Modelación Numérica y Mapas de Afectación por Flujo de Lahares Primarios en el Drenaje Sur del Volcán Cotopaxi

Abstract

Los lahares primarios originados durante erupciones de volcanes nevados, como el volcán Cotopaxi, son el resultado de la combinación de mecanismos físicos relacionados con el fenómeno eruptivo como la expulsión de ceniza, material piroclástico y flujos de lava incandescente que provocan el derretimiento súbito de una porción del glaciar. Afectan directamente asentamientos humanos e infraestructura desarrollada a lo largo de los cauces de los ríos y llanuras que corresponden a los drenajes naturales por donde transitan los lahares. El periodo de recurrencia eruptiva del volcán es relativamente amplio considerando la más reciente erupción significativa que ha sido registrada en junio de 1877. La investigación se enfoca en la modelación numérica unidimensional para flujo no permanente realizada en el programa libre HEC-RAS, considerando información geológica, glaciológica, vulcanológica y cartográfica actual, generada y recopilada en campo durante los últimos años. Estos datos han sido analizados y considerados para la definición de los parámetros iniciales que corresponden a volúmenes e hidrogramas. El modelo numérico calibrado en base al evento histórico de 1877, constituye la base para la simulación de los escenarios probables de ocurrencia. Los resultados obtenidos permiten la generación de mapas de afectación referenciales que constituyen un aporte técnico y práctico, ya que pueden ser utilizados para tomar decisiones acerca de la definición de zonas de afectación, sitios seguros, planificación territorial, planes de concientización, recuperación y mitigación ante procesos eruptivos futuros del volcán Cotopaxi que afecten de manera particular el valle de Latacunga.

Modelling future lahars controlled by different volcanic eruption scenarios at Cotopaxi (Ecuador) calibrated with the massively destructive 1877 lahar

“…Nevertheless, we can compare them to the 1985 Nevado del Ruíz lahars due to similar characteristics related to (i) the total flow volume (~90 × 10 6 m 3 ), (ii) the triggering mechanism (VEI = 3 eruption), which generated pyroclastic flows interacting with the glacier cap, and (iii) the runout distance (70–100 km from the source). Moreover, (iv) recent models for lahars from Cotopaxi (Mothes et al, 2004; Ordóñez et al, 2013; Vera et al, 2019) use a bulking factor of 3 as a rule of thumb throughout all modelled eruption scenarios, which is the mean of the bulking factors of 2 to 4 reported for Nevado del Ruíz lahars (Pierson et al, 1990). However, volumetric growth observed at the 1985 Nevado del Ruiz event may not be directly transferable to Cotopaxi lahars for two reasons.…”

“…A common, but simplified approach to estimate lahar volumes at Cotopaxi (Barberi et al, 1992; Mothes et al, 2004; Ordóñez et al, 2013;Toapaxi et al, 2019 ; Vera et al, 2019) relies on three controls: the two dynamic controls include (i) variable thicknesses of ice and snow melted uniformly from the glacier surface during an eruption as a proxy for the degree of volcano‐glacier interactions, and (ii) the size of the summit glacier. A third, static control for estimation of total lahar volume is (iii) the assumption of a threefold volume growth of the lahar due to incorporation of solids.…”

“…This approach may not only lead to an overprediction of flow discharge and stage height, but also means that the dynamic evolution of syneruptive lahars, characterized by initial flow acceleration, rapid volumetric growth on the steep volcano flanks, and further erosion and deposition during downstream propagation, cannot be traced in the modelling process. Previous simulations for lahars from Cotopaxi using 1-D or 2-D flood-routing models (Aguilera et al, 2004;Barberi et al, 1992;Toapaxi et al, 2019;Vera et al, 2019)…”

“…Regarding the extent of Cotopaxi's glaciers, Cáceres (2017) reported that the surface area has diminished on average by about 52% between the years 1977 and 2016. With the calculation of lahar volumes being based on the melting thickness and the glacier extent, a glacier retreat of about 50% would directly translate into a 50% reduction in initial volumes of potential future lahars compared to the situation during the 1877 eruption (Toapaxi et al, 2019;Vera et al, 2019).…”

“…This approach may not only lead to an over‐prediction of flow discharge and stage height, but also means that the dynamic evolution of syneruptive lahars, characterized by initial flow acceleration, rapid volumetric growth on the steep volcano flanks, and further erosion and deposition during downstream propagation, cannot be traced in the modelling process. Previous simulations for lahars from Cotopaxi using 1‐D or 2‐D flood‐routing models (Aguilera et al, 2004; Barberi et al, 1992; Toapaxi et al, 2019; Vera et al, 2019) demonstrate the fluid‐mechanical component applicable in the distant channels but not applicable to the initial runoff on the steep volcano flanks. Simultaneously, Ordóñez et al (2013) and Pistolesi et al (2014) estimated inundation areas with LAHARZ (Ordóñez et al, 2013; Pistolesi et al, 2014) but could not reproduce initial lahar formation because the model neglects dynamic bulking processes on the volcano flank.…”