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. Debris flows pose a severe threat to settlements and human life in mountainous regions. Typically triggered by violent thunderstorms, a mixture of heavy boulders, woody debris, and water quickly forms and rapidly begins to rush downhill at high velocity. Interestingly, a pulsating flow pattern can spontaneously develop, consisting of several consecutive waves, with the flow coming to a complete stop between waves. This phenomenon has already been studied by other researchers under laboratory conditions and using simulations. However, high‐resolution velocity measurements in the field are very rare. In this study, we present radar observations of two natural debris flows at the Gadria creek (Italy) to investigate how this behavior is reflected in reality. The insights we derived from our data sets can contribute significantly to the understanding of the process and are crucial for the management of this risk through safety measures. PD radar velocity observations of debris flows were validated in a comparative analysis with DPIV and optical methods P