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Lake Cathie Management

Lake Cathie waterways
Lake Cathie Signage

Lake Cathie is an area of high conservation value and serves a variety of uses, from water bird habitat to recreational boating and swimming. A 'balanced' opening strategy facilitates the decision making process with respect to opening Lake Cathie to the ocean and is vital to accommodate the conflicting environmental and social constraints on the lake. This strategy aims to ensure minimal adverse impact on the ecology of the area, minimise build up of silt, sand and flooding, while still providing residents and visitors opportunities for recreational activities.

Background

The lake system alternates between being open or closed to the ocean and is known as an Intermittently Closed and Open Lake or Lagoon (ICOLL). There are about 70 ICOLLs in NSW. ICOLLs, such as Lake Cathie, are separated from the ocean by a sand barrier which forms and breaks down depending on the movement and redistribution of the sand by waves, tides and flood flows. ICOLLs open and close to the ocean naturally in a constant but irregular cycle. It is common for ICOLLs to remain closed for several years at a time.

The lower Lake Cathie estuary is highly utilised by the public for swimming and passive recreational activities and is a greatly valued asset of the Lake Cathie village and Port Macquarie-Hastings (LGA). When the lake closes, Council has historically monitored lake levels and water quality. Flooding can become an issue with development adjacent to the lake and foreshore, commercial premises, roads, parklands and sewerage systems when water levels in the Lake are high.

Lake Cathie Opening Strategy

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 Strategy 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 greater, 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.

 Brochure on the Lake Cathie Opening Strategy1024KB pdf(PDF, 1024KB)

Lake Cathie Water Level

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

Dredging 

Council is responsible for the management and maintenance of the lower Lake Cathie estuary (Lake Cathie Creek), the surrounding foreshore reserves and nearby beaches. Periodic dredging of the lower estuary is undertaken to ensure that recreational amenity is maintained within the waterway, with the dredge spoil being used to nourish nearby areas subject to erosion.

The lower Lake Cathie estuary is periodically dredged with the dredge spoil placed on Lighthouse Beach to create a sand buffer from coastal erosion impacts. In the intervening years after dredging occurs, sand accumulates into the lower estuary and can impact on the amenity of the recreational activities generally undertaken in the lower waterway. The sand buffer on Lighthouse Beach is typically also lost during this time due to the erosional nature of the beach environment.

This work is typically undertaken with a ‘suction/cutter’ dredge. The beach nourishment work involves placement of dredged sand in front of the properties along Illaroo road to form an artificial dune. The placement of this sand is typically undertaken by an excavator with the sand being pumped to the beach via a pipe from the dredge.

To aid in preserving the amenity of lower Cathie Creek, Council has identified periodic maintenance dredging as a suitable means to remove accumulated sand banks within the waterway. The  Dredging Strategy (2007)1MB pdf(PDF, 1MB) proposes ongoing dredging every 5-10 years. The  Lake Cathie Coastal Zone Management Plan (2016)3MB pdf(PDF, 3MB) identifies excess dredged sand to be placed on Lighthouse Beach to act as a mitigation measure for predicted long term coastal recession.

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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. Council staff are of the view that the Strategy does not need to be reviewed. 

Whilst the 2011 Lake Cathie / Lake Innes Estuary Hydrodynamic Model now provides the most relevant, accurate and up-to-date information on the management of the lake, some of the relevant studies for the Lake Cathie estuary are provided below. This list demonstrates the extensive work undertaken on studying the Lake:

The coastline around Lake Cathie is particularly exposed to coastal processes that threaten private and public assets. Coastal erosion has occurred south of the Lake Cathie entrance and has led to exposure of coffee rock that underlie the dune system. Coffee rock is formed by the process of induration - which is the hardening of soft sand. This coastal erosion is likely to increase over time under current predictions for climate change and the projected sea level rise. Lake Cathie has been identified as one of fifteen (15) coastal erosion “Hotspots” along the NSW coast by the Office of Environment & Heritage (OEH)

The Coastal Zone Management Plan (CZMP) process for Lake Cathie came about after the following studies were undertaken.

  1. The Lake Cathie Coastal Hazard Study - completed in 2008 and later revised in 2010 to take into account the introduction of the NSW Government’s sea level rise planning benchmarks (NSW Sea Level Rise Policy, 2009).
  2. The Lake Cathie Coastline Management Study - Stage 1 - completed in 2009 provides a preliminary assessment of 13 potential options to address coastal hazards for this area.
  3. The Lake Cathie Coastline Management Study - Stage 2 - completed in 2012. This report identified the need for Council to address Coastal Management options and the following 4 options were assessed in more detail and included in community consultations.
    1. 400m Revetment protection for Illaroo Road
    2. Groyne
    3. Beach Nourishment
    4. Planned retreat including acquisition/voluntary purchase

Community consultation concluded with an overwhelming support for a revetment wall with beach nourishment.

The Lake Cathie CZMP incorporates a revetment and beach nourishment as the management option in particular for Illaroo Road and will guide the management of the Lake Cathie foreshore into the future to ensure the most beneficial outcome for local residents, the community and the environment.

Please note: It is noted that under Section 55G of the Coastal Protection Act 1979 the CZMP requires certification from the minister. In a letter dated March 2015 the Minister for the Environment requested a number of actions be undertaken before the CZMP would be certified. As such, Council has completed these actions and additional information has been incorporated into the Lake Cathie CZMP including the results of a social and economic assessment of coastal protection options, which supports Council’s preferred option of a revetment as the main long-term management option in the CZMP.

The revised CZMP was adopted by Council in April 2016 and was certified by the Minister for Planning on 1st November 2016.  The CZMP was published in the NSW Government Gazette on 27th January 2017.

Lake Cathie Study Area

Lake Cathie / Lake Innes Coastal Model Development and Investigation

Background Information: why Council conducted these studies

As part of the design works for the revetment wall a Geotechnical Investigation was commissioned by Council. The Geotechnical Investigation project was undertaken by an independent company, Regional Geotechnical Solutions, and included testing to confirm the geological profile and the extent of Coffee rock and other material that may impact on the rate of erosion and the size of the area that is under threat of erosion within the next 100 years.

Due to some concerns from the Lake Cathie community a review of the original Lake Cathie Coastal Hazard Study was done to determine whether the erosion hazard was accurate. The review, undertaken by Cardno Pty Ltd did not recommend any change to the existing coastal hazard planning projections.

Over the years, the Lake Cathie/Lake Innes Estuary has been the subject of numerous environmental investigations that deal with estuarine and catchment processes such as increased sediment and nutrient loads, sediment transport and tidal flushing, increased tidal constriction and bed scour. In August 2005 Port Macquarie Hastings Council resolved to undertake a detailed modelling study to investigate potential management options and improvement works for the Estuary.

This report documents the development of a coastal hydrodynamic model for the Estuary and assesses four possible management options, including widening of Kenwood Drive Bridge, changes to the lake opening strategy, isolating Lake Innes and channel dredging upstream of Ocean Drive Bridge.

The study also discusses the possible ecological threats and impacts that may occur under changed tidal hydrodynamics, water quality and sediment transport conditions following implementation of identified management options.

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 resolved to adopt the findings of the report which contained the following management measures:

a) Kenwood Drive bridge That Kenwood Drive bridge be widened as part of any future asset upgrade or replacement works.

b) Lake Cathie opening strategy That the current opening level of 1.6m Australian Height Datum (AHD) remain unchanged.

c) Isolation of Lake Innes from Lake Cathie That the hydrodynamic model be made available to the NSW Office of Environment & Heritage (OEH), or others, for use in any further investigation of the reversion of Lake Innes to freshwater.

d) Dredging of tidal flushing channels That no dredging works be undertaken between Ocean Drive Bridge and Kenwood Drive bridge. That dredging works be limited to removal of sediment from the entrance downstream of Ocean Drive bridge.

 Lake Cathie Lake Innes Estuary Hydrodynamic Model Development Investigation

 

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This page was last updated on: 17 January 2019