Sediment Transport Dynamics around an Emergent Vegetation Patch in Unsteady Flows

While recent efforts have promoted riverine vegetation because it can create habitats, enhance biodiversity, and improve nutrient dynamics and bank stability, its geomorphic impact when located in the main channel is not fully understood. This dissertation investigates the influence of an in-stream emergent vegetation patch on sediment transport rates and channel bed morphology for two sediment mixtures over repeated unsteady flows, with the overall goal being to assess the potential of vegetation patches as in-stream restoration measures. Three related groups of flume experiments were conducted that measured the steady flow hydraulics downstream of a sparse or dense model vegetation patch without a sediment bed, and the sediment transport and morphological adjustment during repeated hydrographs over sand/gravel and sand/silt bed mixtures with and without a patch. Differencing of digital bed elevation models before and after successive hydrographs enabled volumetric bed change calculations and evaluation of morphological adjustment over extended time scales.
Patch density and bed sediment distribution played dominant roles in determining conditions in which a patch was advantageous. For the sand/gravel mixture, the patch created a spatially variable bed surface that decreased entrainment thresholds and increased sediment yields. In the sand/silt mixture, patches reduced sediment yield by contributing to larger bedforms throughout the channel despite excessive scour adjacent to the patch location. Relationships for predicting bed load transport during unsteady flows were developed for no patch, sparse and dense patch conditions using a separate limb approach that captured temporal variability in grain entrainment and transport. These equations predicted reduced transport rates with a patch present relative to reference shear stress values which were generally reduced by the patch. Changes in reach-scale morphology indicated that the channel would become dynamically stable with a sparse patch in either sediment mixture for low flow hydrograph sequences, which is a small subset of the conditions tested. Thus, this research provides a framework to expand vegetation studies through application of unsteady flow regimes that better simulate natural systems, but until a larger range of sediment, patch, and flow conditions are tested, in-channel vegetation patches should be used cautiously in restoration projects.