Assessing River Dynamics Using a Timeseries of UAV Imagery. A case study of the Petit Buëch river in Hautes – Alpes, Southern France
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The start of photogrammetry can be dated back to the XIX century, but this field has seen a narrow development for almost 150 years, attributable to the poor results obtained despite the huge resources required to achieve them. The digitization of the data and digital acquisition of the aerial imagery led, in the last 20 years, to a fast development of photogrammetric techniques, increasing the quality of data obtained while enormously reducing the costs. In this context, the use of Unmanned Airborne Vehicles (UAVs) is the natural evolution of 150 years of aerial photography. The Petit Buëch is a highly dynamic river system in the south-east of France, with a clear braiding pattern. The erosion of the two catchments studied, at Le Pont du Chabestan and La Bâtie-Montsaléon, is high and endangers nearby anthropic structure and infrastructure. Thus, studying its dynamics is of pivotal importance. In order to monitor and study these dynamics, a series of photos captured by an Unmanned Airborne Vehicle (UAV) will be used, processed and analysed through the application of photogrammetric techniques. The two study areas have already two sets of photos processed and analysed (2014 and 2015), from a previous work in the same area, and these sets will be integrated with the latest available photography from 2016 and 2017. With Agisoft PhotoScan and the Structure-from-Motion algorithm, the imagery will be processed and yield two OrthoMosaics and two Digital Elevation Models (DEMs) that will be finally used to carry out a qualitative and quantitative analysis of the two areas. The qualitative analysis will concern the visual comparison of the two OrthoMosaic from 2016 and 2017, seeking for clear river path change and bank displacement. The quantitative analysis will try to quantify this displacement through a profile analysis of the river in 2016 and 2017, while a volume change analysis will consider also the DEMs from 2014 and 2015. The goal of this thesis project is to understand the river dynamics of the Petit Buëch but also the accuracy and quality of the product delivered in the end. It is noteworthy that the UAV and camera used to capture the imagery were not high-end, posing interesting questions on how cheap tools perform and how high quality the products they yield could be.