XXVIII IAHR Congress, Graz, 1999 - proceedings -

SANDRO LONGO(1), ALBERTO LAMBERTI(2)

1) Dep. of Civil Engineering, University of Parma,

Viale delle Scienze, I-43100 Parma

2) DISTART, University of Bologna,

Viale Risorgimento,2, I-40136 Bologna

In the present paper we report a model of a grain stream moving over an erodible bottom essentially in frictional and quasi static regime. The model is based on several assumptions:

frictional component, due to long contacts between particles, is represented by a Coulomb law, assuming that at least for limited values of the mean stress level the global friction angle experiences a strong reduction immediately after shearing start, due to collapse of grain-grain gears; then it tends to increase with decreasing concentration. Anyway the difficulties in measuring volume concentration of the grains with the necessary precision and the substantial impossibility of checking the results, suggest a different closure for the friction angle. We implicitly assume that collisions between particles are binary and that even multiple contacts between particles in movement immediately generate friction. The collisional contribution to the global stress is expressed as function of the local concentration, local velocity gradient and of the average contact angle between shearing layers. The kinetic contribution is neglected because of its minor relevance at high concentration. The model is applied to the simulation of one of the experiments carried out in a rotating drum built up in the Hydraulic Laboratory of DISTART.

Keywords: rheology of granular systems, debris flows.