Corrasion

Abstract: Using Laser Doppler Anemometry we measured current velocities in the median plane around dead lotic macroinvertebrates in a flume which reproduced natural near bottom hydraulics. We investigated specimens of the gastropods Ancylus, Acroloxus, and Potamopyrgus, the amphipod Gammarus, and the larval caddisflies Anabolia, Micrasema, and Silo of various size, various alignment to the flow or which were otherwise manipulated in order to clarify certain questions of adaptation of shape or case building style to flow, or the effects of flow on field distribution patterns. The steepest velocity gradients close to the animals were found near areas of their bodies protruding furthest into the flow. In such regions the rates of potential diffusive exchange processes, the potential corrasion (abrasion through suspended solids), and, for larger specimens, the lift forces (directed towards the water surface) must be highest. Posterior of these areas growing boundary layers formed above those species whose upper contour was approximately parallel to the upstream-downstream direction of the flow. All specimens removed momentum from the flow and thus experience a drag force (directed downstream). From the complete data set we derived the following general conclusions about the physical effects of potential morphological adaptations, taking into consideration diffusion through boundary layers, corrasion, lift forces, friction and pressure drag forces: The physical significance of these five factors generally depends on the Reynolds number of an animal and is largely affected by flow separation, which was significantly related to the ratio of body length to height and the slope of the posterior contour. A simultaneous effective morphological adaptation to all five factors is physically impossible and, in addition, would have to change from life at low (e.g. a young, small specimen of a species) to life at high (e.g. a fully grown specimen of the same species) Reynolds number.

Abstract: Several erosional forms on bedrock at Cantley, Quebec, differ from well-known glacial abrasion forms. The forms consist of obstacle marks, hollows, depressions, and channels, which are defined by sharp rims, smooth inner surfaces, divergent flow features, and remnant ridges. These forms are found on lee, lateral, and overhung rock surfaces. This assemblage of features is best explained by differential erosion produced by separation eddies along lines of reattachment. Rapid, sediment-laden, turbulent, subglacial melt-water flows likely produced the forms by corrasion and cavitation erosion. Sculpted fluvial forms in terrain subject to flooding in Australia are identical to some of the Cantley forms which confirms their formation by water erosion. Although glacial abrasion may not be eliminated as an explanation for sculpted forms, it is not necessary. Ice-abrasion forms, such as striations, and such plucked forms as gouges and crescentic fractures are also present at the Cantley site. Pitted forms, polishing, and carbonate precipitate are also found. The occurrence of abrasion, pitting, polishing, and carbonate precipitate with meltwater forms suggests that the meltwater flows were subglacial. Decoupling of abrading ice from its bed temporarily suspended glacial abrasion, whereas reattachment of ice to the bed may have led to the rounding of sharp edges and the production of striations superposed on the glacifluvial forms. The association of forms produced both by glacifluvial erosion and ice abrasion suggests that the glacier was alternately lifted from, and reattached to, the bed during periodic subglacial floods. These floods may have affected the dynamics of the ice sheet, and depositional sequences related to catastrophic meltwater outbursts probably were laid down in adjacent basins.