Incipient motion of shells and shell fragments

Troch, Christophe Nicolas Aimé (2015-12)

Thesis (MSc)--Stellenbosch University, 2015

Thesis

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 exposed to the drag, unlike when they are flat on the bed, where the smaller cross section is exposed to the direction of flow. The settling velocity of shells is thus lower than the velocity required to make them erode. A model is presented to determine the critical bed shear stress of various shells at incipient motion lying flat on a horizontal sand bed. This model makes use 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 shells and fragments, in their dirty and clean states. These results are compared to results found in literature. Experiments were conducted in an open channel to determine critical bed shear stresses for incipient motion of the shells using Vanoni's side-wall correction. The lift and drag forces of the mussel and limpet shells are also determined under the flow of water using a strain gauge. The experimental data is compared to the data obtained by the analytical model. The Shields parameter is used in an attempt to relate the erosion characteristics of the shells to non-uniform sand grains on a bed.

AFRIKAANSE OPSOMMING : In hierdie studie word die erosie kenmerke van skulpe en skulp fragmente ondersoek. Wanneer skulpe afsak na die bodem, word, soortgelyk soos blare rangskik onder 'n boom, die grootste deursnit area van die skulp aan die sleurkrag blootgestel, in teenstelling met wanneer dit op die bodem lê, waar die kleiner deursnit area blootgestel is aan die vloei. Die sinksnelheid van skulpe is dus laer as die snelheid wat vereis word om hulle te laat erodeer. 'n Model om die kritieke skuifspanning vir die aanvang van beweging van verskeie skulpe wat op 'n horisontale sandbodem lê te bepaal, word voorgestel. Hierdie model maak van die hidrodinamiese kragte, waar die sleurkrag en hefkrag in ag geneem word, gebruik. Mosselen klipmosselskulpe word ondersoek, asook verskillende vorms van gefossiliseerde skulpe en fragmente van skulpe, in hulle vuil en skoon toestande. Hierdie resultate word dan met resultate soos gevind in literatuur vergelyking. Eksperimente om die kritiese skuifspanningskragte op die bodem te bepaal vir die aanvangsbeweging van die skulpe word in 'n oop kanaal gedoen deur van Vanoni se wandmuur regstelling gebruik te maak. Die sleurkragte en hefkragte van die mossel en klipmossel skulpe word onder die invloed van vloeiende water met behulp van 'n vervormingsmeter bepaal. Die eksperimentele data word met die data wat verkry is deur die analitiese model vergelyking. Die Shield parameter word gebruik in 'n poging om die erosie kenmerke van die skulpe met nie-uniforme sandkorrels wat op die bodem lê te vergelyk.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97940
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