Designing Rain Gardens: A Practical Guide

1 3 Designing rain Gardens: A Practical Guide5 Contents Welcome 5 What are rain Gardens? 7 Designing your rain Garden 10 Checklist 33 Public Engagement 34 Signage 36 Cost & Maintenance 39 Funding 40 Resources 41 Designing rain Gardens: A Practical GuidePublished by Urban Design LondonPalestra197 Blackfriars RoadLondon SE1 rights :/Jamie Kukadia/Madeleine Lundholm/Ian RussellEditor:/Paul DoddWith special thanks to:/ LB Enfield/ LB Waltham Forest/ LB Hammersmith & Fulham/ George Warren, GLA/ Walthamstow Village in Bloom/ GreenBlue Urban/ WSP7 Welcome London s drainage infrastructure is a piped network, with few soft infrastructure measures.

2 Considering the age of the city s drainage system and other pressures such as population growth, climate change and densification, it is easy to understand that the network is reaching the limit of its capacity. A long-term view and alternative approaches are needed to avoid exceeding capacity or the cost of completely replacing the purpose of this document is to provide a Practical Guide to Designing and installing rain gardens in an urban environment. rain gardens are a type of a Sustainable Drainage System (SuDS) that capture rainfall before it enters the piped network and either releases it slowly into the network or allows it to infiltrate into the ground. rain gardens can help improve surface water management whilst performing a range of other functions such as amenity, reduced pollution and improved biodiversity. This Guide is intended to provide you with information on how to design a rain garden in an existing street, and tips on how to avoid common mistakes.

3 The document takes you through the six simple steps involved in Designing a rain garden: 1/ Choose the right location2/ Composition of a rain garden3/ Calculate the size and depth 4/ Consider infiltration5/ Manage flows6/ Select the planting QualityAmenityQuantityMaintain and protect the natural water cycleQuantity QualitySupport the management of flood riskThe Four Pillars of SuDS Design Source: The SuDS Manual 2015, CIRIA, p6 Manage the quality of therunoff to prevent pollutionCreate and sustain better places for natureCreate and sustain better places for peopleAmenityBiodiversity9 What are rain Gardens? rain gardens are shallow landscaped depressions that reduce rainfall runoff and mitigate the impact of pollution. They can be used to enhance the capacity of the surface water piped drainage network by capturing and storing rainfall, allowing it to soak into the ground or release it slowly back into the piped network.

4 rain gardens are flexible in design and are excellent examples of how SuDS components can be integrated into a streetscape without negatively impacting on the primary function of our streets and spaces. The performance of a rain garden can be enhanced by engineering the sub-base to include a gravel layer that helps to filter pollutants and provides more storage capacity for rain water. These engineered rain gardens are often referred to as bioretention systems, however in practice there is no clear distinction between rain gardens and bioretention refer to the SuDS Manual under Resources (p41) for additional information about SuDS and bioretention realm improvementsTraffic managementsCycle routesParking controlsModal filtersSuDS OpportunitiesThere are many opportunities to integrate rain gardens into the public realm, such as:11115522233334411 Vauxhall Walk, LB Lambeth The Vauxhall Walk rain gardens are part of The Missing Link scheme; a broader commitment to improve walking and cycling connections between Vauxhall and Nine Elms, through a series of connecting pocket parks and gardens should be designed to help reduce flood risk and improve water quality, biodiversity and public amenity within the available budget.

5 They can be planted with a wide variety of species depending on the context and demands of the gardens can be implemented wherever space is available. They should not only be considered in areas at risk of flooding as flood risk will be reduced by introducing a rain garden upstream of flood risk areas, as long as they share the same catchment area. Installation costs can be significantly reduced by integrating rain gardens into public realm projects and can provide substantial added public health and wellbeingIncreases amenity spaceAesthetic improvementsImproves air qualityReduces urban heat island effectEnhances wildlife habitat for biodiversityReduces flood riskFilters pollution and improves water qualitySuDS Benefits 15263748 Walthamstow Village Square, LB Waltham Forest13 The next step is to calculate the catchment area that drains into your rain garden. The catchment area is defined by location and topography.

6 Some information on levels is usually required but in many cases a visual assessment will be adequate to determine your rain garden s catchment area. Existing gullies are usually located at local low points. Locating the rain garden at an existing gully, either by incorporating the gully into the footprint of the rain garden or constructing the rain garden just upstream of the gully, provides an in-built Road, LB Enfield A series of enhanced public spaces were created as part of the Cycle Enfield scheme. Different rain gardens were incorporated into the works at Compton Road to make the junction more pedestrian following chapter sets out the six simple steps involved in Designing your rain garden. 1/ Choose the right location2/ Composition of a rain garden3/ Calculate the size and depth 4/ Consider infiltration5/ Manage flows6/ Select the planting Designing your rain GardenDetermine your runoff areaFind your low pointExisting gully can provide a potential overflow1/ Choose the right location The first step is to determine the location of your rain garden.

7 rain gardens are best located at low points where surface water will flow to. However, where rain gardens are being created as part of a larger scheme the overall layout may determine the most suitable location, so some flexibility is minimum depth of 300mm of topsoil is recommended for shrubs and herbaceous plants. A shallower depth of 200mm can be considered for rain gardens that will be turfed or seeded with wildflowers. You should seek advice from a horticulturist, landscape architect or grounds maintenance specialist when specifying your layer of mulch should be added to planted rain gardens to help suppress weeds and reduce competition for water and nutrients whilst the planting becomes sub-base should be 100-500mm depth of sub-base will vary depending on the required storage capacity and budget. A deeper sub-base will help store more water. Typically the sub-base will be 100-500mm deep, and a minimum depth of 100mm is recommended in most instances.

8 In the instance where the sub-soil is free draining or the catchment area is small it may be considered appropriate to eliminate the sub-base layer Composition of a rain gardenA rain garden typically comprises the following features:1/Freeboard 2 / To ps oil 3/Sub-base1/FreeboardThe freeboard provides potential water storage space, above the topsoil. The freeboard depth is measured from the carriageway or footpath level to the top of the top soil. You should aim to create a freeboard depth of 200-300mm to encourage water to flow into your rain garden and to accommodate silt accumulation and build-up of leaf litter. A common mistake is to make the freeboard too shallow, where the topsoil is level with the footpath or carriageway, which does not allow water to flow into the rain garden. The minimum freeboard depth should be / To ps oilTopsoil usually consists of a mixture of soil, sand and compost. Soil permeability can be specified according to the ratio of these three components.

9 rain gardens often have a sandier soil composition than normal to allow faster infiltration. A ratio of approximately 50% sand, 30% topsoil and 20% compost will be suitable for most plants, although not all plants cope well in sandy soil conditions. Cross section showing the typical features of a rain gardenFreeboardTo ps oil Sub-base17In some cases it may be preferable to reconfigure the make-up of the rain garden by including a gravel filled trench as well as, or instead of, a gravel option has the potential to improve the drainage capacity of the rain garden by increasing hydraulic connectivity between the surface and the sub-soil. However, as this may lead to reduced water treatment it is recommended that you provide at least one metre between the rain garden inlets and the gravel trench is provided to act as a filter strip. InfiltrationGravel filled trench1m1mCross section of a rain garden with a gravel filled trenchBuried services can have an impact on the design of your rain garden.

10 rain gardens can still be installed where utilities such electricity, gas and water are located within the proposed construction depth; however, one of the following options will need to be employed:1/ Hand digging Hand digging around utilities can enable you to create your rain garden at the desired depth, however it is time consuming and can lead to significantly increased Stop diggingReducing the construction depth of the rain garden at locations where buried services are present is a cost effective method. However this will reduce the performance of your rain garden. 3/ Dig deeperIn some cases it may be possible to retain the overall volume of sub-base by increasing the depth where there are no services, to compensate for reduced excavation where services are present. This will minimise digging around services whilst maintaining the desired storage capacity. The available budget and the required performance will determine which of these options are best for your rain construction depthBuried servicesCross section illustrating how the construction depth can be increased locally to account for servicesDepth increased19 Newport Street, LB Lambeth The Vauxhall Walk rain gardens are part of The Missing Link scheme.