Preparatory work has begun on dredging Narrabeen Lagoon entrance on either side of Ocean Street Bridge, an operation that will take several months to reduce risk of flooding.
Thousands of tonnes of sand, silt and mud, deposited by successive tides and the four creeks that feed into the lagoon, is restricting the flow of water in and out of the coastal lake.
This natural build-up of sediment increases the risk of regional flooding during storm surges, when freshwater flowing into the lagoon is prevented from escaping into the ocean. Water levels rise around the lagoon’s 55 square kilometres catchment area, impacting housing, infrastructure and the natural environment.
An estimated 40,000 tonnes of sand and mud will be extracted using mechanical diggers, dredgers, pumps and barges.
When Manly Observer visited the site shortly after the operation commenced, an excavator on caterpillar tracks was making long sandbars using sand dug from the lagoon.
A site supervisor explained to this reporter, “The digger makes three fingers of sand out into the lake. From these we can set up the dredging equipment and extract the sand and mud alongside, which is deposited on a barge.
“After we’ve finished one area, the first finger of sand is scooped up too, then we move to the second finger. We expect to extract around 22,000 cubic metres of sand. This will be transported to Collaroy to reinforce the beach around the new sea defence walls.”
Sand, Soil and Silt
Narrabeen Lagoon is an ICOLL: Intermittently Closed and Open Lake and Lagoon. There are approximately 70 along the NSW Coast and four on the Northern Beaches, including Curl Curl, Dee Why and Manly.
Narrabeen Lagoon is fed by Deep Creek, Middle Creek, Mullet Creek, Nareen Creek and Narrabeen Creek, which bring soil and silt from suburbia and surrounding bushland.
The build-up of ocean sand and creek-borne sediment coupled with lowering of the lagoon levels leads to an inevitable blockage of the ocean entrance, from which seawater is prevented from flowing in and out at high tide.
For a hydrological description, Science Direct defines it as “Closure occurs when a subaerial sand berm stabilises across the entrance channel during times of low fluvial discharge.”
In layperson’s terms, the NSW Department of Planning, Industry and Environment (DPIE) description states: “ICOLLs is a generic term for a distinct type of estuary with a tendency for the ocean entrance to close during periods of low freshwater inflow.”
Sedimentary build-up at the entrance to ICOLLs is a natural phenomenon, but it requires regular removal to enable the marine health of the estuarine environment (and reduce risk of flooding).
However, as previously reported in Manly Observer, Northern Beaches Council is still assessing the best long-term management with the Narrabeen Lagoon Entrance Management Strategy (link here).
Northern Beaches Mayor Sue Heins confirmed the current dredging works are part of Council’s strategy and based on flood risk management studies.
“Narrabeen Lagoon is one of our greatest natural waterways but as locals know all too well, it is prone to flooding,” she said. “Council has a strategy to manage the lagoon entrance to minimise the risk of flooding, backed by research and analysis by coastal experts.
“It includes more frequent sand clearance operations, as well as reshaping and revegetating sand dunes to assist with sand stabilisation. We appreciate that these works may be an inconvenience in the short term, but they are necessary to protect our community from flooding in the medium to long term.”
A Council spokesperson added, “To ensure public safety, Birdwood Dune car park will be closed during this time and there will be parking and pedestrian access restrictions during work hours at Mactier and Wetherill Streets.
“The sand will be deposited at Collaroy-Narrabeen Beach between Goodwin and Stuart Streets. Works will be complete by the summer school holidays. To get the job done before the busy summer months, up to 200 truckloads of sand will be carted each day.”
Northern Beaches Council Narrabeen Lagoon management: https://www.northernbeaches.nsw.gov.au/council/news/explainer-narrabeen-lagoon-management
