Impact of a Random Sequence of Debris Flows on Torrential Fan Formation

Abstract

Debris flows with different magnitudes can have a large impact on debris fan characteristics such as height or slope. Moreover, knowledge about the impact of random sequences of debris flows of different magnitudes on debris fan properties is sparse in the literature and can be improved using numerical simulations of debris fan formation. Therefore, in this paper we present the results of numerical simulations wherein we investigated the impact of a random sequence of debris flows on torrential fan formation, where the total volume of transported debris was kept constant, but different rheological properties were used. Overall, 62 debris flow events with different magnitudes from 100 m 3 to 20,000 m 3 were selected, and the total volume was approximately 225,000 m 3 . The sequence of these debris flows was randomly generated, and selected debris fan characteristics after the 62 events were compared. For modeling purposes, we applied the Rapid Mass Movement Simulations (RAMMS) software and its debris flow module (RAMMS-DF). The modeling was carried out using (a) real fan topography from an alpine environment (i.e., an actual debris fan in north-west (NW) Slovenia formed by the Suhelj torrent) and (b) an artificial surface with a constant slope. Several RAMMS model parameters were tested. The simulation results confirm that the random sequence of debris flow events has only some minor effects on the fan formation (e.g., slope, maximum height), even when changing debris flow rheological properties in a wide range. After the 62 events, independent of the selected sequence of debris flows, the final fan characteristics were not significantly different from each other. Mann-Whitney (MW) tests and t-tests were used for this purpose, and the selected significance level was 0.05. Moreover, this conclusion applies for artificial and real terrain and for a wide range of tested RAMMS model rheological parameters. Further testing of the RAMMS-DF model in real situations is proposed in order to better understand its applicability and limitations under real conditions for debris flow hazard assessment or the planning of mitigation measures.

An extreme May 2018 debris flood case study in northern Slovenia: analysis, modelling, and mitigation

“…Moreover, Dietrich and Krautblatter (2019) used μ = 0.16 and ξ = 200 m/s 2 parameters for debris flow modelling in Germany. Additionally, μ parameter value is in the range of the possible values that could be suitable for the alpine environment (e.g., Bezak et al 2019b). Figure 6 shows results of the simulation (i.e., deposited material) using a calibrated set of parameters.…”

Debris Flow Modelling Using RAMMS Model in the Alpine Environment With Focus on the Model Parameters and Main Characteristics

“…the dry-Coulomb type friction μ (Mu) and the viscous-turbulent friction ξ (Xi)). These two parameters are usually calibrated, although other parameters such as stop parameter or simulation resolution also have an effect on the modelling results (Bezak et al, 2019). However, some of these are limited by data availability.…”

“…alluvial fans). These are relatively flat parts of mountainous regions that are often quite heavily populated (Bezak et al, 2019). Reliable debris flow prediction is often not possible due to limited geological information or details about triggering mechanisms such as extreme rainfall event (Takahashi, 2014).…”

Mountain Landslides: Monitoring, Modeling, and Mitigation

“…Bezak et al [6] have presented the results of numerical simulations investigating the impact of a random sequence of debris flows on torrential fan formation. The results confirm that the random sequence of debris flow events have only minor effects on the fan formation (e.g., slope, maximum height), even when changing debris flow rheological properties in a wide range.…”