Flood plume extents for major rivers on GBR based on modelled river tracers

This product shows flood plumes associated with the major rivers along the Queensland coastline flowing into the Great Barrier Reef Marine Park. These visualisations are produced from using currents calculated by the eReefs hydrodynamic model to drive a tracer transport model.

Flood plumes deliver sediments and nutrients into the ocean, both of which can result in detrimental effects on seagrass and reef habitats.

The left most panel shows the near-surface salinity and the wind vectors. The salinity indicates how salty the water is and it is lowered by river run off and rainfall. Thus reduced salinity is a measure of the combined impact of all rivers combined. The wind is shown as arrows overlaid on the salinity. The wind pushes the flood plumes around and has a strong influence on where the plumes go. The other major influence is the impact of the rotation of the Earth on buoyant plumes. The other three panels each show the river tracers for five rivers each. We split up the rivers into multiple panels to limit the amount of overlap in their extent during large flooding events.

In the model, tracers are released at the mouth into the surface flow of each river. These tracers move with the ocean currents, becoming more dilute as they spread out and mix with the ocean water, allowing the concentration of river water to be tracked over time. These tracers show the fraction of the water, at any given location, associated with each river.

The lowest threshold of river water concentration (1%) shown in the visualisation was chosen to align with the visible extent of flood plumes as seen in satellite imagery. At this concentration we can expect organisms on the sea floor to see raised nutrient levels, some fine sediment and a significant reduction in light.

Limitations and spatial accuracy

It should be noted that the river tracer product tracks the concentration of river water. It does not track sediment or nutrient in the water. As a result, it should be used as a guide for the potential area that might be affected by flood water. The primary value of this product is that it attributes flood plume water to its source river allowing the extent of individual rivers to be understood.

To understand more about the actual nutrient and sediment levels refer to the eReefs BioGeoChemical (BGC) model as it directly represents sediment and nutrients from all processes including wind resuspension and biological processes. In the BGC model however, it is not possible to directly attribute components of the sediment and nutrients concentrations to their source rivers.

This visualisation only shows the 15 largest rivers (based on largest historical flow) and so does not fully represent all the flood plumes. The salinity product can be used to see other rivers modelled by eReefs that are not explicitly separated out using the river tracer visualisation product.

The rivers shown in the visualisation were adjusted in their concentration threshold to represent the full flow of water associated with their river basin. In some catchments there are multiple rivers flowing into the ocean and eReefs represents this whole basin with a single river. As a result, the shape of the flood plume close to the coast may not be representative.

As part of the research into determining a suitable river concentration threshold to visualise we undertook many comparisons between the estimated flood plume extent from eReefs and those visible in Sentinel 2 satellite imagery. From this we found that the plume extent from eReefs was generally accurate to within about 10 km, with the most likely reason for the difference being slight errors in the model due to wind. The strength and direction of the wind is the predominant factor in determining the spread of the flood plumes. As a result any small errors in the modelling of the wind will lead to errors in the flood plume boundaries. The eReefs hydrodynamic model is driven by wind data from the Bureau of Meteorology's Access-R weather model, which is a forecast. It has a resolution of 12 km and so it is surprising that the eReefs model is as spatially accurate as it is. Part of the reason for this is that while the wind occasionally pushes the plumes offshore, the main determinant of the distribution is the dynamics of buoyant plumes. The rotation of the Earth acts to deflect to the left (in the Southern Hemisphere) any relative increase in motion between fluid layers. One such relative motion is a buoyant plume flowing over the top of denser ocean water. Deflected left on a river discharging along an east coast means it being push towards the coast. Thus, the plumes are trapped near the coast. The distance to which they spread from the coast is also set by this balance between density driven flow and the Earth’s rotation, something ocean models are very good at.

References.

Baird, M. E., J. Andrewartha, M. Herzfeld, E. Jones, N. Margvelashvili, M. Mongin, F. Rizwi, J. Skerratt, M. Soja-Wozniak, K. Wild-Allen, T. Schroeder, B. Robson, E. da Silva, M. Devlin (2017) River plumes of the Great Barrier Reef: freshwater, sediment and optical footprints quantified by the eReefs modelling system. In Syme, G., Hatton MacDonald, D., Fulton, B. and Piantadosi, J. (eds) MODSIM2017, 22nd International Congress on Modelling and Simulation. Modelling and Simulation Society of Australia and New Zealand, December 2017, pp.1892-1898. ISBN: 978-0-9872143-7-9. https://www.mssanz.org.au/modsim2017/L22/baird.pdf.