Spotting strategic storm drain inlets in flat urban catchments

Abstract

Storm drain inlet blockage causes more flooding incidents than does storm sewer overloading in flat urban catchments. Although inlet clearance has proved to be cost-effective in mitigating pluvial flooding, clearing all inlets in the wet season is unfeasible. No study has sought to partition the role of storm drain inlets because the dynamic flow exchange at every inlet is impossible to monitor at a catchment scale and even hard to calculate. This study presents a proactive approach to identifying strategic (i.e., most influential) inlets to save considerable labor costs in grate inlet maintenance. Our findings show that the tapered Pareto distribution can describe the distribution of inlet interception rates almost invariant to rainfall types. A modified Pareto principle called the 70–25 rule is derived, suggesting that 70% surface runoff is drained by the top 25% inlets. A nested hydrodynamic model, which solves full shallow water equations and captures detailed physical processes, was built to compute surface runoff intercepted by grate inlets in the built environment. The model is composed of a subcatchment scale, 2D-1D dual drainage model embedded in a catchment scale 1D model. The 2D-1D interaction was described by a piecewise function of piezometric head at the grate inlet; the parameters in each subfunction were statistically derived from a series of 3D numerical experiments involving full processes from free-surface flow to pressurized flow. The 2D model was built on a sensitivity-tested, unstructured mesh with allowable triangle areas ranging from 0.15 to 6 m2. The simulations showed a mild (ca.5%) versus radical (40–50%) reduction in runoff interception with the removal of the non-strategic versus strategic inlets under non-extreme rainfalls. The application to the low-lying, ultra-urban test case confirmed the robustness and effectiveness of the method for strategic inlets identification.