From examination of natural events a proposal for risk mitigation of lahars by a cellular automata methodology: a case study for Vascún valley, Ecuador

Abstract

Abstract. Lahars are erosive floods, mixtures of water and pyroclastic deposits, they look the biggest environmental disaster as number of fatalities in the volcanic areas. Security measures have been recently adopted in the threatened territories, by constructing retaining dams and embankments in key positions. Such solutions could involve a strong environmental impact for the works and the continuous accumulation of volcanic deposits, such that equilibrium conditions could lack far, triggering more disastrous events. An improved version of the Cellular Automaton model LLUNPIY for lahars simulations is presented. The growing frequency of lahars, maybe for the climatic change, in the Vascún Valley of Tungurahua Volcano, Ecuador has recently produced smaller and less dangerous events, sometimes favoured by the collapse of temporary ponds, generated by small landslides. An investigation is here performed in order to reproduce such situations in controlled way by the use of LLUNPIY simulations. Using precise field data, points are individuated where dams by backfills, easy to collapse, can produce momentary ponds; LLUNPIY simulations permit to project triggering of small lahars by minor rainfall events or to project in the case of larger rainfalls the anticipation of lahar detachment, avoiding simultaneous and dangerous confluence with other lahars.

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

“…Incorporating the wide range of flow behaviours and the dynamic processes characteristic of lahars in runout analyses poses a challenging task, so that any theoretical model or practical simulation tool necessarily draws on physical and rheological simplifications of varying degrees. Different computer codes simulating lahars have been proposed, including the widely used empirical model LAHARZ (Iverson et al, 1998), which predicts inundation limits (Delaite et al, 2005; Huggel et al, 2008; Lee et al, 2015; Muñoz‐Salinas et al, 2009; Pistolesi et al, 2014; Schneider et al, 2008; Worni et al, 2011), the cellular‐automaton model LLUNPIY (Lupiano et al, 2020; Machado et al, 2015), and a variety of physically based numerical models that apply different rheology assumptions. Among these, 1‐D models are today largely supplanted by multiscale continuum models with depth‐averaged equations for conservation of mass and momentum.…”

“…The simulation of entrainment processes either requires the input of user‐specific growth rates or of process‐based erosion rates as a function of velocity (Fagents & Baloga, 2006) or shear stress (Frank et al, 2015; Iverson, 2012). Although lahars develop a considerable erosive force related to flow height, flow density and shear stress, lahar models have only rarely addressed erosional processes or volumetric changes (Carrivick, 2007; Carrivick et al, 2010; Fagents & Baloga, 2006; Lupiano et al, 2020), largely due to the shortcomings of the models used. These models are forced to start the calculation already with the hypothetical total flow volume, either on the volcano edifice or in channelized topography further downstream.…”