SPATIAL VARIATIONS IN EXCESS SHEAR STRESS IN A GRAVEL-BED RIVER BEND

Detailed topographic and hydraulic measurements were obtained in a bend of the Colorado River in Rocky Mountain National Park, Colorado, to characterize flow through the reach with a two-dimensional model. When compared with the threshold Shields stress required for particle entrainment (tau(c)*), estimates of Shields stress (tau*) obtained from the model provided a distribution of excess stress through the reach for a range of discharges. Magnitudes of excess stress have been related (elsewhere) to transport phase: ratios of tau*/tau(c)* between 1 and 2 imply partial mobility; tau*/tau(c)* >= 2 suggests a fully mobile bed. This study found that at similar to bankfull flow, areas of excess stress (1 < tau*/tau(c)* < 2, and tau*/tau(c)* >= 2) approach roughly 60% and 10% of the bed, respectively. For the peak discharge observed during the field campaign, areal coverage of excess stress probably increased to approximately 85 and 15%, respectively, suggesting that all of the bed was experiencing at least some degree of sediment mobility. These results suggest that small regions of the channel contribute disproportionately to the overall sediment flux at high flows, that the size of these regions increases with stage, and that this effect amplifies with channel curvature.