Tidal effects on groundwater dynamics in unconfined aquifers

TLDR

Tidal fluctuations are often ignored in regional groundwater studies, yet they can cause a quasi‑steady rise in the mean water‑table that may influence regional flow near shorelines. The study investigates how tidal fluctuations affect groundwater hydraulics by modeling a variably saturated aquifer with realistic beach slope, seepage face, and unsaturated zone. The authors assess the impact of these factors on the aquifer velocity field using a variably saturated numerical model. Simulations reveal that tidal fluctuations markedly alter the velocity field near the exit face, raise the water table, and, when coupled with beach slope and capillary fringe effects, significantly influence groundwater discharge and contaminant migration. © 2001 John Wiley & Sons, Ltd.

Abstract

Abstract The variation of seawater level resulting from tidal fluctuations is usually neglected in regional groundwater flow studies. Although the tidal oscillation is damped near the shoreline, there is a quasi‐steady‐state rise in the mean water‐table position, which may have an influence on regional groundwater flow. In this paper the effects of tidal fluctuations on groundwater hydraulics are investigated using a variably saturated numerical model that includes the effects of a realistic mild beach slope, seepage face and the unsaturated zone. In particular the impact of these factors on the velocity field in the aquifer is assessed. Simulations show that the tidal fluctuation has substantial consequences for the local velocity field in the vicinity of the exit face, which affects the nearshore migration of contaminant in coastal aquifers. An overheight in the water table as a result of the tidal fluctuation is observed and this has a significant effect on groundwater discharge to the sea when the landward boundary condition is a constant water level. The effect of beach slope is very significant and simplifying the problem by considering a vertical beach face causes serious errors in predicting the water‐table position and the groundwater flux. For media with a high effective capillary fringe, the moisture retained above the water table is important in determining the effects of the tidal fluctuations. Copyright © 2001 John Wiley & Sons, Ltd.