ARS | Publication request: Grain transport mechanics in shallow flow

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 2, 2009
Publication Date: August 10, 2009
Citation: Prasad, S., Suryadevarda, M.R., Romkens, M.J. 2009. Grain transport mechanics in shallow flow. Ecohydrology. 2:248-256.

Interpretive Summary: Soil erosion by water is a widespread phenomenon that involves many subcomponent processes, including soil detachment by raindrop impact, surface and subsurface flow, sediment transport, and infiltration, and is impacted by soil type, topography, and a host of surface conditions and management practices. One of the most complicated and least understood sub-processes is sediment transport on shallow overland. The prevalent view is that sediment movement in overland flow can be described by a power function of the excess shear stress or streampower relative to their respective soil characteristics-critical values. While these relationships, if properly calibrated, may be useful from an engineering standpoint, the underlying principles of sediment movement are not well understood, leading to inconsistent and highly variable sediment transport prediction results. This article addresses this issue, also referred to as grain mechanics--that is, how transport is affected by interactions among grain particles and between grain particles and boundaries due to kinetic energy loss by collision. The paper presents experimental observations complemented by an analytical treatise.

Technical Abstract: A physical model based on continuum multiphase flow is described to represent saltating transport of grains in shallow overland flows. The two-phase continuum flow of water and sediment considers coupled St.Venant type equations. The interactive cumulative effect of grains is incorporated by a dispersive stress term. The mean fluid thrust on the particle in the saltation layer of grains is expressed in terms of a slip velocity. The continuum model leads to the unexpected but interesting result that particle velocity increases with the solid concentration. This increase predicts monotonic behavior leading to overestimates of particle velocity. To improve the predictions, grain dynamic equations which incorporate bed collision are analyzed. The analysis leads to an improved model for predicting saltation height. Incorporation of the results in the continuum model yields a velocity-concentration relationship that is consistent with experimental observations for increasing concentration. Laboratory flume experiments explore the evaluation of various parameters from the measured particle velocities by photonic probes.