Assessing the Suitability of Video Imaging for Studying the Dynamics of Nearshore Sandbars in Tideless Beaches

Nearshore sandbars, an important natural defense mechanism of the beaches, can be monitored using shore-based video systems. Before studying bar dynamics with video images, we must establish the relationship between the real bar positions and the videoed bar positions (detected by the preferential wave breaking on the shallows). This analysis becomes essential in the two studied tideless beaches of Barcelona due to the critical differences with respect to the sites studied previously. Bogatell beach is terraced (without a trough) in more than 50% of the profiles. There, the videoed barlines are a good proxy of the terrace edge position. In La Barceloneta beach, with dominance of barred profiles, the videoed barlines better represent the bar crest position. On average, the obtained distances between real and videoed bar positions Delta r are 10-15 m, with the videoed barlines located shoreward. Changes in the bathymetric profile shape and the root-mean-squared wave height H-rms induce a variability of Delta r of 16 m in La Barceloneta and 13 m in Bogatell. This apparent variability masks the real changes in bar position and should preferably be reduced before further analysis. As a highly significant correlation between Delta r and H-rms is detected in the two beaches, the proposed reduction method consists of sampling at a specific range of H-rms. This diminishes the variability by 10% to 14 m in La Barceloneta and 11 m in Bogatell. This paper confirms the suitability of using video systems for monitoring bars and terraces in the Barcelona beaches.