Linking bed morphology changes of two sediment mixtures to sediment transport predictions in unsteady flows

Abstract

Flume experiments were conducted to measure bed morphology adjustments in sand/gravel and sand/silt sediment mixtures during repeated hydrographs and to link these changes to sediment transport patterns over multiple time scales. Sediment composition and hydrograph flow magnitude greatly influenced channel morphology, which impacted sediment yield, hysteresis, and transport predictions. Bed load yields were larger and more variable for the sand/silt mixture, as gravel in the sand/gravel sediment inhibited grain entrainment, limited bed form growth, and acted to stabilize the bed. Hysteresis patterns varied due to bed form and surface structure adjustments, as well as the stabilizing effect of antecedent low flows. Using half the data set, a dimensionless fractional transport equation was derived based on excess shear stress. Dimensionless reference shear stresses were estimated in two ways: as bulk values from all transport measurements and by applying a separate limb approach in which values were estimated for each limb of each hydrograph. For the other half of the data set, transport predictions with the separate limb approach were more accurate than those from six existing transport equations and the fractional relationship applied with bulk reference shear stresses. Thus, hydrograph limb-dependent dimensionless reference shear stress links changing bed morphology and sediment transport, providing a parameter to improve transport predictions during individual flood events and in unsteady flow regimes. This approach represents a framework with which to develop site-specific transport relationships for varying flow regimes, particularly in cases where detailed bed morphology measurements are not feasible and heterogeneous sediment complicates bed structure over time. Key Points: Bed morphology and sediment transport studied during unsteady flow sequences Separate limb reference shear stresses improved sediment transport predictions Reference shear stress effectively linked transport to changing bed morphology