Update: Narrabeen Lagoon entrance

Since we posted the story below last week, heavy rainfall raised Narrabeen Lagoon to trigger level, allowing Council to re-open the entrance today and reduce the risk of localised flooding.

Additional rainfall is forecast over the next two days which should result in a wider and more free-flowing opening.

Thursday 15 February

In August 2022 the elected Council, following extensive community consultation, approved a comprehensive entrance management strategy for Narrabeen Lagoon which included more frequent sand excavations to reduce the risk of flooding.

In keeping with this strategy, last year 25,000 cubic metres (around 45,000-50,000 tonnes) of sand was extracted from the lagoon entrance ahead of the busy summer swimming season. <_o3a_p>

Despite these significant works, the entrance closed in early February when a four-metre swell washed sand into the entrance, compounded by a high degree of clockwise beach rotation in the Collaroy Narrabeen embayment. <_o3a_p>

Beach rotation relates to the width of sand on the beach near the entrance, and North Narrabeen Beach is currently the widest it has been in decades due to large volumes of sand moving northwards up the beach. With sand moving northward, natural closure of the entrance is accelerated and difficult to prevent, especially when the wave and tidal conditions are right. <_o3a_p>

Although the entrance has closed, the flood risk to the low‐lying area surrounding Narrabeen Lagoon remains lower than usual as a result of the recent clearance works. <_o3a_p>

The works intentionally dredged a higher volume of sand from the west of the Ocean St bridge compared to past clearances in accordance with expert advice. While the area closer to the beach fills up with sand from the ocean, the deeper area to the west will allow for faster flow of water out of the lagoon should heavy rainfall raise the water level to allow an emergency opening at the entrance, thus reducing the risk of flooding. <_o3a_p>

Once reopened, the works will also significantly increase the chance of the lagoon remaining open for a longer period. <_o3a_p>

Council's coast and flood engineers will continue to closely monitor weather forecasts, lagoon water levels and the state of the entrance and be ready to open it mechanically when lagoon water levels rise and weather, tide and swell conditions allow for a successful opening. <_o3a_p>

To understand how Council manages our lagoons, please watch the video explainer. <_o3a_p>

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