Sediment deposition characteristics in shallow saturation excess overland flow for urban Stormwater applications

Abstract

AbstractOver the last several decades, urban stormwater management has grown to include green infrastructure, such as bioswales. However, little is known about the fate of particulate contaminants after depositing in bioswales and similar systems, in part due to limited understanding of sediment transport dynamics in the shallow (water depths comparable to bed roughness height), near‐saturated flow conditions typical of such systems. To better understand the sediment deposition characteristics, a flume study was performed with natural sediment and flow characteristics similar to those encountered in bioswales. A fluorescent, paramagnetic sediment tracer was used to mimic silt sized particles carried by stormwater flows, and a photographic hood was used to image the tracers deposited on the flume surface after subsequent pulses of tracer slurry, where the image intensity was established to be a reliable proxy for surface tracer concentration. These data were analysed to calculate the sediment trapping efficiency, that is, the rate of sediment deposition onto the soil surface per unit time, and samples of the flume effluent were collected to provide an independent mass balance of the deposited tracer. Experimentally determined sediment trapping efficiencies were found to decrease with increasing bed slope angle, which indicates soils at greater bed slopes will be less capable of retaining particulate‐associated contaminants, and were found to decrease over time as the soil surface became saturated with fine particles. These trapping efficiencies were also compared with mathematical models from the literature, augmented with a formula representing the observed surface‐saturation effect. The results of this study can lead to the development of more realistic models for predicting the removal performance of current stormwater remediation designs in regard to fine sediment associated contaminants.