Bottom Shear Stress in Channel Experiments Involving Fast-Moving Viscous Free-Surface Flows

In situ studies, as well as physical model investigations, are of fundamental importance for understanding fast-moving flows kinds of phenomena, for example, hyperconcentrated flows, pyroclastic flows, mud or debris flows. Related measurements provide insights to better understand flow properties. This work is motivated by recent observations that fast moving flows in low-gradient channels exhibit significant fluctuations in normal and shear stress with possible negative consequences on their erosive capacity, obtaining amplified volumes downstream. The work here presented is related to natural processes which may be well reproduced by single-phase simulation experiments, that is mud flows, pyroclastic flows and, to some extent to hyperconcentrated flows. It presents a laboratory procedure developed for the mean shear stress measurement at the bottom of channelized fast moving viscous free surface flows. Related flow rates were recorded as well. The investigation campaign was conducted using clear water and a solution of water and Sodium Carboxymethylcellulose (Na-CMC), the latter with an obtained viscosity of an order of magnitude greater than that of clear water. The obtained results indicate that the mean shear stresses exerted by the solution of water and Na-CMC at the bottom are higher than the ones measured for clear water, proving the greater resistance exerted by viscous flows.