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An Investigation on the Hydrodynamics and Sediment Dynamics on an Intertidal

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An Investigation on the Hydrodynamics and Sediment Dynamics on an Intertidal An Investigation on the Hydrodynamics and Sediment Dynamics on an Intertidal Mudflat in Central San Francisco Bay by Stefan Andreas Talke B.S., (University of California, Berkeley) 1996 M.S. (University of California, Berkeley) 2001 A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Engineering – Civil and Environmental Engineering in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor Mark T. Stacey, Chair Professor James R. Hunt Professor Stanley A. Berger Spring 2005 The dissertation of Stefan Andreas Talke is approved: Chair Date Date Date University of California, Berkeley Spring 2005 An Investigation on the Hydrodynamics and Sediment Dynamics on an Intertidal Mudflat in Central San Francisco Bay Copyright 2005 by Stefan Andreas Talke Abstract An Investigation on the Hydrodynamics and Sediment Dynamics on an Intertidal Mudflat in Central San Francisco Bay by Stefan Andreas Talke Doctor of Philosophy in Engineering – Civil and Environmental Engineering University of California, Berkeley Professor Mark T. Stacey, Chair On an intertidal mudflat in Central San Francisco Bay, we conducted four field experiments between February, 2001 and October, 2003 to investigate hydrodynamic and sediment transport processes. Not only tides and locally driven wind waves bring energy onto the mudflat, but also remotely forced ocean swell and infra-gravity seiching. These energy sources persist and dominate over different time scales, with variation occurring over minutes, hours, and days. The different hydrodynamic forcing mechanisms superpose upon each other—and interact nonlinearly—in the water column, creating a dynamically varying interaction 1 with the sediment bed. Wind waves produce the largest stresses; however, ocean swell often determines the virtual roughness experienced by the seiche and tidal currents. The boundary layer of the tide is modulated by seiching motions, and is better described by a log-linear profile than a logarithmic profile. However, an analytical log-linear model using acceleration and stratification length scales does not explain the observed velocity structure. Instead, a k-ε turbulence closure model (General Ocean Turbulence Model, GOTM) reproduces the unsteady boundary layer structure of the seiching motion. A simple analytical solution (Smith, 1977) suffices to model the boundary layer of ocean swell. Several methods of separating wave energy and turbulence are compared; results show that TKE and dissipation vary with the total energy climate. Sediment erosion, deposition, and transport also vary on the tidal, seiche, and wave time scales. The tidal component of sediment flux has the largest magnitude, and is often determined by the timing and strength of wind events. Notable fluxes occur at the wave and seiche frequencies. Onshore transport occurs during an ebb tide at the seiche frequency. Sediment concentrations increase when the seiche opposes the ebb current, and decrease when they are aligned. Erosion is not occurring due to a critical stress, but when flow reverses over the rippled bed and causes sediment laden vortices to be ejected. When the seiche opposes the ebb current, waves are more likely to reverse the overall flow and sediment concentration increases. The interactions of multiple frequencies of motion are clearly vital to understanding sediment dynamics on an intertidal mudflat. 2 Table of Contents CHAPTER 1: INTRODUCTION ...............................................................................................................1 1.1. BACKGROUND AND MOTIVATION......................................................................................................1 1.2. RECENT HISTORY OF SAN FRANCISCO BAY AND FIELD SITE ...........................................................7 1.3. STRUCTURE OF DISSERTATION........................................................................................................10 1.4. FIGURES............................................................................................................................................13 CHAPTER 2: EXPERIMENT DESCRIPTION AND METHODS........................................................15 2.0. INTRODUCTION.................................................................................................................................15 2.1. DEPLOYMENT FRAMES ....................................................................................................................15 2.2. INSTRUMENTS...................................................................................................................................20 2.3. EXPERIMENT DETAILS .....................................................................................................................23 2.4. EXPERIMENTAL METHODS ..............................................................................................................30 2.5. EXPERIMENTAL ERROR ...................................................................................................................33 2.6. OBSERVATIONS OF FIELD SITE.........................................................................................................36 2.7. FIGURES............................................................................................................................................41 CHAPTER 3: HYDRODYNAMIC FORCING AND THE INTERTIDAL ENVIRONMENT ...........49 3.1. INTRODUCTION.................................................................................................................................49 3.2. FORCING MECHANISMS ...................................................................................................................51 3.3. IMPLICATIONS OF FORCING MECHANISMS ......................................................................................62 3.4. CONCLUSIONS ..................................................................................................................................75 3.5. FIGURES............................................................................................................................................78 CHAPTER 4: ESTIMATING TKE AND DISSIPATION .....................................................................94 4.1. INTRODUCTION.................................................................................................................................94 4.2. THEORY: CALCULATING TKE, DISSIPATION, AND STRESS............................................................97 4.3. SEPARATING WAVES AND TURBULENCE.......................................................................................102 4.4. PRACTICAL IMPLEMENTATION IN SHALLOW WATER ENVIRONMENT .........................................108 4. 5. RESULTS: TKE AND DISSIPATION ................................................................................................117 4.6. COMPARISONS OF METHODS AND ERROR ESTIMATION ................................................................124 4.7. CONCLUSIONS ................................................................................................................................135 4.8. FIGURES..........................................................................................................................................138 CHAPTER 5: WAVE AND CURRENT BOUNDARY LAYERS .......................................................148 5.1. INTRODUCTION...............................................................................................................................148 5.2. CURRENT BOUNDARY LAYER ........................................................................................................150 5.3. BED STRESS ....................................................................................................................................164 5.4. WAVE BOUNDARY LAYERS............................................................................................................174 5.5. DISCUSSION AND CONCLUSION......................................................................................................196 5.6. FIGURES..........................................................................................................................................199 CHAPTER 6: SEDIMENT TRANSPORT ............................................................................................223 6.1. INTRODUCTION AND THEORY ........................................................................................................223 6.2. SEDIMENT TRANSPORT ..................................................................................................................228 6.3. FLUCTUATIONS OF SEDIMENT CONCENTRATION AT SEICHE FREQUENCY..................................242 6.4. INTERACTION OF TURBULENT EDDIES WITH RIPPLES ...................................................................251 6.5. TRANSPORT OF SEDIMENT DUE TO WAVES ....................................................................................255 6.6. DISCUSSION ....................................................................................................................................258 6.7. FIGURES..........................................................................................................................................262 i CHAPTER 7: CONCLUSIONS AND FUTURE DIRECTIONS.........................................................280 7.1. MAJOR CONCLUSIONS ...................................................................................................................281
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