Narrabeen Lagoon: Bold bid to stop regular flooding of surrounding streets
A concerted push has begun to stop streets surrounding Sydney’s largest coastal lagoon from being regularly flooded.
Manly
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A bold move to slash the risk of homes and businesses being flooded around the edges of Sydney’s largest coastal lagoon has begun.
Northern Beaches Council is searching for a solution to keep the ocean outlet to Narrabeen Lagoon, which is regularly blocked by a build up of sand, open for longer.
It costs the council about $1 million each time it conducts major excavation work — on average once every four years — to remove 40,000 cubic metres of sand to reopen the channel connecting the lagoon to the sea at North Narrabeen Beach.
The lagoon naturally opens and closes depending on the amount of sand at the entrance, tides, prevailing swell conditions and rainfall.
Now, as part of the creation of its draft “Narrabeen Lagoon Entrance Management Strategy” the council has suggested “long term” options such as regular sand pumping and creating a new “ebb-tide channel”.
It is also considering installing “low flow pipes” to keep the water constantly flowing between the lagoon and the sea, even if the entrance is blocked.
The council has already ruled out building a rock breakwall just to the south of the lagoon entrance because it was too expensive and analysis “identified a range of significant environmental, recreational, public safety and aesthetic impacts associated with this option”
Sand pumping involves use of a mobile pumping system, fed with sand excavated from the lagoon entrance and mixed with seawater and then sent through a 2km long and 20cm diameter pipeline, built under the sand or along Ocean St, to deposit sand along Collaroy-Narrabeen Beach.
The creation of a new ebb-tide channel involves using the power of the outgoing tide by building “half-tide training walls” (low-level rock structures) that direct the tide along a channel which will help keep the entrance open for longer.
But Mayor Michael Regan said the council was also calling on the public to have their say in the development of a long-term strategy.
“Managing the entrance to Narrabeen Lagoon to avoid flooding is a complex operation,” Mr Regan said.“As a short term solution, we regularly use machines to open the lagoon to the ocean when we have the right combination of rain, lagoon levels, and ocean conditions.
“Open it at the wrong time and it is likely to close back up quite quickly and even cause additional sand build up, making it more difficult to open the next time.
“Every four years or so we also undertake a much more extensive excavation to clear thousands of cubic metres of sand that has built up in and near the lagoon entrance.”
Mr Regan said the council was keen to hear if the community supported other options such as sand pumping.
“This is the first stage only and we look forward to hearing from our community as we continue investigating the options, considering the science and ultimately developing a strategy which will reduce the risk of flooding for our community.”
The options paper can be viewed online at https://yoursay.northernbeaches.nsw.gov.au/
Feedback is open until March 28.
EARLIER:
By Jim O'Rourke
July 24,2020
Narrabeen Lagoon: Diggers deployed to help stop flooding ahead of wild weather
Authorities are using heavy machinery at the entrance to lagoons on the northern beaches to help stop potential flooding caused by this weekend’s wild weather.Northern Beaches Council has called in the excavators and bulldozers ahead of expected heavy rain and thunderstorms — and possible gusty winds — beginning on Saturday.
As another low pressure looks likely to form off the NSW east coast, the council started clearing sand away from the entrance to Manly Lagoon at Queenscliff Beach on Thursday.
At North Narrabeen Beach this morning a digger and small dozer began creating a channel linking Narrabeen Lagoon to the seas.
Up to 35mm of rain has been forecast by the Weather Bureau for the northern beaches on Sunday with 15mm and thunderstorms predicted for Saturday.
More rain — up to 15mm — is expected on Monday before conditions begin to ease on Tuesday.
The council said the northern beaches has been battered in recent weeks by waves as high as 10m causing erosion at a number of beaches.
“At Narrabeen, the huge swells, high tides and strong winds have forced sand along North Narrabeen Beach and across the mouth of Narrabeen Lagoon, closing the entrance,” it said in a statement.
“Our engineers and environmental scientists have been closely monitoring conditions and with rainfall forecast this weekend and into next week, council is moving machinery into position to reopen the lagoon and reduce the flood risk.”
In order for the entrance to stay open, the council said the water in the lagoon needs to be least 1m above mean sea level, “opening on a falling high tide and should have flow out of the lagoon from rainfall”.
“Without these conditions being met, flow out of the lagoon is very slow and there is a strong chance that the next high tides and swells will push more sand back into the entrance and close the mouth not long after the excavators have done their work.
“However, given the lagoon is at an elevated level, we are determined to create an opening before the forecast rainfall.”
A 23-tonne excavator was working early on Friday morning to dig a channel through the mouth with the aim to open the lagoon on the high tide.
even though the council anticipates the lagoon will remain open, crews will remain on standby across the weekend and in to next week to take further action if required.
Residents who live in flood-prone areas are encouraged to keep an eye on the forecasts and any severe weather warnings.
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