Urban Green DaMS (Design and Modelling of SuDS)

If we don't manage rainfall appropriately, it can lead to flooding. Traditionally, urban areas have been drained using underground sewer systems. These can be expensive and disruptive to build and maintain. Storm runoff collects contaminants as it flows over urban surfaces and through sewer pipes, and is a significant cause of river pollution. In many cities, combined sewers discharge raw sewage into natural water bodies during storm events. Without intervention, growing populations and the effects of climate change will increase the frequency and severity of urban flooding and pollution events.
As an alternative to building more/larger sewers, we are starting to implement SuDS (Sustainable Drainage Systems). SuDS is an overarching term for a 'toolbox' of techniques that aim to deal with the quantity of rainfall, but also to have a positive impact on water quality, amenity and biodiversity. Retrofitting SuDS into urban areas can help to improve stormwater management within our existing urban areas. Vegetated bioretention cells (often referred to as rain gardens) are one of the simplest, practical and most reproducible SuDS options. They can be fitted adjacent to urban streets, dealing directly with road runoff.
Bioretention cells are emerging as a preferred option in the USA and Australia. However, we do not yet have the same understanding of their performance as for traditional measures such as pipes. This is because they have 'living' elements (i.e. plants & soil) whose functionality varies from place to place and over time. The soil has a critical role to play in supporting plant life and managing runoff. Bioretention cells typically use engineered soils or 'substrates' that need to meet specific physical requirements. To reduce the requirements for imported materials, we need to be confident of their performance with locally-sourced substrate components, thereby reducing cost and improving overall sustainability.
Water usage by plants helps to reduce runoff. We will observe plant water usage (evapotranspiration rates) in six full-scale bioretention cells functioning under semi-controlled conditions as part of the Newcastle University's new National Green Infrastructure Facility (funded by UKCRIC: EP/R010102/1). Controlled tests using smaller columns at the University of Sheffield's climate controlled laboratories will allow us to explore more substrate options. We will measure plant respiration in installed SuDS systems to generate a database of evapotranspiration rates for different urban plant types.
Bioretention cells slow down excess flow before it is passed to the sewer. We will carry out a detailed investigation of how the substrate and drainage outlet arrangements affect runoff detention. Information relating to maintenance needs is particularly sparse, with clogging of substrates especially poorly understood. We will use magnetic, fluorescent, tracer particles to explore the vulnerability of substrates to clogging by the dirt and fine particles present in road runoff.
Drainage engineers use hydraulic models to represent catchment runoff and sewer system flows. The new data will allow us to develop a numerical model of bioretention cell rainfall-runoff processes. Our project partners include the developers of the most widely-used drainage network modelling tools. We will work with them to include bioretention cells in their software. We will also update the cutting-edge urban flood risk model CityCAT to incorporate bioretention cells.
Soil and vegetation conditions change over time in response to seasonal weather patterns, and vegetation lifecycles. Furthermore, the hydrological response is sensitive to rainfall duration and intensity, as well as antecedent soil moisture conditions. Conventional approaches to sizing drainage components tend to ignore all these sources of variability. We will develop new SuDS design guidance that uses probabilistic performance functions to address this.

The Urban Green DaMS project will provide direct and immediate benefits to practitioners and hydraulic modellers, and longer term benefits to policy makers and society.

SuDS and Green Infrastructure practitioners: Bioretention cells represent the most practical and reproducible vegetated SuDS option for dealing with urban road runoff, with the potential for widespread application in existing urban cores. Roads represent a significant proportion of the urban impermeable surface, contributing significantly to sewer overloading and urban flood risk. The Urban Green DaMS project will produce a step-change in UK urban stormwater management practices by providing robust scientific evidence to underpin the widespread usage of bioretention cells. In particular, we will focus on understanding how to design these systems to perform efficiently and how to represent them within drainage modelling tools. This will support the UK Water Research and Innovation Partnership goal to grow the UK's global market share in water management from 3% to 10%. The retrofitting of bioretention cells involves a broad stakeholder group, including householders, landscape architects, local authorities and the water utilities. SuDS design and implementation guidance and training in the UK is primarily provided by CIRIA, whose 'SuDS Manual' is written in such a way that it is accessible to all relevant stakeholder groups. CIRIA will be our primary dissemination partner, ensuring that the project's findings are disseminated via the SuDS Manual and relevant CIRIA training.

Hydraulic modelling software developers: The rainfall-runoff model will initially benefit the developers of drainage design and analysis tools, providing specific knowledge and understanding to enable software enhancement. Our project partner Innovyze oversees the development of three products that are widely used by UK and international drainage practitioners: MicroDrainage; InfoWorks and SWMM. The project team will work directly with Innovyze to ensure that key research findings are incorporated within the next generation of urban drainage modelling tools. In the longer term, the implementation of robust model representations within these practitioner tools will enable local authorities and water utilities to confidently evaluate and adopt this highly-transferable approach into policies and practice for catchment-scale stormwater management.

National government and policy makers: To date the UK Government has provided rather limited support or incentivisation for the water utilities to transition from pipe-based to SuDS-based stormwater management. We will utilise national-level professional forums (ICE, CIWEM UDG) to ensure that our studies contribute to evidence required to underpin national policy-level transitions. We also benefit from CoI Dawson's role as a member of the National Infrastructure Commission Technical Expert Group.

Socio-economic benefits to the general public : Urban flood risk is a major societal and engineering challenge. Flood risk will increase as a consequence of the combined effects of climate change (more intense summer rainfalls) and anticipated growth in the urban population. Existing (piped) sewerage/drainage infrastructure frequently has insufficient capacity. Sustainable Drainage Systems (SuDS) are essential to supplement and complement traditional drainage approaches in a cost effective way. Vegetated SuDS options - such as green roofs and bioretention cells - also make a positive contribution to urban landscapes as part of Green Infrastructure. In addition to reduced flood risk, bioretention cells will support improved water quality, reduced urban heat island effects and biodiversity and amenity benefits. There is increasingly strong evidence for the economic benefits of green infrastructure relative to traditional grey infrastructure (e.g. the New York Green Infrastructure plan calculates as $1.5bn saving).