Alluvium was engaged by Melbourne Water to develop a 5‐year program of works for debris flow management in the Upper Yarra and Thomson catchments.
The objectives of the work was to understand how to reduce risk to water supply by assessing the impact of post‐fire debris flow on water supply in the Upper Yarra catchment and investigating and identifying suitable practices to manage debris flow risk areas to minimise the impact on water quality.
The debris flow management plan was developed using the latest modelling from the University of Melbourne, land‐manager consultation, and review of post‐fire erosion control strategies that have been applied and tested elsewhere.
The models used to inform the plan reflect the best available science on post‐fire hydrology in Australia and draws on a large body of data stemming from investment by Melbourne Water in applied forest hydrology research. The model has been developed and tested for geographic areas that are typical of Melbourne’s water supply catchments.
We found that debris flow risk is highly sensitive to fire severity. For a high severity wildfire (crown burn) in the Upper Yarra catchment, there is about 30% chance that the reservoir days of untreatable water will exceed 300 days. For a mixed severity fire the days of untreatable water is much lower.
The modelling shows that the debris flow threat is more concentrated in some area than other. Importantly, this provides Melbourne Water with opportunities to prioritise areas (or hotspots) for erosion control.
Modelling indicates that risk can be reduced by 30‐50% relative to baseline scenario. The work also indicated that erosion control with hillslope treatment (wood mulch and coir logs) and erosion barriers in headwaters can significantly reduce the duration that water is untreatable. Further reduction in risk can be achieved by modifying road embankments to perform a role as check dams. Modelling uncertainties are relatively large and further monitoring and evaluation is needed to understand in more detail how risk can be mitigated. The debris flow management plan recommended that all mitigation strategies are pursued as part of a multipronged approach to risk reduction.