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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 Title: Patterns of grain-size dependent sediment tranpsort in low-ordered, ephemeral-channels

Authors
item Yuill, B -
item Nichols, Mary

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 19, 2010
Publication Date: July 30, 2010
Citation: Yuill, B., Nichols, M.H. 2010. Patterns of grain-size dependent sediment tranpsort in low-ordered, ephemeral-channels. Earth Surface Processes and Landforms. p. 1-13.

Interpretive Summary: Sediment is a primary source of water pollution in surface water in Arizona. Sediment samples are collected on the Walnut Gulch Experimental Watershed in southeastern Arizona to further our understanding of sedimentation processes. Sediment particles travel in differing modes depending on their distribution, and source, and characteristics of runoff. Sediment data were collected during 37 runoff events and were analyzed to determine grain-size dependant factors affecting sediment transport in a low ordered, ephemeral watershed. Two distinct grain-size dependent transport patterns were observed, that of the finer grain-size fraction (< 0.25 mm) and that of a coarser grain-size fraction (= 0.25 mm). The concentration of the fine fraction was correlated to flow duration, peaking near the beginning of a flow event and declining thereafter. The concentration of the fine fraction showed a slight trend with seasonal and recovery period. These differences based on grain size are important because many models of sediment transport are based on a single grain size to represent sediment characteristics. This may lead to erroneous estimates of sediment loads, especially when a large range of sediment grain sizes are present.

Technical Abstract: Sediment data were analyzed to determine grain-size dependant factors affecting sediment transport in a low ordered, ephemeral watershed. Sediment was collected during 37 flow events at the outlet of a 4.53 ha sub-watershed within the Walnut Gulch Experimental Watershed in southeastern Arizona. Measured concentrations ranged from 262 to 90 275 mg L-1 and included grain sizes up to 8.0 mm in diameter. Two distinct grain-size dependent transport patterns were observed, that of the finer grain-size fraction (< 0.25 mm) and that of a coarser grain-size fraction (= 0.25 mm). The concentration of the fine fraction was correlated to flow duration, peaking near the beginning of a flow event and declining thereafter. The concentration of the fine fraction showed a slight trend with seasonal and recovery period. The concentration of the coarse fraction was negatively correlated with instantaneous discharge and was not correlated with event duration. The majority of the sediment yield of the fine fraction was typically transported out of the watershed before the hydrograph peak while the majority of the yield of coarser fraction was delivered after the hydrograph peak. Grain-size dependent transport patterns were likely dependent on sediment source, with the fines contributed by the hillslopes and coarse sediment being supplied from the channel bed material. Because transport patterns differ based on grain size, attempts to define the total sediment yield based on a single transport variable may lead to erroneous results, especially when a large range of sediment grain sizes are present.

Last Modified: 11/25/2014
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