Landslides

Landslide, mudslide, debris flow–it doesn’t matter what you call it, the outcome is the same: normally stable and stationary soil gets a bad case of wanderlust. In most cases, slides are triggered when too much water saturates a steep slope or when an earthquake shakes the ground loose. This can occur almost anywhere, but Central America–with its precipitous hills, frequent heavy rainstorms and unstable volcanic soil–is probably the number one hotspot. Ed Harp, a landslide geologist with the U.S. Geological Survey (USGS), describes the local soil as “like glass shards that interlock with each other.” This cohesion, he explains, enables the soil to adhere to near-vertical slopes without sliding off.

But during earthquakes, the volcanic soil becomes volatile. “If you shake this stuff, it collapses,” Harp says. It also has a sponge-like nature and can absorb a lot of water before it finally and catastrophically comes loose. In 1998, Hurricane Mitch stalled over Central America, and fierce rains caused hundreds of landslides. Mountains of mud buried thousands of villagers living near Nicaragua’s Casita volcano after torrential downpours saturated its slopes and the ground gave way.

Since then, new “hazard maps” of Central America have begun giving local officials a guide to slide-prone areas. To create many of the maps, geologists are using ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer), which rides on board NASA’s Terra remote-sensing satellite. ASTER can snap
pictures of Earth in 14 wavelengths and capture data as fine as 15 square meters per pixel. Closer to the ground, researchers are creating hazard maps of potentially unstable terrain with LIDAR (Light Detection and Ranging). A LIDAR camera-usually mounted to a small plane or helicopter-is like radar but employs pulsed light instead of radio waves.

After Hurricane Mitch, the USGS decided that an on-the-ground weather-monitoring network might be the best tool to save lives in Central America. The agency designed and funded a $3.8-million-dollar network for Guatemala, Honduras, Nicaragua and El Salvador. Today there are 56 stations, each of which measures rainfall intensity and river levels in real time and then transmits the data over satellite via radio uplink.

“We have sites in Central America that can receive the information directly,” explains Mark Smith, a surface-water specialist with the USGS who was involved in setting up the network. “Data transmits every three hours. But we put a threshold on the equipment, so if it starts raining really hard, then it goes into an emergency mode and sends information every five minutes or less.” When the next big hurricane slams into Central America, villagers with homes and farms nestled on unstable hillsides in places such as Tegucigalpa, Honduras, and Zompopera, El Salvador–where some of the worst slides occurred during Hurricane Mitch–might now get a heads-up that their world is about to come crashing down again.