UAV-based monitoring of sedimentary processes at a large-scale revitalisation of an alpine river – concept and outlook

In the 19^th and 20^th centuries, numerous alpine rivers were modified into straightened and monotonous channels by river regulations, which had significant negative effects on the ecology of the river systems and their floodplains. River revitalisations aim to restore river sections in order to create stepping stones in anthropogenic altered rivers. A large-scale example are the measures along the alpine Inn River between Stams and Rietz in Tyrol (Austria) over a length of about 3 river kilometres, which are implemented within the extension project of the hydropower plant Sellrain-Silz. During two low-water periods (October - April), starting in 2021, the existing bank protections were removed to a large extent, the river bed was widened up to 75 meters and a back water zone as well as a branch were created. As a result, the river stretch can develop by its own dynamics and ecologically valuable areas such as shallow water zones as well as gravel and sand banks are supposed to be formed.

In order to assess the morphodynamics and ecological functionality of these measures, while also maintaining flood protection, a comprehensive monitoring program is being conducted. This includes nine photogrammetric surveys using an unmanned aerial vehicle (UAV) between March and October 2022. The UAV data were used to generate ortho-images and digital elevation models. The accuracy was assessed by comparison with airborne LiDAR data from one flight in March 2022. The objective is to quantify the erosion and deposition processes in the area of the measures and to determine the sedimentological processes that occurred during the survey period using the UAV results and hydro-morphological data (e.g., hydrograph, suspended sediment concentration) from nearby gauging stations on the Inn River. In addition, the suitability of high-resolution data from UAV surveys for monitoring sediment and bedload dynamics in alpine rivers will be evaluated. First results show that deposition processes dominated in the area of the measures, while relatively low discharge values were recorded throughout the study period. Based on the data analysis, we elaborate a suggestion for further monitoring in addition to the cross-section surveys already conducted since 1980, i.e., one UAV flight per year at low-flow conditions in order to establish a long term monitoring of morphodynamics.