Tidal Interactions with Local Topography Above a Sponge Reef by Jeannette Bedard BSc, Royal Roads Military College, 1994 A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE in the School of Earth and Ocean Science Jeannette Bedard, 2011 University of Victoria All rights reserved. This thesis may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author. 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While these forms may be included Bien que ces formulaires aient inclus dans in the document page count, their la pagination, il n'y aura aucun contenu removal does not represent any loss manquant. of content from the thesis. ii Supervisory Committee Tidal Interactions with Local Topography Above a Sponge Reef by Jeannette Bedard Bsc., Royal Roads Military College, 1994 Supervisory Committee Dr. Eric Kunze (School of Earth and Ocean Science) Supervisor Dr. Richard Dewey (School of Earth and Ocean Science) Co-Supervisor Dr. Jody Klymak (School of Earth and Ocean Science) Departmental Member iii Abstract Supervisory Committee Dr. Eric Kunze (Department of Earth and Ocean Sciences) Supervisor Dr. Richard Dewey (Department of Earth and Ocean Sciences) Co-Supervisor Dr. Jody Klymak (Department of Earth and Ocean Sciences) Departmental Member The interaction of tidal currents with Fraser Ridge in the Strait of Georgia, B.C., generates an internal lee-wave on each strong flood but, due to the ridge's asymmetry, not during ebbs. Just prior to lee-wave formation, a strong accelerated bottom jet forms with magnitudes up to 0.7 m s-1 forms during barotropic tidal flows reaching 0.2 m s-1. On the steepest slope, this jet forms directly above a rare glass sponge reef, and may prevent the sponges from being smothered in sediment by periodically resuspending and carrying it away. Both the accelerated jet and lee-wave remove tidal energy. At peak flood tide, the lee-wave has energy dissipation rates reaching 10-5 W kg-1 that removes energy at a rate of ~611 W m-1, while the bottom boundary layer at the time of the accelerated jet has energy dissipation rates reaching 10-4 W kg-1 that removes energy at a rate of ~525 W m-1. iv Table of Contents Supervisory Committee.......................................................................................................ii Abstract...............................................................................................................................iii Table of Contents................................................................................................................iv List of Figures.....................................................................................................................vi Acknowledgments.............................................................................................................vii 1. Introduction ...................................................................................................................1 1.1 Glass Sponge Reefs................................................................................................2 1.2 Study Location.......................................................................................................3 2. Theory and Field Studies..............................................................................................6 2.1 Two-Dimensional Flow-Topography Interaction..................................................7 2.2 Three-Dimensional Effects....................................................................................9 2.3 Expectations at Fraser Ridge...............................................................................10 3. Methods.........................................................................................................................12 3.1 16-27 November 2007 Field Program.................................................................12 3.2 June to August 2010 Mooring..............................................................................13 4. Observations.................................................................................................................16 4.1 General Flow Characteristics Around Fraser Ridge............................................16 4.1.1 Horizontal Flow Structure.............................................................................16 4.1.2 Cross-Ridge Evolution of the Flow..............................................................20 4.1.2.1 Weak Flood Tide....................................................................................20 4.1.2.2 Strong Ebb Tide.....................................................................................20 4.1.2.3 Strong Flood Tide..................................................................................24 4.1.2.4 Weak Ebb Tide......................................................................................27 4.1.3 Along-Ridge Evolution of the Flow.............................................................27 4.2 Flow Directly Above Sponge Reef .....................................................................30 4.2.1 Accelerated Jet..............................................................................................30 4.2.2 Bottom Turbulence and Mixing....................................................................34 4.3 Energy Budget.....................................................................................................35 4.3.1 Local Energy Dissipation .............................................................................36 4.3.2 Time-Varying Term .....................................................................................37 v 4.3.3 Bernoulli Drop .............................................................................................39 4.3.4 Energy Budget Summary..............................................................................41 5. Discussion......................................................................................................................42 5.1 Ridge-Scale Flow.................................................................................................42 5.2 Flow at the Sponge-Reef Scale............................................................................43 6. Conclusions...................................................................................................................48 7. Bibliography.................................................................................................................50 vi List of Figures 1. Map and Tides at Fraser Ridge........................................................................................5 2. Parameter Definition Sketch............................................................................................6 3. Fraser Ridge Topography and Transects.......................................................................11 4. Energy Dissipation Rate Calculation Examples............................................................15 5a. Depth and Time Average of Horizontal Velocity 0 – 25 m.........................................18 5b. Depth and Time Average of Horizontal Velocity 25 – 100 m.....................................19 5c. Depth and Time Average of Horizontal Velocity 100 m – bottom..............................19 6a. Time Series of Section B03 - B10................................................................................22 6b. Time Series of Section B10 - B19...............................................................................23 7. Cross-Ridge Section B13...............................................................................................25 8. Cross-Ridge Section E06...............................................................................................25 9. Histograms of North Velocity for Tracks B13 and