Author(s): Ashis Kumar Dey; Tetsuro Tsujimoto; Tadanori Kitamura
Keywords: Headcut; migration rate; non-cohesive soil; stratified soil; plunge-pool erosion; conceptual model; sediment transport
Abstract: Flume experiments were conducted to characterize the headcut erosion process on low bed slope landforms such as floodplain. Two different series of experiments were performed to investigate the headcut erosion and migration phenomenon both in homogeneous and heterogeneous non-cohesive soil. The flume bed was prepared with homogeneous coarse sand of uniform diameter for the first series and a composite fill of coarse sand and fine particles of polyvinyl chloride for the second series. Adhesive spray was applied on the top surface, which resisted the surface erosion and ensured the development of headcut. The effects of hydraulic and geometric conditions on headcut migration rate were analyzed. When all the detached sediments resulting from the erosion process were transported easily far downstream as wash load, then the overland flow rate and the headcut height governed the plunge-pool erosion process and the headcut migration rate as well (second series of experiments). In contrast, when the detached sediment resulting from the erosion process was transported as bed-material load, then the migration rate was governed not only by the upstream parameters but also by the downstream bed morphology (first series of experiments). The experimental results of both series showed that the shape of the plunge-pool remained unchanged with time for a given flow discharge. For a given flow rate and initial headcut-height, variable migration rate was observed in the first series of experiments, whereas the second series of experiments showed a constant migration rate. Conceptual models were proposed to explain the details of headcut migration process for both cases. In addition to overland flow rate and headcut height, the sediment transport capacity as well as the bed slope downstream of headcut was found to be a significant parameter that controlled the migration rate when the detached sediment was transported as bed-material load. The plunge-pool scour-rate that is governed by the overland flow rate, headcut-height and the properties of bed material controlled the migration rate when the detached sediment was transported as wash load. The migration rates calculated using conceptual models reasonably agreed with the experimental data for both cases.