Groundwater-level variation during semidiurnal spring tidal cycles on a sandy beach

Abstract

Observations of two-dimensional variations in groundwater level on beach profiles at South Beach, Wollongong, support and extend observations previously reported. Time-series curves showing water-level change at individual wells along the profiles are markedly asymmetrical and their ranges of oscillation are dependent on tidal range and distance landward of the beach face. The asymmetry is attributed to filtering processes at the beach face and in the beach, that separate the various tidal constituents. Tidally induced groundwater changes are superimposed on a three-dimensional water-table surface that is tied to the beach morphology and to groundwater recharge from the backshore zone. The three-dimensional flow net is raised and lowered as the tide rises and falls. although with some time lag between change in level of ocean water and groundwater response. The groundwater rise begins earliest in shoreline embayments and spreads landwards and outwards to higher water-table surfaces near shoreline salients and in the backshore zone. Groundwater responses, therefore. differ on the salients and in the embayments: a landward water-table slope prevails in the embayments and a seaward slope characterizes the salients.