Frédéric Pouget

Monitoring the Topography of a Dynamic Tidal Inlet Using UAV Imagery

Unmanned Aerial Vehicles (UAVs) are being increasingly used to monitor topographic changes in coastal areas. Compared to Light Detection And Ranging (LiDAR) data or Terrestrial Laser Scanning data, this solution is low-cost and easy to use, while allowing the production of a Digital Surface Model (DSM) with a similar accuracy. Three campaigns were carried out within a three-month period at a lagoon-inlet system (Bonne-Anse Bay, La Palmyre, France), with a flying wing (eBee) combined with a digital camera. Ground Control Points (GCPs), surveyed by the Global Navigation Satellite System (GNSS) and post-processed by differential correction, allowed georeferencing DSMs. Using a photogrammetry process (Structure From Motion algorithm), DSMs and orthomosaics were produced. The DSM accuracy was assessed against the ellipsoidal height of a GNSS profile and Independent Control Points (ICPs) and the root mean square discrepancies were about 10 and 17 cm, respectively. Compared to traditional topographic surveys, this solution allows the accurate representation of bedforms with a wavelength of the order of 1 m and a height of 0.1 m. Finally, changes identified between both main campaigns revealed erosion/accretion areas and the progradation of a sandspit. These results open new perspectives to validate detailed morphological predictions or to parameterize bottom friction in coastal numerical models.

Variability of fresh- and salt-water marshes characteristics on the west coast of France: A spatio-temporal assessment

The degradation of water quality and the multiple conflicts of interest between users make marsh restoration very important. A Water Quality Evaluation System (WQES) was developed for river systems by the European Water Framework Directive (WFD). Some form of biologically-based, habitat-specific reference standard seems absolutely essential for wise management and stewardship of marsh ecosystems. The goal of this study was to develop a statistical method to define and to characterize a water body typology for drained marshes of the Charente-Maritime wetlands on the French Atlantic coast, placing particular emphasis on environmental factors as hydraulic functioning, human activities and pedological substratum. The Charente-Maritime marshes represent a good field study because of his high diversity of types of marshes and of anthropogenic activities in a restrictive area thus erasing spatial climatic effect (latitude effect). The statistical method developed here had permitted to define and characterize 12 different water bodies, 7 in freshwater (F1 to F7) and 5 in salt water marshes for the Charente-Maritime area. This typology demonstrated an important link between the size catchment area, nitrate concentrations, and leaching of precipitation from cultured soils. Even though the Charente-Maritime marshes are strongly impacted by humans, they may still retain the ability to remove nitrate. The increasing gradient of water renewal in the freshwater marshes from F1 to F7 explained the decreasing gradient of eutrophication. A better management of the hydrodynamic of the marshes can avoid eutrophication risk on the coastal sea area. Reliance on the WFD parameter set necessarily placed limits on the kinds of interpretations that could be made and on the studys potential contribution to the basic science of marshes. Ecologically-based insights regarding both external flows (links between ecosystems, meta-ecosystem theory) and internal flows (structure of the planktonic food web) seem an essential prerequisite for further advances in the study of marsh ecosystems.

Évolution spatiotemporelle de l’exposition humaine face au tsunami à Padang - Diagnostic de la vulnérabilité et des capacités d’évacuations à l’échelle infra-urbaine

Cet article resume une demarche integree visant a evaluer l’exposition humaine et ses variations spatiotemporelles en cas de tsunami dans une zone urbaine littorale en Indonesie, ainsi que la capacite d’evacuation vers des refuges. Ce travail de recherche systematise des methodes permettant d’estimer l’effectif de population presente a tout moment de la journee, n’importe quel jour de la semaine et de l’annee, a une echelle microscopique, dans une zone urbaine. Il se fonde pour cela sur une hypothese de rythme de vie controlant les activites et donc la distribution de la population. L’heure d’arrivee d’un tsunami etant imprevisible, ces informations sont tres importantes pour ameliorer les programmes de reduction du risque. Cette demarche permet ainsi de degager des scenarios types de distribution de la population, utilises ensuite pour evaluer la capacite d’evacuation de ces populations. Le modele de simulation issu de cette recherche permet de mesurer l’accessibilite des zones refuges selon certains scenarios, et d’etablir un diagnostic precis des capacites d’evacuation des civils face au tsunami.