Incipient motion of shells and shell fragments

[摘要] ENGLISH ABSTRACT : In the present study, the erosion characteristics of shells and shell fragments are investigated.When shells settle, they settle like leaves, where the biggest cross section is exposedto the drag, unlike when they are flat on the bed, where the smaller cross section is exposedto the direction of flow. The settling velocity of shells is thus lower than the velocity requiredto make them erode. A model is presented to determine the critical bed shear stressof various shells at incipient motion lying flat on a horizontal sand bed. This model makesuse of the hydrodynamical forces, where drag and lift forces are taken into consideration.The mussel and limpet shells are investigated, as well as various shapes of fossilized shellsand fragments, in their dirty and clean states. These results are compared to results foundin literature. Experiments were conducted in an open channel to determine critical bedshear stresses for incipient motion of the shells using Vanoni's side-wall correction. Thelift and drag forces of the mussel and limpet shells are also determined under the flow ofwater using a strain gauge. The experimental data is compared to the data obtained bythe analytical model. The Shields parameter is used in an attempt to relate the erosioncharacteristics of the shells to non-uniform sand grains on a bed.

[发布机构] Stellenbosch University