Incipient Motion of Shells and Shell Gravel

Abstract

Incipient motion of seashells under a unidirectional flow of water in a horizontal open channel over surfaces with differing roughnesses and shell placements is investigated experimentally. Drag and lift forces are measured with strain gauges and, from these measurements, the lift and drag coefficients are inferred. The various shell shapes that are investigated in this study are the Patellogastropoda, commonly known as the limpet shell, and the Mytilus galloprovincialis, commonly known as the blue mussel, as well as fossilized shells and shell fragments obtained through a dredging campaign. The bed shear stresses at incipient motion are measured and determined from an existing model for shells and the results are compared to the measurements and published data. It is indicated that a critical Shields parameter for shells may not always be an appropriate measure to use to estimate the incipient motion of shells.