HR: 15:25h
AN: H53D-08    [Abstracts]
TI: Sand in the cobbles: Laboratory measurements of fine-sediment transport over a coarse and immobile bed
AU: * Grams, P E
EM: grams@cc.usu.edu
AF: Johns Hopkins University, Dept. of Geography & Environmental Engineering 305 Ames Hall, Baltimore, MD 21218 United States
AU: Wilcock, P R
EM: wilcock@jhu.edu
AF: Johns Hopkins University, Dept. of Geography & Environmental Engineering 305 Ames Hall, Baltimore, MD 21218 United States
AU: Wiele, S M
EM: smwiele@usgs.gov
AF: US Geological Survey, 520 N. Park Ave., Tucson, AZ 85719
AB: The transported load in most fluvial systems includes a significant component of fine-grained sediment. Even in gravel- and cobble-bedded rivers, much of the sediment load may consist of sand and finer material that is transported on the bed and in suspension. Thus, the morphology of rivers with coarse, generally immobile beds is largely determined by deposition of fine sediment on the bed and banks. The theoretical and empirical framework for predicting the transport and routing of fine sediment through such rivers is incomplete, making predictions of morphologic change difficult. Existing models for suspended sediment transport focus primarily on sand-covered beds. For a sand bed among coarse immobile grains, adjustments for the effects of fractional sand coverage on the bed and drag exerted by large bed grains are available, but have not been specifically tested. We conducted two sets of laboratory flume experiments with fine-grained suspended transport over large immobile bed grains. The experiments were scaled such that (1) immobile bed particles were much larger than the sediment in transport, but less than 10% of flow depth, (2) transported sediment was in the same size-range as occurs in field settings, and (3) bed shear stresses scaled by the grain size of the transported sediment were also similar to those that occur in the field. The first set of experiments was conducted under conditions of equilibrium transport to evaluate the effect of the near-bed boundary condition on the suspended sediment transport field. In the second set of experiments, non-uniform transport conditions were imposed to examine the migration of sand pulses through a coarse-bedded channel. We measured near-bed sediment concentrations and monitored interstitial sand storage among hemispherical roughness elements for a range of flow and transport rates. Runs with less sand coverage on the bed had higher near-bed sand concentrations compared to runs with greater sand coverage. As sand bed elevation drops among the roughness elements, turbulent wakes shed by the large grains appear to enhance grain entrainment more than the corresponding decrease in bed area covered by sand decreases sand entrainment. These higher concentrations are maintained until the bed is depleted of fine sediment. We also observed that partial filling of interstitial spaces occurred over a narrow range of flow and transport rates, suggesting that a sharp threshold may exist between a bed with no interstitial sand storage and a bed that is completely covered by sand. We are testing these transport thresholds with a morphodynamic model applied to the sand migration observed in the nonuniform transport experiments.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting