Modulation of headcut soil erosion in rills due to upstream sediment loads

Abstract

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 that impinging jet theory can provide an excellent analytic foundation for predicting these erosional phenomena. This research sought to examine systematically the effect of an upstream sediment load on the morphodynamics of actively migrating headcuts in upland concentrated flows. Using a specially designed experimental facility, actively migrating headcuts in packed soil beds were allowed to develop freely when subjected to concentrated, clear‐water flows similar to rills, crop furrows, and ephemeral gullies. After such headcut development and without altering the boundary conditions of the experiment, a sediment load composed of relatively coarser grained sand was introduced into the flow well upstream of the erosional feature. At relatively low upstream sediment loadings, the morphodynamics and sediment discharge of the actively migrating headcuts remained unchanged in time and space. As upstream sediment loadings increased, the size and migration rate of the headcuts decreased markedly, yet sediment discharge was not strongly affected as its sediment texture shifted from headcut‐controlled flux to sediment‐feed flux. The headcut erosion process was completely arrested when the upstream sediment feed rate exceeded a specific magnitude. Analytic arguments are used to explain the reaction time and morphodynamic response of an actively migrating headcut to this upstream sediment loading. This research shows that headcut erosion can be strongly modulated by an upstream sediment load, further complicating the prediction of soil erosion on upland areas dissected by sediment‐laden concentrated flows.