The assessment of flow velocity has a central role in quantitative analysis of debris flows, both for the characterization of the phenomenology of these processes, and for the assessment of related hazards. Large scale particle image velocimetry (LSPIV) can contribute to the assessment of surface velocity of debris flows, provided that the specific features of these processes (e.g. fast stage variations and particles up to boulder size on the flow surface) are taken into account. Three debris flow events, each of them consisting of several surges featuring different sediment concentration, flow stage and velocity, have been analyzed at the inlet of a sediment trap in a stream of the eastern Italian Alps (Gadria Creek). Free softwares have been employed for preliminary treatment (ortho-rectification and format conversion) of video-recorded images as well as for LSPIV application. Results show that LSPIV velocities are consistent with manual measurements on the ortho-rectified imagery and with front velocity measured from the hydrographs in a channel reach approximately 70&thinsp;m upstream of the sediment trap. Horizontal turbulence, computed as the standard deviation of the flow directions at a given cross-section for a given surge, proved to be correlated with surface velocity and with visually estimated sediment concentration. The study demonstrates the effectiveness of LSPIV in the assessment of surface velocity of debris flows, and permit to identify the most crucial aspects for improving the accuracy of debris flows velocity measurements.
Theule, J. I., Crema, S., Marchi, L., Cavalli, M., & Comiti, F. (2018). Exploiting LSPIV to assess debris-flow velocities in the field. Natural Hazards and Earth System Sciences, 18(1), 1–13. https://doi.org/10.5194/nhess-18-1-2018