Effect of debris-flow sediment grain-size distribution on fan morphology

Abstract

Abstract. Knowledge of how debris flows result in the fan-shaped morphology around a channel outlet is crucial for mitigation of debris-flow-related disasters and investigation of previous sediment transport from the upper channel. Therefore, using a flume connected to a deposition area (inundation plane), this study conducted fan-morphology experiments to assess the effects of differences in grain-size distribution within debris flows on changes in fan morphology. Two types of debris-flow material, i.e., monogranular particles comprising monodispersed sediment particles and multigranular particles comprising polydispersed sediment particles, were used to generate monogranular and multigranular experimental debris flows, respectively. By adjusting the average grain size coincident between the monogranular and multigranular flows, we generated two types of debris flow with similar debris mixture hydrographs but different grain-size distributions in the flume. Although the flow depths were mostly similar between the monogranular and multigranular flows before the start of the debris-flow runout at the deposition area, the runout distances of the front of the multigranular flows were shorter than those of the monogranular flows. The difference in runout distance was responsible for the variations in the extent to and location in which the debris flows changed their direction of descent, resulting in the different shapes and morphologies of the fans in response to grain-size distribution. Although the direction of descent of the flows changed repeatedly, the extent of morphological symmetry of the debris-flow fans increased at a similar time during fan formation irrespective of the grain-size distribution. In contrast to this similarity in the rate of change in fan symmetry, the shift of the multigranular flow directions eventually increased the extent of asymmetry in fan morphology and expanded the scale of deviations in fan morphology between experimental test runs. Therefore, wide-ranging grain-size distributions within debris flows likely result in complex fan morphology with a high degree of asymmetry.

Numerical Simulation of Debris Flow Disaster in Yunnan Mountainous Areas Guided by Discrete Element Tracing Method

“…(2021) [21] studied the in uence of particle size distribution of debris ow sediments on the formation process of fan-shaped landforms. Through experiments and numerical simulation, they explored the physical mechanism and geomorphological evolution law of debris ow deposition under different particle size distribution conditions.…”

“…This study provided a new method and tool for debris ow risk assessment and disaster prevention decision. Tsunetaka et al (2022) [23] studied the in uence of debris ow sediment particle size distribution on fan-shaped landform. Through on-the-spot investigation and numerical simulation, they revealed the geomorphological changes in the process of debris ow sediment accumulation under different particle size distribution conditions.…”

Debris‐flow fan development and geomorphic effects in alpine canyons under a changing climate

“…Historical records and modern sedimentary facies are combined to distinguish two dominant geomorphic processes (primary and secondary) on alluvial fans, in which debris flows dominate fan deposition, and stream flows modify the original depositional morphology (Crosta & Frattini, 2004; de Haas et al, 2014; Vincent, 2020; Vincent et al, 2022). Considerable research related to the geomorphological characteristics (Hooke & Rohrer, 1977; Li et al, 2006; Lv et al, 2016; Tang et al, 1991), development processes (Cui et al, 2006; Vincent et al, 2022; Yin et al, 2017; Zhang et al, 2010) and evolutionary mechanisms (de Haas et al, 2018; Hamilton et al, 2013; Leenman & Eaton, 2021; Wasklewicz & Scheinert, 2016) of alluvial fans formed at the outlet of debris‐flow gullies has been conducted, for example, how riverbed property, sediment grain‐size distribution and water content affect the erosion‐deposition processes (e.g., debris‐flow runout and channel avulsions) on the debris‐flow fan and how they further affect the fan surface morphology and spatial distribution (Chen et al, 2022; Schürch et al, 2011; Tsunetaka et al, 2022; Whipple, 1994; Zubrycky et al, 2021). Moreover, the geometrical, morphological and geo‐lithological parameters of the depositional site and drainage basin, such as fan gradient and area, and relief ratio of the source basin area, are often used as discriminant function variables.…”