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Lake Cathie Opening Strategy

Lake Cathie Entrance Opening July 2018
July 2018

Council in consultation with the local community, established the Lake Cathie Opening Strategy in 1995. The Strategy was updated and reviewed in 2001, 2004/5 and 2011.  The  Opening Strategy145KB pdf(PDF, 145KB) 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 & Floodplain Sub-Committee).

Coast Estuary and Floodplain Sub-Committe

Includes representatives from:

  • Local Progress Associations
  • National Parks and Wildlife Service
  • NSW Fisheries
  • NSW Agriculture
  • Department of Lands
  • Community representatives
  • Council staff.

The Sub-Committee has the task of overseeing the ongoing management of the estuary and recommend to Council strategies and actions for the implementation of work within the estuary. Refer to the related information area on this webpage for the Committee Charter and reports.

Preparation of a Draft Coastal Management Program Scoping Study

Council is preparing this now due to be completed in June 2020. The Scoping Study will lead into the Coastal Management Program (CMP) proper which will include a new review of the Lake Cathie Opening Strategy.

The current strategy suggests that an artificial opening of the Lake should take place when conditions prescribed in the strategy are prevalent such as:

  • Water level reaches 1.6m AHD
  • Water level falls below 0.2 AHD (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 occurred on average once every 12 months. An Artificial opening is when Council dredges the sand between the lake and the ocean with an excavator.

This Strategy aims 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.

The Lake Cathie opening strategy outlines that the lake is only to be opened when the lake water level is at, or exceeds, 1.6 metre AHD or falls below 0.2metre AHD with high salinity if optimal conditions allow. Recently water levels fell below 0.2m AHD, however it was decided by stakeholders that optimal conditions did not exist and the preference from State Government agencies was for minimal interference.

If the entrance to Lake Cathie was opened at the moment, it would likely close in a short period of time due to the lack of sufficient volumes of water within the lake and surrounding catchment to keep the entrance open. Onshore sand would be transported into the lake entrance and would quickly close the lake entrance. Coastal waters entering the lake could contain large amounts of red weed (rhodophyta), which, when the lake closed, would be trapped inside the lake. The red weed would then decay and would make water quality conditions within the lake much worse than they are at present. This would endanger aquatic fish and other lake fauna and flora. Optimal conditions are not prevailing.

Council are keeping a close watch on the lake and the local flora and fauna that are supported by the lake. There have been several small fish kills recently, found to be largely the result of high-water temperatures and resulting low dissolved oxygen. There have been similar fish kills around NSW during this time also. The local bird life have however, been enjoying the exposed foreshores and extensive mud flats which are abundant with food.

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 Weir construction feasibility study. 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 in their areas of governance. Representatives from DPIE recently visited and confirmed nature should be left to run its course and not interfering with the lake is the best management option at this time.

This page was last updated on: 06 April 2020