Gully erosion and headcut advance

Scope and method of study. The purpose of this study was to examine the forces and erosion processes contributing to gully headcut advance. The formation and movement of gully headcuts are the dominant causes of damage and thus pose the greatest risk to the stability of earth auxiliary spillways. The hydraulic shear stress transferred to the boundary of an overfall was measured in a fixed-bed model using hot-film anemometry. A large-scale test facility was then constructed to measure headcut advance for a variety of hydraulic and placed soil conditions. This facility allowed the observation of headcut movement at field scale.; Findings and conclusions. Prediction equations were developed for the magnitude, location, and variance of stress and pressure forces acting on the horizontal floor and the vertical wall of an overfall. These equations allow estimation of one of the primary forces causing headcut advance. The large-scale test facility allowed examination of the interaction between the attacking hydraulic forces and the resisting soil forces. The advance rate was found to decrease as the average density and average unconfined compressive strength increased. A sand layer at the base of an overfall was found to dramatically increase the rate of headcut movement when the overburden material was erosion resistant. The overfall height and discharge did not have a strong influence on headcut advance over the range of heights and discharges examined. Intermediate backwater levels downstream of the overfall were found to cause the largest headcut advance rates. The stress measurements support this finding.