HR: 08:45h
AN: H51H-04    [Abstracts]
TI: Response of Bed Surface Patchiness to Reductions in Sediment Supply
AU: * Nelson, P A
EM: pnelson@berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720 United States
AU: Venditti, J G
EM: jgvenditti@yahoo.ca
AF: Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720 United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, CA 94720 United States
AB: In most gravel-bed rivers, and in many gravel-bedded flumes, the bed surface is typically arranged into patchy areas of similar grain size and sorting. Despite the ubiquity of patchiness and its potential importance for benthic habitats and bedload transport calculations, no theory for the development of patches presently exists. Field and flume studies indicate that patches can be distinguished (analogous to bar classification), into `forced patches' and `free patches'. Forced patches arise due to boundary shear stress divergence and cross-stream bed slopes that lead to spatially persistent bed surface textural variations. Free patches are downstream migrating zones of distinct clustering of sediment sizes. Lateral and downstream grain size patchiness has been argued by some to be linked to sediment supply. Here, we investigate the effects of a stepwise reduction in sediment supply on bed surface patchiness using data from two flume experiments in an attempt to better understand the processes responsible for patch development and, ultimately, to guide development of physically realistic bedload flux models. We use small width-to-depth ratio flumes in order to suppress forced patches and focus, instead, on free patches. The first set of experiments was conducted in a 7 m long, 0.3 m wide flume at Tsukuba University, Japan, using a constant discharge, a bimodal sediment, and a width-to-depth ratio of ~3. The second set of experiments was conducted in a larger 30 m long and 0.86 m wide flume at UC Berkeley, using a constant discharge, a unimodal sediment, and a width-to-depth ratio of ~4. For these experiments, we use facies maps of the bed and video of bedload transport to examine patch dynamics in response to changes in sediment supply. We also explore relations between fluctuations in bedload, the water surface, and bed slope and identifiable patches. Because of the low width-to-depth ratios used, bed surface patchiness in both experiments was primarily a longitudinal phenomenon associated with the formation and movement of bedload sheets. Some cross-stream bed surface heterogeneity was evident, however, as coarse zones at the channel margin expanded when the sediment feed was decreased and sediment transport was confined to a relatively fine zone of active transport in the center of the flume. After an extended period with no feed in the UC Berkeley experiments, bedload transport essentially ceased and distinct grain size patches could not be discerned. The use of low width-to-depth ratios highlights the grain-to-grain interactions that may be partially responsible for patch development. In these mobile patches, associated with bedload sheets, we see clear evidence of a `catch and mobilize' interaction between coarse and fine particles, even in unimodal, sand-free sediment.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: Fall Meeting 2005