Hydrographic Survey of scours in river Wienfluss

Hydrographic survey of five scours in the riverbed of the river Wienfluss 1) Hydrographic survey a) Unmanned survey boat for use in very small waters The survey boat includes a semi-autonomous navigation unit with survey-grade satellite navigation unit (GNSS) with double antenna system for precise alignment of the survey boat. b) Multibeam echo sounder (MBES) Norbit iWBMS (minimum water depth under the transducer: 0.2 m) The MBES scans the bottom of the riverbed with 512 beams per ping. The possibility of swiveling the fan allows adjacent areas to be detected which cannot be navigated by boat due to the water depth, but in particular steeply rising bank areas (drop-offs) can be detected. c) Post-processing of the echo sounder measurement data The existing shallow water depths and narrow scour areas cause strong multipath reflections of the signals, which require extensive manual post-processing of the measurement data. This includes manually cleaning the point cloud of multipath reflections and filtering the data sets for subsequent DTM creation. DTM creation of the recorded underwater area with a grid width of 20 cm 2) Terrestrial photogrammetric survey and evaluation of the scour edge areas Due to the greatly reduced water level caused by the pumping out of the Wien River, the scours are no longer covered with water up to their upper edge. This means that, contrary to the original plan, the scour edge areas cannot be recorded with the echo sounder. For this reason, it was decided to carry out a photogrammetric survey and evaluation of the scour edge areas. The photogrammetric images are analyzed using the structure from motion (SFM) method and the surface is displayed as a 3D point cloud. The point cloud is filtered and artifacts from the evaluation are removed manually. In order to merge the underwater DTMs with the surface DTMs of the scour areas, the water-covered areas of the surface model are masked out manually.