Abstract:The urban hydrological model is introduced into the urban flash flood forecasts and warning operation. The data of land use and land cover types and gridded urban drainage network capacity are parameterized in the model. Forced by the radar rainfall estimates, the model simulates the urban surface hydrological response and hydraulic processes. The hydraulic model is based on the shallow water equation, and the alternating-direction-implicit is used to solve the differential equation by two steps in the x direction and y direction, respectively. This solution comprises the back water effects in simulation and indirectly emulates the multiple flow direction methods, recalling the surface water dispersion in turbulence or diffusional effects. The case study demonstrates the online and offline running of the urban hydrological model for the purpose of flash flood warning, partially for the model validation. The online hydrological model takes the case study on the 21 July 2012 thunderstorm in Beijing, in which the radar quantitative-rainfall estimates are forced on the model for reproducing the gridded inundation mappings. The model simulation results resemble the flash flood scenarios of the waterloggings and water inundations on the 21 July 2012 thunderstorm day. The offline model simulation addresses measuring the rainfall intensity threshold for the ranked risks of the storm producing flash floods, especially the rainfall intensity thresholds for floods-susceptible places (FSPs). Therefore, the hydrological model simulation deduces the 1 h, 3 h and 6 h cumulative rainfall thresholds inducing water inundation depths over 0.2 m, 0.5 m, 0.8 m and 1.2 m in more than 49 FSPs, which are the rainfall intensity thresholds of the flash flood warning in the blue, yellow, orange and red signals, respectively.