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Symposium Abstracts: Storm-Dominated Shelves
Waves and currents are two factors that influence the movement of sediments on many shelves, but their relative importance changes with depth and shelf topography. Waves contribute to both the resuspension of finer sediment and to the bed load transport of coarser material, and result in the following processes: 1. shoreward bed load transport of material due to wave asymmetry (important in water < 10 m deep); 2. wave resuspension of the coarse material for short periods (<wave period) and of finer material for considerably longer (> wave period); and 3. wave pumping of fine sediment into suspension due to the pressure gradient beneath the crest and trough of the waves. Currents, tidal or storm-driven, are the major determinants of the direction of both bed load and suspended sediment, but for storm-dominated shelves they seldom initiate the transport. The tidal currents can produce an effective diffusive mechanism for sediment dispersal. Wind-driven currents result from the applied surface wind stress and may involve significant coastal upwelling and downwelling due to Ekman transport. Trapped continental shelf waves can also contribute to the currents on the shelf. The interaction between the waves and the currents has two effects: 1. the boundary layer initiated by the oscillatory wave currents changes the bed roughness ‘seen’ by the steady current and alters the direction of the steady current close to the bed; and 2. bed load transport occurs in a direction determined by the instantaneous wave current plus steady current.
Transport patterns of sediment of different sizes may be quite different, depending on whether the dominant processes result in mainly bed load or suspended load transport. These processes are controlled by the long term climatology of the shelf system, which itself ultimately depends on the frequency and intensity of storms in the region. In deeper water out toward the shelf break, the transport events will become more episodic and increasingly dependent on the few very intensive storms. The statistics of the storms are, however, not well documented and their effects on the sediments are, as yet, unmeasured.
Acknowledgments and Associated Footnotes
1 School of Environmental Sciences, University of East Anglia, Norwich, U.K. NR4 7TJ
Copyright © 2008 by the Canadian Society of Petroleum Geologists