Large scale morphodynamics characterisation of exposed sandy beaches by DGPS

Abstract

During the last decades important stretches of the coastal foredunes in the Aveiro Lagoon exposed sandy beaches (Portuguese west coast) were destroyed by wave action, with strong impact on the local social activities. As the foredune erosion and degree of destruction by overwashes are related with the foreshore and backshore morphology, the study of the morphological evolution of these areas of the beach is of special interest. In the present work, time scale of volumetric changes and recovery periods are analysed. A sector located south of an groin field with two contrasting stretches was selected. In one of those stretches, with about 3 kin extension, artificial sand remobilization from the foreshore was operated to build a sand dike. The second one with 4 km extension has evolved without any direct human intervention. In order to determine the morphological evolution of these two stretches, a regular monitoring program was performed by accurate DGPS measurements provided by a multi-antenna system adapted on a four wheel motor quad. Cross correlation analysis between morphological data obtained from DGPS measurements of the exposed beach and wave parameters was done. The wave breaking parameters were computed with the use of a wave model which includes shoaling and refraction. The average beach volume and the average beach face slope were correlated with the average wave height in the breaking zone for several different periods, since the day before to each survey until three months prior to each survey. From the results obtained we conclude that, the recovery period of the sedimentary volumes in the stretch where sand was artificially removed is lower than in the stretch without any human intervention, although this conclusion does not apply for beach face slope recovery.