ENVIRONMENT REPORT PORT PHILLIP BAY AND WESTERN PORT RECEIVING WATER QUALITY MODELLING: SCENARIOS Publication 1380 June 2011 A report developed for the ‘Better Bays & Waterways’ program, a water quality improvement plan for Port Phillip Bay and Western Port and their catchments. PORT PHILLIP BAY AND WESTERN PORT RECEIVING WATER QUALITY MODELLING: SCENARIOS REPORT STATUS Version Date Status Prepared Reviewed Approved By: By: By: V.1 May 2007 Draft SH, KPB, CB KPB KPB V.2 November 2007 Final SH, KPB KPB KPB V.3 May 2009 Final revised JO, KPB RL KPB Disclaimer: This document was prepared to inform the development of the Better Bays and Waterways Water Quality Improvement Plan and does not necessarily reflect the views of EPA Victoria. EPA Victoria makes no express or implied guarantees or representations to any third party as to the contents of this document. EPA Victoria accepts no liability for any reliance by any third party on the information detailed in this document. Joint EPA and ASR Report No. 2008-EPA1 May 2009 This document was prepared to inform the development of the Better Bays and Waterways Water Quality Improvement Plan. Recommendations included in these interim reports may have been superseded by actions contained in the final plan. Prepared by R. Lee, K. Black and J. Oldman PORT PHILLIP BAY AND WESTERN PORT RECEIVING WATER QUALITY MODELLING: SCENARIOS EXECUTIVE SUMMARY A fully integrated suite of receiving water quality models of Port Phillip Bay and Western Port was developed for the Better Bays and Waterways (BBW) program at the request of Environment Protection Authority of Victoria (EPA Victoria) and Melbourne Water. Four coupled models were created and installed on the EPA (Vic) computer with other baywide supporting simulations and a comprehensive range of graphical and data analysis tools. All software came from the package ‘The 3DD Suite™ of Marine and Freshwater Numerical Models’. The fourfold model coupling involved: • a dynamic catchment model (defining freshwater, nutrient and mud inputs) • three-dimensional salinity and temperature stratified hydrodynamics (for tidal, wind and density-induced circulation) • dispersal (for effluent, sediment transport and pollutant dispersal) • primary production (for nutrient uptake, phytoplankton and zooplankton). The models covered Port Phillip Bay, Western Port and northern Bass Strait. Applications ranged from the impacts of nutrients released into Port Phillip Bay to mud transport modelling in flood events. Global climate change predictions were also made in this broad-ranging study, which is the first environmental overview since the CSIRO Port Phillip Bay Environmental Study in the mid-1990s. In the current study, Western Port is also treated. The models simulate the behaviour and dispersal of key water quality indicators as prescribed by the Department of Water, Environment, Heritage and the Arts for BBW. The indicators include nutrients (total nitrogen and total phosphorus), chlorophyll-a (Chl-a), suspended solids, salinity, toxicants, pathogens and litter. The models address catchment loading in context with in-bay processing through dispersion and exchange, and ecosystem assimilation. The indicators used in this report are by no means an exhaustive assessment of marine condition in terms of broader ecosystem issues. Applications ranged from the mapping of averaged indicators over a two-year simulation period to specific footprints representing event impacts. The 2004–05 simulation period took into account a major nitrogen reduction improvement at the Western Treatment Plant in November 2004 and a significant (1-in-100-year) flood event in February 2005. This simulation period also represented prolonged drought conditions (since 1998) that differ vastly from the modelled (and observed) conditions during the CSIRO Port Phillip Bay Study in the early 1990s, when climate was significantly wetter and cooler. Model scenarios that incorporated global climate change predictions for 2030 and 2070 were run and compared with current conditions to identify regions of heightened vulnerability to climate impact. This report deals with the scenario modelling (Beaches Stage 15), which builds upon the development phase (Beaches Stage 7, see the first three reports in this series) to inform the Port Phillip Bay and Western Port Water Quality Improvement Plan on current and projected conditions. Four aspects were identified for this stage: • assessment of recent (2004–05) water quality conditions in Port Phillip Bay and Western Port • future growth and climate change scenarios • nested studies for priority beaches identified in the companion sampling program (Beaches Stage 9, EPA 2008b) • investigation of key issues for each bay through specialised modelling studies. These modelling tasks incorporate daily catchment flows and water quality loadings generated by the PortsE2 catchment model that enter the bays via 42 discrete points and diffuse discharge locations. Comparisons to in-situ data are clear and the increased spatial resolution of the models enhances the understanding of load dispersion, settlement and decay within the bays. Resultant dispersion patterns of pollutants indicate an overwhelming influence of wet-weather events to transport catchment loads to the bay. Long-term periods of wet and dry conditions, as observed during 1992–96 and 1997–2007 respectively, can cause vastly different dispersion patterns for pollutants discharged to the bay. Climate change and future growth scenarios incorporate 2030 and 2070 projections for rainfall and evaporation in the Melbourne region (using the CSIRO Mk3 A1F1 medium-sensitivity scenario from Whetton and Power 2007) and the Department of Sustainability and Environment’s (DSE) 2030 planning scheme for Melbourne. The 2030 scenarios were adopted by the PortsE2 climate change scenario modelling for the Better Bays and Waterways decision support system. There are significant differences in ocean flushing between Port Phillip Bay (approximately one year) and Western Port (up to about a month), so initial studies isolate the effect of shifting the 1 PORT PHILLIP BAY AND WESTERN PORT RECEIVING WATER QUALITY MODELLING: SCENARIOS rainfall/evaporation balance in Port Phillip Bay. To enable this assessment the models are developed to incorporate an evaporation scheme and calibrated to in-situ salinity data. Models are compared between wetter conditions from the 1990s and projected dry conditions for 2030 and 2070, and mark a two to three PSU shift in salinity conditions from hyposaline (less salty than ocean) to hypersaline (more salty than ocean), which significantly alters the circulation and flushing of the bay. Comparing the extended drought conditions of 2004–05 with 2030 and 2070 projections, there are more subtle changes to salinity and temperature. Differences in pollutant dispersal manifest as constrained footprints from the major discharge points. Projected simulations for Western Port indicate significant increases in temperature (Western arm) and salinity (North-East arm). Companion dispersion modelling shows reduced loads increase the coastal concentration of pollutants near the discharge points and cause a subsequent downstream decrease in concentrations. Beach scenarios assess the potential of impacts at priority beaches from the PortsE2 discharge points. A nested 30 m grid model for the Swan Bay-Queenscliff region confirms observations that source waters from Swan Bay and ‘The Cut’ impact the beach during dry weather conditions. An investigation of the various pathogen sources in this region (bird roosting in Swan Bay, marina discharges, ferry operations) is recommended to assist in determining a mitigation strategy. The Rye-Rosebud and Altona–St Kilda priority beach regions are assessed for prevailing conditions and pathogen dispersal. In the northern region, coastally trapped waters disperse discharges alongshore, accumulating pathogens at the two priority beach regions of Altona and St Kilda. In the SE region, poor circulation in the Rye–Rosebud area causes pathogens to accumulate alongshore in the vicinity of the discharge points. This modelling highlighted the priority beaches as vulnerable regions impacted by accumulative coastal discharge. Configured for a range of local threat sources, further modelling could prioritise and track proposed mitigation strategies. Focus studies are undertaken to assess plankton–nutrient responses in Port Phillip Bay and sediment resuspension in Western Port. Phytoplankton response to current and projected conditions is explored using a coupled nutrient– phytoplankton–zooplankton model (NPZ). Further refinements of the NPZ model have advanced this into a highly evolved tool, allowing dynamic interaction of biota and nutrients within a complex system. While current and projected 2030 scenarios show similar patterns, the model indicates that significant increases in nutrients can lead to substantial changes in Chl-a levels. It has highlighted the need to investigate further the sensitivity of thresholds above which the bay could be subject to substantial and numerous algal outbreaks, particularly during favourable spring and summer conditions. The sediment dispersion model is augmented to include wave mixing and in-bay processes that are able to resuspend and transport sediment originally discharged from the catchment. Significant increases in suspended material occur across the system, with a strong preference for heightened concentrations in the eastern arm. This emphasises