Three-dimensional saturated-unsaturated flow simulation by galerkin finite element method

Abstract

A three-dimensional mathematical model has been developed using Galerkin finite element method for simulation of transient saturated-unsaturated groundwater flow in multiaquifer system. The program deals with a variety of boundary conditions encountered in real hydrogeological situation consisting of simple constant or time dependent prescribed head (surface water bodies) or prescribed flux boundaries (pumping well, recharging well etc:), complicated system-dependent and atmosphere-controlled boundary including the near-surface activities i.e., precipitation, evaporation, root water extraction and surface ponding caused by excess precipitation and also boundary along the river side having variable seepage face conditions. The supporting programs developed are a mesh generator for three-dimensional domain discretization, and two other for checking, interpolating and arranging the input data like precipitation, evaporation, river water level which finally create input files for the main program. For parameterization of the unsaturated soil properties three characteristic functions are included in the main program which are optionally available. To test the different features incorporated in the model, a number of published schematized problems have been simulated by the program. Six such problems are presented in detail in this thesis with simulated results and their comparisons with published results where available. Two problems show the model responses in purely unsaturated conditions. In the first problem, the program simulates the moisture content propagation under the specified constant pressure head at the top of a soil column. The results show increase of pressure head and moisture content in the column with a moving front. In the second problem the imposed flux at the top is varying linearly with time and the simulation results show how the pressure head profiles rapidly changes from their initial hydrostatic distribution. In the third problem a fully saturated steady state confined groundwater flow under pressure head difference between two ends with vertical flux at the top surface. is simulated where the results are compared with the available analytical solution of the problem. The fourth problem deals with the near surface activities due to the heavy precipitation followed by subsequent ponding on the surface after a long dry spell with the evaporation, root water extraction occurring at the same time. The resulting groundwater rise i • " and fall due ta recharge fram infiltratianare exhibited in a layered sail calumn. The fifth and sixth prablems shaw the drainage and replenishment simulatian with river bank situatian. The fifth prablem shaws the drainage fram a hamageneaus sail due ta sudden lawering .of water level in the adjacent surface wat.erbady. Transient pasitians .of the graundwater table and seepage face with the autflaw valume thraugh this face are simulated and campared with published results. The last prablem is clase ta the real river bank situatian where the sail damain cansists .of three layers .of sail with different hydraulic characteristics and the adjacent river water level is changed ta high flaad canditian fallawed by pralanged law level and a secand flaad thereafter. The pragram simulates the replenishment and drainage .of the zane, ' the pressure head cantaurs and the velacity vectar profiles at different flaad and law level canditians. The time and lacatian .of probable bank failure are alsa identified by these results. Ta verify the madel respanses in simulating a camplex real hydragealagical situatian a strip area .of Dhaka city alang the bank .of Burhiganga river is studied. The baundary canditians .of the damain are derived from the river water level and piezometric level data. The simulated damain is campased .of faur layers: upper aquitard, upper aquifer, lawer aquitard and lawer .or main aquifer. Madel calibratian and sensitivity analysis .of the parameters have been perfarmed under this study. The madel is calibrated in trial-and-error appraach with the help .of .observedpiezometric level data. After having a set .of calibrated param'eters, sensitivities .of the different parameters have been examined by perturbing a parameter at a time while keeping the ather parameters canstant. Simulated piezometric head prafiles shaw the trend .of rise and fall with the river water level at the nades nearer the river but this: phenamena diminishes rapidly away fram the river' at the remate nades where cantinuaus declining trend .of piezametric levels are .observed. Simulated water balance campanents identify the river effect as the mast daminating recharge mechanism .of the area which cantributes around 60 percent .of the tatal valume .of abstractian. Urban recharge and starage reductian .of the upper aquifer alsa cantribute ta the abstractian valume .of the area.