< The inaugural Australian Forum for Operational Oceanography (FOO 2015)
07.05.2015 05:50 Age: 3 yrs
Category: SRS, NSW-IMOS, ANFOG

NSW floodwaters seen from space and by an IMOS glider

The storms that hit the Hunter Valley region of NSW on 21-22 April 2015 caused much flooding as well as damage from winds and huge waves. River discharge mixes with seawater to form a buoyant mixture that can take some time to disperse, depending on the influence of winds and ocean currents.


MODIS satellite image for 25 April (1km resolution) of the floodwater plume showing interaction of floodwater with the East Australian Current. Image: IMOS OceanCurrent.

MODIS satellite image for 25 April (4km resolution). Image: NASA.

The glider observations during the latter half of 30 April show how the seaward edge of the plume is over-ridden by the EAC water. Image: IMOS OceanCurrent.

The MODIS satellite image for 25 April says much about the way dispersal works in the ocean. A thin tendril of floodwater, coded yellow-orange in this image, can be seen stretched out along the boundary that already existed between the low-chlorophyll waters of the East Australian Current (coded blue) and the Tasman Sea waters shown in green. Most of the buoyant plume was still close to the coast on 25 April, its seaward edge marked by a sharp but irregular boundary. We will see over the next few weeks where this mass of water goes.

The satellite is not our only way of investigating this significant event. An IMOS Slocum glider was also on the job. Its track is shown in magenta overlain on the MODIS images. Its sensors very clearly distinguish the floodwaters from the ocean waters, especially through the impact on the water's salinity and fluorescence.

Click forward from 26 April to see the glider encounter, then depart from, the buoyant pool of floodwaters. The observations during the latter half of 30 April show how the seaward edge of the plume is over-ridden by the EAC water because the temperature effect on density is winning against the freshness effect. High estimates of Coloured Dissolved Organic Matter (CDOM) in the low-salinity water suggest that the MODIS estimates of Chlorophyll are probably being 'tricked' by high levels of CDOM as well as by the suspended sediments, but nevertheless, can be used to monitor the dispersal of floodwaters.

One very important consequence of the runoff may be the input of nutrients (nitrate, phosphate and silicate) to the Tasman Sea ecosystem. A bloom of diatoms (the preferred prey of many zooplankton, but rarely abundant off NSW) may result from this flood.

The top satellite image from IMOS OceanCurrent provides an excellent example of the extra value provided by IMOS through ocean colour processing to 1km resolution (which underpins the OceanCurrent image).  Australian scientists and other users derive massive benefit from global satellite data made available by NASA.  The additional processing undertaken by IMOS makes this resource even more useful in the Australian region.  In this case it has provided the detail necessary to observe interaction of floodwater with the East Australian Current.