THE COMPUTER MODEL SHARP, A QUASI-THREE-DIMENSIONAL FINITE-DIFFERENCE MODEL TO SIMULATE FRESHWATER AND SALTWATER FLOW IN LAYERED COASTAL AQUIFER SYSTEMS

This report documents the quasi-three-dimensional, finite-difference model, SHARP, which simulates freshwater and saltwater flow separated by a sharp interface in layered coastal aquifer systems. The model accommodates multiple aquifers separated by confining layers, with spatially variable porous media properties. The uppermost aquifer can be confined, unconfined or semi-confined with areally distributed recharge. Temporal variations in recharge and pumping are accounted for by multiple pumping periods. The boundary conditions which can be simulated in the model are: prescribed flux boundaries, constant freshwater head and/or constant saltwater head boundaries, and leaky head-dependent boundaries. For each aquifer, the vertically integrated freshwater and saltwater flow equations are solved. These two equations are coupled by the boundary condition at the interface. Leakage between aquifers is calculated by applying Darcy's law. The resulting system of coupled, non-linear partial differential equations is discretized using an implicit finite-difference scheme. The discretized system of equations is solved using the strongly implicit procedure (SIP). The positions of the interface tip and toe, within the discretized finite-difference grid blocks, are tracked using linear extrapolation of the interface elevations calculated at grid points. This documentation includes an overview of saltwater intrusion modeling approaches and the mathematical formulation of SHARP. The model is verified against experimental and analytical solutions, and sample areal and crosssectional applications are presented.

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