Climate change is creating more extreme weather, and the frequency and severity of both flooding events and droughts is

increasing. Sustainable water resource management is essential to mitigate both effects while improving water quality. This guide provides an overview to managing urban water as a resource

Ferranti, Emma

James, Sue

Sharifi, S

Stevens, A.J.

Stevens, P.J.

University of Birmingham Research Archive, E-papers Repository

Rainwater harvested and used, overflow to rain gardenLarge canopy and mature trees intercept rainfall and provide evapotranspirationDrives, pavements, cycleways and  car parks use permeable paving where appropriate Rainwater stored in underground tank used for irrigationUrban wetlands filter runoff pollutants and slowly drain to watercoursePark designed around connecting people to waterSwales and infiltration basins attenuate water during heavy rainfall Green walls utilising rainwater only, helping cool the buildingIntensive and extensive green roofs capture the majority of rainfallRoad surface runoff directed to tree-lined SuDS Managing Water  as a ResourceFirst  Steps in Urban Water SCHOOL OFENGINEERINGFirst Steps in Urban Water2Rainwater conveyed to sewer as fast as possibleImpermeable surfaces and little vegetation increase surface run offIn drought mains water needed for irrigationSediment and agricultural runoffCombined sewer system overwhelmed by heavy rainfall, can lead to sewer floodingOverflow events following heavy rainfall contaminate river with untreated wastewaterWastewater Treatment WorksIntensively managed uplands result in sparse vegetation and compacted soils, increasing surface water flowCanalised channels and lack of riparian planting increase peak river flow, increasing downstream flood riskClimate change is creating more extreme weather1, and the frequency and severity of both flooding events and droughts is increasing2. Sustainable water resource management is essential to mitigate both effects while improving water quality.Managing water as a problemHistorical land and water practices have modified our river catchments. Rural land uses (eg intensive farming) can increase the volume of rainfall runoff and flow of sediment into rivers; channel straightening moves the water more quickly downstream; and impermeable urban surfaces create runoff and flood easily (Fig. 1). Moreover, traditional combined sewers3 treat surface runoff as wastewater and were not designed for the current or future extreme rainfall events, or urban population densities.  When heavy rainfall events inundate combined sewers, storm overflow pipes permit untreated sewage and wastewater to enter rivers and seas. Between 2019 and 2022, 10 million hours of storm overflows4 polluted rivers and bathing waters. Our water resources face challenges from both climate change1 and population growth5. In the UK, we currently use ~14 billion litres of water per day and will need 4 billion more by 20506, with future water shortages likely7. Managing water as a resource We must slow the flow and improve water quality upstream by altering land management8; reduce urban runoff by intercepting rainfall and increasing infiltration; harvest rainwater and recycle greywater9(Fig. 2). This will reduce flood and storm overflow risk, provide irrigation for hot summers10, and preserve water resources for drinking. Green infrastructure (GI) is fundamental for managing water. It reduces costs compared to traditional drainage11 and provides a multitude of other benefits12. Globally, strategies for managing urban water include site-specific designs – low impact development (LID) and sustainable drainage systems (SuDS); broader frameworks for city or regional management – integrated urban water management and water sensitive urban design (WSUD); nature-centric approaches – nature-based solutions (NbS); expansive city-wide concepts – sponge cities13. In the UK, the 2010 Flood and Water Management Act (FWMA), National Planning Policy Framework (NPPF) and local policies encourage or mandate SuDS. Schedule 3 of the FWMA, proposed for implementation in 2024, makes SuDS mandatory in all new development, with drainage systems needing approval from the SuDS approval body (SAB) before construction work can begin14. River level high or at capacity before reaching urban areaFig. 1   Water as a problem: urban water management without positive interventionsReferences1 Met Office.Link2 Hanlon, H.M. et al. 2021. Climatic Change 166, 50.Link3 Terms highlighted inblue italics are defined  in the Glossary overleaf.4 Environment Agency.Link5 ONS 2022. National population projections.Link6 DEFRA 2023. Link7 Environment Agency 2023.Link8 Environment Agency 2012.Link9 Environment Agency 2011.Link10 Ferranti, E.J.S. et al.  2021.Link11 Susdrain.Link12 TDAG 2010.Link13 For more information on the terminology see Fletcher T.D. et al 2014, Urban Water Journal 12(7) pp. 525-54.Link14 DEFRA 2023.Link15 Environment Agency 2021.Link16 Defra 2015. Link17 Ignition project 2020.Link18 Zhang, Q. et al. 2022. Catena 213. 106223.Link19 Pflug, S. et al. 2021. Ecohydrology 14.Link20 Ciria, 2012. Link21 Natural England 2023.Link22 HM Government Green Finance Strategy 2023. LinkFirst Steps in Urban Water 3Green Infrastructure (GI) for water resource managementBy storing or slowing surface water, the use of SuDS and other GI reduces flood risk15 and improves water quality (Table 1). To design for more extreme rainfall events, it is important to manage exceedance flows (Box 1). SuDS should be designed in accordance with DEFRA’s technical standards16. Table 2 overleaf outlines the key considerations for the planning, delivery, and management of such interventions.Integrated planningIt is important to make SuDS and other GI interventions integral to the overall development design. This enables different design elements to complement each other and can make the individual interventions more cost-effective. For instance, new cycleways and car parks can be integrated into SuDS by using permeable paving  and bioswale tree pits. SuDS can be retrofitted into existing places20, providing an opportunity to increase biodiversity  and improve the urban fabric. Financing improved water managementAs part of their Plan for Water6 the UK government will create a Water Restoration Fund, using money from water company fines and penalties to support habitat improvements and catchment projects. Natural England’s Nutrient Mitigation Scheme21 is being trialled in the Tees catchment whereby developers fund schemes that remove pollution to offset the impact of their development. The Green Finance Strategy22 sets out how private finance will be mobilised to support public sector investment, which can help meet the scale of investment needed to manage water sustainably. On a household scale, capturing rain and surface water on property may reduce sewerage bills23 and some water companies offer free water butts24, 25.Rainwater as a resource, including irrigating trees to maintain evapotranspiration for urban coolingTrees increase efficiency of SuDS featuresUrban wetland stores water close to where it fallsTreated water used  for irrigationWastewater Treatment WorksUrban vegetation including street trees, green roofs and green walls, intercepts and utilises rainfallAfforestation and sustainable land management in uplands stabilises soils and improves infiltrationMeandering river, connected to its floodplain and supported by riparian planting, slows the flow into urban areas and improves biodiversityTable 1  Effectiveness of GI interventions17Green walls Retain up to 75% of rainfall.Remove 33-99% of total suspended solids and 30-83% of nitrates.Street trees An average tree can intercept 3.2m3 of rainfall per year*, with SuDS-enabled trees reducing peak flow by 81%.Greenspace 30% of annual rainfall is infiltrated  or retained, with 42-100% reduction in suspended sediments.Green roofs Can reduce peak flow by up to 88% and delay it by up to five hours.Filter strips and swalesReduce runoff by an average of 70% and total suspended soils by 79%.* There are significant variations between broadleaf and evergreen trees, species, age and maturity.18, 19Fig. 2   Water as a resource: urban water management with positive interventions Box 1  Planning for exceedance26Even with a well-designed system, capacity may be exceeded during extreme or prolonged rainfall events.Planning ahead (or retrofitting) to manage exceedance flows is essential to minimise the damage of surface water flow. Exceedance flows can be directed into localised SuDS features, overflowing to urban wetlands and then into natural watercourses.Beaver dams help slow the flowReferences23 Ofwat.Link24 Anglian Water.Link25 Southern Water 2023.Link26 For more information on managing exceedance see Ciria 2006 (C635F).Linkand Ciria 2014 (C738).Link27 For more information on valuation see Jaluzot, A. and Ferranti, E.J.S. 2019.Link28 Armson, D. et al. 2013. Urban Forestry & Urban Greening 12, pp. 282–286.Link29 ARUP 2015.Link 30 See Appendix C,  Sewage Sector Guidance.Link31 Forest Research.Link32 Susdrain.Link33 TDAG 2014.Link34 TDAG 2018.Link35 Interpave.Link36 BGS SuDS Infiltration Map.Link37 Interlay.Link38 Environment Agency 2023.Link39 Living roofs.LinkOther resourcesCiria 2015. The SuDS Manual (C753F).Link Ciria 2017. Guidance on the construction of SuDS (C768F).Link ICE 2023. ICE Manual of Blue-Green Infrastructure. LinkFirst Steps in Urban Water4Table 2  Ensuring water management interventions are appropriate and effectiveIntervention Planning considerations Delivery Management Case studiesAll SuDS options31, 32–  Suitability of area: Topography, geology, groundwater flows.–  Discharge locations, bypass system for use while cleaning, disposal areas of organic arisings. –  How they fit into wider design – amenity and biodiversity.–  Maintenance plan.–  Use skilled and experienced contractors, following government guidelines16.–  Clearing of inlets and outlets.–  Vegetation management. –  Littler picking.–  Checks and maintenance  of components.LinkTree planting pits33, 34–  Type of system, eg structural soils, rafted, crated.–  Underground services.–  Tree species: right tree right place. Order well ahead of time.–  Irrigation system design.–  Use of mulch to reduce evaporation.–  Adequate quantity and quality of growing media.–  Timing of planting – autumn/winter best for rainfall.–  Ensure contractors have adequate environmental awareness (ISO 9001 certification).–  Irrigation of trees.–  Pruning regime.–  Tree safety inspections.LinkPermeable paving35, 36–  System type: Total, partial  or no infiltration.–  Steepness of land.–  Underground services.–  Contaminated sites, designed not to drain water into water table.–  Porosity of underlying soil.–  Use of type 3 sub-base.–  Use of skilled and experience contractors37.–  Suction sweeping.–  Manual weed removal/weed burner (avoid weed killers as they may enter groundwater).–  Permeable asphalt requires pressure wash or flushing  every quarter.LinkLinkSwales, infiltration basins and rain gardens32, 36–  Soil permeability.–  Connection to and from other features.–  Catchment area – size needed.–  Steepness of slopes.–  Integrate into surrounding landscape.–  Use of appropriate planting – consider biodiversity, and visibility constraints if near highway.–  Grass cutting (less often  if managed as meadow).–  Litter picking.–  Inspection of components.–  Remove silts38 – wildlife piles  or compost.LinkLinkRainwater harvesting –  Sizing of tank using rainwater figures against usage.–  Connect overflow to other systems.–  Mesh/filters to stop insects  and debris.–  Empty before heavy rainfall.–  Clean mesh/filters.LinkGreen roofs  and walls39–  Intensive vs extensive.–  Overflow routes.–  Irrigation system design.–  Appropriate planting for substrate and biodiversity.–  Skilled and experienced contractors.–  Irrigation of plants and general plant maintenance.–  Checks and maintenance of components.LinkLinkLinkDesigning for exceedance23–  Identify the flood pathways, incorporate them into emergency management plans. –  Public awareness of pathways.–  Can alter pathways with groundworks.–  Ensure subsequent works do not block designed pathways.–  Monitoring waterflow during storm events against assumed water pathways.–  Keep pathways clear.LinkGuidance document produced by TDAG and the Birmingham Institute of Forest Research and the School of Engineering  of the University of Birmingham in 2023. Ferranti acknowledges EPSRC Fellowship EP/R007365/1. We are grateful for  the Steering Committee who guided content – in particular Susannah Gill (Mersey Forest) and Sue Illman (Illman Young).This document should be cited as: Stevens, P.J., Stevens, A.J., Ferranti, E.J.S., Sharifi, S. and James, S. First Steps  in Urban Water. 2023. A Trees and Design Action Group (TDAG) Guidance Document. UK: London. Available from:  https://doi.org/10.25500/epapers.bham.00004284 Trees and Design Action Group www.tdag.org.uk© Trees and Design Action Group TrustGlossaryCombined sewers.A sewer system that takes foul water as well  as surface water.Evapotranspiration. Combines evaporation and transpiration. Evaporation is the process by which water from soil and plant surfaces changes from a liquid to a vapour. Transpiration is the process by which trees absorb water through their roots and transfer it up to the leaves where it evaporates into the environment through  leaf pores.Exceedance.When waterflows exceed the system capacity.Green Infrastructure.A strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services.Integrated urban water management (IUWM). Integrated management of all parts of the water cycle within a catchment, including water supply, groundwater, wastewater and stormwater.Low impact development (LID).A “design with nature approach” to hydrology. Term used widely in USA.Nature-based solutions.Actions to protect, conserve, restore, sustainably use and manage natural or modified ecosystems.Sponge city.A holistic strategy to tackle urban pluvial flooding while improving ecosystems and the environment. Term used widely in China.Sustainable drainage systems (SuDS).SuDS are designed to maximise the opportunities and benefits we can secure from surface water management. Water sensitive urban design (WSUD).Integrating water cycle management with the built environment through planning and urban design. Key elements1. Treat water as a resourceValue water: minimise use of potable water by appropriate use of rainwater and filtered and treated greywater.2. Small changes have a large effectThe cumulative effects of multiple small-scale interventions can be substantial.3. Integrated water managementLink water management with urban heat reduction, flood risk management and greenspace provision, using valuation tools27 to maximise societal benefit. Include all key players early – design teams, utilities, planners, highway engineers, local communities, and other stakeholders.4. Use trees wherever possibleTrees improve the performance of GI28  and deliver a wealth of social, environmental and economics benefits12.5. Use of mapping and modellingMake use of geographic information systems and hydraulic modelling software to help assess the most suitable interventions. 6. Design with maintenance in mindHave a maintenance plan in place before supervised construction starts. Include regular inspections, components and how they work, identified disposal sites, action plan for accidental spills, and advice on  how to undertake excavations (eg utilities). Co-design with relevant maintenance regimes where appropriate (eg soft landscape management).7. Plan for SuDS adoption29Water companies have the capacity to adopt SuDS that are predominantly used for drainage from buildings or their paved areas, provided they are designed according to their guidance30. 

First Steps in Urban Water

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First Steps in Urban Water

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