Monitoring Seasonal Morphobathymetric Change of Nearshore Seafloor Using Drone-Based Multispectral Imagery

Abstract

Short-term changes in shallow bathymetry affect the coastal zone and therefore their monitoring is an essential task in coastal planning projects. This study provides a novel approach for monitoring shallow bathymetry change based on drone multispectral imagery. Particularly we apply a shallow water inversion algorithm on two composite multispectral datasets being acquired five months apart in a small Mediterranean sandy embayment (Chania, Greece). Initially, we perform radiometric corrections using proprietary software and following we combine the bands from standard and multispectral cameras resulting in a six-band composite image suitable for applying the shallow water inversion algorithm. Bathymetry inversion results showed good correlation and low errors (< 0.3m) with sonar measurements collected with an uncrewed surface vehicle (USV). Bathymetry maps and true-color orthomosaics assist in identifying morphobathymetric features representing crescentic bars with rip channel systems. The temporal bathymetry and true-color data reveal important erosional and depositional patterns, which were developed under the impact of winter storms. Furthermore, bathymetric profiles show that the crescentic bar appears to migrate across and along-shore over the 5-months period. Drone-based multispectral imagery proves to be an important and cost-effective tool for shallow seafloor mapping and monitoring when it is combined with shallow water analytical models.

The unique value proposition for using drones to map coastal ecosystems

“…In addition to mapping ecological features, drone mapping images can be processed using SfM photogrammetry techniques to build digital terrain models (DTMs) and digital surface models (DSMs). This enables fine-scale, three-dimensional information to help map abiotic features in coastal systems both on land and underwater (Alevizos and Alexakis, 2022;Casella et al, 2020), including tracking topographic change over time (Casella et al, 2016;Lowe et al, 2019;Volpano et al, 2022). Combining the digital elevation models (DSM or DTM) with an RGB orthomosaic of the region provides a unique perspective of coastal ecosystems that is simpler and more cost effective to create than using more traditional aerial survey and satellite techniques.…”