Estuary entrance management

Intermittently closed and open lakes and lagoons (ICOLLs) are estuaries with entrance channels that naturally become blocked by a build-up of sand, then open again when water levels become high enough to overtop this barrier.

Waves and incoming tides move sediments into estuary entrances, and outgoing tides and water movements shift sediment out. The frequency and duration of entrance opening varies considerably across ICOLLs, which have adapted to these variations, including changes in water chemistry such as salinity.

Managing inundation

When entrances close, water levels can increase from rainfall, run-off and wave overtopping. This can cause inundation of low-lying foreshore areas, including urban areas.

Local councils artificially open ICOLLs to manage inundation of private and public infrastructure. However, opening ICOLL entrances comes at a cost and must be weighed against the real or perceived benefits of intervention. For example, entrance manipulation can cause unintended negative impacts to water quality and the physical form and natural functioning of these systems.

Alternatives to artificially opening entrances include relocating, modifying or raising at-risk low-lying assets.

Our role

We work to minimise impacts on sensitive ICOLL environments and protect and improve the health of estuaries.

To achieve this, we support councils by:

• providing technical and financial support to develop coastal management programs (CMPs) for estuaries, including ICOLL entrance management plans
• supporting the relocation and/or raising of assets and low-lying land through the Floodplain Management Program
• working to monitor, evaluate and report on the condition and physical properties of estuaries, which helps in the development of CMPs
• providing a structured approach to help manage the impacts of land-use activities on waterway health with the Risk-based framework for considering waterway health outcomes
• developing water quality and river flow objectives based on community values and uses of waterways across our coastal catchments
• providing technical and funding support through our Coastal and Estuaries Grants Program for estuary management projects, to manage risks from coastal hazards, restore coastal habitats and improve estuary health.

Rainfall, wave climate and estuary entrances

Low rainfall periods, or drought, are usually associated with the formation of sand barriers at estuary entrances. Entrance breakouts occur during periods of high rainfall and flooding, which cause estuary waters to build up behind the barrier and overtop it until they scour an entrance channel into the ocean.

Rainfall and wave climates can be seasonally specific, but they are controlled by global atmospheric conditions. How climate change will influence these atmospheric systems is difficult to predict.

Climate change is expected to reduce overall rainfall in southern Australia and, combined with increases in sea levels, will influence ICOLL entrances and the water levels within them.

Natural versus artificial openings

Under natural conditions an ICOLL entrance will open due to rising water levels from rainfall overtopping the sand barrier or occasionally from surf waves overtopping during storms.

When an ICOLL entrance is closed the water level can become higher than the ocean. Natural breakouts that occur because of this difference in water levels produce a larger channel scour and tend to stay open longer than artificial openings, which is preferable.

Artificial openings:

• are usually created when water levels rise above a ‘trigger level’, which is set to reduce inundation of low-lying properties and other built assets such as roads
• occur at lower water levels than natural openings
• result in far less channel scour compared to natural openings
• are more prone to rapid closure
• can cause changes to hydrology, salinity and inundation regimes, especially if continually created
• have long-term impacts on endangered wetland vegetation communities, which are dependent on the periodic inundation that occurs during ICOLL entrance closures.

Estuary entrances and water quality

Many closed ICOLLs have good water quality and remain suitable for swimming and fishing over time.

Water quality is influenced largely by rainfall and run-off from the surrounding catchment. Urban and agricultural run-off can lead to pollutants entering ICOLLs and causing water quality challenges.

ICOLL water is often a deep red-brown ‘tea’ colour, which is a consequence of run-off containing tannins from surrounding vegetation such as tea trees. This is a natural phenomenon and part of a healthy ICOLL ecosystem.

Artificial openings of ICOLLs do not necessarily improve water quality with tidal flushing. For example, artificial openings can reduce flushing because tidal exchanges are small compared to the water movement facilitated by a deeper and longer lasting natural opening.

Tidal influences within an ICOLL usually only affect the lower parts of the estuary and have minimal effect on water quality further away from the entrance.

The best way to improve water quality in an ICOLL is to improve the quality of water flowing into it and prevent pollutants from entering.

This can be achieved by councils taking the lead and encouraging the community to also change practices, such as:

• wash cars on grass
• pick up dog droppings
• lower use of fertiliser
• treat polluted run-off through stormwater systems or artificial wetlands
• maintain natural vegetation corridors along creeks, rivers and foreshores to act as natural filtration systems.

Permanent training walls

About 70% of ICOLLs in New South Wales are closed most of the time. When run-off into a closed system is not well managed, it can have a negative impact on water quality.

In the past, entrance training walls have been constructed to permanently open some ICOLL systems to the ocean to address water quality issues with tidal flushing. For example, training walls have been constructed on Wallis Lake, Lake Macquarie, Lake Wagonga and Lake Illawarra. All these lakes have demonstrated diverse responses in water quality, but commonly exhibit altered ICOLL environments and unforeseen costs.

Permanently opening ICOLL entrances to the ocean can have ongoing negative effects, such as:

• physical changes from altered tidal patterns and water flows, such as erosion of foreshores, undermining of bridges and jetties, and creation of deeper channels – changes that can take hundreds of years to stabilise
• physical changes that impact the estuarine ecosystem, such as fish habitats
• a reduction in water quality
• an increase or stabilisation of salinity that results in the estuary becoming more like the ocean, which impacts the estuarine ecosystem.

Case study

In response to increasing community concerns about poor water quality, in 2007 the NSW Government supported Wollongong and Shellharbour city councils to construct twin training walls that extend into the ocean to keep Lake Illawarra permanently open.

The training walls have resulted in changes to tide heights, seagrass, the local fishing economy, water quality and saltmarsh communities on the lake foreshore.

In 2021, the NSW Government endorsed the Lake Illawarra Coastal Management Program 2020–2030 developed by the local councils. This coastal management program aims to address historical impacts and enhance the overall health and amenity of the lake.

Watch these videos to find out more about the changes caused by the training walls: