PNNL

Abstract

The landforms of salt marsh systems are rather complex, with meandering channel networks cutting through low-lying, extensive marsh platforms. Some small-scale topographic details are critical to flooding and draining processes but cannot be reflected in the Digital Elevation Model (DEM) due to limited measurement resolution and vegetation bias. A numerical model needs a grid with high enough resolution to resolve the small-scale connectivity to correctly model the small-scale flow processes, making the model computationally unaffordable. In this study, we derived a subgrid model, which couples two flow components, one is the regular subgrid model for simulating the subgrid-resolved flows, and the other is a conductivity-formulated porous flow model for modeling small-scale gut flows unresolved at the subgrid level. The model validation was performed in an idealized case with a narrow slot unresolved in the coupled flow model. Model results were compared with a slot-resolved subgrid model and suggested that, with a proper model parameterization, the coupled flow model is capable of reproducing flooding and draining processes similar to the slot-resolved subgrid model. The model was applied to simulating flooding and draining processes in a meso-tidal salt marsh. The model results demonstrate that the coupled flow model can be used to reduce the impacts of artificial ponding which usually occur in a numerical simulation of salt marshes with numerous narrow gullies and creeks.