Dieter Issler

Possible erosion mechanisms in snow avalanches

Abstract Snow erosion and entrainment processes in avalanches are classified according to their mechanisms, the flow regimes in which they occur, and their spatial position within the avalanche. Simple, but process-specific, models are proposed for erosion by impacts, abrasion, plowing and blasting. On the basis of order-of-magnitude estimates, the first three mechanisms are clearly expected to be important. The fourth mechanism stipulates that the compaction of the snow cover ahead of the avalanche leads to the flow of escaping air just in front of the avalanche that may disrupt the snow cover and support formation of a saltation layer. The effects of this hypothetical mechanism resemble those of the plowing mechanism. All mechanisms depend strongly on the snow properties, but, with plausible parameter values, erosion rates at or above the experimentally found rates are obtained. The entrainment rate of an avalanche is most often limited by the shear stress needed to accelerate the eroded snow to avalanche speed.

Application of a SPH depth-integrated model to landslide run-out analysis

Hazard and risk assessment of landslides with potentially long run-out is becoming more and more important. Numerical tools exploiting different constitutive models, initial data and numerical solution techniques are important for making the expert’s assessment more objective, even though they cannot substitute for the expert’s understanding of the site-specific conditions and the involved processes. This paper presents a depth-integrated model accounting for pore water pressure dissipation and applications both to real events and problems for which analytical solutions exist. The main ingredients are: (i) The mathematical model, which includes pore pressure dissipation as an additional equation. This makes possible to model flowslide problems with a high mobility at the beginning, the landslide mass coming to rest once pore water pressures dissipate. (ii) The rheological models describing basal friction: Bingham, frictional, Voellmy and cohesive-frictional viscous models. (iii) We have implemented simple erosion laws, providing a comparison between the approaches of Egashira, Hungr and Blanc. (iv) We propose a Lagrangian SPH model to discretize the equations, including pore water pressure information associated to the moving SPH nodes.

Submarine mass-wasting on glacially-influenced continental slopes: processes and dynamics

Abstract Submarine slides and debris flows are common and effective mechanisms of sediment transfer from continental shelves to deeper parts of ocean basins. They are particularly common along glaciated margins that have experienced high sediment flux to the shelf break during and after glacial maxima. During one single event, typically lasting for a few hours or less, enormous sediment volumes can be transported over distances of hundreds of kilometres, even on very gentle slopes. In order to understand the physics of these mass flows, the process is divided into a release phase, followed by break-up, flow and final deposition. Little is presently known regarding release and break-up, although some plausible explanations can be inferred from basic mechanics of granular materials. Once initiated, the flow of clay-rich or muddy sediments may be assumed to behave as a (non-Newtonian) Herschel-Bulkley fluid. Fluid dynamic concepts can then be applied to describe the flow provided the rheological properties of the material are known. Numerical modelling supports our assertion that the long runout distances observed for large volumes of sediments moving down gentle slopes can be explained by partial hydroplaning of the flowing mass. Hydroplaning might also explain the sharp decrease of the friction coefficient for submarine mass flows as a function of the released volume. The paper emphasizes the need for a better understanding of the physics of mass wasting in the submarine environment.

Particle densities, velocities and size distributions in large avalanches from impact-sensor measurements

Abstract In winter 1998/99, high-frequency pressure measurements with 10 cm sensors mounted 1–19 m above ground were carried out in the upper run-out zone of the avalanche test site at Vallée de la Sionne, Switzerland. Two large dry-snow avalanches clearly revealed a three-layered structure, with surprisingly low pressures in the suspension (or powder-snow) layer. The height of the saltation layer varied between 1 and > 3 m. From the duration, impulse and frequency of single-particle impacts (observed in the saltation layer and intermittently in the dense flow),particle-size and velocity distribution functions as well as strongly varying saltation-layer densities were found. With improved methods for peak detection and correction for grazing impacts, pressure measurements will become premier tool for testing granular flow models.

Experimental Information on the Dynamics of Dry-Snow Avalanches

This paper is a first step towards a synoptic analysis of the experimental information on snow avalanche flow provided by field observations and dedicated experiments over the past 60 years. Both full-size tests in instrumented avalanche tracks and laboratory experiments with snow or substitute materials are used to extract information on two major questions: (i) Which flow regimes are possible in avalanches and under which conditions do they occur? (ii) By which mechanisms and at which rate do avalanches entrain snow from the snow cover? The major types of sensors used in avalanche experiments are briefly discussed, and it is seen that a large variety of sensors and experimental techniques—including laboratory experiments—have to be combined in order to obtain definitive answers to the open questions.

Exploring the significance of the fluidized flow regime for avalanche hazard mapping

Abstract Observational and experimental evidence suggests that it is important to explicitly account for the fluidized flow regime in avalanche hazard mapping due to its high mobility, intermediate density and high velocity. We explore the differences from conventional runout modelling by implementing an extension of the Norem–Irgens–Schieldrop (NIS) rheology in a simple mass-point model. When the dispersive stresses and the excess pore pressure equal the overburden pressure, the flow height increases and the density diminishes until a new equilibrium is reached, determined by the different density dependencies of the two parameters of the dispersive stresses. Fluidization requires sufficiently steep terrain; when it occurs it leads to substantially higher velocities than compared to the dense-flow regime. The model parameters are strongly constrained by their physical meaning and vary little between widely different avalanches. However, in all test cases we obtained better agreement between simulated and observed runout distances and pressure effects than with conventional models.

Dynamics, Velocity and Run-Out of the Giant Storegga Slide

A huge slide (volume of 2400 km3 and run-out 450 km) was released in the Storegga area off the western coast of Norway during early Holocene, followed by numerous smaller debris flows. We perform numerical simulations of the giant slide using a Bingham model for the clay material. Agreement with present deposit distribution and run-out is found by assuming that the shear resistance between the debris flow and the seabed decreases during the flow, and we suggest sediment remolding or hydroplaning as possible explanations. Debris velocities are predicted and possible applications to the associated tsunami event are investigated.

Inferences on flow mechanisms from snow avalanche deposits

Abstract The deposit structure of 20 very small to large avalanches that occurred in the Davos area, eastern Swiss Alps, during winters 2004/05 and 2005/06 was investigated. Snow-cover entrainment was significant in the majority of events and likely to have occurred in all cases. Evidence was found both for plough-like frontal entrainment (especially in wet-snow avalanches) and more gradual erosion along the base of dry-snow avalanches. Several of the dry-snow avalanches, both small and large, showed a fairly abrupt decrease in deposit thickness in the distal direction, often accompanied by changes in the granulometry and the deposit density. Combined with other observations (snow plastered onto tree trunks, deposit-less flow marks in bends, etc.) and measurements at instrumented test sites, this phenomenon is best explained as being due to a fluidized, low-density flow regime that formed mostly in the head of some dry-snow avalanches. The mass fraction of the fluidized deposits ranged from less than 1% to ∼25% of the total deposit mass. Fluidization appears to depend rather sensitively on snow conditions and path properties.

The 1978 Quick Clay Landslide at Rissa, Mid Norway: Subaqueous Morphology and Tsunami Simulations

The 1978 landslide at Rissa is the largest to have struck Norway during the last century and is world-famous because it was filmed. Swath bathymetry data and seismic reflection profiles reveal detailed information about the subaqueous morphology of the mass-transport deposits (MTD). Results show that the landslide affected nearly 20% of the lake floor and that it exhibits a complex morphology including distinct lobes, transverse ridges, longitudinal ridges, flow structures and rafted blocks. The rafted blocks found at the outer-rim of the MTD travelled a distance of over 1,000 m in the early stage of the landslide on an almost flat basin floor. Simulation of sediment dynamics and tsunami modelling show that the rafted blocks most likely triggered the flood wave with a recorded maximum surface elevation of 6.8 m.

Experimental Studies Of Subaqueous Vs. Subaerial Debris Flows – Velocity Characteristics As A function Of The Ambient Fluid

A series of comparable subaerial and subaqueous debris flow experiments of sand-claywater mixtures has been performed at the St Anthony Falls Laboratory (SAFL) at University of Minnesota. Different compositions were tested and velocities measured in detail using PIV (Particle Image Velocimetry) techniques. The experimental series provides a unique data set highlighting the effects of the ambient and interstitial fluid in comparable subaerial and subaqueous debris flows. Based on our experimental data we emphasize the differences in the dynamical behaviour associated with the two environments and suggest important mechanisms to be included in numerical models.