March 19, 2020 – A new U.S. Geological Survey map of Puerto Rico shows the relative risks of landslides due to the kind of intense rainfall brought on by hurricanes. It identifies 20% of the island as at high risk, 9% at very high risk, and 1% at extremely high risk of landslides under those conditions.
About 40% of the island is identified as at moderate risk, while 30%, mostly flatter terrain near the coasts, is identified as at low risk of landslides after intense rainfall.
After powerful Hurricane Maria crossed virtually the entire island in 2017, deluging its mountainous terrain with heavy downpours, USGS ground-failure experts spent about two weeks crisscrossing the island by road and in helicopters. They evaluated hundreds of the more than 70,000 landslides brought on by the hurricane. “We saw the hardships that landslides caused the islanders,” said USGS research geologist William H. Schulz and geologist Stephen Hughes of the University of Puerto Rico Mayagüez, the map’s co-authors.
Because Hurricane Maria’s devastation was so extensive, it provided the scientists with plenty of evidence to build a reliable computer model of hurricane-induced landslide susceptibility across the entire island, Schulz said. The resulting map can help community planners decide where to site public buildings, emergency shelters, roads, dams and bridges, and can also help with evacuation planning when hurricanes are forecast to strike the island.
“If used properly, I believe it will help reduce loss of life in future storms,” Schulz said. “That was our primary goal in creating it.”
Mountainous, beautiful, and vulnerable to landslides
The Commonwealth of Puerto Rico is about the size of Connecticut and has a similar population, with more than three million residents. “It’s very mountainous, very beautiful and very vulnerable to landslides,” said Jonathan Godt, USGS Landslide Hazards Program Coordinator. Nearly twice a year, some part of the island experiences multiple landslides, according to the report accompanying the new map.
“There are few places on the mainland where you have the confluence of that high frequency of landslides and the density of the population,” Godt said, “and that makes hazard analysis work there particularly important.”
The island is vulnerable because of its steep mountains, its extensive road network, its history of land modification for agriculture, and its geologic and geographic settings. It lies near the boundary of two moving plates of Earth’s crust, so its mountains are still actively rising, and the high temperatures and rainfall found in this tropical environment accelerate weathering of rock. These factors combined with gravity and erosion from streams result in landslides.
“Landslides are a fact of life almost continually there,” Schulz said, “and emergency managers are very well aware of the risks they pose.” The new landslide susceptibility map is intended to provide state and local governments with the detailed information they need for effective planning. “We’re working very closely with the Puerto Rico Planning Board, the Department of Transportation, and multiple municipalities,” along with the Federal Emergency Management Agency, the Department of Housing and Urban Development, and the Department of Health and Human Services.
The report notes that planners may use the map to avoid development in areas with very high landslide susceptibility due to intense rain, or they may require slope stability studies, as well as design and construction methods that account for potential slope instability. They may monitor high-risk areas for landslide warning signs. They may also consider evacuating these areas when intense rainfall is forecast and may target them for emergency response after storms.
Tools to help residents make plans, spot warning signs
While the map isn’t intended for use to evaluate individual properties, residents can use it to get a general idea of how susceptible their area is to landslides during a storm and to make hurricane evacuation plans, Schulz said. An enlargeable web viewer version is designed to run on any smart phone or tablet without special software. And a booklet, compiled by experts from the USGS, the University of Puerto Rico and the University of Colorado and available online in Spanish and English, can help residents spot warning signs that a landslide may be imminent, such as cracked foundations, leaning trees, and broken water or sewer lines.
The USGS report stresses that property owners in high-risk areas should rely on a site visit from a geologist or an engineer with geological training when making decisions about where and how to build. The report highlights one reason why site-specific opinions are important: the single best predictor of an area’s landslide susceptibility is its soil type – something that varies greatly across the island and is difficult for non-experts to determine.
Other factors that played major roles in landslide susceptibility included the amount of moisture retained in the soil and the steepness of the slope. (Landslides were most prevalent in areas where the land surface sloped between 25 and 45 degrees, the researchers found; surface soils most prone to landsliding generally don’t exist on steeper slopes.) The researchers found that land curvature, land cover, the underlying rock types, distance to roads and streams, and mean annual precipitation also helped predict susceptibility to rainfall-induced landslides, and they incorporated those factors into the model and map.
Surveys found few earthquake-caused landslides
The map does not predict the potential for earthquakes to trigger landslides, which are brought about by different causes and can occur on different types of terrain, Godt said.
Experts from the USGS surveyed landslides and other types of ground failure in southwest Puerto Rico after the recent sequence of earthquakes that began there in December 2019, but they identified relatively few earthquake-induced landslides, Godt said. The information collected will help scientists improve their understanding of ground failures due to earthquakes, he said, but “there are no immediate plans to produce a map of the potential for earthquake-induced landslides across the entire island.”