Beach scraping at Surfside’s East Beach in September – sand from the tidal zone was pushed up onto the dunes. Photo: Eurobodalla Shire Council.
An operation to replace the sand on Batemans Bay beaches has drawn to a close earlier than expected, but it won’t be the last time it’s done.
Beach dredging is the science of removing sand and either placing it on the beach you want to restore, or a few hundred metres out so waves then naturally wash it in and over the beach.
The Batemans Bay operation involved removing 30,000 cubic metres of sand from the bottom of the Clyde River, carving it out to keep the channel clear for boats, before dumping it about 600 metres offshore from the badly eroded Surfside Beach.
“In summer, Batemans Bay is subject to larger wave heights due to frequent seasonal storms along the south-eastern Australian coast,” the NSW Government wrote in a nearly 500-page proposal earlier this year.
“These waves generate strong bottom stresses and wave-induced currents, which enhance sediment resuspension and transport toward the inner bay. This increased wave action causes sediment to move onshore from outer to inner bay regions at the bottom.”
The proposal goes on to detail plans to dredge the Clyde River several more times between now and 2035, removing about 25,000 cubic metres each time – but no more than 30,000 cubic metres.
This year’s operation started in late October and was expected to take four weeks, but this week, the NSW Government said the project had wrapped up “ahead of schedule” thanks to the “efficiency of the dredge vessel”.
The vessel in question was MV Port Frederick, which cruises along sucking up sand into its hoppers through a drag head and pipe, which trail along the port side of the vessel. It then takes the sand from where it is not wanted and dumps it where it is wanted.
The government says the Clyde River now has a minimum depth of 2.9 metres (at lowest tide) across a 40-metre wide channel, “making navigation smoother and safer for everyone”.
The MV Port Frederick dredging ship. Photo: The Yard Brisbane (TYB).
The project is also under scrutiny by the University of NSW (UNSW) for a research study, assessing the “effectiveness of proposed dredging and offshore sediment dumping operations in delivering nourishment to Surfside Beach”.
The researchers said this beach has “experienced alarming shoreline retreat in recent years, with erosion threatening residential properties, public infrastructure, and environmental assets”.
Using various models based on water current movements in the area, including during storm events and flooding, UNSW found “strong evidence in support” of dredging.
“Whether under wave-forced or calm conditions, sediment is effectively delivered toward Surfside Beach,” the report read.
“In particular, during … calm scenarios, sediment continues to move shoreward, with notable retention along the erosion-prone eastern section of the beach.
“The project stands as a well-founded and practical solution to the dual challenges of maintaining navigational access and protecting vulnerable coastal zones.”
Researchers were out on several days during the dredging operation, undertaking water and sediment tests.
Callum Shakespeare is an expert in climate and fluid physics at the Australian National University (ANU). He says the method of placing the sand offshore – and relying on the waves to bring it to the beach – rather than putting it directly on the beach comes down to the ship.
“Obviously, putting it on the beach directly would be a more efficient way of getting it on the beach, but because they’re dredging it with a barge, it would be difficult to get too close to the shore,” he told Region.
As for how long the effects last?
“It’s a cycle – because the same waves that take sand away, also bring sand in at other times. Beaches grow and recede, all through the action of water … so they’re putting the sand where you’ve got the waves setting up this inflowing current … about 600 metres offshore.”
Restoring the beaches around Batemans Bay will be a constant task. Photo: NSW Government.
Shakespeare says manufactured beaches – like those around the Gold Coast, where there were originally mangrove swamps – suffer the most loss of sand because the flow of passing currents “doesn’t really support there being sand there”.
“That’s one of the risks when you go around and change the coastal environment,” he says.
But Batemans Bay is still “fairly natural”.
“I don’t think there’s a huge amount of infrastructure that would massively change [sand movement], although obviously they are modifying it through the dredging operation to keep that channel open. And that’s exactly why they’ll have to keep dredging it out.”
The NSW Government says it will now “monitor the conditions at the Clyde River bar and advise when the bar needs dredging again”.
“This is expected to be in a few years’ time.”
