Mathias J. M. Römkens

Experiments on headcut growth and migration in concentrated flows typical of upland areas

Experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well-developed surface seal that minimized surface soil detachment. During overland flow, soil erosion occurred exclusively at the headcut, and after a short period of time, a steady state condition was reached where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A fourfold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. A sediment bed was deposited downstream of the migrating headcut, and its slope depended weakly on flow discharge.

Extension of the Heaslet‐Alksne Technique to arbitrary soil water diffusivities

Note: 28: 2793-2798 Reference EFLUM-ARTICLE-1992-002 Record created on 2005-09-08, modified on 2017-02-23

Wave formation on a shallow layer of flowing grains

The phenomenon of longitudinal waves in shallow grain flows has been studied through laboratory experiments. The transport process of spherical particles on a metallic chute has been characterized for this purpose. The wave mode of material transport could be measured within selected combinations of flow parameters such as the angular inclination of the chute, the mean size of the grains and the mass flow rate. It has been observed that the moving particles tend to redistribute systematically in the direction of mean flow. As a result, nonlinear longitudinal waves evolve on the surface of the chute. Observations of the predominantly rolling mode of particle motion revealed significant particle dispersion away from the wavefronts. The frequency of inter-particle collisions was low in the dispersed flow regions but increased rapidly near the wavefronts to dissipate the excess kinetic energy, thus resulting in a large increase in the average volumetric solid fraction. In order to explain the appearance of discontinuities in the volumetric solid fraction, a theoretical model that preserves the overall balance of energy and allows a discontinuous periodic solution is examined here. The depth-averaged dispersed flow of the grains has been approximated by equations of motion similar to those of shallow fluid flow. The resistance to the rolling motion of the particles is expressed in terms of the hydrodynamic drag force. The theoretical model predicts the flow criterion for which the longitudinal waves would be self-sustaining.

Sediment Transport in Shallow Overland 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 (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 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.