Lahars risk at the Tacaná Volcano Complex (México–Guatemala) from numerical simulations and physical vulnerability analysis

The lahars generated at the active Tacaná Volcanic Complex (Mexico–Guatemala) can be particularly dangerous for three reasons: (1) the high altitudinal gradient between the volcano (4080 m asl) and the surrounding areas (sea level); (2) its location within one of the regions with the highest annual precipitation in Mexico (exceeding 4000 mm) prone to extreme rainfall events; and (3) its capability of ejecting large amounts of pyroclastic material as revealed by its past eruptive activity. In order to estimate the risk, eruptive, climatic, and spatial analyses were carried out at the watershed level associated with the TVC (Coatán, Cahoacán and Suchiate), which constitutes the basis for 89 simulations with nine volumes of lahar (from 3 × 105 m3 to 4.0 × 107 m3) using the numerical model TITAN2D (Two Phase Pitman-Le), considering debris and hyperconcentrated flows. The results show that lahars with volumes greater than 1.0 × 107 m3 and 2.0 × 107 m3 in the Coatán and Cahoacán watersheds, respectively, can reach distances greater than 30 km downstream along the main rivers, affecting Tapachula city and more than 100,000 thousand inhabitants. The threatened areas were spatially correlated with the physical vulnerability associated with the infrastructure (houses, bridges) of the different exposed localities in order to determine the risk level. The risk maps indicate that the localities at the high-risk level represent 25, 6.8 and 5.5% of the communities in the San Rafael Caldera (Guatemala), Cahoacán, and Suchiate (Mexico) watersheds, respectively. The localities at the medium-risk level represent 27.7% in the San Rafael Caldera, and a range of 2.2–2.8% in the rest of the watersheds. Some localities might be affected by lahars originating in different watersheds. This work represents a very valuable instrument for risk management and for preparing mitigation plans for this hazard.