INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Sediment transport in shallow overland flow
Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: July 25, 2011
Publication Date: September 18, 2011
Citation: Suryadevara, M., Romkens, M.J., Prasad, S. 2011. Sediment transport in shallow overland flow. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). 711P0311cd: Paper 11045. 8 pp. CDROM.
Interpretive Summary: Soil erosion is a complicated phenomenon involving many processes. On the contributing source areas of the uplands, soil detachment and transport by rainfall and surface flow are generally recognized as the major component processes. Among these, sediment transport is perhaps the most difficult to evaluate given the multitude of factors affecting this sub-process, such as the different flow regime characteristics, soil type, and surface conditions. The result has been that the many developed relationships differ in concept, vary in structure, require the calibration of parameters, etc. For several years, the National Sedimentation Laboratory has conducted a series of simple laboratory experiments in which particles were seeded into a controlled flow regime in order to better understand the basic mechanisms involved. Emphasis was placed on the interactions between sediment particles and the flow, among sediment particles, and the particle-conveyance boundary. Three modes of coarse sediment transport were observed: saltation, movement by sediment waves, and a sediment meander. The critical variables measured consisted of the sediment concentration and sediment particle velocity. A model was formulated for coarse sediment movement that consisted of a sediment layer and a sediment-free one. The analysis, based on the continuity and conservation of momentum for this system, yielded an expression of the transition of saltation to the sediment wave mode in terms of the solid concentration, at which point the maximum sediment transport capacity was reached. Furthermore, the sediment velocity-concentration relationship increased initially, then reached a maximum, after which a decrease was noted due to particle interactions. These findings for this rather simple system indicate the highly complicated nature of sediment movement in shallow upland flow.
Soil erosion is a highly complicated phenomenon consisting of many component processes. On upland areas, these processes are usually thought of as detachment and transport of soil particles by rainfall and surface flow. One of the most difficult processes to quantify is sediment transport. This process depends on a host of factors including sediment type, size, and size distribution on one hand and the flow regime relative to rates and concentrations on the other hand. The effect of all of these factors is modulated by soil surface cover and surface roughness conditions. The National Sedimentation Laboratory has in recent years conducted a series of laboratory studies to examine sediment movement in shallow overland flow. These experiments involved super-critical flow regimes in a 7 m. long and 10 cm. wide channel in which sand-size material was seeded at the upstream end at controlled rates in a super-critical flow regime with Froude numbers > 1. Total sediment movement was monitored continuously at the downstream end. Particle sizes were coarse sand (1000-1400 µm), medium sand 600-850 µm), and spherical glass beads (600-1000 µm). Measurements included particle velocity measurements and particle concentrations using photonic probes. Three modes of transport were noted: a saltation mode at low concentrations, a strip mode in which sediment moved in regularly spaced strips, and a meander mode. The latter two modes were attributed to particle interactions. The transported sediment was collected at the downstream end by a rotating sampler. A linear increase in transport rate was noted with an increase in seeding rate until a critical saltation limit was reached after which a decrease occurred with the formation of organized sedimentation structures. The small structures were “stripes” with spacing of the order of magnitude of tens particle diameter while the larger scale “meander” had wavelengths of hundreds of particle diameters. The measured pseudo-equilibrium transport rates were smallest in the meander mode followed by the stripe mode. The highest transport rates were observed in the saltation mode with coarse sand. Measurements were complemented with analytical considerations using the conservation of mass and momentum relationships. The analytical model considered a two-layer consisting of a layer with sediment particles, overlain by a layer of clear water. A relationship was obtained that described the transition from the saltation mode to the strip mode in terms of a critical solid concentration.