In This Section
Shoalhaven City Council, in conjunction with the NSW Government, is responsible for the management of a number of river and lake entrances in the Shoalhaven LGA.
Due to the historical development of some floodplains, low-lying properties are at risk of flooding under certain flooding and entrance conditions. These coastal rivers and lakes are diverse, complex ecosystems and comprise highly dynamic areas which are constantly changing, whether they are open or closed to the ocean.
Following the construction of Berry's Canal in 1822, the Shoalhaven River entrance at Shoalhaven Heads is now intermittent and is opened by the occurrence of floods and subject to closure by natural onshore oceanic processes.
The Shoalhaven River entrance is artificially opened in accordance with the Shoalhaven River Entrance Management Plan for Flood Mitigation. Normal flows reach the ocean at Crookhaven Heads via the man-made Berry's Canal.
The Shoalhaven region has a number of coastal lakes that are perched behind a barrier of sand between the lake and ocean when they are closed. These sand barriers can be breached naturally following heavy rainfall or heavy seas, however, prior to this water may build up behind the sand barriers which can lead to flooding in areas where historic development of low-lying private properties and infrastructure has occurred.
These coastal lakes are known as Intermittently Closed and Open Lakes and Lagoons or (ICOLLs) and are common on the south coast of NSW where catchments are generally smaller in size and average rainfall is lower.
The Shoalhaven region contains a large number of ICOLLs, with four (4) of these managed by Shoalhaven City Council:
Shoalhaven City Council recognises that ICOLLs are governed by natural processes and should be maintained with respect to these unless assets are at risk of being flooded.
Council aims to maintain sand barriers and coastal lakes in as close to a natural condition as possible. In doing so, managing lake entrances is a balance between maintaining natural process and minimising flooding of assets (i.e. private property, etc.).
Shoalhaven City Council manages ICOLL entrances in accordance with an Entrance Management Policy developed by Council in partnership with NSW Government agencies and in consultation with local communities.
Shoalhaven City Council intends to not unnecessarily intervene with ICOLL entrances, in order to allow the natural processes to occur; a view shared by the State Government.
However, when the water level within the ICOLL reaches a certain pre-determined 'trigger level' Council will artificially open the lake entrance to the ocean in accordance with the conditions of the Entrance Management Plan. Such works are undertaken following consultation with NSW Government agencies.
All river and lake entrances are located on NSW Crown Lands owned land. Council is only permitted to artificially open entrances in accordance with the levels and conditions contained within Entrance Management Policies and any additional conditions imposed on Council in the relevant NSW Crown Lands licence.
Generally, a lake or river entrance is closed by waves triggered from a severe ocean storm for example an East Coast Low, resulting in washing sand into the entrance channel restricting tidal flow.
When tidal flow is restricted over time this gradually leads to the closure of the entrance channel until the point of complete closure. A series of ocean storms can accelerate this process.
Shoalhaven City Council constantly monitors the condition of managed river and lake entrances which includes regular topographic survey of closed entrance berms.
Intermittently closed and open lakes and lagoons (ICOLL) is a generic term for a distinct type of coastal estuary that opens and closes to the ocean irregularly in response to catchment rainfall and oceanic conditions.
Worldwide, ICOLLs are relatively rare, however NSW is home to almost 70, with most of these situated on the south coast where catchments are generally smaller and rainfall is lower. The Shoalhaven has a number of ICOLLs, some of which are managed by Shoalhaven City Council. These include Swan lake, Burrill lake, Tabourie Lake and Lake Conjola.
ICOLLs have evolved over thousands of years and are as such sensitive to human disturbances. This makes them one of the most complex and difficult coastal environments to manage in urban areas.
It is natural for coastal lakes to be both open and closed, with approximately 70% of NSW ICOLLs predominately closed for most of the time.
Entrances are dynamic in nature and constantly changing, from opened to the ocean after catchment rainfall, to closed due to coastal processes. In between these events, channels can naturally drift and change shape by water and wind constantly moving sand into and around the entrance.
Closure can occur due to a number of reasons:
Openings can be caused by:
The long-term goal of both Shoalhaven City Council and the NSW State Government is to allow for the natural processes to occur for as far as possible within the constraints of property inundation, flooding of infrastructure, and overall to minimise human interference with the natural dynamic nature of ICOLLs. This is a goal reflected within NSW Government legislation which Council must follow.
The significant reasons for not maintaining an open entrance are:
Shoalhaven City Council will mechanically open lakes to alleviate low-level flooding when water levels reach a pre-determined 'trigger level'.
All ICOLLs managed by Council have an Entrance Management Policy, prepared in accordance with NSW legislation and in collaboration with NSW Government agencies and the community. This Entrance Management Policy addresses the when, where and how of undertaking a mechanical opening and is the procedure which Council legally must adhere to.
The main factor governing when a 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.
If a lake requires to be opened outside the criteria stated within the Entrance Management Policy, Council must obtain approval from relevant Government agencies.
Mechanical openings are often short lived due to several factors:
It is natural for ICOLLs to be closed at times. During long periods of closure the water can change colour, this does not mean the lake is dirty or unhealthy. It may simply mean that the tannins and lignins of plants and trees within the ICOLL catchment are creating a brownish/yellow or reddish colouration. During and after heavy rainfall, however, water may be brown due to the rain carrying sediments via stormwater pipes and creeks.
Shoalhaven City Council has ongoing water quality monitoring programs that measure the levels of pollutants within waterways in the Shoalhaven region. When lakes are closed, further water quality monitoring is typically undertaken by Council to ensure the same quality is maintained.
Estuary Health Report Cards were joint initiative between Shoalhaven City Council and NSW Department of Planning and Environment and are used to assess the overall health of the estuary system. These health report cards successfully show water quality has not declined in periods of lake closure. If test results indicate that water quality is unsafe, Shoalhaven City Council will advise the local community.
Water quality of the ICOLLs is normally within the Australian New Zealand Environment Conservation Guidelines. After heavy rain, bacterial levels can increase, this is normal in lakes and estuaries regardless of the entrance is open or closed as pollutants are washed off the catchment during rain events. For this reason, it's best to avoid swimming for up to three days after heavy rain, especially near stormwater outlets. This is a general warning for swimming as part of the statewide Beachwatch program.
Shoalhaven City Council, residents and visitors all play an important part in keeping waterways clean and healthy through monitoring and managing our catchments.
You can find water quality monitoring results on the Aqua Data page.
During periods of lake closure, green algae may appear in shallow areas of the lake, this is 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 is likely to cause unpleasant odours. The algae doesn't affect water quality as shown by Shoalhaven City Council's water quality monitoring results. Reducing the amount of nutrients entering a coastal lake is the best way of managing water quality and algal blooms in the lake.
Council is committed to reviewing and updating Entrance Management Policies. This must be done in accordance with the Coastal Management Act 2016 through the development of a Coastal Management Program (CMP).
The removal of large wooden debris from NSW rivers, streams and waterbodies is listed as a 'Key Threatening Process' under the Fisheries Management Act 1996 (NSW).
Council assess fallen trees and woody debris that have fallen into a water body based on risk to the public and navigational hazards. Council consults with both Transport for NSW and the NSW Department of Primary Industries – Fisheries in relation to the decision to remove or move any wooden debris in a water body.
When ICOLLs are closed for long periods, plants growing around the edges can die from prolonged waterlogging. However, this is a natural process with riparian vegetation gradually advancing and retreating along the foreshore in response to varying water levels over time. Potential loss of riparian vegetation can be managed by ensuring that there are sufficient replacement plants growing landward side of the foreshore (i.e., maintaining or establishing riparian buffer areas) and planting, or allowing natural rehabilitation, of more suitable species that can tolerate wet, salty soils, such as saltmarsh and mangroves.
Under natural conditions, closed ICOLLs break out over a relatively wide range of water levels which is termed the 'natural breakout range'. An occasional artificial opening of the entrance within this range is not likely to have a significant impact on the environment since it falls within the expected natural variation.
However, over the longer term, numerous artificial openings especially at a low water level is likely to have a significant impacts on the environment since the natural frequency and duration of opening and closing to the ocean will be significantly altered.
Artificial openings of ICOLLs results in a rapid lowering of the water level and the exposure of extensive areas of sediment and aquatic vegetation, such as seagrass and wetlands. Decomposition of exposed aquatic vegetation can also result in deoxygenation of the water column (low dissolved oxygen (DO), which can lead to the death of fish from suffocation (i.e. fish kills).
Exposure of sediments to the air and the death of aquatic vegetation can lead to the release of hydrogen sulphide and other foul smelling gases. Over the long term, more frequent openings will lead to shifts in the structure and distribution of fringing riparian vegetation communities.
Shoalhaven's ICOLLs are habited by threatened and migratory shorebirds. ICOLLs provide a diverse range of habitats for foraging, breeding, roosting and sheltering for the migratory shorebirds. Threatened and migratory shorebirds, such as the Little Tern, typically breed in the sand bar of a closed ICOLL entrance, laying their eggs directly on the sand in a simple, shallow nest scrape.
Council takes mitigating steps to minimise disturbance to the threatened and migratory shorebirds such as:
The intermittently open Shoalhaven River entrance and Swan Lake, Lake Conjola, Burrill Lake and Tabourie Lake ICOLLs all have specific trigger levels for the lake that when reached allows the artificial opening of the entrance by Shoalhaven City Council subject to the safety of machinery operators.
The latest water level for the Shoalhaven River at Nowra and Shoalhaven Heads, Lake Conjola, Burrill Lake and Tabourie Lake can be viewed on the Bureau of Meteorology website.
The latest water level at Swan Lake, Shoalhaven River at Nowra and Shoalhaven Heads, Lake Conjola, Burrill Lake and Tabourie Lake can be viewed on the Manly Hydraulic Laboratory (MHL) website.
All levels in metres AHD
Shoalhaven City Council at their Ordinary Meeting on Tuesday 26 October 2021 announced they have been granted a licence from Crown Lands to mechanically open the entrance to Lake Conjola in periods of emergency at lower trigger requirements than previously existed. The licence will assist Council in mechanically opening the entrance when severe weather impacts on the Lake Conjola community.
After consultation with the community, Council have applied for revised trigger levels for the Lake Conjola Crown Land Licence enabling Council staff to seek state government approvals to open the entrance, when it is required to be opened mechanically.
Council had previously prepared a review of environmental factors environmental assessment and submitted a licence application to the Department of Planning, Industry and Environment - Crown Lands. This new application has led to the issue of the subject Licence and has been issued for a five-year period.
It should also be noted that Council has undertaken extensive community consultation in relation to the Lake Conjola Coastal Management Program which is currently in the process of being formulated. The issue of Lake entrance management is a consideration in the formulation of this program. The program is also collecting scientific information to inform the forecasting of requirements and to alter the licence in the future, if required.
Council, in partnership with state agencies and relevant communities, has developed entrance management plans.
The completion of flood risk management plans and coastal management program studies will trigger a review of these entrance management plans.
The feasibility of permanent flood mitigation and entrance management options such as engineered options or strategic planning considering current climate change predictions are investigated in floodplain risk management studies and plans, and the coastal management program.
In the long-term, the aim is to reduce the need for artificial manipulation of the entrance by taking active measures to remove, relocate or otherwise manage items of low-lying infrastructure that currently necessitate breaches below the natural breakout range.
Potential mitigation measures are investigated through a floodplain risk management study and plan, and coastal management program for individual catchments.