Dredges and the environment

Example of the type of picture often used in the campaign against dredging. This picture of dredge “Brisbane” with Cairns in the background was taken by environmental groups (CAFNEC/WWF) and found in many newspapers around the world. The dredge has just dumped its load creating highly turbid water. Of all the ports, this is the closest that dredging gets to the Great Barrier Reef but it is still 15 kilometres away and not in the direction of the current. The muddy water does not reach the Reef in any significant concentration.

Dredging stirs up the sea bed and causes mud and sediment to move, sometimes onto sensitive environments. The first major public concerns about the effect of dredging on the GBR occurred with the major expansion of the Townsville Port channel in 1993 mentioned previously. But it was not until early in the 21st century and the mining boom in Australia that the matter received national and international attention. In the last 15 years, the main coal export terminal at Hay Point was expanded, and new ports were constructed at Abbot Point for coal and in Gladstone for natural gas. In addition, there are major programs to deepen the shipping channels in Townsville and Cairns in order to accommodate the progressively larger ships of today’s cargo and passenger fleets. All these dredging operations have attracted considerable opposition and claims from scientists and environmental groups that they would destroy the GBR.[1]

Unlike the natural loss of coral by cyclones, bleaching, or crown of thorns starfish, where graphic pictures of massive coral loss have been used to capture the world’s imagination about the perceived death of the GBR, no similar images can be found for dredging. As will be seen immediately below, there has been no measurable effect on the GBR. Even coral mortality on the fringing reefs closest to the dredging has been negligible. This finding is the conclusion of hundreds of millions of dollars[2] of intensive environmental monitoring programs in Australia. Fortunately, for those wishing to impress the public with spurious claims of the dangers of dredging, it is a dirty business. It is thus easy to take evocative pictures of muddy water (Figure 4.1) in stark contrast to the blue waters of the GBR. These images have been very effective at convincing the world that there is a serious problem, especially when backed up by statements from eminent scientists and science organisations.[3]

This section will show that the impact of dredging is so small that it has been almost impossible to measure far outside the confines of the ports themselves and there is no possibility that dredging is adversely affecting the GBR. The distance from the ports to the GBR is far too great, usually over 50 km to the nearest reef, and the prevailing water current directions are completely unfavourable for this to occur. The GBR is generally a long way offshore and the ports are, not surprisingly, on the coast.  In addition, the quantities of dredged material are very small compared with natural movements of sediment caused by waves and currents generated by hurricanes and the south-easterly trade winds. It may be possible that the corals of the Mediocre Fringing Reefs close to the shore are affected by dredging due to the close proximity, a few kilometres, of some of the ports. Years of measurements have, however, provided very little evidence of significant damage to these small areas.[4]

Ports are on the coast where the water is often naturally muddy

Waters close to the coast are often quite dirty due to mud on the seabed being stirred up by waves. Figures 4.2 and 4.3 show the coast near Townsville where the muddy water can clearly be seen, as well as the waves. These waves are generated by the “Trade Winds”, a very frequent phenomenon on this coast that blow quite strongly every couple of weeks.  The mud on the seabed is many metres thick and has been deposited over thousands of years. This area was a muddy bay well before European settlement, farming, or dredging.

The coast at Townsville on a windy day with Townsville Port and shipping channel in the background. Waves are stirring up the mud on the sea bed. This happens every couple of weeks. There are fringing coral reefs a few kilometres to the left of this picture. This muddy water causes the shipping channel to fill with mud so dredging is required annually. During a big cyclone the quantity of mud suspended would be far greater than shown here.  (Photo: Port of Townsville Limited)
Cleveland Bay near Townsville with Magnetic Island in the background. Waves are stirring up the muddy sea-bed. This is mud that has been deposited in the bay over thousands of years and is not caused by human activity such as soil erosion from farms or dredging. There are fringing coral reefs on Magnetic Island which are tolerant of this muddy water.  (Photo: Port of Townsville Limited

[1] Brodie, J. (2014). Dredging the Great Barrier Reef: Use and misuse of science. Estuarine, Coastal and Shelf Science, 142, pp.1–3.

[2] Stoddart, J. and Poiner, I. (2014). Some suggestions for the environmental management of harbours during and after dredging programs. Ports Australia Conference.

[3] Hughes, T. (2014). Mounting evidence shows dredge spoil threat to the Great Barrier Reef. [online] The Conversation. Available at: https://theconversation.com/mounting-evidence-shows-dredge-spoil-threat-to-the-great-barrier-reef-29773

[4] Larcombe, P. and Ridd, P. (2015). The Sedimentary Geoscience of the Great Barrier Reef Shelf – context for Management of Dredge Material. Brisbane: Queensland Port Association..

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