Understanding how volume affects the mobility of dry debris flows

Abstract

[1] The prediction of the runout length L of large dry debris flows has long been the subject of a considerable research effort, primarily due to the obvious concern caused by their destructive power. One seemingly well established feature is the increase of the mobility M of a rock avalanche, defined as the ratio of the runout distance to the fall height, with its volume V. The physical nature of this lubrication mechanism remains however controversial. In this paper, we analyse field data and discrete numerical simulations of granular flows and demonstrate the geometrical origin of the apparent enhancement of the mobility with the volume. We evidence the intertwined role of volume and topography and show the existence of two contributions in the runout, defining two flow regimes: one dominated by sliding, in which the runout is independent of V, and another dominated by spreading, in which the runout is strongly dependent on V. In the light of these results, the search of a volume dependent lubrication mechanism appears to be an ill-posed problem. Citation: Staron, L., [Howard, 1973]. Beside an obvious concern for hazard assessment, rock avalanches are also efficient agents of erosion in active orogens, capable of moving large masses of material over kilometre-scale distances instantaneously [Hovius and Stark, 2006;Korup et al., 2007]. In spite of the sustained interest these dramatic natural events have raised in the scientific community, they still escape physical understanding [Iverson, 1997[Iverson, , 2003.[3] Among the various issues raised by these flows, the prediction of their runout length L (see Figure 2a, insert) keeps a first rank position, primarily due to the obvious concern caused by their destructive power. Surprisingly, they can travel over distances several times larger than the height H of the source topography [Dade and Huppert, 1998]. One seemingly established feature is the increase of the mobility M = L/H with the volume V of rock mobilized by the avalanche (Figure 2a). This positive correlation was first noted by Heim [1932]. Yet, the identification of lubrication mechanisms enhanced by volume remains a persisting and challenging issue [Legros, 2002, and references therein]. In this paper, we analyse both field data and discrete numerical simulations of granular flows, and show that the increase of M with V reflects a purely geometrical correlation. In the light of these results, the search for a volume dependent lubrication mechanism appears to be an ill-posed problem.[4] The conventional analysis of the dissipative properties of geological granular flows relies on the hypothesis, first put forward by Heim [1932] and prompted by an analogy with solid friction, that the whole of the initial potential energy of the mass is dissipated by the work of friction forces along the topography. Neglecting centripetal acceleration induced by the topography, and any other energy transfers in the system, we obtain:where m e is an effective friction coefficient quantifying the average macroscopic d...

Frictional velocity-weakening in landslides on Earth and on other planetary bodies

“…In this sense, large landslides are said to present high mobility or long runout distances 5,6,8 . However, as already long discussed in the literature, this interpretation is not as straightforward as it seems 8,9,11,12,14,15 . Confusion between the two independent questions formulated in the introduction has led some to refute the decrease of effective friction with volume because of the questionable meaning of the Heim's ratio 15 .…”

“…However, as already long discussed in the literature, this interpretation is not as straightforward as it seems 8,9,11,12,14,15 . Confusion between the two independent questions formulated in the introduction has led some to refute the decrease of effective friction with volume because of the questionable meaning of the Heim's ratio 15 . Its limitations as a measure of the effective friction have long been recognized in the literature, based on the fact that its definition does not involve the displacement of the centre of gravity 16 , the spreading of mass or the role of the topography 8,9,12,14,15 .…”

Can the collapse of a fly ash heap develop into an air-fluidized flow? — Reanalysis of the Jupille accident (1961)

“…The physical meaning of this parameter is questionable for large flows, in which spreading, more than sliding, accounts for the runout distance (Staron and Lajeunesse, 2009). The 206,300 m³ volume of displaced material in the case study is however below the threshold of 10 6 m³, above which spreading effects dominate (Davies, 1982;Iverson, 2003b).…”

Reconstruction of the Kinematics of Landslide and Debris Flow Through Numerical Modeling Supported by Multidisciplinary Data: The 2009 Siaolin, Taiwan Landslide

“…Though with such simplifications, this type of models has been successively validated to be able to reproduce experimental and natural granular flows (Gray et al 2003;Kuo et al 2009). In addition to be constrained by the frictional experimental results, the Coulomb friction coefficient in the simulation is further referenced to a universal scaling law (Staron and Lajeunesse, 2009) and reconfirmed by an iterative optimization scheme. Inspecting extensive practical landslide data and discrete element simulations, the friction coefficient is found to follow a simple geometrical relation, the universal scaling law, which is inversely proportional to the one-third power of the landslide volume (Staron and Lajeunesse, 2009).…”

“…In addition to be constrained by the frictional experimental results, the Coulomb friction coefficient in the simulation is further referenced to a universal scaling law (Staron and Lajeunesse, 2009) and reconfirmed by an iterative optimization scheme. Inspecting extensive practical landslide data and discrete element simulations, the friction coefficient is found to follow a simple geometrical relation, the universal scaling law, which is inversely proportional to the one-third power of the landslide volume (Staron and Lajeunesse, 2009). Based on the friction angle 6 o found in the Tsaoling landslide with a volume 0.126 km 3 (Kuo et al, 2009), we estimate the friction angle for the Siaolin landslide (about 1/6 in volume) is about 11 o according to the geometrical scaling relation.…”