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A continental shelf bottom boundary layer model : the effects of waves, currents, and a movable bed (2009)

Glenn, Scott M.

Massachusetts Institute of Technology and Woods Hole Oceanographic Institution

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Publication Date: 2022-05-25

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1983

Description: A simple model for the bottom boundary layer on the continental shelf is presented. The governing equations are developed for a stratified, turbulent Ekman layer in a combined wave and current flow over a moveable sediment bed. An eddy diffusivity closure scheme that includes the effect of suspended sediment, temperature, and salinity induced stratification on the vertical turbulent diffusion of mass and momentum couples the resulting unsteady conservation equations for fluid momentum, fluid mass, and suspended sediment mass. The wave velocity, current velocity, and suspended sediment concentration profiles predicted by the simultaneous solution of the conservation equations require the physical bottom roughness and a sediment reference concentrati on to be specified as boundary conditions. The physical bottom roughness associated with biologically generated bedforms, wave generated ripples, and near bed sediment transport are calculated as functions of the flow and sediment conditions. Using expressions for the height of sediment transporting layer and the sediment velocity, an expression for the sediment reference concentration is developed by matching laboratory measurements of sediment transport rates in oscillatory flow. The model predicts that the bottom flow field is highly dependent on (1) the nonlinear wave and current interaction, which increases the boundary shear stress and enhances vertical turbulent diffusion, (2) the effect of the boundary shear stress on a moveable sediment bed, which determines the physical bottom roughness and the amount of sediment in suspension, and (3) the effect of stable stratification, which inhibits vertical turbulent transport and couples the flow to the suspended sediment and fluid density profiles. The validity of the theoretical approach is supported by model predictions that are in excellent agreement with high quality data collected during two continental shelf bottom boundary layer experiments for a wide range of flow and bottom conditions.

Description: Funding for the work resulting in this Thesis has been provided by the American Gas Association (Project No. PR-153-126), the National Science Foundation (Grant No. OCE~8014930), and NOAA-Sea Grant (NA-79AA-D-0010l; NA 79AA-D-00102).

Keywords: Boundary layer ; Sediment transport ; Stratified flow

Repository Name: Woods Hole Open Access Server

Type: Thesis

Format: application/pdf

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High frequency bottom stress variability and its prediction in the CODE region (2017)

Grant, William D. ; Williams, Albert J. ; Glenn, Scott M. ; [et al.]

Woods Hole Oceanographic Institution

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Publication Date: 2022-05-26

Description: High quality bottom boundary layer measurements obtained in the CODE region off Northern California are described. Bottom tripod velocity measurements and supporting data obtained during typical spring and early summer conditions and during a winter storm are analyzed to obtain both velocity profiles and mean bottom stress and bottom roughness estimates. The spring/summer measurements were taken in June, 1981 during CODE-1 at C3 (90 m) by Grant and Williams, WHOI; the winter storm data was taken in November 1980 prior to CODE-1 at the R2 (80 m) site by Cacchione and Drake, USGS. The mean near-bottom (〈 2m) velocity profiles are logarithmic (R2 〉 0.993) much of the time for everyday flows; deviations are primarily due to kinematical effects induced by unsteadiness from internal waves. Stress profiles show the logarithmic layer corresponds to a constant stress layer as expected for the inertial region of a boundary layer. Stress estimates made from dissipation and profile techniques agree at the 95 percent confidence level. Typical z0 values estimated from measurements greater than 30 cm above the bottom have magnitudes of approximately 1 cm; an order of magnitude larger than the physical bottom roughness. Corresponding u* values have typical magnitudes of 0.5-1.0 cm/sec; more than twice as large as expected from a usual drag law prediction (corresponding to over a factor of four in mean stress). These values are demonstrated to be consistent with those expected for combined wave and current flows predicted theoretically by Grant and Madsen (1979) and Smith (1977). The u* values estimated from the CODE-1 data and predicted by the Grant and Madsen (1979) model typically agree within 10-15 percent. Similar results are demonstrated for the winter storm conditions during which large sediment transport occurs. (Typical z0 values are 4-6 cm; typical u* values are 3-6 cm/sec). The waves influencing the mid-shelf bottom stress estimates are 14-20 second swell associated with Southern and Western Pacific storms. These waves are present over most of the year. The results clearly demonstrate that waves must be taken into account in predicting bottom stress over the Northern California Shelf.

Description: Prepared for the National Science Foundation under Grant OCE 80-14938.

Keywords: Ocean waves ; Shear waves ; Boundary layer ; Ocean circulation