Quasi-three dimensional two-phase debris flow model acounting for boulder transport

Abstract

We present a quasi three-dimensional numerical model to simulate debris flows accounting for a continuum non-Newtonian fluid phase composed by water and fine sediments, and a non-continuum phase for large particles such as boulders. Particles are treated in a Lagrangian frame of reference using the 3D Discrete Element Method. The fluid phase flow equations are solved by the RiverFLO-2D computational model which is based on the 2D depth-averaged shallow water approximation and uses the Finite Element Method on a triangular non-structured mesh. The model considers particleparticle and wall-particle collisions, and considers that particles are immersed in a fluid and subject to gravity, friction and drag forces. Bingham and Cross rheological models are used for the continuum phase providing very stable results, even in the range of very low shear rates. Results show that the Bingham formulation proves better able to simulate the stopping of the fluid when applied shear stresses are low. Results from numerical simulations comparison with analytical solutions and data from flume-experiments, show that the model is capable of reasonable approximating the motion of large particles moving in the fluid flow. An application to simulate a debris ow event that occurred in Venezuela in 1999 shows that the model can model the main boulder accumulation reported for the alluvial fan in that event.