Tidally induced groundwater circulation in an unconfined coastal aquifer modeled with a Hele-Shaw cell

Abstract

We use a scaled Hele-Shaw cell to mimic tidally induced nearshore groundwater motion in an unconfined aquifer. The low slope angle of sandy beaches contributes to a strongly asymmetric pattern of fluid exchange across the water-sediment interface during tidal cycles. This asymmetry leads to a time-averaged groundwater circulation centered beneath the low-tide level; horizontal and vertical components of motion are of the same order. Horizontal seaward flow extends a significant depth beneath the interface, consonant with a time-averaged water table that is elevated above mean sea level, providing the hydraulic gradient necessary to drive this deeper export of mass equal to the time-averaged inflow by infiltration. This tidally forced flow behavior likely contributes to observed fluctuations in submarine seepage rates. Moreover, the circulation largely involves a subsurface cycling of seawater that infiltrates during the rising tide. Measurements of submarine seepage near the tidal zone therefore likely involve this cycled seawater as well as freshwater flows.