Assessing UAV survey performance for geomorphological monitoring of mountain rivers

Abstract

<p>Characterising morphological changes in mountain areas is of fundamental importance for science<br>and engineering. Intense floods usually involve massive sediment transport, which may significantly<br>alter basin and river characteristics. Sediment erosion and deposition control the dynamics<br>of morphological structures such as alternate bars and meanders. By using unmanned aerial vehicles<br>(UAV), it has been possible to obtain high-precision bed elevation data at the sediment scale.<br>Our project aims to develop a consistent and optimised methodology for monitoring morphological<br>changes in an Alpine watershed using an UAV. Since 2017, we have been monitoring the Plat de la<br>Lé area drained by the River Navisence (Zinal, canton Valais, Switzerland). In mountainous regions,<br>poor accessibility and light conditions make it difficult to set control points on the ground. We first<br>analysed the relevance and influence of certain ground control points (GCP) on the the accuracy of<br>the digital elevation model (DEM) obtained from the UAV’s images. Errors in the GCP localisation<br>were much larger than the DEM resolution. Not only did the GCP number and flight height affect<br>these errors, as expected, but their positions and orientations also played a part. We then carried<br>out an additional monitoring campaign to understand the influence of these parameters on the DEM<br>accuracy. This campaign was ran on two areas: a steep-slope area with irregular topography and<br>a low-slope area that comprises the river channel and its floodplain. We built DEMs for each area<br>considering different GCP numbers (in the 3–18 range with 14 additional checkpoints) and flight<br>heights (in the 40–140-m range). The present study provides guidelines, including an optimal combination<br>of parameters that significantly reduce errors in the DEM, and a methodology that can be<br>used for monitoring Alpine watersheds on a regular basis.</p>