Sediment Transport in Shallow Overland Flow

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, 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 (Froude numbers > 1). Total sediment movement was monitored continuously at the downstream end. Particle sizes were coarse sand (1000-1400 m), medium sand 600-850 m), and spherical glass beads (600-1000 m). 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 sediment 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.