Hydrodynamics of Bivalve Shell Entrainment and Transport

Abstract

ABSTRACT The relationship between the hydrodynamic properties of disarticulated bivalve shells, and the entrainment and transport of these shells by water currents, was studied by means of flow-visualization experiments and measurements of surface pressure around the shell bodies. The study was performed on eleven bivalve shells, simulated as resting convex-up on the bottom, with their beaks pointing into the flow. The results indicate that bivalve shells respond to hydrodynamic processes in a systematic and predictive manner. Drag and lift were best represented by an exponential relationship between the sum of the projected plan and frontal area of the shells. This relationship supports the argument that greater forces are generated for large, convex and elongated shells. In addition, lift-to drag ratio increased with increasing thickness index (height/length). Roughness elements on shells, such as ribs, produce a drag-reduction transition at high Reynolds numbers, as with spheres, but this transition occurs at lower Reynolds numbers than for spheres. Asymmetry in shells contributes to the development of drag and lift. Large lateral forces were found to be directly related to asymmetry in plan shape. Force-to-shape relationships can be applied to any given shell to determine its transportability and assess the influence of hydrodynamic processes on fossil assemblages. Force resolution was used to calculate entrainment velocities for shells, which were compared with entrainment velocities measured in a recirculating flume. Calculated velocities of 0.2-0.8 m/s were in good agreement with measured velocities. Calculations of entrainment velocity can be used to estimate paleocurrent velocity in the reconstruction of depositional environments.