Information Briefing: Dredging in the Great Barrier Reef Region

Posted by on 27 February 2015


  • Ports Australia has prepared this paper to provide factual information on key issues related to dredging in the Great Barrier Reef region.


  • Dredging of shipping channels and berths is an essential to the safe and efficient operation of ports in the Great Barrier Reef region. Shipping channels are of equal importance to supply chains as are road and rail networks.
  • The safe and efficient operation of shipping is essential in enabling import and export industries to operate and compete in global markets.
  • The value of trade transiting through Queensland ports equates to approximately $50 billion p.a.
  • Like land-based transport networks, channels need to be maintained and developed as trade grows.

2.1 Capital Dredging

  • Required to create new or improve existing channels and berths. Periodic capital dredging has occurred at all Queensland ports in response to the ongoing global trend to move to larger ships to meet the economies of scale of modern competitive markets.
  • For example, the Port of Gladstone has undergone major capital dredging programs approximately every 20-30 years as part of its growth as a major industrial port. This has required channels to be 55% deeper (10.4m to 16.1m) than they were in 1980 to allow access by modern ships.
  • Certain lobby groups have grossly exaggerated the volume of proposed capital dredging that is likely to occur in the Great Barrier Reef region. They have included projects that were nothing more than concepts, those that have been cancelled and several which have been rejected.

Without capital dredging to support seaborne trade, Queensland would struggle to remain globally competitive or provide new opportunities for employment, industries, trades and agriculture.

2.2 Maintenance Dredging

  • All Queensland ports have undertaken maintenance dredging since they were established. Ports cannot function without dredging to ensure safe navigation.
  • Channels silt up because sediments are transported by currents from nearby shallow areas and accumulate in the deeper channels and berths. On occasions, floods and cyclones can add considerable additional sediment.
  • The costs of port closures and inefficient operations can be large. Even small reductions in channel depths can result in ships being unable to carry full loads increasing the cost of importing and exporting goods.
  • Some Queensland ports such as Cairns, Gladstone and Townsville require an annual major maintenance dredging campaign. Others ports (e.g. Hay Point, Abbot Point) need maintenance dredging every 3-10 years (depending on climatic conditions).
  • Seagrass and corals are not associated with maintenance dredging areas. Impacts from maintenance dredging are short term and localised.

Each year approximately 1 million cubic metres of material needs to be dredged from the port channels in Queensland.

This is a relatively minor amount compared to the volumes of sediment transported by coastal processes (e.g. south-east trade winds resuspend a similar amount of material in Cleveland Bay, Townsville every 8-12 days).


  • Careful consideration is given to the best options of where to place dredge material. This consideration takes into account environmental, social, logistical and economic considerations. In many instances there are limitations to what option can be feasibly selected.

3.1 Onshore Placement

  • Dredged material placed on land takes years to dry. The material needs to be stored in a dedicated containment ponds, potentially lined to avoid groundwater impacts, and must be engineered to cater for cyclones and storm surges.
  • Large areas (100s of ha) of flat land close to the port are required if material is to be placed onshore. In most cases, such extensive areas are not available as the land is used for residential purposes or is of high conservation value.
  • The Great Barrier Reef 2014 Strategic Assessment concluded: “that land based disposal of maintenance material for the six major Great Barrier Reef ports was not a viable long term option.”
  • Reclamation is possible in some cases but results in a loss of shallow coastal habitats and may lead to unacceptable changes to coastal sediment transport processes (e.g. cause foreshore erosion). Reclamation is generally only viable in capital dredge projects where additional industrial land area is required.

Land disposal is not possible at most ports and may not be the best environmental solution. It is only viable for small quantities or one-off projects.

3.2 Offshore Placement

  • All maintenance dredging material is disposed of at-sea in designated placement areas. These areas have been used for decades and comprise bare seabed distant from sensitive areas such as coral reefs, seagrass beds or similar.
  • A 2013 review of international practices by the Federal Department of Environment concluded that at sea disposal is the most common practice globally and that Australian management practices for this approach were leading edge.

Only natural, clean dredge material is placed at sea. In many cases this is the most environmentally responsible and economically viable option.

This is particularly the case for maintenance dredging material which would have otherwise made its way over the seabed had it not been trapped by the deepened channels.


  • A ban has been proposed for placing capital dredge material in the Great Barrier Reef Marine Park. This will affect growth plans for a number of ports including the Ports of Cairns, Abbot Point and Gladstone.
  • The potential for a ban on placing dredge material (capital or maintenance) in the World Heritage Area has also been raised. This would effectively prevent any material being placed at sea by ports (given the World Heritage Area boundaries extend to the low water mark).
  • Such a ban would result in a total halt to maintenance dredging because, as noted above, there are no long-term viable land based options at any of the ports. A halt to maintenance dredging would have catastrophic effect on communities, jobs and businesses. For example, a one-month closure of the Port of Townsville would result in lost output of $138 million and 448 jobs.
  • Even if a land based site at a port could be identified, it would cost in excess of $50 million per port and take at least 2 years to establish (and probably much longer considering tenure/native title issues).

The environmental benefits to marine ecosystems of a ban on at sea placement would be negligible whilst the potential effects to land based ecosystems would be substantial.

Widespread land based storage of dredged material would permanently sterilise large areas of the coast from any other use.