Movement of solids in air and water by raindrop impact. effects of drop-size and water-depth variations

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Abstract
Transportation of 200 pm sand grains by impacting raindrops in both air and water was experimentally investigated through the water depth range 0-75 mm, employing four drop sizes. Airsplash movement, greatest for large drops impacting bare sand, fell rapidly with decreasing dropsize and increasing water depth. Shallow surface water inhibited airsplash strongly, yields falling by large factors through the depth range 0-2 mm. Hence, airsplashing of solids, even if randomly directed from any one point, must produce net migration from bare to water-covered surfaces. Drops over about 1.5 mm in diameter disrupt on impacting water at terminal velocity, generating disturbances which can, if water is shallow, suspend soil particles. Smaller drops evidently penetrate water surfaces without disruption, disturbing soil particles more directly and efficiently. If target water flows, even very slowly on low slopes, net downstream particle migration (rain-flow transportation) results. Rain-flow transportation rates rose from zero at zero depth to near-maximum values at 2 mm. Actual maxima were reached at depths of 2-3 drop diameters, transport rates declining rapidly with further depth increases. The combination of airsplash and rain-flow transportation extends transportation to areas where overland flow is either absent or too weak, acting alone, to move solids significantly.

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