Modeling sediment transport in vegetative areas with a two-phase flow approach

In vegetated areas, water flow and sediment transport are highly influenced by their interactions with vegetation. The induced vegetation drag force reduces the water flow velocity but increases the TKE (turbulent kinetic energy). Recent laboratory experiments of flow within array of rigid cylinders have shown that the fluid bed shear stress, and consequently the sediment transport rate, are correlated with the TKE instead of with the depth-average velocity.

In this context, modeling sediment transport in vegetated areas represents a major challenge for the prediction of the geomorphic evolution of coastlines. In this study, a three-phase flow model for sediment transport in vegetation is presented. The governing equations are obtained from a double averaging procedure: a spatial and turbulence (Favre) averaging. The model is based on the sedFOAM solver, in which the particle phase is represented as a continuum with constitutive laws based on the kinetic theory of granular flows and a turbulence model is required for the fluid phase. The vegetation is represented as a passive phase which interacts with the other phases through a drag force.

First, simulations without sediment are performed and compared with measurements from existing laboratory experiments. The model demonstrates a very good capacity to predict the fluid bed shear stress and the turbulence intensity. The model is also compared with new high-resolution field data (Cook’s beach, New-Zealand). Secondly, sediment transport simulations are performed and compared with laboratory experiments. The results of the model are used to analyze the physics of sediment transport within vegetative regions and to discuss the next necessary steps toward larger-scale modeling.