Emerging insights into the dynamics of submarine debris flows

Abstract

Recent experimental and theoretical work on the dynamics of submarine debris flows is summarized. Hy-droplaning was first discovered in laboratory flows and later shown to likely occur in natural debris flows as well. It is a prime mechanism for explaining the extremely long runout distances observed in some natural debris flows even of over-consolidated clay materials. Moreover, the accelerations and high velocities reached by the flow head in a short time appear to fit well with the required initial conditions of observed tsunamis as obtained from back-calculations. Investigations of high-speed video recordings of laboratory debris flows were combined with measurements of total and pore pressure. The results are pointing towards yet another important role of ambient water: Water that intrudes from the water cushion underneath the hydroplaning head and through cracks in the upper surface of the debris flow may drastically soften initially stiff clayey material in the "neck" of the flow, where significant stretching occurs due to the reduced friction at the bottom of the hydroplaning head. This self-reinforcing process may lead to the head separating from the main body and becoming an "outrunner" block as clearly observed in several natural debris flows. Comparison of laboratory flows with different material composition indicates a gradual transition from hydroplaning plug flows of stiff clay-rich material, with a very low suspension rate, to the strongly agitated flow of sandy materials that develop a pronounced turbidity current. Statistical analysis of the great number of distinguishable lobes in the Storegga slide complex reveals power-law scaling behavior of the runout distance with the release mass over many orders of magnitude. Mathematical flow models based on viscoplastic material behavior (e.g. BING) successfully reproduce the observed scaling behavior only for relatively small clay-rich debris flows while granular (frictional) models fail at all scales. For very large release masses, hydroplan-ing or significant softening of the shear layer due to water in-Correspondence to: A. Elverhøi (anders.elverhoi@geo.uio.no) corporation must be invoked to recover the observed scaling behavior; a combination of both effects likely will give the most realistic description of the phenomenon. Detailed studies of the neck behavior and the compositional dependence of the material properties are needed to arrive at a quantitative model. Other related and important open questions concern the rheological model appropriate for sandy debris flows and the suspension rate from the dense body into the associated turbidity current.

Oceanic propagation of a potential tsunami from the La Palma Island

“…The simplest models for modeling landslides are block models [Perla et al, 1980], which may be successfully applied to slides with no or little internal deformation. Debris flows may extend over large geographical areas, their evolution is often simulated using depth averaged models [Locat and Lee, 2002;Elverhøi et al, 2005]. For the tsunamigenic dense part of flowing slides, simple Bingham fluids, generalized Herschel-Buckley rheologies, or bi-linear models have traditionally been used, parameterizing the yield strength as a power law function of the strain rate in the sliding material [Imran et al, 2001].…”

Hydrodynamic modeling of tsunamis from the Currituck landslide

“…It is thought that the former value better accounts for the initial high acceleration of failure, whereas the latter value represents the overall total duration of slide movement in the excavation region. We also examine a very short duration time (7.2 min) to examine the effect on tsunami amplitudes of a slide that is much more mobile than expected from the post-failure analysis (e.g., from low basal shear stress, Elverhøi et al, 2005).…”

“…In the past, a poor understanding of submarine landslide dynamics, in combination with the higher-order hydrodynamic theory needed to model dispersion and nonlinearity, have been major obstacles in understanding landslide tsunamis. Recent research, however, has resulted in new modeling methods to address both of these problems (e.g., Imran et al, 2001;Elverhøi et al, 2005;Lastras et al, 2005) that will undoubtedly lead to accelerated progress in estimating the severity of this natural hazard. Finally, the broad continental shelf also has a significant effect on the wave evolution of tsunamis (Shibata, 1983), particularly those generated from continental slope landslides.…”

Numerical Simulation on the Dynamic Characteristics of a Tremendous Debris Flow in Sichuan, China

“…It is about 80 km northeast of the earthquake epicenter, with a basin area of approximately 400 km 2 . The Yingxiu-Beichuan seismogenic fault of the Wenchuan earthquake goes through the middle part of the river basin.…”

“…From the standpoint of dynamic mechanics, Elverhøi et al [2] performed a numerical model of submarine debris flows with Computational Fluid Dynamics X (CFX). Zakeri et al [3] applied CFX to simulate the submarine debris flow impact on pipelines.…”