Simulation Of Compound Coastal Flooding In Shanghai

Regional integration of the Yangtze River Delta(YRD)is a national strategy for regional development in China.The core city of the YRD,Shanghai that situates on the eastern fringe of the YRD is surrounded by the waters,is facing with the rising risk of the compound flooding due to the coincidence of storm surge,extreme precipitation,high tide and high river discharge in the background of climate change,sea level rise and land subsidence.The purpose of this paper is to construct a new integrated numerical modeling system,which coupled atmosphere(Fujita typhoon),ocean(TELEMAC-TOMAWAC)and coast(MIKE11-MIKE21)processes,was developed to simulate the extreme compound coastal flooding(including four threats of flooding)in Shanghai.The compound flooding scenario in Shanghai was modeled with and without levee,respectively,using Typhoon Winnie(No.9711)as the sample case to estimate the flooding area and spatial distribution and quantitatively evaluate the flooding depth and process.Scenario simulation method was used to quantitatively evaluate the potential risks of the compound flooding in Shanghai using basic parameters of climate change,sea surface temperature and sea level rise as the inputs.The new developed model was calibrated to provide a valid numerical solution for the integrated compound flooding simulation.The main work and conclusions of this research include:(1)Building a numerical model of the compound flooding in Shanghai.A integrated modeling method,which coupled atmosphere,ocean and coast processes was developed,based on historical information and considered the factors of storm surge,extreme rainstorm,astronomical tide,upstream flood,waves,sea level rise,land subsidence,and upstream flood discharges,to accomplish the combination of the Fujita Typhoon Model,the TELEMAC 2-D Ocean Model,the TOMAWAC Wave Model,the MIKE11 1-D River Model and the MIKE21 2-D Hydrodynamic Model.The result,obtained from modeling the influence of Typhoon Winnie on Shanghai and comparing the calibrated parameters with the measured data,indicated that the multi-model-nesting and multi-model-coupling technology developed in this article was effective and it can be utilized in modeling the compound flooding in Shanghai.(2)Modeling the future scenario of the compound flooding in Shanghai.Simulation of the future scenario of the compound flooding in Shanghai was performed under the influence of climate change and multiple hazard factors.The flooding area of Shanghai in 2100 extreme flooding would be 2640 km~2(accounting for 49%of Shanghai’s total area,except Chongming District)and the flooding area of the water depth of 0.2~0.5 m would be 1500 km~2(57%),the area of 0.5~1 m is 842 km~2,>1 m is292 km~2(11%),based on the worst case of future sea level rise and surface subsidence.The compound flooding with the four threats,namely the storm surge,extreme precipitation,high tide and high river discharge,could cause catastrophic effects on Shanghai under the background of climate change and sea level rise.The overbank effect of waves of the Hangzhou Bay,south of Shanghai,would be the main factor of coastal flooding.The increased water penetration effect of the Huangpu River and connected river network system and the coincidence of rain storm and flood within the Taihu Basin would be the main factor of inland flooding.(3)Scenario Simulation with and without levee in Shanghai.Quantitative evaluation was performed on the flooding process using Typhoon Winnie as the weather background.First,the key role of seawall and levee construction in coastal flooding prevention in Shanghai was demonstrated by comparing the flooding conditions before and after the levee upgrades in 1998 with the condition without the levee.The result showed that flooding area decreased significantly to 67.63 km~2 when modeling the condition after the levee upgrades in 1998 compared to the real condition in 1997,and the water depth in most areas were below 0.2 m without large scale flooding which proved that the levee upgrades in 1998 significantly reduced the risk of flooding.Under the extreme scenario of no levee,the flooding area could reach 3230 km~2 caused by the Typhoon Winnie.The 2-D and 3-D dynamic display of the modeling results showed that high-risk areas(>1 m water depth),were mainly located along the Hangzhou Bay,Yangtze River,downtown and upstream of the Huangpu River,under the combined influence of coastal and basin floods.Second,comparison between the traditional method and the method discussed in this article had been performed through calculating the compound flooding areas and spatial distribution under no levee condition.If the traditional method of cutting the terrain with extreme water level(static method)was used,4455 km~2(accounting for90.6%of Shanghai’s total area,except Chongming District)land area in Shanghai was lower than the four threats and high tide elevation(~5.1 m,which is the elevation of Huanghai in 1985).However,using the integrated numerical simulation system(dynamic method)developed in this article,the calculated flooding area was 3230km~2(65.7%)in Shanghai under no levee scenario.This indicated that the extreme water level cutting terrain method was likely to overestimate the flooding area.Accurate estimate of the flooding area’s spatial distribution can provide rational information for the future planning of property insurance and city development.This study utilized the existing high-quality data to improve the accuracy and credibility of the scenario simulation and provided supports for future model building to assess flooding loss in Shanghai,quantitative assessment of flooding risks in key areas(tourist resorts,financial centers,subway transportation system)and development of emergency evacuation plans and flood control plans.