Numerical Study of Tidal Effects on Seawater Intrusion in Confined and Unconfined Aquifers by Time-Independent Finite-Difference Method

TLDR: In this paper, a two-dimensional time-independent finite difference model is presented to simulate the effect of tides on the intrusion of seawater in either a confined or an unconfined aquifer.

Abstract: This study presents a two-dimensional time-independent finite difference model to simulate tidal effects on the intrusion of seawater in either a confined or phreatic aquifer. The model considers a sloped beach face. The irregular beach face and varying ground water surface are mapped to a square domain and the time dependence of the moving boundary is removed by an appropriate coordinate transformation. The finite-difference expression of the governing equations is, therefore, written in a time-independent mesh system, and a time-independent finite difference scheme is established. The numerical technique is confirmed by the reported results of the traditional Henry's and modified Henry's problems. Idealized saltwater intrusion, known as Henry's problem, was extended to simulate seawater intrusion on sloped beach faces accounting for tidal effects. Tidal effects enhance the migration of seawater. The dominant feature of a confined aquifer is that seawater intrudes inland during the flood tide. The saltwater continues to intrude further inland and reaches its furthest upstream position at ebb tide. In an unconfined aquifer, the tidal fluctuation causes the variation of the intrusion profile in the vertical direction only. The fluctuation of the water table oscillates according with the fluctuating tide with a time lag, but the influence of the tide is damped out with horizontal distance from the beach. The variation of the distance through which the seawater intrudes also oscillates with the tide, with a constant time lag of 0.25T, where T is the tidal period. The shape of the aquifer also affects the intrusion of saltwater and the velocity of intrusion. The rate of seawater intrusion and the distance through which the seawater intrudes increases with the slope of the bank.