Debris flow interaction with open rigid barriers A DEM-LBM approach for trapping efficiency and impact force analysis

Debris flow is a dangerous landslide phenomenon occurring after intense rainfall in mountainous regions. It can be defined as a very rapid flow of heterogeneous material of different grain sizes with high water content. Due to its multi-phase nature, in which solid, fluid and air continuously interact, debris flow is a complex phenomenon, difficult both to analyze and to simulate. Because of its rapidity and unpredictability, it can cause loss of lives and extended damages to environment and structures. Thus, efficient mitigation measures are often desirable. Due to the complexity of the phenomenon, the design of barriers is still a challenging problem. Since a proper regulation does not exist, several of them have been designed only by imitating previously built barriers that have exhibited the proper functions during past events. Moreover, different types exist. The present thesis focuses on structural mitigation measures, with particular reference to open rigid barriers. Several Authors suggested that these barriers have to lower the kinetic energy of the flowing mass and to retain coarse sediments, allowing water and fine particles to pass. The main aspects to consider in the design of such barriers are: (1) the filter size problem, i.e. the size of the outlets, (2) the forces exerted on the barrier by the flowing mass during and after its impact. Thus, the present thesis addresses such two problems through a novel numerical method. An existing DEM-LBM code (Leonardi et al., 2015) has been enhanced with a complete friction model, which allows the creation of stable structures among grains. The result, a 3D continuum- discrete two-phase code, is able to consider the three-dimensional behaviour of the granular mass, the influence of the fluid phase, and their effects when they impact on the barrier. The new code has been validated and adopted to study the clogging mechanisms and the outlet geometry that promotes a retention of coarse grains. First, a monosized dry granular mass has been released under the effect of gravity in an inclined channel, at end of which the barrier is set. A complete parametric study on a single outlet barrier has been performed to provide the bases for furthersimulations on multiple-outlets barriers. The influence of the impact angle, of the channel slope, and of the normalized outlet width on both the trapping efficiency and the impact force has been evaluated and critically discussed. Then, progressively weakening the assumption of dry monosized mass, more realistic configurations have been analyzed. On one hand, bidisized dry granular simulations have been performed accounting for the presence of fine particles. On the other hand, a fluid phase, representing water and fine particles, has been added to the monosized dry granular mass. Interesting outcomes have been obtained on both trapping efficiency and impact forces. Starting from the dry monosized material and a single outlet barrier, a geometrical setting which provides a complete clogging of the barrier has been found. For opening width lower than 5 times the mean particle radius, the trapping efficiency is almost 100%. This result can be extended to the multiple-outlets barrier case if the width of the barrier piles is at least 6 times the mean particle radius. Moreover, introducing a bidispersion in grain size, the efficiency of the retaining function of the barrier is preserved up to a 70% in volume of small particles. The addition of a fluid phase, for solid volume fraction greater than 5%, does not affect the results. Considering the impact forces, high stresses are localized in the outlet neighbourhood, and their intensity increases by increasing the outlet width. The presence of bidispersion lowers the global impact forces, almost independently from the fraction of fine particles. Comparing the dry cases with those in which the fluid is added, it is noted that, in the first seconds after the impact, the presence of the fluid slightly lowers the impact forces due to the solid phase. Then, the fluid phase mainly transfers its momentum to the clogged solid phase, rather than directly to the barrier.