Inundation modelling for fluvial and pluvial flooding during typhoons: a case study in Shanghai city

The co-occurrence of storm surge, high tides and heavy precipitation increases flood probability and potential consequences compared to each hazard separately in Delta cities. The Huangpu River (HP) is a tributary of the Yangtze River in China, which drains Tai Lake to the west of Shanghai city, and meanders through downtown Shanghai. During typhoon events, storm surge reverses the river flow and pushes the water level up as far as 100km upstream. If storm surge (1-2 m above msl) co-occurs with high tide (2-3 m above msl), it poses great threats to the HP floodwall (crest level from ~3-5.5m above msl). At the same time, typhoons cause heavy precipitation (up to 70-80mm/hour) in the city, which increases urban drainage discharge in the pipeline system. In order to prevent elevated water levels in the river, stormwater drainage is ceased to effectively work in this situation. Pluvial and fluvial flooding occur simultaneously. The objective of this paper is to develop a hydrodynamic model to simulate simultaneous pluvial and fluvial flooding and to produce inundation maps due to failure of floodwalls and the urban drainage system. We apply the Delft3D FM numerical model to compound flood events in Shanghai. Results raise risk awareness for decision-makers during compound flood events and demonstrate the importance of compound flood modelling at a city scale.