Impact Of Urbanization On Flood Risk In The Yangtze River Delta

The Yangtze River Delta is located in the transition zones between north and south areas of the monsoon climate region in eastern China.The precipitation has significant seasonal and interannual variations,resulting in frequent floods and serious economic losses.At the same time,as one of the regions with the highest urbanization level in China,the fast urbanization in the Yangtze River Delta results in construction land expansion at large scale,which has greatly changed the underlying surface properties.The impervious surface increases rapidly,while water bodies continue to shrink.As a result,flood losses increase rapidly,and flood risk under the background of rapid urbanization is becoming more and more prominent.Therefore,it is increasingly necessary to do research on the flood risk in this region.The urbanization in different regions of the Yangtze River Delta is obviously different,resulting in remarkable difference of flood risk.In order to analyze variations of flood risk in the process of the urbanization,the inner Lixiahe region and Wuchengxiyu region in the Yangtze River Delta was selected,and comparative analysis of the difference in the features of precipitation and water level changes between the two regions in the urbanization process,was conducted.Based on the formation mechanism of the flood,the evaluation model of flood risk under the background of urbanization was constructed to analyze the spatial and temporal variation characteristics of flood risk which was greatly influenced by urbanization.The inner Lixiahe region and Wuchengxiyu region are typical plain river network areas with the similar climate,and abnormal precipitation,low-lying terrain and dense water bodies are the natural geographical basis for frequent floods.The changes of runoff yield and concentration caused by the underlying surface variation are the dominant social factors in the formation of the flood.The precipitation in the inner Lixiahe region and Wuchengxiyu region are relatively abundant,but the precipitation intensity is slightly larger in the former.Rainstorm is the dominant factor of flood,and the contribution rate of rainstorm in the inner Lixiahe region is much larger.The trend of precipitation in the two regions is not obvious both in the flood season and throughout the year,but the precipitation intensity in the flood season has shown a significant increasing trend.A few floods took place simultaneously in the two areas,including 1962,1991,2011.The highest level of Wuchengxiyu region rises(falls)with a longer time than that in the inner Lixiahe region;its water level line is gentler.The interannual variation of flood extreme water level in the inner Lixiahe region is larger than that in Wuchengxiyu region,which is relatively small.The development trend of extreme water level in flood season in the inner Lixiahe region is different,but the extreme water level in Wuchengxiyu region showed a significant increase.The maximum water level in the flood season in the two research areas is the highly correlated with the precipitation,rainstorm amount in the flood season and the maximum 30 days precipitation.Correlation between the highest water level in the inner Lixiahe region and precipitation is higher than that in Wuchengxiyu region;while the hydrologic alteration is more obvious in Wuchengxiyu region than that in the inner Lixiahe region.Flood risk assessment model is established based on the flood hazard(precipitation,terrain,river,construction area)and vulnerability(population,GDP).Result showed flood risk in the inner Lixiahe region is influenced by precipitation,terrain,river and other natural factors,and the contribution of the flood hazard to the flood risk is more obvious.However,the flood risk in Wuchengxiyu region is mainly influenced by land use change,population,and GDP,and the contribution of the flood vulnerability to flood risk is more prominent.The spatial and temporal differences of urbanization are in good agreement with the temporal and spatial variation of flood risk,and the higher the urbanization level,the higher the flood risk.Differences in the urbanization level between regions have more significant impact on the flood risk compared to different precipitation scenarios.In the term of relationship between flood risk and underlying surface change and economic development,these are highly correlated with each other and flood risk increases with urbanization level.