Rippled scour depressions characterization through interferometric data at Barra da Lagoa — Moçambique beaches, Southern Brazil

Abstract

This paper investigates rippled scour depressions on the inner shelf, between 3 and 25 m depth, of Barra da LagoaMoçambique beaches, located on the northeast of Santa Catarina Island, southern Brazil. The data was acquired with an EdgeTech® 4600 interferometric sonar, 540 kHz, which outputs side scan sonar images and swath bathymetry, proving coverage that is 3 to 4 times the water depth. The data covered an area of approximately 12 km² with 100% of coverage and was collected using Hypack®2013 and Discover® software. Side scan images were processed with SonarWiz5® and swath bathymetry with Hypack®. Preliminary results show an inner shelf dominated by fine sediment with the presence of 0.5 to 1.0m deep depressions of rippled coarse grain sediments. Those ripples have wavelengths ranging from 0.70 to 1.20 m and height from 0.3 to 0.6 m. The different bottom types were validated with a ground-truthing station, which consisted in the use of a van veen grab sampler. The coarsest sediments have median size of 1 mm and the finest, 0.2 mm, being classified as coarse sand / fine sand, respectively. These features were identified as rippled scour depressions or sorted bedforms, in accordance with the literature. They are perpendicular to the shoreline and occur from centre to south of Barra da Lagoa -Moçambique beaches, presents in depths that ranges from 20 to 3 m.