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Coastal study clears muddy water

CSIRO : 25 November, 2003  (Technical Article)
A team of environmental detectives using catchment-scale forensics has found that coastal bays muddied by land clearing and farming can be restored to health by controlling soil erosion.
'We have conclusively linked soil erosion on the land with mud deposits in the bay', says CSIRO team leader Dr Peter Wallbrink.

The mystery of the disappearing seagrass meadows in Western Port Bay (Victoria) was the starting point for a three-year study commissioned by Melbourne Water, conducted by CSIRO and supported by EPA Victoria.

'We don't have evidence to prove that the mud caused the initial seagrass decline but it is a prime suspect. There is no doubt mud is affecting the water quality.

'And we need to understand where the mud comes from as part of the effort to aid the recovery of seagrass meadows and the marine life they nurture. (For example seagrass meadows are important habitats for many commercially and recreationally important fish.) We can then fix the erosion sources and thus reduce the amount of mud entering the bay.

'Combining CSIRO's advanced 'forensic' research with the on-ground capacity of state authorities has been a very effective partnership that can be repeated around Australia's coastline', adds Dr Wallbrink. 'CSIRO conducted a similar study of Moreton Bay, Qld for example.'

The Western Port study sets out the priority areas for sediment control in the rivers that drain into the bay.

According to Dr Wallbrink, 'In the short term, stabilisation of the river and gully banks would be an effective option. In the long term, reconnection of channels to floodplains and re-establishment of vegetation along the streams and river corridors should also occur.'

'We also recommend re-establishment of shoreline mangroves if appropriate.'

The CSIRO team used a number of techniques to understand where the sediment was eroding from, how it was moving around the bay and where it was deposited.

'The whole bay is like a big washing machine with the mud swirled around by fast-moving tides so it was quite a challenge to work out what mud came from what river', says CSIRO researcher Mr Gary Hancock.

Three sediment-tracing methods were used to link the sediment in the bay to their erosion source in the rivers.

How much mud contributed by each of the rivers was calculated using a purpose-built mathematical model. The movement and redistribution of the mud in the bay was then identified using analysis of hundreds of mud samples.

According to Melbourne Water Managing Director Brian Bayley, 'A number of studies have been initiated by Melbourne Water and undertaken by CSIRO, including the Port Phillip Bay Environmental Study and a climate change study'.

'Melbourne Water will now work towards reducing the amount of sediment entering waterways and Western Port.'

'Works to reduce sediment inputs from the Upper Lang Lang River catchment are currently being investigated and Melbourne Water has already committed $10 million to a 10-year project to reduce sediment from the Bunyip Main Drain, including the construction of a sediment trap', says Mr Bayley.

Mr Doug Newton from EPA Victoria concludes, 'The study sets out the priority areas for sediment action in the Westernport catchment and it states where coastal action efforts can be targeted. The report also provides valuable insights into future research needs.
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