• Some gullies will only be stabilised with high intensity , hard engineering intervention involving terrain reforming of the whole gully system , while others will require less interventionist measures . Yet others will require a combination of high intensity , hard engineering approaches for some parts of the gully complex and lower-key measures in other parts of the gully as part of an overall integrated gully complex management strategy .
• Given the scale of the alluvial gullies visited in the Bowen River catchment during the workshop , there was broad agreement that methods used in mined site rehabilitation and the scale of such activities , with some adaptation , are highly applicable to alluvial gully rehabilitation . Perhaps the key difference with mine rehabilitation is that mines often have a source of topsoil that is stripped from the site , directly placed on rehabilitation areas or stockpiled ahead of the mining operation . This is then available for re-application . In alluvial gully systems , however , much of the stable topsoil has been removed , so a stable soil surface needs to be reconstructed from the available stable or unstable sub-soil material , and organic material as a contributor for soil stability either manufactured on-site , or imported from elsewhere .
• Whilst seen to be a challenging problem , there was general agreement that the rehabilitation of large active alluvial gully systems is a tractable one , and that rehabilitation activities can be implemented cost-effectively within the scope of current budgets .
• Given the highly concentrated nature of alluvial gully sediment and nutrient sources , there was agreement that such sites could be treated at rates that are potentially cheaper ( when measured in terms of $/ tonne of sediment export reduction ) than any existing sediment mitigation strategies . Given that alluvial gullies are often deep , highly active and highly connected to the drainage network , significant investment in a single concentrated area can be justified because of the substantial sediment reductions that can be gained . At one of the sites visited during the workshop ( Site 1 - which from repeat LiDAR surveys is estimated to be producing ~ 25,000 t / yr of sediment ) an investment of $ 2m with an 80 % effectiveness would deliver sediment reductions at a rate of $ 100 / tonne ( based on a single year ’ s yield reduction ). This figure is at the lower end of available estimates for sediment reduction across the GBR catchments from any source . If the cost-effectiveness of the full life cycle of the gully is taken into account ( i . e . where the gully may continue to erode at its current rate for 100 years or more , the cost effectiveness equation could drop to $ 10 / tonne or less - allowing for continuing maintenance costs ).
• There was general agreement that due to the complexity of alluvial gullies and their significance as major sources of sediment and nutrients to the GBR , that the rehabilitation of such sites should only occur in association with expert advice from a team of experienced practitioners .
• There was some discussion about the need to develop a training and accreditation process for those involved in alluvial gully rehabilitation design and implementation . In the meantime a number of general principles were agreed as requirements for the successful implementation of alluvial gully rehabilitation .
-Assemble a team of experienced practitioners with a range of expertise to study the material characteristics of each site and recommend and oversee intervention design . -Site assessment must include the upstream and downstream controls at each site , and place the management interventions within this context . -Measure or estimate using verifiable techniques a pre-treatment gully sediment export rate , as this will determine value for money of any proposed intervention . -Determine the most cost effective and fit for purpose combination of hard and soft engineering to be used in each particular instance based on gully geometry and subsoil material characteristics . -Key principles of gully rehabilitation include :
▪ Stock exclusion for the medium to long term
▪ Development of short term erosion mitigation measures during the
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