Ports play a vital role in the Australian economy, facilitating 99% of Australia’s import and export trade*.
In the same way that we maintain roads and rail lines for freight transport, we need to look after our port infrastructure to keep trade flowing.
Put very simply, when a ship navigates into port there needs to be a certain amount of distance between the bottom of the ship and the seafloor.
Because the seafloor is always moving, marine sediment builds up and reduces that distance. This can significantly affect the efficiency and safety of port operations.
Maintenance dredging refers to the process of removing that sediment and disposing of it sustainably.
It is undertaken by port authorities around the world and is critical for maintaining port infrastructure.
For those who want to know more, we’ve gone into a bit more detail.
When you look out at a port, you’ll see the ocean, the wharves and maybe some offloading machinery. What you won’t see is the infrastructure beneath the surface.
This includes a shipping channel, swing basins or aprons, and berthing pockets. Without getting too tied down in jargon, all of these are basically manmade depressions in the seabed that allow a ship to manoeuvre into port.
Over time, natural forces like tides, storms and cyclones cause some of the sediment that is constantly shifting through the ocean to settle into these channels and pockets.
This sediment starts to reduce the depth of these holes, all of which have a design depth and a declared depth.
The design depth is the original construction depth that port engineers consider ideal for operating safely and efficiently at both high tide and low tide. The declared depth is the depth designated by the harbor master, acknowledging this sediment build-up.
If maintenance dredging does not take place, the channels, aprons and pockets get shallower. The picture below illustrates the issue more clearly.
As the distance between ship and seabed (known as ‘under keel clearance’) reduces, the port’s day-to-day activities become more cumbersome. The depth necessary for safe loading, manoeuvring and transit of ships is impacted.
A fully loaded ship has less margin for error. The amount of cargo that can be loaded is reduced and tides need to be managed very carefully.
This can cause shipping delays that have significant flow-on effects for local businesses and the broader economy.
Once the sediment is removed from the shipping channel or berth pockets, it has to be relocated somewhere and this can have an environmental impact.
For this reason, maintenance dredging and disposal of dredge material (i.e. sediment) is highly regulated. An international legislative framework is provided through the London Protocol, which was created in 1996, while the National Assessment Guidelines for Dredging 2009 (NAGD) support regulation within Australia.
Crucially, the legislation aims to ensure that:
A strict NAGD criteria determines whether dredge material will be suitable for at-sea disposal or whether it must be placed onshore. From 2015–2017, we conducted further research into sustainable sediment management, using the Port of Hay Point as our base case.
As NQBP operates three ports within the Great Barrier Reef World Heritage Area, we must apply for permits from the Great Barrier Reef Marine Park Authority to conduct any maintenance dredging and at-sea disposal within these ports.
The NAGD also stipulates that dredging is undertaken outside key environmental sensitive cycles and that environmental monitoring is undertaken prior to, during and following maintenance dredging.
NQBP is very proud of its long-term record of extensive environmental monitoring. Our data helps inform our maintenance dredging activities.
We follow a strict process with numerous controls in place to minimise environmental impact:
We currently have 10-year permits for our ports in Mackay and Weipa. We are in the process of applying for 10-year permits for the Port of Hay Point.
We typically use what is called a trailing suction hopper dredger to undertake maintenance dredging at our ports.
It is mainly used for dredging loose and soft soils such as sand, gravel, silt or clay. The diagram below illustrates this more clearly.
In the first step, one or two suction tubes, equipped with a drag head, are lowered on the seabed and the drag head is trailed over the bottom. A pump system sucks up a mixture of sand or soil and water, and discharges it in the ‘hopper’ or hold of the vessel.
Once fully loaded, the vessel sails to the unloading site. The material is then deposited on the seabed through bottom doors or valves, as shown in the second step.
*Imports and exports by gross weight. Australian Bureau of Statistics, Trade through Australia’s ports, 2002