Lake Cathie Opening Strategy

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In consultation with the local community, a Lake Cathie Opening Strategy was developed in 1995. The Strategy was updated and reviewed in 2001, 2004/5 and 2011. The former Opening Strategy was developed under the Lake Cathie-Lake Innes Estuary Management Plan, developed by the Lake Cathie-Bonny Hills Estuary Management Committee (now the Coast, Estuary and Floodplain Advisory Sub-Committee).

Following the drought in 2018/19, advice was received from NSW Crown Land that Council would require a new Environmental Approval (REF) in order to open the lake. This advice effectively makes the opening strategy redundant as it removed Council's power to undertake works to initiate any openings.

Since that advice was received Council has prepared a new REF that permits an opening once water levels reach 1.6m AHD. The opening is only permitted for flood mitigation purposes.

Preparation of a Coastal Management Program

Council is currently preparing a Coastal Management Program (CMP) which will include a review of the Lake Cathie Opening Strategy.

The former strategy suggested that an artificial opening of the Lake should take place when conditions prescribed in the strategy were prevalent such as:

  • Water level reaches 1.6m AHD
  • Water level falls below 0.2 AHD and salinity exceeds 40 pats per 1000 (if optimal conditions allow)
  • There is poor water quality or when high lake levels threaten breeding cycles of critical habitat.

Artificial openings for events which trigger 1.6m AHD have historically occurred on average once every 12 months. An artificial opening is when we dredge the sand between the lake and the ocean with an excavator.

This Strategy aimed to minimise adverse effects on the ecology of the area, minimise build-up of sand in the lake entrance, and reduce impacts of flooding, while providing residents and visitors with opportunities for recreational activities.

Considering the impacts and monitoring the situation

Careful consideration of any changes to the Opening Strategy is needed as hydrodynamic impacts such as altered wave action entering the lower estuary, changes in longshore sand drift, tidal prism and wave environment could result in loss of existing sand shoals and beach areas. Changes in channel alignment and sediment dynamics could also be experienced within the lake which in turn could significantly change the duration of lake opening events and impact on water quality.

Altering estuarine processes will likely create other management issues including impacting on access to the lake and foreshore, safety of wading and/or swimming areas, and altering the saline/freshwater balance within the greater lake system. Environmental issues including habitat loss for wading birds, fish and prawn species could also be potentially impacted by changing the Opening Strategy.

Development and validation of the coastal model was undertaken to investigate and review the current entrance opening strategy and assist with demonstrating justification for any changes to the management approach in the future. Computer modelling was used as a tool to prepare a hydrodynamic model of the Estuary for the purpose of investigating and assessing changes to estuarine / coastal conditions and other specific site related issues including potential management actions identified by the Council and the wider community.

In light of the information and computer modelling undertaken as part of the investigation, the 2011 Hydrodynamic Study did not recommend any changes be made to the adopted Opening Strategy.

Council is collaborating with stakeholders to undertake further Acid Sulphate Soil studies, a Saltmarsh study, review of the 2013 Lake Innes Reversion report and a review of the 2011 Hydrodynamic Model . We will have additional information to understand what is happening in the immediate catchment and within the lake systems themselves, once this work is completed. Council are also in regular contact with the managing stakeholders and other Councils with similar ICOLLs. The overriding principle of managing ICOLLS is that they be left alone and nature should be left to run its course. Not interfering with ICOLLS is widely regarded as the best management option.

Lake Cathie Opening Strategy(PDF, 145KB)