Each year the US spends billions of dollars on coastal and river restoration, making it crucial to ensure that projects will be successful. Wetland plants are widely used in coastal restoration because of their important ecosystem services, e.g. providing habitat, recycling nutrients. In addition, by damping waves and currents, vegetation can protect the coast from erosion. However, the details of how vegetation influences sediment transport is not well understood. Most studies show that vegetation reduces erosion, however, some other studies show that under the same flow velocity, vegetation increases erosion.
In this talk, I will discuss the contradictory roles that vegetation plays regarding sediment transport. On the one hand, vegetation slows down the flow and reduces mean bed shear stress. Because net sediment transport rate has historically been parameterized as an increasing function of the mean bed stress, vegetation was expected to reduce erosion. On the other hand, vegetation generates turbulence, which correlates with the fluctuation of the instantaneous bed shear stress. Thus, under conditions with the same velocity, channels with vegetation have higher instantaneous bed stress than channels without vegetation, and as a result, have a higher net sediment transport rate. I will present recent laboratory experiments on sediment transport in vegetated channels and a turbulence-based model for sediment transport. The model can be integrated with large-scale hydrodynamic models for predicting coastal and river channel evolution and for designing coastal and river restoration plans.