Revegetating the economy

Revegetating the economy

1.02.2013 - Posted by Misko Ivezich

I flew out of Brisbane earlier this week for a quick trip back to Melbourne and from the air I was amazed at the volume of sediment flowing into Moreton Bay following the deluge of rain over the long weekend. Later I saw that Canadian Astronaut Chris Hadfield had posted pictures he’d taken from space of rivers all up the Queensland coast, including the Mary River, Burnett River, Calliope River and Fitzroy River. Each picture shows a similar scene to the one I saw over Moreton Bay, tonnes and tonnes of sediment flowing into the marine environment. This would have always occurred after flood events. However research by the CSIRO has demonstrated that sediment loads have increased orders of magnitude over pre European conditions. The sediment is not only impacting the marine environment, I also read that Brisbane’s water supply was also threatened when the Mt Crosby water treatment plant went offline as it was drawing more mud than water from the Brisbane River.

Mary River, Maryborough

Burnett River, Bundaberg

Calliope River, Gladstone 

Fitzroy River, Rockhampton - all images from Chris Hadfield (you can follow him on twitter @Cmdr_Hadfield)

The source of this sediment has been the subject to some conjecture, it had been thought that much of the sediment loads in Queensland rivers had come from hillslope erosion in grazing lands. However recent research by the Australian Rivers Institute has found that much of the sediment is derived from the river channel, from erosion of the river banks. This is consistent with research by Outhet . Bank erosion rates have increased greatly since European settlement due to the clearing of riparian vegetation and uncontrolled grazing of riparian and instream vegetation by stock.

in Victoria the cost of flood related river restoration work has amounted to $80 million (net present value) over the past 20 years (approx). In addition to this river restoration cost, the cost of repairs to public infrastructure (such as roads and bridges) has been an order of magnitude greater again.

Successive Victorian governments have invested in the revegetation of waterways across the state over the past 15 years or so. These programs had been undertaken based on a consensus of opinion among waterway scientists and managers on the benefits of riparian vegetation for stream ecology and for reducing the extent of erosion in flood events. While there was evidence that remnant quality native vegetation limited flood related erosion, there was limited evidence that revegetation programs could achieve the same outcome. It was not until such revegetation had been installed, had time to establish and had been tested by large floods that evidence could be collected on the success or otherwise of revegetation programs in limiting flood related erosion and related environmental, social and economic costs.

A couple of years ago after major flooding in Victoria I was involved in a study that assessed the resilience of some revegetated rivers. The timing of the flooding in Victoria in 2010 and 2011 let us select a range of sites that had been revegetated between 10-15 years ago and examine whether the revegetation had influenced the erosion outcome for the sites. The investigation comprised a paired site assessment where we examined sites that had and had not been revegetated. We found that the vegetation established through the revegetaton programs played the critical role in limiting channel change at the sites.

Suggesting large revegetation programs after flood events is a sensitive issue. To many in the community vegetation is seen to exacerbate floods by blocking up river channels. At the local scale this can be true, however research by the University of Melbourne has shown that vegetation establishment can also be a tool for flood mitigation. Floods travel as waves and these waves will flow through a vegetated river system more slowly than a cleared one. As a result water will be delivered downstream to rural and urban communities at a slower rate with a lower peak discharge if the upstream riparian corridor has been revegetated. Revegetating the rivers and creeks in the upper catchment could be the difference between a flood breaking its levee or not.

With the Queensland government now talking about large scale flood mitigation programs (such as levees and new dams) the role revegetation can also be examined as a cost effective means of not only reducing the peak discharge (and extent of flooding) but the damage associated with flood related erosion. Large scale revegetation of rivers and creeks across Queensland could have significant benefits to the economy. The benefits would include reduced damage to public and private infrastructure from erosion, potential reduction in peak flood levels and as a result the number of properties inundated following floods, a reduction in sediment being exported to the marine environment which have some big tourism drawcards such as the Great Barrier Reef and Moreton Bay and large reductions in water treatment costs. And after all that we’ll end up with much healthier rivers too.