Submitted to: Transactions of the ASAE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/22/2001
Publication Date: 8/1/2002
Citation: Erpul, G., Norton, L.D., Gabriels, D. The effect of wind on physical raindrop impact and rainsplash detachment. Transactions of the American Society of Agricultural Engineers. 2002. 45(6):51-62. Interpretive Summary: Wind and water erosion processes have traditionally been separately studied, and separate models were developed to predict soil erosion under either wind or rain. In nature, erosive rainstorms are usually associated with high winds, e.g., convective storms. Hence, erosion under wind-driven rain is a process that cannot be ignored in erosion research and prediction model development. A quantification of wind and rain interactions and the wind-driven rain effects on sediment detachment and transport bridges the knowledge gap. In a wind tunnel facility equipped with a rainfall simulator, we studied the effects of wind velocity on the raindrop size distribution, rain impact angle and energy, air-splash and surface-flow transport of eroded sediments on windward and leeward slopes. Wind not only increased the raindrop impact velocity but also altered the angle of raindrop incidence. Slope aspect relative to the wind direction, i.e., windward or leeward, caused significant differences in both air splash and surface-flow sediment transport. On a leeward slope, as the overland flow generation is reduced due to the lowered rain interception, the wind-driven soil particle transport becomes a dominant process. The results can be used to improve the understanding of erosion processes and provide a better estimate of soil detachment and transport under wind-driven rain. The impact will be mainly on the erosion research community, as improved scientific understanding on wind-driven rain processes will eventually lead to better erosion estimates.
Technical Abstract: In wind-driven rains, variations in raindrop trajectory and frequency are highly expected due to the changes in the angle of raindrop incidence. This paper presents experimental data obtained on the effects of horizontal wind velocity on physical raindrop impact and rainsplash detachment. In a wind tunnel facility equipped with a rainfall simulator, windless rains and the rains driven by horizontal wind velocities of 6, 10, and 14 ms-1 were applied to three agricultural soils packed into 20 by 55 cm soil pan at both windward and leeward slopes of 7, 15, and 20%. Rain intensity was directly measured with inclined raingauges with respect to the prevailing wind direction. These measurements showed that the actual amount of rainfall intercepted on the soil surface highly varied depending on the angle of rain incidence, which was a function of the rain inclination and slope gradient and aspect. A two-dimensional numerical model was used to estimate wind- driven raindrop trajectories, considering forces that act on a raindrop falling through a wind field. Rain energy was also measured by a kinetic energy sensor. Theory and measurement showed that an exponential relationship existed between the energy of simulated rainfall and the applied horizontal wind velocity. The experiments yielded results, which led to the conclusion that the wind not only increased the raindrop resultant impact velocity but also altered the angle of raindrop incidence, resulting in variable raindrop impact frequency and impact angle. Accordingly, differential rainsplash detachment occurred depending on the changes in raindrop trajectory and frequency with wind velocity and direction.