There are four 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 and Curl Curl lagoons are all managed by Northern Beaches Council. All four 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 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 four main lagoons 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.
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.
The four lagoons on the northern beaches can fill up like a bath tub 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 1.3m 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 prove unsuccessful, and adversely impact 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 which 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 out flow 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.
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 3-5 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.
Once the lagoon has closed, Council can undertake a mechanical break out where a channel is formed using an excavator allowing the lagoon to drain out to the ocean. In order for a break out to be successful, the water level in the lagoon needs to be rising to at least 1 metre higher than mean sea level with significant rainfall occurring or forecast otherwise the flow in 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 how other coastal lagoons on the Northern Beaches are also 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.
The second management action is an entrance clearance where a large amount of sand is excavated from the entrance area; from west of Ocean Street Bridge to the ocean, and transported back to Collaroy-Narrabeen beach. This ensures the entrance is not constricted with sand for a number of years and tends to keep the lagoon open (depending again on ocean and rainfall conditions). This occurs every few years and takes a few months to complete. This is a long term management option and requires significant planning and funding. It was last completed in December 2018 and prior to that in November 2016.
Council monitors the water level and lagoon entrance conditions through site inspections and digital water level recorders, which are publicly available at Northern Beaches Flood Information Network.
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 greater than what can leave the entrance
- The amount of water washing in from heavy ocean swells and extreme tides
- Some properties are very low lying
An entrance clearance is a project that Council undertakes every few years where a large amount of sand is excavated from the entrance area; from west of Ocean Street Bridge to the ocean, and transported back to Collaroy-Narrabeen beach. This ensures the entrance is not constricted with sand for up to a few years (depending again on ocean and rainfall conditions). This normally occurs every few years and takes a few months to complete. It was last completed in December 2018 and prior to that in November 2016.
Depending on future rainfall and when the lagoon entrance naturally closes following accumulation of sand in the mouth, a renewed entrance clearance will be planned to occur in 2021.
Once the lagoon entrance closes, Council can mechanically break out the entrance when the lagoon water level rises to a level of over 1 metre above mean sea level and there is significant rainfall occurring or forecast. Council cannot open the lagoon while water levels are low because the flow in from the ocean on high tide overcomes the outflow from the lagoon and it will close again. Even after a successful breakout, the lagoon can close up again quite quickly, particularly with the current large volumes of sand at the northern end of Narrabeen Beach (and depending on ocean and rainfall conditions).
Council aims to manage the entrance in a way that balances community needs, flooding risk and the need to maintain the natural process of a coastal lagoon (which includes periodic closure).