Typhoon landslide risk analyzed systematically
The work of civil and environmental engineers has tremendous positive impact on the health and welfare of individuals and communities. One way to expand this impact in the developing world is to transfer technology and engineering expertise globally, while adapting it to local needs. This issue of On Balance looks at such a transfer of technology for hazard and risk assessment to help prevent typhoon-induced landslides, which pose a huge risk for Southeast Asia. Landslides cause thousands of deaths and billions of dollars in property damage each year. In 2006, a landslide destroyed the entire village of Guinsaugon, Philippines, killing most of its 1,857 residents. The mountainous region near Baguio City, Philippines, about 200 kilometers north of Manila, experienced some 65 recorded landslides between 1991 and 2004. Baguio sits 1,500 meters above sea level, has an annual rainfall of 3,648 millimeters, and holds the world record for most rainfall in 24 hours: 1,168 millimeters in July 1911. As a desirable tourist destination, Baguio has become increasingly populous and developed, leaving the area in greater danger of landslides and increasing the human and economic costs when they occur. Blasting for mining, and deforestation from logging and slash-and-burn farming also contribute to the problem, but no system exists for analyzing and managing the landslide risks.
Professor Herbert Einstein of the MIT Department of Civil and Environmental Engineering and graduate student Artessa Saldivar-Sali developed a simple system for determining a landslide risk rating based on commonly available data for a 50-square-kilometer area surrounding Baguio City. Using the area’s landslide history and hazard contributory factors—type of underlying bedrock, slope gradient and vegetation growth—they determined a hazard rating for each slope. They converted that into an overall landslide risk rating by using multipliers based on land use and population.
The researchers concluded that landslide hazard is determined by a combination of two factors: the underlying bedrock and the slope gradient. They found that landslides are less common in areas with limestone bedrock, even though these may be relatively steep. While roughly half of the bedrock in the area is from the Pliocene Baguio Formation, only 5.7 percent of landslides occur on this, and more than half of those took place on moderate slopes. Broadleaf trees provided the least amount of protection on this type of bedrock. But in general, a mix of broadleaf trees or bushes and scrub provide the best protection. The highest incidence of landslides in the area (14.3 percent) occurs on bedrock from the oldest geologic era represented, the Cretaceous Pugo Formation of volcanic rock, which accounts for only 1.4 square kilometers of the total area.
This technology is especially applicable to areas of the world where a detailed landslide risk analysis has not been performed, because the system is based on characteristics that can be assessed in the field or from available records. It could be applied directly in any country with similar topography, geology and climate to the Philippines to help planners improve building codes, determine zoning, and strengthen mitigation measures in mountainous tropical regions hit frequently by typhoons.
Einstein and Saldivar-Sali report these results in the May 2007 issue of Engineering Geology. Einstein recently received the Award for Outstanding Contributions to Rock Mechanics from the American Rock Mechanics Association. For more information, write to OnBalance@mit.edu.
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