UAS hydrometry survey in the Upper Isar River, Germany

The UAWOS team has for the second time taken to the field to perform a UAS hydrometry survey in the Upper Isar River, in Bavaria, Germany, between the 24th and 27th of February 2024. The Isar river has its source in the Alps, and flows through Southern Germany reaching the Danube River after approximately 300 km. The river passes through the city of Munich, and is of great importance to the region, as numerous hydropower plants have been constructed along its length. The focus of the UAWOS flights was in the Upper Isar region, located near the German towns of Mittenwald, Krün, and Wallgau, close to the border with Austria. The river has a high sediment load and can thus pose a flood risk to close by settlements, hence the interest of performing an hydrometry survey at this location.The river is characterized by a rather unstructured morphology, as at some points it splits into two or three concurrent streams, and in other locations, it consists of a unique stream. 

Fig.1: M300 RTK with Doppler Radar.

Fig. 2: M300 RTK with Water Penetrating Radar.

Over the course of the survey, two drone platforms were utilized (DJI Matrice 300 RTK, fig. 1, and DJI Matrice 600) to fly some of the UAWOS developed payloads, which included the Water Penetrating Radar (WPR), the Doppler Radar, and the IWR1443 payload. Using the data collected with these payloads, bathymetry, water surface velocity and water surface elevation are expected to be calculated and compared to ground truth data. Digital Terrain Models (DTM) were also collected using LiDAR and Photogrammetry from high altitude flights. You can watch a video of one of the Isar cross section flights here.

At lower altitudes, the drone was flown close to the water surface when equipped with the WPR and Doppler Radar. Cross section flights with the WPR were particularly challenging, as the drone was flown very close to the river (approximately 50 cm above water level) from one bank to the other, as pictured in the video above. During these flights, the operator had to also account for dangerous objects, such as tree branches and overhanging vegetation, which can provoke a crash if not properly avoided.

Fig. 3: Collection of ground truth data.

Fig. 4: Drone operation from the riverbank.

To validate the drone collected data, ground truth values were collected by the team using RTK bathymetry, RTK water surface elevation, and an Acoustic Digital Current Meter. One of the advantages of the Isar is that it is relatively shallow, facilitating the task of collecting ground truth data, as one can walk on the stream to collect data (fig. 3). Furthermore, the riverbanks are relatively large and flat, which allowed the drone operator to be close to the stream, and always ensure visual line of sight (VLOS) with the drone (fig. 4), particularly upon performing more challenging flights, such as the aforementioned cross section flights.

Fig. 5: Experimental drone platform with LiDAR and Stereo Camera.

Due to the challenges of performing low altitude cross section flights, the UAWOS project also has a focus on developing workflows and implementing algorithms to facilitate the drone operators’ job once on the field. The Isar survey served as an opportunity to deploy an experimental drone with a LiDAR and Stereo Camera payload (fig. 5) to collect data to investigate the feasibility of utilizing these sensors to help automate the flight process and ensure robust collision avoidance.

Fig. 6: The team enjoying a break in between flights.

Fig. 7: Preflight preparations.

The Upper Isar survey was a success as all desired data was collected. The used drone hardware also behaved as expected, as no major technical challenges were faced during operation. The team also gained new insights on aspects to consider for future surveys, to ensure a faster and more efficient workflow. These included considering a different approach to prepare the survey, namely studying multiple access routes to the river beforehand, and performing data collection flights with different flight planning characteristics. Beyond the survey itself, the team thoroughly enjoyed being out in the field and experiencing the beauty of the region!