Physical modelling of laboratory debris flows by using CMC

Debris flows are among the most destructive processes occurring in mountain environments. It is recognized that rheology describing the relationship between stress and strain rates is mainly dependent from interstitial fluid content, pore fluid pressure, sediment concentration, and size and distribution of dispersed grains. However, due to their complex behaviour, it is unlikely that they can be characterized by a prefixed rheological model. Within this framework, we have assessed the feasibility of a double model of similarity of Reynolds and Froude, as attempt to mimicking the interstitial fluid phase at laboratory scale. For the purpose we used the Sodium Carboxymethylcellulose (Na-CMC or briefly CMC) as thickening agent in aqueous solutions. By varying the solute concentration, it was possible to reproduce the pore fluid of a specific viscosity. The related transparency of such solutions allows observing and describing evolution processes by highfrequency digital acquisition. Among the others, the main innovative feature consisted in the possibility of establishing a relationship between laboratory experiments and corresponding assumed prototypes, by means of the proposed model scaling. The experimental campaign being carried out, allowed to establish a state equation for the model reference, that is a relationship among viscosity, solute concentration and temperature. A predominant dependence between viscosity and solute concentration was observed, marking another advantage in using such solutions. The similarity then connect model results to the prototype.