MODELING CHANGES IN THE FLUX AND GRAIN SIZE DISTRIBUTION OF THE BED SEDIMENT IN A LARGE GRAVEL-BED RIVER

Sediment budgets developed for high gradient rivers are subject to large uncertainties in the quantification of source-area and storage terms. Fluxes of the coarse fraction of the supply (bed load) can be constrained from measurements, but the time and effort required to measure these fluxes is prohibitive in most cases. In this presentation, we discuss the development of a sediment routing model that tracks downstream changes in the flux and grain size distribution of the bed load through a 250 km reach of the Colorado River in western Colorado and eastern Utah. The model is formulated for nine reaches where values of discharge, width, reach-average slope and roughness have been measured or verified. The reaches are defined geographically by changes in lithology which affect valley confinement. Bed surface grain size distributions (GSDs) have been measured at 78 locations across the study area, but these distributions are used as a test of the model, not as input, except at the upstream boundary. The model is formulated under the assumption that the coarse sediment load is derived from the bed surface, and transport capacity is determined by local hydraulic conditions and grain size. Estimates of the bankfull bed load transport capacity for each reach are computed for 14 size fractions of the surface sediment, and fractional transport rates are summed to get the bed load transport capacity of that reach. In the adjacent reach, the GSD and fluxes of each size fraction from upstream are used to determine the mean grain size, and fractional transport capacity of that reach. Calculations proceed downstream from one reach to the next, and illustrate linked changes in (1) bed load transport capacity, and (2) grain size distributions of the bed surface. Results show that the model-derived GSDs match the measured GSDs very closely, except for the two most distal reaches where the slope is affected by uplift associated with salt diapirs. The mismatch between modeled and observed GSDs in these reaches occurs because the slope is not self-formed, consequently the model overestimates the local transport capacity in relation to supply. Except for these two reaches, the modeled bed load fluxes seem very reasonable (Q_b ~ 0.5-1.0 kg/m/s at bankfull discharge), and exhibit downstream trends that are consistent with trends reported in two previous studies.