Dredging is often claimed to be killing the GBR. This document (14 MByte ) Sedimentary Geoscience of GBR-Larcombe-Ridd-Final(151202) gives details of the natural sediment movements on the GBR. One of the conclusions (see page one) is that dredging moves a very small amount of sediment compared with natural influences such as resuspension by the south easterly trade winds and cyclones. See sections 8.4 and 8.5 (pages 112 to 115)
The Great Barrier Reef ports and dredging
Along the 2 000 km long coast adjoining the Great Barrier Reef (GBR) there are a dozen or so ports that export mostly minerals and agricultural produce such as coal, copper, zinc, lead, silica, beef and sugar. General cargo and fuel for much of north-east Australia also comes through these ports. In addition, a small number of cruise ships bound for the GBR and the islands of the Coral Sea and South Pacific also use the ports. Many ports are little more than a single wharf that accepts a few ships per month. But Hay Point is one of the largest coal export ports in the world.
Although there has been a rapid growth in coal exports during the last few decades, the shipping traffic in the GBR is sparse in contrast to more populated regions in Asia, Europe and the Americas. The shipping serves a very small population of around 700 000 people over the 2 000 km length of the reef. Similar length coastlines in other parts of the world, such as in Asia and Europe, may serve a hundred or even a thousand times that number.
The ports require dredging to deepen the channels or berths to more than 10 metres depth. In shallow muddy bays such as Cleveland Bay and Trinity Bay, the channels are more than 10 km long. After the initial channel is dredged (called capital dredging), maintenance dredging is periodically required to remove sediment that falls into the channel. Maintenance dredging may occur yearly or at longer periods, but in the event of a cyclone/hurricane, when enormous quantities of sediment are moved around by large waves, dredging may be required urgently to clear the channel.
The distinction between maintenance and capital dredging is significant. Maintenance dredging is composed of recently suspended material and is of similar composition to the sediment naturally moving around in the area. It is often composed of mud and unconsolidated “fluffy’ material that is easy to suspend. Dredging this soft sediment generally results in generation of muddy plumes of water as a large proportion of the material washes out of the dredge hopper, or from the cutters or buckets of the dredge. In addition, muddy plumes are also created when this soft material is dumped on the seabed by the dredge. Capital dredging, initially to widen or deepen a channel, cuts into material that may be thousands of years old, and is generally hard and consolidated. Dredging and dumping of this material results in far less loss of material into the surrounding water.[*]
Ports such as Townsville and Cairns have very large maintenance dredging requirements. The shipping channels go across muddy bays and the mud on the seabed is suspended by waves every couple of weeks during periods of strong south-easterly trade winds, and by cyclonic waves every decade or so.[1] During these strong wind periods, the muddy water moves with the tides, and wind driven currents, across the deep water of the channel where the currents and wave agitation slow. This causes the sediment to settle in the channel. Under these circumstances, the shipping channel is acting as a sediment trap and removing mobile sediment from the system.
There is often considerable opposition to dredging of ports because of perceived damage to the environment. What is forgotten is that a large dredging requirement arises because a very large quantity of sediment is naturally mobile in the first place. Nearby ecosystems will inevitably be naturally subjected to regular high concentrations of suspended sediment.
Such ecosystems will be able to cope with relatively high sediment loads. This characteristic will be examined in relation of Townsville Port.
Other information about dredging
Peter Ridd’s background monitoring dredges
Dredges and the environment – ports are often already in muddy places.
Ports are a long way from the GBR
How do mud volumes from dredges compare relative to natural movements of mud?
[*] For Abbot Point, it is 3% of fines which is 40% of total, i.e about 1.5%. of 1.1 Million m3 capital dredging. Haskoning Australia (2016) Maintenance Dredging Strategy for the Great Barrier Reef World Heritage Area Ports: Technical Supporting Document. Department of Transport and Main Roads, Queensland.
[1] Orpin, A.R. and Ridd, P.V. (2012). Exposure of inshore corals to suspended sediments due to wave-resuspension and river plumes in the central Great Barrier Reef: A reappraisal. Continental Shelf Research, 47, pp.55–67.
Great Barrier Reef Science Commentary 