Susceptibility to Landslides Triggered by Intense Rainfall, Puerto Rico

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Map Depicting Susceptibility to Landslides Triggered by Intense Rainfall, Puerto Rico
by K. Stephen Hughes and William H. Schulz

Puerto Rico is vulnerable to landsliding. This report summarizes creation of a new high-resolution model of rainfall-induced landslide susceptibility for the main island.

Executive Summary and Abstract Excerpts from USGS Open-File Report 2020-1022:

Executive Summary

Puerto Rico is vulnerable to landsliding. This report summarizes creation of a new high-resolution model of rainfall-induced landslide susceptibility for the main island. The main island of Puerto Rico was classified at 5-meter pixel scale into categories of Low, Moderate, High, Very High, or Extremely High susceptibility to landsliding during and soon after intense rainfall, such as is produced during tropical cyclones. The map data can be downloaded as georeferenced files in multiple formats by the public or government agencies and used in a geographic information system platform. The model is intended for use in planning, development, and emergency management. The map highlights areas susceptible to landsliding that may warrant further site-specific evaluation by licensed professionals. This map product highlights an important collaboration between the U.S. Geological Survey and the Department of Geology at the University of Puerto Rico at Mayagüez that materialized after the destructive Hurricanes Irma and María in September 2017. An inventory of more than 70,000 mass wasting sites triggered by Hurricane María represents one of the initial products of this partnership and was an essential dataset in the modeling effort described in the report. Characteristics of the inventoried sites were statistically analyzed to produce Susceptibility Index values for various geospatial factors that were combined to form the new map product. The model output demonstrates how post-disaster data can be used to better understand risks and hazards for the future. The project benefited from diverse feedback among colleagues in the U.S. Geological Survey, at the University of Puerto Rico at Mayagüez, and government agencies in Puerto Rico. In addition, the inventory and present study satisfy sections (a), (b), (c), (d), and (i) of Article 4 in Puerto Rico Law 24 of 2008—Protocol for the Mitigation of Landslide Risks in Puerto Rico. The susceptibility map also represents an important step in understanding erosion, sedimentation, and hazards that arise from heavy rainfall in Puerto Rico and the tropics.

Abstract

Landslides in Puerto Rico range from nuisances to deadly events. Centuries of agricultural and urban modification of the landscape have perturbed many already unstable hillsides on the tropical island. One of the main triggers of mass wasting on the island is the high-intensity rainfall that is associated with tropical atmospheric systems. Puerto Rico’s geographic position and rugged topography render millions of residents vulnerable to widespread landslide events. In this study, a high-resolution (5 meters), high-intensity rainfallinduced landslide susceptibility model was produced using the frequency-ratio method. Datasets utilized in the model included a complete-island landslide inventory created from imagery obtained after Hurricanes Irma and María impacted the island during September 2017, slope inclination, land-surface curvature, soil type, geologic terrane, mean annual precipitation, land use, soil moisture, and distance to roadways and streams. The final data product (plate 1) is a statistically viable representation of where landslides are likely to initiate during or soon after intense rainfall, with a robust receiver operating characteristic area-under-curve value of 0.87. The model output raster pixel values were binned into 100 equal-area quantiles and then classified into Low, Moderate, High, Very High, and Extremely High classes of susceptibility. The Extremely High susceptibility classification represents the most vulnerable 1 percent of the island, whereas Very High, High, Moderate, and Low classifications cover 9, 20, 30, and 40 percent of the island, respectively. The susceptibility map is intended to assist in planning future development, mitigation measures, and post-event emergency response; however, it is not a substitute for site-specific, slope-stability assessments performed by licensed geologists and engineers. Additionally, the map does not portray locations where landslide material may travel after mobilization, and which may be at extreme risk; nor does it necessarily portray where landslides may occur during earthquakes or mass wasting triggered by prolonged, relatively low-intensity rainfall.

Note

The index page for the report includes links to the report, the map sheets, related works, application sites, data, and companion files.

Reference

Hughes, K.S., and Schulz, W.H., 2020, Map depicting susceptibility to landslides triggered by intense rainfall, Puerto Rico: U.S. Geological Survey Open-File Report 2020–1022, 91 p., 1 plate, scale 1:150,000, https://doi.org/10.3133/ofr20201022.