Flume Studies of the Reorientation of Pebbles and Shells

Studies in a laboratory flume of the behavior of groups of pebbles and of two types of shell (a mussel, Mytilus edulis, and a gastropod, Nucella lapillus) under conditions of unidirectional flow within the tranquil regime have demonstrated that different types of orientation pattern may be developed, depending largely on the grain size of the bed material and the nature of the flow. Bed materials used in this experiment were medium sand and sandy silt. The effects of continuous flow ("wet" state) and initial flow impact ("dry" state) were both investigated. Reorientation of pebbles (studied only on the sand bed) induced a unimodal, current-normal distribution under both "wet" and "dry" conditions, although the rate of reorientation is much higher in the second case. The orientation patterns for mussels are more complex, strongly unimodal current-parallel distributions characterizing "dry"-state runs on both bed types; whereas bimodal, mutually perpendicular patterns (primary mode current-parallel on sand, current-normal on silt) occur under "wet"-state conditions. Nucella lapillus provides less reliable orientation patterns, but the behavior of this shell type is of interest since, under all conditions investigated, most of the reoriented shells lie with their apices pointing downcurrent, opposite in sense to the associated mussel shells. Under given conditions of flow and bed type, the mean angular response of reoriented pebbles and shells is primarily a function of flow velocity, and increases reciprocally with that property although in the sand-bed runs this relationship is interrupted by a narrow, critical regressive zone which probably corresponds to a bed-form phase change from ripples to dunes. Increasing depth produces a progressive diminution in reorientation response, although the relationship does not appear to be an entirely simple one. Investigation of pebble parameters suggests that, while effective weight has little influence on reorientation Zingg shape class, sphericity, and possibly roundness play some part in pebble response. The geological significance of these experimental results is briefly discussed, with particular reference to current vector analysis.