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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 Hastings LGA Coast and Estuaries Sub- Committee). The Sub-Committee comprises representatives from various Progress Associations, National Parks and Wildlife Service, NSW Fisheries, NSW Agriculture, Department of Lands, local community and Council. 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.

Council opens the Lake when conditions prescribed in the strategy are prevalent such as; the lake height reaches 1.6m AHD or falls below 0.2 AHD, there is poor water quality or when high lake levels threaten breeding cycles of critical habitat. Artificial openings 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. 

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

 Brochure on the Lake Cathie Opening Strategy2MB pdf(PDF, 2MB)

Water Testing 

Council undertakes water quality monitoring at Lake Cathie to ensure that the system is healthy and is acceptable for recreational purposes (i.e. swimming, fishing).

Water Levels

The NSW Government provides water level data for Lake Cathie, taken from automatic recording equipment. The water level is monitored by Council.

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Original Opening Procedure

Since at least the early 1960's Council has artificially opened the estuary entrance by excavating a channel through the beach dune when water levels inside the lake reaches a specific level. Originally this level was 1.5m AHD (1.5m above mean sea level). The details around the adoption of this level are not known and it is assumed that this original level was not supported by any detailed environmental studies or adopted strategies.

1995 Opening Strategy

The original Lake Cathie/Lake Innes Entrance Opening Strategy and Management Plan was developed in May 1995. Adoption of the Management Plan by Council followed an extensive community consultation program and direct input from key stakeholder groups.The 1995 Strategy nominated an opening trigger level of 1.8m AHD or when poor water quality is experienced or when high lake levels threaten bird breeding cycles.

2001 Opening Strategy Review

In the intervening years after the original Opening Strategy was adopted, several reports were commissioned by Council as a result of economic and social issues raised by the community. Ultimately a review of the Opening Strategy was conducted in 2000 which led to the Strategy being updated in 2001 with changes to the opening trigger level (reduced to 1.6m AHD). This change was mainly due to the results of monitoring of the lake opening levels which showed that in the years between 1995 and 2001 the lake had been recurrently opened at around 1.6m AHD. This change in lake level also responded to Council and community concerns regarding inundation impacts of both private and public land when the Lake rises above 1.6m AHD.

A key component of the 2001 review was the Council resolution to develop the opening ‘brochure’. During the review phase it was clear that the community awareness of the various competing interests in the management of the lake were largely unknown to a large section of the community. The brochure was produced as an education tool to inform the community on the multitude of issuess that need to be balanced in order to ensure that the lake system is sustainably managed.

2004 Opening Strategy Review

Council further investigated the Opening Strategy in 2004. The 2004 report was initially focused on the broader management issues of Lake Cathie (i.e. possible reversion of Lake Innes to freshwater etc). However, due to the large number of submissions made on the Opening Strategy it was decided that the Opening Strategy should be the subject of a further review.

The 2004 report acknowledged that there was a lack of detailed scientific data available to comprehensively conclude whether the management strategy should or should not be changed. Thus a key recommendation of the 2004 report was that the lake system should be further studied via a computer hydrological model which would allow Council to make a more informed decision on whether the current management strategy was appropriate.

In September 2005 Council resolved to undertake a detailed modelling study to investigate potential management options and possible improvement works for the lake and estuary. This culminated in the Lake Cathie / Lake Innes Estuary Hydrodynamic Model which was initiated in 2008 and completed in May 2011.

Throughout the 2004 review the consultants noted that the Lake Cathie and wider community are passionate about the management of the lake and recommended that further community engagement strategies be pursued and implemented. The consultants noted that although the original plan and strategy were prepared with community and stakeholder participation, the community is transient in nature and has changed over the years since its adoption. The consultants also noted that Council has a vital role in communicating and consulting with the current local residents and tourists, providing up to date information and responding to their concerns.

2011 Opening Strategy Review

The 2011 Lake Cathie / Lake Innes Estuary Hydrodynamic Model project involved a comprehensive and detailed study of the Lake Cathie/ Lake Innes estuary. A hydrodynamic computer model was developed to provide Council with comprehensive and definitive scientific justification for any changes in the current management approach of the lake.

The hydrodynamic model of the Lake Cathie/Lake Innes estuary provides Council with a comprehensive management and decision making tool which is based on a scientific understanding of the underlying drivers of the estuarine physical processes (erosion, deposition, shoaling, water quality, estuary health, flood management). The model allows prediction of “cause and effect” impacts of sedimentary, tidal and fresh water flow processes. Council are able to better assess impacts and reach informed decisions in sustainably managing the lake and estuary system.

In consultation with the community, the computer based hydrodynamic model was used to assess four possible scenarios:

  • Management Option 1: Widening of Kenwood Drive Bridge
  • Management Option 2: Changes to the lake Opening Strategy
  • Management Option 3: Isolating Lake Innes
  • Management Option 4: Channel dredging upstream of Ocean Drive Bridge

In order to determine the best management practice for opening the lake three possible mechanical opening levels (eg. 1.4m AHD, 1.6m AHD and 1.8m AHD) were tested. The results of the hydrodynamic model demonstrated that the current lake opening trigger level of 1.6m AHD was the most appropriate and that this level should be maintained.

The Port Macquarie-Hastings Coasts & Estuaries Management Sub-Committee met to discuss the results of the study and recommended that Council adopt the findings of the report. Accordingly, Council adopted the findings of the report at the ordinary Council meeting held on 27 June 2011.

In summary, the Lake Cathie Opening Strategy, since its adoption in 1995, has been reviewed on average every 5 years with significant reviews being undertaken in 2001, 2005/6 and 2011. As a result of the hydrodynamic computer model that was developed as part of the 2011 study, it is considered that Council now has a detailed and accurate scientific understanding of the Lake and its environmental processes. This study confirmed that Council has adopted a balanced approach to the management of the lake and that the most pragmatic solution to the opening of the lake has been achieved.  

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This page was last updated on: 22 May 2019