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.
A need for further knowledge
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 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.