Deriving vertical velocity profiles of natural debris flows at the Gadria creek, Italy

Debris flows, like most gravitational flows, exhibit an extremely diverse flow behavior depending on the relative composition of the mixture. For debris flows, the interaction between the fluid and the solid components as well as the interaction of the solids with each other is of decisive importance for the bulk flow behavior. Combined information of bulk flow properties, material composition and internal deformation is needed to constrain constitutive relations for debris flows.

This study focuses on the measurement velocity profiles in natural debris flows observed at a monitoring station at the Gadria creek in South Tyrol, Italy and to relate these with measurements of grain size distribution, flow depth, basal stress measurements and horizontal velocity distributions. Velocity profiles are measured along the sidewall of a concrete structure in the middle of the channel at 11 levels above the channel bed using pairs of conductivity sensors with a certain horizontal spacing s. Cross-correlating the signals yields a time delay t that allows calculating the velocity v.

The main focus of the first stage of the project BEHAVE is to identify the optimal way to process the conductivity signals for the subsequent continuous velocity determination, as the processing parameters have a major influence on the resulting velocity profiles. In an initial assessment, we find that the step size of the floating window plays tendentiously a more important role for the quantitative velocity value generation than the value size of the floating window (= maximum lag). In contrast, the quality of the velocity values is decisively influenced by the setting of an optimal auto-correlation factor (ACF) value threshold, which indicates the significance of the individual correlations. We finally compare the velocity distributions from selected time periods of two debris flow events with each other.

These results will form the basis for further analysis, such as rheological characterizations of the collected debris flow materials, combinations with horizontal velocity profiles and the comparison with laboratory test data.