Role of Hydrodynamic Forces in Shaping the Dynamics of Channel Evolution in Urban Watershed

Abstract

This study explores the impact of urbanization on stream channel erosion. The effect of changes in flow regimes and the associated changes in hydrodynamic forces were studied using monitored data for nine outfalls in Fairfax County, Virginia. The Stormwater Management Model (SWMM) simulated the flow and velocity hydrographs at each outfall site, using continuous rainfall records for the period 2004–2014. The cumulative stream flow work was used to study the impact of change in hydrology and hydraulics on the dynamics of channel evolution. The cumulative stream flow work at each section was determined by integrating the stream power (discharge time velocity) with time. The results show heterogeneity in the responses of the downstream channel reaches. However, the aggregation of results shows that the cumulative stream flow work accounts for about 70% of the variance in the observed changes in stream cross sectional areas. The cumulative stream flow work was suggested as a method for sizing stormwater detention facilities to meet the channel protection criteria for adequacy of outfalls. The advantage of the method is that it accounts for both the generated runoff at point of discharge and the connectivity of the site to the downstream watercourse. The study suggests that the channel protection criteria for adequacy of outfall should be based on limiting the post-developed cumulative stream flow work to its pre-developed conditions.