Abstract
Experimental data characterising dispersion within Typha latifolia were previously collected in a laboratory setting. This mixing characterisation was combined with previously proposed computational fluid dynamics modelling approaches to predict residence time distributions for vegetated stormwater treatment pond layouts (including a wetland) derived from Highways England design guidance. The results showed that the presence of vegetation resulted in residence times closer to plug flow, indicating significant improvements in stormwater treatment capability. The new modelling approach reflects changes in residence time due to mixing within the vegetation, but it also suggests that it is more important to include vegetation within the model in the correct location than it is to accurately characterise it. Estimates of hydraulic efficiency suggest that fully vegetated stormwater ponds such as wetlands should function well as a treatment device, but more typical ponds with clear water need to be designed to be between 50% and 100% larger than their nominal residence times would suggest when designed against treatment criteria.
Highlights
Stormwater ponds and wetlands are commonly used in sustainable drainage systems, built to treat rainfall run-off before discharging into watercourses (HA, 2006; Shilton, 2005; Woods Ballard et al, 2015)
This paper describes an alternative, empirical, approach to incorporating the smaller scale vegetation-driven dispersion processes within a computational fluid dynamics (CFD) model
Strong jets are formed in the regular basin pond, the pond with island and the wetland
Summary
Stormwater ponds and wetlands are commonly used in sustainable drainage systems, built to treat rainfall run-off before discharging into watercourses (HA, 2006; Shilton, 2005; Woods Ballard et al, 2015). These systems are rarely monitored postinstallation, and sufficient treatment capability on their part is often assumed on the basis of a nominal hydraulic residence time that is described by volume divided by discharge (tn = VQ−1). An alternative approach is to obtain RTDs through the use of computational fluid dynamics (CFD) modelling. CFD modelling has previously been used to evaluate RTDs in a variety of devices, including manholes, stormwater tanks and ponds (Adamsson, 2004; Stovin et al, 2013; Tsavdaris et al, 2013)
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