Coupling Fluvial-Hydraulic Models to Study the Effects of Vegetation on Sediment Transport and Flow Dynamics in the South Platte River, Colorado

This study investigated the effects of riparian vegetation on sediment transport rates and flow dynamics in the South Platte River just downstream of Fort Lupton, Colorado. FaSTMECH, a two-dimensional coupled fluvial and hydraulic model, was used to compute flow characteristics (velocity and depth) in addition to sediment mobility characteristics (shear stress and sediment flux) for four discharge levels ranging from 5% of bankfull flow to bankfull flow (Qbf). Estimates of a dimensionless drag coefficient (Cd) representative of the middle-aged bushy willows found on the river banks at the study site were used to create a spatially variable roughness in the model throughout the river reach. Model results show that during average annual flood events, vegetation on the river banks causes increased drag forces on the flow, leading to an increased proportion of flow being diverted into the main channel and resulting in higher velocities. The spatial distribution of shear stresses collapse under these conditions with an order of magnitude decreases over river banks and significant increases throughout the main channel. Sediment fluxes in the reach increase by nearly an order of magnitude with the presence of bank vegetation, however, the greatest differences occur during Qbf when the highest fraction of the sediment is mobile. Further analysis of vegetation effects was conducted by performing a sensitivity analysis by altering the representative non-dimensional vegetation drag coefficient by as much as +/- 400%. These alterations represent differences in vegetation density, height, orientation, leafy/leafless structure, age, rigidity, and vegetation type. Although there is a relationship between sediment fluxes and changes in Cd, there only exists a 14% increase in transport at Qbf between the two exterior limits of Cd