Jacques Locat

Bio: Jacques Locat is an academic researcher from Laval University. The author has contributed to research in topics: Landslide & Submarine landslide. The author has an hindex of 37, co-authored 171 publications receiving 5062 citations. Previous affiliations of Jacques Locat include University of Lausanne.

Scientific ocean drilling behind the assessment of geo‐hazards from submarine slides, Barcelona, Spain, 25–27 October 2006

Abstract: Submarine slope instability represents a geo-hazard for its destructive potential on nearshore structures and life and offshore seabed structures. Submarine slides may bear a tsunamigenic potential and are capable of methane gas release into the seawater and atmosphere. A recent workshop sponsored by the European Science Foundation (ESF; http://www.esf.org), “Scientific Ocean Drilling Behind the Assessment of Geo-hazards From Submarine Slides,” held in Barcelona, Spain, 25–27 October 2006, reviewed the current state of knowledge on submarine slope failures and how scientific drilling can improve our knowledge of the process and help to mitigate the derived risks (a report with full details of participants and program can be found at http://www.geohazards.no/IGCP511/). The workshop gathered 50 scientists and representatives of private companies, mainly from the European area, representing a wide spectrum of disciplines such as geophysics, stratigraphy sedimentology paleoceanography marine geotechnology, geotechnical engineering, and tsunami modeling. During the workshop, it was agreed that scientific drilling offers the possibility of answering a number of scientific questions, among them, (1) What is the frequency of submarine slides? (2) What was the tsunamigenic potential of past submarine slides, and what is the tsunamigenic potential of unfailed submarine slopes? (3) Do precursory phenomena of slope failure exist? (4) Can we monitor sea-floor gravitational movements such as creep? (5) What makes up weak layers in midlatitude continental margins? And (6) when and under what circumstances do weak layers form? Scientific drilling also offers the possibility of testing at least two existing hypotheses on basic mechanisms of submarine slide generation and of massive releases of gas: (1) focusing of fluids and lateral transfer of stresses under variable overburden on permeable layers and (2) proving the link between methane emissions during rapid climatic changes and submarine slides.

Resistance au cisaillement sous faible consolidation et structuration des argiles marines

Abstract: La mesure de la resistance au cisaillement d'argiles intactes en place ou en laboratoire est parfois impossible, que ce soit a cause des limitations des equipements disponibles ou de la difficulte d'acceder au depot (fonds marins...). Il est encore important d'examiner si, a partir d'echantillons remanies de ces memes depots, il est possible de recueillir des donnees propres au depot. Il est propose ici une methode d'analyse en laboratoire a partir d'echantillons remanies. Ainsi, des depots d'argile marine, mers de Tyrrell et de Champlain (Quebec), maintenant emerges mais relativement jeunes (6000 a 12000 ans) ont ete etudies sur le terrain puis en laboratoire avec des echantillons non remanies. Puis les echantillons furent totalement remanies et consolides en laboratoire sous de faibles contraintes (7 a 150 kPa) ; ces echantillons furent ensuite soumis a des essais de penetration au cone suedois. On a observe que l'evolution de Cu (resistance au cisaillement non drainee) en fonction de la contrainte effective de consolidation se decompose en trois zones distinctes analogues aux zones observees sur les depots d'echantillons intacts ou sur le chantier. Ces resultats montrent que, au moins pour le cas des argiles de l'Est du Canada, caracterisees par une faible degradation de la mineralogie de la roche mere, il est possible de prevoir le comportement des depots d'argile en placce a partir d'essais de laboratoire sur des echantillons remanies. (A). (Titre anglais : Shear strength under low consolidation and structuration of marine clays).

Late Wisconsinan deglaciation and proglacial lakes development in the Charlevoix region, southeastern Québec, Canada

Abstract: The Charlevoix region, in southeastern Quebec, is characterized by a dramatic landscape formed by the junction of the Laurentian Highlands, the Charlevoix Astrobleme and the St Lawrence Estuary. At the Last Glacial Maximum (LGM), the region was completely covered by the Laurentide Ice Sheet (LIS). The complex topography of the region was the stage of many of the major deglacial events of southern Quebec (e.g. Goldthwait Sea Invasion, St Lawrence Ice-Stream, Saint-Narcisse Episode). We present a detailed reconstruction of the pattern of retreat of the LIS in the Charlevoix region based on the interpretation of ice-marginal features (e.g. moraines, fans) and glaciolacustrine landforms and deposits, two extensive field campaigns, and the interpretation of high-resolution 3D digital aerial photographs and LiDAR data. Our results indicate five moraine complexes in the region: the Rochette, the Brulee, the Sainte-Anne, the Saint-Narcisse and the Mars-Batiscan complexes. Deltas, fans, fine-grained sediments, littoral deposits, drainage breaches and deposits were used to identify 91 palaeo-proglacial lakes. The identification of these lakes and their relation to moraine complexes enabled the reconstruction of six stages of lake development during the Charlevoix deglaciation. The development of proglacial lakes occurred in all types of terrain (highlands, lowlands, transitory levels above marine limit). We conclude that local topography had a decisive effect on promoting both moraine deposition and lake development. We suggest that similar topographical regions (hilly-mountainous) that were affected by major ice-margin stabilizations during glacial retreat should have experience small lakes dominating valleys and topographical lows.

Co-evolving delta faces under the condition of a moving sediment source

Abstract: A fan delta developed under the constraints of a moving point of flow-sediment supply is described. The sediment mixtures used in the experiments were chosen to approximate field conditions, with the coarser fractions depositing to form the topset (subaerial delta surface) and foreset, and the finer fractions forming a turbidity current that plunges near the shoreline, overrides the foreset, and continues into deeper water to emplace a bottomset. The migration rate of the feed point was adjusted so that the base of the lateral foreset along one side just touched the flume wall, so leaving open water on that side. The experiments provide insight into the sedimentation processes associated with these three co-evolving structures of the delta, i.e. its topset, foreset, and bottomset. The results indicate that the lateral delta extent can be controlled via the downstream migration of its sediment source.

Étude de la formation des sédiments fins glacio-lacustres du lac Barlow-Ojibway : le site d’Olga, Matagami, Québec

Abstract: Le site choisi est situe a moins de 10 km au nord-est de Matagami, Quebec, sur le Territoire de la Baie-James. Cet article resume les principales informations geologiques obtenues et insiste sur l’analyse detaillee des sediments fins. Au site de la digue d’essai d’Olga, le depot est constitue, a la base, d’un complexe de till et de sable et gravier fluvio-glaciaires, d’un depot glacio-lacustre et lacustre comprenant une zone alteree au sommet recouvert d’une couche de matiere organique. Le tout repose sur des roches metamorphiques et volcaniques de la province du Superieur. Les sediments fins glacio-lacustres comprennent des varves distales pouvant atteindre plus de 2,5 cm d'epaisseur a la base de la section. Les sediments fins ont fait l’objet d’analyses mineralogiques et physico-chimiques detaillees. La mineralogie est dominee par les mineraux non phyllosilicates et le complexe absorbant du sediment est sature en calcium. La teneur en eau moyenne varie de 53% a 74% dans les varves, mais peut atteindre 110% dans les couches foncees. La teneur en eau est legerement au-dessus de la teneur en eau d’equilibre avec la pression des terres. L’etude illustre l’utilisation de l’epaisseur des couches et de la quantite de sediments qu’elles contiennent dans l’analyse de la mise en place et du taux de sedimentation. Les taux de sedimentation et d’accumulation sont non lineaires dans le temps et ont varie, depuis le debut de la formation du depot, de 2,7 cm/an a 0,8 cm/an et de 2,6 g/cm2/ an a 0,7 g/cm2 /an respectivement.

The physical character of subaqueous sedimentary density flows and their deposits

Abstract: 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.

Recommendations for the quantitative analysis of landslide risk

Abstract: 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.

Use of LIDAR in landslide investigations: a review

Abstract: 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.

Bioturbation: a fresh look at Darwin's last idea

Abstract: 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.

Subaqueous sediment density flows: Depositional processes and deposit types

Abstract: 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.