The Use of an Equivalent Porosity Method to Model Flow in Marshes

by Ian P. King,
Lisa C. Roig,

Document Type: Proceeding Paper

Part of: North American Water and Environment Congress & Destructive Water


Two dimensional modeling of marshes under unsteady flow conditions often involves description of sections that alternately wet and dry. The equivalent porosity method (King and Roig, 1988; Roig, 1989) was originally developed to simulate these types of systems and at the same time avoiding the numerical shocks and mass conservation errors that can occur with the use of other types of flooding and draining algorithms (King et al. 1986). The method is based upon the observation that a finite discretization of an intermittently flooded flow domain is not sufficient to describe the continuous movement of the shoreline between discrete nodes. In the case of smoothly varying bathymetry such as a tidal flat, the shoreline moves dynamically as a function of the local water surface elevation and the bed slope. In marshes and flood plains, the overbank surface may be represented by the elevations of the node points, but subgrid scale bathymetric variations can significantly change the way the flood propagates over that surface. The equivalent porosity approach adopts a nodal wetted area curve that describes the fraction of the nodal area that is wet for different levels of flooding. Thus the shoreline is numerically blurred over the scale of the nodal discretization, but numerical mass conservation is successfully preserved.

Subject Headings: Two-dimensional models | Porosity | Water surface | Shoreline protection | Numerical methods | Floods | Bathymetry

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