Measurements of Velocity Profiles in Natural Debris Flows: A View behind the Muddy Curtain

ABSTRACT The internal deformation behavior of natural debris flows is of interest for model development and model testing for debris-flow hazard mitigation. Up to now, only a few attempts have been made to measure velocity profiles in natural debris flows due to the low predictability and high destructive power of these flows. In this contribution, we present recent advances to measure in-situ velocity profiles together with flow parameters like flow height, basal normal stress, and pore fluid pressure. This was accomplished by constructing a fin-shaped monitoring barrier with an array of paired conductivity sensors in the middle of Gadria Creek, Italy. We present results from two natural debris-flow events. Compared to the first event on July 10, 2017, the second event on August 19, 2017, was visually more liquid. Both debris flows exhibited significant longitudinal changes of flow properties like flow height and density. The liquefaction ratios reached values up to unity in some sections of the flows. Velocity profiles for the July event were mostly concave-up, while the profiles for the more liquid event in August were linear to convex. These measurements provide new insights into the dynamics of real-scale debris flows.