Risk from Lahars in the Northern Valleys of Cotopaxi Volcano (Ecuador)

Cotopaxi volcano (Ecuador) is famous for production of large-scale lahars through melting of ice and snow on its summit glacier. The lahar hazard in the northern valleys of the volcano is assessed through numerical simulation of a maximum expected event. Considerations of past activity suggest that an event like that of the 1877 eruption is the maximum expected lahar event. Review of the historical records reveals that northerly flowing lahars initially followed the Rio Pita and Rio Salto; at “La Caldera”, owing to a sharp bend in the channel, the lahar partly overflowed into Rio Santa Clara. The lahars along Rio Pita and Rio Santa Clara were conveyed to the Los Chillos valley. The simulation, using an initial flow volume of 60×106 m3 reproduces the maximum heights reached by the 1877 lahar along the northern valley. The volume of lahar triggered by an eruption similar to that of 1877 is estimated to have a volume about 2/3 of that of 1877. This hypothesized reduction of volume is attributed to shrinkage of the summit glacier over the past century. However, dramatic population growth along valleys exposed to lahar hazard over the past 100 years makes the present risk from lahars higher than in the past. The sharp bend of “La Caldera” represents a crucial site controlling lahar propagation: should a lahar overflow into the Santa Clara valley the risk increases considerably due to the much higher concentration of human settlements along the valley. Results of a lahar simulation in which the entire flow is artificially forced into Rio Pita suggest that construction of a dyke at “La Caldera” to prevent overflow would substantially reduce the general risk in the area.