Novel approach for determining risk of water supply disruptions due to post-wildfire debris flows, A

Abstract

Forested catchments are critical to water supplies in major cities around the world. As wildfire and extreme rainfall become more frequent, water supply systems are facing an increasing threat of contamination from erosion. In southeast Australia debris flows are particularly problematic because they produce sediment loads that are likely to impact water treatability to the point where water supply interruptions are likely. Assessing the threat of water supply interruptions and evaluating mitigation options is complex, because there are many factors that affect the treatability of the water. For example, the magnitude, frequency and spatial extent of debris flows, entry points of sediment into reservoirs, particle size distribution of sediment, reservoir hydrodynamics, and location of the potable water offtake are factors likely to determine to how debris flows translate to treatability of water at the offtake. In this paper we couple a post-fire debris-flow model with a reservoir hydrodynamic model to estimate the probability and duration of water contamination in a water supply catchment burned by wildfire. Central to this paper is the technique of coupling two models into a risk framework that gives probabilities to the number of days that sediment concentration thresholds for water treatment are exceeded at the offtake. The work is set in the Upper Yarra catchment, which supplies a major reservoir for the city of Melbourne (population ~ 4M). The results show that wildfires pose a substantial threat with relatively high likelihood (exceedance probability between 0.2 and 0.5) for water supply interruptions in the order of several months to a year. The cost of such interruptions could be > AU$100 million. The framework presented provides a direct link between geophysical models and metrics that are used by water supply authorities in strategic planning around resource allocation and cost-benefit analysis of alternative mitigation options.