Assessment and modelling of two lahars caused by "Hurricane Stan" at Atitlan, Guatemala, October 2005

Abstract

In October 2005, massive slope failures were triggered all around Guatemala by the heavy rainfall that accompanied Hurricane Stan and unchained a series of landslides, debris flows and lahars. Two communities called Panabaj and San Juan La Laguna located inside the Atitlan caldera in the highlands of Guatemala were seriously affected by these events. Panabaj located in the aprons of the Toliman volcano was completely destroyed and more than 1,200 people were killed by a slope failure that created a lahar. San Juan La Laguna is located at the border of an alluvial fan that is a natural channel for discharge of the San Pedro volcano, a whole neighborhood of the community was destroyed by a lahar created by a small slope failure in the southwestern part of the volcano and by a series of landslides and contributive flows from the steep side of the basin that added material to the lahar. The volcanic lahars happened almost at the same time. A peak of precipitation of nearly 300 mm in 24 hours (54.4 mm, three hours before the lahars occurred), a volcanic setting and steep slopes near the borders of the craters was the scenario where the 2005 events took place. This study gives a background of the two lahar events and simulates them by using two computer programs. Although very close in location, the flows had different types of origin and post failure behavior. A description of the geology, past lahar events and climate of the area is done to give an insight of the context where the events occurred. An important objective of this thesis is also to study the capabilities and use of the two computer programs RAMMS and FLO 2D, for possible future applications at NGI (Norges Geotekniske Institutt) and ICG (International Centre for Geohazards).

The Panabaj lahar was simulated with the RAMMS computer program and the San Juan La Laguna lahar was simulated with the FLO 2D computer program. Different computer programs were selected for this study because of the differences in the nature and behavior of the two lahars. The two computer programs are described with their numerical models. The results of these simulations are analyzed and compared with the field observations. Using a back analysis, a calibration of the parameters that best fitted the event in Panabaj was made with the RAMMS program. A selection of the best suited input parameters and a discharge hydrograph based on the accumulated rainfall was built to produce a simulation of the San Juan La Laguna slide.

In the Appendix A, the chronology of Hurricane Stan in Guatemala is described. In the Appendix B, a simulation of a tsunami that took place in the Lake Atitlan one hour after the Panabaj lahar happened, was done combining the RAMMS and the FLO 2D program. The tsunami was generated by a lahar from the San Pedro Volcano and affected the shores of Santiago and Panabaj communities.