Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: January 3, 2006
Publication Date: April 2, 2006
Citation: Romkens, M.J., Rao, S., Prasad, S. 2006. Sediment transport research in shallow overland flow. Federal Interagency Sedimentation Conference Proceedings, April 2-6, 2006, Reno, Nevada. CDROM.
Interpretive Summary: Soil erosion on agricultural land is a multi-faceted phenomenon that involves many processes and properties such as rainfall, surface and subsurface flow, soil and soil profile properties, and soil surface characteristics. Surface flow is one of the most important processes as it determines the rate of detached soil that can be transported downslope. It has always been assumed that, beyond a critical threshold value, the sediment transport rate is in some consistent manner related to the surface flow rate and gradient, e.g., it is hydrodynamically determined. Our experimental findings suggest that sediment transport is a far more complicated process than thus far has been assumed. Sediment transport can be highly organized, non-uniform, time variant, and sediment concentration dependent. That is, sedimentary fluid mechanics, or the interaction between moving particles, determines the mode and rate process. The implications are extremely consequential in terms of the adequacy of our current sediment transport prediction technology. This article discusses the results of a highly controlled sediment transport study that shows the sediment transport limiting effect due to a high degree of particle interaction. It also shows the different bedform models that developed, namely saltation to a strip mode and ultimately to a meandering condition. It also indicates a mathematical analysis of the change in transport mode from saltation to the strip mode.
Hydraulic induced sediment movement is a highly complex process influenced by many factors of a hydraulic and sediment property nature. This process is even more complicated in shallow overland flow, where the hydraulic regime as well as the sediment characteristics can be highly variable. Research to better understand the micro-mechanical nature of sediment movement in shallow flow has been underway for several years. Studies were conducted to examine this process both in the absence (gravity flow) and presence of water. Experiments consisted of measurements of particle velocity and particle concentration on the mode of transport and the corresponding associated transport rates. Results show that the sediment movement is not a random phenomenon but occurs in a highly organized manner ranging from saltation of sediment particles at very low concentrations to movement in a sediment wave like pattern. These waves may transgress into meanders depending on the channel bed conditions and the grain addition rate into the stream. As a consequence, the sediment transport capacity of shallow flow is severely impacted. This paper discusses the experimental findings from a steady state flow regime to which sediment was added at a controlled rate at the upstream of a 7 m long and 10 cm wide channel of about 1º slope steepness. The analytical interpretations are based on a two-phase flow model involving the St. Venant equations of shallow water flow and granular flow.