Influence Analysis Of Land Subsidence On Flood Risk In Detention Basin

With the development of economic society,the groundwater over-extraction have caused the decreasing of groundwater level and land subsidence.Land subsidence is one of the environmental geological disasters in China.The severe land subsidence would decrease the watercourse’s flood carrying capacity and change the water levelstorage volume relationship and flood risk of detention basin,which make bad effect on flood regulation and flood control.Therefore,it is of great significance to explore the influence of land subsidence on flood control in detention basin.(1)The land subsidence,space distribution characteristics and flood control function of Dahuangbao detention basin are analyzed using GIS spatial information technology.The results reveal that during the year of 1989-2010 in Dahuangbao detention basin,the average land subsidence is 17mm/a and both the inundated area and storage increase comparing the results before land subsidence.When the design water level is 2.87m,storage capacity goes up 14%.(2)The flood routing numerical model of Dahuangbao detention basin is established to model the detention processes of different frequencies floods pre and post land subsidence.The results reveal that land subsidence has large influence on water level and flood concentration time of inundated area.When the 50-year flood is coming,due to land subsidence,the Chenzhaozhuang flood concentration time is 20 min early and flooding level deceases 0.326m.The Gaozhuangzi flood concentration time is 6 hours late and flooding level deceases 0.236m.Moreover,the decreasing of reclamation dam also can influence the detention basin.(3)The flood risk estimation index system of combining nature,economics and technology is established and based on the weight method of three scaling in two steps and drop half ladder diagram method,the membership function is confirmed and the flood risk of different frequency floods pre and post land subsidence is analyzed.The results reveal that the land subsidence would increase the flood risk and as for the case in this paper,the use principle of partition detention would affect the estimation results and the risk of pre flood detention is large but the risk of post flood detention relates to actual flood diversion.(4)The flood damage assessment model based on spatial information grid is established and the flood routing model and ecomonic damage assessment model are combined using MATLAB.On the basis o f assessing the flood damage pre and post land subsidence in Dahuangbao detention basin,the results reveal that with the same frequency flood,the total damage post land subsidence is larger than the damage pre land subsidence.However,with the case in this paper,the use principle of partition detention would affect the damage estimation results and the loss of pre flood detention is large but the damage of post flood detention is small.(5)After land subsidence and all partitions in detention basin are used,the risk and damage of Ⅰ area show increasing trend and Ⅱ、Ⅲ areas exhibit increasing and decreasing trends.In this paper,the flood risk and damage assessment of different flood detention water level in I area are analyzed.The results reveal that when the 50-year flood is coming and partition flood detention water level is 2.62m,the total flood risk and total damage of detention basin are the smallest,which shows that the results is reasonable and rational.Therefore,the total risk would be the smallest if the flood detention water level in detention basin is reduced to 2.62m.