A Phenomenological Study of Sediment Transport in Shallow Overland Flow

Abstract

<abstract> Abstract. 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, size distribution, and concentrations on one hand and the flow regime relative to rates and velocities on the other hand. The effect of all of these factors is influenced 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 to 1400 μm), medium sand (600 to 850 μm), and spherical glass beads (600 to 1000 μm). Measurements included particle velocity 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 of 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.