Study On Coupling Model Of Groundwater And Land Subsidence And Simulation Of Emergency Water Supply In Ningbo Urban Area

Ningbo,located in the south wing of the Yangtze River Delta,is an important port city in China.Due to the exploitation of groundwater,the regional land subsidence occurred in the central urban area of Ningbo in 1964.With the expansion of groundwater exploitation scale,the land subsidence has been increasing,which has long puzzled the urbanization construction of Ningbo.The land subsidence caused by groundwater exploitation is a complex fluid-solid coupling problem.The previous studies on land subsidence in Ningbo mainly focused on the small-scale land subsidence induced by engineering construction.So far,there is no research being carried out on the land subsidence caused by deep groundwater exploitation in large areas.This paper is based on the project of "investigation and evaluation of groundwater source for emergency water supply in Ningbo urban area",taking Ningbo urban area as the research object,on the basis of fully understanding the geological and hydrogeological conditions of Ningbo plain,analyzes and summarizes the main characteristics and interrelations of groundwater level and land subsidence in the study area.The coupling model of three-dimensional groundwater flow and one-dimensional soil deformation in the study area is established.MODFLOW and SUB package are used to solve the model.After identification and calibration,the hydrogeological parameters of the model are determined.Finally,based on the coupled model of groundwater flow and soil deformation in the study area,six reasonable exploitation plans for emergency water supply in Ningbo urban area are formulated,and the dynamic prediction of groundwater level and land subsidence changes in each plan is made.The main conclusions are as follows:(1)The development of land subsidence in the study area corresponds to the exploitation of groundwater in time and space.Both the center of groundwater level funnel and land subsidence funnel are located in Jiangdong Hefeng,and the shape and extension of subsidence funnel are basically the same as that of groundwater funnel.(2)The land subsidence mainly occurs in the first soft soil group,the second soft-hard soil layer,the I aquifer and the fourth hard soil layer.(3)Based on the characteristics of significant vertical electrical difference of rock and soil layers in the study area,the symmetrical quadrupole electrical sounding data in the study area are inverted and analyzed by Geoeletro electrical sounding processing software,and the corresponding relationship between rock-soil layer lithology and apparent resistivity in the study area is established.The three-dimensional hydrogeological structure model of the study area is constructed by combining the resistivity profiles with geological drillings and geological sections.(4)Based on Terzaghi’s one-dimensional consolidation theory and Leake’s "interbed" theory,using the relationship between the skeletal specific storage of the interbed and the soil compressive coefficient,the groundwater flow model and the soil deformation model are coupled by adding the calculated water change of interbed to the flow equation as the source term.The coupling model of three-dimensional groundwater flow and one-dimensional soil deformation in the study area is established.The model considers the elastic and inelastic deformation of the soil layer and the lag phenomenon of drainage and compression.(5)The hydrogeological parameters of the model are identified and calibrated by the observation data of groundwater level and the monitoring data of the land subsidence center in the simulation period.The results show that the groundwater level and land subsidence are well fitted,which proves the reliability of the numerical model,and can better reflect the changes of groundwater level and land subsidence caused by groundwater exploitation in the study area.(6)The pore confined water in Ningbo urban area has good supply conditions and strong water level recovery ability,which can meet the demand of short-term centralized water supply.The shorter the emergency water supply time and the lower the water level control depth,the more people can be supplied by the confined water.If the time of emergency water supply is longer and the water level control depth is lower,the water level funnel area,the subsidence area and the maximum subsidence at the end of water supply will be large.After the completion of water supply,the surface deformation in the study area is affected by the elastic rebound caused by the recovery of groundwater level and the delayed settlement of the lagging interbed.Different areas show different deformation characteristics.Generally speaking,the area near the mining well appears the ground rebound,while the area far away from the mining well continues to increase the subsidence.