Tidal wave propagation in a coastal aquifer: Effects of leakages through its submarine outlet-capping and offshore roof

Summary: This paper considers a leaky aquifer system comprising an unconfined aquifer, a confined aquifer, and a confining layer (the aquifer’s roof). The unconfined aquifer terminates at the coastline, the confined aquifer and its roof extend under the sea for a certain distance. The aquifer’s submarine outlet is covered by a thin layer of sediment (referred to as the “outlet-capping” hereafter) with properties dissimilar to the aquifer. Previous studies either neglected the leakage through the aquifer’s roof or the outlet-capping. This paper considers both effects and gives an analytical solution to describe the tide-induced head fluctuation in the aquifer system. For aquifers with zero or infinite offshore length, or with impermeable roof or outlet-capping, or without the outlet-capping, existing solutions in the literature are obtained. We defined the threshold value for the offshore aquifer length, which was found to be less than 5km for typical coastal aquifer systems. For an aquifer with its offshore length greater than the threshold, the inland head fluctuation becomes insensitive to variations of this length and the outlet-capping’s leakance. The leakage through the offshore roof decreases the threshold value. When the offshore confined aquifer is short, the inland head fluctuation increases with the outlet-capping’s leakance. In this case negative phase shift occurs near the coastline only for aquifers with small leakages through the offshore roof and the outlet-capping. In general, the combined actions of the tidal loading, the leakage and length of the offshore roof, and the leakage of the outlet-capping lead to complex, non-monotonic dependency of the inland head fluctuation on the aquifer parameters. [Copyright &y& Elsevier]