On the use of the calibration-based approach for debris-flow forward-analyses

Abstract. In the present paper the problem of modeling the propagation of potential debris flows is tackled resorting to a numerical approach. In particular, numerical analyses are carried out with the RASH3D code, based on a single-phase depth-averaged continuum mechanics approach. Since each numerical analysis requires the selection of a rheology and the setting of the rheological input parameters, a calibration-based approach, where the rheological parameters are constrained by systematic adjustment during trial-and-error back-analysis of full-scale events, has been assumed. The back-analysis of a 1000 m3 debris flow, located at Tate's Cairn, Hong Kong, and the forward-analysis of a 10 000 m3 potential debris flow, located in the same basin have been used to investigate the transferability of back-calculated rheological parameters from one case to another. Three different rheologies have been tested: Frictional, Voellmy and Quadratic. From obtained results it emerges that 1) the back-calculation of a past event with different rheologies can help in selecting the rheology that better reproduces the runout of the analysed event and, on the basis of that selection, can give some indication about the dynamics of the investigated flow, 2) the use of back-calculated parameters for forward purposes requires that past and potential events have similar characteristics, some of which are a function of the assumed rheology. Among tested rheologies, it is observed that the Quadratic rheology is more influenced by volume size than Frictional and Voellmy rheologies and consequently its application requires that events are also similar in volume

A DEM evaluation of the clogging probability of debris-flow barriers: from the contact parameters to the overall interaction mechanism

In this work, we study the interaction between geophysical phenomena such as debris ow, and retention structures. We propose a numerical tool based on the Discrete Element Method, where the frictional nature of natural grains is reproduced through the implementation of frictional contacts and rolling resistance. The relationship between the contact parameters and the overall behavior of the granular mass is investigated through a set of heap-formation calibration tests, which allow to track the transition from a fluid regime to stable granular structures. The information gathered in the calibration tests is then used to study the stoppage of a granular flow on an incline. This is induced by placing a slit dam with a single vertical opening. The relative size between slit opening and grain diameter determines the overall e ciency of the retention, but also the basal friction controlled by the channel inclination plays a vital role. Numerous threshold are in this way obtained. The grains clog instantly for small relative opening sizes, and flow freely when a very large slit is used. Two secondary thresholds exist in the intermediate range, corresponding to a partial release of the mass through multiple consecutive clogging and ruptures of the grains behind the slit, and to the delayed formation of a single two-dimensional arch at the base. The consequences for the design of structures of this type are then discussed

Debris-flow interaction with slid barriers: efficiency and clogging mechanism

The design of structures for debris flow mitigation is still mostly based on the application of simple, empirical formulas. Standard practice suggests that slit structures might be a convenient solution for partially stopping the flow by filtering out the largest grains, which represent the most hazardous component. However, efforts towards the development of rational approaches for the design of the abovementioned structures have been hindered by the complexity of the debris materials. The interaction between a granular flow composed of monodisperse grains and a slit barrier is in this work reproduced by a discrete numerical model. The model is validated by comparison with a set of experimental runs with an analogous geometry. While the model greatly simplifies reality, it is anyway able to give insight in how clogging, and eventually stoppage, develops. Traditionally, the slit size is prescribed by choosing a multiple of the desired grain size to be stopped. The results highlight how stoppage can be achieved with a relatively wide range of possible slit openings. A family of recurring behaviors is observed, depending on the amount of entrained material and on whether stoppage is instantaneous or delayed. Finally, the initial assumptions are relaxed by moving to a bi-disperse flow. The results of the monodisperse case can be generalized to the bi-disperse case by definition of an equivalent radius

Advance survey and modelling technologies for the study of the slope stability in an Alpine basin

Alpine basins are typically characterised by an amphitheatre shape with steep rocky walls on the upper, a deposition zone of glacial debris in the middle and a channel in the lower part. All different parts are in constant evolution, and different kinds of instability phenomena can be identified: rock fall at the top rocky walls, rotational sliding of the deposit and debris flow in the channel down the valley. The different kinds of instability are somehow connected among them since the rock fall can power the rock debris that can trigger a debris flow. All different phenomena are chained in a global basin evolution also connected with seasonal climate variation that can induce different water presence and different water phase (liquid/solid). Moreover, instability phenomena seam to increase in frequencies and magnitudes in the latest decades possibly connected to climate change. This paper reports a study of the stability condition of an Alpine basin in North-West Italy by applying advance survey and modelling techniques: aerial photogrammetric survey of the rock wall, limit equilibrium methods that take ice presence into account and finally numerical analysis of the debris evolution along the slope. Parametric analyses aimed to quantify the influence of the different most important aspects have also been carried on. The application of advanced tools helped to better understand the study area failure and evolution mechanisms and to identify the main points to investigate in detail

Comparison of depth-averaged and full-3D model for the benchmarking exercise on landslide runout

Two models, with fundamental differences in their approaches, are used for modeling benchmarking exercise. The first, RASH3D, is based on a set of depth-averaged equation, solved in an Eulerian framework. The second, HYBIRD, employs a 3D Lattice-Boltzmann Model (LBM), i.e. the conservation equations are not depth-averaged and therefore multiple velocity measures are available over the depth. The model output are compared for what concerns the Yu-Tung debris flow back-analysis. Two additional cases are then analyzed using RASH3D

From depth-averaging to fully three-dimensional modelling of debris-flow dynamics

Two numerical codes, with fundamental differences in their approaches, are used for modeling the Yu Tung debris flow, which occurred in Hong Kong in 2008. The first code, RASH3D, is based depthaveraged St. Venant equations, solved in an Eulerian framework. The second code, HYBIRD, is fully 3D and based on Lattice-Boltzmann Model (LBM), i.e. the conservation equations are not depth-averaged and therefore multiple velocity measures are available over the depth. The two model output are compared and discussed

Analysis of the load exerted by debris flows on filter barriers : comparison between numerical results and field measurements

The hazard posed by debris flows onto mountainous settlements often requires structural countermeasures, such as barriers, to be installed. There is a worldwide trend in employing barriers that are partially impervious to the flow, trapping the coarsest sediment, and reducing the erosive power of the flow early on. However, many design choices with respect to effectiveness and structural integrity are not trivial, because there is a poor knowledge of the flow-structure interaction mechanism. In this work, we report results from a monitoring campaign on a barrier installed within an experimental site. At the site, the structural response of the barrier is recorded any time an event occurs. However, the results exhibit features that do not fully reconcile with the load model prescribed by the available guidelines. To gather insight, we propose a numerical study based on the use of the Discrete Elements Method for the flow simulation and the Finite Element Method for the structural response of the barrier. The compatibility between site measurements and numerical output validates the use of the DEM-FEM model. It also highlights certain key details on the load pattern on the barrier that are not yet included in existing guidelines

Study of the influence of baffles on an artificial debris flow through back-analysis simulations

In this work, we explore the applicability of a novel approach to the full-scale simulation of debris flows, the Lattice-Boltzmann Method (LBM). The main novelty lies in eliminating the need for depthintegrating the conservation equations, which is still a dominant approach in the field. A full 3D model, both for the topography and for the flow itself is therefore developed and employed. The 3D nature of the model allows to accurately reproduce structural countermeasures. An artificial debris flow, generated in real-scale at an experimental site in Korea, provides the basis for a cross comparison of results. The effects of two arrays of baffles were also tested in the experiment. The flow scale is intermediate between the large natural flows that are usually reported in the literature and a typical experimental apparatus. It is therefore an ideal candidate for an explorative application of the numerical metho