Modeling Syneruptive Lahars at Cotopaxi Volcano, Ecuador: Advantages and Uncertainties

Cotopaxi volcano (5897 m a.s.l.) is one of the most dangerous glacier-capped volcanoes worldwide and is located at the border of the densely populated Inter-Andean Valley. During the last major eruption (VEI 4) in 1877, large-scale lahars were triggered on Cotopaxi's flanks that severely damaged the settlements of Latacunga and Valle de Los Chillos about 45 km downstream. Up to now, lahar models of Cotopaxi do not yet consider confluence and erosion effects along the pathway.

Here, we introduce an erosive lahar model with calibrated parameters, offering new insights into downstream evolution of lahars at Cotopaxi. Modelling is carried out using the RAMMS 2-D numerical simulation code, which includes an entrainment module to determine potential erosion depth. First, we back-calculate the well documented 1877 lahar event using selected historic records along the northern trajectory to Valle de Los Chillos. Careful calibration of release volume, erosion and friction parameters of the Voellmy-Salm rheology allows to evaluate the sensitivity and range of model input parameters as well as to estimate uncertainties of the model. Our results show that historic records match best with parameter combinations of very low Coulomb-type friction μ (0.005 to 0.01) and high turbulent friction ξ (1000 to 1800 ms^-2). Observed low Coulomb friction values and low erosion depth of the modeled lahars compared to RAMMS debris flow applications in literature may be attributed to scaling effects of huge lahars. Finally, we apply the calibrated model to typical eruption scenarios of Cotopaxi (VEI 1 to >4), with release volumes ranging from 3 to 60 million m³, in order to better estimate the intensity of future primary lahars at Cotopaxi volcano.

Our results contribute to the multi-hazard risk assessment in the RIESGOS project (2017-2020) funded by the German Ministry of Education and Research. In this study, we show very carefully calibrated lahar models capable of reproducing previously non-respected effects such as confluence, erosion reach and propagation speed, which provide more accurate representation of key processes shaping lahar hazards.