An Experiment Study On The Groundwater Hydrodynamics Due To Changing Water Head

Affected by tides and waves,mechanism of flow uprush and downwash processes in the swash zone is complicated,which is a difficult and crucial problem needed to be solved in coastal engineering.With the groundwater storage constantly changing inside the beach,the pore-water pressure characteristics and groundwater table fluctuation play an important role in beach erosion and accretion process.Therefore,study of the pore-water pressure variation inside the sand beach is crucial to understand the physical insight of tidal and wave hydrodynamics in the swash zone.In addition,it can provide scientific references for problems related to saltwater intrusion,the disposal of waste water and the mobility of beach sediments.In this study,the variation processes of groundwater pore-water pressure under changing driven water head were investigated using the indoor sand column experiments.For such experiment,capillary effect plays a key role,and has a great influence on the intermal pore-water pressure variation near the interface of the mediums.In order to further study the pore-water variation in a sand column,a series of experiments were carried out,including dewatering(drainage/discharging)experiments,watering(wetting/charging)experiments and oscillatory driven water head experiments.In this study,we discussed the influence of pore-water pressure variation with respect to the sand grain size,the slope of sand surface,capillary truncation factor and oscillating velocity.In dewatering experiments,the pressure head drops suddenly when the water level passes through the sand surface because of capillary effect.The sudden drop value is equal to the height of capillary fringe,which is inversely proportional to the grain size.Speed of this sudden drop is also inversely proportional to the slope of sand surface.For the layered sand column experiment with sand layer staying above the gravel layer,the pore-water pressure descents slowly through the interface of sand and gravel.As for the watering experiment of the shallow groundwater hydrodynamics,truncation factor(TF)makes a great influence.The addition of a minute amount of water results in a disproportionate rapid rise of the pressure head initially,and then a gradually increase to its equilibrium-As for TF S 1,a thin film of water addition produces nearly instantaneous increase in pore-water pressure,corresponding to a rapid rise of groundwater table to the sand surface.Pressure head increment increases linearly with the TF value and reaches its maximum when TF value approaches unity.Considering the vertical distribution of the pressure head,its gradient becomes larger right after the water addition(>1),then decreases to its initial value of hydrostatic condition in the end(=1).In the case of TF>1,there is an unsaturated layer above the groundwater table.The pressure head increases after the minute water addition,whose magnitude however decreases as the increase of TF value.Nevertheless,the final position of the groundwater table is still below the sand surface,and the gradient of the pressure head vertical distribution does not vary significantly throughout the entire process.In oscillatory changing water head experiments,this study investigated the total water head and groundwater table response to different oscillating velocities.The amplitude of total water head variation in sand column is smaller than that of driven water head,which has been shown in previous studies.The larger driven speed of the oscillation is5 the smaller the absolute valley value of the total water head and the higher the peak value.Due to capillary effect,the absolute valley value is greater than the peak value of total water head in sand colvimn.The first 1/4 period of this experiment is similar to the dewatering experiment,whereas the second 1/4 period is similar to the watering experiment.Variation of the total water head and water table in the sand column obviously lags behind the driven water head during the valley phase,which indicates the existence of hysteresis effect.As for the first oscillation period,the groundwater table decreases in the first 0,35 period,and then increases.In this paper,characteristics of pore-water pressure in sand column have been studied through physical model tests,which further extends our understanding of the problem.This study started from the simple uni-directional external driven head experiments,then extended to the oscillatory driven water head condition,which establishes a concrete foundation for further study of the complex and periodic state of the problem.