Hydraulics and rheology of natural hyperconcentrated flows from Draix-Bleone observatory, French Alps

We present an experimental study of natural hyperconcentrated flows of fine sediment from Draix-Bleone observatory. Suspensions of volumic concentration ranging from 29 to 44% were tested in a rheometer and exhibit a Herschel–Bulkley behaviour. Suspensions of volumic concentration ranging from 0 to 31% were tested in flume experiments with varying slope and discharge. Velocity profile measurements indicate a transition from turbulent Newtonian flows to laminar non-Newtonian flows as the concentration increases. Rheological data and flume velocity profiles consistently suggest that a yield stress appears for volumic concentrations higher than 26–29%. Finally, the relation between the Reynolds number and the friction coefficient computed for all flume experiments is found to follow a unique trend, independently of the concentration. This allows the laminar, transitional and turbulent regimes to be identified, and suggests that transitional to laminar conditions can be observed in the field during hyperconcentrated events.