The application and suitability of unmanned airborne vehicle based imagery to determine river dynamics: A case study of the Buëch in France

Abstract

The Buëch is a dynamic river in south-eastern France with active erosion and accretion processes. In this river both alternating braiding and meandering patterns occur. Bank erosion has resulted in the closure of roads and the loss of agricultural lands. Therefore, it is important to monitor and better understand river dynamics. In this study, a method is developed to derive and describe the changes in geomorphological features using Unmanned Airborne Vehicle (UAV) based imagery and the positional accuracy in XYZ directions of the UAV products are determined and evaluated.

Three research locations along the Buëch have been established, providing imagery for multiple years. At each location in 2014 and 2015, a Differential Global Positioning System (DGPS) was used to measure the location of markers that were used as Ground Control Points (GCPs). OrthoMosaics and Digital Elevation Models (DEMs) were created using the Structure from Motion (SfM) algorithm available in Agisoft PhotoScan. The OrthoMosaics have a high resolution (cell size) of approximately 3 cm and the OrthoDEMs have a resolution of approximately 6 cm. The positional accuracy is on average approximately 30 cm in the XY-direction and 2 cm in the Z-direction. The created time series enables description of erosion and accretion processes on a sub meter scale which yields an increase in insight of the river dynamics of the Buëch.

With OrthoMosaics and OrthoDEMs riverbed morphology and patterns can be mapped and quantified. The result show significant changes in geomorphology over the time span monitored (2014-2015). Channel displacements of up to 20 m are observed at La Bâtie-Montsaléon. Bank erosion in general reaches up to 3 m in width. At La Bâtie-Montsaléon, a part of the river bank including trees on top with a length of 150 m and a maximum width of 40 m eroded. This resulted in a volume change of 5853.55 m3. This river bank erosion was not recognised without the OrthoMosaics and OrthoDEMs on basis of the UAV imagery.