The Extraction Process

McCallum Bros Ltd® has a long and proud history operating vessels in the Hauraki Gulf since the late 1800’s and is absolutely committed to protecting the Gulf for future generations.

Historically a variety of techniques have been used to extract sand, including clamshell grabs on the booms of scows, stationary dredge techniques, through to the trailing suction techniques that are used today.

McCallum Bros Ltd® now extracts sand using its modern trailer suction dredging vessel, the William Fraser, which was purpose-built for McCallum Bros Ltd® and commissioned in September 2019.

The William Fraser incorporates the latest Dutch dredging and drive engine technology, which minimises the environmental impact of sand extraction.

When extraction activity is undertaken, the drag head is lowered to the seafloor and acts much like a vacuum cleaner, moving along the bottom picking up sand slurry to a depth of approximately 80mm.

The sand slurry passes up the dredge arm and is delivered to a screen deck on board the vessel. Sand and particles smaller than 2mm pass through the screens and are delivered into the barge hopper via a pipe system alongside the vessel hopper.

Oversized material passes over the screens and is returned to the ocean via a moon pool, which discharges below the hull of the vessel.

As the sand slurry settles into the hopper, the heavier sand particles settle into the hopper whereas the water and fine sediment pass over weir boards and into the sea through one of six moon pools located around the hopper.

Water and sediment analysis has shown that no contaminants, petrochemicals or heavy metals are contained within the sand or discharged water. The moon pools force overflow water under the vessel and, due to its lack of aeration, it settles back quickly on the seafloor. Analysis has shown this happens in as little as seven minutes.

To ensure extraction takes place in the designated area, the William Fraser is fitted with a number of technologies to track the vessel’s location. The Marine Navigation System TimeZero (MAXSea) is the current positioning system used, and a track is recorded wherever extraction has occurred, which is submitted to Council for their records.

The exact distance from shore where sand is extracted varies depending on the Coastal Permit being used at the time.

The inshore permit is dependent on both the nearshore bar location and water depth at the time of extraction, which can move significantly depending on weather conditions, the tide and coastline topography. Extraction, however, generally takes place at least 500 metres from the beach.

Extraction on the offshore application occurs at least 2km offshore, or in no less than 25m water depth, whichever is the greater distance from the shore.

When extracting sand, the William Fraser travels at a speed of approximately 2 to 2.5 knots but this varies depending on the weather conditions at the time of extraction.

Once the hopper is fully loaded, the vessel returns directly to the Ports of Auckland to unload the sand into stockpiles. The sand is then distributed to McCallum Bros Ltd® customer as required.

To reduce the impact on the local community and recreational users of Pākiri beach, extraction generally takes place at night. Records for the last 12 months show more than 90 percent of extraction activity was undertaken at night between 8pm and 3am.