In this study, we describe a novel framework of modeling the city inundation arising from typhoon-induced surge and torrential rainfall by coupling a well-known ocean model (FVCOM) and a river analysis system (HEC-RAS 6.0). The FVCOM model is applied to capture the behavior of surge from deep sea to shallow offshore near PRD. Under the hydrograph of surge and tide supplied by FVCOM, the HEC-RAS model is applied to simulate the potential flooding scenarios caused by the comprehensive effect of surge overtopping and torrential rain during Super Typhoon Mangkhu in 2018. Then, we analyze the interaction between pluvial flooding and fluvial flooding in urban region during the typhoon, and the effect of the spatial heterogeneity of the roughness arising from different land uses in the city. The main conclusions can provide useful guidelines and insights on dealing with the possible storm surge and inundation in future in the cities of PRD.
During events of typhoon, storm surges always pose threat to human lives in low-lying areas near the coast. Apart from storm surge, torrential rainfall usually caused a large area of inundated regions as well as overtopping flood. Overall, typhoon-induced flooding cause massive direct economic losses every year through damaging the houses and infrastructure, and also induce indirect losses through the harmful impact on transportation, tourism and industrialization. With the rapid urbanization in coastal zones in recent decades, the economic losses against typhoon-induced flooding are also increased significantly. Moreover, climate change shows a tendency that future tropical storms will be more intense (Emanuel, 2005) and coastal inundation will likely worsen as a result of rapid sea level rise in flood-prone regions (IPCC, 2016). So, a reasonable plan for coastal flood prevention and mitigation is vital for human sustainable development.
It is well known that typhoon-induced flooding is driven by two sources, (1) storm surge and (2) rainfall-runoff (Torres, et al., 2015). Storm surge, together with astronomical tide and waves, form high water levels which may overtop or breach coastal levee to inundate the low-lying areas and cause catastrophic consequences. Compared to astronomical tide which can be regarded as a deterministic component in the total water level, the storm surges are not so simple to determine due to the complexity of their moving tracks and time-varying characteristics such as centric pressure, cyclone sizes, etc, which has been proved to have significant effect on the storm surge (Irish et al., 2008; Zhang et al., 2021a). Moreover, the combined effect of storm surges and tide leads to more uncertainty while they interact with each other nonlinearly under the shallow topography (Park and Suh, 2012). In coastal and estuarine regions, the storm surge would be transformed under the restriction of coastline and estuary boundary (Ebersole et al., 2010). On the other hand, typhoon can introduce torrential quantities of rainfall as it move inland (Torres, et al., 2015). Differing from most of the normal rainfalls, typhoon-induced rainfall is not uniformly distributed and is featured by short-term intense downpours, causing localized inland flooding. Moreover, rainfall-runoff and storm surge are not mutually exclusive flood hazards. The joint hydraulic processes between the two factors during a typhoon event are of particular importance and interest in the field of coastal flooding prediction and hazard assessment.
