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

A geomorphological analysis of the Veatch slide complex is presented here indicting the presence of at least two major slides presenting different morphological characteristics and involving different type of sediments (in terms of strength and grain size). These slides took place after the erosion of the Veatch Canyon which had been the locus of many bank failures. It is shown that a more refined morphological analysis of similar landslide areas may lead to the definition of a greater number of slides and a greater role of channels both leading to a more complex interpretation on their mobility.

A Geomorphological Analysis of the Veatch Slide Complex Off Massachusetts, U.S.A.

Debris-flows are able to transport large quantities of sediment downslope, producing complex distributions of deposits and eroded surfaces along their flow path. Incorporation of surficial deposits during a debris-flow may change the mechanical behaviour of the flow. This paper presents the results of various rheological tests and numerical modelling for assessing torrential risk scenarios in the Faucon torrential stream where a debris-flow occurred in 1996, after a severe thunderstorm over the catchment basin and the breaking of a natural dam. Grain-size distribution and petrographic analysis have shown that this debris-flow can be characterized as a granular then a muddy debris-flow. Rheological tests using either a parallel-plate rheometer, a coaxial rheometer, slump tests, or a inclined plane were carried out on several samples. Results have shown that the flow behaviour could be described by a Heschel-Bulkley constitutive equation. Rheological response of several natural suspensions collected from quaternary deposits were also investigated. In order to model the runout of the flow, we used the BING code. Model describes well the influence of each type of sediment on the behaviour (runout distance, deposit thickness) of the flow, neither the velocities were overestimated. Different risk scenarios are tested and discussed.

Torrential hazard assessment using a debris-flow runout model. The case of the Faucon stream

The Government of Quebec recently initiated the deployment of a vast groundwater pressures monitoring network in postglacial marine clays to document their variations in time and improve our understanding of the relationship between failure initiation and climate in clay slopes. This project aims at evaluating the impacts of climate change on clay-slope stability and how it can be integrated in landslide risk management to improve public safety. Hydrogeological data will be acquired at sites located throughout the Quebec Province’s post-glacial clay deposits to create a public georeferenced index of typical hydrogeological conditions. The project goes beyond the characterization of groundwater pressures and their variations in clay slopes. Indeed, slope deformation will be measured at several sites. Also, two sites in flat terrain will be instrumented in order to evaluate mechanical properties of clay layers in simple 1-D conditions and groundwater recharge. The unsaturated clay crust in slopes susceptible to superficial landslides will be characterized and instrumented. The current lifetime of the monitoring project has been set to a period of 25 years.

/pdf/development-of-a-long-term-monitoring-network-of-sensitive-3eyl4h058w.pdf

Development of a long term monitoring network of sensitive clay slopes in Québec in the context of climate change

La difficulte d'une analyse de risque a l'echelle regionale se heurte
en general au manque de connaissances permettant d'estimer la probabilite
de rupture des instabilites. Ce deficit est notamment lie a l'absence
d'inventaires des evenements statistiquement representatifs. Tirant
profit de la presence d'un tel inventaire ainsi que de nombreuses
donnees georeferees (ne dit-on pas georeferencees, ou est-ce un anglicisme
?), une methode d'estimation continue de l'alea et du risque le long
du Promontoire de Quebec a ete developpee.
La ville de Quebec est bâtie le long de la rive nord du Fleuve St.
-Laurent, sur un Promontoire dont la hauteur varie entre 60 et 100
metres. Depuis 1775, de nombreux evenements catastrophiques (y-a-t'il
eu de si nombreux evenements catastrophiques ?) ont touche l'autoroute
ainsi que les quartiers de la ville qui se situent au pied de la
falaise. Pres de 90 personnes ont ete tuees et 70 blessees, pour
la plupart avant 1900 (date de mise en oeuvre des premieres mesures
de confortement).
L'estimation de l'alea est basee sur cinq facteurs d'instabilite,
qui ont ete modelises a l'aide d'un Systeme d'Information Geographique
(SIG): (1) l'orientation et (2) la pente de la topographie, (3) l'epaisseur
maximale erodable du massif rocheux, (3) la taille des bassins-versants
topographiques et (5) les resultats de tests cinematiques. Des valeurs-seuils
(?) ont ete definies pour chaque facteur d'instabilite, sur la base
de criteres physiques ou statistiques. Chaque facteur d'instabilite
? ou combinaison de facteurs d'instabilite ? est considere comme
significatif (la valeur "1" lui est attribue) si il (s'il?) respecte
soit un critere unique (i.e. un facteur d'instabilite est superieur
a sa valeur-seuil), soit un critere croise (i.e. plusieurs facteurs
d'instabilites sont simultanement superieurs a leurs valeurs-seuil(s?)
respectives). Une densite d'instabilite relative a ete calculee en
sommant les valeurs attribuees aux facteurs d'instabilite ou a leurs
combinaisons. La methode a ete calibree a l'aide de l'inventaire
des evenements, en utilisant un pas d'echantillonnage de 100 m. L'adequation
entre la densite d'instabilites calculee et celle deduite de l'inventaire
est bonne.
Les principales structures susceptibles de generer des instabilites
ont en outre ete analysees en detail. Des tests cinematiques plus
precis ainsi que des calculs de facteurs de securite ont ete effectues,
permettent d'affiner l'estimation de la densite d'instabilites calculee.
La frequence annuelle d'instabilites le long du Promontoire de Quebec,
deduite de l'inventaire, a ensuite ete repartie par tranches de 100
m, proportionnellement a la densite d'instabilites calculee. Une
probabilite de rupture pour chaque tranche de 100 m a ainsi pu etre
calculee. Comme il est raisonnable de faire l'hypothese que chaque
instabilite qui se detache de la paroi atteint le pied de la falaise,
la probabilite de propagation est egale a 100 %. L'alea est par consequent
equivalent a la frequence.
La probabilite qu'une maison soit touchee est ensuite obtenue en multipliant
la frequence annuelle par le pourcentage de maisons pour chaque tranche
de 100 m consideree. Le risque peut enfin etre calcule en multipliant
cette valeur par la vulnerabilite et la valeur de chaque element
a risque considere.
L'utilisation de valeurs-seuil(s) permet a la methode d'etre utilisee
a la fois pour un degrossissage au bureau a l'aide de documents numeriques
et sur le terrain par une appreciation subjective de la susceptibilite.
Ne devrait-on pas parler davantage des resultats? ?

Estimation continue de l'aléa et du risque à l'aide d'un SIG: L'exemple du promontoire de Québec (Québec, Canada)

The erodability of surficial sediment in Saguenay Fjord, Quebec, was investigated over a three-year period at fourteen sites in Baie des Ha! Ha! and the North Arm using a benthic flume, Miniflume. Previous studies showed that Saguenay Fjord sediment characteristics vary greatly on small and large scales. The critical shear stress (τc) reflect this variability, with τc between 0.07 and 0.48 Pa. The measured critical shear stress and erosion rates were consistent with those of other in situ studies in other area. The variability in the results is linked to the great spatial variability of benthic sediments rather than to temporal variability. Some of the results were integrated in the Parchure and Mehta (1985) erosion law since the upper portion of the active sediment layer is typically a stratified bed. The identification of erosion parameters (critical shear stress, erosion rate, parameters α?and Ef in erosion law) and their high variability is the first step in defining the erosion susceptibility hazard in Saguenay Fjord.

Jacques Locat

Papers

A Geomorphological Analysis of the Veatch Slide Complex Off Massachusetts, U.S.A.

Torrential hazard assessment using a debris-flow runout model. The case of the Faucon stream

Development of a long term monitoring network of sensitive clay slopes in Québec in the context of climate change

Estimation continue de l'aléa et du risque à l'aide d'un SIG: L'exemple du promontoire de Québec (Québec, Canada)

In situ Flume Measurements of Sediment Erodability in Saguenay Fjord (Quebec, Canada)