Submitted to: International Symposium on River Sedimentation
Publication Type: Proceedings
Publication Acceptance Date: 4/15/2007
Publication Date: 8/1/2007
Citation: Rao, S.M., Prasad, S.N., Romkens, M.J. 2007. Post Saltation Sediment Transport by Shallow Super Critical Flows. International Symposium on River Sedimentation, Moscow, Russia, Aug. 1-4, 2007. CD-ROM Interpretive Summary: Sediment movement in shallow overland flow is one of the many sub-processes of soil erosion on upland areas and the main mechanism by which detailed soil particles move offsite into the stream system and surface water bodies. The capacity of excess rain to transport soil as sediment in surface runoff depends on many factors, such as flow rate, flow velocity, the amount of available soil, its particle size characteristics, and soil surface characteristics. In spite of the importance of sediment transport processes on upland soil erosion, only limited efforts have been made to understand the physics of these processes and to arrive at proven and tested relationships for the surface flow transport capacity. This paper summarizes the results of ongoing studies to describe sediment movement in shallow overland flow in order to address these shortcomings. Laboratory experiments have shown that sediment movement in overland flow is not a random phenomenon but occurs in a highly organized manner. At very low sediment concentrations sediment particles move in a saltation mode which, at higher concentrations, transitions into waves and subsequently into a meander of continuous sediment. The data indicates that the maximum transport capacity occurs at the moment that the saltation mode transitions into a sediment wave mode.
Technical Abstract: Laboratory flume experiments of shallow overland flow show that the sediment transport is not a random phenomenon but occurs in a highly organized manner. The organization reveals evolution of bed covered by sediments with several scales ranging from saltation of particles at very low concentrations to movement in wave forms. The wave modes emerge in post-saltation state and the sediment transport capacity of shallow flows is severely impacted. These wave modes consist of stripes, meanders and alternate bars. The temporal evolution of transport rates in the equilibrium stripe mode indicates smaller time scales (~10 sec) as compared to (~10 min) the large spatial scale structures such as meanders. Power spectra of transport rate fluctuations characterize the stripe mode with multiple peaks at different frequencies.