A Perspective of Surge Dynamics in Natural Debris Flows Through Pulse-Doppler Radar Observations

As a consequence of their natural occurrence and the frequent formation of multiple surges with high sediment loads, debris flows are considered one of the most hazardous gravity-driven mass movements in mountain regions. Field measurements of surge dynamics are an essential link in the chain of understanding fundamental process dynamics and engineering protection against debris flows. However, continuous information on the velocities of multiple consecutive surges within a single debris-flow event with high temporal resolution is rare. In this study, we present a new pulse-Doppler radar (PD radar) for high-resolution real-time debris-flow monitoring. We analyze PD radar data sets over a torrent length of 250 m for two debris flows that occurred at the Gadria creek (IT), on 26 July 2019 and 10 August 2020. The radar data were validated with independently derived data from particle image velocimetry and manually tracked velocities. We observe that between surges, the flow frequently comes to a complete halt and is re-mobilized by subsequent surges, resembling erosion-deposition waves in granular flows. In addition, our data confirm that surges can superimpose and merge. We anticipate that the outcomes of this work serve as a blueprint for future high-resolution observations of debris-flow surge dynamics with PD radar and that our findings provide new insights into the physical principles of natural debris flows.