Flood Risk And Adaptation Path Research In Coastal Areas Of China

With global climate change and rapid urbanization,coastal areas are facing increasing flood risk.Scientific understanding of the future coastal flood risk changes and effective adaptation can help mitigate risk and ensure sustainable development.China has a long coastline,and its coastal areas are vulnerable to coastal flooding due to rapid socioeconomic agglomeration and development.There is an urgent need for flood risk assessment and economic feasibility analysis of corresponding coastal adaptation pathways.However,there is no research exploring the spatial and temporal patterns of coastal flooding and analyzing the economic efficiency of adaptation pathways and their urban-rural differences in coastal areas of China at the national scale.To fill this gap,multi-source flood control standards were collected to develop the latest county-level coastal flood control standard dataset.Based on the latest extreme sea level data,coastal elevation data and other basic data,hydrological-hydrodynamic simulation and scenario analysis were used to simulate the spatial distribution and future changes of coastal flooding under multiple return periods.Based on land use data,asset value data,vulnerability curve and other basic data,methods such as risk analysis,costbenefit analysis and variance analysis were used to analyze the spatial pattern,future changes and urban-rural differences of coastal flood risk,and further explore the spatial economic efficiency and urban-rural differences of the adaptation pathway of“maintaining the current flood protection level”(maintaining and upgrading the existing adaptation measures to maintain the current flood protection level according to sea level rise).As follows are the main conclusions:1.The spatial distribution and future changes of coastal flooding under 9 return periods was revealed.With the increase of the flood return period,that is,from a 2-year to 1000-year return period,the inundation area of coastal flooding is expanding and the inundation depth is increasing.Without considering the existing coastal flood protection measures,the flood of a 1000-year return period will inundate 5.3%(71,000km(?))of the coastal areas.And without adopting certain adaptation pathways,this inundation proportion will increase to 6.0–6.8% in 2100 along with future sea level rise of the RCP4.5 scenario.Spatially,the floods of each return period are concentrated in the coastal zones of the Bohai Rim,Yangtze River Delta and Pearl River Delta.2.An updated dataset of county-level coastal flood protection standard in China was constructed and the spatial and temporal patterns of coastal flood risk at the countylevel scale in China were evaluated.27.6%(197)of county-level administrative units were affected by coastal flooding.Considering the flood protection level led to a significant reduction in coastal flood risk compared to not considering the flood protection level,with annual expected damage decreasing from 647.19 billion yuan to18.90 billion yuan,or 97.1%.Spatially,the administrative units with significantly reduced risk are concentrated in the more socio-economically developed urban areas such as the Bohai Rim,Yangtze River Delta and Pearl River Delta.If the existing adaptation measures are not maintained or upgraded,future coastal flood risk will increase rapidly.Under the extreme scenario of RCP8.5 High-end_SSP5,the average annual growth rate can reach 4.2(3.9–4.8)%.Therefore,certain coastal adaptation pathways are necessary to mitigate increasing flood risk.3.The economic efficiency of the coastal adaptation pathway of “maintaining the current flood protection level” under different future scenarios are quantified.First,socioeconomic development and GHG emissions affect the economic efficiency of each county administrative unit.Under different future scenarios,the number of administrative units with NPV>0(or BCR>1)fluctuates between 107 and 137.The number tends to increase with accelerated socioeconomic development and increasing GHG concentrations.Second,the number of administrative units with economic efficiency increases as the discount rate decreases.Therefore,it is necessary to pay attention to future climate change and socio-economic development,continuously assess the economic return of future investments,and make timely adjustments to increase economic efficiency.4.It revealed that the above adaptation pathway is economically robust in 64.0%of the county-level administrative units.Combining the results of 54 cost-benefit analyses under all scenarios,the coastal adaptation pathway of “maintaining the current flood protection level” is economically robust in 105 county-level administrative units(64.0%),i.e.,the benefits of disaster reduction under all scenarios are greater than the costs of disaster reduction;they are widely distributed in the province of Hebei,Tianjin,Jiangsu,Shanghai,Zhejiang,and Guangdong.Shanghai,Zhejiang,and Guangdong.For the remaining 58 county-level administrative units that are not economically robust,22 of them do not have economic efficiency under any of the scenarios,and they are located in the province of Liaoning,Shandong,Fujian,and Guangdong.For these 58 administrative units that are not economically robust,it is necessary to adopt more suitable coastal adaptation pathways based on regional characteristics,either by adding complementary adaptation measures or by incorporating nature-based solutions to constitute hybrid adaptation measures.Reliable information collection and sound analysis techniques can help reduce the uncertainty of cost-benefit analysis.5.The urban-rural differences in coastal flood risk and the economic efficiency of the above adaptation pathway were explored.In terms of coastal flood risk,there is no significant difference between urban and rural areas;however,the absolute risk is higher in urban areas and the relative risk is higher in rural areas.There is also no significant difference in economic efficiency between urban and rural areas using the adaptation pathway of “maintaining the current flood protection level”.Accordingly,while paying attention to urban flood risk,equal attention should be paid to rural flood risk: in rural areas reducing the flood loss rate should be focused and in urban areas the residual risk should be paid attention.Meanwhile,urban areas and rural areas should also be optimized equally: for urban areas with relatively dense assets,the population layout should be reasonably planned to optimize the urban development direction;for rural areas with relatively sparse assets,the focus should be on supplementing nonengineered flood protection measures to improve the ability to adapt floods of a small socially vulnerable population.In addition,nature-based solutions can effectively serve coastal areas to a certain extent.Taking the coastal areas of China as the study area,the paper uses hydrologicalhydrodynamic simulation,flood risk analysis,scenario analysis,cost-benefit analysis and variability analysis to conduct spatial analysis and variability analysis on the economic efficiency of coastal flood risks and adaptation pathways,in order to explore how to prevent floods economically and effectively,and also proposes corresponding coastal adaptation for urban and rural areas,which are innovative and practical.The findings can provide a scientific basis for coastal areas to formulate disaster mitigation strategies,reduce regional differences,and achieve sustainable development.