Abstract
Between 2003 and 2013, drought, large wildfires, and record-breaking rainfall contributed to debris flows in southeast Australia that appear to be unprecedented in spatial extent and density in historical records. Here, we used a debris-flow inventory from this period of dry and wet extremes to examine the processes and climatic controls underlying the regionwide debris-flow response. Results reveal shallow landslides and surface runoff as two distinct initiation mechanisms, linked to different geologic settings and contrasting hydroclimatic conditions. Landslide-generated debris flows occurred in sandy soils, independent of past fires, and were tightly controlled by extreme rainfall causing saturation and mass failure during La Nina periods. In contrast, runoff-generated debris flows occurred in clay-rich soils from short and intense rainstorms after wildfires in dry conditions, often associated with El Nino. Thus, it appears that both ends of the wet and dry climate extremes produce the same general geomorphic response, debris flows, but in different areas and by different initiation processes. Debris-flow activity is therefore at a maximum when amplitude and frequency of climate oscillations are large. Debris flows in southeast Australia are likely to become more frequent and widespread as wildfire activity and rainfall intensity are predicted to increase.
Highlights
Studies show evidence of tight coupling among climate variability, wildfire activity, and the geomorphic processes contributing to denudation (Riley et al, 2015; Meyer et al, 2001)
Debris flows were associated with two distinct processes: (1) runoff-generated debris flows after wildfire in dry conditions (Figs. 2A and 2E), and (2) landslide-generated debris flows during the wet period in 2010–2012, when large areas were subject to P24 >150 mm (Figs. 2A and 2B)
The two processes were related to El Niño–Southern Oscillation (ENSO) modes, with wildfire and extreme rainfall more likely to occur in El Niño and La Niña conditions, respectively (Fisher-exact test; p = 0.49, odds ratio = 9)
Summary
Studies show evidence of tight coupling among climate variability, wildfire activity, and the geomorphic processes contributing to denudation (Riley et al, 2015; Meyer et al, 2001). In southeast Australia, the series of climatic events at the start of the 21st century produced both exceptionally dry and wet conditions (Freund et al, 2017) During this period, debris flows occurred regularly in forested and relatively stable postorogenic mountain ranges, with incidents of both runoff-generated debris flows from short and intense rainfall on burned areas, and landslide-generated debris flows from extended periods of heavy rainfall and saturated conditions (both types described in Meyer et al, 2001). Debris flows occurred regularly in forested and relatively stable postorogenic mountain ranges, with incidents of both runoff-generated debris flows from short and intense rainfall on burned areas, and landslide-generated debris flows from extended periods of heavy rainfall and saturated conditions (both types described in Meyer et al, 2001) In terms of their density, spatial extent, and impact, there are no historical records of such frequent and extensive debris-flow activity. The socioeconomic costs were significant and included disruptions to water supply, a human fatality, and damage to infrastructure costing >US$60 million
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