Numerical Study Of Tidal Loading And Density Effects On Tide-induced Head Fluctuations In Coastal Aquifer Systems

Tide is a natural phenomenon in the coastal area. The periodic fluctuations of the tides will surely result in considerable variation of the pressure in the underlying aquifer system, which is defined as the tidal loading effect on the groundwater flow. Despite numerous analytical studies of the tide-induced head fluctuations in the past 60 years, many of which have considered the tidal loading effect, few numerical studies has considered this effect. It is even not included in the well developed and widely-used numerical models such as MODFLOW and FEFLOW.Here we make advantages of the extended two-dimensional finite element model MARUN which incorporates the tidal loading effect. The MARUN code can simulate groundwater flow and solute transport in variably-saturated porous media, taking into account the effects of salt concentration on fluid density and viscosity. To verify the correctness of the extension of the MARUN code, we consider two coastal aquifer systems considered by previous analytical studies in literature. The first one consists of an unconfined aquifer, a confined aquifer and a semipermeable layer between them with the two lower layers extending a certain distance under the sea. The second one is a coastal confined aquifer with an impermeable roof that extends under the sea for a distance. The aquifer’s submarine outlet is covered by a thin layer of sediment which is regarded as impermeable. Numerical simulations were conduced using different tidal loading efficiency ranging from 0 to 1. The results indicate that, in the confined aquifer, the numerical solutions considering the density effect agree well with the existing analytical solutions which ignore the density effect with the maximum error being 0.21%. In the semipermeable layer, the numerical solutions also agree well with the analytical solutions except for the area near the coastal area. For the second numerical model, numerical solutions also agree well with the existing analytical solutions with the maximum error being 0.12%.Density difference between seawater and groundwater will affect the groundwater flow undoubtedly. We simulated the density effect on tidal head fluctuations with the MARUN code. Two different kinds of aquifer systems are simulated. The first is a single unconfined aquifer. The second is a multi-layered aquifer systems. Both have different beach slopes. Numerical simulations were conducted in two ways. One considers the density effect, and the other ignores the density effect. Then the error between the two numerical solutions with and without density effect is analyzed. Numerical simulation results indicated that, when the beach slope is 0.1, the maximum error is 2.5%, which appears on the sea-land interface. The maximum error increases with the beach slope. When the beach slope is 0.86, the maximum error becomes more than 10%. It shows that the water table fluctuation of the single unconfined aquifer is affected by both the density effect and the beach slope. As far as the multi-layered aquifer systems are concerned, the error in the confined aquifer and semipermeable layer reaches its maximum of 2.5% on the sea-land interface and decreases landward. In this case the beach slope has negligible impacts on the groundwater head fluctuations of the confined aquifer and semipermeable layer.