No abstract

International audienceThe morphology of rocky coastlines is the expression of the interaction between external forcings and the intrinsic rock mass properties, leading to a specific erosional response. Among these rock mass properties the contribution of structural discontinuities is still poorly understood. Nevertheless, recent studies suggest that geological structures may have a first order influence in controlling the coast morphology. Understanding how the structural pattern interacts with erosion processes is fundamental to assess the evolution of these environments. We propose a regional comparison between the orientation of rocky shorelines and faults segments in six different litho-structural units in the Provence and Cote d'Azur area (southern France). Our analysis reveals that in many of these sectors the dominant fault trend (E-W) is also present in the distribution of coastlines orientations. In some particular areas, a complex geological inheritance (Maures) blurs this apparent relationship between faults and coastal morphology. We conclude that, in our study area, the level of complexity of the structural setting seems to significantly control the nature and shape of the coastline

Cet article présente une recherche interdisciplinaire associant sociologie et géologie. L'analyse porte sur la vulnérabilité urbaine vis-à-vis de l'érosion des falaises côtières. Le terrain d'étude est la commune de Carry-le-Rouet dont le littoral est constitué de falaises calcaires associées parfois à de la calcarénite, en alternance avec des niveaux sableux et argileux moins consolidés, d'une hauteur maximale de 40 m. L'analyse révèle que certains des quartiers les plus luxueux de la commune sont les plus exposés au risque d'érosion, échappant au « classique » cumul des vulnérabilités sociales et environnementales. Les riverains n'ignorent pas le risque, mais, attachés à une culture anthropocentrée et techniciste, ils occultent leur trajectoire de vulnérabilité. En outre, la capacité d'action de ces populations aisées leur permet de capter à leur profit les bénéfices de la solidarité territoriale, par l'obtention de travaux de renforcement financés par la puissance publique, tout en revendiquant un entre-soi socialement et ethniquement sélectif.

Terrestrial laser scanners (TLS) are well known for providing an efficient means to monitor coastal cliff erosion. Cliffs along micro-tidal coasts, however, have often escaped quantification because the narrow or absent coastal platforms do not offer stable and embracing vantage points. To circumvent this issue, mobile laser scanning surveys from a boat can be used. We present a case study from Provence-Alpes-Côte-d'Azur (Mediterranean coastsouthern France) to quantify cliff erosion in such micro-tidal environments. Three surveys were sub-contracted over a period of 17 months to monitor a 3.5-km-long cliff of Carry-le-Rouet (15km west of Marseille). Data quality was check independently using man-made planar walls positioned above the cliff face, to retrieve survey precision and change detection thresholds.Boat-borne mobile lidar system was capable of describing planar features with a precision of 3-4 cm (epoch 1 and 2) and improved to 2.6 cm (epoch 3) with point densities around 100 pts/m². Absolute positioning accuracy varied between 0.1 cm and 0.3 cm. Because the coastline is very sinuous, we describe a method to unfold the point clouds using a continuous analytical surface made of vertical planes joined by arcs of cylinders and perform the analysis in 2.5D.Change was detected using a unique, conservative, threshold of 14 cm (99% quantile estimated on plane change) on grids of 10 x 10 cm pixels. Integrated over the entire cliff face, the average annual cliff recession rate at Carry-le-Rouet is 1.1cm/year. In 17 months, erosion was three times more effective in sandstone and marls layers than in calcarenites and conglomerates.Erosion varies vertically with erosion three times more effective in the lower 25 m of the cliffs than above. Despite imperfections, boat-borne laser scanning system are capable of delivering meaningful erosion data even in this low erosion context.

Les politiques modernes de gestion des catastrophes naturelles au niveau national, européen et mondial identifient la connaissance des impacts dommageables des évènements comme un des principaux défis de la prévention. En effet, si celle-ci existe dans beaucoup de pays, elle est trop souvent déconcentrée et morcelée entre des acteurs de la gestion des risques qui dialoguent trop peu souvent, notamment entre les secteurs public et privé. En France, L'Observatoire National des Risques Naturels (ONRN) est un exemple de partenariat public-privé qui réunit Etat, assureurs et réassureur et qui repose sur une gouvernance concertée associant les collectivités représentées par l'AMF et des partenaires de projet volontaires (EPTB, observatoires locaux…) pour partager données, expertises et études. Ceci dans le but d'améliorer la connaissance des risques pour réduire la vulnérabilité des territoires. Il a entrepris plusieurs initiatives dans ce sens en partenariat avec des acteurs locaux de la gestion des risques. L'une d'entre-elles est présentée dans le cadre de cet article. Menée en partenariat avec l'Observatoire Régional des Risques Majeurs en Région PACA, elle avait pour but d'établir un diagnostic de la vulnérabilité du département du Var au risque d'inondation à partir des indicateurs produits par l'ONRN et d'informations fournies par des acteurs locaux. Elle a permis de mettre en regard la situation du territoire et les efforts de prévention, et a vocation à terme à constituer un outil d'aide à la décision aux politiques publiques de gestion du risque d'inondations. Mots-clés : inondations, vulnérabilité, observatoire, public, privé, gouvernance concertée, prévention des risques, indicateur The analysis of the vulnerability to flood hazard in the PACA region: the contribution of observatories Modern policies of natural disasters management at national, European and global level emphasize the fact that the knowledge of the impacts of damaging events is one of the main challenges for prevention. Indeed, while it does exist in many countries, that knowledge is too often decentralized and fragmented between risk management stakeholders that talk too infrequently to each other, especially between public and private sectors. In France, the National Observatory of Natural Hazards (ONRN) is an example of public-private partnership that brings together state, insurers and reinsurers, and is based on participative governance involving the communities represented by the AMF and voluntary project partners (EPTB, local risk observatories..) to share information, expertise and studies. It aims at improving the knowledge of the risks to reduce the vulnerability of territories. It has taken several initiatives in this direction in partnership with local stakeholders involved in risk management. One of them is presented in this article. Conducted in partnership with the Regional Observatory of Major Risks in the PACA region, it aims at diagnosing the vulnerability of the Var department to the risk of flooding, based on the indicato...