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Submitted to: International Symposium on Sediment Dynamics and the Hydromorphology of Fluvial Systems Proceedings
Publication Type: Proceedings Publication Acceptance Date: 7/1/2014 Publication Date: 12/1/2014 Citation: Langendoen, E.J., Wells, R.R., Ursic, M.E., Vieira, D.A., Dabney, S.M. 2014. Evaluating sediment transport capacity relationships for use in ephemeral gully erosion models. In: Sediment Dynamics from the Summit to the Sea, Y.J. XU (ed). IAHS, Wallingford, UK pp. 128-133. Interpretive Summary: Observations of gully erosion on cropland have shown that eroding gullies contribute as much as 40% of the sediment delivered to the edge of the field. Tools to quantify or predict gully erosion at the watershed scale are lacking because of the complexity of gully erosion processes. Scientists at the USDA-ARS National Sedimentation Laboratory have evaluated six commonly used sediment transport relations for their capability to estimate sediment transport for the five sediment size classes (clay, silt, sand, small aggregates, and large aggregates) typically used in ARS soil erosion technology. Analysis showed that the transport rate can be satisfactorily predicted for the sand and large aggregate size fractions using transport relationships based on unit stream power theory. However, improved soil detachment relationships for ephemeral gullies are needed to predict the transport of clays, silts and small aggregates. Findings will be used to improve ARS soil erosion prediction technology, such as RUSLE, used by the USDA Natural Resources Conservation Service. Technical Abstract: Ephemeral gully erosion on cropland in the U.S. may contribute up to 40% of the sediment delivered to the edge of the field. Well-tested, physically- and process-based tools for field and watershed scale prediction of gully erosion are lacking, as ephemeral gully erosion processes, often caused by headward migrating headcuts, are complex. Understanding sediment transport capacity downstream of migrating headcuts is essential, as sediment deposition often leads to temporary storage that controls downstream water elevation, which in turn affects the rate of headcut migration. Current process-based gully erosion prediction technology used by the Agricultural Research Service (ARS) is based on characterizing the headcut migration rate, which however requires the deposition depth as input to the model. Alternatively, the deposition depth can be calculated if downstream sediment transport capacity could be predicted. Experimental, laboratory data sets collected at the ARS-National Sedimentation Laboratory were used to test existing sediment transport relationships for the five sediment size classes (clay, silt, sand, small aggregates, and large aggregates) typically used in ARS soil erosion technology. Analysis showed that the transport rate can be satisfactorily predicted for the sand and large aggregate size fractions using common transport relationships based on unit stream power theory. The fractional content of the sand and large aggregate size classes can be computed using standard relationships, which are based on soil texture, previously developed by ARS. The transport of clays, silts and small aggregates is detachment limited, and must therefore be computed using improved soil detachment relationships for ephemeral gullies. |
