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United States Department of Agriculture

Agricultural Research Service

Research Project: SOIL EROSION, SEDIMENT YIELD, CONSERVATION STRUCTURES, AND DSS FOR SUSTAINABLE LAND MANAGEMENT ON SEMIARID RANGELAND WATERSHED

Location: Southwest Watershed Research

Title: Coarse Bed Material Patch Evolution in Low-Order, Ephemeral Channels 2007

Authors
item Yuill, B. -
item Nichols, Mary
item Yager, E. -

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 18, 2010
Publication Date: N/A

Interpretive Summary: Sediment on the channel bed is a primary source of transported material during flash flows in dryland ephemeral channels. Often, channel bed sediment is arranged in patches that are discernable by particle size and texture. A field experiment was conducted to determine whether and how these patches evolve during successive flows. Twelve coarse (gravel-cobble) sediment patches distributed throughout the channel network within a 4.53 ha watershed in southeastern Arizona were monitored for 2 years. Changes in patch area and grain size were measured, and painted patch grains were monitored to confirm that patch grains were mobilized during flow. Individual coarse bed material patches exhibited variable persistence during flows. While no patch fully dispersed during the study period, two new patches formed. Most coarse patches remained relatively stable in area and grain-size distribution. In general, patch grains that were lost through transport were sufficiently replaced with sediment from upstream. These results provide important information for understanding the sources of sediment transported during flows.

Technical Abstract: In river channel beds composed of a wide range of grain sizes, the bed material is often arranged in discrete patches discernable by texture. These patches are the primary source of entrainable coarse sediment within the channel system and their composition and size influence the composition and rate of sediment transport. Twelve coarse (gravel-cobble) sediment patches distributed throughout the channel network within a 4.53 ha watershed in southeastern Arizona were monitored for 2 years. Changes in patch area and grain size were measured and painted patch grains were monitored to confirm that patch grains were mobilized during flow. Individual coarse bed material patches exhibited variable persistence during flows with return frequencies ranging from approximately 1 year to 4.6 years. While no patch fully dispersed during the study period, two new patches formed. Most coarse patches remained relatively stable in area and grain-size distribution despite the entrainment of patch grains as lost grains were sufficiently replaced with sediment from upstream. Because of the grain replacement process and the effect of other sediment supply dynamics, the changes in patch area and grain size distribution display a complex relationship with the magnitude of predicted grain mobilization within each patch. Results indicate that relative stability varies from patch to patch, influenced by the balance of patch grains transported out of the patch and the deposition of new grains into the patch. Predictive models of sediment transport that assume the channel bed is a fixed source of sediment supply may prove erroneous.

Last Modified: 7/25/2014
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