Prior to the mid 1990s, cross-margin transport in gravity-driven flows was assumed to occur infrequently under present sea-level conditions in the form of autosuspending turbidity currents that maintain their negative buoyancy through resuspension of bottom sediment on steep slopes. Since that time, however, a number of large observational studies on less steep shelves have observed high rates of cross-shelf transport in thin (?(10cm)) layers of highly concentrated fine sediment. These gravity currents are sustained by the combined action of their negative buoyancy and supplemental shear stress provided by surface waves. Since the slope of most continental shelves is not steep enough to sustain gravity-driven flows, the discovery of wave-supported gravity currents (WSGC) identified an important mechanism for cross-shelf sediment transport on many shelves. A complete understanding of WSCG is required to model the export of sediment and associated chemical species from the shelf to the abyssal ocean. Our knowledge of these currents, however, is limited to observational studies that cannot resolve the velocity or density structure within them. In addition, recent studies shows sand fractions may exert a critical role on the dynamics of these systems. The goals of the project are to determine the structure and transport in WSGC using a combination of laboratory experiments and prior field observations, to ascertain the conditions necessary for the occurrence, and transport magnitude of WSGC, and to investigate the sand fraction role on the initiation, transport and termination of WSGC.
Figure from Hooshmand et al., 2015.
(a–e) Reynolds stress for the rough wall and (f–j) the sediment bed runs. The top two figures show the free-stream velocity with the minimum and maximum velocity and flow reversal times indicated by vertical-dashed lines. The rough wall and sediment bed experiments in each row have similar wave orbital velocities (Uorb), which is indicated in the caption for each figure. Solid black lines in each figure show boundary layer heights derived from the maximum velocity.
People: Abbas Hooshmand, Zhuochen Han, Alex Horner-Devine
Funding: National Science Foundation
Published Papers:
Hooshmand, A., A. R. Horner-Devine and M. P. Lamb, 2015. Structure of turbulence and sediment stratification in wave-supported mud layers, J. Geophys. Res. Oceans, 120, doi:10.1002/2014JC010231