Effect of slope on the growth and migration of headcuts in rills

Experiments were conducted to examine soil erosion by headcut development and upstream migration in rills typical of upland areas. Soil material, simulated rain, overland flow discharge, and initial headcut height were held constant in each experiment, but initial slope of the bed varied from 1% to 10%. Air-dried, crushed, and sieved sandy loam to sandy clay loam soil was incrementally packed into a laboratory channel 2 m long and 0.165 m wide to a depth of 0.25 m. Soil bulk density was 1425 kg m(-3) in each experiment. A pre-formed headcut 25-mm high was constructed 1.5 m downstream of the entrance of the flume. Simulated rain, applied at 21 mm h(-1) for 4 h, produced a well-developed surface seal that minimized the detachment of the surface soil. Following the rainfall, overland flow at a rate of 52 1 min(-1) was released onto the bed, soil erosion occurred at the pre-formed headcut overfall, and a scour hole developed, enlarged, and migrated upstream. The rate of headcut migration was constant within each experiment, but higher slopes of the bed generally resulted in lower rates of migration. At slopes on the bed of 2% and smaller, the overfall nappe at the headcut brinkpoint remained submerged, and a steady-state condition was achieved: sediment yield and geometry of the scour hole remained constant as the headcut migrated upstream. For slopes on the bed of 3% and greater, the overfall nappe became aerated, and as the headcut migrated upstream, the depth of scour increased. Higher slopes on the bed resulted in deeper scour holes. Mechanisms of soil erosion included the formation of tension cracks and seal removal at the headcut brinkpoint, soil washout along the aerated headcut face, and plunge-pool scour. The slope of the sediment deposit downstream of the migrating headcut was 2.2% for all experiments, and suggests that flow discharge and not the initial slope of the bed controlled transport capacity and downstream adjustment of the bed. Published by Elsevier Science B.V.