Effects of debris flow composition on runout, depositional mechanisms, and deposit morphology in laboratory experiments

Abstract

Predicting debris flow runout is of major importance for hazard mitigation. Apart from topography and volume, runout distance and area depends on debris flow composition and rheology, but how is poorly understood. We experimentally investigated effects of composition on debris flow runout, depositional mechanisms, and deposit geometry. The small-scale experimental debris flows were largely similar to natural debris flows in terms of flow behavior, deposit morphology, grain size sorting, channel width-depth ratio, and runout. Deposit geometry (lobe thickness and width) in our experimental debris flows is largely determined by composition, while the effects of initial conditions of topography (i.e., outflow plain slope and channel slope and width) and volume are negligible. We find a clear optimum in the relations of runout with coarse-material fraction and clay fraction. Increasing coarse-material concentration leads to larger runout. However, excess coarse material results in a large accumulation of coarse debris at the flow front and enhances diffusivity, increasing frontal friction and decreasing runout. Increasing clay content initially enhances runout, but too much clay leads to very viscous flows, reducing runout. Runout increases with channel slope and width, outflow plain slope, debris flow volume, and water fraction. These results imply that debris flow runout depends at least as much on composition as on topography. This study improves understanding of the effects of debris flow composition on runout and may aid future debris flow hazard assessments.

DebrisInterMixing-2.3: a finite volume solver for three-dimensional debris-flow simulations with two calibration parameters – Part 2: Model validation with experiments

“…The key attributes of the model are its sensitivity to water content, gravel and clay fraction and clay mineralogy on the one hand (also see Haas et al, 2015), and the interaction between the phase-averaged bulk rheology of the mixture and the complex three-dimensional flow structure on the other.…”

Visualization of dominant stress-transfer mechanisms in experimental debris flows of different particle-size distribution

“…However, this numerical modelling can be computationally expensive and such studies suffer from being restricted by one or more of the following: two-dimensional (2D) geometry, use of spherical or round rather than angular particles, limited particle numbers, no interstitial fluid, small range in particle size, restriction to steady flow, and use of periodic boundaries. For these reasons, small-scale laboratory experiments, which enable the fundamental interaction of particles to be observed and related to the constitutive behaviour, are also necessary; accepting the difficulties in scaling debris flows for physical modelling (Chau et al 2000;Bowman and Sanvitale 2009;Sanvitale and Bowman 2012;Kaitna et al 2014;Paleo Cageao 2014;De Haas et al 2015).…”

The ability of tree stems to intercept debris flows in forested fan areas: A laboratory modelling study

“…In addition, as reported by Scheidl (2013), full dynamic similarity of all forces … is not feasible by using the same fluid with the same viscosity. Small-scale experiments, however, make it possible to carry out a large number of observations using a wide range of configurations, making them very useful for studying debris-flow phenomena, which are very difficult to observe at full-size scales (de Haas et al, 2015). In addition, despite the scaling simplifications involved, laboratory simulations can provide a good approximation of reality in some cases, allowing the generalisation of the results to the actual scale.…”

“…In addition, despite the scaling simplifications involved, laboratory simulations can provide a good approximation of reality in some cases, allowing the generalisation of the results to the actual scale. In this respect, D 'Agostino et al (2010) andde Haas et al (2015) conducted their studies with apparatus and procedures similar to those used here.…”

“…Empirical-statistical methods are based on regression analysis of data from past debris-flow events to define relationships between factors such as runout length and sediment volume. Due the difficulties involved in studying debris flows at the field scale, physical scale-model experiments are considered to be very helpful for understanding the runout of hillslope debris flows (Scheidl et al, 2013), and several different scale laboratory experiments have been carried out to simulate debris flows (e.g., Major and Iverson, 1999;D'Agostino et al, 2010;Hürlimann et al, 2015;de Haas, 2015). The majority of these studies have focused on the rheology and flow behaviour of debris flows and the formation of frontal accumulations of coarse debris.…”