Numerical simulations of compound flooding caused by storm surge and heavy rain with the presence of urban drainage system, coastal dam and tide gates: A case study of Xiangshan, China

Abstract: Nowadays, most coastal cities have complex drainage systems (i.e., rivers and pipelines) connected to the sea, affecting the flooding during the co-occurrence of rain and storm surges. In this study, this compound flooding is studied by developing a coupled model based on a one-dimensional (1D) hydrological model SWMM and a two-dimensional (2D) hydrodynamic model ADCIRC. The coupling is achieved by setting the 1D model as a subroutine of the 2D model while the parallel computing scheme of the 2D model is kept using the Message Passing Interface (MPI). The coupled model is applied in Xiangshan, Zhejiang Province, along the eastern coast of China. The model accurately ensures water conservation and reproduces the observed tidal elevation, storm surge, and river water stages during Typhoons Haikui (2012) and Fitow (2013). A series of simulations are carried out to quantify the influences of a coastal dam, drainage system, control of tide gates, rain, and storm surge on the compound flooding. The results show that the drainage system enhances the connectivity between the land and the sea no matter whether the coastal dam exists, leading to faster flooding and draining. Furthermore, when a storm surge occurs, the high surge level can significantly reduce the drainage efficiency of the inland flooding during rain and thus causes more severe flooding hazards. The simulation results also suggest that urban pipelines play an important role in preventing urban flooding caused by rain. And for the prevention of storm surge inundation, an effective gate control strategy is suggested that the gates should be closed when the water levels on both sides of the dam are equal for the first time before the peak of the storm surge and are open when the water levels are equal again. Finally, the flooded area can be divided into the tidal zone, the hydrologic zone, and the transition zone to identify different flood mitigation strategies.