Experimental and numerical investigation on seawater intrusion into unconfined coastal aquifers under instantaneous changes of heads at boundaries

Sea level rise and groundwater extrapolation lead to freshwater head changes at boundaries of coastal aquifers and causes further intrusion of seawater into aquifers. In the present study, the behavior of unconfined coastal aquifer to instantaneous sea level rise and freshwater head decline is investigated by physical and mathematical dispersive modellings. Three laboratory models have been conducted in a sand tank and salt wedge shape has been photographed over time. Salt wedge toe position, area of transition zone and volume of intruded seawater are measured for this purpose. The numerical results are in good agreement with experimental observations. The outputs derived by both modellings indicate that the instantaneous rise of head in sea boundary or significant decline of freshwater head in landside deforms the wedge shape of saline water, so that the seawater encroachment velocity at its tip will be faster than other regions. Moreover, establishment of an inverse hydraulic gradient (hydraulic gradient from sea to land) causes significant seawater delivery from sea into the aquifer. As a result, the salt wedge can even be stretched up to land boundary. Results of dispersive simulations demonstrate that when the velocity of saline water rises inside the aquifer, the advection role will be highlighted rather than dispersion. However, the longitudinal and transverse dispersivity will also increase and that consequently will widen the transition zone. Nevertheless, the increase rate of saline water volume has been achieved much higher than the growth rate of toe position and transition zone area.