Modeling Coastal Flooding due to Compound Ocean Disasters

Intensified activities of tropical cyclones in the Western North Pacific have imposed increasing threats to coastal cities in the context of global climate change. A storm surge superimposed with astronomical high tide, i.e., a storm-tide event, often causes severe flooding in many coastal cities in South China Sea (SCS) region. Meanwhile, the potential tsunami hazard associated with the megathrust in Manila subduction zone has become a serious concern in this region. These two kinds of coastal hazard have been studied independently in the past since they are caused by different physical mechanisms in nature. However, there is no scientific reason to rule out the possibility of concurrence of a storm-tide-tsunami event, which is admittedly rare. This study simulates a group of synthetic events assuming that tsunami waves are generated by a Mw 9 earthquake in the Manila subduction zone during a typhoon, which has the same characteristics as the 2017 Typhoon Hato. A numerical model package originally developed for storm-tide calculation has been modified to simulate a concurrent storm-tide-tsunami event. A variety of scenarios are considered as the tsunamis are superimposed at different phases of the storm-tide event. Their compound impacts on Macau and Hong Kong in Pearl River Delta, China, are investigated. The worst-case scenarios have been identified at Macau and Hong Kong, respectively. In addition, the efficacy of the linear superposition of results obtained separately for each hazard (i.e., typhoon and tsunamis) is also discussed. In general, the linearly superimposed solutions consistently underestimate the maximum water levels and yield delays on the arrival times of peak flooding stage in the Pearl River estuary. It is concluded that for coastal protection and hazard mitigation planning in this region, these extreme situations need to be considered. A fully coupled numerical model package is now available for conducting such studies.