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RT2: Changing Hydrodynamic Conditions (Impacts of Second Entrance) DRAFT FINAL REPORT Draft A 9 September 2005 RT2: Changing Hydrodynamic Conditions (Impacts of Second Entrance) DRAFT FINAL REPORT Draft A 9 September 2005 Sinclair Knight Merz ABN 37 001 024 095 590 Orrong Road, Armadale 3143 PO Box 2500 Malvern VIC 3144 Australia Tel: +61 3 9248 3100 Fax: +61 3 9248 3400 Web: www.skmconsulting.com COPYRIGHT: The concepts and information contained in this document are the property of Sinclair Knight Merz Pty Ltd. Use or copying of this document in whole or in part without the written permission of Sinclair Knight Merz constitutes an infringement of copyright. Draft Final Report Contents 1. Introduction and Background 1 2. A Second Entrance to the Gippsland Lakes 3 2.1 The Concept of a Second Entrance 3 2.2 The Second Entrance and Algal Blooms 3 2.3 The Peel-Harvey Estuary Precedent 4 2.4 Impacts of a Second Entrance to the Gippsland Lakes 4 3. The Values of the Gippsland Lakes 6 3.1 Introduction 6 3.2 Environmental 6 3.2.1 Wetlands 6 3.2.2 Estuarine Systems 7 3.3 Socio-Economic 9 3.3.1 Recreation and tourism 9 3.3.2 Aesthetic values 10 3.3.3 Commercial 11 3.3.4 Other values 12 4. Existing Hydrodynamic and Water Quality Conditions 13 4.1 Introduction 13 4.2 General Considerations 13 4.3 Hydraulic Parameters 13 4.3.1 Bathymetry 13 4.3.2 Tidal Levels 15 4.3.3 Tidal Range 16 4.3.4 Tidal Velocity 17 4.4 Water Quality 18 4.4.1 Catchment Inputs 18 4.4.2 Algal Blooms 19 4.4.3 Water Quality Modelling 25 5. Potential Second Entrance Configurations 29 6. Impacts of Second Entrance on Hydrodynamic and Water Quality Conditions 31 6.1 Introduction 31 6.2 General Considerations 31 6.3 Hydraulic Parameters 31 SINCLAIR KNIGHT MERZ I:\WCMS\Projects\WC03085\Deliverables\R02_dbs_Draft Final Report.doc PAGE 1 Draft Final Report 6.3.1 Bathymetry 31 6.3.2 Tidal Levels 32 6.3.3 Tidal Range 35 6.3.4 Tidal Velocity 35 6.4 Water Quality 36 6.4.1 Hydraulic Flushing 36 6.4.2 Saline Recovery 38 6.5 Impact of Second Entrance 39 6.5.1 Overview 39 6.5.2 Hydraulic Parameters 39 6.5.3 Water Quality 43 6.5.4 Discussion 45 6.5.5 Comparison with CSIRO 2001 Model 46 6.6 Other Considerations 47 6.6.1 Gippsland Lakes Flooding 47 6.6.2 Coastal Processes 47 6.6.3 Navigation and Dredging 48 6.7 Impacts of Second Entrance at Rotamah Island or Lake Reeve 48 6.8 Conclusions 49 7. Risks of a Second Entrance 51 7.1 Overview of Risk Assessment Framework 51 7.2 Ocean Grange 52 7.2.1 Environmental 52 7.2.2 Socio-Economic 63 7.3 Rotamah Island 69 7.3.1 Environmental 69 7.3.2 Socio-Economic 72 7.4 Lake Reeve 72 7.4.1 Environmental 72 Socio-Economic 75 7.4.2 75 7.5 Summary of Risk Assessment 76 8. Conclusion 81 9. References 83 Appendix A Project Brief 87 Appendix B Steering Committee 89 Appendix C Hydrodynamic Modelling 90 SINCLAIR KNIGHT MERZ I:\WCMS\Projects\WC03085\Deliverables\R02_dbs_Draft Final Report.doc PAGE 2 Draft Final Report Document history and status Revision Date issued Reviewed by Approved by Date approved Revision type Interim A 11 April 2005 S Treadwell A Brinkley 13/04/05 Draft Draft Final A 29 July 2005 Issued for A Brinkley 29/7/05 For review by R Grayson external review Draft Final B 8 September A Brinkley A Brinkley 8 September 2005 2005 Distribution of copies Revision Copy no Quantity Issued to Interim A 1 1 (electronic) Chris Barry Draft Final A 1 1 (electronic) Rodger Grayson Draft Final B 1 1 (electronic) Chris Barry Printed: 9 September 2005 Last saved: 7 September 2005 08:48 AM File name: I:\WCMS\Projects\WC03085\Deliverables\R02_dbs_Draft Final Report.doc Author: David Sheehan, Harry Houridis, Melinda Holt, Amy Mannix Project manager: David Sheehan Name of organisation: Gippsland Research Co-ordination Group Name of project: RT2: Changing Hydrodynamic Conditions (Impacts of Second Entrance) Name of document: Draft Final Report Document version: Draft B Project number: WC03085 SINCLAIR KNIGHT MERZ I:\WCMS\Projects\WC03085\Deliverables\R02_dbs_Draft Final Report.doc PAGE 3 Draft Final Report 1. Introduction and Background 1.1 Project Overview The Gippsland Research Co-ordination Group (GRCG) has commissioned Sinclair Knight Merz (SKM) to undertake an assessment of the hydrodynamic, environmental, social and economic impacts of a second entrance to the Gippsland Lakes. The project is being undertaken as part of the Group’s Gippsland Lakes Research and Development Program. The information sheet that accompanied the call for research and investigation proposals for the project is attached as Appendix A. The project is being undertaken under the direction of a Steering Committee, membership of which is listed in Appendix B. The project was originally intended to be undertaken in two Stages. The first stage was to comprise background reviews, establishment of a risk assessment framework, and assessment of hydrodynamic, environmental, social and economic impacts of both existing conditions, and conditions relating to one second entrance configuration/scenario. The need for a second stage was then to be assessed following the completion of the first stage. The second stage, if it proceeded, was to investigate up to a further two second entrance configurations/scenarios. A Draft Interim Report documenting Stage 1 investigations was submitted for consideration by the Steering Committee on 13 April 2005. This was then presented to and discussed with the Steering Committee at a meeting on 22 April 2005. It was agreed at the meeting that full hydrodynamic modelling of further second entrance options should not be undertaken. However, it was agreed that a desktop investigation, without additional hydrodynamic modelling, should be undertaken for two variants to the second entrance option investigated in Stage 1. The project has focussed on assessing the potential risks of a second entrance to the economic, social and environmental values of the Lakes. The Draft Final Report documents the investigations and findings of the Stage 1 second entrance option, and the two variants. 1.2 Draft Final Report The concept of a second entrance is introduced in Chapter 2. The economic, social and environmental values of the Lakes that could potentially be impacted upon by a second entrance are outlined in Chapter 3. The risk assessment has been based where possible on the expected impacts of a second entrance on relevant Lake parameters, particularly water level regime, velocities and salinities. Expected changes to these parameters have then been used to infer the likely impacts of a second entrance on the frequency and severity of algal blooms in the Lakes. Existing hydrodynamic and water quality conditions in the Lakes are summarised in Chapter 4. SINCLAIR KNIGHT MERZ I:\WCMS\Projects\WC03085\Deliverables\R02_dbs_Draft Final Report.doc PAGE 1 Draft Final Report The three potential second entrance configurations investigated during the project, and the rationale for these, are outlined in Chapter 5. The impacts of each configuration on hydrodynamic and water quality conditions, including inferred impacts on frequency and severity of algal blooms, are detailed in Chapter 6. The risk assessment is then presented in Chapter 7, and conclusions presented in Chapter 8. SINCLAIR KNIGHT MERZ 2 I:\WCMS\Projects\WC03085\Deliverables\R02_dbs_Draft Final Report.doc PAGE Draft Final Report 2. A Second Entrance to the Gippsland Lakes 2.1 The Concept of a Second Entrance The Gippsland Lakes are one of Australia’s largest coastal lagoon systems. They represent a unique aquatic ecosystem with a range of environmental values that support a number of beneficial uses that include recreational boating, tourism and commercial fishing. Most of the lakes and lagoons also form part of the Gippsland Lakes Ramsar site and therefore support significant numbers of waders and seabirds. Like most coastal embayments around Australia, the pressures of coastal development and catchment degradation on the Gippsland Lakes has resulted in substantial changes to the ecosystem. There has been a well-documented impact on water quality that includes the regular occurrence of blue green algal blooms and extended periods of deoxygenation in bottom waters and sediments of the lower lakes. In response to the need to investigate strategies for ameliorating the environmental problems of the Gippsland Lakes, and following recommendations from the Gippsland Lakes Environmental Audit (Harris et al. 1998), the CSIRO initiated the Gippsland Lakes Environmental Study (GLES), which assessed the various options available for improving water quality and ecological function of the Gippsland Lakes. One suggested option was to consider the feasibility of a second entrance near Ocean Grange (Webster et al 2001). 2.2 The Second Entrance and Algal Blooms The Gippsland Lakes are susceptible to eutrophication because of a number of factors including poor flushing, high nutrient loads per unit volume of receiving water, and vertical salinity stratification. Algal blooms have the potential to occur all year round, but tend to occur in autumn/late summer when long, dry periods with low river flows during winter are followed by calm conditions conducive to stratification. Blooms sometimes also follow major rain events and floods in spring which are accompanied by high nutrient loads from the catchments. Large scale blooms of the toxic, blue-green alga Nodularia spumigenia, have occurred about every 10 years, although blooms of unspecified algal species including dinoflagellates and diatoms occur more often. Most Nodularia blooms have been recorded from Lake King.