ARS | Publication request: Turbulent Flow Within Headcut Scour Holes in Rills and Gullies

Submitted to: International Symposium on Gully Erosion
Publication Type: Proceedings
Publication Acceptance Date: April 10, 2007
Publication Date: September 17, 2007
Citation: Bennett, S.J., Alonso, C.V. 2007. Turbulent Flow Within Headcut Scour Holes in Rills and Gullies. Proceedings of the IV International Symposium on Gully Erosion. September 17-19, 2007, Pamplona, Spain, J. Casali and R. Gimenez (eds.). Public University of Navarre. pp. 18-19.

Interpretive Summary: On hillslopes and agricultural fields, soil erosion occurs in areas of concentrated flow such as rills, crop furrows, and gullies. Within these relatively small channels, localized erosion often occurs due to the development and upstream migration of headcuts, which are abrupt step-changes in bed elevation. The development and migration of headcuts can significantly increase soil losses and sediment yields. Recent analytical models for headcut erosion in soils treat the eroding flow at the headcut brinkpoint and in the scour hole domain as a sequence of impinging and associated wall jets that act on the erodible boundary. However, no study has yet confirmed that a jet impingement model is the correct analog for headcut scour holes, and no study has sought to collect the velocity, turbulence, and pressure fields within headcut scour holes and compare these directly to existing experimental and theoretical characterizations of plane turbulent impinging jets. The overall goals of this research program were to fill this knowledge gap and to assess the applicability and limits of jet impingement theory to soil erosion in upland concentrated flows. The data required to achieve these goals would be impossible tocollect in live-bed experiments because: (1) sediment-laden flows preclude the use of available velocity sensors; (2) the rate of change of headcut morphology from initial growth to steady-state is quite rapid; and (3) any probe inserted into the flow would disrupt the flow field and thus the observed soil erosion and headcut behavior. Thus the present study utilized twodimensional, rigid-boundary conditions replicated from live-bed experiments. Results from this study support the use and application of a jet impingement approach for modeling flow and soil erosion in upland concentrated flows due to headcut development and migration.

Technical Abstract: Soil erosion remains the principle cause of soil degradation worldwide, and the development and migration of headcuts in rills, crop furrows, and gullies can significantly increase soil losses on hillslopes, upland areas, and agricultural fields. Experiments were conducted to define the time-mean turbulent flow characteristics within fixed headcut scour holes typical of upland concentrated flows and to assess the distribution of these flow and pressure parameters for discrete areas of the scour hole domain. These data show that: (1) flow within headcut scour holes is analogous to plane turbulent reattached wall jets; (2) turbulence maxima are associated with the jet entry, recirculation eddies, and flow reattachment; (3) turbulent velocities are distributed asymmetrically about the free jet axis within the scour hole; (4) turbulent velocities associated with the reattached wall jet display good similarity collapse when scaled with the jet entry velocity; and (5) distributions of wall pressure near reattachment agree well with a similarity argument derived for impinging jets. This study supports the use and application of a jet impingement approach for modeling flow and soil erosion in upland concentrated flows due to headcut development and migration. Moreover, it is suggested that velocity-gradient shear, turbulent shear, and near-bed pressure gradients all are involved in soil erosion within headcut scour holes.