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The Swan Estuary has thus changed
from a predominantly freshwater/brackish
system with low stratification and low
nutrients being flushed annually by
vigorous river flows, to a system held in
a precarious balance between microbial
activity feeding on the dead carcases of
plankton and zooplankton, consuming
oxygen in the process, and the algae
photosynthesising thereby introducing
oxygen into the water column.
Much has been made of the estuary
health indicators not showing a decrease
in ecosystem health over the last few
years. But it must be remembered these
indicators measure only fish diversity
in the upper waters and then only at
selected times. Fish are transitory and
come and go through Fremantle at will,
searching for food and habitat at a time
when the ocean waters are becoming
depleted of both. Suggesting that these
indicators are a reflection of ecosystem
health is a bit like counting visitors to
a cemetery and then suggesting, when
visitor numbers are up, the cemetery
is a healthy place! For indicators to
be meaningful they must include the
abundance and diversity of all the
bottom dwellers, yet little or no work has
been done to include these in the Swan
The danger now is that as the fresh-
water carrying the tannins recedes, the
water will clear, the high nutrient levels
will come to the surface, the water will
warm further – all in all, the perfect
cocktail for a massive algal bloom!
Unfortunately, it is not simply a
matter of reducing the inflowing nutrient
levels. As suggested above, there is now
a store of probably close to 50 years of
food supply locked in the sediments,
available whenever the bottom waters
become anoxic (lacking in dissolved
oxygen). The long term solution lies in
reducing the nutrient inflows. The short
term fix, maybe a semi permanent one,
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is to prevent the salinity stratification
from forming and, instead, maintaining
a homogeneous water column that is
readily mixed by the tide and the wind,
preventing the bottom water from
depleting in oxygen.
The other danger that awaits the
Swan-Canning Estuary – and us in the
future – is global sea level rise. Fig.2
shows simulated flood results based on a
hydrodynamic model of the inundation
level referenced to the Australian Height
Datum (AHD). The model superimposes
the high tides experienced in July 2011,
river flows of about flood values in 1983,
an uncontentious 50 cm sea level rise
due to global warming and a coastal
wave surge of 50 cm, as recorded when
Tropical Cyclone Alby passed along the
coast in 1978. Not included is the known
rate of subsidence observed in Fremantle.
This is a realistic scenario, and may be
compared to the levels reached many
times in the past at Fremantle, most
recently in 1988, 2003 and 2004. What
these simulations show is that such events
will become more commonplace with
even quite small rises in sea level. So
climate change sceptics please read and
think before, yet again, going ballistic!
A Suggested Solution
Clearly, the flooding alone would have
Fig 2. From left: Middle Swan, Upper swan and Upper Canning.
a devastating impact on some of the
iconic areas of Perth. However, equally
important is the realisation that much
of the Swan-Canning Estuary shoreline
would require protective stonewalls,
changing the current gentle landscape to
a much harsher outline.
Is there a solution to these two
catastrophic realisations – the death of
the estuary being an observed fact and
the flooding being a conservative pre-
diction for the next 30 or so years? The
answer lies in preventing the sea from
entering the estuary at Fremantle; this
would prevent the estuary from develop-
ing anoxic bottom waters, while, at the
same time, offering water level control
to mitigate flooding. Such a solution
has been implemented elsewhere – in
Singapore for the control of Marina Bay,
in New Orleans, in Venice through the
construction of the three gates of the
Moses scheme, (Fig 3) the world’s largest
insurance policy against global warming,
and is being considered at numerous
other coastal cities around the world.
Such a lock and barrage could easily
be built somewhere between the two
bridges in Fremantle at an estimated cost
of around A$20M. We need to ask now
what value do we place on the life of
the river that has been the lifeblood
of settlement in Perth?
Fig 3. Venice
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