| The transport processes of subsurface freshwater/saltwater have been widely observed in different problems of various fields,such as hydro-engineering,environmental engineering,underground engineering and other fields.It is very crucial to describe accurately the characteristics and behaviors of subsurface freshwater/saltwater transport in saturated-unsaturated soils for solving properly these problems.The means of numerical simulation can effectively depict the flow and transport processes of various subsurface fluids at different time and space scales under complex conditions,and is an economically feasible and efficient method.The saturated-unsaturated soil with respect to the transport processes of subsurface freshwater/saltwater is a complicated multi-phase flow system,where the liquid phase and gas phase exists simultaneously,containing two or three of the component: air,freshwater,saltwater.That is,this system is a typical liquid-gas two-phase flow system.Most previous studies with respect to the subsurface freshwater/saltwater transport were limited to only the flow features of liquid-phase,and the gas-phase flow and its influence on other fluids flow were neglected.However,for many problems about the freshwater/saltwater transport in unsaturated soils,such as rainfall infiltration and tidal fluctuations,the gas phase is driven by the water-flow,and the influence of generated gas phase pressure deserves to be studied.Therefore,the three-dimensional liquid-gas two-phase flow with multi-components transport model with considering the effects of hydrodynamic dispersion was established in this study,and the effects of airflow induced by rainfall infiltration on the shallow groundwater table fluctuations,and the tide-induced airflow and its effects on salt-freshwater transport were investigated quantitatively.Also,combining the improved micro genetic algorithm(mGA),the simulation-optimization model for the air curtain controlling seawater intrusion(mGA-3DT2DM)was developed,and the layout parameters of air curtain for a typical three-dimensional confined aquifer were optimized.Finally,combining the inverse code of the parameters of groundwater model(UCODE),the inverse model for estimating the unsaturated hydraulic parameter in the liquid-gas two-phase flow system(UCODE-3DT2DM)was developed,and the unsaturated hydraulic parameters of a sandy soil column were estimated inversely based on the infiltration experiment.The main research contents and innovative achievements are as follows:(1)The source code of TOUGH2/T2 DM was improved,and the three-dimensional liquid-gas two-phase flow with multi-components transport model with considering the effects of hydrodynamic dispersion was established.The TOUGH2/T2 DM was employed to solve the liquid-gas two-phase flow with multi-components transport processes with respect to the subsurface freshwater/saltwater transport.However,the mechanical dispersive fluxes only through the Y,Z interfaces were calculated in the T2 DM source code.So the T2 DM was improved firstly by adding the calculation of the mechanical dispersive fluxes through the X interfaces in the subroutine DISF.As a result,the T2 DM was extended to the 3DT2 DM,which provides an effective tool for solving the problems of three-dimensional liquid-gas two-phase flow with multi-components transport with considering the effects of hydrodynamic dispersion.Then,the semi-analytical solution of modified Henry problem with considering both the molecular diffusion and mechanical dispersion was employed to verify the accuracy of the 3DT2 DM.Also,a symmetrical and three-dimensional test case was designed,and was used to inspect further the effectiveness of 3DT2 DM to the problems of three-dimensional hydrodynamic dispersion through examining the symmetry of the distribution of relevant variables in the direction of XZ,YZ and XY.Consequently,the 3DT2 DM can be used to simulate the subsurface freshwater/saltwater transport processes under conditions of rainfall infiltration and tidal fluctuations.(2)The 3DT2 DM was employed to simulate the effects of airflow induced by rainfall infiltration on the shallow groundwater table fluctuations,and the formation and development processes of the Lisse effect and Reverse Wieringermeer effect were revealed.Through the relationship between the gas phase pressure and the water table fluctuations,it is found that the pore-air pressure generated in unsaturated zones causes the pore-water pressure above the capillary fringe to be positive,and the groundwater table and the water level in the observation well rise.This is the cause of the Lisse effect.The effects of airflow caused by rainfall on the groundwater table fluctuations were also studied quantitatively by comparing to the results obtained from a water single-phase flow model.The variations of subsurface seepages and the fluctuations of water table in the zone close to riparian under the condition of rainfall were simulated and analyzed.The Reverse Wieringermeer(RW for short)effect occurring at the toe of a hillslope-riparian zone drives water toward the upslope,and it is interconnected with the downward airflow upslope triggered by the advancing wetting front within a certain region.Moreover,the airflow mitigates the attenuation of RW effect.Changes in intrinsic permeability affect the magnitude of RW effect near the stream,as well as the downward speed of airflow,thereby generating highly complex responses.The capillary fringe is not a controlling factor but an influence factor on the formation of RW effect,which is mainly related to the antecedent moisture.It was demonstrated that RW effect also occurs where an unsaturated zone occurs above the capillary fringe with a subsurface lateral flow.(3)The tidal boundary condition for the liquid-gas two-phase flow system which considers the phase transformation was proposed,and then the periodic changes in relevant state variables were analyzed when the tidally driven unconfined aquifer reached a new dynamic steady state.Influenced by the tidal fluctuations,the pressure and the phase of the boundary elements adjacent to the beach were changed continually.The time-dependent Dirichlet boundary conditions were transformed to the Neumann boundary condition,where the source/sink term was calculated based on the change rate of pressure head within the specified duration.Also,the phase state of the boundary elements was judged every time-step.Consequently,this treatment of tidal boundary conditions coincides with the tidal processes in real field situations.Then,the periodic changes of water-flow,airflow and salinity within a tidal-driven subsurface system were analyzed by using 3DT2 DM.In addition,compared to the case without airflow,the generated airflow in unsaturated zones during a tidal cycle prohibits the fluctuations of water-flow velocity within the saturated zone,causing the magnitude of salinity and its amplitude in the upper salt-freshwater interface to decrease.(4)Combining the improved micro genetic algorithm(mGA),the simulation-optimization model for the air curtain controlling seawater intrusion was developed,and the layout parameter of air curtain for a typical three-dimensional confined aquifer were optimized.Under the precondition that the air curtain can mitigate effectively the seawater intrusion,taking the minimum cost as the objective function,the source codes of the 3DT2 DM and mGA were modified to realize the data transmission about the decision variables and the fitness function.Thus,the simulation-optimization model for the air curtain controlling seawater intrusion under ordinary circumstances(mGA-3DT2DM)was developed.The layout parameter(including the interval between the adjacent boreholes,the applied air pressure,the location and length of air injection zone)of air curtain for a typical three-dimensional confined aquifer were optimized using the coupling model mGA-3DT2 DM,in order to verify the feasibility and effectiveness of the coupling model mGA-3DT2 DM.Furthermore,this coupling model provides the technical support for the layout parameter of air curtain.(5)Combining the inverse code of the parameters of groundwater model(UCODE),the inverse model for estimating the unsaturated hydraulic parameter in the liquid-gas two-phase flow system was developed.Taking the minimum sum of squared weighted residuals between the observed and simulated values as the objective function,the source codes of the optimization algorithms(UCODE)and the forward model(3DT2DM)were modified to realize the real-time data exchange between them with regard to a series of successive experimental processes.As a result,the inverse model for estimation of the unsaturated hydraulic parameter within the liquid-gas two-phase flow system was developed.In accordance with the observed data from an infiltration experiment of a soil column,the unsaturated hydraulic parameters were estimated inversely,and the performance of different empirical models was compared.The obtained results validate the efficiency of the inverse estimation model and the accuracy of the optimal value of parameters.This inverse model provides an effective method for acquiring accurately the unsaturated hydraulic parameter in the numerical simulation of liquid-gas two-phase flow. |
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