ARS | Publication request: Grain transport mechanics in shallow overland flow

Submitted to: National Sedimentaton Laboratory (NSL)- 50 Years of Soil & Water Research in a Changing Agricultural Environment
Publication Type: Proceedings/Symposium
Publication Acceptance Date: August 1, 2008
Publication Date: September 30, 2008
Citation: Prasad, S., Suryadevara, M., Romkens, M.J. 2008. Grain transport mechanics in shallow overland flow. National Sedimentaton Laboratory (NSL)- 50 Years of Soil & Water Research in a Changing Agricultural Environment. CD-ROM.

Interpretive Summary: Soil erosion on upland areas consists of many sub-processes, each of which is guided by its own principles and has its own complexities. One of its most vexing problems in terms of quantification is sediment transport in shallow overland flow. Most sediment movement studies have focused on bulk flow in streams and rivers. Sediment movement in shallow flow on upland areas offers special challenges because of the usually super-critical nature of the flow regimes. Our experimental studies have indicated that grain interactions exercise a strong influence on the transport mode of sediment. This article summarizes key aspects of our experimental findings, including the development of a two-layered model in which the relationships between sediment particle velocity and concentration, particle-particle interactions, and particle-boundary impacts are central themes.

Technical Abstract: A physical model based on continuum multiphase flow is described to represent saltating transport of grains in shallow overland flow. 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 which is obtained by the recent work of Eames et al. (2004). The continuum model leads to the result that particle velocity increases with the solid concentration which is rather unexpected but quite interesting. This increase, however, predicts monotonic behavior leading to overestimates of particle velocity. To improve the prediction, therefore, grain dynamic equations which incorporate bed collision are analyzed to lead to a model of saltation height. Incorporation of the results in the continuum model indicates velocity-concentration relationship in the right direction for increasing concentration. Laboratory flume experiments explore the evaluation of various parameters from the measured particle velocities by photonic probes.