Submitted to: Journal of Fluid Mechanics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 16, 2000
Publication Date: N/A
Interpretive Summary: Soil erosion is a complex phenomenon involving many component processes, such as soil detachment and transport by rainfall and overland flow, infiltration, seepage, etc., and many soil and soil surface conditions. One of the most important processes is transport of soil from the point of detachment to the place of deposition. There are numerous sediment transport relationships, most of them were derived for large flows in channel systems with low velocities. On upland areas, sediment transport involves shallow flow and high velocities. The existing transport relationships are not applicable for those situations. Yet, they are being used in erosion and runoff models for lack of better ones. This article is the first report of a fundamental study which attempts to better understand the mechanism of sediment transport in shallow flow. The approach taken is to simulate and model shallow flow of granular materials on a chute and to follow the development of longitudinal organization of the flow as it moves down the chute. It was observed that the transport process follows a wave mode action, in which the particles went through a process of acceleration and deceleration as the wave front passes. The model yields criteria for which these wave actions occur. It is thought that this flow mechanism is similar to that of shallow liquid flows as is often observed in the form of rolling waves on sloping surfaces with shallow flow. The information obtained in this and successive experiments will be very helpful in arriving at sediment transport relationships which are far more reflective of conditions as they occur on upland areas.
The phenomenon of longitudinal self-organization in shallow flows of granular material has been studied through experiments and theoretical modeling. The unsteady transport process of spherical glass beads on a metallic chute has been characterized for this purpose. The wave mode of the transport process could be distinctively observed and measured within selected combinations of flow parameters such as the angular inclination of the chute, the mass flow rate of the granular solid and the mean size of the particles. It has been observed that a homogeneous mixture of the micro-scale spherical particles (in size ranges between 88-250 microns) tends to redistribute itself systematically in the direction of the mean flow. As a result of this organization, nonlinear longitudinal waves evolve in the flow field so that the transport of the spherical solids is dictated by the periodic movement of a series of progressive waveforms. Though the evolution of the periodic structures is sensitive to narrow ranges of the flow parameters, the formation and subsequent transport of these structures are very robust. In order to explain the occurrence of such wave-like transport process, a physical model of the two-phase flow has been proposed. The nominally two- dimensional shallow flow of the granular solids down the inclined plane has been approximated by the equations of motion of shallow channel flow. The resistance to the rolling motion of the spherical particles has been expressed in terms of the aerodynamic drag force. The theoretical model predicts the flow criteria for which the wave-like longitudinal structures would be self-sustaining. The predicted wave- velocities under different flow conditions agree within the same order