The Impacts of Stormwater Management on Hydromodification and Bedload Sediment Transport in a Gravel-bed Stream

Abstract

Global trends of increasing urbanization have led many researchers to attempt to quantify the resulting impacts on channel morphology and on the ecological health of urban channels. Overall, there has been a lack of research on the effectiveness of stormwater management facilities in reducing the potential for bedload sediment transport within the channel. This study aimed to characterize the impact of stormwater management facilities on bedload sediment transport potential within a particular urban stream. A field study was undertaken along Morningside Creek, a tributary of the Rouge River in Toronto, Ontario. A tracer study using 300 stones with radio frequency identification tags was undertaken over a two year period, tracking the positions of the stones after major storm events. A hydrologic model was then prepared for the catchment area, detailing the stormwater management features. A critical shear stress of 0.043 was determined for the threshold of particle mobilization, using a hiding factor to account for the increase in shear stress in the displacement of larger particles. The travel distances of the tagged particles were shown to follow a non-linear decreasing function with particle size. A variety of hydrometrics were measured based on the high resolution water level measurements taken within the creek. The cumulative excess shear stress was used as the basis for determining bedload sediment potential within the creek. A model of bedload sediment transport was developed by modifying the excess shear stress relationship first proposed by DeVries (2000). The hydrologic model was used to compare a variety of stormwater management scenarios to determine their effectiveness in reducing the potential for sediment transport within the creek. It was determined that the detention ponds in the Morningside Creek catchment provide a 33% reduction in bedload sediment transport potential. Analysis of the hydrologic model revealed that increases in imperviousness lead to a proportionate increase in bedload sediment transport. The hydrologic model also determined that for storms of a similar return period, longer storm durations generate larger bedload sediment transport potential.