DEBRIS-FLOW HAZARDS IN EL SALVADOR—A DIFFICULT PROBLEM AND SIGNIFICANT OPPORTUNITY

Volcanic debris flows in El Salvador pose a significant risk to tens of thousands of people, and to property and important infrastructure. Major cities and nearly a third of the country’s >7 million population are located near San Salvador, San Vicente, and San Miguel volcanoes. Debris flows travelling as little as 4 km from source at these volcanoes put hundreds to thousands of lives at risk. A recent disaster highlights the problem. In November 2009, Hurricane Ida drew moisture from the Pacific Ocean into El Salvador, deluged parts of the country (355 mm of rain in 24 hr; intensities to 80 mm per hr), and triggered several shallow landslides and debris flows at Volcán San Vicente. Debris flows inundated parts of Guadalupe, Verapaz, and Tepetitán, villages located north of the volcano within 6 km of its summit, and a severe flood passed through the village of San Vicente to the northeast. Eight deaths in Guadalupe and 40–50 deaths in Verapaz were attributed to debris flows. In late May 2010, tropical storm Agatha again drew moisture into the country and dropped heavy rainfall (up to 94 mm in 48 hr) on the same areas affected by Hurricane Ida. That storm also triggered shallow landslides and small debris flows, and caused reworking of previous debris-flow deposits. At least 9 deaths were reported, and more than 8,000 people were evacuated to temporary shelters. Such events recur frequently at Volcán San Vicente. Rainfall-triggered debris flows, typically having volumes of 100,000–500,000 m³, have occurred at least five times in the past 90 years and affected some or all of the villages surrounding the volcano. Given the high frequency of debris flows and population density in vulnerable areas, El Salvador’s government faces a significant challenge in providing effective warnings and mitigating hazardous events. They must struggle with four basic questions: where will shallow landslides and debris flow occur; when will they occur, how large will they be, and how far will they travel? Salvadoran scientists and engineers are working to address these questions on an operational basis, and are also instituting public outreach. The recent disaster has spurred an opportunity for the governmental geological agency (SNET) to enhance its operational capacity to mitigate and hopefully avert future disasters.