Angle of Repose of Selected Bivalve Shell Beds
The angle of repose, Ø, is defined as the angle between the tilting bed composed of loose sedimentary material and the horizontal plane when the first faint movement of a particle occurs anywhere on the bed. This angle is required for calculating the critical shear stress for bed erosion. Values of Ø which are not easily available from literature, were determined in laboratory experiments for beds composed of Donax variabilis (coquina), one of Crassostrea virginica (common oyster) and two were composed of mixtures of Chione cancellata (crossbarred chione) and Noetia ponderosa (ponderous ark). The mean value, Ø, ranged from 28.40° to 38.60°. An attempt was made to examine the possibility of representing the distribution of Ø, or that of simple functions of Ø, in the Gaussian form, because such a representation would enable an easily defined statistical description of data on Ø. Tests for normality, however, suggested that none of these distributions could be considered to be Gaussian for any of the beds. It is, therefore, recommended that Ø values be characterized by Ø, the coefficient of variation of Ø and geometric parameters defining shell size and sphericity. Surface roughness, which is also essential in characterizing Ø, must be identified by species, as no easily quantifiable parameter defining roughness is presently available.