Numerical Simulation Of “Typhoon-rainstorm-storm Surge” Compound Coastal Flood Under Changing Environment

Due to the proximity to the sea,the humid area of South China and the high degree of urbanization,the flood disaster mechanism in coastal cities of Guangdong-Hong Kong-Macao Greater Bay Area is often very complex,which is easily affected by typhoon,rainstorm,storm surge,astronomical tide,leading to more severe compound coastal flood disaster.At present,most of the research focuses on modeling single or double factors' flood simulation,while the compound "typhoon-rainstorm-storm surge" flood numerical simulation,referring to many research fields,such as atmosphere,ocean,urban hydrology,were less discussed.In order to explore the “typhoon-rainstorm-storm surge” flood numerical simulation methods,take Guangdong-Hong Kong-Macao Greater Bay Area as case study,establish the coupling urban flood model based on one-dimensional(1D)hydrodynamic model and cellular automata model,combined with atmospheric model and ocean numerical model,discuss the influence of rainfall and underlying surface changes caused by urbanization on “typhoon-rainstorm”flood,and evaluate the effects of sea level rise on compound “typhoon-rainstorm-storm surge”flood.The results are as follows:(1)A coupling model(SWMM/WCA2D)of storm water management model(SWMM)and cellular automata model(WCA2D)is constructed.Taking the rainstorm “20180607” in changban District of Guangzhou as case study,the simulation results are close to the measured inundation extent and depth,which shows that WCA2 D can be combined with SWMM to achieve two-dimensional(2D)flood simulation,and the calculation efficiency is3-5 times higher than traditional 2D hydrodynamic model,which indicates that the urban flood simulation method performs high accuracy and higher computing speed.(2)A weather research and forecasting model(WRF)is established in the study area,combined with SWMM/WCA2 D model,the effects of rainstrom and underlying surface changes caused by high urbanization on “typhoon-rainstorm” flood are analyzed.The results show that the rainstorm volume obviously increased when Typhoon Ewiniar(2018)and Mangkhut(2018)occurred in Chebei River Basin and when Typhoon Ewiniar(2018)and Mujigae(2018)occurred in Bantian Catchment.Taking Chebei River Basin as example,the increasing rainstorm volume and underlying surface impervious rate caused by urbanization will lead to the total inundation area(d>0.05m),inundation area of high water depth(d>1m)and total node flooding volume increase by 22.7 times,74 times and 41 times respectively,which is much higher than the scenario which only considering the increase of underlying surface impervious rate(3.6 times,7 times and 4.8 times).The results show that urbanization can also change rainfall process,and the influence of rainfall change should be considered when discussing the potental effect of urbanization on “typhoon-rainstorm” flood.(3)On the basis of WRF model and coupled SWMM/WCA2 D model,combined with MIKE21,and the compound “typhoon rainstorm storm surge” flood numerical simulation is realized.The results show that the maximum inundation area of compound“typhoon-rainstorm-storm surge” simulation scenario is about 4.49 times higher than that of "typhoon rainstorm" flood,and 90% higher than that of “typhoon-storm surge” flood,and the maximum inundation area of “typhoon-rainstorm-storm surge” scenario is larger than the sum of other two scenarios.The results show that the compound flood model can represent the superposition effects of typhoon,rainstorm and storm surge,and only considering two-factors-induced flood will obviously underestimate the compound flood degree.(4)Based on the above compound flood model,the effects of 4 sea level rise scenarios on compound flood in the study area are studied.The simulation results show that,when the sea level rises by 0.2m/0.5m in 2050,the overflow volumn,total inundation area(d>0.05m)and inundation area with high depth(d>1m)will be 1.25/1.60,1.48/2.75 and 1.54/4.21 times of the current sea level scenario;when the sea level rises by 0.4m/1.2m in 2100,the total overflow volume,total inundation area and inundation area with high depth will be 1.52/2.46 times,2.53/4.35 times and 3.5/10 times of the current sea level scenario.