| Land subsidence is the most prone environmental geological problem in cities.It has the characteristics of irreversibility,long disaster development cycle,and difficult prevention and control.And once it happens,it will induce a series of other disasters such as ground fissures.Therefore,it is very important to conduct research on the prevention and control of ground subsidence geological disasters.Changchun is a medium-sized city short of water.As early as the 1980 s,there were clear signs of subsidence in the city centre as a result of unrestricted groundwater extraction.Therefore,it is of great significance for the rational exploitation of groundwater and the prevention and control of land subsidence disaster in the future to determine the present situation of groundwater level in Changchun city,find out whether there is land subsidence phenomenon,and study the influence of groundwater exploitation on land subsidence.Thus,based on the 2018 Changchun City Water Source Well Survey Project,2019 Changchun City Geological Hazard Risk Assessment Project and other relevant zoning data,this paper investigated the groundwater falling funnel and land subsidence in the city of Changchun.According to the basic data of the study area,a land subsidence model coupled with water and soil was established and solved by using Modflow and SUB software packages in GMS.Then the groundwater monitoring data from 2006 to 2018 were used to complete the identification and verification of the model.Finally,we predict the groundwater falling funnel and land subsidence in the next ten years under different groundwater exploitation conditions.It provides reference for the planning,disposition,management and protection of underground water and the prevention and planning of land subsidence in Changchun city.The main research contents and results are as follows:(1)This paper analyzed the physical geography,regional geology and hydrogeology conditions of Changchun city.According to the relevant data,the historical evolution law of groundwater falling funnel and land subsidence in Changchun city was analyzed.The results show that in the 1980 s,there was a regional groundwater drop funnel caused by overexploitation of groundwater in the study area,and there was an obvious sign of land subsidence in the downtown area.In 2000,the groundwater fall funnel became 13 smaller places,and the water level showed a rising state.Based on the unified measurement data of the underground water level in the urban area of Changchun in 2019,this paper drew the groundwater isolevel map,and confirmed that there is basically no groundwater falling funnel in the urban area of Changchun at present.According to the field investigation of land subsidence and the level monitoring of the main urban area,it is confirmed that there is no land subsidence caused by groundwater exploitation in the study area.(2)On the basis of the above,this paper determined the scope of calculation area,generalize the aquifer structure,groundwater flow field,filling and drainage conditions,boundary conditions and subsidence mechanism in the study area,and established the hydrogeological conceptual model.According to the elevation data of the surface and the roof and floor of each aquifer in the study area,a three-dimensional geometric model was established.After that,this paper sorted out the source and sink data of rainfall,evaporation and groundwater exploitation in the study area from 2006 to mid-2018,and solved the land subsidence model coupled with soil and water by using Modflow and SUB software packages in GMS.(3)After the model was established,the measured water level data from 2006 to2014 were selected for identification,and the measured water level data from 2015 to2018 were used for verification to determine the hydrogeological parameters.According to the existing data,the ground subsidence parameters were determined.Through identification and verification,it is found that most of the observation holes are found within 5 percent of the water level.The results show that the water level of each observation hole is well fitted and meets the requirement of equilibrium,which is in line with the actual situation of the study area.This model can be used to predict.(4)In order to explore the influence of groundwater exploitation on land subsidence in Changchun city,this paper used the established prediction model to predict the development trend of groundwater funnel and land subsidence in the study area ten years later under different groundwater exploitation conditions.The results show that under the current exploitation condition,the groundwater level will rise slowly in the next ten years,and the land subsidence will not be induced.It shows that the exploitation amount and layout of groundwater are reasonable at present.Under the condition of doubling the exploitation of groundwater,the water level shows a downward trend.The accumulative settlement of 37.55 mm in the groundwater concentrated exploitation area of Changchun Institute of Technology.The subsidence amount is very small,so it can be considered that the subsidence phenomenon will not occur under the condition of doubling the exploitation of groundwater.It shows that there is still a certain exploitation potential of groundwater in Changchun.At three times the amount of groundwater being extracted,the groundwater level will decrease further in the next 10 years,which will lead to local subsidence.The maximum accumulative settlement is located in the groundwater concentrated exploitation area of Changchun Institute of Technology,reaching 115.38 mm.The accumulative settlement in the most areas are below 10 mm.At four times the amount of groundwater being extracted,the water table will drop dramatically over the next 10 years.The phreatic water level of each groundwater concentrated exploitation area is basically close to the bottom of the aquifer.The maximum accumulative settlement reached 195.25 mm.It could cause damage to buildings in the city centre. |
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