USGS SHARP Model

In many coastal settings, aquifer systems consist of sequences of layers with varying hydraulic properties. An idealized cross section through a layered coastal aquifer system extending offshore to a submarine canyon outcrop is shown in Figure 8.1. Under natural, undisturbed conditions an equilibrium seaward hydraulic gradient exists within each aquifer, with excess freshwater discharging to the sea (Figure 8.1a). In the uppermost, unconfined aquifer the freshwater flows out to sea across the ocean floor. In the lower, confined aquifers the freshwater discharges to the sea by leaking upward through the overlying layers and/or by flowing out the canyon outcrop. Within each layer a wedge-shaped body of denser seawater will develop beneath the lighter freshwater.