BEACH AND SURFZONE MORPHODYNAMICS OS CASSINO BEACH, RS USING VIDEO IMAGERY TECHNIQUES AND BEACH PROFILES

Abstract

During April and June of 2005, a major nearshore processes experiment was conducted at Cassino beach, RS, Brazil. Temporal changes obtained by 71 daily beach profile surveys, simultaneously with an Argus video imaging database and hydrodynamic data indicated that low and high beach mobility displayed by the beach profile envelope were respectively associated with the subaerial and subaqueous portion of the beach, being the latter caused by bar migration. A 15 m shoreward berm shift was observed during a storm with significant wave height of 3 m and high elevation of sea level above the mean shoreline location. Three bars were identified, being the first an inter-tidal bar, periodically exposed due to wind setup, and the other two sub-tidal bars located at average distances of 99 and 237 m from the shoreline. The first bar location was observed to have high mobility in short time scales (hours), and appears to be controlled by swash and backwash processes. The second bar is more stable in time scales of hours, but has higher mobility in time scales of days. It was observed offshore migration rates as high as 11.6 m/day for this bar, associated with the presence of a rip current over the profile location. Shoreward migration rates reached 9.3 m/day, and were linked to low dynamic conditions, with significant wave height between 0.5 m and 0.75 m. During 13 days of low hydrodynamic conditions, the second bar became attached to the inner bar after migrating 45.5 m, simultaneously to a rhythmic feature development. The whole sequence documented by the Argus station at the field site indicated high wave dissipation during the period of attached bars.