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As a consequence of their spontaneous occurrence, and frequent formation of multiple surges with high sediment loads, debris flows are considered one of the most hazardous gravity-driven mass movements in montane regions. Field measurements of surface velocities are an essential link in the chain of understanding fundamental process dynamics and applied protection against debris flows. In order to measure the velocities of multiple consecutive surges within a single debris-flow event, a PD radar (pulse-Doppler high-frequency radar) sensor for high-resolution real-time debris-flow monitoring has been developed. In this contribution we present PD radar measurements of two debris flows, that occurred at the Gadria creek in Italy on July 26, 2019, and August 10, 2020, over a torrent length of 250 meters. We record over 55 high-amplitude surges that overlap and superimpose at the front of the debris flow, but also subsequently throughout the debris-flow body. Our results demonstrate applicability of a PD radar for debris-flow monitoring and serve as a data source for modelling surge dynamics in debris flows.

The high destructive potential of debris flows poses a challenge for the design of structural mitigation measures to protect vulnerable areas. An essential part of designing such structures is to determine the magnitude and spatial distribution of the impact forces. The impact is expected to be related to the composition of the flow, which can vary from clay to large boulders along a flow event and between events. Experimental studies are prone to scaling bias and benchmark observations at the full scale are rare so far. Here we present measurements of the temporal and spatial variations of the impact of a natural debris flow in the Gadria creek, IT, onto an instrumented barrier structure. The flow event was preceded by a precursory surge, which was then followed by the debris flow with multiple surges. The flow height reached up to 2.3 m. We found that the impact of boulders occurs primarily in the upper half of the flow profile, the highest forces were measured in first part of the flow, and deposition has a strong influence on the lower part of the structure by damping and/or redirection.