Sydney Bay Turning into Mudflat Due to Sediment Buildup

For 12 years, a green waste bin has become a symbol of a Sydney bay that once was home to sailing boats and oyster leases.

Year after year, the bin has been slowly disappearing under the mud just metres from the shore at Oyster Bay.

Residents in this part of the Sutherland Shire say the bay is filling up with sediment and becoming a shallow mudflat.

Oyster Bay local Travis Read said it had been slowly happening since 1970 when the council filled in part of the bay to build a football field and a golf course.

"It was only ever about 4 feet, 1.2 metres, deep and now at low tide, it either dries out or it's 10 centimetres deep," said Mr Read, who is president of the Revive Oyster Bay community action group.

"That changed the shape of the bay, reduced the flushing of the bay and it meant that sediment out of the creek had nowhere to settle before it went into the bay.

Mr Read said at the current rate, the whole bay would become a mudflat within 15 years.

"If we're accumulating even 1cm a year … in 10 to 15 years there's no bay left," he said.

A report by Sutherland Shire Council said about 83,000 square metres of sediment had been deposited in the bay since 1980, about 57 per cent of it coming from the upstream catchment. The remainder likely originated from the Georges River.

The report suggests sediment has been increasing at about 3mm a year since 1980.

Locals say boating now impossible

Mr Read said the mud had become a navigation hazard.

In one instance, the shallow mud once trapped two tour boats carrying children that would have required them to wait until the tide rose again had it not been for quick-thinking residents who evacuated them on longboards.

Harry Taylor, who has lived in Oyster Bay since 1957, remembers when the bay was used for oyster farm leases, fishing and recreational boating.

Mr Taylor said he did not see the bay used for boating or swimming anymore because it had become too muddy and was no longer deep enough.

He took his own boat out of the water when the fields were built.

Liz Adams, who has lived in the area for 55 years, said she watched boating slowly disappear from the bay after the golf course was built out.

"It's just mud everywhere, and you've got to really be at full tide to be able to go anywhere, even in the smallest boat," she said.

"It's not really possible to enjoy the bay now."

Council backs stemming flow of sediments

Sutherland Shire Council recently endorsed a plan to use measures such as improving stormwater treatment, stabilising creek banks and constructing bioretention systems to address sediment build-up in Oyster Bay.

A spokesperson for the council said their studies did not support a "definitive conclusion" that Oyster Bay would become a mudflat, as the Revive Oyster Bay group has suggested.

"Council understands community concerns about the long-term condition of the bay," the spokesperson said.

"These actions are designed to improve water quality, manage erosion, and reduce sediment loads entering the bay from within the LGA [Local Government Area].

The spokesperson said dredging Oyster Bay would not align with the council's policy and would not provide a sustainable or cost-effective solution.

"Council's approach is to consider targeted dredging in specific locations, such as upstream creek tributaries and stormwater outlets, where it can deliver clear benefits," the spokesperson said.

"Broadscale dredging of the entire bay is not proposed and remains as a NSW government responsibility."

However, Mr Read said if the council adopted their plan in full, it would only reduce the inflow of sediment by 5 per cent.

"At least what has accumulated since 1980 needs to be removed from the bay," Mr Read said.

"Unfortunately, with Oyster Bay it needs to be dredged."

Nell Graham from Georges Riverkeeper did not think dredging was a fitting solution as sediment would need testing for contaminants and either be put in landfill or dumped out at sea.

"You're taking one problem and putting it elsewhere," Ms Graham said.

She said solutions to improve stormwater runoff, such as gross pollutant traps and more frequent street sweeping, should be explored.