Morphology evolution of an ebb-tidal delta following a storm perturbation : assessments from remote sensed video data and direct surveys
Résumé
Tidal inlet morphology is strongly dependent on local hydrodynamic conditions that rely on wave/tide dominance. In some places, this hydrodynamic context exhibits a very strong seasonal pattern, showing tide-induced morphologies in summer and wave-induced ones in winter. The Barra Nova tidal inlet in Algarve, South Portugal, presents such characteristics. Short term morphological changes were investigated during an extensive field campaign carried on in February and March 1999, after the winter storm period. This study integrates both topo-bathymetric surveys and remotely sensed video data in order to assess the morphological evolution of the inlet system during the transitional period following storms. Comparison of datasets issued from these two direct and indirect methods has shown a very good agreement (errors usually less than 10 cm in elevation) that allows the quantification of morphological changes of the entire inlet system during the whole period of video observation.
During a transitional period after winter storms, ebb shoals adjust rapidly to the new low wave energy conditions. They migrate eastwards (50 m in two weeks for the main sand bar of the downdrift swash platform), resulting in a global downdrift trending of the whole delta. This evolution, closely linked to the previous development of storm-induced features, tends to close the system. Despite a large retreat of Barreta Island during the storm event before the campaign, simultaneous downdrift migration of the inlet channel was not observed. Our results suggest that such a migration can occur only at the end of the transitional phase once the storm induced morphologies have been destroyed.