INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: A phenomenological study of sediment transport in shallow overland flow
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 6, 2012
Publication Date: March 1, 2013
Citation: Romkens, M.J., Suryadevara, M.R., Prasad, S.N. 2013. A phenomenological study of sediment transport in shallow overland flow. Transactions of the ASABE. 56(2):515-522.
Interpretive Summary: Erosion on upland areas is a highly complex phenomenon consisting of many component processes. One of those is sediment transport by shallow overland flow. Of particular interest to erosion specialists and conservation practitioners is the ability to determine the sediment transport capacity of the surface flow regime. This value determines how much of the detached soil can be delivered by overland flow to the stream system. The capacity term is affected by many factors including sediment parameters such as size, concentration, and the physical, chemical, and mineralogical properties, fluid properties such as rates and velocity, as well as by the soil surface properties and conditions. In this study, sediment transport was studied from a basic mechanistic standpoint under highly controlled, steady state flow regimes. It was noted that at low concentrations sediment movement is by saltation, while at higher volume concentrations sediment moved in sediment waves and in a meander mode. The sediment transport capacity was highest for the saltation mode. A relationship was derived for determining the critical concentration level where the saltation mode transitions into the sediment wave mode. Reasonable agreement was obtained between the experimentally determined and theoretically derived values for the critical concentration.
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 basic laboratory studies to better understand 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. 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, sediment waves in which sediment moved in regularly spaced waves, and a meander mode. The latter two modes were attributed to particle interactions. The transported sediment was continuously collected at the downstream end by a rotating sampler. A curvilinear increase in transport rate was noted with an increase in seeding rate until a critical saltation limit was reached after which a decrease in sediment movement occurred with the formation of organized sediment structures. The small structures were waves with spacings 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 wave mode. A relationship was obtained that described the transition from the saltation mode to the wave mode in terms of a critical solid concentration.