Advisors: Gottfried Mandlburger

In September 2024 eastern Austria, Lower Austria in particular, experienced several days of heavy rainfall, that was followed by several massive flooding events. Because of this, a large amount of sediments was set in motion, leading to morphological changes in the terrain and significant financial damage in affected areas. The focus of this study lies on the documentation and interpretation of hydro-morphological changes along a section of the Pielach following the once in a century flood in 2024. For the documentation of the changes one digital elevation model (DEM) each was derived from two ALB point clouds from 2023 and 2024. To accurately model the terrain, a refraction correction was first applied to the underwater points. The necessary water surface model was generated by compacting Cross-Sections of the water surface along the river axis [6]. The quality of the ground point classification is essential for the spatially variable accuracy of the DEM. The quality of the ground point classification of 2024, however, was not sufficient in certain areas due to the seasonal effects in play during the measurements.  To make up for this the 2024 ground point classification was supplemented and refined through a time-consuming manual process in the focus areas of this study.After this the DEM of Differences (DoD) was calculated from the two DEM. The DoD method makes it possible to rapidly determine terrain changes over a large area. The actual height changes were derived via the local spatially variable DoD-accuracy and the student-t-value of each grid point using a combination of the methods described by Kraus et. al. [5] and Wheaton et. al. [11].The results show that the flooding has caused significant hydro-morphological changes study area. The width of the riverbed increased by over 20m and several large sediment deposits changed the course of the Pielach in its riverbed. Sediment deposits also increased the height of the terrain adjacent to the river by several centimetres. Besides these changes caused by the flood in September 2024 man-made height changes also occurred in the studied timeframe. These can mainly be attributed to redistributed materials and construction work.