Parallelization Of The Wash123D Code-Phase I: 2-Dimensional Overland And 3-Dimensional Subsurface Flows

The parallel WASH123D, which is supported by the DoD CHSSI (Department of Defense Common High Performance Computing Software Support Initiative), is designed to solve watershed problems on scalable computing systems. WASH123D is a first-principle, physics-based model to compute water flow and/or contaminant and sediment transport within a watershed system. In the WASH123D model, a watershed is conceptualized as a coupled system of one-dimensional (1-D) channel network, two-dimensional (2-D) overland regime, and three-dimensional (3-D) subsurface media. It aims to address the environmental issues concerning both water quantity and quality. To reach numerical solutions with reasonable and tolerable computer time for simulations that embrace large meshes, numerical algorithm improvement and code parallelization are two essential tasks. Mathematically, 1-D channel flow and 2-D overland flow are described with the St. Venant equations, which are solved with either the Semi-Lagrangian or the Eulerian finite element method. The 3-D subsurface flow is governed by the modified Richards equation, which is solved with the Eulerian finite element method. The contaminant transport and sediment transport equations, which are solved with the Lagrangian-Eulerian finite element method, are derived based on the mass conservation principle. A parallel in-element particle-tracking algorithm for unsteady flow is applied to backtrack fictitious particles from global nodes to determine the so-called Lagrangian values when the Semi-Lagrangian or the Lagrangian-Eulerian method is used. This paper addresses the parallelization of such a complex numerical model. In phase I, tasks including data structure and software design, software tool development, as well as tool integration are accomplished. The 2- and 3-D flow modules are expected to be employed in the production stage. © 2004 Elsevier B.V