Particle-fluid-structure interaction for debris flow impact on flexible barriers

Flexible barriers are increasingly used for the protection from debris flow in mountainous terrain due to their low cost and environmental impact. However, the development of a numerical tool for the rational design of such structures is still a challenge. In this work, a hybrid computational framework is presented, using a total Lagrangian formulation of the finite element method to represent a flexible barrier. The actions exerted on the structure by a debris flow are obtained from simultaneous simulations of the flow of a fluid-grain mixture, using two conveniently coupled solvers: the discrete element method governs the motion of the grains, while the free-surface non-Newtonian fluid phase is solved using the lattice Boltzmann method. Simulations on realistic geometries show the dependence of the momentum transfer on the barrier on the composition of the debris flow, challenging typical assumptions made during the design process today. In particular, we demonstrate that both grains and fluid contribute in a nonnegligible way to the momentum transfer. Moreover, we show how the flexibility of the barrier reduces its vulnerability to structural collapse, and how the stress is distributed on its fabric, highlighting potential weak points

Review of the mechanisms of debris-flow impact against barriers

Our limited understanding of the mechanisms pertaining to the force exerted by debris flows on barriers makes it difficult to ascertain whether a design is inadequate, adequate, or over-designed. The main scientific challenge is because flow-type landslides impacting a rigid barrier is rarely captured in the field, and no systematic, physical experimental data is available to reveal the impact mechanisms. An important consideration in flow-structure interaction is that the impact dynamics can differ radically depending on the composition of the flow. Currently, no framework exists that can characterize the impact behavior for a wide range of flow compositions. This review paper examines recent works on debris-flow structure interactions and the limitations of commonly used approaches to estimate the impact load for the design of barriers. Key challenges faced in this area and outlook for further research are discussed

Impact of debris flows on filter barriers: Analysis based on site monitoring data

Debris flows are one of the most complex and devastating natural phenomena, and they affect mountainous areas throughout the world. Structural measures are currently adopted to mitigate the related hazard in urbanized areas. However, their design requires an estimate of the impact force, which is an open issue. The numerous formulae proposed in the literature require the assignment of empirical coefficients and an evaluation of the kinematic characteristics of the incoming flow. Both are generally not known a priori. In this article, we present the Grand Valey torrent site (Italian Alps). A monitoring system made up of strain gauges was installed on a filter barrier at the site, allowing the evaluation of impact forces. The system provides pivotal information for calibrating impact formulae. Two debris flows occurred during the monitoring period. We present the interpretation of videos, impact measurements, and the results of numerical analyses. The combined analysis allows a back calculation of the events in terms of forces, flow depth, and velocity. Thus, we investigate the applicability of the impact formulae suggested in the literature and of the recommended empirical coefficients. The results highlight that hydrostatic effects dominated the impact during the first event, while hydrodynamic effects prevailed in the second one

Impact load estimation on retention structures with the discrete element method

The design of countermeasures such as barriers and filter dams needs an accurate estimation of the impact load. However, debris flows typically contain poorly sorted grains, whose size can span several orders of magnitude. Large grains can induce impulsive loads on a barrier, and potentially clog the openings designed to induce self-cleaning after an event. The current modeling techniques, mostly based on continuum-based depth-integrated approximations, cannot accurately describe these mechanisms, and analytical approaches often fail to tackle this complexity. In an effort to reproduce a realistic impact load, a sample flow composed of grains is reproduced with a three-dimensional model based on the Discrete Element Method (DEM). The mass impinges upon a barrier with a prescribed velocity. The barrier design is inspired by a monitored dam built on a catchment located in the Italian Alps, which features multiple outlets. The grains can clog the outlets, forming frictional arches. The load pattern on the barrier is analyzed in terms of single-grain impact and of collective behaviors. The impulse transferred by the granular mass to the structure is then used as input for a structural analysis of the barrier through a Finite Element analysis. The results highlight how frictional chains can induce loads that are substantially different from those determined by standard analytical approaches

Design of active debris flow mitigation measures: a comprehensive analysis of existing impact models

Debris flows occur in mountainous areas characterized by steep slope and occasional severe rainstorms. The massive urbanization in these areas raised the importance of studying and mitigating these phenomena. Concerning the strategy of protection, it is fundamental to evaluate both the effect of the magnitude (that concerns the definition of the hazard), in terms of mobilized volume and travel distance, and the best technical protection structures (that concerns the mitigation measures) to reduce the existing risk to an acceptable residual one. In particular, the mitigation measure design requires the evaluation of the effects of debris flow impact forces against them. In other words, once it is established that mitigation structures are required, the impacting pressure shall be evaluated and it should be verified that it does not exceed barrier resistance. In this paper, the author wants to focus on the definition and the evaluation of the impacting load of debris flows on protection structures: a critical review of main existing models and equations treated in scientific literature is here presented. Although most of these equations are based on solid physical basis, they are always affected by an empirical nature due to the presence of coefficients for fitting the numerical results with laboratory and, less frequently, field data. The predicting capability of these equations, namely the capability of fitting experimental/field data, is analysed and evaluated using ten different datasets available in scientific literature. The purpose of this paper is to provide a comprehensive analysis of the existing debris flow impact models, highlighting their strong points and limits. Moreover, this paper could have a practical aspect by helping engineers in the choice of the best technical solution and the safe design of debris flow protection structures. Existing design guidelines for debris flow protection barrier have been analysed. Finally, starting from the analysis of the hydro-static model response to fit field data and introducing some practical assumptions, an empirical formula is proposed for taking into account the dynamic effects of the phenomenon

Studies of Flexible Barriers Under Debris Flow Impact: An Application to an Alpine Basin

AbstractThe aim of this paper is to analyze the most relevant aspects that influence the interaction between debris flow phenomena and protection barriers. The volume of the debris and its lithological nature are conditioning the barrier size and strength. This system is often complicated by environmental and climate influences that need to be taken into consideration as well; therefore, a correct design of a protection barrier system in an alpine basin is a complex procedure that needs to be rationalized. This paper will concentrate on the barrier dimension design proposing a rational scheme of study of the global problem. The application to an Alpine basin is reported

A unified and modular coupling of particle methods with fem for civil engineering problems

In this work, a modular coupling approach for particle methods with the FEM (finite element method) is presented. The proposed coupled strategy takes advantage from the ability of particle methods of dealing with large displacements and deformations, especially when solving complex fluid–structure and solid–structure interaction problems. The coupling between the FEM and particle methods is done using a co-simulation approach implemented in the open-source Kratos Multiphysics framework. The particle methods considered in this work are the DEM (discrete element method) and the PFEM (particle finite element method). The Lagrangian description of the PFEM is well suited for modeling fluids undergoing large deformations and free-surface motions, and the DEM can be used to simulate rocks, debris and other solid objects. To accelerate the convergence of the coupled strategy, a block Gauss–Seidel algorithm with Aitken relaxation is used. Several numerical examples, with an emphasis on natural hazards, are presented to test and validate the proposed coupled method.Peer ReviewedPostprint (published version

Numerically based design of protection systems against landslides. Workshop Numeric in Geotechnology

fast flow-like landslides are the one of the most dangerous natural hazards. Rigid or flexible structures are often used to stop, deviate or slow down the flow. Because of the complexity of the landslide-barrier interaction, the design of these defence structures is still based on oversimplified and empirical approaches. Numerical methods able to cap-ture the essential features of the phenomenon can offer a valuable tool to gain a better understanding of the impact process and to support the design. This paper shows the potentialities of the MPM in this field. The run-up of the landslide and the dynamic forces on the structure are some of the fundamental parameters to define the properties of the barrier