Joint impact of rainfall and tidal level on flood risk in a coastal city with a complex river network: a case study of Fuzhou City, China

摘要

Coastal cities are particularly vulnerable to flood under multivariableconditions, such as heavy precipitation, high sea levels, and storms. Thecombined effect of multiple sources and the joint probability of extremesshould be considered to assess and manage flood risk better. This paper aimsto study the combined effect of rainfall and the tidal level of thereceiving water body on flood probability and severity in Fuzhou City, whichhas a complex river network. Flood severity under a range of precipitationintensities, with return periods (RPs) of 5 yr to 100 yr, and tidallevels was assessed through a hydrodynamic model verified by data observedduring Typhoon Longwang in 2005. According to the percentages of the rivernetwork where flooding occurred, the threshold conditions for flood severitywere estimated in two scenarios: with and without working pumps. In FuzhouCity, working pumps efficiently reduce flood risk from precipitation withina 20-yr RP. However, the pumps may not work efficiently when rainfallexceeds a 100-yr RP because of the limited conveyance capacity of theriver network. Joint risk probability was estimated through the optimalcopula. The joint probability of rainfall and tidal level both exceedingtheir threshold values is very low, and the greatest threat in Fuzhou comesfrom heavy rainfall. However, the tidal level poses an extra risk of flood.Given that this extra risk is ignored in the design of flood defense inFuzhou, flood frequency and severity may be higher than understood duringdesign.