Perspectives on Compound Flooding in Chinese Estuary Regions

Extreme surges and rainfall represent major driving factors for compound flooding in estuary regions along the Chinese coast.The combined effect of extreme surges and rainfall(that is,compound floods)might lead to greater impacts than if the drivers occurred in isolation.Hence,understanding the frequency and severity of compound flooding is important for improving flood hazard assessment and compound flood resilience in coastal cities.In this study,we examined the dependence between extreme surges and corresponding rainfall events in 26 catchments along the Chinese coastline during typhoon and non-typhoon seasons using copula functions,to identify where the two drivers more often occur together and the implication for flood management in these locations.We found that the interaction between flood drivers is statistically significant in 10catchments located around Hainan Island(south)and Shanghai,where surge peaks occur mainly during the typhoon season and around the Bohai Sea(north),where surge peaks occur mainly during the non-typhoon season.We further applied the copula-based framework to model the dependence between surge peaks and associated rainfall and estimate their joint and conditional probability in two specific locations—Hainan Island and the Bohai Sea,where the correlation between flood drivers is statistically significant.We observed that in Hainan Island where most of the surge peaks occur during the typhoon season,extreme rainfall events during the typhoon season are generally more intense compared to annual maxima rainfall.In contrast,around the Bohai Sea where surge peaks occur mainly outside the typhoon season,rainfall is less intense than annual maxima rainfall.These results show that the interaction between extreme surges and rainfall can provide valuable insight when designing coastal and urban infrastructure,especially in highly populated urban areas prone to both coastal and pluvial flooding,such as many Chinese coastal cities.

Damage Curves Derived from Hurricane Ike in the West of Galveston Bay Based on Insurance Claims and Hydrodynamic Simulations

Hurricane Ike,which struck the United States in September 2008,was the ninth most expensive hurricane in terms of damages.It caused nearly USD 30 billion in damage after making landfall on the Bolivar Peninsula,Texas.We used the Delft3dFM/SWAN hydrodynamic and spectral wave model to simulate the storm surge inundation around Galveston Bay during Hurricane Ike.Damage curves were established through the relationship between eight hydrodynamic parameters(water depth,flow velocity,unit discharge,flow momentum flux,significant wave height,wave energy flux,total water depth(flow depth plus wave height),and total(flow plus wave)force)simulated by the model and National Flood Insurance Program(NFIP)insurance damage data.The NFIP insurance database contains a large amount of building damage data,building stories,and elevation,as well as other information from the Ike event.We found that the damage curves are sensitive to the model grid resolution,building elevation,and the number of stories.We also found that the resulting damage functions are steeper than those developed for residential structures in many other locations.