HR: 09:00h
AN: H41G-05    [Abstracts]
TI: The Effect of Large Roughness Elements on Local Flow and Bedload Transport
AU: * Yager, E
EM: yager@seismo.berkelely.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720
AU: Schmeeckle, M
AF: Department of Geography, Arizona State University, Tempe, AZ 85287
AU: Dietrich, W E
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720
AU: Kirchner, J W
AF: Department of Earth and Planetary Science, University of California, Berkeley, CA 94720
AB: Most mountainous drainage networks contain large roughness elements (boulders, large woody debris) that alter reach-scale flow and sediment transport. The obstacles create large spatial deviations in the flow shear, so bedload transport equations based on the total boundary shear stress do not apply in these channels. Thus it is difficult to determine, for a given roughness configuration, the effect of sediment supply on channel morphology. We have previously developed a transport equation that partitions the total shear stress between immobile bed elements and the finer, more mobile sediment. This approach improves predictions of sediment flux, but does not explain the mechanics of sediment transport around large obstacles. We investigated the mechanics of sediment transport in a flume set at a constant gradient and water discharge. Sand was transported over two beds: mobile sand grains and regular arrays of immobile spheres. A high-speed video camera, mounted above the flume, recorded the variation in transport rate around the spheres. Particle imaging velocimetry (PIV) was used to determine 2-D velocity fields (downstream and vertical) around the roughness elements. The large spherical particles generated a horseshoe vortex flow pattern with downward flow along the lower half of the front and side of the particle. Turbulence structures in the main flow (probably sweep events) intermittently interacted with this vortex pattern to produce high near-bed downstream velocities that generated the bulk of the transport. Preliminary results show that the downstream transport rate varied by an order of magnitude from immediately adjacent (123 grains/cm/s) to halfway between the immobile grains (14 grains/cm/s). This large variation in sediment flux and shear stress led to significant morphologic changes; scour holes and sand deposits developed adjacent to and midway between the immobile elements, respectively. Furthermore, the immobile grains increased local stresses and therefore caused sediment transport that did not occur on a bed of only sand. In contrast, previous steeper gradient (10%) experiments showed that large grains trap sediment that would otherwise be highly mobile. In these very shallow and steep flows, the total boundary shear stress was much greater than the critical stress of the mobile fraction. Thus, the effect of roughness elements on sediment transport and bed morphology may vary depending on their position in the channel network. We are currently testing our stress partitioning model and the correlation between local sediment flux and stress.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting