Using a Sponge Layer as an Open-Ocean Boundary Treatment in Barotropic Tide and Storm-Surge Models

In numerical modeling of coastal circulation, a computational domain is truncated at locations away from areas of interest and boundary conditions are prescribed on these boundaries in order to complete the model description. It is important that the boundary conditions accurately introduce the desired information and minimize reflections of any outgoing waves. A widely-used open-ocean boundary condition simply clamps a surface water elevation. Although sufficient to some degree, this boundary condition is a reflecting boundary and could introduce unwanted numerical artifacts. We investigate the use of a sponge layer, which is able to absorb outgoing waves and introduce a prescribed reference solution into a calculation, as an alternative boundary treatment in a barotropic coastal ocean model, in particular, in realistic basin-scale tide and storm-surge applications. The investigation is conducted within the context of a massively-parallel finite-element-based shallow water flow solver. Numerical results demonstrate that the sponge layer treatment provides satisfactory results and improves the robustness of the numerical model significantly.