Extending Coral Reef Elevation-Change Studies from Regional- to Centimeter-Scale with a New High-resolution, Underwater Imaging System

Healthy coral reefs serve as natural barriers that protect adjacent shorelines and coastal communities from coastal hazards, such as wave impacts from storms. Recent predictions, however, assert that anthropogenic impacts will cause the global degradation of coral reefs, leading to a net-erosional state by 2050. In 2017 we published the first measurements of regional-scale (100s km²) elevation changes for coral reef environments. Such measurements are critical to understanding the various consequences of reef degradation, including structural framework, hydrodynamic flow, and sediment transport over large areas. These elevation change analyses are typically limited to 1-2 m horizontal and 15-cm vertical resolutions owing to the resolution of the underlying lidar or multibeam-sonar data. We have extended our elevation change analyses to sub-centimeter horizontal and vertical resolutions by using Structure-from-Motion (SfM) techniques and the recently developed SfM Quantitative Underwater Imaging Device with 5 cameras (SQUID-5). This towed system with integrated survey-grade GPS positioning simultaneously acquires 5 images of the seafloor, each from different view angles, as fast as 4 Hz and at tow speeds up to 1.5 m/s. We field tested SQUID-5 in July 2019 at Crocker Reef in the Florida Keys over a variety of reef habitats and 3 sediment elevation tables (SETs). Preliminary analyses of the geo-referenced 3D reef models and comparison with the millimeter-resolution SET measurements clearly indicate that elevation measurements with sub-centimeter resolutions are achievable in water depths up to ~6 m. The combination of rapid data acquisition and high resolution makes detailed reefscape-scale elevation studies feasible, complementing larger areal, lower resolution lidar and multibeam surveys. Selecting and monitoring coral restoration sites, fine-scale hydrodynamic modeling, and localized sediment transport analyses are a few applications of the resultant 3D models.