AbstractAquatic ecosystems are threatened by a variety of contaminants contained in road runoff. Pollutant mitigation is therefore an important function of sustainable drainage systems such as vegetated ponds. However, design is predominantly based on “black box” approaches. In addition, most studies do not evaluate alternative design layouts for a given location and hydrological regime. This research project evaluates a small vegetated pond, by means of experimental and numerical methods. The vegetated wet detention pond (two flow balancing basins separated by a berm) had 1.6m maximum depth, 304m³ storage capacity, a hydro-brake controlling the outflow, and was equipped with sediment traps and stage loggers (at the inlet/outlet) for monitoring purposes. It received road runoff after a bio-retention area and a swale (L=80m) adjacent to an urban road. Experimental methods included the collection of water/sediment from strategic parts of the system and subsequent analysis. Water quality investigation included BOD5, COD, TSS, VSS, pH, heavy metals and other elements. Sediment quality analysis included particle size distribution, accumulation rates, volatile substance content and heavy metal/elements concentrations in different size fractions. Numerical methods included the evaluation of the current design under extreme flow conditions, in terms of flow distributions, followed by investigating alternative geometries for the same footprint in the interest of promoting sustainable flow regimes and sedimentation potential.
The storm events exhibited first flush patterns in the inflow, but linear associations between many pollutants in the inlet and the outlet, suggested short-circuiting was affecting effluent quality during storm events. The pond system showed variability and complexity in the behaviour of pollutants with influences due to spatial, seasonal, and site-specific effects. The pollution levels in the pond (water, sediments) were low compared to standards and the pond seemed to have relieved the River Wallington (receiving water-body) from a great amount of pollution. However, increased salinity and low biodegradability of material found in the pond could be of concern in the long-term. The system promoted sedimentation although there was indication of post-depositional re-suspension especially under high flows. Vegetation probably encouraged buffering while increasing the levels of biogenic debris in the pond water. The main factors contributing to the water and sediment quality fluctuations were identified during this study. These factors were of a broad-spectrum of variables related to environmental stressors and design properties.
The modelling of the vegetation was very intricate although the CFD code modelled relatively accurately the flow distributions within the vegetated domain. The simulations suggest that the excessive vegetation diverged the flow from the basins to the un-vegetated banks and therefore, increased the re-suspension potential of settled material. The response of this particular vegetated pond in terms of sedimentation potential and velocity distributions (during the design flow) was highly influenced by the geometry and the vegetation cover of the pond system. In addition, simulations indicated that the most appropriate design layout for the given flow regime was an elliptical pond with a submerged/emergent island placed at a central location. The response of this pond layout, in respect of velocity distributions, could be up to 30% more efficient than the existing configuration.
The research findings of this study contributed to greater knowledge of the dynamic nature of treatment mechanisms in vegetated SuDS and offered innovative information on how to simulate (vegetated) SuDS ponds using CFD codes.
|Date of Award||Sep 2015|
|Supervisor||John Williams (Supervisor) & Catherine Mant (Supervisor)|