Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/16/2003
Publication Date: 8/1/2004
Citation: Erpul, G., Gabriels, D., Norton, L.D. 2004. Wind effects on sediment transport by raindrop-impacted shallow flow: a wind tunnel study. Earth Surface Processes and Landforms. 29:955-967. Interpretive Summary: In rain storms that cause erosion, wind can occur at the same time and quite often at high speeds. In addition to changing the direction of the raindrops, the wind can also change the energy the drops have when they strike the soil surface. We studied this effect in a wind tunnel using wind at different speeds on three agricultural soils, examining erosion when the soil was facing into the wind (windward) and the soil was facing away from the wind (leeward). The effect of wind on the soil erosion by the raindrops was greater on the windward sloping soil because of an increase in the raindrop pressure forces acting on the soil. This research impacts scientists, modelers, soil conservationists and others who need to better understand how sediment is detached and transported by wind and water, better model these physical erosion process, and also need to develop or improve soil conservation practices. This information can assist in building new mathematical model relationships, so that we can improve our predictions of soil erosion and of practices to control this erosion. Knowledge of the affect of windward and leeward locations on soil detachment and transport can also aid in the design and placement of buffer zones, wind breaks, or other erosion control practices.
Technical Abstract: In wind-driven rains, wind velocity and direction is expected to affect not only energy input of rains but also shallow flow hydraulics by changing roughness induced by raindrop impacts with an angle on flow and the rainsplash trajectories of soil particles within flow. A wind-tunnel study under wind-driven rains was conducted to determine the effects of horizontal wind velocity and direction on sediment transport by the raindrop-impacted shallow flow. 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. Based on the interrill erosion mechanics, raindrop impact pressure (T) as a rainfall parameter and product of unit discharge and slope in the form of q*bS*co as a flow parameter were used to explain the interactions between impact and flow parameters and sediment transport (qs). Flow depth was calculated from the measured discharge and slope using the Darcy-Weishbach friction coefficient (f). The results of flow depth indicated that the ratios of mean drop size to the flow depth (d50/y) were always greater than unity and ranged from 2.13 to 6.37 and from 1.89 to 11.86 for windward and leeward slopes, respectively. Although, within these ranges, soil detachment by shallow waterflow was assumed minimal, and a large raindrop effect on detachment was expected, statistical analysis of power law models showed that had much smaller exponent values when compared to those of q and So, indicating that flow parameters better explained the variations in the sediment transport. Further analysis of the Pearson correlation coefficients between T and qSo and qs also showed that wind velocity and direction significantly affected the hydraulic roughness, and f, which didn't account for wind effects on roughness, resulted in unrealistic flow depth calculations in windward slopes where not only reverse within-flow particle trajectories but also reverse lateral raindrop stress with respect to the shallow flow direction occurred.