Determining ridge and runnel longshore migration rate using Spot imagery

• Published in: Oceanologica acta 2002-05, Vol.25 (3), p.149-158
• Main Authors: Lafon, Virginie, Dupuis, Hélène, Howa, Hélène, Froidefond, Jean-Marie
• Format: Article
• Language: English
• Subjects: Aquitaine coast; Coastal oceanography, estuaries. Regional oceanography; Coastline; Couleur de l’océan; Côte aquitaine; Déplacement des barres littorales; Earth, ocean, space; Exact sciences and technology; External geophysics; Marine; Ocean colour; Physics of the oceans; Remote sensing; Ridge and runnel movement; Trait de côte; Télédétection
• ISSN: 0399-1784
• DOI: 10.1016/S0399-1784(02)01190-8

High-resolution colour satellite imagery is used to map the morphology and to determine the longshore movements of intertidal ridge and runnel systems that characterise the Atlantic southwest coast of France. A couple of Spot images recorded during the summer of 1989 are processed to extract accurate geographical maps of the coastline defined, in this study, as the level corresponding to the lowest astronomical tide. The coastline extraction method is based on a reflectance model, providing depth measurements that were calibrated in the field. This method enables us to map the coastline regardless of the tide level above the lowest astronomical tide at the moment of the image capture. As this model can be extrapolated to a large area when sea water is clear (considering a restricted range of suspended matter concentration) and bottom colour is homogeneous, the variability of both water turbidity and bottom colour have been analysed on the images. From the remotely sensed coastlines, a statistical analysis of ridge and runnel system length and longshore migration has been performed for a 35 km long coastline located to the north of the Arcachon lagoon inlet. It has been observed that, during the summer of 1989 characterised by fair weather conditions, the typical ridge and runnel morphology remains nearly conservative. Whereas the ridge and runnel mean wavelength remains constant during the summer between 435 and 430 m, about 84% of the systems move southward at the mean rate of 2.4 m d –1. These results derived from Spot image analysis were validated by in situ topographic surveys undertaken during similar hydrodynamical conditions. L’imagerie satellitaire à haute résolution spatiale est utilisée afin de caractériser la morphologie et les déplacements des systèmes de barre–baïne intertidales situés sur la côte sud-ouest de la France baignée par l’océan Atlantique. Deux images Spot, enregistrées durant l’été 1989, ont été analysées afin de cartographier précisément le trait de côte défini, dans cette étude, comme le lieu du zéro hydrographique. La méthode d’extraction du trait de côte est basée sur un modèle de réflectance calibré sur le terrain, qui met en relation la réflectance avec la profondeur. Cette méthode permet d’obtenir une cartographie du trait de côte, quelle que soit la hauteur d’eau au dessus du zéro hydrographique. Comme ce modèle peut être appliqué à un long linéaire côtier lorsque la turbidité de l’eau varie peu et que la couleur du fond e