Published On
09/04/2024
Central Coast Council have been pro-actively and actively managing The Entrance Channel to minimise flood risk and maximise ecosystem health. In 2022 Council developed a new Tuggerah Lakes Interim Entrance Management Procedure which described flood-minimisations actions to be undertaken based on water level predictions in consideration of entrance channel conditions. Management of The Entrance channel is now determined based on a complex system of water level predictions, rather than a trigger level simply being reached.
The Entrance Channel is currently classified as being in a heavily constricted state, as ocean tides have been pushing sand into the entrance channel due to prolonged periods of low catchment rainfall. This is actually a very important natural phenomenon for perched estuaries, preventing the net loss of water that would occur from the lake system if the channel was wide open during prolonged periods of low rainfall.
Based on this heavily constricted classification, Council pro-actively chose to undertake optional non-flood entrance berm maintenance works in February. These included secondary pilot channel preparation works involving construction of a 40m wide pilot channel through the berm, with the end plug to the ocean kept in place. This meant that once flood prediction and lake level triggers were met, the process of fully opening this second channel was very efficient.
Timeline of events
Thursday 4th April 2024
- The BOM issued a severe weather warning for high rainfall for large parts of NSW, including the Central Coast and in accordance with the Entrance Management Procedure Council placed machinery resources required for entrance works on standby at Karagi Point.
- We continued to monitor forecasts, rainfall, ocean conditions and lake water levels, undertook scenario modelling and conducted site inspections.
- At this time the lake level was about 0.4m ADH and the primary channel was open.
Friday 5th April 2024
- Overnight there was a forecast prediction for a 25% chance of water levels reaching the 1.3m AHD trigger point within the next 4 days. As a result, the pre-flood Entrance Management Procedure was enacted which included pre-flood berm preparation works immediately adjacent to the existing primary channel opening, lowering sand levels by mechanical scraping along the banks. This resulted in widening of the already open primary channel north of the rock shelf to enhance flow of water out of the lake.
- At this time the lake level reached 0.68m ADH and the primary channel was open.
Saturday 6th April 2024
- Council's predictive model indicated that the lake was likely to exceed the minor flood level of 0.9m late morning Saturday, with a peak just above moderate flood level at 1.32m on Sunday afternoon.
- Our procedure typically requires a 1.1m lake height to establish optimal flow through the secondary pilot channel, however the Tuggerah Lakes Interim Entrance Management Procedure states that Council may wish to proceed with undertaking the entrance procedures on a precautionary basis in the event where there is a high degree of uncertainty in forecast rainfall and/or peak flood levels.
- Considering conditions, we chose to open the secondary pilot channel by removing the plug at low tide on Saturday which was 12:30pm.
- At this time lake levels were about 0.8m. The primary channel and secondary channel were now both open.
Sunday 7th April 2024
- By morning, the secondary pilot channel was nice and wide, however sand shoaling occurred during the high tide overnight, and the flood waters weren’t high enough to push through, having still not met even the minor flood threshold of 0.9m. This shoaling of the channel emphasises the challenges and risks in early opening.
- Works were conducted by Council crews at low tide on Sunday, with machinery successfully increasing the depth of the secondary pilot channel. Outward flows were successfully established at 12noon.
- At this time the lake height was 0.88m and the primary channel was open. The secondary channel had closed over and then been re-opened.
Monday 8th April 2024
- Lakes levels reached the Minor Flood threshold of 0.9m on Monday morning as the result of continuing flow of stormwater into the Lake from Yarramalong Valley and Ourimbah Creek.
- By low tide the levels had dropped back to less than 0.9m (below flood height), and there was an expectation that lake levels would continue to slowly recede as floodwaters exited to the ocean through the primary channel.
- However, Monday nights high tide and the continuation of flows in from the catchment pushed levels up to 0.92m and they remained above the 0.9m Minor Flood threshold. The primary channel remained open; however, the secondary channel had closed over due to ocean forces pushing sand in.
Tuesday 9th April 2024
- Machinry was again deployed to the secondary channel and a flow of water out of the lake at low tide was re-established thanks to the work of Council staff who successfully increased the depth of this secondary channel at low tide.
- Overnight high tides reached an unusually large peak at 1.14m AHD, significantly higher than lake levels which were sitting around 0.88m. This caused water to flow into the lake system and pushed large volumes of sand into the secondary channel. This was exacerbated by a Tasman low bringing strong to severe gale winds and huge 4m close range southerly swells.
- Lake levels continue to sit just under the Minor Flood level of 0.9m. The primary channel remained open, and the secondary channel had closed over and been re-opened.
Wednesday 10th April 2024
- Machinery was again deployed to the secondary pilot channel and flows were again re-established thanks to the work of Council staff who were able to successfully increase the depth of this secondary channel at low tide.
- Lake levels continue to sit just around the Minor Flood level of 0.9m. The primary channel remained open, and the secondary channel had closed over and been re-opened.
Why is the lake level taking so long to recede?
There has been a temporary plateau in water levels in Tuggerah Lakes whilst storm water continues to flow into the Lake from Yarramalong Valley and Ourimbah Creek, with outward flow limited by a combination of tidal influences, restrictions on the effectiveness of the primary channel due to its location and shoaling of the secondary channel at each high tide.
One of the issues Council has encountered in response to this flood event is the extreme southward location that the primary channel has chosen to follow. This is an unusual phenomenon occurring in recent years since construction of the groyne at The Entrance Beach, which has resulted in sand deficits on the north side of the groyne. The repositioning of the primary channel at this location means that the depth of the channel is limited by the rock shelf, reducing its capacity to operate effectively. The Entrance Beach groyne is a NSW Government asset, and Council has recently made representation to the NSW Government asking for further consideration to be given in regards to the impact of the groyne on the effective operation of The Entrance Channel and flooding.
As has been seen in recent days with efforts to realign the channel through a new pilot channel, establishing a successful channel in a more northwards location can be difficult. As expected, sand shoaling has occurred in the secondary pilot channel during each high tide, and the flood waters haven’t been high enough to push the incoming sand back out. Due to the marginal difference between the height of the lake and the ocean, continuing flow of the secondary pilot channel is not occurring without intervention.
Council will continue to intervene and make efforts to assist in release of floodwaters from the lake until satisfied that the lake levels remain in the normal range below the minor flood height.
Why didn’t the secondary channel get opened earlier?
There are many benefits to keeping the plug on the secondary channel until triggers are met. In order for effective sand scouring to take place upon opening, there needs to be enough water depth in the lake to create adequate pressure. Concurrently, ocean tide and wave conditions need to be carefully considered as large swell or high tide conditions may result in infilling of both water and sand into the system.
Allowing the lake levels to rise slightly higher than normal also allows the lake’s fringing wetland habitats to be immersed, assists with the distribution of wrack and improves mixing of the water between shallow nearshore areas and deeper parts of the lakes. The fluctuations in lake levels also help to free and lift wrack that can get trapped near the shoreline and place it in areas where it can aerobically break down in the dry sun (such as saltmarshes, fringing wetlands, and foreshore reserve areas), reducing odour.
