<p>After a heavy rainfall event on August 31<sup>st</sup>, 2019, a debris flow at the Dawinbach in the municipality of Strengen (Tyrol, Austria) caused a blockage of the culvert below the provincial road B-316 and deposition in the residential area. The debris deposition raised up to 2 to 3 meters on the road and led to property damage to real estate. The total volume of the debris flow was approximately 15 000 cubic meters.</p><p>In order to control a further debris flow of this magnitude, the Austrian Service of Torrent and Avalanche Control started to construct mitigation measures. They include a channel relocation in order to significantly increase the channel crosssection. Hence the construction company STRABAG is also relocating the provincial road bridge.</p><p>Since the risk for this road section and for the workers on site is particularly high during the construction period, a combined monitoring and early warning concept was developed and implemented by the BOKU, Vienna and the company IBTP Koschuch.</p><p>The monitoring site consisting of a pulse compression radar and a pull rope system was installed 800m upstream from the fan. The combination of the two sensors now results in three major advantages.</p><ul><li>At sensor level, the system operates redundantly.</li> <li>A more reliable differentiation between increased discharge or debris flow is given.</li> <li>In the event of a false alarm, the system provides easier diagnosis and assignment of the fault.</li> </ul><p>Two events of increased runoff occurred during the deployment period. Both were successfully detected by the pulse compression radar. Here, the first event was used for threshold validation of the radar unit. Thus, an alarm could already be sent out automatically for the second one. The road is controlled by an integrated light signal system consisting of three traffic lights. A siren near the construction site can warn workers of an impending event by means of an acoustic signal. The reaction time after the alarm has been triggered is between 75 and 150 seconds, depending on the speed of the debris flow. The responsible authorities are informed by sending an SMS chain, which includes details about the type of process and the type of the activated triggering system.</p>

<p>The surface velocity of debris flows is constantly subject to strong temporal and spatial fluctuations. These are amplified by the pulse-like occurrence of surges throughout the event and by the high variance of the solids fraction. However, continuous information on velocities of multiple consecutive surges within a single debris-flow event with high temporal resolution is rare. In this study, we test a pulse-Doppler (PD) radar over a total torrent length of 180 m. The PD radar utilizes pulsed transmission and provides spatially resolved cells, called range gates, that extend over a width of 20 meters. Doppler velocity spectra composed of velocity classes and echo intensities are obtained at 4 Hz for each range gate. From these, we derived continuous velocity-time data sets. We present PD radar data for three debris flows that occurred on 05.06, 30.06, and 08.09.2022 at the Illgraben creek, Switzerland. The radar data were validated at the first event with a velocity data set obtained from a LiDAR scanner installed at the same location. This novel method collects high-resolution 3D point clouds at 10 Hz. This data was used to derive a high-resolution velocity vector field for one of the events. We isolate a ~ 2x2 m box in the middle of the channel and compare the LiDAR derived velocities at this location to those measured by the PD radar. Our comparison shows a strong correlation between the two data sets, with a coefficient of determination of 0.85. In addition, we note a minor consistent offset in the two velocity data sets of 0.5 m/s. We attribute this to the nature of the different measurement methods and conclude that the two methods may be sensitive to different features of the surface of the flow. However, our results show the high effectiveness and reliability of both methods in debris-flow monitoring. We anticipate that further analysis of the data sets will provide new insights into the geophysical principles of debris flows.</p>

<p><strong>Comparison of the surface velocity of a debris flow at the Gadria creek using pulse compression radar and digital particle image velocimetry (DPIV).</strong></p><p><strong> </strong></p><p>Tobias Schöffl, Georg Nagl, Johannes Hübl</p><p>Institute of Mountain Risk Engineering, University of Natural Resources and Life Sciences, Vienna, Austria</p><p> </p><p>A central aspect of protection against debris flows is the understanding of the process. The flow velocity is an important parameter which is used, for example, in the dimensioning of protective structures, for technical building protection and for early warning systems. The measurement of the surface velocity which is regarded as the maximum velocity occurring within a debris flow, is therefore an essential link in the chain of fundamental process research and applied protection against natural hazards.</p><p>Due to the further development of various technologies such as video technology and high-frequency radar technology, the non-contact measurement of the surface speed of a debris flow has improved significantly in recent years. Radar technology provides a wide aspect of applications in alpine mass movements such as debris flows, avalanches and rockfall and is able to detect such processes up to a range of 2500 meters in distance. An additional beneficial feature is the possibility of non-contact measurement of the surface velocity. In the catchment area of the Gadria basin (South Tyrol, Italy), the measuring station, which has been in operation since 2016, has been extended by a pulse compression radar and a new HD video camera. On July 26, 2019 a debris flow consisting of several surges was recorded with both the radar and the HD video camera. To obtain surface velocity data from the video material, the material was analyzed and evaluated using digital particle image velocimetry by making use of the MATLAB software and its freely accessible ADD-On "PIVlab".</p><p>The results of the compared surface velocity data showed a value of up to 0.74 according to the statistical mean of the coefficient of determination. The results demonstrate the high effectiveness of the pulse compression radar and the DPIV analysis in a wide range of the assessment of surface velocity of natural debris flows. There is great potential in both measuring systems and the chosen comparative analysis provides a blueprint for future recorded debris flows.</p>