Tide-induced groundwater level fluctuation in a U-shaped coastal aquifer

The prediction of groundwater level fluctuation due to tidal waves propagation in coastal aquifers is important for the planning and management of water resources in coastal areas. A two-dimensional (2-D) analytical solution is derived to describe the tidal groundwater fluctuation in an aquifer bounded by three water-land boundaries that form a U-shaped coastline. Two opposite sides represent estuary-land boundaries on which the amplitude attenuation and phase shift of the tidal movement in the estuary are considered while the third side is an ocean-land boundary. The effects of wave interaction due to the propagation of oscillating oceanic tides in the cross-shore direction inland and the transmission of the two opposite estuarine tides in the along-shore direction are investigated. Three existing head-fluctuation solutions can be considered as special cases of the present solution; one is for one-dimensional flow and the other two are for 2-D flow. A transition distance ranging from 10 to 15 times of tidal propagation length along the shoreline can be estimated based on the solution. This distance can be used to judge whether the interaction among tides is significant. The influences of hydraulic properties on the tidal fluctuations within the aquifer can therefore be assessed quantitatively. Based on sensitivity analyses, one can conclude that the tidal head is most sensitive to the transmissivity and storativity of the aquifer, and least to the damping coefficient of tidal amplitude and wave number along the estuary. The sensitivities of head fluctuation to the changes of transmissivity and storativity depend on the shoreline length and whether the interaction among waves is significant. On the other hand, the sensitivities of head fluctuation to the changes of damping coefficient and wave number increase with diagonal distance from the entry of estuary and reach the largest magnitude near the estuary far away seashore.