Capital vs Maintenance dredging
“Capital dredging” is new dredging – it makes a new channel or changes the shape of an existing one. It is a once-off.
“Maintenance dredging” is routine work to remove material that has drifted into the channel over time and limits the access of ships. In Townsville, maintenance dredging has to be done every year – and it is a big job. On average (prior to this proposed expansion), 400,000 cubic metres of dredge spoil has to be removed from the channel every year! If the expansion were to go ahead, this would increase to around 450,000 cubic metres.
Types of dredges
Dredging is undertaken with a variety of dredges. In Townsville, we are used to seeing the dredge ‘Brisbane’ undertaking on-going maintenance dredging in the Platypus and Sea channels.
The Brisbane is a ‘trailer suction hopper dredge’ (TSHD) which trails its suction pipe when working. The pipe, which is fitted with a dredge drag head, loads the dredge spoil into one or more hoppers in the vessel. When the hoppers are full, the TSHD sails to a disposal area and either dumps the material through doors in the hull or pumps the material out of the hoppers.
One of the problems with this type of dredge is that, to ensure that the hopper does not include a lot of water (rather than sediment) as it steams off to drop its load, it uses ‘overflow’ dredging. This enables the water with the fine (and most damaging) sediment to overflow from the hopper and make more room for the heavier sediment.
Little of the capital dredging for the proposed expansion will use a TSHD dredge.
For ten of the staggering ten and a half years that the dredging would go on for (according to the Additional Environmental Impact Statement on the expansion project), dredging would be done with a backhoe dredge. This is the equivalent of a huge backhoe on a floating barge, which digs up the seabed and dumps it into another barge alongside it. In the current expansion project, this barge would be taken back to the land, where the spoil would be unloaded to create 152 hectares of new land for the Port where once was water.