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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
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
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
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