Debris-flow monitoring and warning: Review and examples

Abstract

Debris flows represent one of the most dangerous types of mass movements, because of their high velocities, large impact forces and long runout distances. This review describes the available debris-flow monitoring techniques and proposes recommendations to inform the design of future monitoring and warning/alarm systems. The selection and application of these techniques is highly dependent on site and hazard characterization, which is illustrated through detailed descriptions of nine monitoring sites: five in Europe, three in Asia and one in the USA. Most of these monitored catchments cover less than ~10 km 2 and are topographically rugged with Melton Indices greater than 0.5. Hourly rainfall intensities between 5 and 15 mm/h are sufficient to trigger debris flows at many of the sites, and observed debris-flow volumes range from a few hundred up to almost one million cubic meters. The sensors found in these monitoring systems can be separated into two classes: a class measuring the initiation mechanisms, and another class measuring the flow dynamics. The first class principally includes rain gauges, but also contains of soil moisture and pore-water pressure sensors. The second class involves a large variety of sensors focusing on flow stage or ground vibrations and commonly includes video cameras to validate and aid in the data interpretation. Given the sporadic nature of debris flows, an essential characteristics of the monitoring systems is the differentiation between a continuous mode that samples at low frequency ("non-event mode") and another mode that records the measurements at high frequency ("event mode"). The event detection algorithm, used to switch into the "event mode" depends on a threshold that is typically based on rainfall or ground vibration. Identifying the correct definition of these thresholds is a fundamental task not only for monitoring purposes, but also for the implementation of warning and alarm systems.

Debris flows recorded in the Moscardo catchment (Italian Alps) between 1990 and 2019

“…The mean frequency between 1990 and 2019 is 1.0 event per year including three debris floods ( Table 2). The frequency is similar or lower than other instrumented basins in the Alps if the monitoring stations at the basin outlet or on the alluvial fan are considered: Réal (France): 1.0 event/year (Hürlimann et al, 2019), Illgraben (Switzerland): 3-5 debris flow every year plus some debris floods (Hürlimann et al, 2019), Gadria (Italy): 1.3 events/year between 2011 and 2017 (Comiti et al, 2014;Coviello et al, 2020).…”

“…The Moscardo Torrent dataset could contribute to further analysis, in addition to those already carried out and reported in previous studies and those briefly outlined in this paper. We mention here the comparison of triggering rainfall and basic flow variables (depth, velocity, volumes) with other basins instrumented for debris-flows monitoring under different climate and geolithological conditions (Hürlimann et al, 2019).…”

“…Starting from early studies in Japan and China (Okuda et al 1980;Zhang 1993), many papers presented and analyzed debris-flow data collected in instrumented channels; a recent review (Hürlimann et al, 2019) discusses achievements and open problems in debris-flow monitoring.…”

comment to nhess-2020-243

“…3) "Since being greater than zero is the only constraint of the other parameters, for homogeneity this CV value is considered suitable for all parameters". Field evidences and data show that a variation coefficient equal to 5% strongly underestimates the real variability of both basin area and deposited volume (Hürlimann et al, 2019). In…”

Replies to Velio Coviello

“…As correctly claimed by Velio Coviello, with the term "stony debris flow" we refer to a debris flow in which the presence of silt and/or clay in the mixture is negligible and the stress is dominated by the collision of the particles (e.g. Takahashi, 2009, Stancanelli et al, 2015, Bernard et al, 2019, Hürlimann et al, 2019. We will clarify this definition in the revised manuscript.…”