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United States Department of Agriculture

Agricultural Research Service

Research Project: INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS Title: Effect of soil texture, tailwater height, and pore-water pressure on the morphodynamics of migrating headcuts in upland concentrated flows

Authors
item Wells, Robert
item Bennett, Sean -
item Alonso, Carlos

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 25, 2009
Publication Date: October 1, 2009
Citation: Wells, R.R., Bennet, S.J., Alonso, C.V. 2009. Effect of soil texture, tailwater height, and pore-water pressure on the morphodynamics of migrating headcuts in upland concentrated flows. Earth Surface Processes and Landforms. 34:1867-1877.

Interpretive Summary: Soil erosion and sedimentation by water are major problems that reduce cropland productivity, degrade water quality, and clog water conveyance structures. The objectives of the current study were to determine experimentally the effect of: (1) the amount of sand, silt, and clay in the soil; (2) the depth of water downstream of the channel headcut or knickpoint; and (3) the degree of saturation within the soil mass on the development, upstream movement, and erosion associated with actively migrating headcuts in flows associated with upland and agricultural areas. This was accomplished using an experimental facility specially designed to create actively migrating headcuts within packed soil beds under controlled conditions. The soil texture experiments showed that scour depth varied by a factor of three, while both headcut migration rate and sediment discharge varied by several orders of magnitude. These experimental observations were related to the soil’s erodibility indices (index of the susceptibility of a soil to erode), deduced analytically via standard model calibration procedures. By altering the tailwater height the flow depth within the scour pool was increased, stopping all erosion processes. By manipulating the amount of water within the soil mass, a drier soil bed resulted in lower headcut migration rates and sediment discharges but deeper scour holes, and soil beds with elevated water tables resulted in greater headcut migration rates and sediment discharges but smaller scour holes. These responses were correlated to a reduction (dry case) and an increase (wet case) in the soil’s erodibility index. This information is important to scientists in understanding gully processes in the development of erosion control technology needed to assess the effect of conservation practices on soil erosion.

Technical Abstract: Rill and gully erosion in upland and agricultural areas can result in significant soil degradation worldwide, and headcuts are the primary mechanism by which this landscape dissection occurs. Experiments were conducted to further examine the morphodynamic behavior of actively migrating headcuts in upland concentrated flows with varying boundary conditions. The effect of soil texture greatly modified the erodibility of the select soils, and headcut migration rates, scour depths, and sediment discharges all correlated to these erosion indices. The effect of increasing tailwater height greatly reduced the nappe entry angle at the brink of the headcut, completely arresting the soil erosion processes. Altering subsurface pore-water pressures markedly changed the erodibility coefficients of the select soil, which caused greater headcut migration rates and sediment discharges, yet shallower scour holes, in the presence of a subsurface water table. Current analytic formulations based on jet impingement theory successfully predicted these experimental observations, further demonstrating the utility of such analytic formulations in advancing watershed management technology.

Last Modified: 12/20/2014
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