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ARS Home » Southeast Area » Oxford, Mississippi » National Sedimentation Laboratory » Watershed Physical Processes Research » Research » Publications at this Location » Publication #225142
Title: Measurements and analysis of sediment transport capacity in shallow overland flow

Author
item PRASAD, S. N. - UNIV. OF MISSISSIPPI
item SURYADEVARA, M. R. - UNIV. OF MISSISSIPPI
item Romkens, Mathias

Submitted to: International Soil Conservation Organization (ISCO)
Publication Type: Proceedings
Publication Acceptance Date: 4/15/2008
Publication Date: 5/18/2008
Citation: Prasad, S., Suryadevara, M., Romkens, M.J. 2008. Measurements and analysis of sediment transport capacity in shallow overland flow. International Soil Conservation Organization Conference Proceedings. May 18-23, 2008, Budapest, Hungary. CD-ROM.

Interpretive Summary: Sediment movement on upland areas is a highly complex process influenced by many factors of a hydraulic, surface topography, and sediment property nature. For several years, research has been underway to better understand the micro-mechanical nature of sediment movement in shallow flow. Experiments have been conducted that consist 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 grain addition rate into the stream. Sediment measurements indicate a sudden loss in transport capacity during the transition from saltation to bed configurations. This paper discusses the experimental findings from a steady state flow regime to which sorted sediment was added at a controlled rate at the upstream of a 7 m long and 10 cm wide channel of about 1 degree slope steepness. The experiments were complemented with an analysis of two-layered flow regime using the continuity and momentum equations. The bottom layer consists of sediment-laden flow, the top layer of clear water. An expression was derived for the critical concentration at which sediment waves, that have formed following saltation, transition into a meander. Reasonable agreement was obtained between predicted and observed solid concentrations.

Technical Abstract: Investigations on sediment transport in laboratory flume reveal a significant decrease in transport capacity when bed sediment conditions exhibit structured modes such as stripes and meanders. This decrease is 20% for the stripe mode and as much as 94% in large-scale modes such as meanders. Optical probe measurements of solid concentrations reveal the characteristics of transport modes such as saltation, stripes, and meanders. Sediment transport rates evaluated from these measurements agree with the transport rates measured by grab samples obtained at the downstream end using a rotating sampler. The power spectra of solid concentration fluctuations are in reasonable agreement with the transport rate measurements. Moreover, these spectra of solid concentrations fingerprint the intrinsic nature of bed patterns, which are not easily identifiable by transport rate measurements. A simple analysis on the trajectory motion of grains in flight revealed a condition for the onset of grain clustering as a function of the saltation mean free path, which is a key parameter for the study of sediment transport in general.