Monitoring Fluvial Geomorphic Change Using Unmanned Aircraft System (UAS) Photogrammetry and Laser Scanning

Advancements in digital photogrammetry such as structure-from-motion (SfM) and affordable unmanned aircraft system (UAS) platforms to collect imagery have revolutionized the ability to capture fine-resolution topography. A particularly promising application is the detection of geomorphic change along river corridors, which previously posed challenges for regular monitoring given the difficulties of ground access. Monitoring these areas is important since streambank erosion is a significant source of fine sediments and associated nutrients in many river systems including within the Lake Champlain basin.

To evaluate the effectiveness of UAS-based photogrammetry for monitoring geomorphic change, we surveyed 20 km of river corridors in central Vermont multiple times over a two-year period (2015-2017) using a fixed-wing UAS. Streambank cross-section data were derived from UAS imagery and shown to compare well to ground survey data collected by a terrestrial laser scanner at seven streambank monitoring sites. Mean error between UAS and ground survey was as low as 11 cm in early spring conditions. Digital elevation models (DEMs) and DEMs of Difference (DoDs) helped quantify the volumetric change along selected portions of the survey area where notable erosion occurred. UAS surveys were also compared to previously collected airborne lidar surveys to estimate longer term changes. Results showed that UAS was capable of collecting high quality topographic data at fine resolutions even along vegetated river corridors provided that the survey timing and conditions were optimal. The ability to monitor geomorphic change and quantify bank erosion using flexible and efficient survey methods such as UAS will help inform watershed management plans and monitor progress in reducing sediment loading associate with bank erosion.