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Offshore Controls on Nearshore Circulation

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Offshore Controls on Nearshore Circulation AN ABSTRACT OF THE THESIS OF Joseph W. Long for the degree of Master of Ocean Engineering in Ocean Engineering presented on March 10, 2005. Title: Offshore Controls on Nearshore Circulation Redacted for Privacy Abstract approved: H. Tuba Ozkan-Haller The rip current field resulting from the transformation of surface gravity waves over offshore submarine canyons is studied. Employing a wave transformation model and a wave-induced circulation model over observed bathymetry we find that rip current circulation cells exist with alongshore spacing of O(lOOm) even though the nearshore bathymetry displayed no variations at these length scales. Further, these simulations display skill in predicting observed rip currents during the Nearshore Canyon Experiment (NCEX). Motivated by these simulations we study the relationship between O(lOOm) scale variations in offshore bathymetric contours and the resulting rip current field in the nearshore. To isolate the roles of possible bathymetric features, we construct a series of idealized case studies that include site characteristics found at NCEX that are conducive of rip current development, such as a curved shoreline, an offshore submarine canyon and undulations in the canyon contours. Our results show that the first two components are unable to produce the observed short-scale circulation systems, while wave refraction over undulations in the canyon walls at length scales of O(lOOm) provided a sufficient disturbance to generate alongshore wave height variations that drive multiple rip currents for a variety of incident wave conditions. Rips are not generated when the wave period is short, or when the angle of incidence is large. Analysis of the alongshore momentum balances further demonstrates that the rip current locations are also strongly influenced by inertial effects. Hence, nonlinear processes are important within the rip current circulation cell and we find that nonlinear advective acceleration terms balance a large portion of the driving alongshore gradient in the mean water surface elevation in the vicinity of the rip currents with bottom friction accounting for the remainder. Away from the rips, the balance is between the wave forcing and the pressure gradient outside the surf zone and wave forcing and bottom friction inside the surf zone, as expected. ©Copyright by Joseph W. Long March 10, 2005 All Rights Reserved Offshore Controls on Nearshore Circulation Joseph W. Long A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Ocean Engineering Presented March 10, 2005 Commencement June 2005 Master of Ocean Engineering thesis of Joseph W. Long presented on March 10, 2005. APPROVED: Redacted for Privacy Major Professor, representing Ocean Engineering Redacted for Privacy Head of the Department of Civil, Construction, and Environmental Engineering Redacted for Privacy Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes Redacted for Privacy Redacted for Privacy Long, Author ACKNOWLEDGEMENTS I'll never forget my last day of work before packing up our belongings and moving our lives out west. A friend of mine handed me a piece of paper with three small yet powerful words; follow your dreams. I must admit at the time I didn't even know what my dreams were, and even if I did, the past two and half years have helped me develop new ones. I have to start by thanking the person that gave me this opportunity. My advisor, Tuba Ozkan-Haller is one of the most upbeat and encouraging professors that I have ever known. It was her enthusiasm that attracted me to this program in the first place, and it is her inquisitive nature that motivates me to want to know more. Thank you for listening and providing me with all the opportunities I ever wanted; including fieldwork, laboratory experiments, and even the bitter sweet teaching assistant position. Your guidance and patience is much appreciated. I'd also like to thank the support of my other committee members, Rob Holman and Mick Haller who have provided such useful feedback in my research throughout the past couple years. I thank John for all his computer assistance and for always being there to lighten the mood in the CTh. To my friends Jason, Joel, Jamo, Dave, Anthony, and Chris who without I may have never made it past my first term back in school I am indebted. Our late nights, although maybe not entirely productive, always made it worth going on. Thank you to Chris for opening my eyes to what true friendship can be and for helping me grow in knowledge and faith. The NCEX experiment was an unbelievable experience thanks mostly to Tom Lippmann and the crew from theother OSU.Tom taught me invaluable lessons about working in field but most of all I learned that even tough guys can listen to Bette Midler sometimes. Being cramped in a trailer all day, surveying the surf zone at night, and eating fish tacos with Gabe, Jason, and Jennifer cultivated lasting friendships. This research and my NCEX experience were sponsored by the Office of Naval Research, Coastal Studies Program under Grant N00014-02-1-0198. There are always those people in your life that truly believe you can do no wrong. Mostly they are limited to your parents and mine are no exception. My dad taught me the importance of a good work ethic and that quality work is vital in having pride in what you do. As for my mom, the importance I place on my education is because she instilled in me the desire to learn. She was always there to encourage me when I was at my worst and celebrate me when I was at by best. They never stopped believing in me even when I made mistakes and for that I am grateful. Finally, I devote this thesis to my princess buttercup, Myra. I never imagined the love I could feel for another person until I met you. You sacrificed your desires to come here and support me in my dreams and you never looked back or thought twice. Your ability to forgive, to listen, and to put up with all my moods are qualities unimaginable in most people. The only reason I am able to write this thesis is because of your love, support, and heart of gold, and I thank God each day that he put you in my life. TABLE OF CONTENTS ge Chapter 1: General Introduction..............................................................................1 1.1 Literature Review...........................................................................................2 1.2 Motivation......................................................................................................8 Chapter 2: Nearshore Canyon Experiment............................................................11 2.1 Site Description............................................................................................11 2.2 Data Collection.............................................................................................12 2.3 Bathymetric Surveys....................................................................................15 2.4 Offshore Wave Climate................................................................................21 2.5 Remote Sensing Observations (Argus)........................................................22 Chapter 3: Offshore controls on nearshore rip currents.........................................25 3.1 Abstract........................................................................................................26 3.2 Introduction..................................................................................................27 3.3 Overview of the Nearshore Canyon Experiment.........................................30 3.3.1Site Description........................................................................................30 3.3.2Bathymetric Surveys................................................................................32 3.3.3Offshore Wave Spectra............................................................................33 3.3.4Remote Sensing Observations (Argus)....................................................33 3.4 NCEX Model Simulations...........................................................................34 3.4.1WaveModel.............................................................................................34 3.4.2Circulation Model....................................................................................35 3.4.3Model-Data Comparisons........................................................................37 3.4.4Results for October 10, 2003 at 1900 GMT.............................................38 3.4.5Results for October 31, 2003 at 1500 GMT.............................................41 TABLE OF CONTENTS (Continued) 3.5 Idealized Bathymetry .48 3.5.1Oblique Submarine Canyon.....................................................................50 3.5.2Undulating Canyon Contours...................................................................52 3.5.3Time-Averaged Momentum Balances.....................................................54 3.5.4Effects of Varying Spectral Parameters...................................................62 3.6 Discussion and Conclusions.........................................................................70 3.7 Acknowledgments........................................................................................73 3.8 References.................................................................................................... Chapter 4: Lagrangian Particle
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