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The Influence of Different Water Levels and Water

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The Influence of Different Water Levels and Water THE INFLUENCE OF DIFFERENT WATER LEVELS AND WATER HEIGHT IN WAVE DRIVEN CIRCULATION OVER AN IDEALIZED NINGALOO REEF Thesis by HELEN REYNOLDS Centre for Water Reserch The University of Western Australia 2001 Abstract Ningaloo Reef is located off the western coast of Australia, stretching from NorthWest Cape to Gnaraloo Bay (Environment Australia 2000). Coral reefs are complex ecological systems closely connected to their physical environment. Therefore, effective management of a reef system requires an understanding of the physical oceanographic processes controlling the movement of water over and around the reef. This is increasingly important in the Ningaloo region as the tourism industry grows and population pressures, such as waste disposal and boating, increase. Studies undertaken to date have provided a basic description of the general oceanographic characteristics of the Ningaloo Reef system (D’Adamo & Simpson 2001) They have also included an analytical assessment of possible forcings on water circulation within the backreef lagoons. These studies have concluded circulation within coral reef lagoons is largely driven by wave pumping of water across the reef (Hearn et al. 1986, Hearn 1999). This project conducted a preliminary investigation of wave driven circulation over an idealized version of Ningaloo Reef. The work considered the effects of different water levels and wave conditions using the 2D vertically integrated numerical model FUNWAVE. The results of the numerical modelling were used to describe wave setup and the magnitude and direction of flow over the reef in an idealized Ningaloo lagoon. Preliminary estimates were made of flushing times for the idealized lagoon under wind, tidal and wave forcing. Numerical modelling produced results that agreed well with observed and theoretical values of current speed and wave setup published in the literature. The relative importance of wave-driven flushing was reconfirmed. However, the project was highly idealized and field data for model verification was not available. This limited the conclusions that could be drawn using the magnitude of modelled setup and currents. ii Table of Contents Acknowledgements _______________________________ Error! Bookmark not defined. Abstract _______________________________________________________________ ii 1 Introduction _______________________________________________________ 1 2 Literature Review ___________________________________________________ 4 2.1 Regional Oceanography and Climate of Ningaloo Reef_________________________4 2.1.1 Climate and Meteorology ___________________________________________________ 4 2.1.2 Large Scale Currents _______________________________________________________ 5 2.1.3 Tides ___________________________________________________________________ 7 2.1.4 Waves __________________________________________________________________ 8 2.1.5 Temperature and Salinity __________________________________________________ 10 2.2 Reef Geomorphology ___________________________________________________11 2.2.1 General Coral Reef Morphology _____________________________________________ 11 2.2.2 Geomorphology of Ningaloo Reef ___________________________________________ 13 2.3 Circulation within Coral Reef Lagoons_____________________________________17 2.3.1 Wind Driven Circulation ___________________________________________________ 18 2.3.2 Tidally Driven Circulation__________________________________________________ 19 2.3.3 Wave Driven Circulation___________________________________________________ 20 2.4 Numerical Modelling of Wave Driven Flow in Coral Reefs_____________________30 3 Methodology ______________________________________________________ 33 3.1 Numerical Model ______________________________________________________33 3.1.1 Wave Generation and Breaking______________________________________________ 33 3.1.2 Bottom Friction __________________________________________________________ 34 3.2 Model Inputs __________________________________________________________34 3.2.1 Bathymetry _____________________________________________________________ 35 3.2.2 Water Levels ____________________________________________________________ 37 3.2.3 Wave Forcing ___________________________________________________________ 38 3.3 Simulations ___________________________________________________________39 3.4 Model Outputs ________________________________________________________39 3.4.1 Surface Elevation ________________________________________________________ 40 3.4.2 Velocity Vectors _________________________________________________________ 40 3.4.3 Wave Gauges ___________________________________________________________ 41 4 Results and Analysis________________________________________________ 43 4.1 Modelled Results_______________________________________________________43 4.2 Comparisons with Experimental Results ___________________________________43 4.3 Surface Elevation ______________________________________________________45 4.3.1 Wave Setup _____________________________________________________________ 45 4.3.2 Comparison with Experimental Results________________________________________ 49 4.3.3 Wave Measurements at the Gauges ___________________________________________ 50 iii 4.4 Velocity ______________________________________________________________53 4.4.1 Velocity Direction________________________________________________________ 54 4.4.2 Velocity Magnitude_______________________________________________________ 58 4.4.3 Cross-Reef Velocity ______________________________________________________ 60 4.4.4 Velocity at the Wave Gauges _______________________________________________ 63 4.5 Discharge_____________________________________________________________65 4.5.1 Cross Reef Discharge _____________________________________________________ 65 4.5.2 Total Discharge __________________________________________________________ 67 4.6 Flushing Times ________________________________________________________69 4.6.1 Wind Driven Flushing_____________________________________________________ 70 4.6.2 Tidal Flushing ___________________________________________________________ 71 4.6.3 Wave Driven Flushing_____________________________________________________ 71 5 Discussion________________________________________________________ 73 5.1 Wave Setup ___________________________________________________________73 5.2 Wave Induced Currents and Discharge ____________________________________75 5.3 Flushing Times ________________________________________________________78 5.4 Influence of Other Factors _______________________________________________79 5.4.1 Wave Period ____________________________________________________________ 79 5.4.2 Bottom Friction __________________________________________________________ 80 5.4.3 Irregular Waves__________________________________________________________ 80 6 Conclusions ______________________________________________________ 81 7 Recommendations _________________________________________________ 83 7.1 FUNWAVE and Wave-driven circulation __________________________________83 7.2 Other Forcings ________________________________________________________84 8 Bibliography ______________________________________________________ 85 9 Appendix A _______________________________________________________ 89 10 Appendix B _______________________________________________________ 91 11 Appendix C _______________________________________________________ 96 12 Appendix D ______________________________________________________ 100 iv List of Figures Figure 1.1 Location of Ningaloo Reef on Western Australian Coastline showing location of the continental shelf (200m isobath) (Taylor & Pearce 1999) ________________________________________________ 1 Figure 2.1 Large-Scale Current Regime at Ningaloo (redrawn from Taylor & Pearce 1999) ___________ 6 Figure 2.2 : Timeseries of Wave Height and Current Speed at Ningaloo Reef. For location of wave rider and current meter, refer to Figure 2.4. (Hearn 1999) _________________________________________ 9 Figure 2.3 Main Geomorphological Features of a Coral Reef __________________________________ 12 Figure 2.4: Schematic map of a section of Ningaloo Reef (Hearn 1999)___________________________ 14 Figure 2.5 Division of Ningaloo Reef into sectors based on topographical features (Hearn et al. 1986) __ 16 Figure 2.6 Transect taken in the northern sector of Ningaloo Reef, (Hearn et al. 1986)_______________ 17 Figure 2.7 Definition Diagram for Wave Setup (Massel & Gourlay 2000) _________________________ 21 Figure 2.8 Setup on a Berm or a Reef _____________________________________________________ 22 Figure 2.9 Idealized reef defining theoretical model parameters (redrawn from Symonds et al. 1995)____ 23 Figure 2.10 Correlation between Hs and Current Speed, (Hearn 1999) ___________________________ 28 Figure 2.11 Current speeds indicated by drogue tracking under prevailing southerly-south easterly winds at Turquoise Bay, (Sanderson 1996) ______________________________________________________ 29 Figure 3.1 Transect of Turquoise Bay (Sanderson 1996) ______________________________________ 35 Figure 3.2 Idealized Bathymetry of Ningaloo Reef Used in FUNWAVE ___________________________ 36 Figure 3.3 Location of Wave Gauges relative to bottom contours________________________________ 42 Figure 4.1 Modelled Wave Setup ________________________________________________________ 46 Figure 4.2 Change in Maximum Setup with water depth and wave height _________________________ 48 Figure 4.3 Non-dimensional Comparison of Setup Results _____________________________________
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