Land and Water Link
November 2003
Murky Secrets Revealed
Draining the Koo Wee Rup Swamp seemed like a good idea at the time. They
cut down the trees and made drains across the swamp to reclaim fertile
land for market gardens to supply nearby Melbourne.
But that was in the 1890s and today, soil erosion from those drains and
more recently-formed erosion gullies seems the likely cause of changes
in Western Port Bay. Mud, for example, now occupies large areas that were
previously inhabited by ecologically important seagrass meadows –
areas that have suffered serious decline over the last 30 years.
‘Time, winds, tides – and the resulting sediment transport
wait for no-one’, says Dr Peter Wallbrink, CSIRO Land and Water.
‘Over the last century, and particularly in the last 25 years, a
significant amount of mud from the drainage channels has been distributed,
in a clockwise fashion, around French island within the bay.’
‘But before we can begin to set priorities for rehabilitation,
we need to understand sediment movement processes – where is the
sediment coming from within the catchment, where is it accumulating and
how is it moving within the waters of the bay?’
In search of the answers, a team of nine scientists from CSIRO Land and
Water undertook a three-year study in conjunction with Melbourne
Water Corporation and the Victorian
Environment Protection Authority (EPA). And they deployed a whole
swag of scientific techniques (see ‘The
Science behind the Western Port Bay Sediment Study’ page 2).
These techniques, together with landscape based mathematical models and
the researchers’ own knowledge of sediment and nutrient transport
processes, allowed the team to describe the sources, accumulation and
redistribution of sediment in the bay.
‘Armed with this knowledge, we were able to suggest priorities
for action plans – both short and long term – for each of
the major sediment sources: Bunyip and Lang Lang rivers, north-eastern
shorelines, drainage channels and gullies’, says Dr Wallbrink. ‘If
acted upon, these will reduce the sediment and nutrients entering Western
Port Bay.’
‘In the short term, stabilisation of the river and gully banks
would be an effective option.’
‘In the long term, we recommended re-establishment of shoreline
mangroves if appropriate, reconnection of channels to floodplains and
re-establishment of riparian vegetation along the streams and river corridors.’
Now it is up to the residents of Western Port Bay, and the catchment
and water authorities involved, to decide how to balance the use of the
Western Port catchment for agriculture against its impact on the bay.
More on the science
of the Western Port Bay Sediment Study.
Reports on this study are available on the CSIRO
Land and Water website.
CSIRO contact:
Dr Peter Wallbrink
Ph: +61-2-6246 5823
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