Runoff‐generated debris flows: Observation of initiation conditions and erosion–deposition dynamics along the channel at Cancia (eastern Italian Alps)

Abstract

In the Dolomitic region, abundant coarse hillslope sediment is commonly found at the toe of rocky cliffs. Ephemeral channels originate where lower permeability bedrock surfaces concentrate surface runoff. Debris flows initiate along such channels following intense rainfall and determine the progressive erosion and deepening of the channels. Sediment recharge mechanisms include rock fall, dry ravel processes and channel-bank failures. Here we document debris flow activity that took place in an active debris flow basin during the year 2015. The Cancia basin is located on the southwestern slope of Mount Antelao (3264ma.s.l.) in the dolomitic region of the eastern Italian Alps. The 2.5km 2 basin is incised in dolomitic limestone rocks. The data consist of repeated topographic surveys, distributed rainfall measurements, time-lapse (2s) videos of two events and pore pressure measurements in the channel bed. During July and August 2015, two debris flow events occurred, following similarly intense rainstorms. We compared rainfall data to existing rainfall triggering thresholds and simulated the hydrological response of the headwater catchment with a distributed model in order to estimate the total and peak water discharge. Our data clearly illustrate how debris entrainment along the channel is the main contributor to the overall mobilized volume and that erosion is dominant when the channel slope exceeds 16°. Further downstream, sediment accumulation and depletion occurred alternately for the two successive events, indicating that sediment availability along the channel also influences the flow behaviour along the prevailing-transport reach. The comparison between monitoring data, topographical analysis and hydrological simulation allows the estimation of the average solid concentration of the two events and suggests that debris availability has a significant influence on the debris flow volume.

Reply on RC1

“…14c) and influx of debris flow material from the slope. Though the present study noted the influence of channel morphology on the debris flow characteristics, other studies have observed the changes in channel morphology caused by the debris flows (Remaître et al, 2005;Simoni et al 2020).…”

The Use of Rain Gauge Measurements and Radar Data for the Model‐Based Prediction of Runoff‐Generated Debris‐Flow Occurrence in Early Warning Systems

“…(2020); Simoni et al. (2020) for 20 of the 41 DFs (Table 3). Therefore, for the sake of homogeneity, we used the entire precipitation of the event in all the cases.…”

“…For identifying the rainfall beginning time, we applied the definition of burst used by Coe et al (2008). The timing of occurrence is known by the monitoring stations of Bernard, Berti et al (2019); Berti et al (2020); Simoni et al (2020) for 20 of the 41 DFs (Table 3). Therefore, for the sake of homogeneity, we used the entire precipitation of the event in all the cases.…”

A New Method to Evaluate the Combined Performance of Check Dams and Afforestation on Debris Flows Mitigation in a Dry-hot Valley

“…The estimation of the sediment volume mobilized by a debris ow can be carried out by the differences of pre-and post-events DEMs (Simoni et al, 2020). But the DEMs before and after a debris ow event is di cult to be obtained timely (Mohammad and Ali, 2017;Hassan and Banihabib 2016;Schürch et al 2011).…”

“…Debris ows are mixtures of soil, sand, stones, and water that transport large amounts of sediment downstream, which can cause serious damage and economic losses (Lyu et al, 2020). Debris ows often start from landslides and channel bed failures (Gabet and Mudd, 2006;Gregoretti and Dalla Fontana, 2008;Simoni et al, 2020), moraine lake outbursts (McCoy et al, 2012), and glacial melting (Lyu, 2017b).…”