Compound Flooding Assessing And Mitigating Across Coastal City ——A Case Study In Shanghai

Under the dual effects of climate change and rapid urbanization,coastal cities are extremely vulnerable to storm surges during typhoon periods,extreme rainfall and upstream discharge,and other disasters,which would lead to severe compound floods and cause huge damage.How to scientifically build compound flood scenarios,quantitatively assess compound floods caused by typhoons,and rationally plan flood control facilities,so as to more effectively deal with compound flood disasters and improve disaster prevention and loss reduction capabilities,is an advanced research question in the international academic community.Based on the coupling Copula function and D-Flow FM hydrodynamic model,we discussed the mechanism of compound flood in the coastal megacity:Shanghai.First,we study constructed compound flood scenarios and systematically simulate the compound flood inundation under different return periods.Then,we evaluated the direct economic losses corresponding to compound flood scenarios under different return periods.After that,we compared the disaster prevention and loss reduction capabilities of the gray engineering and blue-green non-engineering adaptation measures.In the end,we proposed spatial adaptation countermeasures for compound flood.The main innovative insights of this study are as follows:（1）In order to evaluate the compound impact of typhoon storm surge and extreme rainfall on Shanghai,we coupled the Holland model and D-Flow FM hydrodynamic model,to set up and validated the storm surge model and urban overland model using the typical compound flood events in historical（the typhoon"Winnie”in 1997,typhoon“Fitow”in 2013 and typhoon“Likima”in 2019）.The results show that the Holland model,the storm surge model,and the urban overland model can simulate wind speed,wind direction,astronomical tide,storm surge,inundated area,and inundation depth with good quality under the historical compound flood event in Shanghai.We can use these models to simulate the compound flood events for further research.（2）Based on the Holland model and the storm surge model,we reconstructed 210storm surge events in Shanghai during the typhoon period from 1961 to 2018 and analyse the impact of relative sea level rise（RSLR）on the peak water level at the Wusongkou gauge.The results indicate that the astronomic tide is the leading driver of peak water level,followed by the contribution of storm surge.In the longer term,the RSLR has significantly amplified the peak water level.Compared with the scenario without RSLR,the influence of RSLR pushes up the median value of peak water level from 5.21m to 5.53m,with an increase of 0.32m over the historical 58 years.While it is true that extreme peak water level become more likely with RSLR due to the asymmetric effect of even small shifts to rare events,it has also been implied that the peak water level is becoming more variable in the sense of each year drawing from a broadening probability distribution.（3）A copula-based methodology is applied to calculate the joint probability of peak water level and accumulated rainfall during the historical typhoon period and to characterize the dependency structure and interaction of each driver.The Gaussian copula is selected as the best fit for the peak water level and accumulated rainfall under the effect of RSLR.The maximum likelihood method was used to estimate the probability design values of the joint distribution function under different return periods.Under the 100-yr return period,the peak water level and accumulated rainfalls are 6.3m and 183.36mm,respectively.In the end,based on the typical compound flood event（typhoon"Winnie"in 1997）,we employed the Copula function and the same frequency amplification method to generate the compound flood scenarios under different return periods.The results indicated that the Copula method could express the interaction and dependence structure between peak water level and accumulated rainfall,provide more scientific and reasonable compound flood scenarios for the simulation and adaptation.（4）Use the storm surge model and the urban overland model to simulate single-driven flood（fluvial and pluvial flood）and multi-driven compound flood scenarios under different return periods.The results show that the inundation area caused by storm surges is mainly concentrated in the central city,along the Huangpu River,the south of Songjiang District,and the southwest of Qingpu District.The inundation area caused by rainstorms is mainly located in low-lying areas.The results indicated that the inundation depth is decided by the storm surge and the inundation area is influenced by the rainstorm.By comparing the inundation area and inundation volume of single-driven floods and compound floods,it can be found that the traditional single-hazard factor frequency analysis methods overestimate or underestimate precipitation and tide levels.In this research,we found that the relationship of the inundated area between single-driven and compound floods is"1+1>2",while the relationship of inundated volume between single-driven and compound floods is"1+1<2".（5）Based on the spatial inundation distribution of compound flood scenarios under different return periods,we employed the damage curve of different land-use types to calculate direct economic losses in Shanghai.The results show that the spatial distribution of the direct economic losses under compound flood scenarios is consistent with the spatial inundation distribution caused by storm surges.Under the 100-yr and200-yr return period scenarios,the direct economic loss of compound flood increases due to the area of pluvial and fluvial flood continues to expand.Huangpu Park was the most serious region and suffered the biggest loss.Due to the intense distribution of commercial land and residential land with high value,most regions in central Shanghai suffered more than 1,500 yuan/m~2.On the other hand,Qingpu,Jinshan,Fengxian,and other districts suffered economic loss below 500 yuan/m~2 even under the most severe compound flood scenario,mainly because they have more agricultural land and green space.（6）Combined with the urban planning of Shanghai,using parameterized different adaptation measures were formulated in the urban overland model,we evaluated the effects of different adaptation measures when dealing with the compound flood hazards.By adding the Huangpu River tide barrier and increasing the flood wall behind the barrier by 2m,Shanghai can mitigate the extreme storm surge event effectively.In order to deal with the pluvial flood,Shanghai would take a combination of adaptive measures such as improving the drainage capacity,enabling the deep tunnel alone Suzhou River,increasing permeable pavement and green roofs,etc.The results indicated that the adaptive strategy for dealing with compound floods in Shanghai should mainly use gray engineering measures,and pay more attention to blue-green non-engineering measures when doing urban planning.