Numerical Simulation of Inundation Caused by Storm Surge and Rainfall at Masan Coastal Area

Abstract

The southern coast of Korea is annually vulnerable to storm surge and rainfall by typhoon, and the damages especially to Busan, Masan and Yeosu City in the southern coast are great. In recent years, the frequency and scale of typhoon is increasing due to extreme weather, sea level rise and torrential rain arising from global warming, which in turn increases the size of damages to the coastal areas. Because a risk of storm surge as a critical factor of coastal disaster increases, various studies of costal disaster are actively under way. Existing storm surge numerical models, however, have calculated only the inundation due to storm surge, yet excluded the inundation due to rainfall. The causes of coastal inundation by storm surge can be classified into two categories: the inland inundation by the rise of sea level due to typhoon and the inundation by the rainfall within the area inside the coastal embankment that is not discharged into the sea via storm-water sewer system. In general the storm accompanies rainfall. Thus, it is essential to consider the effect of rainfall for conducting the numerical simulation of coastal area inundation by storm surge. In this study, to determine the inundation area due to storm surge that accompanies rainfall, we consider the effect of rainfall in the nonlinear shallow water equation. The storm-water sewer numerical model developed by Lee (2013) was added to the storm surge numerical model for more accurate result. Through this storm-water sewer numerical model, the water inundating the inland is drained to sea. However, if the typhoon is reached near the study area, a back water flow may occur by the rise of sea level. We compared the numerical simulation results of the inundation by the combined storm surge and rainfall and the results of inundation by only storm surge. The results showed that the area and depth of inundation with rainfall were significantly greater than those calculated using the existing storm surge model. Furthermore, to examine the efficiency of a sea wall proposed by the Masan City to reduce the inundation due to typhoon, we additionally examined the results of inundation assuming 4 meter high sea wall in Masan coastal area. The numerical results showed that a sea wall is effective in preventing direct inundation, yet its resulting diffraction results in the inundation in the low lying areas around the both ends of sea wall, which in turn leads to the inundation of Masan coastal area. Furthermore, we observed that the rainfall could not flow out into the sea due to the sea wall, which in turn resulted in a greater inundation.