SuDS Design Guidance for Hertfordshire March 2015 V2. Publication date 1 st April 2015
Contents 1. Introduction... 1 1.1 SuDS design guidance for Hertfordshire... 2 1.2 Guidance structure... 2 2. Context... 4 2.1 Legislation & policy... 4 2.1.1 The Flood & Water Management Act... 4 2.1.2 National Planning Policy Framework4 2.1.3 Local Policy... 4 2.2 Sustainable Drainage Systems... 6 2.3 SuDS design process... 8 2.3.1 SuDS Masterplanning... 9 2.3.2 SuDS concept... 9 2.3.3 Outline SuDS design... 11 2.3.4 Detailed SuDS design... 11 3. Technical & Spatial Framework... 12 3.1 The Hertfordshire environment... 12 3.1.1 Geology & soils... 12 3.1.2 Hydrology... 13 3.1.3 Landscape... 15 3.1.4 Land contamination & instability... 16 3.1.5 Biodiversity... 16 3.1.6 Historic environment... 17 3.2 Quantity and quality criteria... 18 3.2.1 Runoff destination... 18 3.2.2 Runoff rates... 19 3.2.3 Storage volumes... 19 3.2.4 Flood risk... 20 3.3 Site constraints & opportunities... 21 3.3.1 Topography... 21 3.3.2 Land availability... 22 3.3.3 Land remediation... 23 3.3.4 Groundwater... 23 3.3.5 Ground permeability... 24 i
3.3.6 Flood risk areas... 25 4. Local Design Principles... 26 4.1 SuDS network... 26 4.1.1 The management train... 26 4.2 Water quality... 27 4.2.1 The treatment train... 27 4.2.2 Treatment principles... 29 4.3 Local distinctiveness... 29 4.3.1 Detailed design & materials... 30 4.4 Biodiversity... 30 4.5 Happy & healthy communities... 31 4.5.1 Multifunctional spaces... 31 4.5.2 Natural security... 32 5. SuDS Features... 34 6. DELIVERY... 38 6.1 Construction... 38 6.1.1 Construction method... 38 6.1.2 Phasing... 38 6.1.3 Affordability... 38 6.2 Operation & maintenance... 38 6.2.1 Management plan... 39 6.2.2 Safe access... 39 6.2.3 Waste management... 40 7. APPENDIX... 41 7.1 Signposts... 41 7.2 Quantity & quality criteria... 44 7.2.1 Peak flow & runoff rates... 44 7.2.2 Greenfield runoff rates... 44 7.2.3 Previously developed land runoff rates... 46 7.2.4 Flow controls... 46 7.2.5 Storage volumes... 47 7.2.6 Quantity data template... 48 ii
7.3 SuDS features guidance sheets... 54 7.3.1 Green roofs... 55 7.3.2 Permeable pavements... 57 7.3.3 Rainwater harvesting... 59 7.3.4 Ponds & wetlands... 61 7.3.5 Retention & infiltration basins... 64 7.3.6 Bio-retention... 67 7.3.7 Swales & filter strips... 69 7.3.8 Filter drains... 74 7.4 Sample maintenance schedule... 76 7.5 Glossary... 80 This Guidance was originally produced by Hertfordshire County Council in April 2014. In March 2015, it was updated to reflect changes in the planning system regarding surface water management. iii
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1. Introduction The effects of climate change, population growth and urbanisation are placing significant pressure on the effective management of surface water. Changing patterns of rainfall and a growing area of impermeable development are giving rise to high rates and volumes of surface water runoff. Conventional drainage systems constructed from constrained networks of underground pipes and storage tanks are struggling to cope with overwhelming quantities of surface water runoff, resulting in problems such as flooding and pollution. Sustainable Drainage Systems (SuDS) manage surface water based on an understanding of the hydrologic cycle the movement of water within the natural environment. They are an environmentally-friendly and cost-effective approach to drainage that avoid the problems associated with conventional drainage systems by managing water at the surface, replicating the flow routes and stopping places that water finds in the natural landscape. This integrated approach to SuDS and the landscape is referred to as the SuDS philosophy, and provides an opportunity for the delivery of multiple environmental, economic and social benefits. The SuDS philosophy is to replicate, as closely as possible, the natural drainage from a site before development. 1 This should be achieved in line with the delivery of three objectives, Quantity, Quality and Amenity & Biodiversity. Each objective should be considered in equal measure, however their delivery will vary according to the constraints and opportunities presented by a site and the local community. Southern Country Park, St Michaels Meadow 1 CIRIA SuDS Manual (C697) 1
1.1 SuDS design guidance for Hertfordshire This guidance is for developers involved in the design and development of SuDS in Hertfordshire. It promotes an integrated approach to SuDS and landscape design, and establishes a set of local design criteria to help shape the development of SuDS in respect of the County s unique environmental context. SuDS design should be in line with best practice. This document will be reviewed and updated periodically to reflect current legislation, policy and guidance and should be read with reference to the following: National Planning Policy Framework https://www.gov.uk/government/publications/national-planning-policyframework--2 Sectorial Guidance (Supporting Information for) the Non-Statutory Standards for Sustainable Drainage Non- statutory SuDS standards to give direction on the design, construction, maintenance and operation of sustainable drainage systems. The SuDS Manual (CIRIA C697) Best practice guidance on the planning, design, construction, operation and maintenance of SuDS http://www.ciria.org/resources/free_publications/the_suds_manual.aspx Preliminary Rainfall Runoff Management for Developments Joint Defra/Environment Agency R&D Technical Report W5-074/A/TR/1 Rev E http://evidence.environmentagency.gov.uk/fcerm/libraries/fcerm_project_documents/rainfall_runoff _Management_for_Developments_-_Revision_E.sflb.ashx Local Flood Risk Management Strategy for Hertfordshire http://www.hertsdirect.org/services/envplan/water/floods/floodrisk/lfrmsherts / Hertfordshire Building Futures Best practice guidance on sustainable design and construction in Hertfordshire http://www.hertslink.org/buildingfutures/ 1.2 Guidance structure This guidance follows a logical approach to the design and delivery of SuDS, and mirrors the three key stages of the SuDS design process as promoted 2
Detail SuDS Design Process Outline Concept within best practice guidance The SuDS Manual (CIRIA C697) (refer to section 2.3): CONTEXT Summary of key drivers for SuDS: Legislation & policy context An introduction to SuDS and the benefits they deliver The SuDS design process TECHNICAL & SPATIAL FRAMEWORK Collate existing data and information and establish a framework for SuDS design: Establish design criteria : o Environmental context o Quantity & quality criteria Site constraints & opportunities LOCAL DESIGN PRINCIPLES Develop SuDS design in line with local design principles: SuDS network Water quality Local distinctiveness Biodiversity Happy & healthy communities DELIVERY Implementation and monitoring of SuDS: Construction Operation & maintenance APPENDIX Technical detail and signposts to important sources of data and information 3
2. Context 2.1 Legislation & policy 2.1.1 The Flood & Water Management Act The Flood and Water Management Act 2010 established Hertfordshire County Council as a Lead Local Flood Authority covering the area of Hertfordshire. In February 2013 the first Local Flood Risk Management Strategy for Hertfordshire was published and this included a specific policy relating to the approval and delivery of Sustainable Drainage Systems (SuDS) in Hertfordshire. 2.1.2 National Planning Policy Framework The National Planning Policy Framework (NPPF) promotes the delivery of SuDS for the management of surface water in all new major developments. 2 It also requires that all new development in areas at risk of flooding prioritise the use of SuDS. In determining planning applications, local planning authorities should consult with the relevant LLFA on the management of surface water, and be satisfied that sufficient arrangements are in place for the provision, operation and maintenance of SuDS for the lifetime of the development. 3 2.1.3 Local Policy In addition to the Local Flood Risk Management Strategy for Hertfordshire, SuDS should be delivered in line with the relevant Local Plan policies. 2 Developments of 10 dwellings or more; or equivalent non-residential or mixed development (as set out in Article 2 (1) of the Town and Country Planning (Development Management Procedure) (England) Order 2010) 3 Unless demonstrated to be inappropriate 4
Green and Blue Infrastructure The delivery of SuDS is strongly aligned with the provision of Green and Blue Infrastructure (GI), which is defined as the network of natural and semi natural features, green spaces, rivers and lakes that intersperse and connect villages, towns and cities. 4 A fundamental aim of GI is to optimise land use, ensuring that it supports the widest range of functions, and delivers the greatest number of benefits, in a sustainable way. For example, the integration of SuDS with public open space for the management of surface water can support climate change adaptation efforts to improve the built environment s resilience to changing temperatures and patterns of rainfall. SuDS can also help deliver ecosystem services that regulate air, soil and water quality and micro-climates, provide carbon sequestration, and replenish soil moisture profiles and groundwater resources. The above diagram illustrates a GI approach to SuDS, integrating the layout of SuDS features (blue infrastructure) with the provision of green open space (green infrastructure). By implementing the SuDS philosophy, natural drainage processes are replicated via a network of linked water features for the controlled management of water and flood risk, increased water security, and enhanced biodiversity and amenity value. 4 Green Infrastructure Landscape Institute Position Statement 2013 5
2.2 Sustainable Drainage Systems SuDS mimic natural drainage processes through a series of features that collect and convey water at or near the surface. SuDS slow the flow of water (attenuate) encouraging natural losses through infiltration and evapotranspiration, and the removal of pollution through filtration and deposition, before discharging the remainder into the nearest watercourse. 6
The integration of SuDS with the landscape provides an opportunity for the delivery of multiple local benefits: Regulate water quantity Reduction in quantity of water within conventional drainage systems and watercourses through natural losses and re-use of rainwater, thereby lowering the risk of flooding. Reduction in demand for water from mains supply through the re-use of rainwater Replenishment of soil moisture and groundwater through infiltration Regulate water quality Purification of water through natural filtration and deposition Conserve and enhance local distinctiveness Delivery of high quality, and high value, public realm through the creation of attractive landscape features Creation of places with strong local character and identity through the use of local materials, traditions and craftsmanship Conserve and enhance biodiversity Enhanced biodiversity through the creation of rich and diverse wetland habitats The creation of permeable habitat networks to support the movement of wildlife within a site and the surrounding area Support happy & healthy communities Creation of attractive places for people to enjoy, for recreation and amenity, through the integration of SuDS and public open space Provision of safe environments that are naturally secure by design and mitigate the risk of flooding Improves visibility and awareness of the water environment, encouraging people to interact with water for education and play Cost effective and efficient Reduction in capital and maintenance costs compared to conventional drainage systems Reduction in carbon and embodied energy compared to conventional drainage systems 7
Planning process Pre-Application Outline/Full Detail 2.3 SuDS design process The SuDS design process reflects the three stage approach to SuDS design as promoted within best practice guidance The SuDS Manual (CIRIA C697). The design process is broadly consistent with the planning process as illustrated below. SuDS design should be considered as early as possible in the development planning process, to ensure the delivery of the most efficient and cost-effective SuDS scheme. SuDS design process SuDS Masterplanning Assemble SuDS design team Establish design criteria SuDS Concept (stage 1) Site survey & analysis SuDS conceptual drainage plan Initial SuDS design statement Outline SuDS Design (stage 2) Demonstrate design criteria Detailed SuDS design statement Detailed SuDS management plan Detailed SuDS Design (stage 3) Detailed technical and spatial information 8
2.3.1 SuDS Masterplanning Masterplanning is a comprehensive and iterative method for taking account of the many factors that affect a site and its development. SuDS should be considered within the Masterplanning process, as their siting and design can strongly influence the layout and character of a development. An integrated approach to SuDS, open space and landscape design is critical to ensure efficient use of land and the delivery of biodiversity and amenity benefits. SuDS design team A multidisciplinary design team should be assembled, in addition to the Local Planning Authority, it should engage a range of specialists such as Landscape Architects, Ecologists, Archaeologists, and SuDS design consultants. It is important that all parties involved in the design and delivery of SuDS have a shared understanding of the drainage system design. The design team should forward plan for cost-effectiveness and efficiencies relating to the construction, operation, and maintenance of SuDS (refer to Chapter 6). Design criteria Design criteria should be established at the beginning of the SuDS design process.. These are a set of conditions that need to be satisfied through the design and include the quantity and quality criteria set out in the standards for sustainable drainage 5 and the local design criteria set out in this guidance. 2.3.2 SuDS concept The conceptual SuDS design should be developed at the pre-application stage of the planning process. Survey & analysis A site survey and analysis should be carried out at the beginning of the SuDS design process to identify the quantity and quality criteria and any environmental constraints and opportunities. This should provide the technical and spatial framework within which the SuDS design evolves. A combination of desk-based studies and on-site surveys should identify the existing ground conditions and drainage characteristics of the site. Soil infiltration rate tests should meet Building Regulations and be carried out in accordance with BRE Digest 365. 5 Sectoral Guidance (Supporting information for) the non-statutory standards for sustainable drainage, Government 9
Assets or features of importance for landscape, biodiversity and the historic environment should be identified and treated in accordance with the relevant planning requirements. The survey and analysis should consider the following: Sub-catchments, flow routes and discharge destinations (see below) Environmental constraints and opportunities (refer to section 3.1) o Topography (site levels & contours) o Land availability o Land contamination and stability o Geology & soils (ground permeability and infiltration rates) o Historic, landscape and biodiversity features and assets Quantity and quality criteria (refer to section 3.2) o Surface water runoff rates and storage volumes o Flood Risk Areas (surface water and groundwater) o Water quality (groundwater, onsite and receiving water body(s)) Sub-catchment & Flow Route Analysis The sub-catchment and flow route analysis is based on the topographical survey and identifies the drainage characteristics of a site including the location of catchments, flow routes, and discharge destination(s). The analysis should guide the SuDS layout, using existing natural flow paths and low lying areas for the attenuation, conveyance and storage of surface water wherever possible. Routes should be identified to accommodate everyday flows, overflows, and exceedance events. It is important that the analysis is carried out before the development layout is fixed so that it can be adjusted to accommodate important flow routes where necessary. Conceptual drainage plan A conceptual drainage plan should be informed by the site survey and analysis. All parties involved in the SuDS design process should review the concept proposal and confirm that it is acceptable in general terms. Design statement An initial design statement is required to give a general explanation of the SuDS design intent. 10
2.3.3 Outline SuDS design The outline SuDS design develops the proposals further. It should demonstrate that the proposed drainage system meets the design criteria. The SuDS design statement should give a detailed explanation of the spatial and technical design, and provide information not readily conveyed on the plans. In particular, it should explain how the drainage system is integrated with the landscape. Management plan A management plan should demonstrate that the proposed drainage system is practical and include information regarding its operational and maintenance requirements (refer to section 6.2). 2.3.4 Detailed SuDS design The detailed SuDS design includes all of the information required at the outline/full application stage, plus more detailed information required as reserved matters or to satisfy conditions. This should include a fully detailed drainage strategy, plus technical details with accompanying engineering drawings, for example, showing SuDS components, flood mitigation, and maintenance and management measures. 11
3. Technical & Spatial Framework 3.1 The Hertfordshire environment SuDS design should respond to Hertfordshire s unique historic, built, and natural environment context. SuDS design should ensure that the development conserves and enhances local landscape/townscape character and quality, benefits biodiversity, and creates attractive and safe places that support the daily needs of local communities. The environmental context of Hertfordshire is summarised in the following section. Signposts to sources of information and data are listed in the Appendix. 3.1.1 Geology & soils Hertfordshire s geology is largely chalk of the Cretaceous period, overlain in the south and east by London Clay. A sand and gravel belt runs north-south through the centre of the county, and small areas of Gault Clay are evident in the north and north-west. Throughout much of Hertfordshire superficial deposits overlay the solid geology and complicate the picture. These include clay-with-flint across the west of the county, including the Chilterns dip slope; boulder clay over central and eastern areas; and gravels in the Vale of St Albans and the river valleys. 6 Spatial distribution of soils and geology in Hertfordshire 6 50 Year Vision for the Wildlife and Natural Habitats of Hertfordshire: HMWT, April 1998 12
There are two kinds of soil in the county. In the north-east soils are predominantly alkaline or neutral chalky soil, and in the central and western parts, soils are more or less acid leached soils. 3.1.2 Hydrology Hertfordshire is host to a variety of waterways and enclosed water bodies. Many have played important roles in the history of the county, serving local industry and providing important trade routes. Today they are an important recreational resource and haven for wildlife. The majority of the county is drained southwards by the Thames Catchment via two major river systems, namely the Colne valley in the west, and the Lee valley in the east. The northern part of the county is drained northwards by the Great Ouse Catchment, with a small area in the west drained by the Thame Catchment. The major river systems generally originate as chalk streams flowing down into the lowland clay. Major rivers and aquifers in Hertfordshire Significant water features are summarised below: The Lea (or Lee) is an important recreational route running through the Lea Valley Regional Park, once a natural river it has been canalised along much of its length. The Grand Union Canal, fed by a group of reservoirs at Tring, flows through the western half of the county between London and Birmingham. The New River is an artificial waterway, and unique industrial relic, that transports fresh drinking water from springs near Ware to London. The New Town of Stevenage was designed to incorporate a sustainable drainage system, including conveyance ditches, a series of water meadows for the temporary storage of storm water, and more permanent water storage at Fairlands Lake. 13
Water bodies include Tring reservoirs, less well known lakes at Aldenham, Stanborough, London Colney and Broadwater in Hatfield Park, and various restored gravel pits. Nature reserves provide wildlife havens at Rye Meads and Great Amwell reserves, among many others. Groundwater The effects of climate change, population growth, and urbanisation are placing significant pressure on the supply and demand for groundwater resources. Groundwater flow is generally down slope into the Colne Valley and Lee Valley river systems 7 towards a large chalk aquifer and groundwater source protection zone. Boreholes and wells driven through the London Clay extract water from the underlying London Basin Chalk Aquifer, supplying much of Hertfordshire, London, and the wider Thames Valley. Water abstraction is generally rising, and over the last ten years demand has varied between 20,000 and 40,000 Ml/year. 8 Flood risk Hertfordshire is one of the driest counties in the East of England, which in turn is the driest region of the UK. The average annual rainfall ranges between 600mm in the north, and 750mm in the west, this compares with a national average rainfall of 838mm. 9 However, despite a relatively low average annual rainfall, the area is still at risk of flooding: Fluvial flooding from watercourses occurs to various extents across the county. In general, there is a significant risk of flooding along the River Lea in the east, and additional flood risk along the River Colne in the west. Under extreme flooding conditions, the majority of rivers are not likely to cause widespread flooding of the floodplain, however may result in some localised flooding or out of bank flow. Pluvial flooding, the accumulation of surface water as a result of intense rainfall, is the major source of flood risk within urban areas. National statistics suggest that 53,400 properties in Hertfordshire are at risk from flooding to a depth of 300mm with a 0.5% probability in any one year.10 Groundwater flooding occurs as a result of the water table breaking the surface. This happens in parts of the county where the chalk aquifer, and other permeable geology such as sand and gravel, is present. 7 Harrow and Hillingdon Geological Society http://www.hhgs.org.uk/ 8 Management of the London Basin Chalk Aquifer 9 Source: Met Office (1971 2000 averages) 10 Hertfordshire County Council Preliminary Flood Risk Assessment June 2011 14
Water quality Chalk streams and underlying aquifers in Hertfordshire are highly sensitive to pollution and are under increasing pressure from activities such as water abstraction, urban and infrastructure development, effluent discharges, agriculture, land drainage and flood defences. 11 Hertfordshire has one of the largest areas of groundwater contamination in the UK, in the form of a plume 20km long, arising from a former Bromate works at Sandridge. 12 3.1.3 Landscape Hertfordshire is host to a variety of landscapes, each with its own distinct character. For example, the valley meadowlands at Hoddesdon have a different sense of place to the chalk hills and scarps of the Chilterns. SuDS should respect local landscape character and reflect the identity of the local surroundings and materials. They should improve landscape quality, and demonstrate how they meet the relevant strategies for managing change. Across the county there are several designations and strategic areas with strong landscape objectives such as: Lee Valley Regional Park Watling Chase Community Forest Landscapes of local importance Registered parks and gardens and other landscapes of historic interest The Chilterns AONB overlaps Hertfordshire in the northwest of the county near to Hemel Hempstead and Hitchin. Consideration should be given to conserving landscape and scenic beauty in this area, and the conservation of wildlife and cultural heritage. Landscape character At a National level, Natural England has produced National Character Areas that include a description of the key ecosystem services provided by each area and opportunities for positive environmental change. At a regional level, Landscape East has produced a landscape typology for the East of England that identifies common types of landscapes such as chalk hills and scarps and lowland settled farmlands. The spatial distribution of landscape types across Hertfordshire is illustrated below. 11 Source: UK BAP 12 http://www.groundwateruk.org/downloads/phil%20bishop.pdf 15
Spatial distribution of landscape character types (Landscape East) At a local level, the Hertfordshire Landscape Character Assessment identifies 232 distinct landscape areas across the County. For each area the assessment provides a description and evaluation of the landscape character and condition, followed by an overarching strategy and set of guidelines for managing change. 3.1.4 Land contamination & instability In Hertfordshire the potential for local land contamination exists as a result of animal husbandry, silage clamps, waste and composting facilities, and in connection with motor vehicle use and maintenance. Land instability occurs for a variety of reasons, for example due to the presence of historic mines, often causing landslides, subsidence or ground heave. In Hertfordshire land stability is commonly influenced by the modification and/or concentration of groundwater flows. For example in areas of chalk, swallow holes, or underground caverns are formed by the dissolution of chalk along fractures and fissures and can cause the ground to collapse. 3.1.5 Biodiversity Hertfordshire has a number of designated sites of importance for biodiversity. The following hierarchy of sites have wetland importance: 16
Internationally important RAMSAR site, encompassing Sites of Special Scientific Interest (SSSI), at Turnford, Cheshunt Pits, Rye Meads, and Amwell Quarry. Chalk streams are a rare and fragile habitat of international importance. They are sensitive water bodies that are highly susceptible to pollutants and changes in temperature.13 Site of European importance: Lee Valley (for birds) and Wormley- Hoddesdon Park Woods Special Protection Areas, and Chilterns Beechwood Special Areas of Conservation Sites of Special Scientific Interest (SSSI) with specific wetland or associated habitat interest: Ashwell Springs Blagrove Common Croxley Common Moor Frogmore Meadows Hertford Heath Hunsdon Mead Moor Hall Meadows Patmore Heath Sarratt Bottom Water end Swallowholes Sawbridgeworth Marsh Tewinbury Thorley Flood Pound Tring Reservoirs 11 Local Nature Reserves with wetland interest Wildlife Sites supporting floodplain meadows, fens, springs, wet woodlands, and associated wildlife, which are dependent upon the maintenance of the water table and clean water. 3.1.6 Historic environment Hertfordshire has a rich historic and cultural association with water. Historic settlements, industry and agriculture have evolved alongside springs and river systems, which were an important source of power and vital trade routes. Today a range of traditional drainage systems, such as ditches, moats and ponds, remain evident in areas of poorly draining geology. In East Herts, fords are a common means of crossing rivers and in places there are distinctive reinforced ditches adjacent to hillside roads. The historic environment not only refers to water features, but includes a range of assets including listed buildings, conservation areas, registered parks and gardens, and scheduled ancient monuments. Development should identify any heritage assets and ensure the conservation and enjoyment of the assets and their setting. Heritage assets with archaeological interest may require desk based assessment and field evaluation. 13 Chalk streams are also under increasing pressure from activities such as water abstraction, urban and infrastructure development, effluent discharges, agriculture, land drainage and flood defences. Source: UK BAP 17
A typical Hertfordshire ford 3.2 Quantity and quality criteria SuDS design is required to meet the quantity and quality criteria set out within the standards for sustainable drainage 14, to ensure the controlled discharge of clean water. 3.2.1 Runoff destination Re-use water first It is required to make as much use of surface water runoff as practicable, before it enters the drainage system. The SuDS design should maximise opportunities to collect and re-use water locally. Techniques, such as rainwater harvesting, which re-use rainwater for non-potable uses such as irrigation and toilet flushing, reduce the amount of water entering the drainage system whilst also lowering demand on the water main. Water re-use techniques can vary in scale and complexity depending on local requirements. At a domestic scale roof water can be directed into rain gardens, or collected in water butts for general household use. At a larger 14 Sectoral Guidance (Supporting Information for) the non-statutory standards for sustainable drainage, Government 18
scale water can be collected as irrigation for public realm planting schemes, community allotments, or recreational facilities such as golf courses. Runoff destination hierarchy The standards for sustainable drainage promote a hierarchy of destinations for the discharge of surface water runoff. At the top of the hierarchy, the preferred options are those which mimic natural processes, promoting losses through infiltration and conveyance across the surface to water bodies. At the bottom of the hierarchy, the least preferred options include discharge to sewers, highways drains and other conventional drainage systems. Run-off should never discharge to the foul sewer. The discharge destination can vary for each sub-catchment serving a development. Whatever the approach, the SuDS design should ensure the controlled delivery of clean water and should not negatively impact upon the status of the receiving water body. 3.2.2 Runoff rates The SuDS philosophy is to replicate, as closely as possible, the natural drainage from a site before development. 15 To do this, it is necessary to calculate the infiltration rates and surface water runoff rates and volumes which occur across the site in its undeveloped greenfield state. It is important to take account of the whole site, including both permeable and impermeable areas. This will identify where infiltration techniques can or cannot be implemented, and help determine the quantity of water 16 that the drainage system will need to manage within conveyance and storage features. 3.2.3 Storage volumes SuDS features offer a range of storage techniques depending on the frequency, rate and volume of surface water that the sub-catchment is required to manage. Calculations are required to take account of climate change and changing patterns of rainfall, and urban creep: which is the loss of permeable area to incremental development such as driveways. Overall, sub-catchments should be sized to ensure that the drainage system can manage the quantity of surface water runoff efficiently, using appropriate flow control mechanisms, without the need for excessive storage volumes. 15 CIRIA SuDS Manual (C697) 16 Known as the allowable peak flow 19
Interception storage Reduces the frequency of surface water runoff by mimicking natural drainage and intercepting the first 5mm of rainfall. Flows are sufficiently slowed to allow natural losses through infiltration and evapo-transpiration. Interception storage features include green roofs, and permeable pavements. Awaiting image Attenuation storage Following most rainfall events, infiltration and flow rates will be lower than the total runoff rate from hard surfaces. Temporary storage is therefore required to reduce the risk of flooding. Attenuation storage should be designed to slow the flow, and control the rate of discharge, in line with the natural drainage from a site before development. (Photo: Retention Basin, Hoddesdon) Long term storage Long term storage can be used for sites that do not have the capacity to manage the quantity of water through interception and attenuation. Long term storage mimics natural drainage by holding water for a longer period of time, one of the primary drivers for the provision of long term storage is the protection of floodplains. (Photo: Wetland, St Michaels Meadow) 3.2.4 Flood risk SuDS design must mitigate any negative impact of surface water runoff from the development on flood risk outside of the development boundary. It should also ensure that flooding does not occur on any part of the site for a 1 in 30 year rainfall event, or in any part of a building for a 1 in 100 year rainfall event. Temporary storage / sacrificial space Temporary storage, or sacrificial space, can be provided to accommodate water from infrequent yet extreme rainfall events. Spaces such as car parks, 20
sports pitches, or landscaped areas, can be designed to store shallow water for a temporary period, allowing it to dissipate over a number of days. This approach can contribute to the sites calculated requirement for storage, avoiding the need for more costly and sophisticated storage systems. The risk to people, and the economic viability of local businesses, is a key consideration. Exceedance It is accepted that it is not possible to design a drainage system that never floods, therefore provision should be made for the management of flows that exceed the design standard of the drainage system. Exceedance routes should work with the existing landscape, and utilise existing natural flow routes and low-lying areas where possible. Any new routes should follow existing linear landscape features, such as hedgerows and trees. In designing for exceedance, public health and safety is a critical consideration, care should be taken to ensure that emergency service vehicular access is not hindered by temporary flow routes or storage areas. 3.3 Site constraints & opportunities SuDS design should work with the natural landscape and take opportunities to incorporate existing site features and assets, particularly where they serve drainage functions. Constraints such as steep slopes, a lack of space, or the presence of pollution, should not prevent the delivery of SuDS where they can be overcome with innovative design. Common site issues alongside potential solutions are summarised below. 3.3.1 Topography Site topography strongly influences the natural drainage characteristics of a site and should be used to inform the sub-catchment and flow route analysis (refer to section 2.3.2). The SuDS design should incorporate natural flow routes and low-lying areas for the conveyance and attenuation of water wherever possible. In designing new SuDS components, a detailed understanding of site levels is critical for shallow features, such as swales and basins, which rely on accurate slope profiles to control flow rates, provide safe access and egress, and support structured vegetated margins. 21
Issue It is difficult to provide shallow SuDS components on steep slopes. Water flows rapidly down slope and ponds at the lowest point Opportunity A series of terraced SuDS components, such as ponds, can be used along the contours to slow the flow of water. 3.3.2 Land availability SuDS collect and manage water at the surface. Keeping water above ground requires space, which is often at a premium and under competing pressure to deliver a range of functions. The delivery of SuDS can be fully integrated with the provision of public open space, making efficient use of land alongside the delivery of mutual benefits for biodiversity, recreation and amenity. Issue The development is high density and does not have adequate open space for surface water features, or open space is not well located for use as SuDS Opportunity Where space is limited, SuDS components that utilise the built environment, such as green roofs, permeable paving, rills and stepped canals, can offset the need for dedicated open space. 22
3.3.3 Land remediation Land use within a site can vary greatly over time. It is therefore important that on-site investigations are carried out to identify any pockets of contaminated land. SuDS design should prevent the spread of contamination, especially within the soil profile and into groundwater resources, and provide opportunities for land remediation where appropriate. Issue Contaminated land is present and there is significant risk of mobilising contaminants Opportunity In areas of contamination the potential to infiltrate to ground may be restricted. Shallow surface SuDS components that minimise disturbance to the underlying soils, and do not allow infiltration can be implemented, reducing requirements for treatment of arisings. Some SuDS components can be lined with an impermeable membrane or clay to prevent infiltration. 3.3.4 Groundwater SuDS design should take account of groundwater levels and Groundwater Protection Zones designated to protect groundwater from contamination. 23
Issue The groundwater levels are too high. SuDS components are likely to become damaged or flooded reducing the volume of storage available. Opportunity In areas with a high water table, shallow surface SuDS components that do not allow infiltration can be implemented. Some SuDS components can be lined with an impermeable membrane or clay to prevent infiltration. 3.3.5 Ground permeability Geology and soils, and groundwater levels, strongly influence ground permeability affecting infiltration and surface water runoff rates. Permeability can vary greatly over a relatively small area, it is therefore important that onsite investigations are carried out to identify the site-specific conditions. Most soils are permeable to an extent, although in general chalks, sands and gravels tend to be more permeable than clays. In some chalk areas high groundwater levels saturate the ground and give rise to surface water bodies such as winterbournes, or new springs. Issue In areas of poor permeability, where there is no infiltration under Greenfield conditions, infiltration is not required as part of the SuDS design. Opportunity Shallow surface SuDS techniques that do not rely on infiltration can be used; furthermore opportunities to infiltrate to a greater depth can be explored. 24
3.3.6 Flood risk areas A site-specific Flood Risk Assessment (FRA) should identify existing flood risk in the site and surrounding areas. SuDS design should demonstrate how it can manage flood risk within the site, including the provision of exceedance measures, and should not negatively impact upon flooding outside the site. Issue Areas that experience natural flooding events, such as floodplains, are not appropriate for SuDS. Storage components are likely to become overwhelmed and potentially eroded Opportunity Some SuDS components can be implemented in areas prone to flooding for the management of everyday rainfall events. Components that are resistant to flooding and avoid point discharge should be considered. Attenuation features should be designed to empty within 24 hours in anticipation of further rainfall events. 25
4. Local Design Principles 4.1 SuDS network A SuDS network should be integrated at every scale of development from small scale residential properties to large scale public open space, linking into the landscape beyond. Networks should be resilient, and ensure that if any part of the drainage system fails, the wider network can continue to function efficiently. 4.1.1 The management train SuDS mimic natural drainage processes through a series of SuDS features that collect and convey surface water. This sequential approach to surface water management is known as a management train, which consists of three stages of control (source, site and regional), each linked by conveyance routes. The SuDS Management Train Each stage of the management train is summarised in the following table. Source control At the top of the management train, source controls, such as green roofs and permeable paving, intercept rainfall at the point at which it falls. Awaiting image The SuDS design should maximise the number of source controls within the upper reaches of a (sub) catchment in order to control the 26
quantity of surface water within the drainage system and reduce the risk of flooding. Site control Site controls collect water into features such as ponds and basins from across the development via conveyance features, such as swales, for discharge into the final stage of the drainage system. (Photo: Blackwell House, Bushey) Regional control Regional controls collect water from the entire site(s). They are landscape-scale features such as wetlands, often located outside the site boundary and are an effective approach to managing large quantities of water that cannot be accommodated within the development. Conveyance Conveyance components transport water between controls, and should utilise existing natural watercourses and flow routes wherever possible. (Photo: St Michaels Meadow) Awaiting image 4.2 Water quality Surface water discharge must not negatively impact upon the quality of the receiving water body. SuDS design provides an opportunity to implement a series of drainage techniques that encourage the passive removal of pollutants and ensure the delivery of clean water. 4.2.1 The treatment train The treatment train promotes a series of stages that incrementally remove pollution from surface water as it travels through the drainage system. This approach mirrors the management train and they should be designed in consideration of each other. 27
The SuDS Treatment Train First treatment stage At the top of the treatment train, SuDS components that have a combined collection and treatment function are most efficient, e.g. green roofs and permeable paving. Impermeable surfaces direct surface water to the edge for collection and treatment by filter strips, and storage/conveyance components such as swales and detention/retention basins. Second treatment stage The second stage includes SuDS features with a combined collection and treatment function to transport water through the site into the final stage of the drainage system. Conveyance routes that can also perform a treatment function include swales and filter drains Third treatment stage At the final stage SuDS features, such as ponds and wetlands, that provide adequate residency time to allow the removal of any remaining pollutes and the deposition of silts, before final discharge into the receiving water body should be utilised. 28
Surfacewater runoff / source 4.2.2 Treatment principles The number of treatment stages required is based upon an evaluation of the risk of the pollution 17 and takes into account the source of surface water runoff and the sensitivity of the receiving water body. The table below summarises the typical number of treatment stages required for common types of development. Roof s only Receiving water body / sensitivity Low Medium High 1 1 1 Residential roads, parking areas, commercial zones 2 2 3 Refuse collection, industrial areas, loading bays, lorry parks, highways 3 3 4 A variety of treatment techniques should be used in series to ensure the removal of a range of pollutants, from liquid chemicals and hydrocarbons, to solid silts and sediments. In general, most pollution should be removed in the first treatment stage, in particular heavy metals as they can have a negative impact upon more sensitive habitats such as wetlands and chalk streams further down the drainage system. SuDS design should aim to remove pollution from the first-flush, the calculated quantity of polluted surface water runoff (generally between 10 and 15mm). As a general rule, rainwater should be collected and treated at source : at or near to where it falls. Multiple source techniques, spread throughout the development, should maximise the quantity of surface water that can be effectively treated, reducing the concentration of pollution further down the system. Attenuation techniques should slow the flow of water and maximise residency time, ensuring that all contaminants can be effectively removed. 4.3 Local distinctiveness SuDS can be fully integrated with the built and natural environment. A considered design should conserve and enhance the local character and quality of a site and its surroundings, creating locally distinct, legible, and attractive places. The integration of SuDS with open space provides an opportunity to incorporate existing landscape features, such as trees and hedgerows, that 17 CIRIA SuDS Manual (C697) 29
can help bring meaning to a place as well as provide instant benefits for amenity and biodiversity. SuDS design can take inspiration from local drainage traditions which have developed in response to local ground conditions. Whilst it may not be appropriate to use historic features such as moats, ponds and ditches, they can inform the design approach. SuDS can contribute to the positive planning and management of valued landscapes, such as the Chilterns AONB, helping to meet local environmental objectives and create special landscapes of local importance. 4.3.1 Detailed design & materials Detailed design is vital to reinforce an area s unique identity. Standard approaches should be avoided, and instead, design should draw on local traditions and craftsmanship using a palette of native plant species and high quality materials and finishes. For example, permeable paving can utilise local paving units and patterns that reflect the local vernacular. Rills and channels set within pavements can add a point of interest and help delineate public and private areas. Many SuDS features, such as swales and basins, require sloping profiles and can be shaped to create interesting symbols and patterns, such as mazes, in the landscape, encouraging interaction and play. Hard aspects of the SuDS design, such as inlets and outlets, should be appropriately sized and visually interesting or neutral. Care should be taken to ensure that structures are not over-engineered or create trip hazards. 4.4 Biodiversity The delivery of SuDS is fully compatible with the conservation and enhancement of biodiversity. The management of water at the surface creates opportunities to deliver a rich diversity of habitats for wildlife, at little or no extra cost. Biodiversity should be enhanced at every scale of the development, from urban street trees to large scale wetlands, to add biodiversity value and support important ecosystem services such as the regulation of air, water and soil The integration of SuDS with open space provides an opportunity to create a linked network of habitats, or stepping stones, which support the movement of wildlife throughout a development and its wider landscape setting. SuDS planting design should specify a diverse planting structure using native species of local origin, or allow natural colonisation, to support local wildlife. Species that attract undesirable predators or are susceptible to disease should be avoided. 30
Risks to wildlife Pests and disease SuDS design should take measures to prevent nuisance species such as gnats and midges that can bite. Wetlands should support habitats that in turn support wildlife species that prey on nuisance species and their larvae. Risk to wildlife SuDS components that pose a physical risk to wildlife, and can trap them with no means of escape, should be avoided. The timing of maintenance regimes should not negatively impact upon wildlife. Herbicides, pesticides or fertilizers that can damage habitats should be avoided. Flocking birds and wildfowl The choice and design of large scale SuDS features, such as wetlands, should be carefully considered within 13 miles of an airport as flocking birds and wildfowl can be a hazard to aircraft. Their design can reduce this risk by avoiding areas of short grass favoured by geese; creating smaller pools and pond edges that allow access by predators such as foxes; and ensuring planting design does not encourage roosting by birds in large numbers. New wetland planting at St Michaels Meadow, Bishop s Stortford 4.5 Happy & healthy communities 4.5.1 Multifunctional spaces 31
An integrated approach to SuDS and landscape design provides an opportunity to create multifunctional spaces, combining a functional drainage system with the provision of public open space and other communal areas. The approach to SuDS design should complement the character of the space and its function. For example, hard SuDS features, such as rills and permeable paving, are fully compatible with more formal urban streets, squares and courtyards. Whereas softer features, such as swales, ponds and wetlands, may be more appropriate within informal green open spaces, sports and play areas. SuDS water features such as rain slides, chains and fountains can be used to enliven public spaces and deliver public art. Where people are encouraged to interact with water, public health is an important design consideration and the use of appropriate treatment stages for the delivery of clean water is vital. Enabling people to access and interact with water in their neighbourhood can promote mental and physical health benefits. 4.5.2 Natural security Natural security is a key principle of SuDS design and can be defined as providing safe and secure surroundings in an environmentally friendly way, without increasing isolation from each other or our environment. 18 The creation of surface water bodies can be associated with a degree of risk to humans and wildlife, however they should not be ring fenced or create inaccessible anti social areas. The perceived danger of open water should be managed through reasonable requirements, for example toddler high fencing in school grounds. A considered design approach and robust management plan can address issues such as drowning, falling, disease and flooding. Effective community consultation can also help to raise local awareness and reduce fear of SuDS. Communities that understand how the systems serve to protect them from flooding, as well as providing amenity and biodiversity benefits are likely to have a lower perception of risk and value what they consider an attractive and safe environment. Health & safety Safe entry and exit SuDS features should be designed to allow safe and easy access and egress (see section 6.2.2 Water depth should be waist deep (up to 500mm), a comfortable depth for individuals entering the water. Barriers Avoid high fences as they create a visual and physical barrier. The 18 http://www.hertsdirect.org/infobase/docs/pdfstore/natsecprevu.pdf 32
awareness of risk develops between the first 3 to 5 years of life so where young children are present, toddler proof fencing can be installed if needed (see diagram below). Toddler fencing should be 600-750mm high preventing access for toddlers whilst allowing adults to step over it as necessary. Fences should have a vertical rail construction; horizontal rails should be avoided as they can be more easily climbed. Signs and safety apparatus Signs, rings and other rescue apparatus can suggest a level of danger that is not proportionate to the actual risk. All SuDS design should be inherently safe, where safety apparatus and warning signs are proposed they should be clearly justified. Trip and slip hazards SuDS components such as inlets, outlets, headwalls and other structures should be designed so that they do not present a trip hazard and should be located away from open water. Toddler fencing around SuDS feature 33
5. SuDS Features SuDS features and their functions are summarised in the table below. SuDS features should be designed in line with best practice guidance 19 and take account of the additional practical advice presented in the Appendix. Key: SuDS management train Source control Collect and manage rainwater at or near to the point at which it falls Site control Collect and manage surface water from across the site Regional control Collect and manage surface water from the site control(s) Conveyance Transport surfacewater between site and regional controls Key: SuDS benefits Storage Attenuates and stores surfacewater Biodiversity Habitats and wildlife benefits Treatment Removes pollution Infiltration Discharges surface water into ground Evapo-transpiration Encourages natural losses through evaporation and transpiration Landscape & Visual Amenity Creates locally distinct attractive places Happy & Healthy Communities Provides opportunities for education and/or recreation & play 19 CIRIA SuDS Manual (C697) 34
Green roofs Green roofs use a series of drainage layers to intercept and store rainwater. They are ideal for large scale public, commercial and educational buildings such as schools. Awaiting image Permeable pavement Permeable pavements use a series of construction layers to intercept, treat and store rainwater. They are ideal for driveways, car parks and other lightly trafficked areas. Awaiting image Rainwater harvesting Rainwater harvesting collects and reuses rainwater, Harvesting schemes can range in scale and complexity depending on local requirements. Small scale domestic schemes include the collection of roof water in water butts for use around the garden or community allotments. Small ponds can provide a water resource for agricultural and recreational land uses. Awaiting image 35
Ponds & wetlands Ponds and wetlands are areas of open water designed to accommodate rising water levels during periods of heavy of rainfall. They also provide an effective mechanism for the treatment of water and the removal of silts and pollution. Their design should enhance visual amenity and provide biodiversity interest. (Photo: St Michaels Meadow) Infiltration & retention basins Infiltration and retention basins are open grass depressions, they are normally dry however during periods of heavy rainfall provide temporary storage for floodwater, before slowly allowing water to dissipate into the ground, or releasing it into the next stage of the drainage system via an outlet. (Photo: Blackwell House, Bushey) Swales Swales are grassed or vegetated channels with a flat base that can collect, treat, store and convey water (Photo: Upton, Northampton) 36
Under-drained swales Under-drained swales are grassed or vegetated channels with a flat base that can collect, treat, store and convey water with a filter drain below the surface. (Photo: Upton, Northampton) Filter strips Filter strips are grass/vegetated verges located along the edge of hard surfaces that allow water to flow as a sheet to another SuDS feature, such as a swale or filter drain. Awaiting image Filter drains Filter drains or French drains are trenches filled with open graded stone that allow runoff to flow laterally into the structure and either infiltrate directly into the ground, or travel along the drain to an outfall. Awaiting image 37
6. DELIVERY 6.1 Construction The SuDS design must be approved by the Local Planning Authority before construction of the development can begin. 6.1.1 Construction method SuDS can be a new and unfamiliar construction process for most contractors. It is therefore important to support the SuDS design with a comprehensive Construction Method Statement to include clear and detailed construction drawings, specifications, and an inspection regime. To ease the process further, industry standard materials and construction techniques can be used, however, they should be applied innovatively in order to avoid hard and over engineered design solutions. In general, construction should be carried out in line with best practice and the relevant industry standards. Where SuDS rely on infiltration techniques, construction methods should serve to protect ground permeability and avoid compaction and/or erosion. Materials management should ensure that loose materials and silts do not clog permeable areas, and are not washed into the drainage system. 6.1.2 Phasing For large developments, construction phases that are consistent with subcatchment areas can be most efficient and cost-effective. This approach ensures that each sub-catchment is served by an independent SuDS system, so that on completion of each phase the catchment can be drained without the need for expensive temporary drainage measures. 6.1.3 Affordability SuDS design can deliver affordable drainage solutions. They generally do not require additional land take above that typically required for external works, or costly excavations for subsurface pipes and storage tanks. Overall SuDS should be less expensive than an equivalent conventional drainage system meeting the same design criteria. 6.2 Operation & maintenance SuDS should operate efficiently from day to day, and be cost effective to maintain for the lifetime of the development. 38
SuDS design should be resilient and utilise passive drainage techniques and gravitational flows wherever possible, avoiding the need for mechanical components, subsurface pipes, and storage tanks that are costly to construct and maintain, and are prone to blockage and flooding often resulting in system failure. Where mechanical components such as inlets/outlets and flow control structures are necessary, they should endure throughout the design life of the drainage system 20 with minimal maintenance and without the need for replacement. 6.2.1 Management plan SuDS can be a new experience for most land managers and maintenance personnel. The SuDS design should be accompanied by a Management Plan, to include maintenance schedules for both surface and sub-surface components, and ensure that all parties involved in the operation and maintenance of SuDS are fully aware of the system requirements. Most management tasks can be carried out as part of the general landscape management, or nature conservation regime. Typical tasks include litter picking, grass cutting and vegetation clearance, waste removal, and the inspection of mechanical components and structures. Typical pond/wetland construction profile 6.2.2 Safe access SuDS layout and design should allow safe and easy access for the monitoring and maintenance of the drainage system and its constituent parts. Inspection 20 Building Regulations usually require 60 years 39
points or chambers can be used for the visual surveillance of mechanical components and structures, and should also be easy to clean and repair where necessary. Where SuDS features have sloping profiles, such as swales, basins, ponds and wetlands, gentle gradients and flat benches should enable safe access and egress, with water levels not in excess of 500mm, as illustrated on the previous page. 6.2.3 Waste management SuDS generate three main types of waste: litter, silt and green waste. Most waste management tasks can be carried out as part of the general landscape management regime, for example litter can be collected as part of the regular street cleaning operations. Inorganic silts are likely to accumulate slowly at the entry/exit of inlets, outlets and flow control structures. Mechanical components should be designed to minimise the accumulation of silts and allow its easy and safe removal. The provision of adequate source controls and pre-treatment structures should remove silts at the beginning of the drainage system to avoid clogging downstream. Dedicated silt ponds or forebays that reduce flow rates and encourage the deposition of silt, can also be used. Organic silts in ponds and wetlands, from the accumulation of plant remains in water with low oxygen levels, can be controlled by appropriate vegetation clearance. Green waste generated by grass cutting, pruning and vegetation clearance may need to be removed from site however it can be more affordable and sustainable, to compost this type of waste as a valuable resource for the community, or to create wildlife piles. 40
Green & Blue Infrastructure Legislation, Policy & Guidance 7. APPENDIX 7.1 Signposts Please note, the following list of reference material is not exhaustive and there may be additional sources of relevant information. CONTEXT (Chapter 2) Flood and Water Management Act http://www.legislation.gov.uk/ukpga/2010/29/contents National Planning Policy Framework https://www.gov.uk/government/publications/national-planning-policyframework--2 Planning Practice Guidance and SuDS National Standards, DEFRA & DCLG The SuDS Manual (CIRIA C697), best practice guidance on the planning, design, construction, operation and maintenance of SuDS http://www.ciria.org/resources/free_publications/the_suds_manual.aspx Local Flood Risk Management Strategy for Hertfordshire http://www.hertsdirect.org/services/envplan/water/floods/floodrisk/lfrmsherts/ Building Futures guidance for sustainable and high quality development in Hertfordshire http://www.hertslink.org/buildingfutures/ Hertfordshire Strategic Green Infrastructure Plan, March 2011 http://www.hertsdirect.org/docs/pdf/s/ship.pdf Green Infrastructure, an integrated approach to land use, Landscape Institute Position Statement 2013 http://www.landscapeinstitute.co.uk/policy/greeninfrastructure.php Natural England Green Infrastructure Guidance http://www.naturalengland.org.uk/ourwork/planningdevelopment/greeninfrast ructure/default.aspx UK Climate Projections http://ukclimateprojections.metoffice.gov.uk/ Climate Change Adaptation, Building Futures http://www.hertslink.org/buildingfutures/16557273/16766372/ 41
Land Stability Contamination Water Quality Flood Risk Groundwater Geology & Soils TECHNICAL & SPATIAL FRAMEWORK (Chapter 3) British Geological Survey (BGS) https://www.bgs.ac.uk/ Infiltration SuDS Map, British Geological Society http://www.bgs.ac.uk/products/hydrogeology/infiltrationsuds.html Groundwater Source Zones, River Quality data, and incidences of watercourse contamination, Environment Agency http://www.environment-agency.gov.uk/business/topics/water/31785.aspx Management of the London Basin Chalk Aquifer http://publications.environment-agency.gov.uk/pdf/geth0710bsvt-e- E.pdf Catchment Flood Management Plans, Anglian Region http://www.environment-agency.gov.uk/research/planning/114303.aspx Local Flood Risk Management Strategy for Hertfordshire 20013-2016 http://www.hertsdirect.org/services/envplan/water/floods/floodrisk/lfrmsherts/ Flood Hazard and Flood Risk Maps, Environment Agency http://www.environment-agency.gov.uk/research/planning/135518.aspx Managing urban flooding from heavy rainfall encouraging the uptake of designing for exceedance http://www.ciria.org/resources/free_publications/c738a.aspx Water Framework Directive, River Basin Management Plans, Water Quality http://maps.environmentagency.gov.uk/wiyby/wiybycontroller?x=357683.0&y=355134.0&scale=1&la yergroups=default&ep=map&textonly=off&lang=_e&topic=wfd_rivers#x=531 713&y=210473&lg=1,7,8,9,5,6,&scale=5 River quality data and incidences of watercourse contamination can be obtained from the Environment Agency. Details of contaminated land can be found on the relevant local authority register, with the exception of some Brownfield sites that may not be listed yet but could have contaminants present. Where contaminants are found to be present early communication with the Environment Agency and Hertfordshire County Council is strongly advised. Chalk mines in Hatfield, Welwyn Hatfield Borough Council http://www.welhat.gov.uk/index.aspx?articleid=356 42
Ops & Maintenance Construction Historic Environment Biodiversity Landscape Character The Hertfordshire Landscape Character Assessment http://www.hertsdirect.org/services/leisculture/heritage1/landscape/hlca/ The East of England Landscape Typology http://landscape-east.org.uk/ Building Futures - Landscape and Biodiversity Module & Built Environment Module http://www.hertslink.org/buildingfutures Key Biodiversity Areas, Building Futures Landscape and Biodiversity Module http://www.hertslink.org/buildingfutures SuDS and the Historic Environment, English Heritage (CIRIA and Essex County Council are currently working partnership to produce this guidance) http://www.susdrain.org/news/articles/survey_suds_historic_environment.ht ml The Hertfordshire Historic Environment Record http://www.hertsdirect.org/services/envplan/archaeology/sites/ Building Futures - Built Environment Module http://www.hertslink.org/buildingfutures DELIVERY Cost comparison examples, DEFRA http://www.defra.gov.uk/consult/files/suds-consult-annexf-ia-annex1-111220.pdf National Building Specification http://www.thenbs.com/ Management and Maintenance of Sustainable Drainage Systems (SuDS) landscapes, Interim Technical guidance Note 01/2014, Landscape Institute March 2014 http://www.landscapeinstitute.org/knowledge/technical_guidance.php Environment Agency Regulatory Statement, The deposit and dewatering of non-hazardous silts from sustainable drainage systems on land, Ref MWRP RPS 005 Version 1.0 issued March 2010 43
7.2 Quantity & quality criteria 7.2.1 Peak flow & runoff rates Peak flow should be calculated to determine the volume of water that needs to be managed and discharged from the site. Where peak flow exceeds the allowable discharge rate, volume control and attenuation storage mechanisms should be used to control flows in line with the allowable rates. In any catchment the rate of runoff will rise in line with the increasing intensity and frequency of rainfall events. The return periods that should be considered are: 1 in 1 year 1 in 2 year 1 in 30 year 1 in 100 year 1 in 100 year + climate change 7.2.2 Greenfield runoff rates The Greenfield Runoff Rate (GF) is the runoff rate for a site in its natural state, prior to development. The Urban Runoff Rate is the runoff rate for a site as developed and can be 20 to 30 times greater than GF rates. The GF rate is the benchmark against which the urban rate should be measured, to calculate the additional flow rates. This in turn will inform the size of volume controls and attenuation storage components required to ensure that the allowable discharge rate is not exceeded, and that the overall rate of flow entering the watercourse is not increased. One of two approaches can be used for the calculation of GF runoff rates: 1. The Institute of Hydrology (IH) Report 124 Flood Estimation for Small Catchments (1994) method can be used to estimate the Greenfield site flow rate, QBAR (the Mean Annual Flood). 2. The Index Flood, QMED (the median of the set of annual maximum flood peaks) (FEH method) can also be used where the appropriate parameters are known or can be derived/ estimated. For developments less than 50 ha, the analysis for determining the Greenfield index flood flow rate should use 50 ha in the formula and linearly extrapolate the flow rate value based on the ratio of the development area. The table below shows the acceptable Greenfield runoff rates for Hertfordshire These runoff rates are an initial guide and give the anticipated flow control allowance for a site. It is accepted that the flow settings are generalised and may not suit all situations. Where a developer wishes to deviate from the flow rates provided, reasonable and justifiable reasons must be given with supporting calculations. 44
Soil type 1 1 2 3 3 4 4 4 SAAR (mm) 700 800 700 600 650 600 650 700 QBAR (l/s/ha) 0.6 0.7 2.8 4.4 4.8 5.6 6.2 6.8 Q1yr (l/s/ha) 0.5 0.6 2.4 3.7 4.1 4.8 5.3 5.7 Q30yr (l/s/ha) 1.4 1.6 6.4 9.9 10.9 12.8 14.0 15.3 Q100yr (l/s/ha) 2.0 2.3 8.9 13.9 15.3 18.0 19.8 21.6 Q100yr + cc (l/s/ha) 2.60 3.002.3 11.68.9 18.13.9 19.95.3 23.418.0 25.719.8 28.11.6 Table: Allowable Greenfield runoff rates for Hertfordshire (litres per second per hectare) SAAR Q1yr Q30yr Q100yr Q100yr + cc Standard Average Annual Rainfall Discharge rate 1 in 1 year storm Discharge rate 1 in 30 year storm Discharge rate 1 in 100 year storm Discharge rate 1 in 100 year storm adjusted to account for climate change The Greenfield runoff rates are calculated using the The Institute of Hydrology (IH) Report 124 Flood Estimation for Small Catchments (1994) method. Rates are expressed in litres per second per hectare. 45
7.2.3 Previously developed land runoff rates Previously Developed Land (PDL) relates to areas where runoff rates have been increased due to previous development, such as Brownfield and re-development sites. On PDL, a Greenfield rate must still be achieved. Where Greenfield rates cannot be achieved, a Betterment Rate should be achieved. The betterment rate is a reduction factor in the peak runoff rate of the existing site. In order to calculate the peak runoff rate, one of the following methods should be used: The Modified Rational Method The fixed UK runoff model The Variable UK runoff model For calculation purposes the peak intensity of rainfall should be taken as 75mm/hour. In cases, that a reduction in peak runoff rates to Greenfield rates cannot be achieved, the betterment rate will act as the flow control rate for the SuDS. Storage calculations are then carried out as per the method for Greenfield sites. Signpost: Calculating Greenfield runoff rates, SuDS National Standards The SuDS Manual CIRIA (C697), Section 4.5.5 Long-term storage https://www.google.co.uk/#q=ciria+c697 Preliminary rainfall runoff management for developments - R&D Technical Report W5-074/A/TR/1 Rev E 2012 (Table 1) Calculating previously developed land runoff rates, SuDS National Standards Preliminary rainfall runoff management for developments - R&D Technical Report W5-074/A/TR/1 Rev E 2012 (Table 1) 7.2.4 Flow controls Flow controls are required to control discharge and meet the peak flow rate criteria. There are a number of different methods for controlling flows, for example; weirs, perforated risers, orifice plates and hydrobrakes. Regardless of the mechanism used, the minimum final discharge from a site should be set at 5l/s. The provision of multiple site controls is not advised as it would artificially increase the rate of allowable discharge. 46
Where storage components are provided a flow control is required. A drainage system may require a number of flow controls, incrementally increasing in flow rate capacity as they progress down each stage of the management train (MT). At the beginning of the MT (source control components) flow rates are likely to be <5l/s. Lower flow rates will be subject to detailed design. Flow controls should have the capacity to accommodate peak flow rates from the whole drained area of the site, including all green space. Green space has a response in line with the natural drainage pattern and therefore runoff from these areas does not have to be stored. Flow rates depend on whether a variable or fixed flow control is used, which relates to the approach to storage. Where long term storage is provided a variable flow rate (1 in 1 1 in 100 GF runoff rates) will be allowed. 21 Where long term storage is not provided the peak flow rate will be restricted to the QBAR rate. 22 Flow controls should be designed so that they are easily accessible, maintainable and not susceptible to blockage. 7.2.5 Storage volumes Storage components should be allocated throughout a site, making full use of existing landscape features. This should reduce the flow rates and minimise the volume of storage required. The general methodology for the calculation of storage volumes is set out below. Regardless of how calculations are derived, storage volume inputs and outputs must be presented on the template below for evaluation. The runoff rate used to calculate the required storage volume should be proportionate to the proposed area of impermeable development, and not the overall area of the site. 100% runoff should be assumed for impermeable development (paved and roofed surfaces) and 0% from permeable areas. 23 Where interception storage is provided a 5mm reduction in runoff can be allowed within the storage volume calculations. Where long term storage is provided a reduction in storage volumes can be allowed taking account of climate change and urban creep where appropriate (there should be no double allocation of storage). Where source control components are not provided, and it is proposed to convey peak flows through the drainage system, conveyance components should have the capacity to accommodate peak flows. A series of rainfall events* should be analysed to determine critical duration, this is the largest volume of storage which is calculated within a given return period. The SuDS design must show that critical duration has been achieved by demonstrating 21 SuDS National Standards, Approach 1 22 SuDS National Standards, Approach 2 23 The Water Industry manual Sewers for Adoption 7th ed. (2012) 47
that peak volume has been calculated, with reducing volumes for subsequent rainfall events. Volume calculations should take account of climate change, and factor in a 30% increase in rainfall for both the 1 in 30 and 1 in 100 year rainfall events. A drain down check is required to ensure that there is at least 50% attenuation storage volume available within 24hours of the rainfall event having ceased, to allow sufficient storage volume for subsequent rainfall events. *Rainfall data can be sourced from the FEH CD-ROM for spreadsheet analysis or alternatively software packages such as InfoWorks or MicroDrainage can be used to source and analyse rainfall data. The use of hydraulic software can be necessary for the analysis of more complex sites. Signpost: SuDS National Standards The SuDS Manual CIRIA (C697), Section 4.5.4 Attenuation storage https://www.google.co.uk/#q=ciria+c697 Preliminary rainfall runoff management for developments - R&D Technical Report W5-074/A/TR/1 Rev E 2012 7.2.6 Quantity data template The following template identifies the flow and volume information for a proposed SuDS scheme. It is recommended that the reader review the associated notes and HCC Building Futures website prior to providing information. 48
Quantity data template 1. Site details 1.1 Planning reference number... 1.2 Site name...... 1.3 Total site area (Note 1)... ha 1.3 Site area which is positively drained (Note 2)... ha 1.4 Developed area (Note 3)... ha 1.5 Predevelopment use (Note 4)... Greenfield / Brownfield / Mixed* 1.6 Site constraints (Note 5).................. 1.7 Type of discharge... Infiltration / watercourse / sewer / mixed* 2. Flow control 2.1 Flow control type (Note 6)... Fixed / Variable* 2.2 Greenfield flow Q1 (Note 7)... l/s/ha, l/s for the site 2.3 Greenfield flow Q100 (Note 8)... l/s/ha, l/s for the site 49
3. Site storage volume 3.1 Source control provided (Note 9)... Yes/No* 3.2 Approach used to calculate storage (Note 10)... 1 / 2 / PDL* 3.3 Storage - 1 in 1 year (Note 11)... m3/m2... m3 for the site 3.4 Storage - 1in 30 year (Note12)... m3/m2... m3 for the site 3.5 Storage - 1 in 100 year plus CC (Note 13)... m3/m2... m3 for the site 3.6 Long term storage (Note 14)... m3/ha... m3 for the site 3.7 Total site storage (Note 15)... m3 4. Design checks 4.1 Time taken for 50% of storage to drain down (Note 16)... hours 4.2 All SuDS storage located outside Q100 floodplain (Note 17)... Yes/No* 4.3 Provision for blockage / design exceedance (Note 18)... Yes/No* (Note *= delete as appropriate) 50
Notes 1. All area with the proposed site boundary to be included. 2. The site area which is positively drained includes all green areas which drain to the SuDS system and the area within which the vegetated SuDS reside. It excludes large open green spaces which do not drain to the SuDS system. 3. The developed area is identified as all areas which are developed as impermeable surfaces, including, roofs, pavements, driveways, paths. Green roofs and permeable pavements should also be included within this area. All green areas (landscaped areas, gardens, public open space and vegetated SuDS features) and open water body areas are excluded from the calculation. All developed areas to be assumed as 100% runoff for the purposes of storage calculations. Advice should be sought from an appropriate authority for reduction in runoff factors for Green roofs and permeable pavements. 4. Discharge from the site by means other than infiltration, means that the site cannot be fully drained by infiltration, or site constraints such as high groundwater table or contaminated land prevent infiltration. The present use of the site (Greenfield / Brownfield) and the way that storage is calculated (Approach 1 / Approach 2/PDL) will dictate the allowable discharge flow rate from the site. Where the site is mixed use, calculations should be developed for the respective sections of the site. 5. Main site constraints which impact on the design of SuDS drainage should be listed. Examples include, high ground water table, contaminated land, clay sites with no potential for infiltration. Site constraints do not mean/infer that a SuDS approach cannot be applied. Site constraints will assist in defining the most appropriate SuDS components for the site. 6. A fixed flow control means that the outflow from the site remains constant regardless of the amount of rainfall falling on the site or amount of storage used. A variable flow control means that as rainfall severity increases and storage starts to fill, the rate of flow leaving the site will increase. (See Note 8 for further information) 7. Q1 Greenfield runoff is the rate of runoff which would flow from the site in a natural Greenfield state for the 1 in 1 year rainfall event. This is the primary discharge allowance for the site. When utilising Approach 2, the flow rate for the 1in 100 year +CC event will also be restricted to Q1 Greenfield runoff. 8. Q100 Greenfield runoff is the rate of runoff which would flow from the site in a natural Greenfield state for the 1 in 100 year rainfall event. This flow will be approximately 3.2 time larger than the Q1 Greenfield runoff rate. When using Approach 2, this row should be left blank as all flows would be restricted to Q1 Greenfield runoff rate. 51
9. Where Source Control is provided, 5mm rainfall depth can be subtracted from attenuation storage calculations. For this volume to be omitted from calculations the following must be demonstrated; a. The site has been split up into sub-catchments. b. A SuDS component acting as a Source Control technique is located within each sub-catchment. 10. The National SuDS Standards sets out a series of approaches to calculate site storage. These relate to attenuation storage. A brief description is provided below; Building Futures website (Section xx) should be referred for further information. a. Approach 1- Provides a variable flow rate leaving the site which increases with rainfall return period. In order to utilise the higher outflow rates, Long Term storage must be provided. Overall storage requirements for Approach 1 are normally lower than that for Approach 2. b. Approach 2- Requires a fixed flow rate leaving the site which is set to Greenfield Q1 flow rate. There is no need to provide Long Term storage, however Approach 2 will normally result in a larger storage volume than Approach 1. c. Previously Developed Land runoff from PDL should be restricted to Greenfield runoff rates. Where restriction to Greenfield runoff rates cannot be achieved discussion will be required as to the reduction of peak runoff which will be required to facilitate redevelopment to agree a suitable betterment rate. As a minimum Peak runoff from the development will not be allowed to exceed the 1 in 1 year and 1 in 100 year peak runoff from the site prior to re-development. Where possible, the rate of runoff should be reduced to the equivalent GF rate for the site. Peak runoff from the site can be calculated using hydraulic modelling software, and should be accompanied with a Modified rational method calculation. 11. The 1 in 1 year storage defines the minimum provision of Source Control SuDS techniques. Note that all volume requirements for the sub-catchment can be provided within the sub-catchment, or provided elsewhere within site controls. 12. The 1 in 30 year storage defines the minimum storage and conveyance requirement (depending upon function) of all SuDS components. In much the same way that conventional drainage systems are designed to not flood for 1 in 30 year rainfall, SuDS components should not flood for the 1 in 30 year event. Note that standing water within SuDS components such as ponds, basins and swales is not classified as flooding. 52
13. Flows from up to the 1 in 100 year plus Climate Change rainfall will not be allowed to leave the site in an uncontrolled way. Flows will either be stored as attenuation (Approach 2) or permitted to temporarily flood pre-determined parts of the site as Long Term storage (Approach 1), see Note 13. Temporary flooding will be to shallow depths (150mm-300mm max) with due consideration given to health and safety within design. 14. Long-Term (LT) storage is specifically aimed at runoff during extreme events to limit flood impact downstream. Flow diverted to LT storage should be infiltrated to the ground or, if this is not possible, discharged to the receiving water at very low flow rates (maximum 2 l/s/ha). LT storage would not be allowed to empty directly back into attenuation storage and would be expected to drain away over 2-10 days. Typically LT storage would be provided on multi-functional open space or sacrificial car parking areas. 15. The total site storage is the full storage provided by the SuDS on site. Care should be taken not to duplicate storage within the calculation. The following examples are provided. a. App 1 Site storage = Int. Storage + 1 in 100 year plus CC storage + LT Storage b. App 2 Site storage = Int. Storage + 1 in 100 year plus CC storage 16. Wherever attenuation storage is filled due to extreme rainfall falling, there must be sufficient volume available, in the event of a subsequent rainfall event falling within the next 1-2 days. The rule of thumb check is for 50% of the attenuation storage volume to be available within 24 hours after rainfall event having finished. 17. SuDS storage should not be provided within the Q100 Fluvial Floodplain. The location of storage should be checked against the EA flood maps, relevant SFRA or a site specific site FRA. Consideration should also be given to the implications on SuDS storage, whenever discharging under surcharge (watercourse or sewer). Shallow storage and un-surcharged discharge will result in less storage being required. 18. Consideration must be given to the potential for blockage or design exceedance at all inlet, outlet and flow control points within the SuDS management train. Proposals should demonstrate that control points can be easily unblocked and that there is an alternative / overflow route between the SuDS components. This should be identified as the flood exceedance route. 53
7.3 SuDS features guidance sheets Key: SuDS Management Train Source control Regional control Site control Conveyance Key: SuDS Benefits Storage Attenuates and stores surface water Treatment Removes pollution Evapo-transpiration Encourages natural losses through evaporation and transpiration Biodiversity Habitats and wildlife benefits Happy & Healthy Communities Provides opportunities for education and/or recreation & play Infiltration Discharges surface water into ground Landscape & Visual Amenity Creates locally distinct attractive places 54
7.3.1 Green roofs Description Green roofs use a series of drainage layers to layers to intercept and manage rainwater. The top layer, known as the substrate, consists of a free draining growing medium (usually soil) supporting a vegetated layer. The underlying layers include a drainage layer to collect and store water as it leaves the substrate. From here, water leaves the roof via an outlet such as a downpipe, before entering the next stage of the drainage system. Awaiting image Issues & opportunities Ideal for large scale public, commercial and educational buildings such as schools Substrate depth can be varied to enhance visual and biodiversity interest, however, a minimum depth of 100mm is required to provide an efficient drainage function and support healthy vegetation Plant species should be chosen to maximise wildlife value and complement the local context. Additional storage layers can be included to increase storage capacity, helping to further reduce the requirement for ground level storage Maintenance Planting and maintenance specification and schedules should be agreed Safety arrangements for inspection and maintenance should be agreed Signpost Green roofs, Building Greener, CIRIA Report C644, published 2007 http://www.ciria.com/buildinggreener/ Green Roof Tool Kit, Environment Agency http://www.environmentagency.gov.uk/business/sectors/91967.aspx 55
Green Roof Construction Detail 56
7.3.2 Permeable pavements Description Permeable pavements use a series of construction layers to intercept and manage rainwater. At the surface, individual paving units such as concrete blocks are shaped to allow water to pass between them into an underlying treatment layer. The treatment layer consists of a grit bedding material which removes pollutants from the water as it passes into the final storage layer. The storage layer, or sub base, is constructed from crushed stone. Water is stored in the voids between the stones; at this point infiltration into the ground should be encouraged before controlled discharge, via a perforated pipe or fin drain, into the next stage of the drainage system. Other types of construction that allow water to pass directly through the surface into a storage layer, or the ground, include porous tarmac, open gravel and reinforced grass surfaces. Awaiting image Issues & opportunities Ideal for driveways, car parks and other lightly trafficked areas. Surface water flows from adjacent areas can wash onto the pavement surface clogging the gaps between units and reducing permeability. Areas adjacent to permeable pavements should fall away and be turfed, or finished 50mm below the pavement edge and planted with surface rooting ground cover species. Maintenance Competent construction and regular inspection is critical to ensure correct installation. Maintenance of hard surfaces should include regular sweeping and suction treatment, and occasional specialist maintenance such as regritting Gravel and grass surfaces may need scarification and mowing depending on location Free draining surface soils require an appropriate root zone specification Signpost BS 7533-13:2009 Pavements design guide Source control using constructed pervious surfaces, CIRIA http://www.ciria.org/service/web_site/am/contentmanagernet/c ontentdisplay.aspx?section=web_site&contentid=8936 Permeable pavement, Interpave manual 6th Edition http://www.paving.org.uk/commercial/permeable.php 57
Permeable Pavement Construction Detail 58
7.3.3 Rainwater harvesting Rainwater harvesting collects and reuses rainwater, harvesting schemes can range in scale and complexity depending on local requirements. Small scale domestic schemes include the collection of roof water in water butts for use around the garden or community allotments. Small ponds can provide a water resource for agricultural and recreational land uses. Less formally SuDS can be used to direct water to trees and shrubs to assist their watering, although this needs careful consideration to avoid water logging species which cannot tolerate this. In an urban setting proprietary geocellular structures have been developed which assist with watering the tree root zone. Awaiting image Raingardens Raingardens are a simple and effective domestic scale system for reusing low pollution risk rainwater, such as roof runoff. Rainwater is collected and fed into the garden supporting a diverse range of herbaceous and shrub plants that enhance visual amenity and provide biodiversity interest. Description The garden is constructed within a shallow depression up to a maximum depth of 450mmm. The floor of the depression is lined with a drainage layer of permeable soil at a minimum depth of 400mm to support healthy vegetation. The soil specification should allow for slow infiltration and maximise the drainage capacity. Roof water enters the garden via a surface channel or pipe; it then filters through the soil drainage layer before finally dispersing into the ground. In periods of exceedance, a flow controlled outlet should direct overflow to the next appropriate stage of the drainage system. Issues & opportunities Raingardens generally do not meet the adoption criteria and require maintenance by the site/property owner Local communities can develop a sense of ownership and take an active role in management and maintenance. Maintenance Overflow is required usually back to the sewer Inlets and outlets are simple to check Refer to maintenance for basins 59
Raingarden Construction Detail 60
7.3.4 Ponds & wetlands water down allows silts to drop to the bottom of the pond/wetland and gives plants, and the organisms they support, an opportunity to remove pollution. The residency time can be enhanced by extending the route of water; wetlands often have a longer length to width ratio than ponds, however, in all cases a minimum length to width ration of 4:1 should be adopted. Description Awaiting image Ponds and wetlands are areas of open water designed to accommodate rising water levels during periods of heavy of rainfall. Vegetation cover is an important component of any pond/wetland design, where vegetation cover exceeds 75% of the ponds surface they are usually referred to as wetlands. The design of the pond/wetland plan and profile is critical and should maximise biodiversity interest without compromising health and safety. Wetlands are generally shallower than ponds, up to maximum depth of 150mm. In any case, permanent water depth should not exceed 400mm, and maximum storage levels should not exceed 600mmm. A level dry bench, and gentle slopes, up to a maximum gradient of 1:3, should allow safe access to the water edge. A wet bench at permanent water level should aid egress from the water, as well as support a range of marginal and aquatic plants. The longer water is held within the water body, known as the residency time, the more effectively it can be treated before discharge into the next stage of the drainage system. Slowing 61
Issues & opportunities SuDS design should ensure that source control components are used at earlier stages of the drainage system, to remove pollution, ensuring that any water entering ponds and wetlands is as clean as possible. Pond and wetland design should enhance visual amenity and provide biodiversity interest. Open water features should be designed to enable safe access and egress. Flow inlets should be at the surface, or through shallow pipes if necessary, to avoid deep construction and erosion issues. An overflow is also required for blockage or exceedance. Maintenance Silt and/or vegetation removal on annual rotation (25%- 30% of area per year) Intensive maintenance should only be carried out between Sept and Nov to minimise impact upon wildlife On-site wetland vegetation and silt disposal Safe access for maintenance Refer to maintenance for basins Signpost Site Handbook for the Construction of SuDS, CIRIA C698 http://www.ciria.org/service/am/contentmanagernet/default.a spx?template=/taggedpage/taggedpagedisplay.cfm&tplid =66&ContentID=16011&TPPID=5891&AspNetFlag=1&Sectio n=free_publications&thispage=2 Ponds, pool and lochans: Guidance on good practice in the management and creation of small water-bodies in Scotland, SEPA, 2000 The Pond Book: A guide to the management and creation of ponds, Ponds Conservation Trust, Oxford 1999 62
Pond and Wetland Plan 63
7.3.5 Retention & infiltration basins Retention Basins Infiltration Basins to convey water if the basin becomes overwhelmed or there is a blockage. During the storage period, infiltration basins allow water to dissipate into the ground, whereas retention basins slowly release water into the next stage of the drainage system via an outlet. A raised outlet with a micro-pool can reduce risk of blockage Description Retention and infiltration basins are open grass depressions, they are normally dry however during periods of heavy rainfall provide temporary storage for floodwater. Basins have a limited ability to treat water; therefore a good SuDS design should ensure that source control components are used at earlier stages of the drainage system, to remove pollution and silts, ensuring that any water entering basins is as clean as possible. Flood waters enter the basin via an inlet. Vegetated surface components, with a gentle gradient, should be used to convey water into basins wherever possible. Otherwise shallow pipes with erosion control can be used; deep inlet structure should always be avoided as they are difficult to maintain. The basin slopes should allow safe access at a maximum gradient of 1:3. An exceedance flow route should be included Issues & opportunities All basins should be designed for multifunctional use, including public open space such as play areas. Good design should maximise biodiversity interest, and incorporate wetlands where appropriate. Maintenance 64
Outlet structure design to protect flow control structure and prevent blockage with simple access for inspection Issues described for swales should also be addressed 65
Basin Construction Detail 66
7.3.6 Bio-retention Description Bio-retention components are generally used in urban areas to collect and treat rainwater, and involve the modification of landscape features to perform a drainage function. A free draining growing medium (usually soil), or root zone, supports a vegetated layer. Water filters through the root zone into the underlying drainage layer before discharge into the next stage of the drainage system. Bioretention (Awaiting image) The competent construction of drainage layers should include the provision of measures to prevent uncontrolled root invasion and siltation. An overflow should be installed for exceedance. Maintenance Surface mulch replenishment Overflow structure design to protect prevent blockage with simple access for inspection Landscape maintenance 67
Bio-Retention Construction Detail 68
7.3.7 Swales & filter strips Swales Description Swales are grassed or vegetated channels with a flat base that can collect, treat, store and convey water depending on the design. They should meet the following design criteria: Maximum linear slope 1:50 to prevent erosion Minimum retention time 10 minutes for pollution control Maximum side slopes 1:3 for safe access and maintenance Minimum height of vegetation 100mm for flow control and treatment Maximum height of grass 50mm, consider wildflower turf for extended cut intervals Design should follow site contours, where this is not possible cascades and check dams can be used Maintenance Rounded shoulder profile should allow safe mowing Mowing regime should be 100mm for grass, and occasional for other habitat Signpost Swales, Abertay University, MacDonald and Jefferies 2003 http://www.ciria.org.uk/suds/pdf/2011/overview%20of%20su DS%20performance%20%282009%29.pdf 69
Swale Construction Detail 70
Under-drained swale or basin Filter strips Description Under-drained swales or basins integrate the profile of a normal swale or basin with a filter drain below the surface. A free draining soil or root zone is needed to convey water through the surface layer to an underlying drainage layer. From here water is discharged into the ground or onto the next stage of the drainage system. Description Filter strips are grass/vegetated verges located along the edge of hard surfaces that allow water to flow as a sheet to another SuDS feature, such as a swale or filter drain. Runoff flows unimpeded across the edge, usually with a 20-25mm drop. The filter strip should have a greater fall than the hard surface for at least 1m. Haunches should be a minimum of 100mm deep to allow good grass growth to the edge. Topsoil depth between 100mm and 150mm is generally required for robust grass growth. Awaiting image Maintenance Protection is required to prevent uncontrolled siltation and maintenance issues. Outlets require safe and easy access for maintenance See maintenance for swales and basins Maintenance Siltation during the construction period should be dealt with before handover of the site for adoption. Litter picking and grass cutting at 100mm average height. 71
Under-drained Swale Construction Detail 72
Filter Strip Construction Detail 73
SuDS Design Guidance for Hertfordshire April 2014 7.3.8 Filter drains Awaiting Image(s) Description Filter drains or French drains are trenches filled with open graded stone that allow runoff to flow laterally into the structure and either infiltrate directly into the ground, or travel along the drain to an outfall. Filter drains can be lined to prevent infiltration where more than one treatment stage is required. Where infiltration is possible the drain may have a high level perforated pipe as an overflow but in impermeable ground a perforated pipe in the base ensures drain down of the structure. The trench should be sized to manage the first flush volume of runoff. Runoff should enter the drain laterally along its length, crushed stone rather than pea gravel fill enhances treatment process. A level edge will prevent point erosion and containment of the stone. A sacrificial surface stone layer may be used or a protective grass filter strip should enhance the design life. Maintenance Outlets require safe and easy access for maintenance 74
SuDS Design Guidance for Hertfordshire April 2014 Filter Drain Construction Detail 75