Narrabeen Lagoon entrance: Council move to reduce risk of flooding of homes

New moves to reduce the risk of homes flooding by better managing how the entrance to Narrabeen Lagoon is kept open have been released for public comment.

Northern Beaches Council has rejected heavy engineering solutions to the recurring problem of sand blocking the entrance of Narrabeen Lagoon.

It wants to instead continue periodic dredging to reduce the risk of flooding and use the sand to replenish the Collaroy-Narrabeen beachfront.

But the council will investigate clearing the entrance more frequently.

It will also investigate pumping the sand south — via pipelines — instead of making as many as 6000 truck movements each time the entrance is dredged.

Dredging in Narrabeen lagoon west of the Ocean St bridge in November 2021. Picture Manly Daily

The proposals are set out in a draft Narrabeen Lagoon Entrance Management Strategy, which the council voted this week to put on an extended public exhibition period from May 6.

But critics, including independent councillor Vincent De Luca, said a decision needed to be made quikcly because the issue had been dragging on for two decades.

Sand being removed west of the Ocean St bridge in 1979. Picture: Supplied

Cr De Luca was also critical of a move to extend the exhibition period from 28 to 42 days. He told Tuesday night’s council meeting that “our community is desperate that it be finally addressed and something substantive actually happen”.

In developing the draft strategy, the council looked at options including the continued use of mechanical means to open the lagoon to reduce the risk of flooding.

Residents being evacuated by Surf Rescue from their home in Mactier S, Narrabeen, after the lagoon flooded in February, 2020. Picture: Damian Shaw

In recent decades, the area east and west of the Ocean St bridge has been dredged every three to five years with between 30,000 and 50,000 cubic metres of sand removed. It cost more than $1 million each time.

The most recent dredging was between September and December last year leaving a $1.47 million bill.

Narrabeen Lagoon being opened in March 2020. Picture: Manly Daily

The council looked at long-term strategies, including a breakwater to prevent sand moving into the entrance of the lagoon from the beach; an ebb-tide channel with submerged structures to enhance the natural scouring action of water leaving the lagoon and; low-flood pipes that allow the tide to enter the lagoon when the entrance is closed.

All three options were considered impractical for various reasons.

An excavator works to create a channel from Narrabeen Lagoon to the ocean ahead of an expected east coast low to affect the northern beaches. Picture: Jim O'Rourke

Instead, the council will look at the use of a mobile sand-pumping system that would pump sand south from the lagoon entrance to replenish Collaroy and Narrabeen beaches.

A pipeline would be permanently installed along the top of the foredune or through a cut-and-cover operation under Ocean St.

Pumping sand south would remove more than 6000 truck movements during each dredging operation.

The council also recommends revegetating and maintaining the Birdwood Park sand dune to assist sand stabilisation and prevent its movement into the lagoon.

Dredging the Eastern Basin of Narrabeen Lagoon in October 2018. Picture: Manly Daily

Northern Beaches Mayor Michael Regan said the main objective of the draft strategy was to reduce the risk of flooding on the Narrabeen Lagoon flood plain.

“Residents on the flood plain know too well the stress that comes every time there is forecast high rainfall, large swells or flood warnings,” he said.

Narrabeen was ordered to evacuate from around Narrabeen Lagoon on March 8 2022. Picture: Jim O'Rourke

“The development of this draft strategy is about making sure we’ve canvassed all feasible options to reduce that stress, to reduce the risk and to make sure our lagoon management continues to follow best practice.”

Cr De Luca told the council meeting that the extended exhibition period will cause more “anxiety because people had suffered great hardship during our floods”.

Narrabeen lagoon empties after huge amounts of water flowed into it causing minor flooding. Picture:John Grainger

“We have continually seen floods, we have continually seen significant amounts of money spent on ad hoc mechanisms to open the lagoon, which sometimes closes immediately … that’s a waste of ratepayers’ money.”

Cr Ruth Robins wanted the exhibition extension because it would give people a greater opportunity to have their say.

The draft strategy will be on exhibition from May 6 on the council’s Your Say page.