Forecasting non-tidal water level in Port Phillip Bay, Australia

When Viva Energy Australia asked MetOcean Solutions to model water levels and storm surges in Port Phillip Bay (Victoria, Australia), we jumped at the challenge. With its large surface area, shallow depth and restricted entrance, resolving the hydrodynamics of Melbourne’s main coastal inlet required careful thought.

“Viva Energy is focussed on maintaining safe under keel clearance required for ships entering Port Phillip Bay,” explains Senior Oceanographer Dr Rafael Soutelino, the project leader. “The DUKC® system recently commissioned by the Victorian Regional Channels Authority is already delivering Viva Energy thousands of tons of extra cargo on every shipment, however, improved forecasting of non-tidal water levels has the potential to further increase safe vessel draughts and increase operational planning windows. Port Phillip Bay is a complex location subject to a variety of different forcings, and discerning the interaction between all the different influences on water level was crucial if we were to make accurate forecasts for their operations.

“The main issue for Viva Energy is negative storm surge when water depth is reduced due to the complex interplay between the meteorological and hydrodynamic processes. There are particular shallow patches and sand banks along the shipping channel, which can get too shallow for larger vessels to pass. Viva Energy required us to set up a robust forecast model for the Bay, generating alerts when water depths were likely to dip below safe levels. The detailed water level forecasts we produce will also be combined with recent observations and used in the DUKC® to provide improved planning advice to all port users.”

To generate accurate forecasts, the modelling had to include a variety of forcings, including the inverse barometric effect on water levels, the important phase lag effects caused by the constrained embayment, and the local and regional meteorological effects due to wind. The model also had to capture the behaviour of the progressive coastal-trapped waves that propagate eastwards across Australia’s extensive southern coastline.

“We used the Regional Ocean Modeling System (ROMS) for this work,” continues Rafael. “To translate the wide forcing into the local area of the Bay, we downscaled from a 5 km resolution parent domain covering most of the Australian South coast to a local 300 m resolution domain covering Port Phillip Bay. The model is forced by the ECMWF forecast winds, and has an external boundary condition prescribed by the Mercator solution.”  

The ROMS 2D model grid setup. The upper panel shows the parent 5 km domain. The lower panel shows three different zoom levels of the child nest bottom topography, grid nodes and connectivity between parent and child models.

The ROMS 2D model grid setup. The upper panel shows the parent 5 km domain. The lower panel shows three different zoom levels of the child nest bottom topography, grid nodes and connectivity between parent and child models.

Viva Energy provided a host of water level data for the model validation.

“Previous studies have shown that storm surges generated in the southwest of the Australian coast propagate eastwards toward Port Phillip Bay as coastal-trapped waves. On many occasions, that travelling patch of high water level is consistently re-intensified by the storm which is also propagating eastwards, providing the perfect set of ingredients for storm surge build up.”

A significant extra-tropical storm event in June 2014 caused a strong and extensive storm surge while propagating from west to east off the south coast of Australia. Water elevations relative to mean sea level are shown in red/blue shades and mean sea level pressure (HPa) in black contours.

A significant extra-tropical storm event in June 2014 caused a strong and extensive storm surge while propagating from west to east off the south coast of Australia. Water elevations relative to mean sea level are shown in red/blue shades and mean sea level pressure (HPa) in black contours.

The final model reproduced water level variations inside Port Phillip Bay very well, and MetOcean Solutions now runs an operational forecast for the location to help Viva Energy and the Port of Geelong safely manage shipping operations.

This work was carried out in collaboration with our science partners at MetraWeather and OMC International.