Management of intermittently closed and open lakes and lagoons

Why do only three of the estuaries have set levels for intervention?

• Arrawarra Creek, Woolgoolga Lake and Nautilus Creek are the estuaries that open and close regularly and where infrastructure and property can be at possible risk of inundation.
• All other estuaries never, or only rarely close, or are in predominantly natural areas where property and infrastructure are not at high risk of flooding.

Why can’t we open the entrances early when the lake levels start to rise?

• Opening ICOLLs at low water levels is generally not recommended as there is not enough water or pressure built up to keep the channel entrance open, and it will close over again quickly.
• While a small rise in water level may not seem like a lot, over an entire water body it can equate to a significant water volume which can greatly improve the hydraulic scour on the entrance leading to longer lasting and more successful openings.
• The longer an ICOLL is left undisturbed the more likely it is to break open naturally without intervention, which is the preferred outcome.
• Also, based on past history the lakes generally break out naturally when they reach a certain level before they experience major flooding and have affected infrastructure or services.

What environmental damage can occur if we open estuaries too regularly?

• This can cause damage to fringing vegetation which may rely on temporary inundation as part its lifecycle.
• It can cause major changes in oxygen levels and associated water quality impacts.
• It can cause shock to fish and other aquatic life not anticipating the change in water levels and environmental conditions.
• The receiving waters can also be impacted by the water quality discharging from the ICOLL.

How did we determine the levels at which to intervene?

• These levels have been carefully selected based on extensive surveying of low-lying assets and features in the waterbody catchment.
• They have been selected to give the estuary the longest time possible to break out naturally without intervention, which is the most desired environmental outcome.
• They have been selected so that the entrance will open before identified high value infrastructure is affected.
• It is recognised that there will be levels of “nuisance flooding” that may temporarily inundate low lying areas such as farmland and yards.

What is “nuisance flooding”?

• This refers to temporary flooding and inundation of low-lying areas that does not result in long term damage to infrastructure such as housing.
• It may involve temporary inundation of drains, yards, and roads but generally not flooding above floor levels of buildings.

Why are ICOLLs so important?

• These areas are important nurseries and breeding areas for many aquatic organisms including fish, crabs and prawns.
• They are often also very important areas for birds, many of whom rely on the mudflats and sandflats for foraging.
• Some of these birds are critically endangered and some of the migratory species have travelled from thousands of kilometres away on the northern hemisphere and call into the estuaries to feed and refuel.

Should we be dredging the estuaries?

• Infilling of the estuaries is a natural and cyclical process. At the mouth this is most often sand coming from the ocean and not from the catchment. Any holes dug will just fill in again over time.
• All the relevant Plans and Studies recommend against either dredging of these waterways or artificial opening to reduce sediment levels.  Regular artificial entrance opening can trigger increased marine sand ingress and shoaling within the lake.

What about the water quality?

• Water quality is not a trigger for opening of the ICOLLS. The City will not make a decision to open the ICOLLS based on water quality concerns unless advised to do so by authorities. If a fish kill occurs the NSW Department of Primary Industries (Fisheries) will investigate and advise if any action is required.
• ICOLLs may look like they have water quality issues because of visual observations such as surface scum and algae, but these are often natural features in still water bodies and not a sign of poor water quality. These systems and the plants and animals that call them home have evolved over thousands of years to adapt to long periods of being closed to the ocean.
• The water quality of the City's estuaries is generally good and has been monitored on and off for the last few years by the EcoHealth project. Water quality information can be found at: https://www.coffsharbour.nsw.gov.au/Environment/Compliance-and-reporting/Ecohealth-a-waterways-monitoring-program

What about sea level rise?

• Regardless of the entrance management regime, lake heights and flood risk for low lying properties will increase with the impacts of climate change.  Increased sea levels, tide and storm surge alone will pose an increased risk even in the absence of rainfall events.
• It is anticipated that infrastructure such as stormwater and sewage systems will be affected by rising water levels. A slow transition is required to prepare and plan for the future.

Does flooding only occur when the entrances are closed?

• No. During a catchment flooding or storm surge inundation event (or a combination), high water levels can still occur regardless of if the entrance is open or closed. Hence, there may be occasions where berm management / entrance opening works may be ineffective in alleviating flooding.
• Planning for and managing these hazards are dealt with separately in the City's Floodplain Risk Management studies and Coastal Management Programs (specifically the Emergency Action Sub Plans).