3DSALT: A Three-Dimensional Finite Element Model of Density-Dependent Flow and Transport Through Saturated-Unsaturated Media

Abstract : This report presents the user's manual of 3DSALT, A Three-Dimensional Finite Element Model of Density-Dependent Flow and Transport Through Saturated- Unsaturated Media. The model is designed for generic application. The model is developed specifically for solving seawater intrusion problems in subsurface media. It consists of a flow module and a transport module. In comparison to conventional finite element (including both Galerkin and upstream finite elements) or finite difference (including both central and upwind finite differences) models, the transport module of 3DSALT offers several advantages: (a) it completely eliminates numerical oscillation due to advection terms, (b) it can be applied to mesh Peclet numbers ranging from 0 to infinity (conventional finite element or finite difference models typically impose unduly severe restriction on the mesh Peclet number), (c) it can use very large time- step sizes to greatly reduce numerical dispersion (in fact, the larger the time- step, the better the solution with respect to advection transport; the time-step size is limited only by the accuracy requirement with respect to diffusion/ dispersion transport, which is normally not a very severe restriction), and (d) the hybrid Lagrangian-Eulerian finite element approach is always superior to and will never be worse than its corresponding upstream finite element method. Because of these advantages, 3DSALT is ideal for simulating density-dependent flow and advection-dominant transport. Density-dependent flow, Lagrangian- Eulerian approach, Finite element, Saturated-unsaturated flow, Groundwater