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.

/pdf/recommendations-for-the-quantitative-analysis-of-landslide-1culjw5db2.pdf

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

Quantitative mineralogical analyses of sediment samples from 18 sites in Quebec indicate that plagioclase is the most abundant mineral in all soils and that chlorite is generally the dominant clay mineral followed by illite and expandable clays consisting of mixed-layer clays minerals involving vermiculite. The study also illustrates how specific surface area and constitutive water content can be used to evaluate departures from average in the mineralogical composition of sensitive clays. The relationship between activity and specific surface area of sensitive clays shows that when compared to soils from different sedimentary basins that have different mineralogy, they are characterized by a much lower activity and specific surface area.

Nature of Sensitive Clays from Québec

As part of the STRATAFORM project, a series of cores were obtained from the Eel River Margin area of Eureka, California. The geotechnical analysis of intact specimens and of reconstituted samples provides some insight on the development of shear strength with burial. The results show the effect of bioturbation in the early part of the lifetime of a sediment. SEDCON tests were used to proposed various relationships which help predict the changes in density, liquidity index, and strength as a function of depth. These relationships are found useful from near the water sediment-interface down to a depth of at least 400 m in the sediment column.

Shear Strength Development with Burial in Eel River Margin Slope Sediments

Strain Rate and Structuring Effects on the Compressibility of a Young Clay

Landslide hazard assessment can be improved by using new digital documents such as digital elevation models (DEM). Geographic Information System (GIS) and geo-referenced documents being more and more available, the geomorphologic theories and methods that were developed before the computer age are now being updated and improved. They yield new insights for landslide hazard assessment. Slope processes and associated morphology were extensively studied during the last two centuries (Gilbert 1880, De La Noe & De Margerie 1888). Terrain slope is the first way to qualify the topography (Strahler 1950) and the basic information used for slope modeling (Young 1972). Terrain slope depends on many factors such as weathering and rock types (Young 1961). In rock slopes, joints play an important role on the slope shaping (Terzhaghi 1962, Scheidegger 1980, Selby 1982). Slope stability is dependent on rock mechanical properties. On one hand, landslides in loose materials (for example in sand piles) are often located within a defined terrain slope angle range (Rautela & Thakur 1999). On the other hand, rock slope stability is strongly controlled by discontinuities that decrease rock strength. Thus, the analysis of the topography reflects the mechanical and structural features of the slope (Locat et al., 2000). Digital elevation models (DEM) makes it possible to perform more detailed and systematic morphological analysis. Using the orientation of each single cell, a DEM can be represented by 3D shaded relief that displays one color for each dip and direction of dip. This kind of representation permits a very simple slope analysis providing quick information for slope hazard assessment. Different cases in the western Swiss Alps are presented.

New insight of geomorphology and landslide prone area detection using digital elevation model(s)

Le fjord du Saguenay est un laboratoire geoscientifique naturel exceptionnel. Sa physiographie et ses caracteristiques geomorphologiques, revelees par les leves multifaisceaux et de sismique-reflexion ainsi qu’une serie de carottages, nous fournissent un eclairage nouveau sur l’evolution du fjord du Saguenay a partir du debut de la deglaciation du fjord, il y a environ 11 000 ans, ainsi que sur l’enregistrement des differentes catastrophes naturelles majeures qui se sont produites dans la region. De plus, a cause de sa morphologie, le fjord du Saguenay est aussi un endroit de predilection pour l’analyse des changements climatiques dans cette region du continent nord-americain. A partir de l’analyse morpho-sedimentologique du fjord du Saguenay, nous proposons de le subdiviser en trois parties : le superieur, le moyen et l’inferieur. Le Saguenay inferieur correspond a la zone comprise entre Tadoussac (kilometre zero), a une profondeur d’environ 20 m, et jusqu’a environ 42 km en amont. Ce secteur est domine par une morphologie glaciaire avec tres peu d’accumulations postglaciaires. Le Saguenay moyen comprend la partie profonde du Saguenay comprise entre l’ile St-Louis et la confluence de la baie des-Ha! Ha! et du Bras‑Nord. On y retrouve surtout des accumulations hemipelagiques, turbiditiques et des coulees de debris. Finalement, il y a le Saguenay superieur qui correspond a la zone amont qui inclut le Bras‑Nord et la baie des‑Ha! Ha!. Ce secteur represente une zone d’accumulation et de comblement, surtout dans le Bras‑Nord ou on retrouve le delta de la riviere Saguenay. Le fjord, en apparence tranquille, nous a revele plusieurs traits geologiques interessants, dont la presence de plusieurs glissements sous-marins, la presence importante de gaz dans la partie profonde et des evidences d’activites sismiques reliees aux seismes de 1663, 1870, et possiblement 1988.

/pdf/le-fjord-du-saguenay-une-physiographie-et-un-registre-3ia4zn7iju.pdf

Jacques Locat

Papers

Nature of Sensitive Clays from Québec

Shear Strength Development with Burial in Eel River Margin Slope Sediments

Strain Rate and Structuring Effects on the Compressibility of a Young Clay

New insight of geomorphology and landslide prone area detection using digital elevation model(s)

Le fjord du Saguenay : une physiographie et un registre exceptionnels