GBR1 - Salinity and Wind during 2019 North Queensland Floods

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    This page shows modelled (eReefs GBR1 Hydrodynamic model v2.0) freshwater plumes along the Great Barrier Reef (GBR) as a result of major flooding in January and February 2019.

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    Background

    During this event the monsoon trough remained stable over Townsville and inland north Queensland for well over a week, funnelling moist air off the ocean on to the land, resulting in sustained heavy rainfall. This led to historic flooding in Townsville and large amounts of rain over the Haughton (just south of Townsville) and Burdekin river catchments. The monsoon can be seen in the wind map as a convergence between winds from the north and south. The monsoon can be best seen on the wind map of the 4 km version of this product as it shows a broader area. The flood plumes from this event can best be seen by zooming into the Burdekin region map

    Ocean salinity and freshwater

    Normal sea water has a salinity of 35 PSU (3.5% salt by weight) and so any lowering of this indicates the introduction of freshwater. This can occur due to rainfall over the ocean or from river runoff. The maps on this page show the salinity scaled to show detail of the low salinity in the flood plumes. If you wish to see the extent of even small amounts of freshwater see our normal salinity maps. The maps on this page show the salinity levels between 22 - 32 PSU to help identify areas where freshwater coral bleaching may have occurred.

    Plume movement

    Freshwater has a lower density than seawater and as so tends to float on the surface. This reluctance to mix can be seen in the difference between the surface water salinity at -2.3m and at depth (-5 m and -18 m). At the surface the freshwater plume extends out to the mid shelf reefs on some days, however at -5 m the plume is much smaller and at -18 m there is very little freshwater intrusion.  

    The movement of the flood waters is dominated by the surface ocean currents, which are in turn, dominated by the direction and strength of the wind. The flood waters are generally blown north west making them hug the coast line, however when the wind drops or blows out to sea (as seen on the 12 - 14 February) the plume spreads out over the mid and outer reefs.

    Freshwater coral bleaching

    While it is well known that freshwater runoff can lead to coral bleaching and mortality (van Woesik et al. 1995), relatively little is known about the level of exposure (as a dose time response) needed to cause bleaching across a wide range of coral species (Berkelmans et al. 2012). More details on the freshwater exposure and coral bleaching can be found on the map of freshwater exposure.

    Model limitations

    The results on this page are generated from a model and thus are not direct measurements of the salinity. They should be thought of as an estimate of the conditions that occurred. See the freshwater exposure 1km model page for a more detailed discussion on the accuracy of the freshwater plumes in this event.

    References

    Berkelmans, R., Jones, A.M. & Schaffelke, B. (2012) Salinity thresholds of Acropora spp. on the Great Barrier Reef. 31: 1103. Coral Reefs. https://doi.org/10.1007/s00338-012-0930-z

    van Woesik R, De Vantier LM, Glazebrook JS (1995) Effects of Cyclone ‘Joy’ on nearshore coral communities of the Great Barrier Reef. Mar Ecol Prog Ser 128:261–270. https://doi.org/10.3354/meps128261