Quantifying streambank erosion: a comparative study using an unmanned aerial system (UAS) and a terrestrial laser scanner

Streambank erosion is a common non-point source contributing to suspended sediment and nutrient loading of waterways, and recently has been estimated to account for 30-80% of sediment loading into receiving waters. There is interest in developing reliable methods to quantify bank erosion in watersheds, so effective management strategies can be devised. However, current methods can be either cost prohibitive or unreliable. Direct measurement approaches (surveys and erosion pins) are labor intensive and yield site-specific measurements that are limited for extrapolation to larger scales. Similar issues arise with analytical methods such as slope stability analysis, which require material parameters that are resource intensive to determine. Newer approaches such as use of aerial LiDAR data have proved effective for watershed level assessment, but come with long turnaround times and high cost. Terrestrial laser scanning (TLS) is also effective and offers high accuracy, however collection over large areas is impractical and post-processing is labor intensive. New technology in the form of unmanned aerial systems (UAS) has the potential to significantly enhance the ability to monitor channel migration and quantify bank erosion at variable scales. In this study, 20 km of the Mad and Winooski Rivers in Vermont were flown using a senseFly eBee UAS. Flights were made in spring and fall 2015 in leaf-off conditions with selected portions also flown after large storms in the summer. Change in bank profiles between spring and fall flights provide a comprehensive estimate of bank erosion along the study reaches. Six sites with varying bank heights, erosion sensitivity, and vegetation conditions were selected for simultaneous surveying using a TLS. Point cloud data from both the TLS and UAS were compared to assess the accuracy of the UAS for capturing the bank profile. Changes in bank cross-sections and in volumes calculated from 3D digital surface models were used to compare the UAS and TLS methods. Ascertaining the accuracy of UAS in measuring streambank profiles will determine the appropriateness of the technology for long-term monitoring of bank erosion. The ability for rapid deployment, covering large areas, and fast post-processing allow for more comprehensive monitoring of streambank erosion in river systems.