Abstract: Stormwater outlets transfer surface runoff to the sewer network, playing a key role in urban flood control. To understand the stormwater discharging mechanism of the outlets, this paper summarizes the outlet discharging capacity in previous experimental studies and compares the pros and cons of different calculation methods and their application scopes. Then, we modify the source code of SWMM to create a dynamic link library (DLL), develop an urban flood model, and improve the calculations of stormwater outlet discharge. Through application in a case study of the Fengxi new town in Xi’an, different calculation methods are evaluated quantitatively in terms of the accuracy in the outlet discharge volume and the depth of depression storage over a catchment. Results show that for a typical catchment under a typical rainfall event, the total outlet discharge volume is 38% - 48% of the rainfall volume, as predicted by different methods. An 18% difference in discharge coefficient among the different formulas is amplified up to a 55% difference in the simulated depth of depression storage, and the maximum depth over the study area is in a range of 0.1-0.6 m due to insufficient capacity of the outlets.

Key words: urban flooding, surface runoff, stormwater drainage discharge, urban flood model, depth of depression storage