This paper presents an analytical solution of groundwater head response to dual‐tide fluctuation in the transect of an island leaky aquifer system comprising a confined aquifer and its overlying semipermeable confining layer. Both layers terminate at the coastlines on two sides of the island. Solution analysis indicates that the tidal waves from the two sides of the island transect interfere at the middle of the island and the interference decreases to zero as the horizontal length of the aquifer increases to infinity. The leakage of the overlying confining layer enhances the landward attenuation of the tidal head fluctuation and shortens the time lag between the head and tide fluctuations. The solution agreed well with the observations in eight piezometers in Garden Island on the continental shelf of Western Australia reported by Trefry and Bekele (2004).
A numerical study was conducted to investigate the fate of solute in a laboratory beach in response to waves and tides. A new temporal upscaling approach labeled ''net inflow'' was introduced to address impacts of waves on solute transport within beaches. Numerical simulations using a computational fluid dynamic model were used as boundary conditions for the two-dimensional variably saturated flow and solute transport model MARUN. The modeling approach was validated against experimental data of solute transport due to waves and tides. Exchange fluxes across the beach face and subsurface solute transport (e.g., trajectory, movement speed, and residence time) were quantified. Simulation results revealed that waves increased the exchange fluxes, and engendered a wider exchange flux zone along the beach surface. Compared to tide-only forcing, waves superimposed on tide caused the plume to be deeper into the beach, and to migrate more seaward. The infiltration into the beach was found to be directly proportional to the general hydraulic gradient in the beach and inversely proportional to the matrix retention (or capillary) capacity. The simulations showed that a higher inland water table would attenuate wave-caused seawater infiltration, which might impact beach geochemical processes (e.g., nutrient recycle and redox condition), especially at low tide zone. The concept of biochemical residence time maps (BRTM) was introduced to account for the net effect of limiting concentration of chemicals on biochemical reactions. It was found that waves shifted the BRTMs downward and seaward in the beach, and subsequently they engendered different biochemical conditions within the beach.
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