Rain impact soil crust. I. Formation on a granite derived soil

Abstract

The formation of soil crusts caused by raindrops (rain-impact soil crusts), on a granite-derived aqualf, was studied experimentally with the aid of optical microscopy. Crust development was initiated when rigid 10-50 pm particles were concentrated on the surface by preferential removal of both coarser and finer material in the airsplashing environment. The resulting silt concentrate was spread over the surface by lateral outflow sheets from the drops, forming a closely packed, dilatant, bed-load layer about 0-5 mm thick. This layer strongly resisted deformation by raindrop impacts and also suppressed hydraulic penetration, which tends to enhance loose packing. Compaction of the underlying soil, to a depth of about 5 mm, followed the establishment of the silt layer. This compaction was attributed to the action of stress waves, notably primary waves, propagated when drops impacted the effectively near-rigid surface of the silt layer.