Environment The urban environment has changed the way in which natural water systems operate. The development of open land and vegetated areas to an urban environment has reduced water storage capabilities by replacing infiltration areas with impermeable structures and materials, such as buildings, roads, car parks and pavements. This has led to an increase in surface water runoff as the amount of water able to infiltrate the ground is reduced. Traditionally, runoff from developed areas is managed by directing the flow into underground pipes and drains, where it discharges rapidly to the nearest watercourse. Through this method, large quantities of surface water are removed away from built up areas to alleviate upstream pressures and prevent local flooding. Once a highly effective method, development pressure is producing greater volumes of greywater to be drained away using the same fixed capacity drainage system. Increased urban runoff subsequently increases the water volume and flow rate in waterways. This leads to the erosion of streambeds, destabilisation of banks, and increased siltation. Another effect of channelling urban runoff to waterways is the concentration of land-based pollutants such as oil along point sources of waterways, affecting the functioning biodiversity quality of the watercourse and its surrounding environment. 4.2 WHERE SuDS MAY BE INAPPROPRIATE? There are several factors that an applicant needs to be aware of, that may affect the successful implementation of SuDS, but through applied engineering solutions may be possible to overcome. These are explained below. Soil Structure - The soil composition directly effects drainage. The amount of water that can be held in the soil is called "porosity" and the rate at which water flows through the soil is dependant on its "permeability". Different surfaces hold different amounts of water and absorb water at different rates, and as a surface becomes less permeable, an increasing amount of water remains on the surface, creating a greater potential for flooding. The soil in Croydon generally consists of two main types, clay and chalk. Clay is less porous so water passes through slowly, while chalk lets water pass through quickly. Depending on the location and the type of soil structure in an area, the implementation of some or all types of SuDS may be impaired by the ability of the surrounding area to infiltrate surface water (Table 1). Table 1: Soil composition and their relative infiltration rates (Sandy) Soil Infiltration Rate (mm/hr) (Silt/Cla (Loam) (Silty) y) (Clay) Extended detention pond? Wet pond? Infiltration trench? Infiltration basin? Porous pavement? Swale/filter strip??

20 Source: Protecting the Quality of our Environment: A guide to Sustainable Urban Drainage: SEPA/Environmental Agency Water Table - The height of the water table also affects the ability of the ground to allow infiltration of surface water. If an area has a naturally high water table there is less capacity for the absorption of surface water, as the ground becomes saturated closer to the surface. Adding additional surface water in these areas will increases the height of the water table and potentially leads to surface flooding. Where there is a deep year-round water table this is not a concern. Site Characteristics - In higher density areas, the ability to implement any practical type of drainage system is often confounded by a lack of space or the close proximity of other buildings. The size of a proposed SuDS is important, as the allocation of storage area depends on how much surface water is produced. If the correct proportions are not achieved there is a probability of failure in a rain event. A system s water reservoir also needs to be situated away from buildings so that there is no danger posed to building foundations. In many cases design considerations may not be enough to be able to provide practicable outcomes for SuDS applications. Water Volume Crucial to the design and application of any SuDS is the management of water generated by the system. In locations that have restrictions on infiltration and those that contain larger non-permeable surfaces it may not always be practicable to apply SuDS effectively. If engineered SuDS cannot meet the water quantity and associated requirements (infiltration) then traditional options may remain the desirable option. Aquifers - The location of SuDS in relation to any sensitive aquifers is important in order to protect the use and quality of the aquifer. An aquifer is a layer of rock or soil that is able to transmit water. As water passes through the soil reaching the aquifer, many pollutants are filtered out, and therefore this water is of high quality. It is important to ensure that if the water in aquifers is tapped into, we minimise the risk of pollutants entering the aquifers especially in sensitive areas where they are close to the surface, thereby protecting water purity. For this reason in some areas it may not be acceptable to construct SuDS especially at those sites where the incidence of runoff from car parks containing petrol, oil and diesel is high, however, the opportunity may still exist to adopt a permeable car park surface, sealing the base to prevent possible infiltration. 14 It is important that if SuDS are to be successful, all the above issues are considered at the earliest point of the design stage. From here the most appropriate system can be selected to suit the circumstances of a particular development.

21 4.3 SUSTAINABLE DRAINAGE TECHNIQUES With the increased commitment towards sustainable development and practice, supported by greater environmental awareness and Government Policies, there is a renewed emphasis on managing activities on a sustainable basis, and achieving environmentally friendly outcomes. The SuDS approach to drainage incorporates a wide variety of techniques. As a result, there is no one correct drainage solution for a site, and in most cases, a combination of techniques, based on the Surface Water Management Train principle (Figure 3) will be required depending on the extent of the proposal, whether a small residential development or a larger commercial application. 15

22 Figure 3: The Surface Water Management Train Principle (Source: The management train starts with prevention, or good housekeeping measures, for individual premises; and progresses through local source controls to larger downstream site and regional controls. Runoff need not pass through all the stages in the management train. It could flow straight to a site control, but as a general principle it is better to deal with runoff locally, returning the water to the natural drainage system as near to the source as possible. Only if the water cannot be managed on site should it be conveyed elsewhere. This may be due to the water requiring additional treatment before disposal or the quantities of runoff generated being greater than the capacity of the natural drainage system at that point. Excess flows would therefore need to be routed off site. Table 2: The ability of some of the different SuDS designs to deal with specific types of pollutants. 16

23 4.3.1 Prevention Source: Protecting the Quality of our Environment: A guide to Sustainable Urban Drainage: SEPA/Environmental Agency SuDS are just one aspect in the management of surface water runoff (Figure 3). Prevention as the old adage says is often better than the cure and in the management of surface water this proves the case. The collection, storage and reuse of surface water along with the recycling of grey water and general good site management practices, can significantly minimise runoff. As these matters are not part of the SuDS process but part of the wider sustainable water and wastewater philosophy, they are addressed in SPG No. 18 Sustainable Water Usage (2004). Just as in a natural catchment, drainage techniques can be used in series to change the flow and quality characteristics of the runoff in stages (Figure 3). These are described below Green Roofs Green roofs can reduce water runoff, flow rates, and improve water quality. A green roof in place of a conventional roof supports a layer of vegetation which absorbs a majority of the rainfall that would otherwise become runoff. There are two basic types of green roof; Intensive and Extensive; Intensive green roofs are roof gardens and like any garden provide space for people. They require the intensive management of a ground level garden, are usually based upon a thick soil or substrate layer and require artificial irrigation. Preferred plants are selected and the roof conditions created to support them. They are heavy systems and thus can have major structural implications for the building. Extensive green roofs are not usually recreational (although extensive planting can be integrated with intensive systems). They have low management requirements and do not usually require artificial irrigation. Plants are selected that will succeed with only minimal modification of the normal roof conditions. Planting styles are usually naturalistic with the objective of establishing a self-sustaining plant community on the roof. Based on a thin layer of soils or substrates, they are lightweight systems with minimal structural implications for the building. Green roofs also have other benefits; visual appeal, creating habitats for native flora and fauna, providing noise and energy insulation, cleansing the air of dust and other pollutants and moderating the heat island effect. 17

24 The strength and slope of a roof determine the amount and type of vegetation that can be situated on it. Before any vegetation is added, the roof must be sealed and a waterproof layer put down on which growing medium and vegetation can be planted. In addition to reducing peak water runoff, green roofs can also help cool cities in summer, filter dust from the air, regulate building temperatures, and improve the aesthetics with minimal maintenance Soakaways Perhaps one of the most common and basic sustainable drainage methods, the soakaway has been a long-time favoured option for managing residential building runoff especially where there have been no accessible wastewater systems in place. The basic soakaway uses an infiltration system, consisting of a hole in the ground back filled with porous material to let water drain through, such as crushed rock, or broken brick. The down piping from the roof is trenched and connected to the hole, and the entire hole buried and grassed with lawn or other suitable material. More elaborate soakaways may include precast concrete chambers with facilities for inspection and cleaning. The porous material allows water to be absorbed quickly and with little resistance into the soakaway, and drain it away from the surface. Once water enters the soakaway, it can then be absorbed into the surrounding earth. The location of a soakaway is usually situated away from a building so that water does not soak back in under the foundations of a house. The main advantages of using soakaways, is that they are effective, cheap and can be easily installed. However, their suitability for a specific site/development will ultimately depend upon a number of factors including, 18

25 the depth to groundwater, aquifer status, geology, source, and protection zones Permeable Footpaths and Car Parks Traditionally, areas such as open car parks, footpaths and driveways have been constructed of impervious surfaces such as concrete and asphalt which increase surface runoff. Replacing these with permeable surfaces, which can store and disperse surface water, alleviates this problem, reducing runoff. Permeable surfaces may consist of reinforced grass, block and sand, gravel, space pavers, or porous pavers. Water passes through the surface to the permeable fill where it is stored, infiltrates the soil or is transported away. The use of a permeable fill also aids in trapping sediment and can provide an environment for bacteria to help break down pollutants such as oil. Petrol interceptors should also be considered for large car parking areas. The volume of water that can be managed depends on the depth and type of base fill used and the rate of infiltration. In addition to relying solely on the process of infiltration to cope with amounts of water when the volume in the store is high, it can be piped out to additional storage areas or to an existing water drainage system. There are a variety of porous surface materials available that could be incorporated into the design of most developments while maintaining their function. By their nature, permeable surfaces with filter drains ensure an efficient use of space and a dual purpose Swales and Vegetation Filter Strips Swales are an infiltration technique often used in conjunction with filter strips for the effective treatment of water runoff. Porous paving and parking areas near Purley Way, Croydon. A swale is typically a gentle sloping culvert beneath which lies a layer of porous material that helps to store and aid the infiltration process. Water passes down the slopes of the swale through grass and vegetation, infiltrating the ground before reaching the bottom of the culvert At this point there may be additional opportunity for infiltration in Road Swale Footpath 19

26 the basin of the swale. If the water in the bottom of the swale pools on the surface, it will travel along the bottom of the swale as it would in a normal watercourse. At intervals along or at the end of a swale another infiltration or storage device can be placed to assist water management, or the remaining water runoff can be directed into an existing drainage system, at an otherwise much reduced rate. Vegetation filter strips can be used throughout a swale and consists of surface cover or varying degrees of vegetation that impede and slow the flow of water and allow greater infiltration to occur. Vegetation also helps to filter runoff, catching much of the suspended sediment load, allowing it to settle and rejoin the soil. Vegetation and filter strips are very effective on gentle slopes such as that on a swale. Swales are often implemented along roadsides and are effective in dealing with the associated runoff. They are also commonly used in public spaces and have a high amenity potential. The use of vegetation and filter strips in swales can vary, and can improve amenity values and support the local biodiversity and wildlife habitat. In addition, swales can avoid curbing and guttering costs associated with traditional roads, making them a costeffective method for managing road runoff. 20

27 4.3.6 Infiltration Trenches Infiltration trenches are similar to soakaways but have been used specifically for roadways and car parking areas where there are often traces of pollution and increased sediment levels in runoff. They differ from soakaways through the inclusion of a few additional design elements: a filter strip and/or a sump or holding area, is used to settle as much silt as possible and prevent it from entering and blocking the infiltration trench; a geotextile layer, which prevents polluted substances from entering into the subsoil, and aids in their bacterial Infiltration trenches have been used along sections of the tram link in conjunction with loose fit paving breakdown, and an overflow pipe that runs along the length of the trench. In time of high flows the overflow pipe permits filtered water to travel to another area in which it can be stored in a reservoir or infiltrated. Grass Filter Strip Small sump to trap additional sediment Overflow pipe Porous fill Geotextile layer to contain/filter pollutants Infiltration Retention Ponds and Basins Retention ponds and basins offer effective storage and infiltration solutions for urban runoff, particularly where large non-permeable hard surfaces are unavoidable, such as substantial road provision. Both systems require a reasonable area in which to be applied, as they are open drainage options (i.e. not diverted or screened underground). This generally rules out smaller applications, but the Runoff Additional runoff t Capped Pond Infiltration Overflow same principles could be applied to water collected from a house, if there were room in the garden. The use of these techniques depends heavily on the scale of the project, the area available and quantities of water being handled. 21

28 A retention pond remains as a pond year round, and is capable of taking additional runoff for example, heavy rain. A retention basin is a reservoir where excess water or overflow can be stored when necessary but is otherwise dry. Retention ponds and basins serve two primary functions: water storage and regulation at times of peak water flow and then releasing water steadily back into the system, preventing flooding; and infiltration. The benefits of either option is that in a holding capacity the silt and other suspended load in the water is allowed to settle, so if water is released back into the drainage system it is without the extra sediment load. The use of a wet pond in Woodside accepts all drainage water from a local residential development Retention ponds are not limited to ponds but can also apply to wetlands and other areas that have the ability to retain and balance water runoff. Retention ponds have an important ecological role and contribute towards the biodiversity of an area, providing an environment for aquatic species as well as functionally absorbing excess water and helping filter and breakdown pollution when necessary. Particularly in public spaces, retention ponds have the ability to provide wider amenity values and benefits rather than just being limited to the storage and treatment of water runoff. Retention basins tend to be large open areas free from any development and usually double as playing fields or open recreational areas. Retention basins and ponds can offer development savings by reducing the amount of piping and infrastructure costs because of their ability to cope with large volumes of runoff. However retention systems need to include appropriate provisions in design for an overflow in case of significant event occurring to avoid flooding. 22 This open drainage basin near Selhurst allows retention and infiltration of excessive surface water in times of need

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30 5. Further Information: 5.1 REGULATIONS 24 There are various design regulations that may apply to SuDS depending on what method is used. Key regulations are included but not limited to the following: Sustainable urban drainage systems design manual for England and Wales Published by DETR and CIRIA The Building Regulations 2000: Approved Document H- Drainage and Waste Disposal Part H3 Rainwater Drainage (effective April 2002) BRE Digest 365 Soakaway design BS EN 752 Drains and Sewer Systems 5.2 USEFUL CONTACTS The list of techniques and systems described in this SPG is not exhaustive or entirely appropriate for every site/development, e.g. alleviating flood risk. The organisations listed below can provide further up-to-date information on sustainable drainage systems as they evolve and specific advice on tailoring them to your situation/budget. Croydon Council Website Address Taberner House Park Lane Croydon CR9 1JT Telephone Fax For specific advice you may want to talk to someone in the following areas: Drainage Department contact Bob Hucks on extn Building Control contact Dave Lee on extn Planners contact Charlie Fossett on extn CIRIA Website Documents C522 Sustainable urban drainage systems design manual for England and Wales C523 Sustainable urban drainage best practice handbook Environment Agency Website Telephone

31 Documents Sustainable Drainage: An introduction Sustainable Drainage System (SuDS) A guide for developers Policy and practice for the protection of groundwater. Bristol. (2nd edition) In my backyard - An Environment Agency website search facility by postcode. Video Designs that hold water: Sustainable urban drainage systems explained. Scottish Environmental Protection Agency Website Thames Water Website Telephone Sutton and East Surrey Water Website Telephone Considerable information on sustainable water systems is available on the Internet and can be yielded by using a reliable search engine and searching for key words such as sustainable drainage or SuDS. This should also produce sources for further reading, consultants and services offered. 25