INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Effect of Upstream Sediment Inflow on the Morphodynamics of Headcuts
Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: January 1, 2010
Publication Date: June 27, 2010
Citation: Wells, R.R., Bennett, S.J., Alonso, C.V. 2010. Effect of upstream sediment inflow on the morphodynamics of headcuts. Federal Interagency Sedimentation Conference Proceedings. CD ROM 2010.
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 upstream sand sized sediment on the development, upstream movement, and soil losses associated with actively migrating gully headcuts in flows typical of upland and agricultural areas. This was accomplished using a laboratory flume specially designed to create actively migrating gully headcuts within soils packed under precisely controlled and quantifiable boundary conditions. The results of the study demonstrated that increased upstream sediment inflow into ephemeral gullies reduced gully dimensions, but did not impact the sediment yield downstream from the gully. Clear-water experiments resulted in steady-state soil erosion wherein a headcut developed and attained a constant rate of migration, shape, and sediment discharge. As the sediment inflow rate of medium sand increased above a certain threshold, the size and migration rate of the headcut decreased markedly, thus arresting local soil erosion. This research demonstrated that gully headcut erosion is influenced by upstream sediment sources and as a result, the development of gully erosion prediction technology should include headcut erosion components that consider upstream sediment sources. As conservationists implement and select appropriate erosion control practices they should carefully consider sediment source effects on the formation and migration of gullies as a result of these practices.
Headcut erosion can severely accelerate soil loss in upland concentrated flows and lead to significant soil degradation in agricultural areas. Previous experimental work has demonstrated that actively migrating headcuts display systematic morphodynamic behavior, and impinging jet theory can provide an excellent theoretical foundation for this erosional phenomenon. This research sought to systematically examine the effect of an upstream sediment inflow on the morphodynamics of actively migrating headcuts in upland concentrated flows. Using a specially-designed experimental facility, actively migrating headcuts were allowed to develop, and then they were subjected to an upstream sediment load composed of sand. As the upstream sediment feed rate increased, the size and migration rate of the headcut decreased markedly, but sediment discharge was less affected. The headcut erosion process was arrested as sediment inflow rate increased above a threshold value. Also, as sediment feed rate increased, the particle size distribution composing the discharged sediment shifted from fine to coarse sediment. This research suggests that headcut erosion can be greatly modulated by an upstream sediment source, emphasizing the need for further enhancements of existing ephemeral-gully models.