Lahar modeling represents an excellent tool for designing hazard maps. It allows the definition of potential inundation zones for different lahar magnitude scenarios and sediment concentrations. Here, we present the results obtained for the 2001 syneruptive lahar at Popocatépetl volcano, based on simulations performed with FLO2D software. An accurate delineation of this event is needed, since it is one of the possible scenarios considered if magmatic activity increases its magnitude. One of the main issues for lahar simulation using FLO2D is the calibration of the input hydrograph and rheological flow properties. Here, we verified that geophone data can be properly calibrated by means of peak discharge calculations obtained by the superelevation method. Digital elevation model resolution also resulted as an important factor in defining the reliability of the simulated flows. Simulation results clearly show the influence of sediment concentrations and rheological properties on lahar depth and distribution. Modifying rheological properties during lahar simulation strongly affects lahar distribution. More viscous lahars have a more restricted aerial distribution and thicker depths, and resulting velocities are noticeably smaller. FLO2D proved to be a very successful tool for delimitating lahar inundation zones as well as generating different lahar scenarios not only related to lahar volume or magnitude, but also taking into account different sediment concentrations and rheologies widely documented as influencing lahar-prone areas.

On the basis of rotating drum analogue experiments, we describe a fragmentation process acting within debris flows during transport and its influence on rheologic behavior. Our hypothesis is based on a detailed textural analysis including granulometry, clast morphology, and rheologic properties of the fluid matrix. Results of the experiments point out that breakage of certain granulometric classes produces fine particles like fine sand and silt. The population growth of fine clasts with time leads to an increase in yield strength and viscosity that progressively modifies the rheologic behavior. From a textural point of view, this is reflected in a bimodal granulometric distribution. Up to now this characteristic has been explained as the effect of bulking and/or sedimentation processes during transport. Our experimental results show that the type of fragmentation depends on particle size and is the consequence of strong clast‐clast interaction and clast‐fluid interactions. Coarse particles develop small fractures which cause the loss of sharp edges and asperities. Medium‐sized particles develop through‐going fractures that cause them to break apart. The latter process explains why intermediate granulometric classes progressively diminish with time in debris flows. Analogue experiments enable us to study the efficacy of clast fragmentation in modifying the textural character and flow behavior of debris flows without the influence of external factors such as erosion and sedimentation. The obtained results constitute the base of a new approach for modeling debris flow dynamics.

A good understanding of the frequency and magnitude of lahars is essential for the assessment of torrential hazards in volcanic terrains. In many instances, however, data on past events is scarce or incomplete, such that the evaluation of possible future risks and/or the planning of adequate countermeasures can only be done with rather limited certainty. In this paper, we present a multiidisciplinary approach based on botanical field evidence and the numerical modelling of a post-eruptive lahar that occurred in 2012 on the northern slope of the Pico de Orizaba volcano, Mexico, with the aim of reconstructing the magnitude of the event. To this end, we used the debris-flow module of the rapid mass movement simulation tool RAMMS on a highly resolved digital terrain model obtained with an unmanned aerial vehicle. The modelling was calibrated with scars found in 19 Pinus hartwegii trees that served as paleo stage indicators (PSI) of lahar magnitude in a sector of Jamapa Gorge. Using this combined assessment and calibration of RAMMS, we obtain a peak discharge of 78 m³ s⁻¹ for the 2012 lahar event which was likely triggered by torrential rainfall during hurricane “Ernesto”. Results also show that the deviation between the modelled lahar stage (depth) and the height of PSI in trees was up to ±0.43 m. We conclude that the combination of PSI and models can be successfully used on (subtropical) volcanoes to assess the frequency, and even more so to calibrate the magnitude of lahars. The added value of the approach is particularly obvious in catchments with very scarce or no hydrological data at all and could thus also be employed for the dating and modelling of older lahars. As such, the approach and the results obtained can be used directly to support disaster risk reduction strategies at Pico de Orizaba volcano, but also in other volcanic regions.

Monitoring the state of active volcanoes is the foundational component of volcanic risk reduction strategies. To a large extent, these responsibilities rest with volcano observatories. Based on the 11 Reports that constitute this Special Issue—“Volcano Observatories in Latin America”—we provide a comprehensive overview of the work that has been carried out by the observatories in Latin America, a region in which tens of millions of people are exposed to volcanic activity. Since the first steps taken in the 1980s, volcano observatories of the region have made significant progress in assessing and monitoring volcanic activity. Currently, 17 institutions officially contribute to monitoring 135 volcanoes in 10 countries. Along with the improvements in the instrumental, technical, and operational capabilities, advancements have been made in long-term hazard assessment and hazard communication. But despite all the progress accomplished, several challenges and limiting factors still remain, such as the lack of financial resources and training opportunities. Efforts should be focused on increasing the number and quality of monitoring networks. El monitoreo del estado de los volcanes activos es un componente fundamental de las estrategias para la reducción del riesgo volcánico. En gran medida, estas responsabilidades recaen en los observatorios volcánicos. A partir de los 11 Reportes que constituyen este Número Especial –“Observatorios volcanológicos en América Latina”– brindamos un detallado resumen del trabajo llevado adelante por los observatorios en Latinoamérica, una región con decenas de millones de personas expuestas a la actividad volcánica. Desde sus primeros pasos a principios de 1980, los observatorios volcanológicos de la región han logrado avances significativos en la evaluación y vigilancia de la actividad volcánica. Actualmente, 17 instituciones contribuyen oficialmente al monitoreo de 135 volcanes en 10 países. Junto con las mejoras en sus capacidades instrumentales, técnicas y operativas, se produjeron avances también en la evaluación y comunicación de peligros a largo plazo. A pesar del avance logrado, aún persisten desafíos y factores limitantes, como la falta de recursos económicos y oportunidades de capacitación. Los esfuerzos futuros deben centrarse en aumentar el número y la calidad de las redes de monitoreo.