Abstract
Abstract. Coastal cities are particularly vulnerable to flood under multivariable conditions, such as heavy precipitation, high sea levels, and storms. The combined effect of multiple sources and the joint probability of extremes should be considered to assess and manage flood risk better. This paper aims to study the combined effect of rainfall and the tidal level of the receiving water body on flood probability and severity in Fuzhou City, which has a complex river network. Flood severity under a range of precipitation intensities, with return periods (RPs) of 5 yr to 100 yr, and tidal levels was assessed through a hydrodynamic model verified by data observed during Typhoon Longwang in 2005. According to the percentages of the river network where flooding occurred, the threshold conditions for flood severity were estimated in two scenarios: with and without working pumps. In Fuzhou City, working pumps efficiently reduce flood risk from precipitation within a 20-yr RP. However, the pumps may not work efficiently when rainfall exceeds a 100-yr RP because of the limited conveyance capacity of the river network. Joint risk probability was estimated through the optimal copula. The joint probability of rainfall and tidal level both exceeding their threshold values is very low, and the greatest threat in Fuzhou comes from 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 in Fuzhou, flood frequency and severity may be higher than understood during design.
Highlights
Flood is the most frequently occurring natural disaster in the world that poses high physical risk and economic damages (Chang et al, 2008; Adelekan, 2010)
The areas flooded by the river that were modeled match the observations very well, the same part shown with thick overlays in yellow color and four small differences indicated by the red overlays
The current work was performed to assess the joint impact of rainfall and downstream tidal level on flood risk in a coastal city with a complex river network
Summary
Flood is the most frequently occurring natural disaster in the world that poses high physical risk and economic damages (Chang et al, 2008; Adelekan, 2010). In China, most coastal cities, such as Fuzhou, Wenzhou, and Shanghai, are located in low-lying areas with a complex river network and a receiving water body downstream, whose tidal level is affected directly or indirectly by upstream flow and tidal effect. In these cities, episodes of flood are mainly river inundations caused by large amounts of rain run-off, which the river network cannot accommodate. Flood risk is influenced by rainfall, the tidal level of the receiving body of water, and the drainage system, including the drainage and river networks and other drainage facilities. The threshold values for flood severity and joint probability, which are keys to the design of future drainage systems, should be considered to assess and manage flood risk
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