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

Cotopaxi volcano (Ecuador) is famous for production of large-scale laharsthrough melting of ice and snow on its summit glacier. The lahar hazard inthe northern valleys of the volcano is assessed through numerical simulationof a maximum expected event. Considerations of past activity suggest that anevent like that of the 1877 eruption is the maximum expected lahar event.Review of the historical records reveals that northerly flowing lahars initiallyfollowed the Rio Pita and Rio Salto; at ``La Caldera'', owing to a sharp bendin the channel, the lahar partly overflowed into Rio Santa Clara. The laharsalong Rio Pita and Rio Santa Clara were conveyed to the Los Chillos valley.The simulation, using an initial flow volume of 60 × 106 m3reproduces the maximum heights reached by the 1877 lahar along the northernvalley. The volume of lahar triggered by an eruption similar to that of 1877 isestimated to have a volume about 2/3 of that of 1877. This hypothesized reductionof volume is attributed to shrinkage of the summit glacier over the past century.However, dramatic population growth along valleys exposed to lahar hazard overthe past 100 years makes the present risk from lahars higher than in the past. Thesharp bend of ``La Caldera'' represents a crucial site controlling lahar propagation:should a lahar overflow into the Santa Clara valley the risk increases considerablydue to the much higher concentration of human settlements along the valley. Resultsof a lahar simulation in which the entire flow is artificially forced into Rio Pita suggestthat construction of a dyke at ``La Caldera'' to prevent overflow would substantiallyreduce the general risk in the area.