Lagoons

There are five coastal lagoons (or estuaries) along the Northern Beaches coast, all of which open and close naturally depending on rainfall and ocean conditions. Narrabeen, Manly, Dee Why, Pittwater and Curl Curl lagoons are all managed by Northern Beaches Council. All of the waterway lagoons are home to a wide variety of plants and animals, specially adapted to a mix of freshwater and saltwater conditions. Too much of either freshwater or ocean water can damage these fragile ecosystems. Maintaining this delicate balance is complicated by storm water pollution and unauthorised lagoon openings.

Water Quality

Water quality for swimming at ocean beaches and Birdwood Park is monitored by NSW Beachwatch on a weekly basis.

It is recommended you avoid swimming during, and at least one day after, heavy rain at ocean beaches, and for three days at estuarine (lagoon) swimming areas.

Avoid swimming if there are signs of water pollution such as:

Discoloured water
Fast flowing or strong smelling drains
Street litter such as drinking straws, food wrappers or leaves floating in the water or on the tide line

Lagoon Health Checks

We undertake ecological monitoring on our five waterways assessing 'chlorophyll a' and 'turbidity' as a measure of their condition. An annual report card is produced allocating a condition ranking for the lagoons in comparison to other NSW lagoons.

Pollution Sources

Stormwater, or rainwater run off, from our roads, footpaths and carparks flows into creeks, estuaries and coastal lagoons and finally to the ocean.

Pouring chemicals down the drain, using herbicides, pesticides and fertilisers in the garden all harm our waterways.

Sweeping leaves down the drain, washing the car on the driveway and using chemical products around the home all damage the water quality in our creeks, lagoons and beaches.

Lagoon Flooding

The four lagoons on the Northern Beaches can fill up like a bathtub when it rains. If it continues to rain, they will overflow and cause flooding to surrounding land.

Council manages the entrances of the lagoons to reduce the risk of flooding to surrounding properties. Once the water in each lagoon reaches a specific level, Council mechanically opens the entrance to let the water drain into the sea.

The opening levels for each lagoon are:

Narrabeen Lagoon 1.0-1.3m AHD
Dee Why Lagoon 2.2m AHD
Curl Curl Lagoon 2.2m AHD
Manly Lagoon 1.4m AHD

Attempting to open the lagoons below these predetermined levels often proves unsuccessful and adversely impacts the ecosystem. Several other natural factors affect Council’s ability to mechanically open the lagoons, including the forecast rainfall, tide, ocean swell and the height of the sand berm at the entrance. It is important to note that in times of very heavy rainfall and/or strong ocean swell, mechanically opening the lagoon may not prevent flooding.

Council in partnership with Manly Hydraulics Laboratory and the NSW Government have developed a flood warning webpage that provides real-time information on rainfall conditions and water levels in a number of our creeks and lagoons.

Unauthorised Lagoon Openings

It’s both illegal and dangerous for anyone else to open lagoons. The water outflow often develops into ‘standing waves’, with particularly dangerous downward pressure. This can trap even strong swimmers. After heavy rains, contaminated water carries serious health risks to humans and marine life. Unauthorised lagoon opening carries fines up to $1,100.

How does Council manage Narrabeen Lagoon entrance?

Narrabeen Lagoon naturally opens and closes to the ocean depending on how much sand has accumulated in the entrance and how much rain we’ve had. During the last 7 years the volume of sand at the northern end of Collaroy-Narrabeen Beach has significantly increased to the point where North Narrabeen is the widest it has been for decades. The sand drift cycles (long-shore drift) have pushed more sand in front of the entrance and there hasn’t been sufficient rainfall to clear it.

Council has two main methods of managing the entrance: mechanical break outs and entrance clearance.

If the lagoon has closed and there is potential flooding forecast, Council can undertake a mechanical break out where a channel is formed using an excavator, allowing the lagoon to drain out to the ocean. Break outs are more likely to be successful when the water level in the lagoon is at least 1 metre higher than mean sea level, with significant rainfall occurring or forecast. Otherwise the inflow from the ocean on high tide overcomes the outflow from the lagoon and it closes again.

Even after a successful breakout, the lagoon can close up again quite quickly if there is unfavourable swell and rainfall conditions, particularly with the current large width of North Narrabeen Beach. As such, this is only a short term management option undertaken primarily for flood mitigation. This is also how other coastal lagoons on the Northern Beaches are managed, however Dee Why and Curl Curl Lagoons are not opened until the water level is over 2m above mean sea level, which means they open with a much greater amount of energy and flow. As with Narrabeen, they stay open until the ocean sand closes them again.

The second management action is an entrance clearance where a large amount of sand is excavated from the entrance area, from the shoal west Ocean Street Bridge through to the ocean, and transported back to Collaroy-Narrabeen beach. This reduces the sand constriction in the entrance and helps to keep the lagoon open for a number of years (depending on ocean and rainfall conditions).

How often does Council undertake entrance clearance?

Council excavates the entrance area every few years, with clearance operations taking a few months each time. This is a long term management options and requires significant planning and funding. It was last completed in November 2023 and prior to that, in December 2021 and December 2018.

Clearance has been required more frequently in recent years as we have been in a period of clockwise beach rotation. Beach rotation is a decadal cycle, where clockwise rotation results in a wider beach and a higher volume of sand at the northern end of the beach, caused by changes in wave currents moving increased volumes of sand northwards along the beach.

How does Council reduce flooding?

Council monitors the water level and lagoon entrance conditions through site inspections and digital water level recorders, which are publicly available through Manly Hydraulics Laboratory.

Once water levels are high enough and rain is forecast, a mechanical breakout can be undertaken in line with mechanical breakout guidelines. In most cases this will allow the lagoon to drain and reduce the risk of flooding. In heavy weather conditions however, many properties may be prone to flooding even when the entrance is open because:

The volume of water flowing down from the catchment can be held back by high ocean levels
Heavy ocean swells and extreme tides can result in such high ocean levels that huge volumes of water can flow into the lagoon and cause flooding even without much rainfall
Some properties are very low lying

When will Council next mechanically break out the entrance?

Council aims to manage the entrance in a way that balances community needs, flood risk and the need to maintain the natural process of a coastal lagoon (which includes periodic closure).

What were the findings of the Narrabeen Lagoon Entrance Management Strategy?

The final adopted strategy includes a range of priority actions which we will now begin to implement including:

trialling more frequent but smaller sand clearance operations (every two-three years rather than four-five years)
developing a more flexible set of the conditions which trigger Council intervention to open the lagoon if required
adjusting the alignment for the pilot channel
reshaping and revegetating the denuded part of Birdwood Park dune to assist with sand stabilisation
investigating the financial viability of mobile sand pumping as a longer term alternative to trucking.

View the Adopted Strategy