Deposition of sediment from suspension in emergent vegetation

Abstract

Emergent instream vegetation influences the transport and deposition of suspended sediment in rivers, and hence their morphology and nutrient dynamics. An experimental laboratory study has shown how emergent vegetation stems promote sediment deposition. The suspended transport and the extent of longitudinal deposits from suspension within emergent stems is enhanced by increased flow depth and reduced by increased sediment grain size and stem density. The shear zone between longitudinal vegetation strips and adjacent unvegetated channel flow induces diffusion of sediment into the vegetated zone. The transverse extent of the resulting deposit is enhanced by increased flow depth and stem density, and reduced by increased sediment grain size. Values of sediment diffusivity for the two experimental situations were inferred by application of two-dimensional formulations of the diffusion-convection equation. These applications indicate that vertical diffusivity is considerably reduced and transverse diffusivity in the shear zone considerably increased by the stems.