Submitted to: Earth Surface Processes and Landforms
Publication Type: Proceedings
Publication Acceptance Date: 12/16/2003
Publication Date: 1/1/2004
Citation: Erpul, G., Gabriels, D., Norton, L.D. 2004. Wind effects on sediment transport by raindrop-impacted shallow flow. Earth Surface Processes and Landforms. 29:955-967. Interpretive Summary:
Technical Abstract: In the raindrop-impacted sediment transport by shallow waterflow from interrill areas, the basic processes considered are detachment by raindrop impact and transport by runoff. Wind velocity and direction affects not only the energy input of rains on the soil surface but also shallow flow hydraulics by changing the rainsplash trajectories within flow and by introducing reverse / advance tangential shear stress in flow-air interface. A wind-tunnel study under wind-driven rains was conducted to determine the effects of horizontal wind velocity and direction on the raindrop-impacted sediment transport by shallow waterflow. Windless rains and the rains driven by horizontal wind velocities of 6 ms-1, 10 ms-1, and 14 ms-1 were applied to three agricultural soils packed into a 20 by 55 cm soil pan placed both windward and leeward slopes of 7%, 15%, and 20%. During each rainfall application, sediment and runoff samples were collected at 5-min intervals at the bottom edge of the soil pan with wide-mouth bottles and were determined gravimetrically. Each treatment was replicated three times and performed on a smoothed surface to minimize micro-topographic effects and to ensure a broad sheet flow. Based on the interrill erosion mechanics, raindrop impact pressure as a rainfall parameter and boundary shear stress as a flow parameter were used to explain the interactions between impact and flow parameters and sediment transport. Rain intensity was measured by inclined rain gauges, and raindrop impact velocity was estimated from the kinetic energy measurements by a piezoelectric sensor. Flow depth was calculated from the measured discharge and slope along with the Darcy-Weishbach friction coefficient (f). The results of f indicated that the ratios of mean drop size to the flow depth (d/y) were always greater than unity and ranged from 2.71 to 6.37 and from 2.45 to 11.86 for windward and leeward slopes, respectively. Within these ranges, soil detachment by shallow waterflow might be assumed minimal, and a large raindrop effect on detachment might be expected. Conversely, Pearson correlation coefficients between rainfall and flow parameter and the sediment transport rate showed that the raindrop impact pressure had very poor coefficients with the rates in the windward slopes. A significant difference in shallow flow hydraulics occurred with the different aspects, and our flow depth calculation using f resulted in unrealistic values in the windward slopes, where there were reverse rainsplash particle trajectories within flow and tangential shear stress of wind in the flow-air interface. Clearly, a reversal of particle course and wind shear stress induced roughness, which was not accounted by f, and thus deepening the flow depth and decreasing raindrop impact contribution to rainsplash detachment and sediment transport in these slopes. The impact of this research is to provide a basis for developing better erosion control methods.