Numerical Analysis of a Potential Debris Flow Event on the Irazú Volcano, Costa Rica

Abstract

The active Irazú Volcano is the highest of several composite volcanic cones which make up the Cordillera Central in Costa Rica, close to the city of Cartago. The top of the volcano is strategic for the Country, since at the height of over 3400 m sit 84 telecommunication towers used by government agencies and several TV and radio stations, which guarantee the station coverage of more than 60 percent of the national territory. Since December 2014, a series of minor tremors, or microseisms, occurred at the Irazú and some open and deep fissures formed on the upper part of the volcano associated with formation of landslides. More research is needed to determine if these fissures are directly related to recent seismic activity. However, the landslide formation has made it necessary to relocate the towers and there is evidence of the possible destabilization of a volume of about 3.5 million cubic meters of material. In particular, if the landslide triggers in conjunction with heavy rains the movement could evolve into a huge debris flow that could affect Cartago city, similar to the debris flow disaster of December 1963. The dynamics of this potential event have been analyzed using the numerical code RASH3D. The calculated flow intensities and flow paths could be used to support hazard mapping and the design of mitigation measures. The reliability of the obtained results are a function of assumptions regarding source areas, magnitudes of possible debris flows and calibration of rheological characteristics, but also digital terrain model (DTM) quality. As to this last aspect, a systematic comparison of numerical results, DTM and air photos enabled identification of various weak points of the digital terrain model and identified potentially critical zones due to the presence of man-made structures.