Physical and numerical modelling of debris flows

Debris flows are geohazards consisting of an irreversible deformation of sediment-water mixtures. They have a high destructive potentiality and, therefore, geotechnical engineers have to find effective countermeasures to reduce the risk. The understanding of their behaviour is fundamental to be able to predict the flow and find the best solutions. However, relevant data are difficult to be obtained from natural debris flows: physical modelling is an effective tool to study these mass movements in detail. Laboratory tests are performed within this thesis. The following issues are analysed:- Influence of release volume and sediment concentration on the flow behaviour.- Representativeness of small-scale experiments compared to natural debris flows.- Numerical simulation of some tests and influence of sediment concentration on the rheological parameters.Several debris flows are tested on a 5 m long runout channel, sloped 17°, and a 4 m long deposition zone, sloped 2°, aiming at representing 1/20 scale. Height sensors, pore pressure sensors and cameras are used to study the flow. A mixer is used to release the debris mass, avoiding the segregation of water and sediments. The material used in the tests is well graded and mainly made of sand, but with non-negligible contents of gravel and lime.The peculiar morphological characteristics of debris flows - i.e. longitudinal grain segregation, coarser flow front, liquefied tail, levees formation - are well observed in the experiments. It is found that an increase of sediment concentration decreases the runout distance and the flow velocity. Release volume mainly affects flow height, by increasing it. The test results are compared to natural debris flows, through Froude number and dimensionless numbers (Savage, Bagnold and friction numbers). High values of Froude number are obtained (from 4, for the highest sediment concentration, to 8, for the lowest sediment concentration), representative of thin and fast flows, compared to natural debris flows with Froude numbers of around 3. The calculation of dimensionless numbers indicates that the three main mechanisms of stress generation (collisional, viscous, frictional) have the same importance. This is in contrast with natural debris flows, where the frictional behaviour dominates on the viscous one which dominates on collisions. The dimensionless numbers have the same order of magnitude of small-scale experiments found in literature: the overestimation of collisional stresses is therefore a common issue of these laboratory tests. An increase of the importance of the frictional mechanism was however found for the highest solid concentration.Numerical simulation of these small-scale tests is performed with the RAMMS::Debris flow software and using the Voellmy rheology. The sensitivity to mu and xi is analysed: xi is found to be a turbulent-viscous parameter which mainly affects the flow velocity; mu is a friction parameter which controls the deposition behaviour. The software is able to well simulate the dimensionless parameters along the flume and the runout distance. However, the deposit shape cannot be simulated. The calibrated values of mu and xi indicate again that the flow is turbulent and with low friction. Finally, the parameter mu is found to be approximately equal to the tangent of the slope angle of the deposition area.