Using unmanned aircraft system (UAS) photogrammetry to monitor bank erosion along river corridors

Excessive streambank erosion is a significant source of fine sediments and associated nutrients in many river systems within the Lake Champlain basin. Geomorphic change detection using high-resolution topographic data is a useful method for monitoring the extent of bank erosion along river corridors. Advances in unmanned aircraft system (UAS) and structure from motion (SfM) photogrammetry techniques have provided a powerful new tool for capturing high resolution topographic data. To evaluate the effectiveness of UAS-based photogrammetry for monitoring bank erosion, a fixed-wing UAS was deployed to survey 20 km of river corridors in central Vermont multiple times over a two-year period (2015-2017). Evaluation of the UAS-based topographic data was performed by comparison of streambank cross-section data to ground survey data at seven streambank monitoring sites. Mean error between UAS and ground survey as low as 11 cm were achieved in early spring conditions. Digital elevation models (DEMs) and DEMs of difference were utilized to quantify the volumetric change along selected portions of the survey area where notable erosion occurred. Longer term estimates of change were also made by comparison of UAS surveys to previously collected airborne lidar surveys. 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. UAS survey data compared well to existing airborne lidar surveys and allowed robust quantification of significant geomorphic changes along rivers. The ability to monitor geomorphic change and quantity bank erosion using a flexible and efficient survey method such as UAS will help to inform watershed management plans and monitor progress in reducing sediment loading due to bank erosion.