Building a safe haven for our society against natural hazards

Objectius de Desenvolupament Sostenible::11 - Ciutats i Comunitats Sostenibles::11.5 - Per a 2030, reduir de forma significativa el nombre de morts causades per desastres, inclosos els relacio­nats amb l’aigua, i de persones afectades per aquests, i reduir substancialment les pèrdues econòmiques directes causades per desastres relacionades amb el producte interior brut mundial, fent un èmfasi especial en la protecció de les persones pobres i de les persones en situacions de vulnerabilita

Effect of density and total weight on flow depth, velocity, and stresses in loess debris flows

Debris flows that involve loess material produce important damage around the world. However, the kinematics of such processes are poorly understood. To better understand these kinematics, we used a flume to measure the kinematics of debris flows with different mixture densities and weights. We used sensors to measure pore fluid pressure and total normal stress. We measured flow patterns, velocities, and depths using a high-speed camera and laser range finder to identify the temporal evolution of the flow behavior and the corresponding peaks. We constructed fitting functions for the relationships between the maximum values of the experimental parameters. The hydrographs of the debris flows could be divided into four phases: increase to a first minor peak, a subsequent smooth increase to a second peak, fluctuation until a third major peak, and a final continuous decrease. The flow depth, velocity, total normal stress, and pore fluid pressure were strongly related to the mixture density and total mixture weight. We defined the corresponding relationships between the flow parameters and mixture kinematics. Linear and exponential relationships described the maximum flow depth and the mixture weight and density, respectively. The flow velocity was linearly related to the weight and density. The pore fluid pressure and total normal stress were linearly related to the weight, but logarithmically related to the density. The regression goodness of fit for all functions was >0.93. Therefore, these functions are accurate and could be used to predict the consequences of loess debris flows. Our results provide an improved understanding of the effects of mixture density and weight on the kinematics of debris flows in loess areas, and can help landscape managers prevent and design improved engineering solutions.Peer ReviewedPostprint (published version

Experiencias con drones para el estudio de movimientos de ladera

La tecnología de los vehículos aéreos no tripulados o drones (UAV) se ha desarrollado de forma exponencial los últimos años. Su uso para la toma de fotografías y el posterior tratamiento con técnicas de fotogrametría digital, permiten la generación de modelos 3D de alta resolución del terreno. Estas nuevas herramientas permiten realizar trabajos de caracterización geométrica de una forma más eficaz, más segura y con mayor resolución. En esta comunicación se presentan experiencias con esta tecnología en el campo del control de erosión y deposición o cambios en el tiempo, la caracterización geométrica para inventariar un desprendimiento rocoso, así como la identificación y caracterización del patrón de discontinuidades de un frente rocoso para el análisis de estabilidad y la cuantificación de volúmenes inestables.Postprint (published version

Critical rainfall conditions for the initiation of torrential flows: results from the Rebaixader catchment (Central Pyrenees)

Torrential flows like debris flows or debris floods are fast movements formed by a mix of water and different amounts of unsorted solid material. They generally occur in steep torrents and pose high risk in mountainous areas. Rainfall is their most common triggering factor and the analysis of the critical rainfall conditions is a fundamental research task. Due to their wide use in warning systems, rainfall thresholds for the triggering of torrential flows are an important outcome of such analysis and are empirically derived using data from past events. In 2009, a monitoring system was installed in the Rebaixader catchment, Central Pyrenees (Spain). Since then, rainfall data of 25 torrential flows (“TRIG rainfalls”) were recorded, with a 5-min sampling frequency. Other 142 rainfalls that did not trigger torrential flows (“NonTRIG rainfalls”) were also collected and analyzed. The goal of this work was threefold: (i) characterize rainfall episodes in the Rebaixader catchment and compare rainfall data that triggered torrential flows and others that did not; (ii) define and test Intensity–Duration (ID) thresholds using rainfall data measured inside the catchment by with different techniques; (iii) analyze how the criterion used for defining the rainfall duration and the spatial variability of rainfall influences the value obtained for the thresholds. The statistical analysis of the rainfall characteristics showed that the parameters that discriminate better the TRIG and NonTRIG rainfalls are the rainfall intensities, the mean rainfall and the total rainfall amount. The antecedent rainfall was not significantly different between TRIG and NonTRIG rainfalls, as it can be expected when the source material is very pervious (a sandy glacial soil in the study site). Thresholds were derived from data collected at one rain gauge located inside the catchment. Two different methods were applied to calculate the duration and intensity of rainfall: (i) using total duration, Dtot, and mean intensity, Imean, of the rainfall event, and (ii) using floating durations, D, and intensities, Ifl, based on the maximum values over floating periods of different duration. The resulting thresholds are considerably different (Imean = 6.20 Dtot-0.36 and Ifl_90% = 5.49 D-0.75, respectively) showing a strong dependence on the applied methodology. On the other hand, the definition of the thresholds is affected by several types of uncertainties. Data from both rain gauges and weather radar were used to analyze the uncertainty associated with the spatial variability of the triggering rainfalls. The analysis indicates that the precipitation recorded by the nearby rain gauges can introduce major uncertainties, especially for convective summer storms. Thus, incorporating radar rainfall can significantly improve the accuracy of the measured triggering rainfall. Finally, thresholds were also derived according to three different criteria for the definition of the duration of the triggering rainfall: (i) the duration until the peak intensity, (ii) the duration until the end of the rainfall; and, (iii) the duration until the trigger of the torrential flow. An important contribution of this work is the assessment of the threshold relationships obtained using the third definition of duration. Moreover, important differences are observed in the obtained thresholds, showing that ID relationships are significantly dependent on the applied methodology.Peer ReviewedPostprint (author's final draft

Monitoring of rainfall and soil moisture at the Rebaixader catchment (Central Pyrenees)

The instrumental monitoring of torrential catchments is a fundamental research task and provides necessary information to improve our understanding on the mechanisms of debris flows. While most monitoring sites include meteorological sensors and analyze the critical rainfall conditions, only very few contain soil moisture measurements. In our monitoring site, the Rebaixader catchment, 11 debris flows and 24 debris floods were detected during the last nine years. Herein, the initiation mechanisms of these torrential flows were analyzed focusing on the critical rainfall conditions and the soil water dynamics. Comparing the temporal distribution of both rainfall episodes and torrential flows, the Kernel density plots showed maximum values for rainfalls at the beginning of June, while the peak for torrential flows is at July 20th. This means that highest probability of debris flows and debris floods triggering is about 1.5 months later than the one of rainstorms in the catchment. Thus, the antecedent rainfall and especially the soil moisture conditions may influence the triggering of torrential flows. In a second step, a new updated rainfall threshold was proposed including total rainfall duration and mean intensity. The analysis of soil moisture data was more complicated and no clear trends were observed in the dataset. Therefore, additional data has to be recorded in order to quantitatively analyze the role of soil moisture on the triggering of flows and for the definition of thresholds. Some preliminary results show that the soil moisture at the beginning of a rainfall event affects the maximum increase of soil moisture, while a slight trend was visible comparing the initial soil moisture with the necessary rainfall amount to trigger a torrential flow.Postprint (published version

Sistema de alerta para procesos torrenciales a escala regional combinando mapas de susceptibilidad y datos del radar meteorológico

Los procesos torrenciales como las corrientes de derrubios o flujos hiperconcentrados causan frecuentemente daños importantes e incluso muertos en zonas montañosas. Para afrontar este peligro, los sistemas de alerta son una herramienta muy útil en la mitigación de los impactos de estos procesos hidro-morfológicos. El presente estudio ha desarrollado, implementado y validado un sistema de alerta a escala regional que determina en tiempo real el nivel de alerta en cada subcuenca de una zona seleccionada. Los principales datos de entrada son un mapa de susceptibilidad y la situación meteorológica que se obtiene mediante el radar. Mediante la aplicación de la técnica de lógica difusa y funciones de pertenencia, el mapa de susceptibilidad y el campo de lluvia se determina en cada subcuenca una de tres posibles clases, y finalmente con una matriz de alerta se calcula uno de los tres niveles de alerta (baja, media o alta). Los resultados del sistema de alerta y su validación han sido muy positivos y demuestran la gran utilidad de estas herramientas.Postprint (published version

Análisis simplificado del potencial de reactivación de coladas de tierra

Mudslides are landslides which usually reactivate. A simplified method for the assessment of the reactivation potential of narrow mudslides is proposed here. The method uses the factor of safety, calculated by the limit equilibrium technique, but incorporating an additional strength component due to friction in the lateral margins of the mudslide, a component which typically is neglected. The approach requires a detailed geomorphological reconnaissance of the mudslide to locate the sliding surface from transversal cross-sections. The method was applied to three mudslides in the eastern Pyrenees. The safety factor was calculated hypothesizing the depth of the water table in the mudslide body. The results show that the lateral strength component is highly important and that the three mudslides will probably reactivate in the future. The technique was checked by comparing the safety factors calculated with the frequency of past reactivations, which was obtained independently by dendrochronological dating.Postprint (published version

Estimación de la erosión producida por las corrientes de derrubios mediante técnicas de minería de datos con datos de campo y topográficos

La erosión del lecho es un proceso común de las corrientes de derrubios, y aumenta considerablemente el volumen de éstas. En este estudio se analizan datos geomorfológicos y topográficos obtenidos de 17 torrentes donde se han producido corrientes de derrubios. Sobre la base de datos se aplicaron dos técnicas de minería de datos: regresión lineal múltiple y árbol de decisión, con la intención de desarrollar un modelo para predecir las tasas de erosión. Los modelos obtenidos mediante las dos técnicas fueron validados e implementados en grupos de datos independientes con la finalidad de evaluar su efectividad. El modelo del árbol de decisión mostró más ventajas que la regresión lineal múltiple. Finalmente se propuso un modelo, basado en el árbol de decisión, para estimar la tasa de erosión, que incluye tres factores condicionantes: la disponibilidad de sedimento, el pendiente del lecho y la forma de la sección transversal.Peer ReviewedPostprint (published version

Experiences of rapid landslides monitoring and warning at catchment scale in the Pyrenees: the example of the Rebaixader test site

Monitoring of rapid landslides, as rockfalls and debris flows, improves the understandings of these processes and also provides fundamental information for an efficient early warning and alarm system (EWAS). The Rebaixader site is a typical high mountain catchment, where sort of torrential phenomena occur with a sub-annual frequency. To investigate the triggering conditions and the post-failure behaviour of the rapid landslides, a monitoring system consisting of five stations was set up progressively since July 2009. Two stations monitor landslide propagation characteristics by using eigth geophones, an ultrasonic device and a video camera and infrared spot lights. Until March 2013, six debris flows, eleven debris floods and four rockfalls (two of them were large boulder falls, 55 and 18 m3) were recorded at the site. The analysis of ground vibration signal allowed defining preliminary thresholds for distinguishing between debris flows, debris floods and rockfalls. The identification of processes was checked by the analysis of the video images and the post-event field evidences. Field surveys, ground vibration records and the movies recorded at the video camera provided unique information on the rapid landslides occurrence and dynamics, and gave an excellent opportunity to show the importance of monitored data for calibrating of simulation models. The monitoring results show that the data gathered by the geophones provides valuable information for the design of alarm systems for the event detection and distinction between processes.Postprint (published version

Numerical modelling of Val d’Arán landslide with Material Point Method

Flow-like landslides in mountainous areas can cause extensive damages due to their high velocity and long run-out distance. This work presents a real case of landslide occurred on 11 May 2018 in Val d’Arán (Catalonia, Spain). It involved about 50.000 m3 of glacial and colluvial material, travelling about 250 m until the valley floor and climbing about 100 meters on the opposite hillside. With the aim to assess the capabilities of the Material Point Method (MPM) [1,2] plane strain analyses with the 2-phases 1-point formulation [3,4] are conducted on a representative section. First, the slope material is described with the Mohr-Coulomb criterion. Second, a constitutive model based on the critical state theory (Ta-Ger [5,6]), is adopted. The simulations results led to conclude that an advanced constitutive model, able to simulate the strength loss of the material during the movement, is required to reproduce flow-like landslides and to obtain realistic results in terms of long run-out distance