Gold Coast Broadwater

Transcription

1 Gold Coast Broadwater Preliminary Coastal and Hydrodynamic Investigations for Cruise Ship Terminal Options July 2013

2 Gold Coast Broadwater Preliminary Coastal and Hydrodynamic Investigations for Cruise Ship Terminal Options Prepared For: Prepared By: Gold Coast City Council BMT WBM Pty Ltd (Member of the BMT group of companies) Offices Brisbane Denver Mackay Melbourne Newcastle Perth Sydney Vancouver

3 DOCUMENT CONTROL SHEET BMT WBM Pty Ltd BMT WBM Pty Ltd Level 8, 200 Creek Street Brisbane 4000 Queensland Australia PO Box 203 Spring Hill 4004 Tel: Fax: ABN Document : Project Manager : Client : Client Contact: R.B ScenarioAssessm ent.docx Ian Teakle Gold Coast City Council Luke Adair Client Reference LG314/621/13/033 Title : Authors : Synopsis : Gold Coast Broadwater: Preliminary Coastal and Hydrodynamic Investigations for Cruise Ship Terminal Options Ian Teakle Jesper Nielsen Dean Patterson This document describes the preliminary investigation of Cruise Ship Terminal option impacts on coastal hydrodynamic processes. REVISION/CHECKING HISTORY REVISION NUMBER DATE OF ISSUE CHECKED BY ISSUED BY 28/05/2013 DCP IAT 28/06/2013 DCP IAT 25/07/2013 DCP IAT 30/07/2013 DCP IAT DISTRIBUTION GCCC BMT WBM File BMT WBM Library DESTINATION REVISION PDF PDF PDF PDF PDF PDF PDF PDF PDF PDF PDF PDF

4 CONTENTS I CONTENTS Contents List of Figures List of Tables i iii v 1 INTRODUCTION Background Earlier Cruise Ship Terminal Assessments Study Scope NUMERICAL MODEL DESCRIPTIONS Waves (SWAN) Hydrodynamics (TUFLOW-FV) Model Domain, Mesh and Bathymetry Model Configuration Boundary Conditions Coastal Processes TUFLOW-FV Wave Coupling Sediment Transport EVO Shoreline Model NUMERICAL MODEL CALIBRATION & VALIDATION Waves Tidal Hydrodynamics /5 Calibration Validation Sediment Transport EVO Validation TUFLOW-FV Littoral Transport Calibration EXISTING ENVIRONMENT Tidal Water Levels Tidal Currents 4-1

5 CONTENTS II 4.3 Waves Morphology Estuarine Sediment Transport Littoral Sediment Transport Conceptual Sand Budget SCENARIOS ASSESSED Dredged Channel Dimensions Initial Assessments Configuration Shortlist HYDRODYNAMIC ASSESSMENTS Methodology Tidal hydrodynamics Storm Tide hydrodynamics Flood hydrodynamics Results Option S Option S Option S Discussion RECYCLED WATER DISPERSION Methodology Results COASTAL PROCESS ASSESSMENTS Capital Dredging Impact Assessments Waves Littoral Sediment Transport Estuarine Sediment Transport Discussion IMPACT MITIGATION SUMMARY REFERENCES 11-1

6 LIST OF FIGURES III APPENDIX A: HYDRODYNAMIC MODEL CALIBRATION 2004/5 A-1 APPENDIX B: HYDRODYNAMIC MODEL VALIDATION, 2009 B-1 APPENDIX C: WAVE MODEL VALIDATION C-1 LIST OF FIGURES Figure 1-1 Cruise Ship Terminal Options from Study Brief 1-4 Figure 2-1 SWAN Model Nested Domains 2-2 Figure 2-2 Regional Hydrodynamic Model Domain 2-3 Figure 2-3 Hydrodynamic Mesh Detail in the Study Area 2-3 Figure 2-4 EVO Gold Coast Model Domain 2-7 Figure 3-1 SWAN Model Calibration to Seaway 2010 Measurements 3-1 Figure 3-2 SWAN Model Calibration to Narrowneck 2011 Measurements 3-2 Figure 3-3 Water Level Comparison at two of the 2004/5 Data Locations 3-3 Figure 3-4 ADCP Flow Comparisons for Three Transects from the 2004/5 Campaign 3-4 Figure 3-5 Flood and Ebb Tide Transect Comparisons from the GCCM 2009 Monitoring 3-5 Figure 3-6 ADCP Flow Comparisons from GCCM February 2009 Monitoring 3-6 Figure 3-7 EVO Shoreline Predictions Compared with Narrowneck Data 3-7 Figure 3-8 Comparison of Calibrated TUFLOW-FV and EVO Longshore Transport Predictions 3-9 Figure 3-9 Seaway Surveys; 19 th February 2009 (Top), 16 th June 2009 (Middle) and Difference (Bottom) 3-10 Figure 3-10 Predicted Bed Elevation Change During May 2009 Simulation 3-11 Figure 4-1 Water Level Timeseries Outside and Inside the Seaway 4-1 Figure 4-2 Flood Tide 3D Current Patterns 4-2 Figure 4-3 Ebb Tide 3D Current Patterns 4-2 Figure 4-4 Northern Wall Recycled Water Release Snapshot Showing Flood Tide Flow Separation Eddy 4-3 Figure 4-5 South-Easterly Wave Transformation Example 4-4 Figure 4-6 Easterly Wave Transformation Example 4-4 Figure 4-7 Gold Coast Seaway Morphological Features 4-5 Figure 4-8 Net Tidal Sediment Transport 4-6 Figure 4-9 Net Littoral Sediment Transport for South-Easterly Waves 4-7 Figure 4-10 Net Littoral Sediment Transport for East-South-Easterly Waves 4-7 Figure 4-11 Seaway Bypassing and Modelled Longshore Sand Transport 4-9

7 LIST OF FIGURES IV Figure 4-12 Conceptual Sand Budget Model 4-10 Figure 5-1 Option S1 5-2 Figure 5-2 Option S2 5-3 Figure 5-3 Option S3 5-3 Figure 6-1 Water Level Timeseries 6-7 Figure 6-2 Coomera River Mouth Current Timeseries 6-7 Figure 6-3 Nerang River Mouth Current Timeseries 6-7 Figure 6-4 Figure 6-5 Figure 6-6 Figure 6-7 Figure 6-8 Figure 6-9 Figure 6-10 Figure 6-11 Figure 6-12 Figure 6-13 Figure 6-14 Figure 6-15 Figure 6-16 Figure 6-17 Figure 6-18 Figure 6-19 Figure 6-20 Figure 6-21 Figure 6-22 Option S1 - Tidal Water Level Impacts. Low Tide (Left), High Tide (Right) 6-8 Option S1 Flood Tide Currents. Base (Right), Developed (Middle) and Impacts (Left) 6-9 Option S1- Ebb Tide Currents.. Base (Right), Developed (Middle) and Impacts (Left) 6-10 Option S1 Coast Crossing Cyclone Peak Surge (Left) and Impact (Right) 6-11 Option S1 Coast Parallel Cyclone Maximum Surge (Left) and Impact (Right) 6-12 Option S1 Flood Simulation Peak Water Level (Left) and Impact (Right) 6-13 Option S2 - Tidal Water Level Impacts. Low Tide (Left), High Tide (Right) 6-14 Option S2 Flood Tide Currents. Base (Right), Developed (Middle) and Impacts (Left) 6-15 Option S2- Ebb Tide Currents. Base (Right), Developed (Middle) and Impacts (Left) 6-16 Option S2 Coast Crossing Cyclone Peak Surge (Left) and Impact (Right) 6-17 Option S2 Coast Parallel Cyclone Maximum Surge (Left) and Impact (Right) 6-18 Option S2 Flood Simulation Peak Water Level (Left) and Impact (Right) 6-19 Option S3 - Tidal Water Level Impacts. Low Tide (Left), High Tide (Right) 6-20 Option S3 Flood Tide Currents. Base (Right), Developed (Middle) and Impacts (Left) 6-21 Option S3- Ebb Tide Currents. Base (Right), Developed (Middle) and Impacts (Left) 6-22 Option S3 Coast Crossing Cyclone Peak Surge (Left) and Impact (Right) 6-23 Option S3 Coast Parallel Cyclone Maximum Surge (Left) and Impact (Right) 6-24 Option S3 Flood Simulation Peak Water Level (Left) and Impact (Right) 6-25 Approach Channel Only Case - Tidal Water Level Impacts. Low Tide (Left), High Tide (Right) 6-26 Figure 7-1 Recycled Water Release Locations 7-3

8 LIST OF TABLES V Figure 7-2 Figure 7-3 Figure 8-1 Recycled Water Tracer Dilution 50 th percentile. Base Case (Left), Option S1 (Middle) and Impacts (Right) 7-4 Recycled Water Tracer Dilution 90 th percentile. Base Case (Left), Option S1 (Middle) and Impacts 7-5 Existing Case and Option S1 Wave Height Adjacent Wave Break Island 8-8 Figure 8-2 Plots of Developed versus Existing Modelled Wave Conditions. 8-8 Figure 8-3 Wave Model Results 12/05/2009. Base Case (Left), Option 1 (Middle) and Impacts (Right) 8-9 Figure 8-4 Figure 8-5 Figure 8-6 Figure 8-7 Figure 8-8 Figure 8-9 Figure 8-10 Figure 8-11 Figure 8-12 Net Littoral Sediment Transport during March Base Case (Left), Option S1 (Middle), Impact (Right) 8-10 Net Littoral Sediment Transport during May Base Case (Left), Option S1 (Middle), Impact (Right) 8-11 Net Littoral Sediment Transport during August Base Case (Left), Option S1 (Middle), Impact (Right) 8-12 Net Bed Elevation Change (DZB) During March 2006 Event. Base Case (Left), Option S1 (Middle), Impact (Right) 8-13 Net Bed Elevation Change (DZB) During May 2009 Event. Base Case (Left), Option S1 (Middle), Impact (Right) 8-14 Net Bed Elevation Change (DZB) During August 2011 Event. Base Case (Left), Option S1 (Middle), Impact (Right) Net Estuarine (Tidal) Sediment Transport. Base Case (Left), Option S1 (Middle), Impact (Right) 8-16 Net Estuarine (Tidal) Sediment Transport. Base Case (Left), Option S2 (Middle), Impact (Right 8-17 Net Estuarine (Tidal) Sediment Transport. Base Case (left), Option S3 (Middle), Impact (Right) 8-18 LIST OF TABLES Table 2-1 Structures Dynamically Added to EVO Simulation 2-7 Table 5-1 Capital Dredging Volumes (Mm 3 ) 5-2 Table 6-1 Modelled Tropical Cyclone Parameters 6-2 Table 8-1 Selected Coastal Process Simulation Periods 8-2 Table 8-2 Option S1 Event Channel Sedimentation Volumes (m 3 ) 8-4 Table 8-3 February to June 2009 Surveyed Volume Changes in Option S1 Footprint (m 3 ) 8-4

9 INTRODUCTION INTRODUCTION Gold Coast City Council (GCCC) commissioned BMT WBM Pty Ltd to undertake preliminary hydrodynamic and coastal process assessments of cruise ship terminal options inside the Gold Coast Broadwater. This report describes the historical context for the study, the methodologies applied, scenarios assessed and outcomes including areas for further investigation. 1.1 Background The Gold Coast Broadwater is the proposed location of a Cruise Ship terminal that forms part of an overall Broadwater Marine Project (BMP), which is being coordinated by Queensland Government and Gold Coast City Council. Following submission of Expressions of Interest a Request for Detailed Proposal (RFDP) has been extended to a short-list of proponents. This preliminary impact assessment has been commissioned to inform the government and short-listed proponents during the BMP tendering process. The Gold Coast Seaway was constructed in 1986 to provide for safer navigation and to stabilise the Nerang River entrance location. The project provided for a dredged and trained navigation channel into the Broadwater, a sand bypassing system to maintain net littoral transport and construction of Wavebreak Island to protect the western shoreline from wave attack. The Seaway development resulted in a more hydraulically efficient connection between the Broadwater and the ocean than the previous natural system, which resulted in an increase of the tidal range within the Broadwater from around 60% of the ocean range to greater than 90%. Canal developments (prior to and after Seaway construction) have also had a substantial influence on the tidal system, resulting in an increased tidal prism. WBM Oceanics (2001) undertook a review of Gold Coast Seaway coastal processes over the 15 year period following the construction of the Seaway. Analysis of survey data indicated that the ebb delta bar initially grew at a rate of more than 450,000m 3 /year for the first 4 years ( ) and that the rate of increase had dropped to below 200,000m 3 /year by More recent analysis by GCCC for the period (GCCC, pers. comm.) indicates that ebb delta growth continues but the rate has further dropped. More than 1,000,000m 3 of sand has additionally been dredged from the Broadwater and placed in the beach system from Surfer s Paradise to Narrowneck since the Seaway was constructed. Following construction, the Seaway entrance channel has scoured from the initial constructed bed elevation of around -6m to -10m LAT to currently have channel centreline bed elevations generally in the range of -10 to -18m LAT. The rate of scour from the main Seaway channel was initially estimated at 150,000m 3 /year and had reduced to less than 10,000m 3 /year by 1998 (WBM Oceanics, 2001). The curvature of flow entering the main Seaway channel and the imbalance between north and south channel flows has resulted in a spatially variable pattern of scour. In particular one scour hole has developed along the central section of the southern training wall and another toward the seaward tip of the northern breakwater. There has been sand accretion along the central section of the northern training wall.

10 INTRODUCTION 1-2 The south wall scour hole has been closely monitored due to implications for the sand bypassing pipeline crossing and the southern training wall stability. WBM Oceanics/BMT WBM have analysed the scour development and wall stability on three occasions (most recently BMT WBM, 2012). This analysis suggests that the entrance channel may be continuing a slow rate of scour of around 20,000m 3 /year with considerable annual fluctuations around his average value. The scour holes have not deepened significantly since about 1996, however their spatial extent has continued to slowly increase. It is uncertain whether the depth limitation has occurred due to encountering a nonerodible stratum. The geotechnical stability of the wall currently lies in the Factor of Safety range Therefore, remedial works options are being considered and future development works will need to carefully consider the potential for worsening the wall stability. 1.2 Earlier Cruise Ship Terminal Assessments In 2003 an interim options for cruise ship terminal development report was undertaken by Worley Parsons and WBM Oceanics (Worley & WBM, 2003). This report looked at 5 options for cruise ship terminal sites and considered a range of technical, operational and environmental issues. Twodimensional hydrodynamic, wave and some sediment transport modelling was undertaken to inform the quantification of impacts for these options. An analysis of the littoral transport system predicted that average sedimentation of the approach channel through the ebb delta bar may be of the order 200,000m 3 /year, with the potential for up to 60,000m 3 to be deposited in a single event. Channel and swing basin sedimentation within the Broadwater was estimated to be in the range 150, ,000m 3 /year. In 2006 a Notional Seaway EIS (GHD, 2006) considered a single Cruise Ship terminal. Twodimensional hydrodynamic and wave modelling and a conceptual analysis of sediment transport were used to assess impacts of the development. An annual rate of 350,000m 3 /annum maintenance dredging was estimated based on this assessment, with 200,000m 3 /annum inside the Seaway and 150,000m 3 /annum in the outer approach channel. In November 2012 a navigation simulation exercise was undertaken for GCCC at Smartship Australia s full mission bridge simulator facility in Brisbane (GCCC, 2012). The simulations considered the feasibility of navigating selected cruise ships to and from a proposed terminal within the Gold Coast Broadwater. The simulations proved that cruise ship operations were feasible subject to a number of assumptions and constraints regarding vessel configuration, dredged channel dimensions and operable wind/wave/tide conditions. 1.3 Study Scope The current study is intended to provide a high-level, preliminary assessment of potential impacts associated with cruise ship terminal options inside the Gold Coast Broadwater. These assessments have used suitably validated numerical models to consider: Tidal regime changes; Storm tide impacts; Flood impacts; Morphodynamic changes; and

11 INTRODUCTION 1-3 Sedimentation potential. Initial modelling assessments considered 6 potential cruise ship terminals, as shown in Figure 1-1, which was reduced to a shortlist of 3 options (S1, S2 & S3) for more detailed modelling. The northern channel option and southern most option were removed from further assessment primarily due to generating significantly higher impacts than the other options. The western most swing basin option was removed from ongoing assessment due to the constraint of relatively shallow rock in this area. The completed assessments have intentionally been performed at a high level, with a large number of different assessments being undertaken to inform a broad range of hydrodynamic and morphodynamic impacts. It should be noted that the degree of model calibration, validation and detail of the impact assessments described here may be less than would be required of a detailed EIS study. However, the purpose of this preliminary investigation is to provide initial results to inform the Broadwater Redevelopment Plan tendering process and to highlight any particular constraints and challenges which the detailed proposals will need to investigate in further detail.

12 INTRODUCTION 1-4 Figure 1-1 Cruise Ship Terminal Options from Study Brief

13 NUMERICAL MODEL DESCRIPTIONS NUMERICAL MODEL DESCRIPTIONS Multiple numerical models have been used to undertake the hydrodynamic and coastal process assessments of the cruise ship terminal options, as described below. 2.1 Waves (SWAN) The wave modelling component of these assessments has been undertaken using the spectral wave model SWAN. SWAN (Delft University of Technology, 2006) is a third-generation spectral wave model, which is capable of simulating the generation of waves by wind, dissipation by whitecapping, depth-induced wave breaking, bottom friction and wave-wave interactions in both deep and shallow water. SWAN simulates wave/swell propagation in two-dimensions, including shoaling and refraction due to spatial variations in bathymetry and currents. This is a global industry standard modelling package that has been applied with reliable results to many such investigations worldwide. The SWAN model of the Gold Coast has previously been developed as part of a commission by GCCC. This project included validation of the SWAN wave model predictions using ADCP measurements conducted in April-May 2011 at 5 locations in approximately 8-10m water depth (GCCC, 2011). The Gold Coast SWAN model is comprised of a series of nested regular grid wave models, ranging in resolution from 500m at the regional scale and 20m at the nearshore domain finest grid scale as shown in Figure 2-1. In this particular study SWAN was used to model the transformation of incoming waves from offshore deep water into the nearshore Gold Coast study area. Specifically the SWAN wave model was used to transform measured directional wave data from the Brisbane (Point Lookout) Waverider Buoy into shallow water. The wave model validation results are presented in Section Hydrodynamics (TUFLOW-FV) The hydrodynamic modelling component of these assessments has been undertaken using the TUFLOW-FV software, which is developed and distributed by BMT WBM ( TUFLOW-FV is a numerical hydrodynamic model for the two-dimensional (2D) and three-dimensional (3D) Non-Linear Shallow Water Equations (NLSWE). The model is suitable for solving a wide range of hydrodynamic systems ranging in scale from open channels and floodplains, through estuaries to coasts and oceans. The Finite-Volume (FV) numerical scheme employed by TUFLOW-FV is capable of solving the NLSWE on both structured rectilinear grids and unstructured meshes comprised of triangular and quadrilateral elements. The flexible mesh allows for seamless boundary fitting along complex coastlines or open channels as well as accurately and efficiently representing complex bathymetries with a minimum number of computational elements. The flexible mesh capability is particularly efficient at resolving a range of scales in a single model without requiring multiple domain nesting. TUFLOW-FV can also simulate the advection and dispersion of multiple scalar constituents within the water column. This capability has been used in the present study to model suspended sediment transport and to model the transport and dilution of recycled water streams.

14 NUMERICAL MODEL DESCRIPTIONS 2-2 Figure 2-1 SWAN Model Nested Domains Model Domain, Mesh and Bathymetry The broad scale model domain shown in Figure 2-2 extends from Double Island Point in the North to Kingscliff in the south and includes Moreton Bay, the Gold Coast Broadwater and all major rivers entering these water bodies extending upstream to their tidal limits. The model resolution at the offshore boundary is approximately 2.5km (cell side length), increasing to around 20m in the vicinity of the Seaway as shown in Figure 2-3. This figure also shows how the developed case dredge footprints have been included in the mesh in order to facilitate the accurate assessment of impacts (developed case minus base case) for the shortlisted configurations. A larger scale model domain with the same north-south extents but extending more than 100km offshore was used for the storm tide assessments, while a cut down version of the domain extending from Point Danger to north of Sovereign Island was used for the coastal process assessments in order to improve computational efficiency. Water level boundary conditions for the cut down model were extracted from the Regional model domain predictions. The model bathymetry has been derived from the following sources, listed in decreasing order of priority: GCCC 5m Broadwater DEM; BMT WBM 20m DEM developed for Moreton Bay RWQMv3 project; and BMT WBM 10m Gold Coast shoreline DEM developed for GCCC.

15 NUMERICAL MODEL DESCRIPTIONS 2-3 Figure 2-2 Regional Hydrodynamic Model Domain Figure 2-3 Hydrodynamic Mesh Detail in the Study Area

16 NUMERICAL MODEL DESCRIPTIONS Model Configuration Unless otherwise stated, the hydrodynamic model validation and impact assessments described in this report have used a 3D configuration of TUFLOW-FV. A hybrid z-coordinate vertical discretisation has been used with layer thickness between 1-2m in the top 20m of the water column. The model has used the following configurations and parameterisations: Smagorinsky model to estimate horizontal turbulent and sub-grid mixing; Coupling with GOTM ( to derive vertical turbulent mixing; Bottom drag derived from application of the log-law (to bottom most cell); and Bottom roughness length-scales of 0.01m. With the exception of the recycled water discharge assessments, the current modelling has not included the simulation of temperature or salinity as density coupled. This is considered to be a reasonable simplification within the Gold Coast Broadwater and littoral zone, where fresh water and temperature induced stratification are generally not likely to be maintained against the tidal, wind and wave-driven mixing mechanisms Boundary Conditions Tidal water level variations have been obtained from the TOPEX database of the 8 primary astronomic tide constituents derived from inverse modelling analysis of satellite water level altimetry measurements ( Wind boundary conditions are based on the measured Gold Coast Seaway wind record supplied by the Commonwealth Bureau of Meteorology. Wave boundary conditions have been derived from the SWAN model/s described in Section 2.1. These are applied as spatially and temporally varying wave fields that are then interpolated onto the TUFLOW-FV flexible mesh. Both un-coupled and fully-coupled wave models have been used, with the latter described in further detail in Section Freshwater inflows have not been included in the current modelling scope, except for the flood simulations. In general the Broadwater system is expected to have close to ocean salinity levels and to be well-mixed. However, this won t be the case during and for a period after significant fluvial flow events and in these circumstances the freshwater induced stratification will be expected to have an influence on vertical mixing and general circulation patterns. The Tropical Cyclone simulations undertaken in this study have used a parametric Holland (1980) cyclone pressure and windfield model to apply these spatially and temporally varying boundary conditions to TUFLOW-FV. One coast-crossing and one coast-parallel severe tropical cyclone were derived with reference to the recent climatology analysis undertaken for the Gold Coast and described in GHD (2013).

17 NUMERICAL MODEL DESCRIPTIONS Coastal Processes TUFLOW-FV The coastal process assessments in this study refer to the modelling of sediment transport driven by currents and waves. This has been undertaken using the sediment transport and morphology module within TUFLOW-FV Wave Coupling A dynamic 2-way coupling between the SWAN wave model and TUFLOW-FV has been implemented to provide the necessary littoral zone forcing of currents by the waves, as well as provide temporally and spatially varying bed elevation, water level and current fields to SWAN. The dynamic 2-way coupling of SWAN and TUFLOW-FV occurs within the inner SWAN model nested region shown in Figure 2-1. Outside this region an un-coupled wave model forcing has been applied, which does not feature dynamic variations in bed elevation, water level and current fields. The (short) wave model derived radiation stress gradients provide a source of momentum to the long wave model, which primarily drives the longshore currents in the surfzone. In addition the short wave motion Stokes Drift induces an additional mass transport in the direction of wave propagation that is applied to the hydrodynamic (long wave) model. Along an approximately straight and uniform coastline, the onshore mass transport is approximately balanced by an offshore directed current (or undertow ). The (short) wave model also provides wave parameter fields (H sig, T p, Direction) to the TUFLOW-FV sediment transport module Sediment Transport The TRANSPOR model (van Rijn, 2004) has been used to predict sediment transport within TUFLOW-FV. The TRANSPOR model is capable of representing multiple fraction sediment transport including wave and current related bedload and suspended load. The calculated bedload component is a direct input to the TUFLOW-FV morphological bed update scheme, while the suspended load component is converted to an equivalent sediment pickup rate (Nielsen, 1992), which provides a suspended sediment source term to the TUFLOW-FV water column advection-dispersion scheme (and corresponding sink term to the bed). Suspended sediment settling provides a sink term to the water column (and corresponding source term to the bed). TRANSPOR represents the interaction of both current and wave related sediment transport. The presence of waves can enhance sediment pickup and therefore also the rate of transport by the local currents. TRANSPOR also includes the prediction of wave-related sediment transport due to processes such as wave velocity skewness and wave boundary layer streaming. These (and other) processes can generate a net transport in the direction of (or against) wave travel, even in the absence of a local current. In the context of littoral zone morphological processes it is useful to make a distinction between longshore and cross-shore sediment transport. In an environment such as the Gold Coast longshore sediment transport is generally dominated by current-driven suspended sediment transport enhanced by additional wave pickup. Net cross-shore sediment transport is often comprised of an onshore contribution due to wave-related processes, more or less balanced by an offshore contribution from

18 NUMERICAL MODEL DESCRIPTIONS 2-6 the undertow current. It is important to appreciate that the complexity of modelling cross-shore sediment transport processes is inherently higher than the longshore transport component and for this reason the uncertainty associated with predicting cross-shore morphological response is correspondingly higher. Coastal bars (such as the Gold Coast Seaway) develop due to the interaction of both longshore and cross-shore sediment transport processes with the tidal (or fluvial) dominated estuarine system. Consequently morphological predictions of coastal bar systems are inherently difficult, and even using state-of-the-art modelling approaches the unavoidably high level of uncertainty in the results should be appreciated. A single sand fraction with median grain size D 50 =0.22mm has been adopted for the modelling assessments. The internal routines in TRANSPOR have been used to calculate bed roughness values based on sediment and hydrodynamic parameters. All other parameters have adopted the default values described in van Rijn (2004), except that a calibration factor has been applied directly to the total sediment transport as described in Section EVO Shoreline Model The littoral process and shoreline evolution model EVO, developed as part of an earlier project commissioned by GCCC, has been used in the current study as a tool for calibration of the littoral transport predictions of the TUFLOW-FV SWAN TRANSPOR modelling system. EVO is a shoreline evolution model that can represent the response to a range of processes of varying timescales: Short term processes (e.g. storm response); Medium term processes (e.g. long-shore transport gradients); and Long term processes (e.g. sea level rise). Key features of EVO are: Suitability for medium- to long-term simulations (years to centuries); Continuous (timeseries) forcing by offshore wave climate and water levels; Represents complex shorelines using a flexible Curvilinear grid; Offshore wave transformation tables, e.g. based on SWAN simulations; Inshore wave transformation to breaking using linear theory; Coupled long-shore and cross-shore shoreline response; Longshore transport based on CERC formula; Cross-shore response based on Equilibrium Profile Concept ; Representation of controls e.g. Groynes and Headlands; and Representation of seawalls and reefs. The Gold Coast EVO model extends approximately 50km from Letitia Spit in the south to Jumpinpin in the north as shown in Figure 2-4. The model has a grid resolution of 200m (or better) in the region from the Tweed River to the Gold Coast Seaway. Offshore wave boundary conditions are derived from the Point Lookout waverider buoy measurements, which provide the necessary directional data

19 NUMERICAL MODEL DESCRIPTIONS 2-7 post For simulations outside the period , the available wave data has been looped. Representative offshore water level boundary conditions are derived from the Gold Coast Seaway tide gauge measurements. A constant 550,000m 3 /annum sediment supply rate is input to the southern model boundary, while the northern model boundary is pinned. The Gold Coast model is run using an hourly timestep in order to have sufficient temporal resolution to respond to storm events. Mechanical bypassing by both dredging and trestle systems is included as sink/source terms model while nourishment from offshore borrow areas are included as source terms only. An offshore supply of sand to the Spit (north of Narrowneck) as identified in Patterson (2012) has also been included as a littoral zone source. During the simulation the most significant littoral zone structural works were dynamically added to the model as summarised in the following table. Table 2-1 Structures Dynamically Added to EVO Simulation Date Name 1963 Tweed River Walls 1972 Kirra Point Groyne 1973 Currumbin Groyne 1975 Little Kirra Groyne 1978 Tallebudgerra Groyne 1984 Palm Beach Groynes 1986 Gold Coast Seaway Figure 2-4 EVO Gold Coast Model Domain

20 NUMERICAL MODEL CALIBRATION & VALIDATION NUMERICAL MODEL CALIBRATION & VALIDATION 3.1 Waves The SWAN wave model has previously been validated against various measurements, including: Gold Coast waverider buoy data (DEHP, ); Gold Coast Seaway and Palm Beach ADCP measurements (GCCC, 2010); and Gold Coast nearshore ADCP measurements (GCCC, 2011) at the following locations: Kirra, Tugun, Palm Beach and Narrowneck. In all cases the model was forced with boundary wave parameters derived from the Point Lookout waverider buoy dataset. Calibration of the SWAN model/s primarily involved tuning of the following model parameters: Incoming wave directional spreading parameter; and Bed friction coefficients. The most relevant validation results for the current study, being for the Southport Seaway in 2010 and Narrowneck in 2011 are shown in Figure 3-1and Figure 3-2. It should be kept in mind that the Brisbane Recorded data is effectively a model input and that the model prediction and recorded data at the inshore point are being compared for goodness of fit. Both of these results demonstrate the good capability of the calibrated SWAN model at predicting nearshore wave conditions in the vicinity of the Gold Coast Seaway. 4 3 Southport Seaway Modelled Southport Seaway Recorded Brisbane Recorded Hsig (m) 2 Direction (degrees) /05 07/05 12/05 17/05 22/05 27/05 01/06 06/06 11/06 Time (2010) Southport Seaway Modelled Southport Seaway Recorded Brisbane Recorded 02/05 07/05 12/05 17/05 22/05 27/05 01/06 06/06 11/06 Time 2010 Figure 3-1 SWAN Model Calibration to Seaway 2010 Measurements

21 NUMERICAL MODEL CALIBRATION & VALIDATION Narrowneck Modelled Narrowneck Recorded Brisbane Recorded Hsig (m) 2 Direction (degrees) /04 22/04 27/04 02/05 07/05 12/05 17/05 22/05 27/05 Time (2011) Narrowneck Modelled Narrowneck Recorded Brisbane Recorded 17/04 22/04 27/04 02/05 07/05 12/05 17/05 22/05 27/05 Time (2011) Figure 3-2 SWAN Model Calibration to Narrowneck 2011 Measurements 3.2 Tidal Hydrodynamics The TUFLOW-FV tidal model has been first calibrated to a dataset from with water level and ADCP flow measurements at a large number of locations within the Broadwater system, and then validated to an ADCP transect dataset from the Seaway entrance and adjacent channels in The 2004/5 and 2009 datasets were both collected by the Griffith Centre for Coastal Management (GCCM) /5 Calibration The 2004/5 datasets included water level measurements at 18 locations throughout the Broadwater system from Russell Island in the north to Burleigh Waters in the south. The dataset also included 14 ADCP flow transects. The instrument and transect locations along with the complete set of water level and flow calibration plots are provided in Appendix A. Water level comparisons at Southport Seaway and at the Royal Pines site in the upper tidal reaches of the Nerang River are shown in Figure 3-3. Taking into account that the model is forced by astronomic tide only boundary conditions, the calibrated model shows a reasonably good capacity to predict the tidal range at all of the 2004/5 water level measurement locations. Flow comparisons for the Seaway main, north and south channels are provided in Figure 3-4. The flow comparisons also generally demonstrate that the model is accurately reproducing the measured tidal flows at a wide number of locations across the Broadwater. One apparent exception is in the Seaway northern channel during the peak flooding tide, where the model predicts significantly higher flows than the data. However, subsequent model validation at the same location (refer Figure 3-6), as well as closer inspection of the flow summation across the main Seaway channel and the north and south branches indicates that some of the recorded data may be questionable.

22 NUMERICAL MODEL CALIBRATION & VALIDATION 3-3 Figure 3-3 Water Level Comparison at two of the 2004/5 Data Locations

23 NUMERICAL MODEL CALIBRATION & VALIDATION 3-4 Figure 3-4 ADCP Flow Comparisons for Three Transects from the 2004/5 Campaign

24 NUMERICAL MODEL CALIBRATION & VALIDATION Validation The 2009 GCCM Seaway monitoring dataset was made up of boat-mounted ADCP data for 3 transects (main Seaway channel, north and south channels) on 3 occasions in February, March and April The measurements covered both spring and neap tidal ranges. The complete set of flow comparisons and some further velocity distribution comparisons are provided in Appendix B. Figure 3-5 shows peak flood and ebb tide transect comparisons for the Seaway main channel transect. The model comparisons are generally good and the model can be seen to accurately reproduce the flood tide flow recirculation (eddy) along the northern training wall. The ebb tide current distribution with strongest current speeds in the northern half of the channel is also well reproduced by the model. Figure 3-6 shows spring tide flow comparisons at the Seaway main, north and south channels and indicates that the model is doing a good job of representing the flow split distribution in this key area of the current study. Figure 3-5 Flood and Ebb Tide Transect Comparisons from the GCCM 2009 Monitoring

25 NUMERICAL MODEL CALIBRATION & VALIDATION 3-6 Figure 3-6 ADCP Flow Comparisons from GCCM February 2009 Monitoring

26 NUMERICAL MODEL CALIBRATION & VALIDATION Sediment Transport EVO Validation Validation of the Gold Coast EVO model has been undertaken in two stages, the first considering the period from , which was a period that included significant shoreline impacts associated with the construction of structures within the littoral zone. During this simulation the most significant littoral zone structural works were dynamically added to the model as summarised in Table 2-1, and the results were compared with the volume analysis undertaken by Macdonald & Patterson (1984). The second validation stage compared shoreline response predictions for the period (for which directional wave boundary condition data was available) with data derived from the regular ETA line surveys conducted by GCCC. An example of the EVO model validation to surveyed shoreline change at Narrowneck is shown below. Figure 3-7 EVO Shoreline Predictions Compared with Narrowneck Data TUFLOW-FV Littoral Transport Calibration TUFLOW-FV SWAN TRANSPOR littoral zone sediment transport predictions were calibrated to the EVO model longshore transport predictions for a number of simulations including significant storm (wave) events. A factor of 0.25 was derived and applied to the morphological model total sediment flux predictions in order to obtain a best-fit of the EVO model transport predictions. The comparison of the two model predictions for 3 month-long periods is shown in Figure 3-8 below. Given the very different models being compared here (one using deterministic sediment transport calculations and the other based on the CERC formula) the degree of agreement is generally very good across a wide range of conditions. No further calibration of the morphodynamic modelling system has been possible within the scope of the current preliminary impact assessments. Further work is to be undertaken to compare the predicted estuarine sediment transport predictions with knowledge about the evolution of the Broadwater following the Seaway construction in Further work could also be undertaken to tune individual components of the TRANSPOR model to best match surveyed bed changes. A validation of the model morphodynamic evolution predictions on the Seaway bar was undertaken using MSQ hydrographic surveys from the 19 th February 2009 and 16 th June These surveys are shown in Figure 3-9 along with the derived bed elevation change during the intervening period. A

27 NUMERICAL MODEL CALIBRATION & VALIDATION 3-8 large storm event occurred in May 2009, which may have contributed the majority of the bathymetric changes during the period. The equivalent period was simulated using the morphodynamic modelling system, with the predicted bed elevation changes shown in Figure Some similarities and differences are immediately apparent in comparing this figure with the surveyed data. For instance, the pattern of onshore migration from the outer bar is evident in both model and data. However, there are significant differences between the modelled and surveyed bed level changes in the vicinity of the southern breakwater and adjacent bar, in that the model has not predicted substantial growth and translation of a shoal with crest elevation ~-4m LAT across the Seaway entrance. This may relate to deficiencies in the standard TRANSPOR model predictions that may require more comprehensive calibration of coefficients than has been possible within the present scope.

28 NUMERICAL MODEL CALIBRATION & VALIDATION 3-9 Figure 3-8 Comparison of Calibrated TUFLOW-FV and EVO Longshore Transport Predictions

29 NUMERICAL MODEL CALIBRATION & VALIDATION 3-10 Figure 3-9 Seaway Surveys; 19 th February 2009 (Top), 16 th June 2009 (Middle) and Difference (Bottom)

30 NUMERICAL MODEL CALIBRATION & VALIDATION 3-11 Figure 3-10 Predicted Bed Elevation Change During May 2009 Simulation

31 EXISTING ENVIRONMENT EXISTING ENVIRONMENT This section uses the validated model to investigate the existing hydrodynamic and morphodynamic behaviour of the existing Gold Coast Seaway environment. 4.1 Tidal Water Levels The Seaway development and subsequent scouring has created relatively efficient hydraulic connection between the ocean and the Broadwater. As shown in Figure 4-1 the tidal range inside the Seaway is currently around 96% of the ocean tide. Figure 4-1 Water Level Timeseries Outside and Inside the Seaway 4.2 Tidal Currents Tidal current patterns in the Seaway are governed by the curved channel geometry and relative flow imbalance between the north and south channels (roughly 67% and 33% of total tidal flow respectively). Figure 4-2 shows a typical peak flood tide current distribution during a spring tidal range while Figure 4-3 shows a typical ebb tide. The ebb and flood tide current distributions show significantly different horizontal and vertical variations, however both exhibit peak spring tide surface speeds of around 2m/s. Due to the channel curvature the currents typically exhibit significant helical flow patterns, whereby the surface currents generally have a component towards the outside of the bend and the bottom currents have a substantial component towards the inside of the bend, as shown in the plan view and curtain plots below. The Seaway flows are complicated by the presence of two bends with diverging flow on the flood tide and converging on the ebb tide. A stronger helical flow is generated within the Seaway by the ebb tide flows.

32 EXISTING ENVIRONMENT 4-2 Figure 4-2 Flood Tide 3D Current Patterns Figure 4-3 Ebb Tide 3D Current Patterns

33 EXISTING ENVIRONMENT 4-3 The flood tide currents typically experience a flow separation from the northern wall, with eddies of varying size developing along the northern bank depending on the stage and strength of the tidal variation. Accurate modelling of this flow recirculation feature requires a relatively high model resolution, and in the case of this study ~10-15m cell sides were used in this area. The north wall flood tide recirculation is an important flow characteristic for the Pimpama/Coombabah STP release operations. Releases from the northern Seaway recycled water diffusers commence approximately 2 hours before high tide and relies on the flow recirculation to minimise dispersion of recycled water into the northern Broadwater as illustrated in Figure 4-4. The ebb tide currents are typically stronger along the northern wall than the flood tide currents due to no significant flow separation. Figure 4-4 Northern Wall Recycled Water Release Snapshot Showing Flood Tide Flow Separation Eddy 4.3 Waves Refraction of waves over the Seaway ebb delta bar bathymetry causes wave focussing at The Other Side (TOS) surf break on South Stradbroke Island. The TOS surf break amenity is further enhanced by the localised discharge of sediment from the bypassing plant which helps define the sand banks. Examples of the wave focussing effect at TOS are shown for south-easterly waves in Figure 4-5 and for easterly waves in Figure 4-6. The wave energy penetrating the Seaway and dissipating against the Wave Break Island shoreline is likely to have evolved in response to first the scouring of the entrance and secondly to the ongoing growth of the ebb delta bar. SWAN modelling of the present configuration (based on 2009 bathymetry) predicts that significant wave heights incident to Wave Break Island are generally less than 0.3m and can get as high as 0.5m for a large easterly wave event such as occurred in May 2009.

34 EXISTING ENVIRONMENT 4-4 Figure 4-5 South-Easterly Wave Transformation Example Figure 4-6 Easterly Wave Transformation Example 4.4 Morphology The Gold Coast Seaway has evolved to its current state following construction of the trained entrance in Following construction, the Seaway entrance channel has scoured from the initial constructed bed elevation of around -6m to -10m LAT to currently have channel centreline bed elevations generally in the range of -10 to -18m LAT. The evolution of the Seaway morphology occurred rapidly until the mid-1990 s and since then has continued to slowly evolve. The curvature of flow entering the main Seaway channel and the imbalance between north and south channel flows has resulted in a spatially variable pattern of scour and shoal development as shown in Figure 4-7. Scour holes have formed adjacent to the middle of the southern training wall, at the tip of the northern wall and off the north east tip of Wave Break Island. A triangular sand shoal has formed

35 EXISTING ENVIRONMENT 4-5 off the eastern shoreline of Wave Break Island and persistent tidal shoals have formed in the north and south navigation channels, which require periodic maintenance dredging. Analysis of ebb delta survey data (WBM Oceanics, 2001) indicated that the ebb delta bar initially grew at a rate of more than 450,000m 3 /year for the first 4 years following Seaway construction ( ) and that the rate of increase had dropped to below 200,000m 3 /year by More recent analysis by GCCC for the period (GCCC, pers. comm.) indicates that the rate of ebb delta growth has further dropped. The ebb delta growth has been fed by a combination of export of sand from the Seaway/Broadwater and by littoral supply leaking past the bypassing jetty. The ebb delta morphology is highly dynamic, with unstable shoals continually growing/shrinking and shifting. Three dominant shoal features can be seen in Figure 4-7; a sand bar extending from offshore of the bypassing plant to past the southern breakwater tip; a crescentic outer ebb delta bar; and a mid ebb delta shoal located to the north east of the entrance. As illustrated in the survey analysis of Figure 3-9, the height and position of these shoals can change dramatically, in particular associated with storm wave events. Figure 4-7 Gold Coast Seaway Morphological Features 4.5 Estuarine Sediment Transport The modelled net tidal sediment transport is shown in Figure 4-8 and indicates a net ebb dominant transport capacity due to tidal flows. There is a gradient from flood dominant transport along the southern wall to stronger ebb dominant transport along the northern wall, which can be understood in terms of the flood/ebb current distribution asymmetries discussed in Section 4.2. The tidal transport potential from the northern channel is much stronger than the transport capacity from the southern channel. While there is a strong ebb transport capacity shown at the Seaway entrance it should be noted that the actual sediment transport in the outer part of the Seaway will be due to the combined influence of

36 EXISTING ENVIRONMENT 4-6 both tidal currents and wave driven processes. The wave driven processes will generally deliver sand into the estuary entrance, while the ebb dominant tidal transport will attempt to export sand onto the ebb delta bar. There has been an overall export of sand from the entrance and the Broadwater since Seaway construction over 25 years ago. Initial morphological evolution occurred rapidly following construction, however since the mid 1990 s the system has continued to evolve relatively slowly. Therefore the system in its current state (i.e. Figure 4-7) is unlikely to have ongoing strong net sediment flux gradients, which would imply significant ongoing scour/accretion potential. Acknowledging that predictive models are never going to be perfect and that representing a dynamic equilibrium regime is particularly demanding of model predictive skill, the existing case modelled net sediment fluxes shown in Figure 4-8 are generally consistent with a system that is not too far removed from a dynamic equilibrium, with a net potential to export sediment from the system and in particular the north channel. Further analysis of the morphological evolution trends seen in survey data since Seaway construction, in tandem with numerical modelling analysis, is necessary in order to gain further more detailed understanding of the historical and future behaviour. Imposed changes to the system (such as from dredging) have the potential to disrupt the current state of (relative) balance and again lead to a period of more rapid morphological evolution as the system tries to re-establish an equilibrium. Predictive models have a particularly important role in assessing the likely nature of the system response to imposed changes. Figure 4-8 Net Tidal Sediment Transport 4.6 Littoral Sediment Transport The littoral sediment transport system is predominantly driven by waves along the straight coastlines to the north and south of the Seaway. In the vicinity of the seaway the wave driven currents and sediment transport interact with the tidal currents entering/exiting the Seaway entrance. The hydrodynamic and morphodynamic processes in the area of the ebb delta bar are extremely complex and therefore difficult to predict, even using state of the art modelling tools. Nevertheless

37 EXISTING ENVIRONMENT 4-7 considerable insight into the processes can be obtained through the exercise of undertaking and critically appraising numerical modelling. Figure 4-9 shows the pattern of net sediment transport for a period of predominantly south-easterly wave conditions. The results demonstrate the prevailing northerly transport of sand along the Southport Spit and predict a capacity for delivering sand across the Seaway entrance. The model results indicate only a relatively limited transport capacity on the south-eastern portion of the ebb delta, which may indicate that the bar has developed in an environment where the net littoral supply from the south has been entirely bypassed by mechanical means over the past 15 years. Figure 4-10 shows the net littoral sediment transport for a period of predominantly east-south-easterly waves. The northerly transport along the spit is considerably lower for this wave direction, however some capacity to deliver sand into the Seaway from the south remains. A much stronger potential to transport sand back into the Seaway from the north is also apparent. Figure 4-9 Net Littoral Sediment Transport for South-Easterly Waves Figure 4-10 Net Littoral Sediment Transport for East-South-Easterly Waves

38 EXISTING ENVIRONMENT 4-8 The EVO model (described in Section 2.3.2) was used to hindcast longshore sediment transport rates at the northern end of the Southport Spit for the period for which directional wave data was available. The following statistics were derived from the continuously modelled longshore transport: Net longshore sediment transport ~640,000m 3 /year; Net northerly transport ~690,000m 3 /year; Net southerly transport ~50,000m 3 /year; Gross longshore transport ~740,000m 3 /year; Maximum monthly transport ~120,000m 3 /month (occurred in March 2006); and 90%ile monthly transport ~80,000m 3 /month. The annual rate of sand bypassing at the Seaway since commissioning in 1986 is shown in Figure 4-11 along with the modelled longshore transport for the period since Prior to 1997 the bypassing plant averaged around 440,000m 3 /year, which is likely to have been around 200,000m 3 /year less than the longshore transport during this period. It is worthwhile noting that the ebb delta bar was estimated to initially grow at a rate of around 450,000m 3 /year immediately following Seaway construction dropping to around 200,000m 3 /annum by the end of this period. It is likely that this volume gain was fed by both significant export of sand from the Broadwater and significant leakage of sand past the bypassing plant. For the period the estimated net longshore sediment transport was 640,000m 3 /year and the average pumped bypassing rate was slightly higher at around 660,000m 3 /year. It is interesting to note the high degree of correlation between the modelled and bypassed sand transport timeseries in Figure 4-11, which is understood to be a consequence of the sand bypassing plant capacity being limited by the littoral systems ability to deliver sand to the intake facility. The ebb delta growth rate is understood to have reduced since it was last analysed in detail in the early 2000 s (WBM Oceanics, 2001). It should be noted that the period 1996 to 2012 is not necessarily representative of the long term climate and is likely to be somewhat biased to the El Nino phase of the Southern Oscillation cycle. Therefore the available period of wave data is likely to have somewhat lower energy wave statistics than the longer term climatology and this bias is likely to affect the derived longshore transport statistics provided above. The modelled littoral transport patterns (shown in Figure 4-9 and Figure 4-10) would support a hypothesis that the Seaway bar has developed into a form which is in dynamic equilibrium with the situation (since 1997) of almost 100% pumped bypassing of the littoral supply. That is the net transport capacity of the ebb delta may be close to zero, despite the gross transport throughout the system being substantial. That is, there is substantial movement of sand both in the littoral supply to/from the bar and within the ebb delta without necessarily any substantial net transport across the system. The important point is that any dredged channel will be subject to sedimentation due to the local gross transport and the net transport does not necessarily determine the potential for channel infilling. Further survey data analysis, model development, validation and application would be required to further test this hypothesis.

39 EXISTING ENVIRONMENT Conceptual Sand Budget A conceptual sand budget for the contemporary (circa 2013) Gold Coast Seaway system is shown in Figure The sand budget model illustrates: Net longshore transport into the system of 590,000m 3 /year; (this is higher than the estimated long term average of 550,000m 3 /year due to substantial updrift nourishment at Surfers Paradise and Narrowneck); ~40,000m 3 /year supply to The Spit littoral system from the remnant Nerang River bar sand lobe (this rate is diminishing with time); Sediment export from the Broadwater of ~80,000m 3 /year; Net longshore transport at the bypassing jetty of ~630,000m 3 /year; Contemporary sand bypassing rate of around 630,000m 3 /year; Ebb delta bar growth of around ~100,000m 3 /year; and Minor net transport through the bar system but substantial gross transport (with the potential to infill a dredged channel). Figure 4-11 Seaway Bypassing and Modelled Longshore Sand Transport

40 EXISTING ENVIRONMENT 4-10 Figure 4-12 Conceptual Sand Budget Model

41 SCENARIOS ASSESSED SCENARIOS ASSESSED This preliminary assessment has initially considered 5 options before undertaking a broader range of impact assessments for 3 shortlisted options. 5.1 Dredged Channel Dimensions Dredged channel dimensions have been adopted as firstly specified in the 2006 EIS (GHD, 2006) and subsequently refined during the 2012 Navigation Simulations. The modelled dredged channel specifications, including sand trap and overdredging allowances are: Approach channel (through offshore bar): 140m width + 30m extra sand trap width (each side). Navigation depth -12m LAT + 1m overdredge allowance. 1:10 batter slopes. Seaway entrance channel: 140m width. Navigation depth -12m LAT + 0.5m overdredge allowance. 1:4 batter slopes. Channel bends: 210m width. Navigation depth -11m LAT + 0.5m overdredge allowance. 1:4 batter slopes. Swing basins: 500m diameter. Navigation depth -9.2m LAT + 0.5m overdredge allowance. 5.2 Initial Assessments Preliminary assessments were undertaken during the early project stages for 4 scenarios from the study brief (Option 1, Option2, Option 3 and Option 4 from Figure 1-1) plus 1 additional option in the northern channel adjacent to Wave Break Island. Tidal hydrodynamic impacts were assessed for these options with an early version of the hydrodynamic model. Based on this preliminary impact analysis, the most southern and northern options were dropped due to substantially larger tidal impacts than the other options. The most westerly swing basins (Option 2 and Option 6b) were dropped for other reasons, partly due to the shallow depth of bedrock in the western Broadwater. Following the presentation of the preliminary assessment results, an additional option utilising a reconfigured Marine Stadium precinct for berthing was added for consideration in the detailed assessment stage of this study.

42 SCENARIOS ASSESSED Configuration Shortlist Three options were shortlisted for detailed analysis as shown in Figure 5-1 to Figure 5-3 below. These options are referred to as Option S1, S2 and S3 throughout the remainder of this report. Options S3 was developed with the principal objective of reducing impacts by minimising the dredge footprint within the Seaway/Broadwater channels. The breakdown of the capital dredging volumes for these options is provided in the table below. The derived option S1 dredge sub-volumes are broadly consistent with those summarised in the Notional Seaway Project EIS (GHD, 2006), with the exception of the Swing Basin volume of 0.8Mm 3, which is significantly less than the previously derived 1.46Mm 3. This is due to the assumed swing basin depth (plus overdredge allowance) being 1.3m shallower than in the earlier assessment, in addition to a location further east requiring less dredging of the Wave Break Island sand shoal. Option Outer Channel Table 5-1 Capital Dredging Volumes (Mm 3 ) Seaway Channel Inner Channel Swing Basin S S Total S * * * Removal of existing land and new reclamation has been excluded from calculations. Figure 5-1 Option S1

43 SCENARIOS ASSESSED 5-3 Figure 5-2 Option S2 Figure 5-3 Option S3

44 HYDRODYNAMIC ASSESSMENTS HYDRODYNAMIC ASSESSMENTS The calibrated hydrodynamic model has been used to undertake the following impacts assessments for the 3 shortlisted options: Tidal water levels; Tidal currents; Storm tide water levels; and Flood water levels. The following sections firstly describe the methodology and then summarise the results of these impact assessments. 6.1 Methodology Tidal hydrodynamics A 14 day spring-neap cycle (01/01/ /01/2009) was simulated for the base (existing) case and the 3 shortlist options. High tide level impacts were assessed by deriving and comparing the 95 th percentile water level, while low tide level impacts were similarly assessed using the 5 th percentile water level. The impacts to peak ebb and peak flood spring tide currents were assessed by comparing base and developed case depth-averaged current field snapshots. The tidal water level impacts are presented in the following figures: Option S1: Figure 6-4; Option S2: Figure 6-10; and Option S3: Figure The tidal current impacts are presented in the following figures: Option S1: Flood tide Figure 6-5 and Ebb tide Figure 6-6; Option S2: Flood tide Figure 6-11 and Ebb tide Figure 6-12; and Option S3: Flood tide Figure 6-17 and Ebb tide Figure It should be noted that the hydrodynamic impact assessment modelling presented in Section 6 does not include forcing by waves, and therefore current patterns outside the Seaway entrance are unlikely to be representative of prevailing conditions. The absence of wave forcing is unlikely to make a substantial difference to water level or current predictions inside the Seaway entrance channel or Broadwater Storm Tide hydrodynamics The impact of the Cruise Ship terminal dredging on the penetration of storm surges into the Broadwater was assessed by simulating 2 severe tropical cyclones; one crossing the coast

45 HYDRODYNAMIC ASSESSMENTS 6-2 approximately 50km north of the Seaway and the other tracking parallel to the coast approximately 50km offshore. The tropical cyclones were simulated using a Holland (1980) cyclone pressure and windfield model and parameters were derived with reference to GHD (2013) cyclone climatology analysis. The simulated tropical cyclone central pressure was 940hPa, which is estimated to be a Maximum Probable Intensity (MPI) event for the Gold Coast. The probability of experiencing an MPI tropical cyclone in the South-East Queensland region was estimated by GHD (2013) at around a 70 year Average Recurrence Interval (ARI). However the likelihood of such a system making a track within 50km of the Gold Coast would be much lower than this. Without making a formal assessment of the probability of the modelled events, it is reasonable to expect that these particular tracks would generate a storm surge with an ARI well in excess of 200 years. The other adopted Holland model parameters are summarised in the table below. Table 6-1 Modelled Tropical Cyclone Parameters Parameter Coast Crossing Coast Parallel Bearing ( true) Minimum distance from Seaway (km) Central Pressure (hpa) Radius to maximum winds (km) Windfield peakedness parameter The storm surge simulations were performed with a 2D depth-averaged version of the hydrodynamic model, using a more extensive domain that extended at least 100km offshore. Tidal forcing was not included in the tropical cyclone simulations. Only wind and pressure driven surge was represented as simulations were not coupled with a wave model. The peak surge level for the coast crossing event is approximately 0.9m in the vicinity of Wave Break Island, while the coast parallel track produces a lower peak surge of around 0.5m. Peak water level in the upper reaches of the river and canal systems is probably substantially overestimated as no topographic reduction factor has been applied to the winds over land (i.e. the windspeeds are representative of open ocean winds) Flood hydrodynamics GCCC s Waterway and Flood Management group supplied critical duration 1% Annual Exceedance Probability flood hydrographs for the following systems draining into the Broadwater: Nerang River, including tributaries; Biggera Creek; Coomera River; Pimpama River; and Logan & Albert Rivers; The flood simulations undertaken for the purpose of cruise ship terminal impact assessments simulated a simultaneous flood in all 5 of these catchments. It is acknowledged that this is an extremely unlikely event in reality; however it greatly simplified this particular impact assessment, which was expected to show minor flood level benefits due to the dredging.

46 HYDRODYNAMIC ASSESSMENTS 6-3 It should be noted that the TUFLOW-FV model used for these assessments was primarily developed for simulation of tidal hydrodynamics and does not include representation of the extensive floodplains that will act as significant storages and overland flowpaths during extreme events such as those simulated here. Therefore the modelled levels are expected to be somewhat higher in the lower reaches of the river systems, and potentially much higher in the upper reaches than would occur in reality, or would be simulated using a model including the floodplain. However, a more refined assessment is unlikely to arrive at significantly different impacts (benefits) than those shown here. 6.2 Results Option S1 The hydrodynamic impact assessments for Option S1 are presented in Figure 6-4 to Figure 6-9. Low tide water levels (as represented by the 5 th percentile water level) are predicted to be lowered by around 0.5-1cm. High tide water levels (as represented by the 95 th percentile water level) are predicted to be increased by around 0.5cm. As illustrated in the timeseries plot of Figure 6-2 the overall tidal range is predicted to experience only a relatively small increase due to the Option S1 dredging. Substantial current speed changes (>0.1m/s) are generally confined to the dredge footprint or immediately adjacent areas. Flood tide currents are substantially reduced within the dredge footprint and at some localised areas immediately outside its perimeter. The spatial extent and strength of the northern wall recirculation is reduced. Similarly ebb tide currents are also reduced within the dredge footprint, with localised increases around the perimeter. The strength of the ebb jet currents are reduced by the substantially deepened flowpath through the ebb delta. Note that this simulation did not include wave driven currents. Aside from the deeper channel depths, navigability of the Seaway would be expected to be generally enhanced by the current speed reductions. Navigability of the bar would in particular be enhanced for all vessel sizes by the deep channel that would limit wave breaking. As shown in the timeseries plot of Figure 6-2 and Figure 6-3, Option S1 current speed impacts are predicted to be negligible in the Nerang River system (upstream of the Highway bridge) and in the Coomera River system. This, along with the minor tide range impacts would suggest that canal revetment stability would be unlikely to be affected by Option S1. The coast-crossing cyclone storm surge is slightly reduced by around 5cm due to the Option S1 dredging. This cyclone track produced a peak surge height of approximately 0.9m inside the Seaway. In the case of a coast-crossing cyclone making landfall to the north of the Seaway the surge tends to propagate southwards within the Broadwater under the influence of northerly winds as the cyclone makes landfall. In this particular case the dredged Seaway more efficiently drains the elevated Broadwater surge offshore, resulting in the peak surge reduction. The coast-parallel cyclone storm surge is increased by up to 0.1m immediately inside the Seaway entrance. The coast-parallel cyclone track produced a peak surge height of around 0.5m inside the Seaway, which is significantly lower than the peak surge for the coast-crossing case. In this particular case the surge is primarily driven by northerly currents and elevated water levels offshore of

47 HYDRODYNAMIC ASSESSMENTS 6-4 the Broadwater, which are able to more efficiently propagate into the Broadwater due to the dredging, resulting in the peak surge increase. The simple flood modelling assessment undertaken as part of this study, indicates that flood water levels will be slightly reduced (by around 1cm) in the most downstream portion of the Broadwater system between Sovereign Island (in the north) and the Gold Coast Highway, Nerang River bridge (in the south). It should be noted that this simulation did not include a moving tidal downstream boundary condition and that in reality the increased tidal range due to Seaway dredging may partially offset the marginally increased flood drainage efficiency Option S2 The hydrodynamic impact assessments for Option S2 are presented in Figure 6-10 to Figure Low tide water levels (as represented by the 5 th percentile water level) are predicted to be lowered by around 2-3cm, with a 1cm reduction extending to the upper tidal reaches of the Nerang River. High tide water levels (as represented by the 95 th percentile water level) are predicted to be increased by up to 2.5cm, with a 0.5-1cm increase extending to the upper tidal reaches of the Nerang River. Current impacts for Option S2 are much more extensive due to the dredging of the south channel. Flood and ebb tide current increases of more than 0.1m/s occur both adjacent to the dredge footprint and further afield. Under this option, the Labrador Channel, which runs north-south to the west of Wave Break Island, carries a higher flow with associated increased current speeds. As shown in Figure 6-3 Option S2 generates some slight (~2cm/s or less than 5%) current speed impacts in the Nerang River at the Highway Bridge. Current impacts in the Coomera River are predicted to be imperceptible for Option S2. The coast-crossing cyclone storm surge is more significantly reduced (by around 0.1m) in the Nerang River system. The coast-parallel cyclone storm surge increase is slightly more substantial than for Option S1 at around 0.1m. The flood water level reductions for this option are substantially more extensive within the Nerang River system due to the south channel dredging. Peak flood level reductions of in excess of 5cm extend slightly upstream of Chevron Island Option S3 The hydrodynamic impact assessments for Option S3 are presented in Figure 6-16 to Figure The tidal, storm surge and flood hydrodynamic impacts predicted for Option S3 are very similar to Option S1 due to the similar extent of dredging in the Seaway entrance. The re-configured Marine Stadium basin remains closed at the southern end and therefore does not attract tidal flow entering the Broadwater. 6.3 Discussion This section has described the methodology and results from a broad range of hydrodynamic impact assessments for 3 cruise ship terminal options inside the Gold Coast Seaway.

48 HYDRODYNAMIC ASSESSMENTS 6-5 These results demonstrate that dredging of a channel into the Seaway will increase the efficiency of tidal exchange and will result in a modest tidal range increase within the Broadwater and attached tidal rivers. Similarly, the efficiency of storm surge propagation into and out of the Broadwater will be increased by the dredging, as will the release of flood water. Comparison of the 3 options clearly demonstrates that the two options with the least amount of dredging footprint within the Broadwater (Options S1 and S3) have a significantly smaller hydrodynamic impact than Option S2, which involves around 1.7km of further dredging of the southern channel. A hypothetical case with only the outer approach channel dredging (up to the Seaway training walls) has been simulated in order to provide some perspective on the relative contribution of the inner and outer dredging to the hydrodynamic impacts. The tidal range impacts of this hypothetical case are shown in Figure The channel only dredging has negligible impacts on low tide levels, while the high tide impacts are broadly similar to Options S1 and S2. Impacts to tidal currents and water levels within the Nerang River and Coomera River systems are predicted to be only minor (imperceptible) for the cases that minimise the dredge footprint within the Broadwater (S1 and S3), and therefore no adverse effects on canal revetment stability would be anticipated for these options. Any substantial current speed impacts are generally confined to the dredge footprint or immediately around its perimeter. Navigability of the Seaway would generally be improved for all vessel sizes (up to the maximum design vessel), due in part to a reduction in peak current speeds and in particular due to the benefits of a deep and defined channel through the outer bar. Navigation simulations were beyond the scope of the present study, however it is recommended that detailed consideration be given to the complex current patterns that could potentially be generated by various combinations of tide, wind and wave conditions in the ebb delta approach channel. It is possible that certain combinations of conditions could generate significant cross-currents in the approach channel. Due to the broad ranging scope of this preliminary study the analysis of storm tide impacts has been limited to two hypothetical tropical cyclones, representing one coast-crossing and one coast-parallel track. An estimated MPI central pressure of 940hPa (GHD, 2013) was adopted for the preliminary impact assessment which therefore represents two very low probability (high ARI) scenarios. A formal assessment of the ARI of the modelled events is beyond the scope of the present study, however it is recommended that in order to aid future more detailed studies GCCC considers the development of formal assessment guidelines for storm tide impacts within its waterways. The two storm tracks produced significantly different peak storm surges within the Broadwater; 0.9m and 0.5m for the coast-crossing and coast-parallel tracks respectively. The impacts were also significantly different with a reduction in peak surge level of m for the coast-crossing track and similar increase for the coast-parallel track. These results demonstrate the sensitivity of a complex system such as the Gold Coast Broadwater to the cyclone track parameters (direction and landfall location) and why a comprehensive assessment methodology may need to be based around a probabilistic approach, such as outlined in the Ocean Hazard Assessment Stage 1 Report (Queensland Government, 2001).

49 HYDRODYNAMIC ASSESSMENTS 6-6 The University of Queensland have undertaken field measurements at the Seaway which indicate that wave setup is not a large contributor to storm tide levels within the Broadwater (Nielsen, 2010). However, the matter of relatively large tidal residual measurements at the old Seaway tide recorder (formally located opposite Wave Break Island) has not been resolved to date (GHD, 2013). In particular, during Tropical Cyclone Roger in March 1993 a peak tidal residual of around 0.8m was measured at the Seaway tide recorder while at the other regional tide gauges (Mooloolaba, Offshore Tweed and Brisbane) the residual during this event was generally below 0.4m. The flooding analysis has also adopted a simplified methodology of simulating all catchment runoff events concurrently. This analysis has predicted minor reduction in flood levels within the lower Broadwater system, with imperceptible reductions within the river and creek systems. In reality tides, tidal anomalies (storm surges) and fluvial flooding will all simultaneously interact within the Broadwater. While the present study has deliberately separated the analyses of these processes, a more detailed assessment should consider how these processes may combine (in terms of probabilities) and how they may interact (possibly non-linearly) within the system. Coastal infra-gravity waves ( long waves ) have not been considered in this preliminary assessment, however under certain conditions they may conceivably generate a significant hydrodynamic response within the Seaway, which may be of consequence to cruise ship mooring. It is recommended that an analysis of available water level and current measurement data be undertaken to investigate whether they exhibit any signals that may indicate the influence (or absence) of coastal infra-gravity waves. Modelling studies might be necessary to investigate whether the proposed dredging may have any impact on coastal infra-gravity wave occurrence.

50 HYDRODYNAMIC ASSESSMENTS 6-7 Figure 6-1 Water Level Timeseries Figure 6-2 Coomera River Mouth Current Timeseries Figure 6-3 Nerang River Mouth Current Timeseries

51 HYDRODYNAMIC ASSESSMENTS 6-8 Figure 6-4 Option S1 - Tidal Water Level Impacts. Low Tide (Left), High Tide (Right)

52 HYDRODYNAMIC ASSESSMENTS 6-9 Figure 6-5 Option S1 Flood Tide Currents. Base (Right), Developed (Middle) and Impacts (Left)

53 HYDRODYNAMIC ASSESSMENTS 6-10 Figure 6-6 Option S1- Ebb Tide Currents.. Base (Right), Developed (Middle) and Impacts (Left)

54 HYDRODYNAMIC ASSESSMENTS 6-11 Figure 6-7 Option S1 Coast Crossing Cyclone Peak Surge (Left) and Impact (Right)