Long term evolution and internal architecture of a high-energy banner ridge from seismic survey of Banc du Four (Western Brittany, France)

International audienceThe recent completion of a coupled seismic and swath bathymetric survey, conducted across the sand ridge system of the Banc du Four located on the Atlantic continental shelf of Brittany (Mer d’Iroise, France), provided new data for the study of the long term evolution of deep tidal sand ridges. Five seismic units are distinguished within the ridge, separated by pronounced major bounding surfaces. The basal unit is interpreted to be shoreface deposits forming the core of the ridge. It is overlaid by a succession of marine sand dunes fields forming the upper units. Sandwave climbing, which combines progradation and accretion, is the major process controlling the growth of the ridge. The elevation of the preserved dune foresets reaches values of about 20–30 m within the ridge. The foresets indicate a combination of giant dunes characterized by numerous steep (up to 20°) clinoforms corresponding to a high-energy depositional environment. Moreover, the presence of scour pits linked to the 3D geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines. All of the radiocarbon ages of the biogenic surficial deposits of the Banc du Four range from 10,036 to 2748 cal years B.P. and suggest the Banc du Four has grown during the last sea-level rise. The apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non-conservation of recent deposits. In contrast, the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge. Moreover, the long-term evolution of the ridge appears strongly controlled by the morphology of the igneous basement. The multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy, consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement.Therefore, the Banc du Four should be thought of as a representative example of a large-scale high-energy banner bank

Morphological analysis of the upper reaches of the Kukuy Canyon derived from shallow bathymetry

We present preliminary results on the morphology of the upper reaches of the Kukuy Canyon and Selenga shelf in front of Proval Bay (Lake Baikal), derived from newly acquired, high-resolution bathymetry. Numerous and varied erosional and transport features provide an interpretation framework for source to sink transfer and gravity flow processes in this shallow and active tectonic environment, suggesting on-going gravity instabilities and sediment-laden flows. Scarps in the canyon head are likely signatures of retrogressive incision of the western tributary and eastward lateral migration of the western tributary, the latter coming within about 1 km of the shoreline. Immature gullies incising the upper-slope feedings of the Kukuy Canyon indicate gravity flows with low erosional power. Large arcuate scarps on the break of the narrow shelf east of Proval Bay reveal gravity instabilities. The morphological connection between the Selenga Delta and the Kukuy Canyon suggests a direct pathway for fluvial sediment focused through breaches in the Sakhalin sand shoal, with likely occurrence of hyperpycnal flows into canyons heads during high sediment discharges. The neotectonic activity affects both the accommodation space around the prograding delta via earthquake-induced subsidence of coastal areas, and the location of incisions through slope instability triggering. Subsequent surveys allowing diachronic analysis would help determining the influence of tectonic and climatic factors controlling sediment transfer across the land-lake continuum and interpreting the morphological signature of the associated gravity processes shaping the delta and surrounding shelf and canyons

Morphology of pillow-hollow and quilted-cover bedforms in Lake Geneva, Switzerland

Extensive areas of the bottom of Lake Geneva are covered with bedforms that have been referred to as pillow-hollows and quilted-cover pattern by prior observers, as well as with sediment waves and trenches. These structures are decimeter to meter scale. A large dataset of video recording and stereographic camera imagery of the lake bottom was recently acquired during a campaign with the Russian MIR submersibles. We present a classification of the different types of sediment structures with a focus on distinctive morphological characters. The variations in the observed lake-bottom structures reveal a continuous range of morphologies between the aforementioned bedforms. Although the role of the bottom-dwelling Burbot fish in at least maintaining the bottom landscape has been suggested in previous studies, the origin of the observed bedforms is unclear. On the basis of our preliminary observations, other candidate mechanisms are briefly mentioned, including bottom currents generated by internal waves. Understanding the formation and evolution of lake-bottom morphology is important since the transport pathways of lake sediment condition to a large extent the fluxes and cycle of pollutants

A strategy for monitoring systemic vulnerability to marine erosion and flooding

Littoralisation, or the concentration of people and activities in coastal areas, associated with the intrinsic mobility of coasts and with the context of climate change, tends to increase the vulnerability of coastal areas. This article presents a new interdisciplinary approach towards the concept of vulnerability that makes it possible to move beyond the nature/society dichotomy, and an inter-sectorial researcher-manager method for the development of a series of monitoring indicators for the four components of systemic vulnerability: hazards, stakes, management and representations. These indicators are precursors of an integrated observatory that will act as a source of data for research and inform public policy for coastal areas.Le phénomène de littoralisation du peuplement et des activités, associé à la mobilité intrinsèque des côtes et au contexte de changement climatique, tend à accroître la vulnérabilité des territoires côtiers. Cet article propose, d’une part, une approche interdisciplinaire renouvelée du concept de vulnérabilité permettant de dépasser la dichotomie nature/société. D’autre part, il présente une méthode intersectorielle chercheurs-gestionnaires de construction d’une série d’indicateurs de suivi des quatre composantes de la vulnérabilité systémique (aléa, enjeux, gestion et représentations). Ces indicateurs préfigurent un observatoire intégré, à la fois source de données pour la recherche, et au service des politiques publiques pour les territoires côtiers

Tectonic controls on nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, Southern California

CHIRP seismic and swath bathymetry data acquired offshore La Jolla, California provide an unprecedented three-dimensional view of the La Jolla and Scripps submarine canyons. Shore-parallel patterns of tectonic deformation appear to control nearshore sediment thickness and distribution around the canyons. These shore-parallel patterns allow the impact of local tectonic deformation to be separated from the influence of eustatic sea-level fluctuations. Based on stratal geometry and acoustic character, we identify a prominent angular unconformity inferred to be the transgressive surface and three sedimentary sequences: an acoustically laminated estuarine unit deposited during early transgression, an infilling or “healing-phase” unit formed during the transgression, and an upper transparent unit. Beneath the transgressive surface, steeply dipping reflectors with several dip reversals record faulting and folding along the La Jolla margin. Scripps Canyon is located at the crest of an antiform, where the rocks are fractured and more susceptible to erosion. La Jolla Canyon is located along the northern strand of the Rose Canyon Fault Zone, which separates Cretaceous lithified rocks to the south from poorly cemented Eocene sands and gravels to the north. Isopach and structure contour maps of the three sedimentary units reveal how their thicknesses and spatial distributions relate to regional tectonic deformation. For example, the estuarine unit is predominantly deposited along the edges of the canyons in paleotopographic lows that may have been inlets along barrier beaches during the Holocene sea-level rise. The distribution of the infilling unit is controlled by pre-existing relief that records tectonic deformation and erosional processes. The thickness and distribution of the upper transparent unit are controlled by long-wavelength, tectonically induced relief on the transgressive surface and hydrodynamics

AGEO : Natural hazard prevention and awareness raising through citizen observatories

ABSTRACT:The Platform for Atlantic Geohazard Risk Management (AGEO) is a new project co-financed under the Interreg Programme for the Atlantic Area which aims to launch five Citizens’ Observatory pilots on geohazards according to regional priorities.info:eu-repo/semantics/publishedVersio

A study of the dynamics of saturated granular assemblages and its implications for transport, stress propagation and failure in marine sediments

The subject of small-scale marine sediment mechanics is an area of active research, motivated by its key role in beach and shelf processes. Much of the existing small-scale work has focused on two-phase flows and particle behavior at the sediment interface, and their role in sediment transport and shaping seabed morphology. The study presented here considers the behavior of unconsolidated sediments below the seabed, 100 to 1000 grain diameters deep. Motivated by field observations off La Jolla Shores Beach that suggest dynamical behavior at such depths, a series of laboratory experiments were conducted to study the behavior of unconsolidated, saturated granular assemblies. Careful measurements of the apparent mass at the base of dry and saturated, glass beads pilings were acquired to examine stress redirection effects. The experiment protocol included slowly lowering the piling base to achieve reproducible piling configurations through wall friction mobilization. Results are analyzed using the Janssen and Oriented Stress Linearity (OSL) models, the latter allowing interpretation of the arching behavior in terms of arch direction and friction angle along arches. The data show qualitative similarities between dry and saturated granular assemblies, suggesting a commonality in their underlying physics, but quantitative differences in the intensity of the arching mechanism, probably associated with lubrication effects. Analysis of the piling quasistatic descent shows evidence of a progressive upward decompression punctuated with dynamic piling reconfigurations interpreted as relaxation oscillations. Both features are associated with spatial and temporal stress heterogeneities and have implications for modeling the mechanics of granular assemblies. Granular matter theories are expected to reproduce such observations occurring under minute deformations in these laboratory experiments. This work also describes a study of nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, using new seismic data and bathymetry. Isopach maps of the sedimentary sequences overlying the bedrock reveal the critical role of the tectonic landscape on the spatial distribution of modern stratigraphic units. A detailed examination of the morphological characteristics of the Scripps and La Jolla canyons refines the relationship between canyon location and development and regional tectonic deformation, and provides insight on the stability of unconsolidated, saturated marine sediments

A study of saturated granular assemblages and its implications for transport, stress propagation and failure in marine sediments

The subject of small-scale marine sediment mechanics is an area of active research, motivated by its key role in beach and shelf processes. Much of the existing small- scale work has focused on two-phase flows and particle behavior at the sediment interface, and their role in sediment transport and shaping seabed morphology. The study presented here considers the behavior of unconsolidated sediments below the seabed, 100 to 1000 grain diameters deep. Motivated by field observations off La Jolla Shores Beach that suggest dynamical behavior at such depths, a series of laboratory experiments were conducted to study the behavior of unconsolidated, saturated granular assemblies. Careful measurements of the apparent mass at the base of dry and saturated, glass beads pilings were acquired to examine stress redirection effects. The experiment protocol included slowly lowering the piling base to achieve reproducible piling configurations through wall friction mobilization. Results are analyzed using the Janssen and Oriented Stress Linearity (OSL) models, the latter allowing interpretation of the arching behavior in terms of arch direction and friction angle along arches. The data show qualitative similarities between dry and saturated granular assemblies, suggesting a commonality in their underlying physics, but quantitative differences in the intensity of the arching mechanism, probably associated with lubrication effects. Analysis of the piling quasistatic descent shows evidence of a progressive upward decompression punctuated with dynamic piling reconfigurations interpreted as relaxation oscillations. Both features are associated with spatial and temporal stress heterogeneities and have implications for modeling the mechanics of granular assemblies. Granular matter theories are expected to reproduce such observations occurring under minute deformations in these laboratory experiments. This work also describes a study of nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, using new seismic data and bathymetry. Isopach maps of the sedimentary sequences overlying the bedrock reveal the critical role of the tectonic landscape on the spatial distribution of modern stratigraphic units. A detailed examination of the morphological characteristics of the Scripps and La Jolla canyons refines the relationship between canyon location and development and regional tectonic deformation, and provides insight on the stability of unconsolidated, saturated marine sediment

A study of saturated granular assemblages and its implications for transport, stress propagation and failure in marine sediments

The subject of small-scale marine sediment mechanics is an area of active research, motivated by its key role in beach and shelf processes. Much of the existing small- scale work has focused on two-phase flows and particle behavior at the sediment interface, and their role in sediment transport and shaping seabed morphology. The study presented here considers the behavior of unconsolidated sediments below the seabed, 100 to 1000 grain diameters deep. Motivated by field observations off La Jolla Shores Beach that suggest dynamical behavior at such depths, a series of laboratory experiments were conducted to study the behavior of unconsolidated, saturated granular assemblies. Careful measurements of the apparent mass at the base of dry and saturated, glass beads pilings were acquired to examine stress redirection effects. The experiment protocol included slowly lowering the piling base to achieve reproducible piling configurations through wall friction mobilization. Results are analyzed using the Janssen and Oriented Stress Linearity (OSL) models, the latter allowing interpretation of the arching behavior in terms of arch direction and friction angle along arches. The data show qualitative similarities between dry and saturated granular assemblies, suggesting a commonality in their underlying physics, but quantitative differences in the intensity of the arching mechanism, probably associated with lubrication effects. Analysis of the piling quasistatic descent shows evidence of a progressive upward decompression punctuated with dynamic piling reconfigurations interpreted as relaxation oscillations. Both features are associated with spatial and temporal stress heterogeneities and have implications for modeling the mechanics of granular assemblies. Granular matter theories are expected to reproduce such observations occurring under minute deformations in these laboratory experiments. This work also describes a study of nearshore sediment accumulation and submarine canyon morphology offshore La Jolla, using new seismic data and bathymetry. Isopach maps of the sedimentary sequences overlying the bedrock reveal the critical role of the tectonic landscape on the spatial distribution of modern stratigraphic units. A detailed examination of the morphological characteristics of the Scripps and La Jolla canyons refines the relationship between canyon location and development and regional tectonic deformation, and provides insight on the stability of unconsolidated, saturated marine sediment

Apport de la cartographie géomorphologique et géographique dans l'évaluation du risque côtier d'érosion des falaises meubles en Bretagne (France)

International audienceLes falaises meubles formées d'altérites et/ou de formations superficielles sont particulièrement présentes sur les littoraux des latitudes tempérées ayant connu l'alternance de climats froids et tempérés de l'ère quaternaire. Ainsi, sur l'ensemble des pays de l'Union Européenne, elles représentent 11 833 km de côte, soit 11,7 % du linéaire côtier (EUROSION, 2004 ; Carpenter et al., 2012). Alors qu'en Bretagne, cette proportion dépasse 20 % (Hénaff et al., 2018), la dynamique des falaises meubles, qui relève à la fois de processus continentaux et marins, est encore mal cernée. Pour autant, régionalement, la croissance des enjeux exposés à leur recul pose désormais tout autant de questions relatives à leur gestion que sur les côtes d'accumulation. Dans ce contexte, six sites localisés dans les départements du Finistère et des Côtes d'Armor ont été sélectionnés pour faire l'objet d'une étude sur les risques côtiers d'érosion des falaises meubles. L'approche retenue pour cette étude repose sur l'analyse croisée des aléas, des enjeux et de la gestion, et intègre la mise en place depuis le printemps 2017 d'un suivi à long terme des paramètres environnementaux visant à apporter une meilleure compréhension des modalités et des vitesses de recul. Dans le but d'identifier les zones concernées par le risque d'érosion, une cartographie géomorphologique des formations meubles délimitant notamment leur extension en arrière des côtes en considérant l'ensemble du versant, a été réalisée sur chacun des sites. Les formes géomorphologiques observées ainsi que les processus naturels et les facteurs anthropiques générateurs de retrait ont été représentés. Les enjeux (habitations, routes, sentier côtier, champs agricoles…) ont ensuite été identifiés sur chaque secteur. Définissant les zones à risque pour l'aléa érosion côtière comme le croisement de la localisation et de l'extension des formations meubles avec les éléments exposés, des cartes d'enjeux ont été générées sur chaque site. Cette analyse du risque se différencie de l'approche classique consistant à calculer une distance de recul à partir d'un taux d'érosion moyen. La méthode proposée permet de s'affranchir des incertitudes sur la vitesse de recul, variable dans le temps et dans l'espace, et elle permet de circonscrire le risque en délimitant l'ensemble des zones qui sont assujetties aux risques d'érosion côtière et où il semblerait prudent d'interdire toutes nouvelles constructions. Cette démarche est donc adaptée à une gestion sur le long terme des territoires littoraux à falaises meubles, qui est essentielle pour s'adapter aux impacts des changements globaux sur le littoral