Promoting Sustainable Drainage Systems

Transcription

1 Promoting Sustainable Drainage Systems Design Guidance for Islington

2 Contents Purpose of this Guidance Introduction Managing rainfall in the city SUDS characteristics and design criteria The design process Adoption and maintenance Designing SUDS in schools Introduction Case study: Elizabeth Garrett Anderson School Adoption of SUDS within schools Designing SUDS in new developments Introduction Case study: Caledonian Road Adoption of SUDS within new developments Retrofitting SUDS in existing housing estates Introduction Case study: Ashby Grove Estate Adoption of SUDS within existing housing SUDS and highways Introduction Case study: Skinner Street Adoption of SUDS on highways SUDS and parks Introduction Case study: Kings Square Park Adoption of SUDS within parks Frequently asked questions Glossary Further information sources Appendix 1: London Plan policy relating to SUDS List of sketches - SUDS features Figure 15: Swale and raingarden basin in open space Figure 21: Biodiversity-based green roof with additional box storage Figure 23: Urban SUDS pond Figure 28: Retrofitted raingarden within housing estate Figure 31: Filter strip and underdrained basin Figure 34: Cross section of permeable paving Figure 37: Bird s eye view of a roadside kerb extension planter...24 Figure 38: Cross section through roadside raingarden planter...24 Purpose of this Guidance Promoting Sustainable Drainage Systems (SUDS) in Islington will bring a range of benefits. SUDS manage runoff from development in an integrated way to reduce the quantity of water entering drains and therefore to reduce surface water floodrisk an important consideration in a dense urban area like Islington, particularly given the increase in heavy rainfall likely as a result of climate change. SUDS also improve the quality of runoff from development, bringing clean water back into use in our urban environment to create attractive places for people and wildlife. This note provides guidance on the application of SUDS within Islington. It has been produced in collaboration with a range of stakeholders within the Council and its partner organisations in order to identify and raise awareness of opportunities for SUDS and to identify how they could be effectively implemented and managed. The note is split into two major parts. Section One provides a summary of the rationale for SUDS, basic SUDS principles and outlines design criteria which the Council will apply in developing or regulating SUDS schemes. It also covers adoption and maintenance issues and, together with the frequently asked questions, addresses other commonly perceived barriers to implementing SUDS. Sections Two to Six focus on the major opportunities for incorporating SUDS within Islington, as identified in an initial stakeholder workshop. These are schools, new developments, existing housing estates, highways and parks. Each chapter is based around an Islington case study scheme which demonstrates how SUDS can be incorporated within that particular context. The conceptual designs have been produced in conjunction with relevant partners via a design workshop for each site which presented draft designs for feedback and discussion. The designs presented in these chapters are those which were agreed to be technically deliverable and otherwise acceptable to relevant partners, subject to available budgets and further consultation with residents and other stakeholders. The chapters also include a number of illustrations of techniques and supporting photographs of where these techniques have been implemented in practice. This guidance aims to provide information and ideas for decision makers and designers within the Council, developers and partner organisations to support the application of SUDS in a range of contexts across Islington. This guidance has been produced by Robert Bray Associates in partnership with Islington Council. Images are by Robert Bray unless otherwise noted. 2

3 1. Introduction Figure 2: Rivers and springs in Islington 1.1 Managing rainfall in the city In the beginning Once upon a time, rain would have fallen on a forest landscape where the borough of Islington now stands. Rainfall filtered gently through a canopy of leaves before falling to the woodland floor and soaking into the ground before finding its way through the surface and soil pathways to marshy places, ponds and streams. The shallow winding streams led eventually to a wide, marsh fringed river of many channels that would later be tamed and called the River Thames. The city grows As agricultural settlements developed in Islington, this natural condition began to change. Water was drained to ditches and then along straightened streams to a river confined by banks. As London grew, development in Islington increased, much of it focusing around the many streams, rivers and springs in the area (see figure 2). As London s population grew further and existing watercourses became polluted, the New River was developed, bringing clean drinking water from Hertfordshire, via Islington, into London. The Victorian period led to major changes in London s water landscape as the demands of London s burgeoning population, escalating land values and the pollution caused by human waste resulted in development of the London sewer system. This network of pipes collected sewage and rainwater under the city for transport to treatment works and then to the sea. Most of London s watercourses were also undergrounded during this period and are no longer visible; however, clues to the past importance of water on Islington s development remain in evidence in many of Islington s street names (see figure 1). Figure 1: Water and Islington place names Early habitation in Islington grew up around springs and shallow wells and there remains evidence of this in many of the borough s place and street names: Clerkenwell and Goswell both take their names from ancient springs in the area - Clerk s Well and Good Well or God s Well. Sadler s Wells is also named after a spring, which became the home of Islington Spa. The spa was famous for the quality of its water which was said to rival that at Tunbridge Wells in its healing properties. Turnmill Street follows the course of the River Fleet, which was also known as Turnmill Brook owing to the number of mills along it. A line of ponds once marked the edge of a gravel terrace carried by glacier meltwaters this boundary is now known as Ball s Pond Road. 2 3

4 Unintended consequences The problems of conventional drainage are inherent in the way it deals with water that flows from hard surfaces in the city. Water is collected and conveyed away from where it falls as quickly as possible, causing flooding when the drainage system cannot cope with the large volumes of water entering it or if it is blocked. This can also cause sewer overflows containing foul waste which create a range of additional problems. A cocktail of oils, sediments, spillage, animal droppings, herbicides, pesticides, road cleaning chemicals and debris is washed into the drainage system directly to streams or treatment works. In times of low flow this causes severe pollution of streams, while in times of high flow streams are damaged through erosion or silt build up. All the benefits that derive from use of clean water locally, for example in creating attractive water features or wildlife habitats and naturally irrigating landscapes, are foregone as it disappears underground in the drainage system. Clean water cannot soak into soil naturally to recharge groundwater. The problem in Islington Islington suffers from many of the problems described above. The borough is highly urbanised with few permeable surfaces and a very dense population. As a result it is deemed to have a high risk of surface water flooding, which is likely to be increased by further growth and intensification of the built environment as well as increasing risk of heavy rainfall due to climate change. As a result of its location and characteristics, the Environment Agency has identified Islington as a one of a number of priority areas for action on surface water flooding. Constraints on the amount of water which filters into the soil also creates a number of problems in Islington. Natural watering of trees and other vegetation is restricted, and we may increasingly have to rely on artificial watering as our summers become hotter and dryer. Drying of soils due to a lack of infiltration has led to significant subsidence problems in some areas. We are also missing major opportunities to bring water back into Islington s environment and to enjoy the amenity benefits it can bring, both as part of our urban landscape and for play and recreation. Water could also be harnessed to create a range of habitats which would support and enhance biodiversity. A new approach to managing rainfall It has now been widely recognised that a new way of managing rainfall is needed to deal with the problems created by the traditional pipe drainage system we have inherited from the past. The inspiration for this new approach, known as Sustainable Drainage Systems, or SUDS, can be found in the way nature intercepts rain through vegetation and soils, allowing water to flow slowly into the ground or to wetlands, ponds and streams. SUDS is a more environmentally friendly way of dealing with runoff from development that uses landscape techniques that mimic nature to control flows, prevent pollution, and provide attractive water features that enhance wildlife and provide benefits for the local community. SUDS provide robust, easily managed and usually cheaper ways of dealing with rainfall. The shift towards this new approach is reflected in a range of policy drivers which promote SUDS; these are outlined in figure 3. Benefits of SUDS In Islington, SUDS offer a number of benefits: Figure 3: SUDS Policy Drivers There are a range of drivers for SUDS in Islington, ranging from national to local, including: Pitt Review This review took place following the floods in Summer 2007 and recommended a range of changes needed to prevent similar problems in the future. It recommended that Local Authorities and the Environment Agency should be given greater responsibility for managing surface water flood risk. Flood and Water Management Bill The Bill includes measures to address flood risk, including by clarifying responsibilities around flooding and encouraging sustainable drainage systems. The Bill will end the automatic right to connect to sewers for surface water drainage and require developers to put SUDS in place in new developments. Connection to the drainage network for surface water will be conditional on meeting new national standards on SUDS. Local Authorities will be responsible for approving SUDS schemes as well as their adoption and maintenance. Planning policy a range of national, regional and local planning policies, including PPS 25 and the London Plan, promote the incorporation of SUDS within developments and more widely across existing neighborhoods. Policy 4A.14 of the London Plan states that any development should incorporate SUDS with the aim of achieving greenfield run-off rates (see Appendix 1). National Indicators 188 (climate change adaptation) and 189 (flooding) both promote the use of SUDS to address flood risk. Islington has been identified as a priority borough for support on NI189 following the Environment Agency s initial assessment of surface water flood risk. Islington s Climate Change Adaptation Strategy and Biodiversity Action Plan SUDS will support delivery of both of these strategies, as well as wider objectives around physical regeneration and improvement of the public realm. Reduces the risk of surface water flooding and related sewer surcharging by providing on site storage and a reduction of rate of runoff into the combined sewer. Improves the water quality of runoff from a site in Islington this is particularly important where this water will be reused to provide local benefits. Improves the quality and attractiveness, and in many cases the value, of public realm and private developments by creating attractive landscape features. Enhances biodiversity through the creation of habitats such as wetlands, ponds and planted raingardens. Reduces the need for artificial watering of trees and landscaped areas by offering techniques for naturally irrigating landscapes. Provides clean water for reuse by residents or businesses, either through an outdoor rainwater butt or internal rainwater recycling, for example to flush toilets. Reduces capital and maintenance costs associated with conventional drainage systems. Provides education and play opportunities by making the water cycle visible. Provides a lower carbon drainage solution by reducing embodied energy in hard infrastructure and the volume of water requiring energy intensive treatment downstream. Increases flexibility of the drainage system, for example to adapt to climate change. 4

5 1.2 SUDS characteristics and design criteria SUDS in the city SUDS use a variety of techniques in sequence to slow the flow of runoff and improve the quality of water by natural cleaning processes. Where there is enough space in development, this can be achieved in open vegetated features like swales (flat bottomed grass channels), basins, ponds and wetlands. But in urban areas it is usually necessary to use more engineered techniques to make the most of confined space. Every urban surface must be considered as a rainfall collector, allowing water to pass through to a drainage layer below or flow to a soakage area so that volumes do not build up to cause problems downstream. Pollution is trapped and treated where it is generated. Important techniques include green roofs, permeable surfaces, raingardens and bioswales, where water can soak into planted areas, or any other underdrained surface that intercepts water at source near to where it falls as rain. SUDS manage runoff from development in an integrated way to reduce the quantity of water entering drains, especially at peak periods, to improve the quality of runoff and promote amenity and biodiversity benefits from using the water in the urban environment. These objectives are detailed further below, along with the design criteria which SUDS schemes in Islington will be expected to meet. The principles of SUDS are the same in urban areas as less dense development, but whereas runoff should infiltrate or flow to natural watercourses where possible outside the city, it is usual for runoff to enter the combined or storm sewer in London. This condition informs the collection and control of runoff and particularly the rate at which water leaves development (see quantity section below). It also means that where water is available for amenity or wildlife interest, ensuring the water is clean through the management train treatment system is critical. These issues are described further below. Figure 4: SUDS in urban environments: urban wetland, Western Harbour, Malmo and combined accessible and green roof at Ropemaker, Islington Design criteria (a) Quantity - managing the flow The rate that water flows from development together with the volume of water falling on a site needs to be managed to reduce the impact of runoff on surrounding development, local watercourses or the combined sewer. The measures used to achieve this protect both the development itself, and the downstream environment from flooding and other effects of uncontrolled runoff. The role of SUDS in reducing flood risk is likely to become increasingly important in Islington as climate change increases the frequency of heavy rainfall and as ongoing development and intensification of the borough lead to additional water (both waste water and surface water) being drained to the combined sewer. Retrofitting SUDS in Islington would reduce local surface water flood risk in a variety of ways. SUDS reduces the frequency with which runoff occurs by encouraging water to soak into the ground or be lost through evaporation. Although much of Islington has clay soils, some infiltration will still take place and will offer a range of benefits. SUDS also reduce the rate of runoff entering the sewer by slowing the flow of water as it passes through planting, soil or layers of crushed stone below permeable surfaces, and by providing additional storage for runoff onsite. In this way SUDS can help prevent local flash flooding caused when runoff is channelled quickly off hard surfaces by conventional gratings and pipes, causing sewers to fill with water quickly and resulting in unpredictable overflows of contaminated water from the combined sewer when it surcharges. In addition, SUDS features intercept rainfall at source removing silt and debris that can block conventional pipe systems and result in flooding. In order to ensure SUDS are designed to control the flow of rainwater leaving any site, the basic characteristics of rainfall events must be understood. Controlling the rate of runoff and the volume of water falling on a site will depend on: the storm probability or the chance of a storm of given magnitude happening in any year, known as the storm return period the storm duration or period of time over which it rains the storm intensity or the depth of rainfall over a period of time There are a number of storm durations and intensities for any given return period. For example, a very short duration storm may have a very high intensity of rainfall and these are more likely to occur in summer, washing pollution from hard surfaces and causing blockage of pipe inlets. Long duration storms with lower intensity can also occur within the same return period but are more typical of winter storms. The critical duration event is the storm that causes the largest peak flow within a particular return period. The nationally agreed return period which should be designed for is a 1 in 100 year with a 30% allowance for climate change. The largest peak flow is calculated through computer simulation of a range of storms likely to occur within this period to see which one creates the greatest storage volume requirement. This will determine the volume of water which needs to be stored on a site. 5

6 The two key elements of quantity are: 1. Runoff rate The natural flow from an undeveloped site is called greenfield rate of runoff and is usually between 3-8 litres/second/hectare. In Islington the Environment Agency have agreed a figure of 8 l/sec/ha. In urban areas most existing development generates much higher uncontrolled flow rates, often between l/sec/ha, as gullies and pipes collect rainfall almost instantly and carry the water away quickly in the sewer. (b) Quality - preventing pollution Whereas the quantity element of SUDS is assessed numerically, the quality of runoff involves a qualitative assessment of how runoff is managed using a sequence of techniques that control and clean flow as it passes from one stage to the next. This is called the management train and is shown in figure 6 below. Figure 6: SUDS Management Train This increase in runoff rate leads to a range of problems which SUDS aims to address by reducing flows of water through the landscape to a greenfield rate of runoff. The London Plan (policy 4A.4) states that developers should aim to achieve greenfield run off from their site. On sites where it can be demonstrated that this is not possible, an estimation of the previous runoff rate can be undertaken and this reduced by at least 50% to a rate agreed with the Local Authority and regulators, in line with the Mayor s essential standard (see Appendix 1 for full policy details). 2. Storage volumes SUDS should be designed to provide an agreed volume of storage on site. This storage volume should be equivalent to the largest peak flow expected within a 1 in 100 year (plus 30% allowance for climate change) return period. This is calculated through modelling of the storm which would create a flow, known as the critical duration event (see above). The amount of storage required also depends on the rate that water can leave the site and can be affected by the underlying geology. The amount of water which will need to be stored on any site in Islington, with different runoff rates, is shown in the table below. Figure 5: Storage volumes for Islington with different return periods and runoff rates (m 3 /m 2 ) Runoff rate Return period (yrs) % 8//sec/ha l/sec/ha l/sec/ha As the table shows, the depth of water that needs to be stored for any area of hard surface in a worst case situation of a 1 in 100 year (+30%) return period varies from 62mm for 8l/sec/ha flow rate down to 25mm for 100l/sec/ha. The total storage volume requirement for the site can then be calculated by multiplying the area of hard surface on the site by the relevant depth of water to be stored (ie 25mm, 35mm or 62mm). The table also demonstrates the benefits of a hierarchical approach to providing storage. For example, day to day rainfall (such as a 1 in 1 year event, requiring only 2-10mm storage) can be easily stored within smaller SUDS features such as swales or green roofs. Storage for moderate rainfall events (such as a 1 in 30 year event) can be provided through features which will be required periodically, for example detention basins. Larger volumes that only occur very infrequently (for example in a 1 in 100 plus 30% return period) can be accommodated in multifunctional spaces such as playing fields, play grounds, car parks or amenity green space which can be designed to provide storage for short periods after extreme rainfall. The sequence begins with prevention measures to reduce the risk of contamination at source control stage, where features, such as green roofs or permeable pavement, intercept and deal with runoff as close as possible to where it falls as rain. Source control is critical to SUDS as it ensures that contamination and silt are dealt with before they enter the drainage system. Volumes that cannot be managed at source, flow slowly to storage or site control features, such as detention basins, ponds or wetlands, usually found toward the edge of a development. These volumes should be clean enough to provide amenity and biodiversity interest for the SUDS scheme. Open conveyance like swales and vegetated low flow channels provide additional cleaning of runoff by trapping and treating any pollution that has passed through the source control stage. In some developments where public open space is available, then regional control (or community control ) can be incorporated to provide additional storage and polishing of runoff. 6

7 As part of this approach, an appropriate number of treatment stages should be provided within the management train to ensure runoff is of sufficient quality. The number of treatment stages incorporated should be based on a risk assessment of pollution. Roofs and housing areas require at least 1 treatment stage, roads and commercial sites need 2 stages, and high risk areas like industrial sites, HGV parks and fuel storage areas 3 or more. At least one treatment stage must be provided before runoff is used in open landscape features, ponds or wetlands to protect wildlife and public amenity. Treatment of the polluted first flush should also be addressed this is the volume of runoff (10-15mm) that flows from hard surfaces during short rainfall events or the beginning of storms, carrying with it accumulated silt and pollutants. It is important that this polluted volume is intercepted for treatment via small sub-catchments or rain collection areas. Figure 7: Planted filter strip at Exwick School, Exeter and multifunctional wetland courtyard as part of SUDS at Western Harbour, Malmo (c) Amenity and biodiversity maximising use and pleasure Unlike traditional drainage that directs rainfall underground into a network of pipes, SUDS provide a controlled flow of clean water at or near the surface to deliver benefits for the community and wildlife. Even where water flows just below the ground in permeable pavement or SUDS planters, the visual quality of the surface can be attractive and provide biodiversity benefits. Keeping water at or near the surface is one of the features of SUDS design as it makes the water story legible, is safer, cheaper and offers a range of benefits, as long as it has been cleaned through source control features and sufficient treatment stages. Amenity and biodiversity benefits include: attractive, safe SUDS features which make use of clean water at the surface to enhance landscape design and create a sense of place provision of multi-functional spaces such as sport, recreational and wildlife areas creation of ecological habitats such as ponds, wetlands and other planted areas enhanced wellbeing and educational opportunities as the water cycle is made a part of people s everyday lives water reuse opportunities, from naturally watered landscapes to rainwater recycling well designed SUDS details including rills, channels, canals, spouts, cascades and pools visually acceptable and safe inlets, outlets and control structures (see figure 9). Figure 8: Islington SUDS Design Standards Schemes should clearly demonstrate, through design drawings and a SUDS design statement, how the following standards have been met: 1. Quantity Runoff rate - Schemes should be designed to reduce flows to greenfield rate of runoff - 8l/sec/ha for Islington. In cases where it can be demonstrated that this is not possible due to site constraints, a higher runoff rate may be agreed with the Local Authority and regulators, which should provide a minimum of a 50% reduction on the existing runoff rate. Storage Volume - The volume of runoff to be stored on site should be based on the largest peak flow of any storm expected within the nationally agreed return period of 1 in 100 years plus a 30% allowance for climate change. 2. Quality Schemes should demonstrate they follow the management train, maximising the use of source control, providing the relevant number of treatment stages and identifying how the first flush will be dealt with. 3. Amenity and Biodiversity The design must maximise amenity and biodiversity benefits such as those described above, while ensuring flow and volumes of runoff entering public open space are predictable and water at the surface is clean and safe. 7

8 1.3 The design process It is important that SUDS proposals are developed based on an understanding of the SUDS design process to ensure that key design elements have been demonstrated at the correct stage in the process. The key stages in design are described below, while figure 10 demonstrates how these stages fit into the wider design and planning process. Key stages in designing SUDS 1. Identifying flow routes through the site SUDS design begins with an assessment of the natural drainage pattern for the site and how this will be modified by development. This will be determined largely by topography and geology, together with a review of historical drainage measures that have modified the original pattern including land drainage, culverts and the sewer network. The flow route analysis should identify how original flow routes are modified by proposed development and create a framework for appropriate SUDS techniques to collect, clean and store runoff in a management train before discharge to an outfall. The identified flow routes should provide corridors for day to day low flows, overflows that can operate when surcharge or blockages occur and exceedance pathways when exceptional rainfall overwhelms the SUDS. This should begin with a consideration of the major components of the site, for example roofs, pedestrian areas and carparking, and the drainage opportunities these provide. Appropriate SUDS solutions should be considered for each surface, for example permeable paving, green roofs and areas of soft landscaping. Once these major surfaces have been assessed to perform a drainage function, then each component can be linked by surface conveyance in the form of rills, channels, linear wetlands or other surface features. This element of the design process is particularly important on very small sites, where it may not be possible to distinguish flow routes and sub-catchments. At convenient points along the path taken by the controlled flow of clean water, it can be used to create biodiversity or other amenity features. Spouts, cascades and other devices can be used to enhance the urban and landscape design and to make visible water s journey through the site. Reuse of water to provide watering of the landscape or to flush toilets could also be built into the design. Figure 9: Example SUDS design details within the urban landscape - T-piece control as pond overflow at Springhill, Stroud and final slot weir to SUDS at Riverside Court, Stamford The flow route analysis should also show the destination of runoff after collection, cleaning and storage. This should be to infiltration wherever possible, to a watercourse where present, and finally to the storm or combined sewer as a last resort. In Islington, due to limitations on infiltration and lack of watercourses, the outflow is normally to the combined sewer. 2. Identifying sub-catchments Identification of flow routes will be likely to lead to designation of sub-catchments, particularly on larger sites. These are small, discreet drainage areas, each with their own drainage characteristics. A sub-catchment deals with its own rainfall using a management train with appropriate treatment stages, first flush volume and often a separate flow control. Each sub-catchment should manage its own flows and volumes wherever possible. The characteristics of the sub-catchment will influence the selection of SUDS techniques and the amount of water which can be stored within its boundary. In many cases a certain amount of water will be stored in each sub-catchment with a control point to hold back runoff during heavy rain. Large volumes of clean water from bigger storms may bypass the control to reach storage structures further down the management train. 3. Building up the design Once the flow pattern is determined and any sub-catchments established, the SUDS features that best suit the conditions they need to drain are selected. The SUDS scheme should be built up around the flow routes and sub-catchments in a way which meets the agreed design criteria for quantity, quality and amenity/biodiversity. By following the above design process, the SUDS design should demonstrate how it will meet quantity criteria, follow the management train including the use of source control techniques that characterise SUDS schemes, and should be integrated with the urban and landscape design to maximise amenity and biodiversity benefits. 8

9 Design and planning process The flowchart below explains how these design stages fit into a wider development design and planning process for a particular site. This diagram has been tailored to the Islington context to reflect the types of schemes generally seen in the borough. Figure 10: Design process for SUDS schemes CONCEPTUAL DRAINAGE DESIGN A conceptual drainage design which identifies opportunities, constraints and basic criteria for drainage design should be produced at pre-application stage. The developer should enter into initial discussions with Islington Council to agree basic SUDS design criteria and adoption requirements. A drawing showing flow routes and sub-catchments (where feasible) with likely SUDS features and a preliminary SUDS design statement will be appropriate at this stage. OUTLINE DRAINAGE PROPOSALS An outline drainage proposal drawing and SUDS design statement should be provided at application stage. These should develop the conceptual design to demonstrate how the design criteria for quantity, quality and amenity/biodiversity (described on page 7) will be met and provide further detail on proposed techniques. The proposals should also describe how the drainage system will be integrated into the landscape design and the methods that will be used for linking systems together and managing flows in excess of the design event. 1.4 Adoption and maintenance Planning for who will adopt, or be responsible for, SUDS and who will maintain them to ensure they continue to provide a drainage function and contribution to the landscape, are important considerations. While many SUDS features, such as permeable pavement, have been shown to still function with very little maintenance, it is important that management considerations are addressed early on to get the best performance from SUDS. SUDS are essentially landscape features even when they are part of pavement design. Maintenance therefore comprises simple tasks like litter collection, grass cutting and inspection of simple inlets and outlets at normal monthly visits for open space management. Occasionally small amounts of silt and wetland vegetation may need to be removed as site waste. Design of SUDS should minimise maintenance requirements. For example pipe connectors should be shallow and short, allowing simple jetting to keep them clear. Inlets and outlets and control structures should be at or near the surface to allow day to day care by landscape contractors or site managers. This simple maintenance of surface SUDS features can therefore be easily incorporated into landscape management. This would be carried out by the landscape managers for any site, who may range from a private management company for a new development, the local authority parks contractor, school facilities managers or highways maintenance teams. Figure 11: SUDS features managed as part of new development, Riverside Court, Stamford DETAILED DRAINAGE DESIGN Detailed drainage design proposals, including necessary calculations, detailed drawings and a management plan, should be submitted to the Planning Authority in order to discharge relevant conditions. The final design should meet the overall design objectives described in this guide and the SUDS Manual. A full SUDS design statement should be provided at this stage. Designing the case study schemes Each of the case study designs within this document has followed the above process as far as possible to demonstrate good practice SUDS design. While they are only worked up to a conceptual stage, and therefore would need further detailed calculations to demonstrate how they have met all of the design criteria in detail, the solutions presented show how it is possible to meet the design criteria in a simple, cost-effective way. 9

10 Promoting Sustainable Drainage Systems in Islington 2. Designing SUDS in schools 2.1 Introduction Schools present a number of opportunities for SUDS, which can be used to enhance the school landscape design and provide a range of educational and play opportunities. Car parks are usually the highest pollution risk on school sites so permeable pavement is often the best solution where open SUDS are difficult to provide due to lack of space. A significant advantage of permeable pavement is its ability to collect, clean and store incident rainfall together with some adjacent surface flows. The voided, crushed stone subbases below play space, hard surface games areas such as Multi-Use Games Areas (MUGAs) must also be considered for storage even if the whole play surface is not permeable. All roofs in urban spaces must be considered for green roofs to collect and clean polluted city rain and reduce storage by 40-60%, depending on their design. 2.2 Case study: Elizabeth Garrett Anderson School The Elizabeth Garret School redevelopment comprises a westerly sloping site of 1.9 hectares. Vehicular access is from the east with pedestrian access along the southern and western boundaries. The site can be divided into a number of sub-catchments with different characteristics. Figure 13: Green roofs and permeable MUGA as part of SUDS, Exwick Heights School, Exeter Once these major surfaces have been assessed to perform a drainage function, then each component can be linked by surface conveyance in the form of rills, channels, linear wetlands or other surface features. Spouts, cascades and rainchutes can all add to a vocabulary of interesting devices that tell the water story from where it falls as rain to its destination in a river or the sea, via the sewer system. At convenient points along the path taken by the controlled flow of clean water it can create biodiversity or other educational features allowing constructive play and learning by children who would otherwise have little direct connection with the water cycle. Figure 12: Rainslides as part of SUDS at newly completed Fort Royal School, Worcester Identifying flow routes and sub-catchments As a first stage in the design process, surface water flow routes were identified. There are 3 primary flow routes through the site that follow landscape corridors, as shown on the SUDS plan for the site (figure 18): 1. Flow route 1 begins at the vehicular entrance to the site and flows northwest along the access road into a proposed landscape space and finally along the northern boundary to Rodney Street. 2. Flow route 2 is a short route beginning at the top of the steps between the MUGA and the Piazza block and flowing out to Rodney Street. 3. Flow route 3 starts at the Art and Technology Building and flows through a landscape space leading to the main pedestrian entrance from Donegal Street. It continues along the southern boundary to the southwest corner with Rodney Street. 10 1

11 These flow routes can be broken down into sub-catchments as shown in figure 18. These sub-catchments can be treated as separate but linked micro SUDS schemes which utilise a range of different techniques. A major advantage to splitting the site into sub-catchments is that runoff attenuation is distributed around the site within construction profiles that provide cleaning at source, minimise the risk of failure and reduce costs. The SUDS design process continues with location of appropriate techniques along the flow route within subcatchments ensuring the correct number of treatment stages are included in the management train. Building in SUDS techniques Flow route 1 Sub-catchment 1 comprises car parking, adjacent hard surfaces and the first part of the entrance road. This sub-catchment has the greatest risk of pollution and therefore requires two treatment stages. Permeable block paving provides an appropriate hard surface with excellent treatment characteristics and an ability to store 1/3 by volume of water in the subbase subject to careful design on the slope. A controlled flow from the permeable paving discharges runoff to sub-catchment 2. Figure 14: Swale and basin at entrance to Exwick Heights School, Exeter The landscape area within sub-catchment 2 (as well as other sub-catchments) provides opportunities for a range of attractive, educational spaces which make use of controlled flows of clean water from previous sub-catchments. These flows should be conveyed in attractive surface low flow channels, such as rills or linear wetlands, with runoff from the sub-catchment itself being cleaned and stored before it joins the onward flow through the site. The low flow channels, swales and basins will come alive when it rains, but at other times be attractive, usable landscape details in their own right. This flow of clean water should be controlled before it travels onward to sub-catchment 3. Sub-catchment 3 provides a final conveyance for Flow Route 1 as an open wetland channel potentially leading to a pond in the biodiversity area. An example design is shown in figure 15 below. It also includes a MUGA (multi-use games area) that should be a pervious construction with an open stone base which should collect runoff from adjacent hard surfaces and provide overflow storage for Flow Route 1 and part of Flow Route 2. A simple kerb upstand around a level permeable hard surface can provide mm of storage during very heavy rainfall when everyone is sheltering from the rain. Once the rain stops, these surface storage areas immediately begin to drain down to the level of the playing surface. This is a cost effective, sustainable way to manage occasional intense storms. Figure 15: Swale and raingarden basin in open space Sub-catchment 2 includes the end of the road and turning circle leading to a multi-functional landscape space yet to be designed. The turning circle should not use permeable block paving due to turning of vehicles and excessive cutting of blocks. The central island could be designed to act as a central runoff collector and cleaning area, or water could flow directly to a swale or basin and then into a low flow channel in the landscape area. Flow Route 2 Sub-catchment 4 includes a terrace, steps, roofs and landscape space towards Rodney Street at the western site boundary. SUDS techniques should include green roofs on all roofs where feasible, some permeable surfaces and the use of plant beds to control and clean runoff. This short route may also be a good location for an educational water feature and pond before controlled discharge to a link with the combined sewer on Rodney Street. 11

12 Flow route 3 Sub-catchment 5 begins the hard flow route starting with an existing Art and Technology building. Although retained and currently connected directly to the combined sewer, this could be disconnected and be included in the flow route management train. A green roof could be retrofitted to the building to attenuate and filter runoff. The sub-catchment also includes landscape with a number of hard surface areas. The plant beds within the subcatchment should be designed to collect at least the first flush volume of runoff (10-15mm) from adjacent impermeable areas and the roof with excess clean water directed to storage structures. A controlled discharge from these structures could then flow in an open feature rill or channel along the southern boundary to the southwest corner of the site. a shallow permeable surface designed to accommodate at least the first flush volume to provide cleaning with excess flowing to adjacent storage structures. Figure 17: Children playing in detention basin and swale maze, Red Hill School, Worcester Sub-catchment 6 is proposed as a hard surface area but could collect runoff in protected filter drains at the edge of ramps and pathways. The creation of some permeable surface near the existing trees would assist with water stress in summer and provide a surface link for overflows. The collection features should again deal with the first flush volume with excess flows directed through filters to the storage structures proposed in Sub-catchment 8. Figure 16: Planted filter drain cleans and stores runoff from play areas, Exwick Heights School Summary of design benefits As this case study demonstrates, SUDS offer significant opportunities for enhancing school landscapes and encouraging learning about water and the environment. At EGA, water is collected in interesting ways within each sub-catchment, for example using spouts and cascades, and conveyed in swales, rills and channels to storage in multifunctional open space or within the construction profile. Wherever possible, water then flows to attractive biodiverse wetlands and ponds before entering the combined sewer. In addition to meeting quantity criteria, including by achieving a greenfield runoff rate, the design delivers clean water which can be used to create an attractive, biodiverse and educational landscape. 2.3 Adoption of SUDS within schools Sub-catchment 7 is roof and a raised platform to form a Piazza area. Runoff from surrounding green roofs and direct roof water to the Piazza should be dealt with within the Piazza area. This could be done through design of a proportion of the Piazza surface being Generally SUDS would be managed by school facilities managers as part of the wider landscape and buildings maintenance programme. As described in Section 1.4 above, this could be done as part of the ongoing landscape management contract or by the existing facilities managers. 12

13 Figure 18: Conceptual SUDS plan for Elizabeth Garrett Anderson School 13

14 3. Designing SUDS in new developments 3.1 Introduction Development in Islington almost always takes place on previously developed land with an existing connection to the combined sewer. Most developments in the borough are also of very high densities with little outdoor space. In these situations, flow routes have been detroyed by previously developed urban features and natural drainage may be difficult to replicate. However, despite the challenges presented by this context, there are a number of opportunities to incorporate SUDS within new or refurbished developments in a way which meet design standards discussed in Section 1 and provide a range of benefits for the development and the wider environment and local area. In urban areas, particularly in very dense developments where green space is minimal and may be completely absent, every hard surface becomes a rainwater collector and the construction profile must be considered for runoff management. As demonstrated in the case study in Section 3.2, with good multifunctional design, quantity and quality criteria can be met and clean water be used to provide amenity and biodiversity benefits within even the smallest, densest development. 3.2 Case study: Caledonian Road The case study development at Caledonian Road is a major residential development which provides around 50 residential units with some ground floor commercial units, around a small central courtyard space. There are three surfaces that intercept rainfall at Caledonian Road: the roof, the courtyard space and car parking. Each surface must be considered as a SUDS feature as there is no other destination for the water. The design process for SUDS at on the Caledonian Road scheme considers how each of these surfaces can be used as part of a wider SUDS scheme to meet agreed design criteria and create an attractive environment for residents. Figure 20: Biodiversity-based green roof, Islington Council Municipal Offices, Upper Street Figure 19: Bioretention planter within an urban development, Portland, Oregon Roofs as SUDS features Green roof technology is now well established as a technique to collect, clean and attenuate flows from roof surfaces with an agreed 40 60% reduction in runoff co-efficient depending on the roof construction, in particular the substrate depth. Green roofs are particularly appropriate for new buildings where management is assured and the whole life benefits can be demonstrated. 14

15 Additional storage can be provided by green roofs through the inclusion of a shallow rainfall storage box, as shown in figure 21 below. This may be particularly important where space is very limited and underground attenuation is not possible. In addition to benefits relating to SUDS, green roofs also offer a range of other benefits to biodiversity, in cooling the building and mitigating the urban heat island effect, providing additional insulation and reducing noise and local air pollution. Figure 21: Biodiversity based green roof with additional box storage There are a number of SUDS features that can be used in courtyard design: permeable pavement with many design opportunities (see figure 34) raingarden and bioretention features that use planting areas as drainage structures under-drained grass surface using filter drains below the surface enhanced storage below surface collectors using geocellular boxes or other storage devices simple basins where space allows urban wetlands and ponds (see figures 22 and 23). Examples of these features within new developments are shown throughout this document. An example of an urban pond design as part of a wider SUDS scheme is illustrated in more detail in figure 23. These images illustrate the value that can be added to new development through the use of clean water at the surface, both for biodiversity and for local residents. These features can be designed in a number of ways to fit with the character and aspirations of the landscape design, to provide biodiversity improvements which support local priorities and to ensure a healthy, safe environment. Figure 22: SUDS features in urban developments: pond at Springhill Housing, Stroud (left) and wetland at Riverside Court, Stamford Water from a green roof is clean and flows off the roof in a controlled manner. This water can be stored in boxes at roof level or be managed at ground level, where it arrives already clean and at a controlled rate ready for use or storage. The water can be delivered directly into surface water features or can be fed into permeable paving or other storage features. Urban courtyards and pedestrian space Urban courtyards and other pedestrian space must also be considered as a rainfall collector with integrated cleaning and storage. An advantage of pedestrian space is that traffic loadings and serious oil spillage do not need to be taken into account in the design. Pedestrian courts only require a single treatment stage before rainfall is stored to be released slowly to the combined sewer or used for amenity in water features in the landscape. This treatment stage can be provided by the green roof or other features at ground level. 15

16 Figure 23: Cross section of urban SUDS pond Figure 24: Permeable paving in new development, Riverside Court, Stamford and outside Islington Town Hall Car parking or other vehicular access/servicing areas Where parking or servicing and access areas are provided in new residential or commercial development these should use permeable pavement to provide an effective primary treatment stage and full storage. Ideally the water should receive a final polishing treatment stage in a surface landscape feature before release to the sewer or use in the landscape. Other servicing or access areas may also provide opportunities for permeable paving. Where permeable paving is not feasible, runoff should be directed into other SUDS features for cleaning and storage. Summary of design benefits The above case study demonstrates how SUDS can be designed into even very dense developments, by making use of all surfaces in innovative ways. Even where space is very limited, storage for runoff can be provided under green roofs or paved areas to meet quantity standards outlined in this guidance. SUDS also have the potential to add significant value to new developments by creating attractive landscape features which make the most of water at the surface to provide benefits for residents and wildlife. SUDS features described above can boost a development s overall sustainability strategy through secondary benefits, for example in improving energy efficiency of buildings, providing water for reuse and creating new habitats to support local Biodiversity Action Plan targets. As well as providing a cost effective way of meeting planning requirements around reducing runoff, a number of studies have suggested water features which often form part of SUDS schemes can significantly increase the sales or rental value of properties. 3.3 Adoption of SUDS within new developments Many new developments, like Riverside Court at Stamford (see figure 22 and 24), are looked after by management companies. This is a common situation in London and it is relatively straight forward to include the maintenance of SUDS in the management plan, as part of the general landscape management. Where areas of new development are adopted by the Local Authority, such as areas of open space or highways, the SUDS features within these should be maintained as part of ongoing management of these spaces. 16

17 Figure 25: Conceptual SUDS plan for a new development 17

18 4. Retrofitting SUDS in existing housing estates 4.1 Introduction Figure 27: Retrofitted SUDS features within housing estate at Augustenborg, Malmo Existing housing estates in Islington provide significant opportunities for retrofitting of SUDS as they often include large areas of passive green space without defined function. These spaces can be used to reduce floodrisk both on the estate itself and the wider local area by collecting and storing water. SUDS can also be used to encourage infiltration of rainwater into the ground in order to reduce drying out of clay soils which has led to subsidence problems on Homes for Islington (HFI) estates (see figure 26). SUDS can also enhance social spaces within estates in an attractive and useful way. Figure 26: SUDS and subsidence In Islington, like a large proportion of the south east of England, much of the existing housing stock is built on shrinkable clay soils. When the moisture content of the clay soils is reduced the clay shrinks causing downward movement. This extraction of moisture from the clay may occur in a number of ways, but the main causes are either prolonged periods of dry weather (a combination of high average temperatures and low average rainfall) or tree roots extracting the moisture from the soil in close proximity to a property. The most extreme cases occur when a combination of the two causes is evident. Over the past few years the UK has seen some of the driest periods of weather on record which has meant that clay soils have not been able to replenish their seasonal moisture loss during the winter months. This has caused an increase in the number of cracks appearing in properties and a rise in the number of subsidence claims made against insurance companies. As a result of these issues, HFI has experienced subsidence at a number of sites, such as Highbury Quadrant; problems have also been exacerbated by historic issues, for example lack of backfilling after bomb damage or of old basements. SUDS can help address problems of subsidence by facilitating natural rehydration of clay soils, particularly in summer. SUDS techniques which offer options for retrofitting within existing housing estates include: disconnection of downpipes diverted to swales, basins or rain gardens diversion of flows from hard paving to similar features road runoff diverted to a primary cleaning feature like a basin or swale with an underdrain road runoff collected through a permeable surface into storage before release where surface water is undesirable, the use of an under-drain and underground storage may be needed. A seminal example of this approach has been demonstrated in Augustenborg, Malmo in Sweden where simple disconnection of downpipes and kerb crossings have diverted water into public landscape. The result is an attractive and functional landscape that cleans day to day rainfall and takes the peak out of large storms. 4.2 Case study: Ashby Grove Estate Ashby Grove housing estate comprises a number of housing blocks in a serviced green space with some gardens and access roads within the development. The site is contained by main roads on two sides, with Ashby House on the southern boundary and a convenient line dividing the internal space into two landscape spaces. Identifying flow routes and sub-catchments The natural flow routes on the site have been severely disrupted by development and the position of discharge points to the sewer; however three pathways for water can be identified within the catchment, as explained below and shown in figure 32. In the event of surcharge, water will either flow to Ashby Grove or south to Cannonbury Crescent and the estate car park. 1. Flow route 1 collects roof water around the outside of the housing blocks where they adjoin the main road. A grass and service strip is located within this area, as is characteristic of this type of development. 18

19 2. Flow route 2 collects roof water within the soft landscape area and carries it between housing blocks to the lowest point locally for discharge to the sewer. 3. Flow route 3 intercepts road runoff along parallel to route 2 and carries water to the sewer discharge point. This discharge point is surrounded by a static space which can be considered a parking or access court. 4. Flow route 4 makes use of a currently unattractive paved court. In the event of environmental improvement with new paving and planting, the use of permeable paving and bioretention planting should be considered to deal with this area. There are a number of benefits of this simple approach: the rate, volume and frequency of runoff is reduced clay soils are rehydrated naturally especially during summer the rain garden can provide an attractive amenity for the development site planting including large trees are watered naturally raingardens are cost effective to construct and maintain Figure 29: Raingarden, Leeds Council Flood Prevention Garden Designing in SUDS techniques Flow route 1 As a first step, a review of the suitability of any roof for retrofitting with a green roof should be undertaken on all retrofit sites. In this case, the corner building, now a church, would be considered for this SUDS feature, although other roofs on the site are too steeply sloping to incorporate green roofs. The passive space that separates the main roads from the building frontage offers a route for rainwater from part of the roof and small hard surface areas around entrance lobbies. A simple disconnected downpipe leading roofwater to a planted depression would create a classic rain garden, as shown in figure 28 below. Most day to day rainfall will collect for short periods of time in the raingarden and soak into the ground even on clay soils. When heavy rainfall occurs, there is a simple grated overflow that directs surplus water to the sewer. Figure 28: Retrofitted raingarden within a housing estate Flow route 2 Rain water from the rear of buildings can be intercepted and carried through back gardens in a number of ways, either in elevated gutters and channels for visual effect, or in pipes to the garden edge. Provision of simple water butts such as those shown in figure 30 could also be considered to provide additional amenity benefits for residents. A simple but attractively designed swale with bridges across to back gardens takes the flows through the public open space. At any point along the flow route, it will be possible to create an attractive water feature based on a simple basin design. This could be a dry play space that occasionally floods, a wetland maze or even a pond feature if this is considered to be desirable. This approach has been used successfully in Augustenborg, Malmo, Sweden, as shown in figure

20 The flow eventually reaches the end of the greenspace where it can discharge to the combined sewer. Again the advantages of this solution are similar to the rain gardens along flow route 1. Figure 30: Rainwater butt and urban rill as part of a SUDS scheme at Springhill, Stroud and retrofitted SUDS in play area within housing estate, Augustenborg, Malmo refurbishment is proposed for development. The benefits of storage and cleaning provided by permeable pavements are well known and they should always be considered wherever new paved surfaces are proposed. Figure 31: Filter strip and underdrained basin Summary of design benefits Flow route 3 This flow route addresses the problem of urban road runoff. Roads and parking constitute a high risk of pollution and require a second treatment stage to ensure clean water leaves the SUDS or is available for amenity. The first SUDS feature proposed is a bioretention area, a variation on the raingarden that provides enhanced treatment before water leaves the basin. Bioretention areas include a filter mechanism for the first flush volume that carries most pollution when rain falls on hard surfaces. A free-draining topsoil allows the first polluted part of runoff to soak into a drainage layer underneath the ground. The basin can fill to mm deep with an overflow to the sewer when severe storms occur. An additional advantageof this technique is that oils and other organic pollutants are trapped and treated just below the surface to avoid contamination that could cause a problem for amenity use. Rain gardens and bioretention features, including the varients shown in fitures 31 and 38, should be used wherever there is space to collect roofwater or runoff from hard surfaces as they are easily retrofitted to existing housing areas with public open space. The second SUDS feature that can be considered for parking areas in particular is permeable pavement (see figure 37). This will normally only be possible where a full The Ashby Grove example demonstrates the benefits retrofitting of SUDS within housing estates could offer to Islington. Landscape improvements such as those described above provide a cost-effective way of reducing local flood risk by reducing pressure on the sewer system. This is likely to be particularly important as climate change brings heavier rainfall and intensification of the borough continues. While retrofitted SUDS schemes within housing estates are unlikely to be able to achieve the quantity standards which would be expected of new developments, schemes could achieve significant betterment over existing runoff rates and lead to a significant cumulative impact. In addition, SUDS could also reduce risk of subsidence, deliver low maintenance improvements to landscape quality and provide wider amenity benefits to residents, such as rainwater reuse. 4.3 Adoption of SUDS within existing housing The existing housing estates considered for SUDS retrofit in this case study are already managed by Homes for Islington, an arms length management organisation linked to the Local Authority. Adoption of SUDS retrofit systems requires an understanding and commitment by the current management team but should be seen in the same way as environmental improvement where existing landscape features require maintenance. The mechanism for adoption therefore already exists in these SUDS retrofit housing schemes. SUDS can be designed to fit with existing management regimes, including to be low maintenance. 20

21 Figure 32: Ashby Grove Conceptual SUDS Plan 21

22 5. SUDS and highways 5.1 Introduction Roads and associated traffic areas are particularly important in SUDS as they comprise large areas of impermeable surface that contribute significant pollution to the runoff from urban areas. The integration of roads and SUDS has proved a challenge because of difficulties in collection of runoff from linear trafficked surfaces and the perceived conflicts between SUDS design requirements and current highway management practices. However, there are now many examples of the use of SUDS technology in road design with exemplars both in Britain and overseas., Figure 33: Street SUDS planter, Portland ( Environmental Services, Portland, Oregon) Collection of rainfall from road surfaces can be separated into two distinct mechanisms: permeable surfaces intercepting rainfall in-situ impermeable surfaces shedding runoff to adjacent SUDS features. Permeable Pavements Use of permeable paving is not new; techniques used are very similar to the road construction used by the Romans and to the original Macadam road specification before tar was introduced as a surface binder. Water percolates through an open road surface to the road into a crushed stone sub-base below, as shown in figure 34 below. The re-discovery of this open construction for roads has caused concern for conventional road designers because water is deliberately introduced into the construction rather than excluded from a water susceptible sub-base. However, there are now many examples of permeable pavement in England with some Highways Departments in Local Authorities adopting the surfaces subject to agreed design criteria. The use of permeable surfaces in urban SUDS design is critical because space is at a premium and permeable pavement, along with green roofs, are the only SUDS techniques that require no additional land take to function effectively. Runoff that has passed through permeable pavement is clean and usually available for use within the urban landscape as it is stored within mm of the surface. Permeable surfaces can accommodate additional volumes, collecting up to twice the surface area of the permeable paving, allowing adjacent impermeable surfaces, including service strips, to drain effectively. Design criteria should be agreed with the Local Authority as demonstrated by Councils pioneering SUDS adoption, such as Oxfordshire County Council. Figure 34: Cross section of permeable paving Interception of rainfall from roads Runoff must be dealt with locally to where it falls to provide source control before clean water flows into public open space for amenity or urban biodiversity. Source control also reduces flow, volume and frequency of runoff from roads. 22

23 Runoff control within the road boundary There is now a body of experience in collecting runoff directly to adjacent SUDS features for control of both flows and volumes together with reduction of pollution. In less dense development the runoff may flow directly to open swales or basins but in urban areas a more engineered design solution is needed. The experience gained in Portland, Oregon, USA has been used as a design source for this section. Landscape structures used to intercept, collect, clean and store flows from impermeable surfaces must meet certain design criteria: the structure must drain the road surface the existing road construction must be protected runoff must drain down within hours the SUDS feature must contribute to the urban landscape road SUDS must be cost effective to construct and maintain. Street SUDS planters introducing planted SUDS between the road and building elevations (see figures 33 and 36) Road SUDS planters using incidental hard surfaces, such as street intersection areas or traffic islands, as bioretention planters Bioretention planters modifying existing landscape to include engineered topsoil and underdrains Raingardens modifying existing landscape to collect water at the surface in planted basins Figure 36: Street SUDS planter, Portland ( Environmental Services, Portland, Oregon) Figure 35: Controlling runoff using a combination of permeable paving, planters and filter drain, Portland, USA Surface collection of road runoff to adjacent public open space The categories of green street structures identified based on the Portland model comprise: Permeable surfaces used wherever possible Kerb extension planters extending SUDS features into the road as part of environmental street design In certain circumstances, it is practical to collect road runoff directly and convey it to public open space for treatment and amenity use. Runoff should be collected as near as possible to the surface to avoid deep pipes and difficult management. Open cross kerb inlets provide the best collectors for road runoff but kerb gullies or chute gullies can be used to transfer polluted runoff to a SUDS management train beginning with a silt interceptor and source control treatment stage before water flows to any amenity feature. Further information on use of open spaces is provided within the Parks Section of this guidance in Section Six. 23

24 5.2 Case study: Skinner Street Figure 38: Cross section through roadside kerb extension planter A level survey of the existing street landscape and a review of major service corridors is the first requirement before retrofit can be considered for roads and streets. This will identify likely flow routes and opportunities for in-street modification. The camber of the road should be established and whether this can be modified to allow proposed SUDS improvements. An evaluation of the road layout at Skinner Street identifies a number of possibilities: Permeable surfaces the cycle route is isolated from heavy traffic and could be resurfaced with permeable block paving or even a porous asphalt Kerb extension where a reduction in on-street parking or carriageway width is acceptable, then the resulting space can become bioretention features Street planters between the road and building frontages may be difficult to create in this situation due to road levels, although space is available The intersection with Corporation Row provides space within the road area for a road SUDS planter to collect runoff in this difficult location The public park offers a SUDS retrofit opportunity as it is lower than surrounding road surfaces, can receive shallow runoff collection, and provide SUDS amenity within the park before release of remaining water to the sewer using a bioretention basin and raingarden. It is important to recognise that all these green street SUDS features will reduce the amount of water entering the sewer and provide amenity opportunities for the community when designed in accordance with the SUDS Manual and Guidance from Islington Borough Council. Figure 37: Bird s eye view of a roadside kerb extension planter Summary of design benefits The Skinner Street example demonstrates the potential benefits of retrofitting SUDS within Islington s highways, in combination with parks and open spaces (see Chapter 6). The incorporation of SUDS within ongoing landscape improvements and highways works where feasible will contribute to wider efforts to reduce local surface water flood risk. While retrofitted SUDS schemes within highways may be unable to achieve the quantity standards which would be expected of new developments, individual schemes could achieve significant betterment over existing runoff rates and therefore have a significant cumulative impact. In addition to these benefits, SUDS could also deliver improvements to landscape quality and reduce management requirements associated with watering of trees and planting, particularly as climate change leads to reduced summer rainfall. 5.3 Adoption of SUDS on highways SUDS features within highways would be adopted by Islington Council and maintained as part of the wider highways maintenance programme. There are Local Authorities currently adopting permeable pavement, swales and other SUDS features, subject to certain conditions. Currently there is little agreement on adoption issues but there are some councils, such as Oxfordshire, which are leading the way in doing this. At present the primary road surface is seldom considered for permeable pavement but many adjacent area may be considered for adoption. These areas include parking courts, parking bays, home-zones and, in the case of Skinner Street, a facility like the cycleway. Where the remit of the Local Authority highways engineer is flexible, these areas provide an opportunity to create an integrated sustainable drainage strategy for urban space. 24

25 Figure 39: Conceptual SUDS Plan for Skinner Street 25

26 6. SUDS and parks 6.1 Introduction Parks and public open space offer one of the few opportunities in cities for managing large volumes of rainfall in surface features like swales, basins, ponds and wetlands. They can intercept flows from surrounding development, and provide a full management train with a range of amenity possibilities to enhance the park landscape. Where the levels of the public open space allow, then large volumes of urban runoff can be stored when heavy rainfall occurs. The principles of source control and interception of first flush volumes ensure that only clean water should enter these public spaces and occasional inundation of the park landscape has minimal effect on planting and hard surfaces. Park design may require simple measures to accommodate stored water but in general the benefits of a dynamic landscape will outweigh any short term inconvenience. Figure 40: Surface water storage in Manor Park, Sheffield both taken during flood conditions, Summer 2007) 6.2 Case study: Kings Square Park Kings Park is an example of a significant public open space surrounded by development that generates large volumes of runoff from roofs and adjacent hard surfaces. The park is bounded to the south by a main road with an access road to the east and large residential buildings to the west and north. Levels are critical in assessing the potential for surface water management in public open space. The park is relatively level falling gently to the east and the access road in front of St Bartholomew s Church. There is also a local depression at the western end. Each surrounding landscape element is considered as a source of runoff and a strategy is developed to collect, clean, store and use the water for amenity: A double tower to the west brings rainfall to the ground in a few downpipes which can be intercepted on the eastern side and brought on the surface into the park. A green roof should be considered for the buildings however as a first option. A housing block along the northern boundary is located a storey below the park level but downpipes service the building on the park elevation. Following an evaluation for green roof suitability, the feasibility of intercepting rainfall at first floor level should be considered. The carrier pipes could be seen as part of a back garden enclosure strategy with rainwater conveyed on the top of a garden screen into the park. The main road to the south appears too low for direct connection to the park but the concept of interception of local road runoff is important because of the treatment opportunities that public open space offers. The access road to the east of the park provides a unique opportunity for the park as it is proposed to introduce permeable pavement to the surface. Although it would be normal to discharge the stored volume of clean water to the sewer, in this situation it would be possible to pump water as it was collected into the park for amenity purposes. All surface runoff from park structures, pathways and other hard surfaces should be collected at the surface within the park for storage and use. Flow routes through the park The natural flow routes across the park should be developed to carry intercepted flows in an interesting way through the features that comprise the park landscape. Flow route 1 As a first consideration, green roofs should be installed on flat roofs of surrounding housing estates where feasible to provide source control. Runoff from both the double tower and the housing block along the northern boundary should then be intercepted via downpipes and channeled into the park, for example using simple rainchutes or surface channels. Permeable paving could also be retrofitted within hard surfaced areas, for example on the road to the eastern park boundary, and could also be used to clean and collect rainwater which could be pumped intermittently into the park to augment the flow of water from roofs. 26

27 Water entering the park should be conveyed in swales or channels to a variety of features, such as basins, wetlands or ponds. This is appropriate as water from both sources, roofs, and access roads will be clean following a minimum of one full treatment stage. SUDS features could be designed to be dry for a large part of the time, and thus be usable for recreation or play, and to fill up or flow with water following rainfall, adding interest to the park. Where significant volumes of water can be channeled into the park, a larger permanent water feature, such as a wetland or pond, could be created. In this way SUDS can support delivery of biodiversity improvements within open space. Water can also be used to create play features, such as those shown in figure 41 below. Figure 42: SUDS within landscape design, The Sustainable Village, Davis County, USA now over 30 years old Flow route 2 This flow route would be more difficult to implement on this site, but would be important if adjacent road runoff could be intercepted, cleaned and stored before flowing to the south east corner of the park. The flatness of the park allows for a very flexible routing of water and all runoff could flow through a variety of features, such as bioretention areas, basins or formal water features, to the north-east or south-east corner for discharge to the sewer. The critical requirement for use of runoff in the park is that water is clean and the flow routes and volumes can be predicted to ensure a safe environment for the public. These requirements are set out in the design criteria included within this note, as well as in wider SUDS Guidance. Figure 41: Pforzheim Water Playground - play features can be incorporated as part of SUDS design (Image Waterscapes: Planning, Building and designing with Water Edited by Herbert Dreiseitl, Dieter Grau and Karl H. C. Ludwig: Birkhauser) Summary of design benefits Parks provide a major opportunity for cleaning, controlling and storing runoff from surrounding roads and buildings. Retrofitting of SUDS measures within parks should provide a cost effective way of reducing flood risk within Islington, particularly where these are implemented as a part of ongoing parks improvements works. SUDS also offer opportunities for open space enhancements by bringing water in a controlled way into open space for use in a variety of ways, whether as permanent water features or multiuse spaces which incorporate a drainage function. SUDS can also be designed into parks to reduce watering requirements, naturally irrigating trees and planted areas, and therefore reducing management requirements. 6.3 Adoption of SUDS within parks SUDS features within parks should be adopted by the Local Authority and managed as part of the ongoing management of parks. Management tasks are likely to be covered by existing maintenance regimes, for example mowing grass, weeding as well as other simple tasks such as general checks to inlet and outlet structures. 27

28 Figure 43: Kings Square Conceptual SUDS Plan 28

29 7. Frequently asked questions 1. Is it feasible to incorporate integrated SUDS schemes in a high density, urban environment? The SUDS approach can be used on any site. In high density urban sites the use of more engineered SUDS, including green roofs and permeable pavement, must be explored and supported by all parties to achieve an acceptable design. For example, the Riverside Court development in Stamford demonstrates a high density of 106 units/hectare by using permeable pavement augmented with sub-base replacement storage as the primary technique to meets SUDS standards. Surface conveyance using rills and canals provides amenity, some biodiversity interest and a robust exceedance route should the site be inundated. All surfaces must be considered from a drainage perspective to allow collection, cleaning and storage within the development boundary. 2. What evidence is there around whether SUDS work? SUDS, called Stormwater Management or BMPs in the US, have been used in many States for over 30 years. There is an extensive literature about the benefits and robustness of SUDS with ongoing research both abroad and in Great Britain. There are very few failures documented and where these occur it is generally due to poor design. For further detail please see list of further information sources in Section Are SUDS feasible in Islington given the ground conditions, particularly the impermeable clay substrate? SUDS mimics natural drainage patterns as they would occur with a range of ground conditions. Where soils are impermeable and limited infiltration is possible, SUDS will collect, clean and store rainfall before channelling it into available watercourses or to the sewer. Even on clay soils, some infiltration will occur and will bring associated benefits for reducing subsidence and providing irrigation. Other water losses will occur through transpiration or evaporation. However, appropriate SUDS design which assumes a realistic level of infiltration will be needed to create a SUDS scheme which works with the existing ground conditions. 4. How easy is it to manage and maintain SUDS systems and who would be responsible for this? SUDS are easy to manage because they are landscape features and generally at or near the surface. The problem of who takes responsibility for the systems can be troublesome because of historical arrangements in place designed to manage a pipe system. Once the administrative blockages are removed then care of SUDS will be straightforward and will generally not incur significant cost (see below). 5. How do the costs of SUDS compare to conventional drainage, both in terms of capital and maintenance costs? The cost of building SUDS, comparing like for like in terms of storage, are nearly always 10-30% cheaper than conventional drainage. Similarly the maintenance of SUDS is simple using landscape management techniques integrated with general site care. Costs of management can be difficult to confirm due to difficulty in allocating maintenance to a specifically SUDS function but it is again nearly always cheaper than for piped systems and may provide additional savings in reducing the need for artificial irrigation. 6. Is it safe to create areas of open water or wetlands, particularly in housing estates and parks where children are likely to play? The concept of risk is changing as we recognise that recent urban design has resulted in a sterile environment. All open water, wet places or features that can change in character when it rains must be evaluated based on a risk assessment. However judgement is a critical element in this process and where water has been reintroduced into peoples lives the response from local people has nearly always been positive so long as SUDS are well designed and well maintained. 7. How do the longevity of SUDS features, such as permeable paving, compare to conventional drainage features? Increasingly the whole life costs of SUDS are comparing well with conventional systems. Careful choice of techniques by designers and competent construction by contractors should result in good whole life costs. Concerns around longevity of permeable pavement are now appearing unfounded, as siltation seems to remain in the block joints rather than migrating into the structure. This allows straightforward reinstatement should blockage occur using brushing/jetting and suction rather than lifting blocks. 8. Does Thames Water accept SUDS techniques and calculations as a way of meeting their drainage requirements? Following meetings with Thames Water it is clear that there is a consensus on the need for SUDS and it is the detail that must be agreed to meet all parties concerns. 9. How can permeable paving be combined with services within roads? One important consideration when using permeable pavement is to ensure that services are located in areas that allow access or will not damage the performance of SUDS when maintenance is undertaken. Therefore pavement design, both visual and functional, must take this requirement into account. Permeable pavement will accommodate up to twice its own surface area of water so the design of a pavement sympathetic to services is usually possible. 10. How can SUDS be combined with rainwater harvesting? Rainwater harvesting is different to SUDS in that it seeks to hold water for as long as possible rather than discharge it within 48 hours to be ready for the next storm, like SUDS. However rainwater harvesting is being successfully integrated into SUDS, particularly in schools, and overflows can discharge directly into SUDS storage like any other collection system. Permeable surfaces are being used as a convenient filter in a number of rainwater harvesting systems with water stored in shallow boxes within sub-bases. This demonstrates the crossover between the two technologies. 29

30 8. Glossary pollutants, and the first flush portion of the flow may be the most contaminated as a result. Amenity Attenuation Balancing pond Base flow Basin Biodiversity Bioretention area Catchment Combined sewer Control structure Conventional drainage Conveyance Design criteria Detention basin Detention pond Filter drain Filter strip Filtration First flush The quality of being pleasant or attractive; agreeableness Reduction of peak flow and increased duration of a flow event. A pond designed to attenuate flows by storing runoff during a storm and releasing it at a controlled rate during and after the storm. The pond always contains water. The sustained flow in a channel or drainage system. A ground depression acting as a flow control or water treatment structure that is normally dry and has a proper outfall, but is designed to detain stormwater temporarily. The diversity of plant and animal life in a particular habitat. A depressed landscaping area that is allowed to collect runoff so it percolates through the soil below the area into an underdrain, thereby promoting pollutant removal. The area contributing surface water flow to a point on a drainage or river system. Can be divided into sub-catchments. A sewer designed to carry foul sewage and surface runoff in the same pipe. Structure to control the volume or rate of flow of water through or over it. The traditional method of drainage surface water using subsurface pipes and storage tanks. Movement or water from one location to another. A set of standards agreed by the developer, planners, and regulators that the proposed system should satisfy. A vegetated depression that is normally dry except following storm events. Constructed to store water temporarily to attenuate flows. May allow infiltration of water to the ground. A pond that has a lower outflow than inflow. Often used to prevent flooding. A linear drain consisting of a trench filled with a permeable material, often with a perforated pipe in the base of the trench to assist drainage. A vegetated area of gently sloping ground designed to drain water evenly off impermeable areas and to filter out silt and other particulates. The act of removing sediment or other particles from a fluid by passing it through a filter. The initial runoff from a site or catchment following the start of a rainfall event. As runoff travels over a catchment it will collect or dissolve Flood routing Flow control device Geocellular structure Geomembrane Geotextile Green roof Greenfield runoff Gully Habitat Infiltration Infiltration basin Infiltration device Interception storage Management train Permeable pavement Porous paving Public sewer Raingarden Rainfall event Design and consideration of above-ground areas that act as pathways permitting water to run safely over land to minimise the adverse effect of flooding. This is required when the design capacity of the drainage system has been exceeded. A device used for the control of surface water from an attenuation facility, e.g. a weir. A plastic box structure used in the ground, often to attenuate runoff. An impermeable plastic sheet, typically manufactured from polypropylene, high density polyethylene or other geosynthetic material. A plastic fabric that is permeable. A roof with plants growing on its surface, which contributes to local biodiversity. The vegetated surface provides a degree of retention, attenuation and treatment of rainwater, and promotes evapotranspiration. The surface water runoff regime from a site before development in Islington assumed to be 8litres/second/hectare. Opening in the road pavement, usually covered by metal grates, which allows water to enter conventional drainage systems. The area or environment where an organism or ecological community normally lives or occurs. The passage of surface water into the ground. A dry basin designed to promote infiltration of surface water into the ground. A device specifically designed to aid infiltration of surface water into the ground. The capture and infiltration of small rainfall events up to about 5mm. The management of runoff in stages as it drains from a site. A permeable surface that is paved and drains through voids between solid parts of the pavement. A permeable surface that drains through voids that are integral to the pavement. A sewer that is vested and maintained by the sewerage undertaker. A planted basin designed to collect and clean runoff. A single occurrence of rainfall before and after which there is a dry 30

31 Rainwater butt Recharge Retention pond Return period Rill Runoff Sediments Sewer Silt Soakaway Source control Sub-base Sub-catchment Surface water Swale Treatment Watercourse Water quality treatment volume Wetland period sufficient to allow its effect on the drainage system to be defined. Small scale garden water storage device which collects rainwater from the roof via the drainpipe. The addition of water to the groundwater system by natural or artificial processes. A pond where runoff is detained for a sufficient time to allow settlement and biological treatment of some pollutants. Refers to how often an event occurs. A 100-year storm refers to the storm that occurs on average once every hundred years. In other words, its annual probability of exceedance is 1% (1/100). An open surface water channel with hard edges, used to collect and convey runoff. They can be planted to provide a cleaning function. Water flow over the ground surface to the drainage system. This occurs if the ground is impermeable, saturated or rainfall is particularly intense. Sediments are the layers of particles that cover the bottom of waterbodies such as lakes, ponds, rivers and reservoirs. A pipe or channel taking domestic foul and/or surface water from buildings and associated paths and hard-standings from two or more cartilages and having a proper outfall. The generic term for waterborne particles with a grain size of 4-63mm, ie. between clay and sand. A sub-surface structure into which surface water is conveyed, designed to promote infiltration. The control of runoff at or near its source. A layer of material on the sub-grade that provides a foundation for a pavement surface. A division of a catchment, to allow runoff to be managed as near to the source as is reasonable. Water that appears on the land surface ie. lakes, rivers, streams, standing water, and ponds. A shallow vegetated channel designed to conduct and retain water, but may also permit infiltration. The vegetation filters particulate matter. Improving the quality of water by physical, chemical or biological means. A term including all rivers, streams, ditches, drains, cuts, culverts, dykes, sluices, and passages through which water flows. The proportion of total runoff from impermeable areas that is captured and treated to remove pollutants. Flooded area in which the water is shallow enough to enable the growth of bottom-rooted plants. 9. Further information sources Further information on SUDS can also be found at: CIRIA provide a range of advice and publications on SUDS, including the SUDS Manual, Sustainable Drainage Systems design manual for England and Wales and Sustainable Water Management in Schools Environment Agency provides a range of guidance on SUDS, including planning advice Interim Code of Practice for SUDS - provides advice on design, adoption and maintenance issues and a full overview of other technical guidance on SUDS Cambridge Sustainable Drainage Design and Adoption Guide North London Strategic Flood Risk Assessment, including Section 9 Guidance for Developers ork/evidence_base/sfra.asp Waterscapes: Planning, Building and designing with Water. Edited by Herbert Dreiseitl, Dieter Grau and Karl H. C. Ludwig: Birkhauser Key planning policies relating to SUDS include: Planning Policy Statement 25: Development and Flood Risk, inlcuding Annex F and Practice Guide ningpolicystatements/planningpolicystatements/pps25 Planning Policy Statement: Planning and Climate Change (Supplement to Planning Policy Statement 1) London Plan (consolidated with Alterations since 2004), Policy 4A.14 and the accompanying Sustainable Design and Construction SPD (May 2006) and The full policy wording is provided in Appendix 1. 31

32 Appendix 1: London Plan policy relating to SUDS London Plan (consolidated with alterations since 2004) Policy 4A.14 Sustainable drainage The Mayor will, and boroughs should, seek to ensure that surface water run-off is managed as close to its source as possible in line with the following drainage hierarchy: store rainwater for later use use infiltration techniques, such as porous surfaces in non-clay areas attenuate rainwater in ponds or open water features for gradual release to a watercourse attenuate rainwater by storing in tanks or sealed water features for gradual release to a watercourse discharge rainwater direct to a watercourse discharge rainwater to a surface water drain discharge rainwater to the combined sewer. London Plan Sustainable Design and Construction SPD (May 2006) Water Pollution and Flooding Standards (2.4.4) Essential Standards Use of Sustainable Drainage Systems measures, wherever practical Achieve 50% attenuation of the undeveloped site s surface water runoff at peak times Mayor s Preferred Standard Achieve 100% attenuation of the undeveloped site s surface water runoff at peak times 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 run-off as possible on site and explore sustainable methods of managing the remainder as close as possible to the site. The Mayor will encourage multi agency collaboration (GLA Group, Environment Agency, Thames Water) to identify sustainable solutions to strategic surface water and combined sewer drainage flooding/overflows. Developers should aim to achieve greenfield run off from their site through incorporating rainwater harvesting and sustainable drainage. Boroughs should encourage the retention of soft landscaping in front gardens and other means of reducing or at least not increasing the amount of hard standing associated with existing homes. 32

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34 Promoting Sustainable Drainage Systems Design Guidance for Islington Promoting Sustainable Drainage Systems (SUDS) in Islington will bring a range of benefits. SUDS manage runoff from development in an integrated way to reduce the quantity of water entering drains and therefore to reduce surface water floodrisk an important consideration in a dense urban area like Islington, particularly given the increase in heavy rainfall likely as a result of climate change. SUDS also improve the quality of runoff from development, bringing clean water back into use in our urban environment to create attractive places for people and wildlife. This note provides guidance on the application of SUDS within Islington. It highlights opportunities for SUDS and identifies how they could be effectively designed, implemented and managed. Each chapter is based around an Islington case study scheme (a school, a new development, an existing housing estate, a highways project and a parks scheme) which demonstrates how SUDS can be incorporated within that particular context. The guidance aims to provide information and ideas for decision makers and designers within the Council, developers and partner organisations to support the application of SUDS in a range of contexts across Islington.