Coastal Management Plan - 13 August 2023
Background to Management Action - EM1 Entrance Management Policy
We asked Council this week to provide a quick summary of the 4 options embedded in their Management Action - EM1 Entrance Management Policy. See below in italics:
The options for management of the entrance to Lake Conjola are considered to fall into four categories, involving progressively greater intervention, as follows:
· Option 1: The entrance area is allowed to behave naturally. Mechanical intervention in the form of excavation of a pilot channel occurs only in response to certain triggers
· Option 2: The entrance area is managed by way of a dry notch approach whereby the sand levels in the entrance area (above water level) are regularly mechanically groomed to facilitate an easier mechanical opening when required, i.e., less excavation is necessary to open the lake when trigger levels are met.
· Option 3: The entrance area is managed by way of occasional dredging whereby a channel is sustained in the position of the natural ebb tide channel, aligned behind the sand spit and directed towards the Cunjurong shoreline. Excavation of a pilot channel would also form part of this approach. It could also be combined with maintenance of a dry notch. The aim of this approach is to avoid loss of the ebb tide channel, which would otherwise necessitate a much longer pilot channel thereby impacting adversely on response times and effectiveness of a mechanical breakout.
· Option 4: Engineering works are constructed in the entrance area, such as entrance breakwaters, to create a permanently open entrance, in which case there would be no requirement for a pilot channel.
CCA – Comments and Assessment of each Option
We note that the trigger levels are water level triggers, and that option 1 is least intervention (it is also current policy) and option 4 is greatest intervention. Also please note that the sustained rains we have had for the last 3 years have helped keep the lake open. Now it is not raining so much, lake water level triggers will not support opening the lake.
It is important to note that Council is NOT being driven by a requirement to keep the lake open, but rather flood mitigation and environmental outcomes in line with the relevant State and Federal legislation that they must comply with. As other Councils have done with their CMPs, we are keen for our Council to go beyond legislative compliance to achieve optimal outcomes for Lake Conjola.
Option 1 – CCA comments
Option 1 is current policy (since 2019) and it is an improvement on the previous (2010/2011) policy, as it has lower trigger height levels, a larger area for clearance works and a five-year licence. This option is still reactive rather than proactive (nothing happens till after the lake closes, possibly well after) and hasn’t been put to the test since adoption in 2018/2019. La Nina conditions have helped keep the entrance open since the February 2020 near record flood event (due to a closed entrance). This management option relies on Council to determine when to mobilise clearance works, and their ability to get it done during periods of heavy flooding.
Option 2 – CCA comments
A dry notch is basically a sand berm (or build-up of sand), placed strategically to allow the lake to close and a barrier dune to form in the northern entrance location. In theory the geometry of the barrier dune will be managed to allow rapid clearance works when lake water trigger height is reached. The CCA believes this is worse than current policy. A dry notch in place at Shoalhaven Heads has been the subject of complaints by their local association related to maintenance of the dry notch and flood related impacts over the past decade.
The CCA is concerned that this option will not mitigate flooding or maintain lake water clarity and quality during times of near or actual closure. Hydraulic calculations conducted as part of the “BMT WBM Lake Conjola Flood Study July 2007” showed that entrance channel geometry at the location of the constriction created by the shoaled sand berm was the critical parameter determining flood levels in Lake Conjola – as evidenced by the 1971, 1975 and 1992 floods occurring when the lake was heavily shoaled (or closed) and the sand berm (the dry notch) height exceeded 1.0m AHD (ie above sea level). Subsequent to this 2007 flood study, the floods of 2011, 2012, 2013, 2014, 2015, 2016 and 2020 all occurred for the same reason.
Maintaining a dry notch at or below 1.0m AHD is likely to be costly given that 40% of the sand that enters the lake is windblown and the time needed for clearance works will likely be beyond current warning times. In summary, the dry notch needs to be well designed, well maintained and some excavation is likely still required once water levels exceed trigger levels. It also does not manage the ebb tide channel.
Option 3 – CCA comments
Option 3 - occasional dredging with water height trigger levels agreed - is an upgrade on Option 1 (current policy) and Option 2 (dry notch) in that this option proposes maintenance of the southern ebb tide channel. It appears based in part on the scientifically validated Managed Entrance option which was recommended by Patterson Britton and Partners (PBP) in their 1999 Entrance Study report.
Option 3 (as it stands) falls short of PBP’s recommendation however, in that the entrance is allowed to close and dredging works are based on trigger levels which apply once closed.
The Managed Entrance option recommended by PBP uses the Decision Support Tool (DST) within the M2 framework developed by Manly Hydraulics Laboratory (MHL). This monitors entrance conditions and provides detailed action requirements through 4 phases that track the narrowing of the entrance opening (as shown below). This makes it very clear when clearance of the entrance and dredging of the southern ebb tide channel should occur and provides a set of prescribed actions to manage the lake’s entrance and southern ebb tide channel. The M2 framework is already in place and MHL continues to monitor the lake.
A little history re the Managed Entrance Option (aka Option 3 with the DST)
In 1999 PBP and MHL provided Council a scope of works to clear the entrance and configure the southern ebb tide channel to a width of 20m and depth of 2m. Council followed this in part, clearing the entrance and widening the southern ebb channel to 10m and deepening it to 1m; removing 10,000 cubic metres of sand which was placed at the foot of the southern ancient sand dune to stabilise it. Although only half done (in terms of its width and depth), it was effective in providing an open entrance for 12 years, from 1999 to 2011 with only one one minor storm surge flood event (1.3m AHD) in this timeframe.
Option 4 – CCA comments
Permanent entrance with hard structures… a break wall. This is an often-discussed preference by many, but it has two significant downsides – i) prohibitive cost (which makes it less likely to happen) and ii) it will restrict outflowing flood waters and associated time to recede. The lake has a natural break wall on the northern side of the entrance which restricts the movement of sand past this point and provides scouring through the entrance opening. On the southern side there is a flexible sand spit which scours out during flood water outflows allowing a greater volume of water to move through the entrance in a much shorter time. This is referred to as a ‘fuse plug’ that pops out much like a champagne cork.
All 4 options on offer were previously assessed (including a cost/benefit analysis) in the PBP 1999 Entrance Management Study. The Managed Entrance option scored highest –they concluded that “the managed entrance option, which has a relatively low cost, and a positive flood mitigation benefit, is the only option which potentially returns a net benefit”. The Managed Entrance Option (Option 3 with the DST) has also been validated as evidenced by an open entrance for 12 consecutive years with only one minor storm surge flood (1.3m AHD) between 1999 and 2011. Since 2011 Council has reverted to previous protocols and practices of allowing the entrance to close with emergency openings. This has resulted in at least 9 significant flood events between 2011 to now.
If Option 3 is chosen for the CMP without use of the M2 framework and the DST, it remains the CCA’s preferred option because it includes occasional dredging into the southern ebb channel. Options 1 and 2 are restricted to clearing the entrance only, not maintaining the southern ebb channel. Management of the configured southern ebb channel is critical to maintaining an open entrance for an extended period because adequate velocity outflows are required to transport sand back out of the lake and scour the entrance at the same time. The important point is that the entrance geometry conditions always determine the extent of flooding. So, if the entrance is constricted and/or closed then we can expect higher levels of flooding.
Thanks again for reading! If you need help, have any questions or disagree, feel free to contact us at email@example.com and we are happy to assist and discuss. Please complete the survey with your preferred option.
A few additional comments… ICOLL – open or closed?
With the introduction of the Conjola Regional Sewerage Scheme in the early 2000’s, Council may no longer see a requirement to keep the lake open, due to less risk of contamination from septic tank seepage. However, the risk of incursion of algal bloom from the wastewater treatment plan is an increased risk to the lake when the entrance closes.
In terms of any objections to keeping the lake open, reference is also often made to the lake as an ICOLL (an intermittently closing and opening lake or lagoon). In other words, the lake closing is a natural event and NSW government policy encourages, in the long term, an ‘as natural’ regime as possible for estuary openings. However, within the subcategories of ICOLLs, the lake is a wave dominated barrier estuary and is open over 85% of the time (nearer 93% before 1920). Lake Conjola is not a typical ICOLL for this reason, and its other physical attributes include being 4 times larger than a typical ICOLL with water depths much deeper than 400mm (up to 10 metres in depth). Negative environmental impacts associated with opening ICOLLs (fish kill/water column separation) have never been recorded when Lake Conjola has been opened mechanically.
Water quality testing
Regardless of entrance conditions, Council undertake water quality testing year-round, increasing the frequency during times of a closed entrance. Shoalhaven Water contribute financially to the water quality testing and maintenance/monitoring of boreholes in key locations (receptors) around the lake as a condition of the Sewerage Scheme operation licence. It is a costly exercise to monitor the extent of nutrient plumes which emanate from the sand filtration dune directly above the aquifer) and appear to expand when the lake is closed and contract when the lake is open. We understand that an open lake assists Shoalhaven Water with their operations and management of the nutrient plumes, consistent with mapping of the nutrient plume within the Earth2Water reports.
The BLUE button is a link to the Manly Hydraulics website with 3 charts:-
1. The M2 Tidal Constituent (essentially the health of the tidal flow),
2. Last 7 days rainfall for Lake Conjola, and
3. Last 7 days water level - gauge is adjacent Holiday Haven Caravan Park.
The date range can be changed by clicking on the date in a top left hand corner of a chart, selecting the required date range and clicking the "APPLY" button.
The YELLOW button displays the last 7 days water level and rainfall for all inlets/lakes (which have an measuring gauge) in the Shoalhaven Council area.
Lake Conjola Home Page
Welcome to the website for the Lake Conjola District which is located just north of the townships of Milton-Ulladulla and Mollymook on the NSW South Coast, Australia.
Conjola is a natural paradise with 660 hectares of pristine lake surrounded by the Conjola National Park, beautiful unspoiled beaches and four small villages around the shores of the lake - Lake Conjola, Conjola Park, Fisherman's Paradise, and Cunjurong Point.
This perfect environment is the ideal location for water sports, bush or beach walking, mountain biking, or to simply relax and revitalise in the various accommodation facilities.
-Conjola Improvements Survey Results
-Jetty Replacement at Yooralla Bay
-Survey - how to spend donated money
- Lake Entrance Licence Oct21
- CCA Information Update Sep21
- CCA Information Update May21
- Conjola Coastal Mgt Program Feb21
- Proposal for Long Term Open Lake
- Conjola Sewerage Scheme Review
- Review Entrance Mgt Policy Feb19
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