p:\csmc\manglesbay\reports\mangles_bay_study.doc 16/09/2002; 3:26 PM THE INFLUENCE OF THE GARDEN ISLAND CAUSEWAY ON THE ENVIRONMENTAL VALUES OF THE SOUTHERN END OF COCKBURN SOUND Prepared for: COCKBURN SOUND MANAGEMENT COUNCIL Prepared by: DAL SCIENCE AND ENGINEERING PTY LTD AUGUST 2002 REPORT NO. 02/247/1 CONTENTS EXECUTIVE SUMMARY __________________________________________________ iv 1. INTRODUCTION________________________________________________________ 9 1.1 GENERAL_________________________________________________________________________9 1.2 BACKGROUND TO THIS STUDY ____________________________________________________10 1.3 STUDY SCOPE AND APPROACH ____________________________________________________10 1.3.1 Scope ________________________________________________________________________10 1.3.2 Approach_____________________________________________________________________11 2. REVIEW OF COCKBURN SOUND CIRCULATION AND MODELLING ______ 14 2.1 CIRCULATION IN COCKBURN SOUND ______________________________________________14 2.2 DETERMINATION OF EXCHANGE (FLUSHING) ______________________________________15 2.3 INFLUENCE OF THE CAUSEWAY ___________________________________________________16 3. CAUSEWAY CIRCULATION MODELLING _______________________________ 18 3.1 APPROACH ______________________________________________________________________18 3.2 MODEL FORCING_________________________________________________________________18 3.3 MODEL GRID_____________________________________________________________________18 3.4 CAUSEWAY CONFIGURATIONS MODELLED ________________________________________19 3.5 THE INFLUENCE OF THE CAUSEWAY ON COCKBURN SOUND ________________________20 3.5.1 The effect of the Causeway on the flushing of Cockburn Sound ___________________________20 3.6 VALIDATION_____________________________________________________________________25 3.6.1 Low Frequency Forcing _________________________________________________________26 3.7 THE INFLUENCE OF THE CAUSEWAY ON THE MANGLES BAY REGION ________________28 3.7.1 Flushing______________________________________________________________________28 3.7.2 Bottom Shear Stress ____________________________________________________________30 4. FIELD SURVEY ________________________________________________________ 32 4.1 INTRODUCTION __________________________________________________________________32 4.2 HYDODYNAMIC INTERPRETATION ________________________________________________32 5. THE EFFECTS OF VARIOUS CAUSEWAY CONFIGURATIONS ON ECOLOGY, COASTAL PROCESSES AND HUMAN USES IN COCKBURN SOUND________________________________________________________________ 37 5.1 ECOLOGY _______________________________________________________________________37 5.1.1 Water quality__________________________________________________________________37 5.1.2 Seagrass _____________________________________________________________________41 5.2 COASTAL PROCESSES ____________________________________________________________49 5.2.1 Effects due to changes in wave energy ______________________________________________49 5.2.2 Longshore sediment transport and shoreline change ___________________________________50 5.3 HUMAN USES ____________________________________________________________________53 5.3.1 Current uses __________________________________________________________________53 5.3.2 Future uses ___________________________________________________________________55 6. CONCLUSIONS AND RECOMMENDATIONS _____________________________ 57 6.1 CONCLUSIONS ___________________________________________________________________57 6.1.1 Flushing and circulation characteristics_____________________________________________57 6.1.2 Ecology ______________________________________________________________________57 6.1.3 Coastal processes ______________________________________________________________58 6.1.4 Human uses ___________________________________________________________________59 6.1.5 Overview _____________________________________________________________________60 6.2 RECOMMENDATIONS _____________________________________________________________61 7. ACKNOWLEDGMENTS ________________________________________________ 62 8. REFERENCES _________________________________________________________ 63 DALSE:CSMC: INFLUENCE ON THE GARDEN ISLAND CAUSEWAY i TABLES Table 2.1 Flushing times for Cockburn Sound ________________________________ 15 Table 3.1 Summary of simulations undertaken using the EFDC model _____________ 20 Table 3.2 The effect of the Causeway on the flushing of Cockburn Sound (summer and autumn) ___________________________________________________ 21 Table 3.3 The effect of the Causeway on the flushing of various areas of Cockburn Sound in summer_______________________________________________ 23 Table 3.4 The effect of the Causeway on the flushing of various areas of Cockburn Sound in autumn _______________________________________________ 24 Table 5.1 Median chlorophyll values in various areas of Cockburn Sound, summer 2001/2002 ____________________________________________________ 39 Table 5.2 Relative impact of various Causeway configurations on summer water quality at the southern end of Cockburn Sound _______________________ 41 Table 5.3 Relative impact of various Causeway configurations on seagrasses. Predictions on shear stress based on peak current velocities during a spring tide in summer ___________________________________________ 48 FIGURES Figure 3.1 Bathymetry (water depth in metres) of the southern region of Cockburn Sound with each of the four Causeway configurations, with model grid overlay _______________________________________________________ 19 Figure 3.2 Changes in dye concentration in Cockburn Sound in summer and autumn, with and without the Causeway_____________________________ 21 Figure 3.3 Areas of Cockburn Sound examined for flushing characteristics __________ 22 Figure 3.4 Changes in dye concentration in various areas of Cockburn Sound in summer, with and without the Causeway ____________________________ 23 Figure 3.5 Changes in dye concentration in various areas of Cockburn Sound in autumn, with and without the Causeway_____________________________ 24 Figure 3.6 Location of current meters used in the validation study _________________ 25 Figure 3.7 Comparison of modelled and field data in the southern entrance of Cockburn Sound _______________________________________________ 27 Figure 3.8 More detailed comparison of modelled and field data in the southern entrance of Cockburn Sound ______________________________________ 27 Figure 3.9 Spectral analysis of modelled and field data for the current meter in the southern entrance of Cockburn Sound_______________________________ 28 Figure 3.10 Initial dye distribution used to investigate the circulation within Mangles Bay. Dye concentration varies between 0 (blue) and 10 (red) ____________ 29 Figure 3.11 Instantaneous dye concentration distribution generated by circulation transporting and mixing the initial dye distribution on the 26th of January 17:30, 1998 ___________________________________________________ 30 DALSE:CSMC: INFLUENCE ON THE GARDEN ISLAND CAUSEWAY ii Figure 4.1 Transect location _______________________________________________ 33 Figure 4.2 Salinity profiles obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon____________________________________ 34 Figure 4.3 Temperature (degrees Celsius) obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon ___________________________ 35 Figure 4.4 Dissolved oxygen (% saturation) obtained for Mangles Bay transect at 6.00 AM, 8.00 AM, 10.00 AM and 12.00 noon _______________________ 36 Figure 5.1 Changes in dye concentration with different Causeway configurations at selected sites in summer (refined grid) ______________________________ 41 Figure 5.2 Current velocity vectors during a spring tide in summer in the Mangles Bay region: comparison between present Causeway, and doubling of existing Causeway openings ______________________________________ 43 Figure 5.3 Current velocity vectors during a spring tide in summer in the Mangles Bay region: Comparison between present Causeway, and additional Causeway openings _____________________________________________ 44 Figure 5.4 Current velocity vectors during a spring tide in summer in the Mangles Bay region: Comparison between present Causeway configuration, and with the Causeway removed ______________________________________ 44 Figure 5.5 Bottom shear stress during an incoming spring tide in summer (kg/cm/s2), in the absence of the Causeway __________________________ 45 Figure 5.6 Bottom shear stress during an outgoing spring tide in summer (kg/cm/s2), in the absence of the Causeway____________________________________ 46 Figure 5.7 Changes in bottom shear stress (relative to the ‘No Causeway’ configuration) caused by various Causeway configurations: incoming spring tide, summer simulation ____________________________________ 46 Figure 5.8 Changes in bottom shear stress (relative to the ‘No Causeway configuration) caused by various Causeway configurations: outgoing spring tide, summer simulation ____________________________________ 47 Figure 5.9 Refraction of (a) south west and (b) north west swell-waves through South Channel (Modified from FPA, 1972) __________________________ 51 Figure 5.10 Refraction of north east wind-waves through Mangles Bay (Modified from FPA, 1972)._______________________________________________ 52 DALSE:CSMC: INFLUENCE ON THE GARDEN ISLAND CAUSEWAY iii EXECUTIVE SUMMARY Cockburn Sound is a