After three years of being open to the sea, the Durras Lake entrance closed on the 30 August 2018.

 

Information sourced from NSW DPI

What is an ‘ICOLL’?

Many coastal lakes and lagoons alternate between being open or closed to the ocean. These are known as Intermittently Closed and Open Lakes and Lagoons (ICOLLs). About 70 of the coastal lakes and lagoons in NSW are ‘ICOLLs’.

Why do ICOLLs open and close to the ocean?

ICOLLs are separated from the ocean by a sand beach barrier or berm. This entrance barrier forms and breaks down depending on the movement and redistribution of sand and sediments by waves, tides, flood flows and winds.

ICOLLs open and close to the ocean naturally in a constant but irregular cycle. When there is sufficient water flowing into the lake or lagoon from the catchment area (usually following heavy rainfall), water levels in the ICOLL will rise. The water levels in ICOLLs can often rise rapidly (within hours or days depending on the catchment size) in response to higher rain events (e.g. 50mm+). Eventually the water in the ICOLL will spill over the entrance sand berm and drain to the ocean. The force of the backed up water then quickly scours an entrance channel through the beach and reopens the ICOLL to the ocean. When ICOLLs are open they become tidal with seawater moving into and out of the estuary with the daily tidal cycle.

ICOLLs close when the ocean waves and tides push sand from offshore into the entrance, which gradually closes the entrance channel. Without further large freshwater flows into the estuary from the catchment, the ICOLL will remain closed to the sea. When ICOLLs are closed they do not exchange water with the ocean and water levels within them fluctuate depending on rainfall, catchment inflows and evaporation.

During wetter times many ICOLLs remain constantly open to the ocean. In times of drought and reduced rainfall ICOLL entrances close more frequently and stay closed for longer periods of time. It is common for many ICOLLs to remain closed for several years at a time. As well as rainfall, specific local conditions such as catchment size and headlands near the entrance determine if an ICOLL is mostly open or mostly closed. However historical records show that about 70% of the ICOLLs in NSW are closed for the majority of the time.

Are closed ICOLLs ‘unhealthy’?

Closed ICOLLs can often cause community concern over issues such as perceived ‘poor ‘ water quality, unpleasant smells and odour, flooding of low lying areas of the foreshore and the health of fish stocks. It is often suggested that ICOLL entrances should be opened to flush out the ‘dirty’ water and replenish it with ‘clean’ seawater. However, just because an ICOLL is closed does not necessarily mean that it has poor water quality. Many closed ICOLLs maintain good water quality and remain suitable for swimming, water sports and fishing for long periods of time. Furthermore, artificially opening an ICOLL entrance will not always improve the water quality due to the limited tidal movement or ‘flushing’ in most ICOLLs.

After heavy rainfall the water quality in a closed ICOLL can temporarily worsen as a result of runoff containing sediment, nutrients and pollutants entering the ICOLL from creeks and drains throughout the catchment. In many instances opening the entrance is not the solution and consistent poor water quality is better addressed by improving the management of catchment inputs to ICOLLs.

When ICOLLs are closed for long periods some plants growing around the lake edges (such as Casuarinas) may 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 in ways other than artificially opening the entrance, including ensuring there are sufficient replacement plants growing on the 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.

It should be remembered that flooding and drying are natural components of the hydrological and ecological processes operating within ICOLLs. Coastal lakes and the life they support have evolved in response to these forces and to maintain a ‘healthy’ lake ideally it should be left to operate as close to natural as possible.

What are I&I NSW’s policies on ICOLL management?

I&I NSW’s fish habitat management policies and guidelines for the management of ICOLLs are:

1. Any proposals for artificial opening of ICOLLs must seek the approval or concurrence of I&I NSW under the Fisheries Management Act 1994.
2. The Department supports minimal interference with ICOLL entrance barriers and advocates natural processes being allowed to operate to the greatest extent possible.
3. The Department does not support the artificial opening of an ICOLL unless the proponent (i.e. Council or other agency) can demonstrate that the social, environmental and economic benefits greatly outweigh any potential adverse impacts.
4. The Department supports using estuary management plans and environmental assessment processes to analyse the issues relating to opening a particular ICOLL, and to develop an entrance management plan or entrance management policy. Proposals for artificial openings which are to be carried out according to a formulated entrance management plan or policy are more likely to be approved by I&I NSW.