Érosion du littoral

Résumé

L’érosion du littoral, appelée aussi érosion côtière, est un phénomène naturel ou anthropique qui se produit en de nombreux endroits du monde. Différents facteurs peuvent induire cette érosion non compensée par l'engraissement local : facteurs planétaires telles que la légère montée des océans et les phénomènes de rééquilibrage isostatique, et facteurs locaux qui déséquilibrent le bilan sédimentaire et la disponibilité de sédiments : conditions hydrodynamiques, courants marins, altération mécanique qui résulte de la pression des mouvements ondulatoires de l'eau (action des vagues et de la houle) particulièrement lors de grandes marées et de fortes tempêtes, suintements d'eau douce, ruissellement d'eau de pluie liée à la dénudation des surfaces favorisée par plusieurs facteurs anthropiques : artificialisation et imperméabilisation des sols, circulation de véhicules, surpiétinement des randonneurs, diminution des apports terrigènes en sable à la suite de la construction de nombreux bar

Source officielle

https://fr.wikipedia.org/wiki/%C3%89rosion_du_littoral

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Effects of entrance conditions on tidal hydrodynamics in idealized prismatic estuaries under sea level rise

Steve Hottinger

Understanding tidal hydrodynamics response to sea level rise (SLR) in estuaries is essential to predict future potential issues such as shoreline erosion, more frequent and intense flooding, coastal wetland threatening, and their economic and ecological associated losses. By going beyond the classic static or “bathtub” approach, this study uses the RMA-2 modelling software to capture the full energy dynamic resulting from SLR and changes in entrance conditions. Indeed, entrance restrictions are seen as a key factor controlling tidal propagation within the entire system and only little knowledge exists on their effects on the flow under SLR. An innovative numerical approach using idealized prismatic estuaries is undertook to fill this gap. More than 150 simulation cases are performed, varying the channels geometry, restrictions dimensions and tidal forcing to capture most of existing prismatic estuarine dynamics. The resulting tidal flow is described using tidal ranges (TR), but also flow velocity, tidal asymmetry, wave types, tidal prism, etc. to find out potential interesting trends. In short channels, tidal reflection is found to be the dominant process influencing tidal propagation while in long channels the flow is mostly controlled by bottom friction. TR are amplified up to 10% per metre SLR, exacerbating the future flooding problematics linked to a higher mean sea level. However, strong TR attenuation is observed when entrance restrictions are implemented, leading to potential insights on SLR effects mitigation using narrower inlets. SLR acts on the flow velocities distribution and on the tidal asymmetry by increasing medium velocities and decreasing flood dominance. Both will impact sediment transport, leading to stronger coastal erosion. This increases therefore the pressure on sensitive ecosystem already struggling to stay in pace with SLR. SLR can bring the channels closer or further from their natural period of sway. This change in tidal resonance can lead to modifications of the flow parameters behaviour. When channels become more resonant with SLR, amplification of the above phenomena are usually seen, while when they are brought away from resonance, attenuation or even reverse effects can be observed. All these issues are not captured by the bathtub approach and arise from changes in tidal dynamics. Furthermore, instead of focusing on specific estuaries as most of the previous studies, this project is using an idealized approach allowing a global evaluation of the future tidal hydrodynamics. Understanding tidal response to sea level rise in estuaries is crucial to develop efficient coastal management plans able to mitigate this threat on estuarine ecosystems and populated areas. In particular, over 130 port cities having a population higher than 1 million people are concerned by SLR issues in estuaries.

2019

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Mercury isotope evidence for Arctic summertime re-emission of mercury from the cryosphere

Jenny Thomas

During Arctic springtime, halogen radicals oxidize atmospheric elemental mercury (Hg0), which deposits to the cryosphere. This is followed by a summertime atmospheric Hg0 peak that is thought to resultmostly fromterrestrial Hg inputs to the ArcticOcean, followed by photoreduction and emission to air. The large terrestrial Hg contribution to the Arctic Ocean and global atmosphere has raised concern over the potential release of permafrost Hg, via rivers and coastal erosion,with Arctic warming. Here we investigateHg isotope variability of Arctic atmospheric,marine, and terrestrial Hg.We observe highly characteristic Hg isotope signatures during the summertime peak that reflect re-emission of Hg deposited to the cryosphere during spring. Air mass back trajectories support a cryospheric Hg emission source but nomajor terrestrial source. This implies that terrestrial Hg inputs to theArcticOcean remain in the marine ecosystem, without substantial loss to the global atmosphere, but with possible effects on food webs.

2022

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The workshop on mapping with drones for natural and built environments has been organised in the framework of the Brazilian-Swiss Joint Research Programme (BSJRP), which supports research activity and academic cooperation. The Ecole polytechnique fédérale de Lausanne (EPFL) is the leading house of the BSJRP and had approved the project proposed by the Geodetic engineering laboratory (TOPO) from EPFL with the Brazilian partners: Universidade Estadual Paulista Júlio de Mesquita Filho (UNESP), Universidade de São Paolo (USP) and Universidade Federal de Pernambuco (UFPE). This group of researchers have developed a joint programme on the use of unmanned aerial vehicles (UAV) for mapping applications. This project is composed of five main tasks with a state of the art, an inventory of analysis tools, the preparation of the workshop and its execution in Recife and finally the writing of these proceedings. The workshop has focused on the potential of new mapping systems based on drones. This topic has been discussed in the framework of potential scientific applications in the field of natural and built environments in Brazil. The main output of the workshop is the identification of the requirements and the research needs in monitoring of natural phenomena (coastal erosion, landslide) and the assessment of transport infrastructures.

2016

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Effects of entrance conditions on tidal hydrodynamics in idealized prismatic estuaries under sea level rise

Steve Hottinger

2019