Processes Impacting on Lake Macquarie

Channel Challenges
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In the last decade or so, the concept of improving water quality through increased tidal flushing has been the subject of numerous investigations and review by researchers and experts in this field.

I am sometimes asked why we can’t simply increase the size of the Lake entrance to improve water quality.

There are two main reasons that increasing the size of the Channel opening is problematic. The first is that it is in fact unlikely to result in significant water quality improvements. The second is that the side effects of such a move would have more serious social an environmental implications than the ones it intended to fix.

Many people believe incorrectly that by increasing the tidal flushing of Lake Macquarie, the issue of poor water quality would be solved automatically. However, because the Lake contains such a huge volume of water, increasing the tidal exchange would have little impact on water quality.

Recent studies have revealed that even if the Lake entrance was increased by 20% the impact on the tidal exchange would only be 0.2%.

However, an increased tidal range between high and low tide within the Lake itself would result and this would cause significant problems such as flooding of low-lying areas around the Lake foreshore, increased channel bank erosion, possible destruction of seagrass beds and increased exposure of mud flat areas at low tide.

Independent investigations into flooding around the Lake conducted by both the Government’s Manly Hydraulics Laboratory and consultants Webb, McKeown and Associates, found that ocean level is one of the main determinants of flood levels in the Lake. Therefore, any significant opening or enlarging of the Channel will raise flood levels particularly when ocean levels are high, such as during storm events. The frequency of flooding in low-lying areas such as Marks Point, Swansea Flats and Belmont South would be increased.

A related suggestion that is often raised with the aim of increasing water exchange through the Channel is the removal of Elizabeth Island.
A range of options were recently investigated and computer modelling conducted, to determine the best strategies for the long term management of the Channel. The option of removing the islands to a depth of -0.5 metres was considered and computer modelled. The results predicted that this action would have minimal impact on the tidal range and tidal flushing of the Lake.

The investigation of this option by WBM Oceanics concludes that from an ecological perspective this action would be detrimental to the Lake’s health.
Large estuary engineering works carried out world-wide in the last century, often undertaken as a ‘quick fix’ solution, have generally had limited success. Many have generated serious negative flow-on consequences for the estuary due to the interactions with the complex natural systems.

The strategy for improving Lake water quality recommended by the Estuary Management Plan and the Premier’s Task Force is to ensure that stormwater entering the Lake does not contain excessive sediments, nutrients and other contaminants. We are starting to see some early signs of improvement in Lake health, although it is a long term task and we still have a long way to go.

 

Editiorials
Channel Challenges
Channel & Water Quality
Constant Change
Seagrass
Seawalls
Threats to the Lake
Vegetated Swales
Water Sensitive Urban Design
Wetlands
Wrack & Ruin
Introduction