Debris‐flow volumes in northeastern Italy: Relationship with drainage area and size probability

Abstract

A dataset of 809 debris flows that occurred in 537 basins in mountainous areas of northeastern Italy between the mid‐19th century and 2015 is collected and analyzed. A remarkable increase in the number of events is observed in the last four decades and is mainly ascribed to more systematic data collection. The correlation between debris‐flow volume and drainage basin area is obtained assuming a power‐law relationship. The exponent of the power‐law curve at the 50th percentile (0.67 ± 0.02) indicates negative allometry, which means that basin area grows out of proportion to debris‐flow volume. In contrast, the exponents at the 98th and 99th percentiles are close to one, implying that debris‐flow volumes grow linearly with basin areas. The isometric relationship between the largest debris flows and the corresponding basin areas is due to the enhanced debris supply provided by the activation of widespread sediment sources, the extent of which is proportional to the basin size. The probability density function of debris‐flow volume for a subset of the collected dataset is calculated using the kernel density estimation function, which permits estimating the probability of the occurrence of debris flows that exceed a given threshold volume. The comparison with debris flows in other hydroclimatic regions shows that, although debris‐flow volumes in northeastern Italy may attain high values, they are often exceeded by debris flows that occur in seismically active regions and/or are triggered by more intense rainstorms. © 2018 John Wiley & Sons, Ltd.

Pronounced increase in slope instability linked to global warming: A case study from the eastern European Alps

“…Because the number of intense rainfall events has not significantly changed (Table 2), the increase in debris flows registered in the last 10 years may be related to the lack of events in the previous two decades (possibly associated with the drier 1995–2009 period), which led to an increased accumulation of unconsolidated debris at the potential initiation areas. Overall, this suggests that downstream sediment transport by debris‐flow activity in the Sulden basin is limited to some extent by sediment availability and/or geomorphic connectivity, as observed in several other Alpine mountain catchments (Buter, 2021; Marchi et al, 2019; Micheletti et al, 2015).…”

“…The systematic recording of natural hazards in South Tyrol started in 1998 and has been maintained by the Autonomous Province of Bozen (see also Marchi et al, 2019; Schlögel et al, 2020; Scorpio et al, 2020). The available data indicates that the most common processes in the Sulden catchment are debris flows and debris floods, together with different types of small‐ to medium‐sized landslides (Figure 1d).…”

“…still challenges our understanding of the mechanisms triggering slope failures, hence limiting our ability to predict mountain‐slope instability (Beniston et al, 2018; Damm & Felderer, 2013; Fischer et al, 2012; Huggel et al, 2012). In this respect, the main challenges we face can be summarized as follows: (i) databases of natural extreme events are often incomplete (e.g., Allen & Huggel, 2013; Marchi et al, 2019; Sass & Oberlechner, 2012); (ii) a time lag may exist between the impact of climatic forcing and the failure event (e.g., Crozier, 2010; Haeberli & Beniston, 1998; Huggel et al, 2012; Noetzli & Gruber, 2009); (iii) ground surface temperature and soil water content are often unknown at regional scales (e.g., Gruber et al, 2004b; Gubler et al, 2013; Luetschg & Haeberli, 2005); and (iv) the role of snow cover/snow depth has not yet been clarified (e.g., Draebing et al, 2014, 2017a, 2017b; Harris & Conte, 1992; Salzmann et al, 2007; Staub & Delaloye, 2017; Zhang, 2005). Each of the these aspects is briefly discussed later.…”

“…Firstly, while databases for large landslides and rock avalanches tend to be more complete and cover longer time periods (Eisbacher & Clague, 1984; Porter & Orombelli, 1981), the quality of the database for small‐ to medium‐scale events may depend on available recording technologies, on when and where the event occurred (i.e., low/high altitude; season during which witnesses were present or not), and on whether damage to infrastructure or human losses were caused (Allen & Huggel, 2013). Furthermore, the recent increase in awareness of these natural events, in the application of new technologies, and in the intensive use of high‐elevation areas may cause these databases to be inherently biased toward events that have taken place in recent decades (Cavalli et al, 2017; Marchi et al, 2019, 2020; Sass & Oberlechner, 2012).…”

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

“…Debris-flow volumes observed between 1990 and 2019 range from 730 to 89 500 m 3 (Table 2). Marchi et al (2019) have explored the relationship between catchment area and debris-flow volume in northeastern Italy using quantile regression. Notwithstanding the large availability of loose debris in the source areas and the abundant precipitation in the Moscardo area, even the largest debris flows observed between 1990 and 2018 lie well below the debris-flow volumes corresponding to the highest percentiles: for the 98th percentile, the central value is 195 894 m 3 , with uncertainty bounds between 170 902 and 223 211 m 3 .…”