Natural hazard assessment for strategic infrastructures: a study of Cotopaxi lahars’ impact upon the Hidroagoyán Dam in Ecuador

In the aftermath of the catastrophic 1877 eruption of Cotopaxi volcano, Ecuador, lahars triggered in the summit cone, after making havoc of the city of Latacunga, flowed into the Pastaza River gorge, eventually reaching the Amazon lowlands. Presently, just downstream the town of Baños, the Hidroagoyán Dam impounds the water of the upper Pastaza, creating a reservoir of about two million m³, and annually producing 2520 GWh of energy, or about the 10% of the national demand of Ecuador. Should an 1877-scale Cotopaxi eruption occur nowadays, which is not unlikely after the 2015 reactivation of the volcano, similarly originated lahars might impact the dam, overwhelming the protective bypass system designed to contain anomalous flood waves of the Pastaza river. We present here an assessment of the hazard that such lahars may imply to the very functioning of Hidroagoyán. The investigation exploits the predictive power of LLUNPIY, a Cellular Automata model for primary and secondary lahars, already validated when simulating the 1877 Cotopaxi north and southward lahars as far as Tumbaco and Latacunga, respectively. Specifically, the present preliminary simulation succeeds for the first time to describe the flow of the lahars along the Pastaza gorge, thus reaching the dam in Baños and beyond. LLUNPIY simulates lahars in a discretized space-time, where the values of altitude, erodible soil depth, lahar thickness, kinetic head and lahar outflows are updated for each cell at each step according to the following processes: 1) Lahar flows determination and shift, 2) Detrital cover erosion, 3) Energy dissipation by turbulence, and 4) Melting of Cotopaxi ice cap by pyroclastic bombs, the latter process being limited to the cells corresponding to the glacier. Simulation inputs are morphology, erodible pyroclastic cover, extension of the Cotopaxi ice cap, pyroclastic bombs’ duration and frequency; by modifying their values we are able to predict several different hazard scenarios, which as a whole represent a reliable forecast of what might happen to the Hidroagoyán dam and the energy production of Ecuador in the case of a novel eruption of Cotopaxi volcano.

References

Lupiano V. et al. (2018). Revisiting the 1877 Cataclysmic Lahars of Cotopaxi Volcano by a Cellular Automata Model and Implications for Future Events. CSAE'18.

Lupiano V. et al. (2021). LLUNPIY Simulations of the 1877 Northward Catastrophic Lahars of Cotopaxi Volcano (Ecuador) for a Contribution to Forecasting the Hazards. Geosciences 2021, 11, 81.

Frimberger T. et al. (2021). Modelling future lahars controlled by different volcanic eruption scenarios at Cotopaxi (Ecuador) calibrated with the massively destructive 1877 lahar. Earth Surface Processes and Landforms.