The complexity of flow and wide variety of depositional processes operating in subaqueous density flows, combined with post-depositional consolidation and soft-sediment deformation, often make it difficult to interpret the characteristics of the original flow from the sedimentary record. This has led to considerable confusion of nomenclature in the literature. This paper attempts to clarify this situation by presenting a simple classification of sedimentary density flows, based on physical flow properties and grain-support mechanisms, and briefly discusses the likely characteristics of the deposited sediments. Cohesive flows are commonly referred to as debris flows and mud flows and defined on the basis of sediment characteristics. The boundary between cohesive and non-cohesive density flows (frictional flows) is poorly constrained, but dimensionless numbers may be of use to define flow thresholds. Frictional flows include a continuous series from sediment slides to turbidity currents. Subdivision of these flows is made on the basis of the dominant particle-support mechanisms, which include matrix strength (in cohesive flows), buoyancy, pore pressure, grain-to-grain interaction (causing dispersive pressure), Reynolds stresses (turbulence) and bed support (particles moved on the stationary bed). The dominant particle-support mechanism depends upon flow conditions, particle concentration, grain-size distribution and particle type. In hyperconcentrated density flows, very high sediment concentrations (>25 volume%) make particle interactions of major importance. The difference between hyperconcentrated density flows and cohesive flows is that the former are friction dominated. With decreasing sediment concentration, vertical particle sorting can result from differential settling, and flows in which this can occur are termed concentrated density flows. The boundary between hyperconcentrated and concentrated density flows is defined by a change in particle behaviour, such that denser or larger grains are no longer fully supported by grain interaction, thus allowing coarse-grain tail (or dense-grain tail) normal grading. The concentration at which this change occurs depends on particle size, sorting, composition and relative density, so that a single threshold concentration cannot be defined. Concentrated density flows may be highly erosive and subsequently deposit complete or incomplete Lowe and Bouma sequences. Conversely, hydroplaning at the base of debris flows, and possibly also in some hyperconcentrated flows, may reduce the fluid drag, thus allowing high flow velocities while preventing large-scale erosion. Flows with concentrations <9% by volume are true turbidity flows (sensuBagnold, 1962), in which fluid turbulence is the main particle-support mechanism. Turbidity flows and concentrated density flows can be subdivided on the basis of flow duration into instantaneous surges, longer duration surge-like flows and quasi-steady currents. Flow duration is shown to control the nature of the resulting deposits. Surge-like turbidity currents tend to produce classical Bouma sequences, whose nature at any one site depends on factors such as flow size, sediment type and proximity to source. In contrast, quasi-steady turbidity currents, generated by hyperpycnal river effluent, can deposit coarsening-up units capped by fining-up units (because of waxing and waning conditions respectively) and may also include thick units of uniform character (resulting from prolonged periods of near-steady conditions). Any flow type may progressively change character along the transport path, with transformation primarily resulting from reductions in sediment concentration through progressive entrainment of surrounding fluid and/or sediment deposition. The rate of fluid entrainment, and consequently flow transformation, is dependent on factors including slope gradient, lateral confinement, bed roughness, flow thickness and water depth. Flows with high and low sediment concentrations may co-exist in one transport event because of downflow transformations, flow stratification or shear layer development of the mixing interface with the overlying water (mixing cloud formation). Deposits of an individual flow event at one site may therefore form from a succession of different flow types, and this introduces considerable complexity into classifying the flow event or component flow types from the deposits.

The physical character of subaqueous sedimentary density flows and their deposits

This paper presents recommended methodologies for the quantitative analysis of landslide hazard, vulnerability and risk at different spatial scales (site-specific, local, regional and national), as well as for the verification and validation of the results. The methodologies described focus on the evaluation of the probabilities of occurrence of different landslide types with certain characteristics. Methods used to determine the spatial distribution of landslide intensity, the characterisation of the elements at risk, the assessment of the potential degree of damage and the quantification of the vulnerability of the elements at risk, and those used to perform the quantitative risk analysis are also described. The paper is intended for use by scientists and practising engineers, geologists and other landslide experts.

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Recommendations for the quantitative analysis of landslide risk

This paper presents a short history of the appraisal of laser scanner technologies in geosciences used for imaging relief by high-resolution digital elevation models (HRDEMs) or 3D models. A general overview of light detection and ranging (LIDAR) techniques applied to landslides is given, followed by a review of different applications of LIDAR for landslide, rockfall and debris-flow. These applications are classified as: (1) Detection and characterization of mass movements; (2) Hazard assessment and susceptibility mapping; (3) Modelling; (4) Monitoring. This review emphasizes how LIDAR-derived HRDEMs can be used to investigate any type of landslides. It is clear that such HRDEMs are not yet a common tool for landslides investigations, but this technique has opened new domains of applications that still have to be developed.

https://link.springer.com/content/pdf/10.1007%2Fs11069-010-9634-2.pdf

Use of LIDAR in landslide investigations: a review

Bioturbation refers to the biological reworking of soils and sediments, and its importance for soil processes and geomorphology was first realised by Charles Darwin, who devoted his last scientific book to the subject. Here, we review some new insights into the evolutionary and ecological role of bioturbation that would have probably amazed Darwin. In modern ecological theory, bioturbation is now recognised as an archetypal example of ‘ecosystem engineering’, modifying geochemical gradients, redistributing food resources, viruses, bacteria, resting stages and eggs. From an evolutionary perspective, recent investigations provide evidence that bioturbation had a key role in the evolution of metazoan life at the end of the Precambrian Era.

Bioturbation: a fresh look at Darwin's last idea

Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.

Subaqueous sediment density flows: Depositional processes and deposit types

Submarine landslides are becoming recognized as a potential source of damaging local tsunamis. However, there are presently few documented case studies of landslide events that have caused historic tsunamis or likely caused prehistoric tsunamis. We present three case studies of submarine landslide environments off the west coast of the United States, including Alaska. Each environment has been imaged using multibeam technology allowing excellent resolution of the morphology of the seafloor. Based on this imagery and the historic record, we document the character of these environments and the resulting tsunamis. In the case of one of the failures, we present a model of the motion of the landslide and the size of tsunami that this motion would have produced.

Characteristics of Several Tsunamigenic Submarine Landslides

In July 1996, a flash flood resulted in the input of 9 million m3 of sediment toward the Bay of Ha! Ha!, leading to the elimination, partly or totally, of the benthic fauna of the bay. In this stud...

Utilisation de la scanographie pour l'étude des sédiments : influence des paramètres physiques, chimiques et biologiques sur la mesure des intensités tomographiques

Submarine mass movements pose a threat to coastal communities and infrastructures, both onshore and offshore. They can be found from the coastal zone down onto the abyssal plain and can take place on slope angles as low as 0.5°. They can move at velocities up to 50 km/h and reach distances over 1000 km. Their volume can be enormous, as illustrated by the 2.5 × 103 km3 Storegga slide. Similar to their sub-aerial counterparts, submarine mass movements can consist of soil or rock and can take the form of slides, spreads, flows, topples or falls, but in addition they can develop into turbidity currents. Their main consequences are linked either to the direct loss of material at the site where the mass movement is initiated or along its path and to the generation of tsunamis.

Submarine Mass Movements and Their Consequences: An Overview

Landslides in sensitive clays represent a major hazard in the northern countries of the world such as Canada, Finland, Norway, Russia, Sweden and in the US state of Alaska. Examples of catastrophic landslides in sensitive clays that impacted populations are numerous: e.g., Saint-Jean-Vianney in 1971 (Tavenas et al. 1971; Potvin et al. 2001), Rissa in 1979 (Gregersen 1981; L’Heureux et al. 2012), Finneidfjord in 1996 (Longva et al. 2003), Kattmarka in 2009 (Nordal et al. 2009) and St-Jude in 2010 (Locat et al. 2012). In order to respond to the societal demands, the scientific community has to expand its knowledge of landslide mechanisms in sensitive clay to assist authorities with state-of-the-art investigation techniques, hazard assessment methods, risk management schemes, mitigation measures and planning.

Landslides in sensitive clays : from geosciences to risk management

The increasing availability of digital elevation models (DEM) makes
it possible to perform quick slope hazard assessments using semi-automatic
procedures. The main morphological and structural features of a landscape
can, for example, be identified using a DEM, taking into account
its mesh size.
The geomorphological concept of base level, which is defined by the
lowest level that can be eroded by a stream, can be useful for landslide
identification and hazard assessment. Assuming that erosion by landsliding
can affect only a limited thickness of the slope ? i.e. from 0 to
approximately 50 m ? during a period of 1,000 to 10,000 years, a
short-term local base level is thus defined. This concept means that
all slope volumes that are not supported at their bottom can slide
rapidly or slowly down towards the valley.
The computation of the base level uses the streams as invariant levels
of the topography. The base level is thus anchored to the lines defined
by the streams. Depending on the position of the upper part of the
streams compared to the crest, the highest crests of the mountain
range can also be considered as invariant. At the term scale of landslide
activity, only rock-falls can affect the flanks of the crest. Large
landslides occur rarely, because of the low force involved: only
large landslides affecting the entire slope can affect crests. If
necessary, an erosion function can be used to undercut the slope
in order to cause its destabilization.
The calculation of the short-term local base level is derived from
the procedure used to trace the background of a physical signal such
as an X-Ray diffraction spectrum. Different possibilities which can
be divided into two categories exist: 1) static procedures, which
converge to a base limiting short term local base level, and 2) dynamic
approaches that take erosion processes leading to a dynamic base
level depending on the duration of the process of ?base level definition?
into account.
Let us consider that all the slope volume located above the short-term
local base level can slide on this base level surface. Different
procedures allow to compute the ?weight? of the pixels (volume in
excess above the base level) that potentially bring additional stress
directly or indirectly on a pixel situated below. This leads to a
first, simple approximation of the force acting on each pixel. The
procedure for computing the involved pixels is based on a routine
similar to the one for watershed analysis. The streams can be determined
artificially by a routine, in order to avoid the problem of the sedimentation
and/or to take into account the erosion by the streams.
Results indicate a good agreement between highly stressed zones and
observed active rockfalls areas. In order to refine the hazard assessment,
the highly stressed zones can be matched with other parameters such
as fracturing or estimated water table level. The rock-fall activity
indicated by active scree deposits must also be crossed with the
results of the above method that indicates the high likelihood of
large rock-falls.

Jacques Locat

Papers

Characteristics of Several Tsunamigenic Submarine Landslides

Utilisation de la scanographie pour l'étude des sédiments : influence des paramètres physiques, chimiques et biologiques sur la mesure des intensités tomographiques

Submarine Mass Movements and Their Consequences: An Overview

Landslides in sensitive clays : from geosciences to risk management

Toward preliminary hazard assessment using DEM topographic analysis and simple mechanic modeling