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Journal of Sedimentary Research, Section
A: Sedimentary Petrology and Processes
Vol. 67 (1997)No.
3. (May), Pages 514-526
Hydrodynamics of Bivalve Shell Entrainment and Transport
Amalia M. Olivera (1*), William L. Wood (2)
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.
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