The influence of articulated and disarticulated cockle shells on the erosion of a cohesive bed

Quaresma, V.S., Amos, C.L. and Bastos, A.C. (2007) The influence of articulated and disarticulated cockle shells on the erosion of a cohesive bed. Journal of Coastal Research, 23 (6), 1443-1451. (doi:10.2112/05-0449.1).

Abstract

An annular flume (Lab Carousel) was used to investigate the influence of shells on the erosion rates of an artificial laboratory cohesive bed (potters clay) under unidirectional flow. Specimens of articulated and disarticulated cockle shells (Cerastoderma edule), separated into six size classes (of maximum diameter from 13 to 26 mm), were used. The fluid-transmitted shear stresses and shell-induced bed erosion for the six classes were measured in triplicate, as well as shell settling velocity, sedimentation diameter, and shell drag coefficient. Bed erosion began after the shells started to move: erosion rates were influenced by shell size and the mode of transport. Two different modes of shell transport were observed: (i) rolling and saltation of articulated shells, and (ii) sliding and saltation of disarticulated shells. Peak bed erosion rates were associated with rolling and sliding modes of transport. Higher values of the solid-transmitted shear stress were calculated for saltating shells (in some cases two times higher), but contact with the bed was less frequent than rolling or sliding. The presence of cockle shells in an environment with hydrodynamic conditions strong enough to induce shell transport could significantly increase the loss of mass (erosion) of the surface bed or cliff. At Hythe intertidal area, where the shell specimens were collected, the presence of miniature furrows containing shells deposits in the troughs is widespread, and the troughs and walls show tool marks indicating shell abrasion.

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