The Process And Numerical Simulation Of Variable Saturation Infiltration Induced By Suspended Solid Clogging During Surface Recharge For Groundwater Using Urban Rain-flood Water

In recent years,the problems of urban waterlogging and the shortage of freshwater are serious,and the groundwater artificial recharge（GAR）with stormwater is a good solution to deal these problems.However,the clogging problems due to suspended solids is the most common reason for malfunction of the GAR systems.The infiltration processes of the GAR with stormwater are complicated,often characterized by unsteady flow and variable saturation.So far,there are only some researches about the clogging problems in saturated aquifers,while few attempts have made on such unsteady and variable saturated flow combined with clogging.This thesis focuses on the possible clogging problem that may occur during groundwater recharge with stormwater.Several methods were applied in this study,such as field investigation and observation,lab experiments,computer modeling,etc.The main work includes the investigation of urban storm runoff samples,the potential analysis of different clogging type,the mechanism of suspended clogging development during groundwater recharge with stormwater,modeling and calculation of the clogging process due to suspended solids.Then,the conclusions are described as follows:（1）Except for some samples whose concentration of NH₄⁺was out of the standard,other samples of the five types of stormwater runoff had good water quality to meet groundwater recharge standard,and the rainwater had the best water quality.From the perspective of clogging risk induced by water characteristics,the potential of chemical clogging and biological clogging was much lower than the potential of physical clogging dominated by suspended solids clogging.（2）The non-expandable inorganic mineral particles of greater than 5μm size accounted for the largest percentage of the suspended solids in stormwater（up to 90%）,such as feldspar and quartz.The road runoff samples had the most inorganic suspended solids that may induced highest clogging potential.The lawn runoff samples had the most organic suspended solids which indicated complex clogging may occur.Among all types of water,the rainwater samples had the lowest clogging potential of suspended solids.（3）Indoor experiments results shows mixed clogging develops in stages.The first step is internal clogging,followed by surface clogging.During the processes of mixed clogging,the aquifers changed from unsaturated to saturated,and then to unsaturated.During the processes of the internal clogging,the position of clogging was in the range of 0-6.4cm,and the aquifers remained saturated（water content was at 80%-100%）,but the hydraulic parameters of the aquifer were greatly affected.When the surface clogging started,aquifer began to transition to an unsaturated state until it reached a stable state（water content was at 60%）.At the end of the experiment,the thickness of the sediment layer was 1.7cm.（4）Batch experiments results shows that the permeability of the aquifer was decline with the rising thickness of the sediment layer and the smaller the particle size of the sediment layer medium under the vertical downward flow condition.When the particle size of the suspended particles is smaller than the effective pore diameter of the aquifer medium and the flow speed is larger than the cranking speed of suspended solids,the suspended solids could enter the aquifer.And the smaller the particles of the sediment layer medium,the greater the possibility of suspended solids entering the aquifer.（5）The mathematic model for suspended solid clogging in unsaturated aquifers was derived from the theories of groundwater dynamics,suspended solids migration,etc.The software called“COMSOL Multiphysics”was applied to solve this model.Then,the comparison figures of the simulations with the experimental data illustrated that modeled results had a reasonably good agreement with the observation data within the process of experiment.（6）Establishing and calculating a numerical model of mixed clogging with different concentrations of suspended solids,and the conclusions are described as follows:During the internal clogging process,the suspended solids concentration rising led to the maximum of clogging increasing,but,it had little influence on the saturation of the aquifer.During the surface clogging process,aquifers released water,and remained unsaturated.And the higher the concentration of suspended solids,the faster the growth of the sediment layer.（7）Establishing and calculating a numerical model of surface clogging with different intermittent recharge cycles,and the conclusions are described as follows:The method of intermittent recharge and intermittent recharge cycles had little effect on the clogging process,but it made a difference on the seepage characteristics of different cycles.The permeability coefficients of the sediment layer and aquifer affected the decline rate of the seepage velocity caused by the thickness of the sedimentation layer increasing.Generally,when the thickness of the sediment layer is over 5 cm,the flow speed is close to 0,and the aquifer is almost completely clogged.