Residence Times in Vegetated Stormwater Ponds

Storm water runoff typically contains and transports a wide range of pollutants, resulting in negative environmental effects with potential threats to ecosystems and health. Hundreds of runoff treatment ponds intended to moderate these impacts are likely to be delivering sub-optimal (and perhaps actually below legally required) levels of improvement in water quality due to poor understanding of flow patterns and the effects of vegetation. This proposal will generate a unique dataset to describe the influence of different types and configurations of vegetation on the pond's fundamental flow - and treatment - characteristics. We will also deliver a validated set of vegetative resistance and mixing parameters that are essential if 3D numerical modelling tools are to be used with confidence. These tools will ensure that future pond designs meet all their water quality and ecosystem services objectives for current legislation and the increasingly stringent EU regulatory framework anticipated over the next decade.

Stormwater ponds take run-off from urban areas, highways and agricultural land, providing detention and attenuation of peak storm discharges and improving water quality. Stormwater ponds are able to provide protection to downstream drainage components and receiving waters by holding or treating run-off at or near the source and provide additional nature conservation and amenity benefits. Within the Highways Agency Asset Inventory System alone there are currently over 800 stormwater ponds. Pond performance (pollutant treatment efficiency) is directly related to hydraulic residence time, a function of the internal flow field, which in turn is controlled by the pond geometry and the distribution and type of vegetation present.

The prediction of water quality improvements within drainage features is gaining importance with stormwater professionals. However, performance prediction is complex since water quality processes are functions of the pond hydraulic residence time. Current evaluations employ the nominal retention time which assumes plug flow through the pond, as the design consideration. It is accepted that the nominal retention time (pond volume/discharge) provides a poor estimate of the actual mean (or median) residence time, with overestimates of treatment times of 100% or more not being uncommon. However, it is still in use, even 'the norm'.

In wastewater treatment wetlands, treatment is good since a high degree of engineering is adopted in creating an efficient, often linear, shape with uniform, dense, vegetation. In contrast, stormwater ponds must fit into existing water courses or urban environments. Together with the additional requirements for biodiversity and ecological function, this leads to pond layouts that may be less than ideal from a hydraulic perspective.

Vegetation can have either a positive or negative role in water quality treatment within stormwater ponds. It provides the appropriate environment for the support of biofilms and the colonisation by algae, enhancing treatment, yet variable spatial distribution influences the spread of the hydraulic residence time. This proposal seeks to better understand and quantify the physical, vegetation-driven, flow mechanisms occurring within a stormwater pond and to develop a robust physically based modelling tool. The research proposed here will deliver improved understanding of the effects of vegetation (type: emergent, floating and submerged; physical characteristics: porosity and spatial distribution) on flow patterns and residence time distributions within stormwater ponds. The validated numerical modelling approach will permit the assessment of short circuiting, a measure of poor performance, and provide estimates for vegetation contact times, sediment deposition regions and rates. This will provide a tool for predicting the treatment efficiency of vegetated stormwater ponds.

The EU Water Framework Directive promotes sustainable water use based on the long-term protection of available water resources. It lays down a requirement that, in developing River Basin Management Plans, pressures on water quality from point and diffuse sources are accounted for and that appropriate measures, including the management of water quality to meet ecological objectives, are implemented. Ponds represent a potentially sustainable water quality treatment measure, but pond performance is strongly influenced by residence time characteristics, which are controlled by - amongst other factors - the presence and type of vegetation. This proposal combines detailed, controlled, laboratory vegetation parameterisation studies with the development of CFD modelling methodologies and full-scale field validation to provide a framework for quantifying pond residence time distributions. This research is fundamental in nature, and as such will provide a foundation for a wide range of applications based on vegetated treatment systems. In the short term, as emphasised in our Project Partners' Statements of Support, this research is expected to have an immediate and significant impact in two specific contexts: (i) Sustainable Drainage System (SuDS) design and (ii) Source control of pesticides in agricultural runoff.

(i) SuDS Design - Urban drainage planners and managers (e.g. Highways Agency, Water Utilities and Local Authorities) are responsible for the design, maintenance and management of drainage systems. SuDS approaches now represent the preferred mechanism for dealing with storm runoff from roads and urban developments, and this research will support the ongoing development of Defra's National Standards for Sustainable Drainage. Suitably designed and maintained vegetated ponds can provide a cost-effective runoff treatment option, whilst also delivering a range of other ecosystem services and amenity benefits. Through the provision of design and management guidance and modelling methodologies, this research will enable ponds to be used with confidence to comply with current legislative requirements. As noted in their statement of support, the HA often has to find solutions to runoff quality problems within restricted parcels of land. The provision of modelling methodologies will provide a flexible framework for the development of innovative design options. Improved guidance should significantly reduce the risk of inadequate designs that fail to meet water quality objectives and which may consequently incur financial penalties and may lead to undesirable environmental impacts and societal health risks.

(ii) Source control of pesticides in agricultural runoff - The outputs of this project will benefit the CRD (Chemicals Regulation Directorate), the agrochemical industry, farmers and water supply companies by providing evidence for the potential to control problematic land to water transfers of pesticides, thereby supporting their continued use. This may, in turn, allow agronomically important pesticides to remain on the market. Direct economic benefit will be derived by farmers faced with high reductions in yield should use of these chemicals be restricted and/or faced with mandatory changes in crop rotation or cultivation practices (e.g. in proposed Water Protection Zones). The project will provide significant benefits to water companies by reducing treatment costs and the risk of compliance failures. The project will deliver clear social benefits since public opinion dictates that it is better to prevent pesticides contaminating the watercourses than to rely solely of methods of removal from drinking water at a later stage. Containment and remediation of pesticides applied to fields also delivers a clear environmental benefit as it reduces pesticide exposure for in-stream wildlife, however low the ecotoxicological risks may be.