A two-phase shallow debris flow model with energy balance

Guillaume Meyrat, Brian McArdell, Ksenyia Ivanova, Perry BarteltWSL Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland&#160;Keywords: Debris flows, multi-phase models, dilatancy, shear stress, density distribution&#160;To implement an accurate numerical tool to simulate debris flow hazard is a longstanding goal of natural hazard research and engineering. In Switzerland the application of numerical debris flow models has, however, been hampered by many practical and theoretical difficulties. One practical problem is to define realistic initial conditions for hazard scenarios that involve both the rocky (granular solid) and muddy (fluid) material. Still another practical problem is to model debris flow growth by entrainment [1]. These problems are compounded by theoretical uncertainties regarding the rheological behavior of multi-phase flows. Recent analysis of debris flow measurements at the Swiss Illgraben test-site [2] (shear and normal stresses, debris flow height) show that the shear force, and therefore the entire debris flow behavior, is largely influenced by the debris flow composition, i.e. the amount of solid particle and muddy fluid at any specific location within the debris flow body (front, tail, etc.). The debris flow composition is, in turn, determined by the initial and entrainment conditions for a specific event. As a consequence, we have concluded that the very first step to construct a robust numerical model is to accurately predict the space and time evolution of the solid/fluid flow composition for any set of initial and boundary conditions. To this aim, we have developed a two-phase dilatant debris flow model [3, 4, 5] that is based on the idea that the dispersion of solid material in fluid phase can change over time. The model is thus able to predict different flow compositions (rocky fronts, watery tails), using shallow-water type mass, momentum and energy conservation equations. This helps to predict when the solid phase deposits, and when muddy fluid washes and channel outbreaks in the runout zone can occur. The parameters controlling the evolution of debris flow density and saturation have been derived by direct comparison to the full-scale measurements performed at the Illgraben test site.&#160;References&#160;&#160;

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Back calculation of the 2017 Piz Cengalo–Bondo landslide cascade with r.avaflow: what we can do and what we can learn

Natural Hazards and Earth System Science ◽

10.5194/nhess-20-505-2020 ◽

2020 ◽

Vol 20 (2) ◽

pp. 505-520 ◽

Cited By ~ 11

Author(s):

Martin Mergili ◽

Michel Jaboyedoff ◽

José Pullarello ◽

Shiva P. Pudasaini

Keyword(s):

Debris Flow ◽

Flow Model ◽

Rock Avalanche ◽

Simulation Software ◽

Process Models ◽

Model Parameters ◽

Two Phase ◽

Three Phase Flow ◽

Glacier Ice ◽

Future Work

Abstract. In the morning of 23 August 2017, around 3×106 m3 of granitoid rock broke off from the eastern face of Piz Cengalo, southeastern Switzerland. The initial rockslide–rockfall entrained 6×105m3 of a glacier and continued as a rock (or rock–ice) avalanche before evolving into a channelized debris flow that reached the village of Bondo at a distance of 6.5 km after a couple of minutes. Subsequent debris flow surges followed in the next hours and days. The event resulted in eight fatalities along its path and severely damaged Bondo. The most likely candidates for the water causing the transformation of the rock avalanche into a long-runout debris flow are the entrained glacier ice and water originating from the debris beneath the rock avalanche. In the present work we try to reconstruct conceptually and numerically the cascade from the initial rockslide–rockfall to the first debris flow surge and thereby consider two scenarios in terms of qualitative conceptual process models: (i) entrainment of most of the glacier ice by the frontal part of the initial rockslide–rockfall and/or injection of water from the basal sediments due to sudden rise in pore pressure, leading to a frontal debris flow, with the rear part largely remaining dry and depositing mid-valley, and (ii) most of the entrained glacier ice remaining beneath or behind the frontal rock avalanche and developing into an avalanching flow of ice and water, part of which overtops and partially entrains the rock avalanche deposit, resulting in a debris flow. Both scenarios can – with some limitations – be numerically reproduced with an enhanced version of the two-phase mass flow model (Pudasaini, 2012) implemented with the simulation software r.avaflow, based on plausible assumptions of the model parameters. However, these simulation results do not allow us to conclude on which of the two scenarios is the more likely one. Future work will be directed towards the application of a three-phase flow model (rock, ice, and fluid) including phase transitions in order to better represent the melting of glacier ice and a more appropriate consideration of deposition of debris flow material along the channel.

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A two-phase debris flow model with boulder transport

International Journal of Safety and Security Engineering ◽

10.2495/safe-v1-n4-389-402 ◽

2011 ◽

Vol 1 (4) ◽

pp. 389-402

Author(s):

C. Martinez ◽

R. Garcia-Martinez ◽

F. Miralles-Wilhelm

Keyword(s):

Debris Flow ◽

Flow Model ◽

Two Phase

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A general two-phase debris flow model

Journal of Geophysical Research Atmospheres ◽

10.1029/2011jf002186 ◽

2012 ◽

Vol 117 (F3) ◽

pp. n/a-n/a ◽

Cited By ~ 188

Author(s):

Shiva P. Pudasaini

Keyword(s):

Debris Flow ◽

Flow Model ◽

Two Phase

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A two-phase dilatant debris flow model based on full scale shear stress and pore pressure measurements

10.5194/egusphere-egu2020-21752 ◽

2020 ◽

Author(s):

Guillaume Meyrat

Keyword(s):

Shear Stress ◽

Debris Flow ◽

Pore Pressure ◽

Debris Flows ◽

Flow Model ◽

Fluid Composition ◽

Geophysical Research ◽

Two Phase ◽

Non Linear ◽

Fluid Pore Pressure

Since 2004, observations of shear and normal stresses have been collected at the base of naturally-triggered debris flows at the Illgraben observation station (Wallis, Switzerland) [1].&#160;&#160; Because flow height and the normal force are simultaneously measured, and limited observations of basal fluid pore pressure are available, it is possible to investigate how the solid/fluid contents of the flow influence the measured shear stress.&#160; The experimental results have emphasized two debris flow properties: (1) Debris flows are characterized by rocky or boulder-rich front, following by a fluidized tail. Consequently, the mass density varies from large values at the front of the flow to lower values towards the tail. A comparison between different debris flow events, however, likewise reveals that the streamwise change in density can vary dramatically between two different events. (2) The relationship between the measured shear and normal tress is highly non-linear.&#160;Operating on the assumption that the streamwise change in density (or equivalently change in streamwise composition) is primarily responsible for the observed non-linear stress behavior, we develop a rheological model describing two-phase debris flow motion. The underlying idea behind the model is that the granular content of the flow can dilate, changing the solid/fluid composition of the flow, and thereby alter the bulk flow density. The model allows us to estimate the correct debris flow composition for different classes of debris flow varying from granular to muddy fluid. Based on these results, we are then able to reproduce the measured shear stress data when we simulate the measured events numerically.&#160; The results appear to confirm dilatant-type flow models proposed by Takahashi [2], and later developed in detail by Iverson and George [3]. The model is used to back-calculate recent debris flow events that occurred near Davos Switzerland in 2018/2019.&#160;&#160;&#160;REFERENCES<ol><li>McArdell, B.W., Bartelt, P. and Kowalski, J. (2007): Field observations of basal forces and fluid pore pressure in a debris flow, Geophysical Research Letters, Vol. 34, No. L07406.</li> </ol>&#160;<ol><li>Takahashi, T. (2007): Debris flows: mechanics, prediction and countermeasures, Taylor and Francis / Balkema, 448pp.</li> </ol>&#160;<ol><li>George, D. L., & Iverson, R. M. (2011). A two-phase debris-flow model that includes coupled evolution of volume fractions, granular dilatancy, and pore-fluid pressure. Italian journal of engineering geology and Environment, 43, 415-424.</li> </ol>

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