Effect of rockfall fragmentation on exposure and subsequent risk analysis

Rockfalls are frequent natural processes in mountain regions with the potential to produce damage. The quantitative risk analysis (QRA) is an approach increasingly used to assess risk and evaluate the performance of mitigation measures. In case of the fragmentation of the falling rock mass, some of the hypothesis taken in the QRA estimation for rockfalls have to be modified since a single block or rock mass can produce several fragments thus modifying the runout probability, the impact energies and exposure of the elements at risk. In this contribution, we present a procedure to account for the exposure in QRA analysis along linear paths using the fragmental rockfall propagation model RockGIS (Matas et al. 2017). The procedure is applied at the “Monasterio de Piedra”, Spain as part of a QRA.Postprint (published version

Quantitative analysis of risk from fragmental rockfalls

The final publication is available at Springer via http://dx.doi.org/10.1007/s10346-018-1087-9Rockfalls are ubiquitous diffuse hazard in mountain regions, cliffs, and cutslopes, with the potential of causing victims and severely damaging buildings and infrastructures. A vast majority of detached rock masses break up when impacting the ground, generating multiple trajectories of rock fragments. In this paper, we present the quantitative risk analysis (QRA) of fragmental rockfalls. Fragmentation in rockfalls requires the redefinition of the probability of reach and the evaluation of the effect of multiple rock blocks trajectories on the exposure. An example of QRA was carried out at the Monasterio de Piedra, Spain, using RockGIS, a rockfall propagation model that takes fragmentation into account (Matas et al. Landslides 14:1565–1578, 2017). The results show that fragmentation has a significant but contrasting effect in the calculation of risk. The risk is reduced if the slope where blocks propagate is sufficiently long and gentle. The reason for this is that, compared to the unfragmented rock masses, the new fragments generated travel shorter distances with lesser kinetic energy. The effect disappears in case of large rockfalls. Conversely, the risk increases if the rock fragments propagate over steep slopes. The reason is that few blocks stop along the way while the generation of a cone of fragments increases the exposure. Our simulations also show that assuming a continuous flow of visitors or segregating the flow in groups of different number of people has only a minor influence on the results. Finally, we observed that the capability of the protection barriers to stop rockfalls of up to a few tens of cubic meters increases with fragmentation.Peer ReviewedPostprint (author's final draft

3D analysis of a fragmental rockfall

The final authenticated version is available online at https://doi.org/10.1007/978-3-030-60706-7_15.Fragmentation in a rockfall event influence the total number of fragments, the mass distribution, and the impact energies and runouts. Then, the probability of impact and hazard characterization should consider fragmentation. A fractal fragmentation model has been proposed in order to reproduce the phenomenon. The Rockfall Fractal Fragmentation Model has been implemented in a 3D rockfall propagation simulator named RockGIS. We present the analysis of a fragmental rockfall that occurred in Mallorca, Spain. Fieldworks are carried out in order to obtain the block size distribution of the rockfall deposit. A 3D terrain model is obtained using UAV surveys and digital photogrammetric techniques. The obtained 3D point cloud is cleaned of vegetation and used to create a Digital Elevation Model (DEM). The fragmentation model parameters and the propagation simulator coefficients have been calibrated to accomplish both, the resultant block size distribution and the runout distance of the blocks. The obtained results show a good reproduction of the fragmental rockfall studied. After the calibration is accomplished, older and future rockfalls in the cliff may be analyzed, considering thousands or single blocks simulations with or without fragmentation.This study was supported by the research project RockModels (Ref.BIA2016-75668-P, AEI/FEDER.UE) funded by the Spanish Ministry of Economy and Competitiveness. Acknowledge also, to the local government of Mallorca (Balearic Islands, Spain).Postprint (author's final draft

Implementation of a fixed-location time lapse photogrammetric rock slope monitoring system in Castellfollit de la Roca, Spain

When monitoring deformations in natural hazards such as rockfalls and landslides, the use of 3D models has become a standard. Several geomatic techniques allow the generation of these models. However, each one has its pros and cons regarding accuracy, cost, sample frequency, etc. In this contribution a fixed-location time lapse camera system for continuous rockfall monitoring using photogrammetry has been developed as an alternative to Light Detection and Ranging (LiDAR) and ground-based interferometric synthetic-aperture radar (GB-InSAR). The usage of stereo photogrammetry allows the obtention of 3D points clouds at a low cost and with a high sample frequency, essential to detect premonitory displacements. In this work the designed system consists of three digital single-lens reflex (DSLR) cameras which collect photographs of the rock slope daily controlled by a Raspberry Pi computer using the open-source library gPhoto2. Photographs are automatically uploaded to a server using 3G network for processing. This system was implemented at Castellfollit de la Roca village (Girona province, Spain), which sits on a basaltic cliff that has shown significant rockfall intensity in recent years. The 3D models obtained will allow monitoring rockfalls frequency, premonitory displacements, and calculate the erosion rate of the slope. All technical decisions taken for the design and implementation on this specific site are discussed and first results shown.This work has been carried within the framework of the research project Georisk “Avances en el análisis de la cuantificación del riesgo (QRA) por desprendimientos rocosos”, PID2019-103974RB-I00 funded by MCIN/AEI/ 10.13039/501100011033. We want to acknowledge the ICGC team for their recommendations during the design of the system and during its implementation on the field. Also, to Vilarrasa S.L enterprise for allowing the placement of the cameras on its buildings. Finally, we appreciate all help provided by the Montagut i Oix local council.Postprint (published version

Capturing rockfall kinematic and fragmentation parameters using high-speed camera system

This paper presents a procedure for tracking rockfall trajectories and extracting kinematic parameters from both the impacts and the resultant fragments. A set of full scale rockfall experiments was performed in a quarry located in Vallirana, Barcelona (Spain). The study site was chosen due to the presence of a rigid discontinuity surface, inclined at 42° in the middle of the slope, whose configuration was expected to favor the breakage of the blocks. The trajectories of the blocks released and of the resultant fragments were recorded with three video cameras. A C++ program was specifically developed to track the 3D trajectory of blocks and fragments, and measure velocities before and after the impact. Two different modules were implemented, one for the blocks that break and one for those that do not. The trajectory of a non-fragmented block is obtained by comparing it to its 3D model. In this way, both the center of mass position and the orientation of the block are tracked. For fragmented blocks, the local coordinates of the fragments determined from the images are converted to terrain coordinates using the program we developed. A total of 16 blocks and 36 rock fragments after impact were tracked. The parameters obtained were georeferenced and linked to a common system of 3D terrestrial coordinates. The captured parameters allow obtaining the velocity distribution of fragments, the coefficient of restitution, and energy balance for the blocks that break. To our knowledge, this is the first attempt to capture kinematic parameters of rock fragments that result from the impact and breakage of rock blocks in full-scale tests. Although the analysis of the rockfall fragmentation phenomenon is beyond of this work, we have compared the performance of the fragmented and unbroken blocks. To this purpose, we have built 3D models of the rock fragments generated using images captured with a drone. The results indicate that blocks that fragment show higher rebound velocities and coefficients of restitution than the blocks that do not although there exists a certain overlap between the two groups. Despite the experiment is carried out on the same discontinuity surface and with small variations in the impact velocities, impact kinetic energies and impact angles, the coefficients of restitution obtained present a wide range of values, both for the blocks that break and for those who do not. The number of tested blocks is too small to draw generalizable conclusions, but they highlight the stochastic nature of the rebound process and the necessity to consider additional parameters for its understanding. Finally, the results confirm the relation between the dissipated energy and, especially the impact energy and the new area created by fragmentation. Furthermore, the blocks that hit the ground with the face are those that generate the most new area while those that hit the vertex generate less.Peer ReviewedPostprint (published version

Simulación de desprendimientos rocosos con fragmentación mediante RockGIS

La fragmentación en desprendimientos rocosos es un fenómeno complejo y poco comprendido que afecta a la evaluación del peligro y del riesgo. Sólo algunos modelos de simulación de caídas de rocas lo consideran explícitamente. RockGIS es un programa de simulación numérica de desprendimientos rocosos que considera la fragmentación, empleando una aproximación puntual de la masa y un modelo de rebote mediante coeficientes de restitución. Su principal innovación es la implementación del modelo de fragmentación fractal (Rockfall Fractal Fragmentation Model, RFFM) para distribuir la masa al romper los bloques. Los parámetros del modelo de fragmentación se calculan según las condiciones cinemáticas de cada impacto para reproducir diferentes escenarios de fragmentación. El funcionamiento de RockGIS ha sido verificado y validado mediante ensayos a escala real y en diversos desprendimientos rocosos naturales. En esta contribución presentamos la evolución de RockGIS en los últimos años y su aplicación a diversos casos de estudio.Este estudio ha sido desarrollado en el marco del Proyecto de Investigación “Caracterización y modelado de desprendimientos de rocas” con el acrónimo RockModels, financiado por el Ministerio de Economía y Competitividad de España, y cofinanciado por la Agencia Estatal de Investigación (AEI) y el Fondo Europeo de Desarrollo Regional (FEDER) sobre el marco del Plan Estatal de Investigación Científico-Técnica e Innovación con código de referencia BIA2016-75668-P (AEI/FEDER,UE) y el proyecto de investigación “Avances en el análisis cuantitativo del riesgo de caída de rocas (QRA) incorporando desarrollos en geomática (GeoRisk)” con referencia PID2019-103974RB-I00, financiado por MCIN/AEI/10.13039/501100 011033, Ministerio de Ciencia e Innovación (MCIN) y la Agencia Estatal de Investigación (AEI).Postprint (published version

RockGIS: a GIS-based model for the analysis of fragmentation in rockfalls

A rockfall is a mass instability event frequently observed in road cuts, open pit mines and quarries, steep slopes and cliffs. After its detachment, the rock mass may disaggregate and break due to the impact with the ground surface, thus producing new rock fragments. The consideration of the fragmentation of the rockfall mass is critical for the calculation of the trajectories of the blocks and the impact energies and for the assessment of the potential damage and the design of protective structures. In this paper, we present RockGIS, a GIS-based tool that simulates stochastically the fragmentation of the rockfall, based on a lumped mass approach. In RockGIS, the fragmentation is triggered by the disaggregation of the detached rock mass through the pre-existing discontinuities just before the impact with the ground. An energy threshold is defined in order to determine whether the impacting blocks break or not. The distribution of the initial mass between a set of newly generated rock fragments is carried out stochastically following a power law. The trajectories of the new rock fragments are distributed within a cone. The fragmentation model has been calibrated and tested with a 10,000 m3 rockfall that took place in 2011 near Vilanova de Banat, Eastern Pyrenees, Spain.Peer ReviewedPostprint (author's final draft

Modelización de desprendimientos rocosos considerando la fragmentación

Determinar el riesgo al que está expuesto un edificio o una infraestructura debido a los desprendimientos rocosos mediante la Evaluación Cuantitativa de Riesgo requiere estudiar la peligrosidad, que integra la magnitud del fenómeno y la probabilidad de ocurrencia. Hoy en día la mayoría de metodologías disponibles para realizar estudios de peligrosidad no consideran la fragmentación de los bloques durante su propagación por la ladera. Este proceso tiene implicaciones directas en el estudio de la peligrosidad ya que se pueden incrementar el número de fragmentos y las energías cinéticas de los bloques disminuyen al reducirse su tamaño. En esta contribución se presenta el diseño e implementación de un módulo de fragmentación en un programa de simulación de desprendimientos rocosos llamado RockGIS y su aplicación a un caso de estudio.Postprint (published version

Rockfall fragmentation analysis: Vilanova de Banat case study

Fragmentation is a critical mechanism for the calculation of the trajectories of the blocks and the impact energies, for the assessment of the potential damage and for the design of protec-tive structures, although few rockfall models account for it. In this contribution we present an application of the trajectory simulation tool RockGIS, which explicitly accounts for fragmen-tation, to a recent rockfall event occurred near Vilanova de Banat (Spain). All parameters of the model controlling the kinematics of the propagation and fragmentation have been calibrat-ed in order to reproduce the number of fragments generated and trajectories followed by the blocks. Several performance criteria have been considered and simulations with and without accounting for fragmentation have been performed to assess their influence. The results con-sidering fragmentation show a reasonable matching with the observations in the field.Postprint (published version