Secondary Lahars Impacting on Building Structures at Chimborazo Volcano: A Retrospective and Scenario-Based Modeling Approach

The intense melting of glacial ice and permafrost can increase the presence of temporarily stored liquid water in dynamic high-alpine environments. A sudden release of this water, especially in volcanic settings, might trigger a process chain of severe consequences. During a period of increased periglacial degradation between 2015 and 2017, several large-volume (> 6.0 × 10⁵ m³), outburst-related secondary lahars damaged local infrastructure on the populated southeastern slopes of Chimborazo volcano in Ecuador. The insufficient understanding of secondary lahars associated with the sudden outburst of water complicates the identification of initiating processes and hinders the ability to decipher the governing mechanisms involved during propagation.

In this study, we present how we (1) identified initiation mechanisms of past secondary lahars at Chimborazo, (2) numerically back-calculated these events, (3) developed future lahar scenarios, and (4) quantified their impact on the local population. We performed a retrospective calibration approach to simulate a secondary lahar using the physics-based model RAMMS::Debris Flow. By introducing a novel two-stage outburst scenario development concept, we were able to predict potential future lahars. Finally, applying a standards-based verification of the structural components of residential development allowed us to evaluate the physical impact of potential lahars on infrastructure. We also assessed how increasing the wall thickness affects high- and low-risk areas.

Our results show that the observed secondary lahars can be numerically reproduced with a set of frictional parameters of µ = 0.028 (Coulomb-type friction) and ξ = 600 ms^-2 (turbulent friction). The model shows high agreement with locally obtained data (Vasconez et al., 2021) on total lahar volume, flow distance, discharge, and flooded area (deviation from target value = 20 %). By comparing the climatic and topographical situation of similar events at other study sites with the conditions at Chimborazo, we assume that glacial/periglacial destabilization processes may have accompanied the initiation of past lahars. Through deciphering the past initiation processes, our scenarios resulted in volumes between 2.7 × 10⁵ m³ (high probability) and 10.8 × 10⁵ m³ (very low probability) for a climatically derived reference period of 180 years. The structural validation of the component resistance identified high risk for approximately 24 % of the entire runout area. The adjustment to 11 cm wider bricks reduces this area by 5 %.

Only a precise quantification of the ice content and dynamic behavior within the source region enable to estimate the influence of destabilization processes on lahar initiation. However, this work makes an important contribution to supporting informed decision-making in land use planning by implementing an interdisciplinary methodology for analyzing the impacts of mass movements.

In this study, we showed that a retrospectively calibrated numerical model enables the simulation of future outburst-triggered lahars, and we further provided a quantification of their impact on downstream communities.

Vasconez, F.J., Maisincho, L., Andrade, S.D., Cáceres Correa, B.E., Bernard, B., Argoti, C., Telenchana, E., Almeida, M., Almeida, S. & Lema, V. (2021): Secondary Lahars Triggered by Periglacial Melting at Chimborazo Volcano, Ecuador. – Revista Politécnica, 48: 19–30.https://doi.org/10.33333/rp.vol48n1.02