What is an ICOLL?

Many coastal lakes open and close naturally and irregularly - they are known as Intermittently Closed and Open Lake and Lagoons or ICOLLs.

An ICOLL can be any lake or lagoon that can intermittently close to the ocean. The term ICOLL also describes estuaries that are classed as both intermittent and wave-dominated barrier, as estuaries that are classed as wave-dominated barrier do occasionally close. 

World-wide, ICOLLs are quite rare. About 70 of the coastal lakes in NSW are ICOLLs and most occur on the south coast where catchments are smaller and rainfall is lower. Shoalhaven has a number of ICOLLs, some of which are managed by Shoalhaven City Council, the larger ones being Swan Lake, Burrill Lake, Tabourie Lake and Lake Conjola.

ICOLLs are very sensitive to human disturbance. This makes them one of the most complex and difficult coastal environments to manage in urban areas.

Why do ICOLLs open and close?

It’s natural for the Lake to be closed at times. In NSW, about 70% of ICOLLs are closed most of the time.

Lake Conjola entrance is constantly changing, from being open to the ocean after a big flood, to being completely closed due to drought and severe coastal storms. In between these events, the entrance channel naturally drifts north until it’s against Cunjurong Point.

The ongoing changes are caused by water and wind constantly moving sand into, and around, the entrance.

·  Coastal storms with big swells wash offshore sand over the spit and into the entrance

·  Rainfall in the catchment may ‘refresh’ the channel by increasing flows that scour sand build up in the entrance and carry some sand offshore

·  Floods dramatically and quickly wash sand out to sea, and scour a more central channel

·  Wave action carries sand from south to north along the beach (littoral drift), pushing the entrance channel north

·  Tides carry sand in and out of the Lake entrance. The flood (incoming) tide carries more sand in, than the ebb (out-going) tide carries out, resulting in a nett gain in sand volume in the entrance

·  The wind, which blows predominantly from the south-east, carries large amounts of sand into the entrance.

Why doesn’t Council keep the Lake open?

The long-term goal of the NSW Government and Shoalhaven City Council is, as far as possible, to progressively allow ICOLL entrances to open and close naturally, within the constraints of property inundation and flooding of infrastructure. This goal is reflected in NSW Government legislation which Council must follow.

The main reasons for not maintaining an open entrance are:

·  An open entrance lets the ocean in (both normal tides and storm surge) and ocean flooding is as big a risk to Lake Conjola as catchment flooding. The April 2013 flood demonstrated this when the Lake naturally opened, and flooding occurred from both the ocean and the catchment

·  Flood models show that only modest reductions in peak flood levels are achieved with an open

entrance (Lake Conjola Floodplain Risk Management Study and Plan 2013)

·  A more permanently open entrance will change the Lake’s natural ecology. Possible impacts include loss of seagrass and saltmarsh, mangrove colonisation and decline in recreational species such as prawns

·  Increased tidal flushing does not necessarily mean overall improvement to water quality or water clarity, as water quality is largely a function of catchment runoff

·  The natural vegetation around the Lake is adapted to changing water levels and protects the foreshores. Lower water levels would expose additional foreshore and potentially lead to erosion, as well as impacting saltmarsh that is dependent on periods of higher water levels

·  An open entrance would reduce low level flooding in the short term. This flooding is already managed by The Entrance Management Plan. In the longer term, sea level rise will lead to increased water levels and inundation of low lying areas (Lake Conjola Floodplain Risk Management Study and Plan 2013).

When does Council open the Lake?

Council mechanically opens Lake Conjola in order to alleviate low level flooding and this occurs when the ‘trigger level’ is reached.

As with all Council managed ICOLLs, Lake Conjola has an Entrance Management Policy (2013). The policy was prepared in accordance with NSW legislation and in collaboration with NSW Government agencies and the community. The when, where and how for a mechanical opening is laid out in this policy and Council must adhere to it.

The main factor governing when the Lake can be mechanically opened is the ‘trigger level’. Each Lake has its own specific trigger level appropriate for managing the low level flooding at each location. In the case of Lake Conjola, the trigger level is 1.0m AHD for a planned opening and 1.2m AHD for an emergency opening.

If the Lake needs to be opened outside the criteria stated in the Entrance Management Policy, Council must obtain approval from all relevant Government agencies - Department of Industry - Crown Lands, NSW Office of Environment & Heritage, Department of Primary Industries - Fisheries and, where threatened nesting shorebirds are present, National Parks & Wildlife Service.

Why does the Lake often close again quickly after mechanical openings?

Mechanical openings are often short lived due to several factors:

·  The water level at the time of opening is too low to create the surge required for effective

scouring of the channel. There is insufficient ‘hydraulic head’.

·  Opening the Lake at a level lower than the trigger level will lead to an even more rapid closure. The better the scour, the longer the Lake stays open; and

·  Storm wash-over carries offshore sand back into the entrance so, if a storm follows on closely after a mechanical opening, it may fill the opening very quickly. 

What’s the best location for a mechanical opening?

A central to northern location is favoured because:

·  Green Island protects the northern area from wave energy and storm wash-over

·  Littoral drift causes the spit to move northwards, naturally forcing the entrance channel towards Cunjurong Point

·  With a channel located along a rocky shoreline, such as Cunjurong Point, there are benefits to both scouring and persistence of the opening

·  Threatened migratory shorebirds nest on the sand spit at Lake Conjola and are endangered.

During the nesting season from September to March, Council must minimise disturbance to the nesting area. Only a northern intervention can be considered at this time.

When the Lake is closed is the water still clean?

It’s natural for ICOLLs to be closed at times. During long periods of closure the water can change colour. This doesn’t mean that the Lake is dirty or unhealthy.

When the Lake is closed, weekly water quality monitoring is undertaken by Council at three locations and results are posted on Council’s website. From 14 November 2018, Council increased the number of water quality monitoring locations to seven. From then until now, test results have consistently rated as ‘Good’ or ‘Fair’ which indicate that the water is suitable for swimming. If test results indicate that water quality is unsafe, then Council will advise the community.

Sediment and pollutants are washed into the Lake by heavy rain and water quality is affected whether the entrance is open or closed. For this reason, it’s best to avoid swimming for up to three days after heavy rain, especially near storm water outlets.

Council, residents and visitors all play a part in keeping the catchment and the Lake clean and healthy.

What is the Green Weed in the Lake?

During periods of Lake closure, green algae may appear in shallow areas of the Lake. It’s a macroalgae and appears in many south coast ICOLLs.

The macroalgae is not toxic or harmful to human health. However, as it breaks down in the sun, it’s likely to cause unpleasant odours. The algae doesn’t affect water quality as shown by Council’s water quality monitoring results. Reducing the amount of nutrients entering the Lake is the best way of managing water quality and algal blooms in the Lake.

Will Council review the Lake Conjola Entrance Management Policy?

Yes, Council is committed to reviewing and updating the Policy. This must be done in accordance with the Coastal Management Act through the development of a Coastal Management Program (CMP). Entrance and lake processes, entrance management to manage low level flooding, environmental, social and economic factors will be considered.

You can find out more about CMPs at www.environment.nsw.gov.au/topics/water/coasts/coastal-management/programs

Is Council preparing a stand-alone Lake Conjola Coastal Management Program (CMP)?

Yes, Council has applied to the NSW Coastal and Estuary Grants Program to seek funding for a Lake Conjola CMP. Council sought input from the Conjola Community Association in the preparation of the grant application.

Is Council seeking approval from the State Government to open the Lake now because it has been closed for so long, even though the Lake has not reached the policy trigger levels?

Yes, Council applied for approval to construct a pilot channel and effect an opening of the lake on 12 February 2019, however, the approval was not granted because the lake level was below the trigger levels set in the Lake Conjola Interim Entrance Management Policy. Council resolved to continue to seek approval (See minutes from Council's Ordinary Meeting - 26 March 2019 - MIN19.143, page 6) and a further application was submitted on 12 April 2019.

Is the water quality good?

While the lake is closed, Council samples once a week at 7 sites. These sites are in locations where most people swim, boat and fish.

Council also tests the water quality at sites throughout the lake once a month. The water quality is generally good in comparison to Australian New Zealand Environment Conservation Guidelines. Sometimes after heavy rain the bacterial levels in the lake increase. This is normal occurrence in lakes and estuaries as pollutants are washed off the catchment during rain events and it is recommended that people do not swim in lakes for up to 3 days after heavy rain. After a few days, the bacteria levels go back to low levels. This is a general warning for swimming as part of the Statewide Beachwatch program. For more information see www.environment.nsw.gov.au/topics/water/beaches/beachwatch-water-quality-program

View the Lake Conjola Beachwatch results at www.shoalhaven.nsw.gov.au/Environment/Coastline-and-waterways/Beachwatch

You can find water quality monitoring results on Council’s website at https://www.shoalhaven.nsw.gov.au/Environment/Aqua-Data

What does a dry notch do?

The preparation and maintenance of a dry notch is to eliminate the need for a pilot channel arrangement as per the adopted interim entrance policy. In a flood event, the dry notch hydraulically will perform a similar function to a pilot channel without the need for extensive intervention.

Opening the lake entrance will not prevent flooding. Even if the entrance is fully open at the start of a large flood, part of the Lake Conjola township and surrounds will be flooded. The dry notch will aim to reduce, not eliminate, the impacts of low level flooding.

Under ocean flooding scenarios an open entrance allows for greater penetration of ocean swell through the inlet into the lake. An open entrance also increases the likelihood of foreshore inundation due to wave run-up. 

Are mosquitos a problem at Lake Conjola?

Please refer to the information provided in the Mosquitos – Background and Updates article. 

How does Council manage the remaining onsite sewage management systems?

Please refer to the information provided in the On-site Sewage Management Systems – Background and Updates article. 

How does Council manage trees and/or shrubs that fall into the Lake?

Please refer to the information provided in the Foreshore Vegetation – Background and Updates article. 

Should there be concerns of contamination from the old Lake Conjola landfill site?

Please refer to the information provided in the Old Tip Site Monitoring – Background and Updates article.