The impact of dredging in Cleveland Bay

The dredge ‘Brisbane’ at work in Cleveland Bay

The latest plans for the Port expansion involve the removal of 11.4 million cubic metres of seabed using mostly a backhoe type dredge. The spoil will be transferred from the seabed to a barge and then taken to the Port to create 152 hectares of new Port land. The expanded channels will increase the amount of annual maintenance – increasing it from an average of around 400,000 cubic metres to around 450,000. Each year this amount will be dumped in Cleveland Bay, about halfway between Cape Cleveland and the northeast tip of Magnetic Island.


The light blue shape in the top right of the above image is the official Dump Material Placement Area (DMPA) for Townsville Port. Notice how close most of the capital dredging work will be to The Strand.

It is absolutely indisputable that dredging of the seabed stirs up loose sediment. This sediment creates plumes in the water that can travel significant distances before becoming deposited. The deposited sediment can also be re-suspended.

In 2014, Dr Russell Reichelt, Chair and CEO of the Great Barrier Reef Marine Park Authority, told 1Extract from Official Committee Hansard Senate, Environment and Communications References Committee, Great Barrier Reef, Wednesday 23 July 2014, Townsville a Senate Inquiry into Management of the Great Barrier Reef:

On the issue of disposal of dredge spoil, the available science does list it as a significant risk in a local setting. It does change the regions up to perhaps 10 kilometres away from the port. That is visible if you wander out and look at the Townsville port and have it explained to you where the mangroves and mud banks came from on southern and western Magnetic Island, for instance, or the expansive mud flats at Cairns. There is no question that they have a significant local effect.

In 1993, the Port dredged 1 million cubic metres of seabed over just 15 weeks.

Recognising that, ‘… major port development works such as dredging have the potential to cause lasting impacts on the natural environment’, the Port Authority ‘commissioned a comprehensive multidisciplinary monitoring program’, ‘scientists studied the coral reefs and seagrasses… and examined waves, current and sediment patterns’, ‘detailed management guidelines were designed’, ‘vast volumes of information were collected’ and ‘TPA commissioned Sinclair Knight Merz to produce a book which summarised the Environmental Monitoring Program’2Townsville Port Authority, Townsville Port Authority Capital dredging Works 1993 Environmental Monitoring Program, 1994. It was all for nothing.

In 2014, Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies, wrote3Prof. Terry Hughes, ‘Mounting evidence shows dredge spoil threat to the Great Barrier Reef’, The Conversation, 2014:

In 1987, prior to major dredging of the channel, the Queensland Department of the Environment established a popular reef walking trail in Geoffrey Bay, designed as a recreational and educational amenity. At low tide, visitors could walk along the reef flat and use a map with designated viewing points to experience a diverse mix of species including branching and brain corals.

Today, the walking trail has been long abandoned, and the intertidal reef is covered in a layer of mud. Whatever the complex reasons for these declines, near-shore fringing reefs and sea grass beds along the Queensland coast are fast disappearing and they can ill-afford further cumulative impacts from unprecedented dredging programs.

The studies undertaken for the EIS and AEIS for the current expansion plan (involving not one but 11.4 million cubic metres of dredge spoil) similarly rely on a ‘comprehensive reactive monitoring’ management system. We can expect a similar or worse outcome.

Dislodged sediment covers corals and seagrasses, and decreases the amount of light reaching them by increasing water turbidity. These impacts limit the growth of corals and seagrasses and, at worst case, kills them. Other marine species, ranging from fish to dugongs, suffer as a result of loss of habitat and food sources.

Dredging can also stir up toxins in the water and increase underwater noise.

For further information on the impact of dredging and dumping in the Great Barrier Reef, click here to download Dredging, dumping and the Great Barrier Reef, from Fight for the Reef (pdf).

References   [ + ]

1. Extract from Official Committee Hansard Senate, Environment and Communications References Committee, Great Barrier Reef, Wednesday 23 July 2014, Townsville
2. Townsville Port Authority, Townsville Port Authority Capital dredging Works 1993 Environmental Monitoring Program, 1994
3. Prof. Terry Hughes, ‘Mounting evidence shows dredge spoil threat to the Great Barrier Reef’, The Conversation, 2014