Shear-thinning is a common flow-feature of fine sediment suspensions. Mixed with gravel, Coulomb friction drives the energy dissipation between small grains while collisions become more and more important with larger grains. The interaction of the flow with local geometries of the channel can enforce each of these three key features, making the design analysis of channel sections with obstacles a highly back-coupled system. This paper addresses the numerical simulation of debris flow material under extreme flow conditions at planned protection measures. Mixtures with small grain sizes are modelled with a single calibration parameter using the 3D CFD phase mixture software debrisInterMixing and compared with laboratory experiments. To further investigate the scaling of the results, a coupled code of YADE and debrisInterMixingLP is applied accounting for the 4-way coupling to the coarse boulders at the front with resolved CFD-DEM, reaching beyond the possibilities of debris flow experiments.