Influencing Factors And Mitigation Studies Of Urban Torrential Rains

In the context of climate change and rapid urbanization,urban pluvial floods pose an increasing threat to human wellbeing and security in the cities of China.In China,especially in some coastal cities,urban pluvial floods occurs frequently.Located in the low-lying Yangtze River Delta,Shanghai has suffered the impacts of pluvial floods in recent years.Due to the complicated urban structure,the mechanisms of floods have not yet been clearly accomplished.The effect of mitigation measures on urban pluvial floods remains to be studied.Basis valuable aid to managing flood problems lies in understanding the roles of environmental factors in influencing the occurrence of pluvial floods.Assessing the effectiveness of concave green spaces in reducing pluvial floods can help us understanding the practical effects of low impact development(LID).The present study takes Shanghai as research area to conduct an empirical study.The followings are the main conclusions of this study:1.This research investigated the spatial distribution of urban pluvial floods and environmental factors.This research presented a spatial analysis of records of inundated streets in the inner city of Shanghai during 1997–2013.The frequency of street inundation over the 17-year period ranged from 0 to 19,which indicated roads that experienced flood inundation were frequently clustered together.Based on the urban drainage system,digital elevation data and land use data,this research extracted the spatial characteristics of explanatory variables that related to pluvial floods.An ordinary least square(OLS)-based exploratory regression was employed to select the significant explanatory variables of pluvial floods.A combination of six factors is found to have a relatively higher performance than other combinations of factors,which is judged by the R2?VIF and corrected Akaike information criterion(AICc).The six explanatory factors are: adjusted altitude,river density,pipeline density,shantytown ratio,green ratio,and road/square ratio.Results from the OLS model show that the inundation frequency is negatively related to adjusted altitude,pipeline density,green ratio and river density,and inundation frequency is positively related to road/square ratio and shantytown ratio.2.This research quantitatively analyzed the relationship between observed pluvial floods and possible explanatory variables.A geographically weighted regression(GWR)was employed to examine the spatially explicit relationships between inundation frequency and spatial explanatory factors,and an ordinary least squares regression was used to validate the GWR results.Results from the GWR model show that the inundation frequency is negatively related to elevation,pipeline density,and river density,and inundation frequency is positively related to road/square ratio and shantytown ratio.The green ratio is another significant explanatory factor for inundation frequency,and its coefficients range from-1.11 to 0.81.In comparison with the OLS model,the GWR model has better performance as it has higher R2,and lower AICc and mean square error values,as well as insignificant spatial autocorrelation of the model residuals.Additionally,the GWR model reveals detailed site-specific roles of the related factors in influencing street inundation.These findings demonstrate that the GWR model is a useful tool for investigating spatially explicit causes of disasters.3.This research explored the spatial differences in pluvial floods under different return period.Based on the rainfall,land use/cover and soil data,a SCS model was employed to analyze the spatial differences in urban direct runoff.Results show that the increasing urban impervious surface is the main cause which leads to the increasing urban direct runoff.Inundation volume,depth and area were compared and analyzed in 2015 under different return periods.The results show that with the increase of return period,inundation volume and area is increasing.Compared to the 5 year return period,inundation volume is nearly 10.89 times,and inundation area is 2.2 times in 50 year return period.The inundation depth shows strong spatial differences under different periods.4.This research evaluated the capacity of concave green spaces to mitigate pluvial floods and the distribution of the benefitted urban population.This research simulated three scenarios,including Null-concave Green space,concave green space with 10 cm,and concave green space with 20 cm.Based on the simulation of urban pluvial floods and the supply and demand analysis of ecosystem service,this research defined the differences of inundation under concave and null-concave scenarios as the supply services of ecosystem provided by the mitigation of concave green spaces to pluvial floods.Under concave and null-concave scenarios,the differences of benefitted region and population were the demand/benefitted services of ecosystem provided by the mitigation of concave green spaces to pluvial floods.The results show that the capacity of concave green spaces to reduce pluvial flood inundation volume is between 35.16% and 79.80% in different return periods.The reduced area is between 26.09% and 82.41%.The benefitted population is between 0.4 million and 1.03 million.The benefitted older adults are between 37.1 thousand and 121.9 thousand.The benefitted population and older adults are mainly clustered in the urban surrounding areas.The capacity of concave green spaces to reduce inundation depth shows a regional difference.These findings demonstrate that the cancave green space is a useful measure to mitigate pluvial flood,especially in 10 year return period.It is necessary and rational to establish LID measures in Shanghai.This research explored the environmental factors of urban pluvial flood,and analyzed the spatial relationship between inundation frequency and explanatory variables using GWR model.This research also explored the effectiveness of concave green spaces on the urban pluvial floods,and analyzed the distribution of the benefitted urban population and older adults.However,due to complicated process and causes of pluvial floods,it needs more work to strengthen the environmental factors and clear the feasibility and cost-benefit analysis of mitigation measures.Nevetheless,this research discussed the urban pluvial floods and environmental factors,and also explored the effectiveness of concave green spaces on the urban pluvial floods.It provides the guidance for policy makers aiming to minimize the flood hazard risk.