Tidal effects on groundwater dynamics in coastal aquifer under different beach slopes

The tide induced groundwater fluctuation and the seawater intrusion have important effects on hydrogeology and ecology of coastal aquifers. Among previous studies, there were few quantitative evaluations of the joint effects of the beach slope and the tide fluctuation on the groundwater dynamics. In this article, a numerical model is built by using the software FEFLOW with consideration of seawater intrusion, tide effects, density dependent flow and beach sloping effects. The simulation results are validated by laboratory experimental data in literature. More numerical scenarios are designed in a practical scale with different beach slopes. Results show that the groundwater fluctuation decays exponentially with the distance to the beach, i.e., A = β1e−γx, and our simulation further shows that the beach slope influence β1 can be expressed in the form of a logarithm function. While for the same location, the amplitude increases logarithmically with the beach angle in the form A = β2In(α) + γ, where β2 and γ are related with the horizontal distance (x) in the form of a logarithm function. The beach slope has no influence on the phase lag, although the latter increases regularly with the distance from the sea. The beach slope effect on the seawater intrusion is investigated through the quantitative relationship among the relative intrusion length (λ), the relative enhancement of the tide induced seawater intrusion (κ) and the beach angle (α). It is shown that the tide effects on a milder beach is much greater than on a vertical one, and both λ and κ can be expressed in logarithm functions of α. The tidal effect on the flow field in the transition zone for a particular mild beach is also studied, with results showing that the tide induced fluctuation of Vx is similar to the groundwater table fluctuation while Vz shows a distinct variation along both directions.