In Puerto Rico, the combination of mountainous terrain and frequent intense rainstorms results in a landscape where landslide features are commonplace. The risk of landsliding during intense or prolonged rainfall is high in steeply-sloping interior municipalities such as Comerío, where 77 percent of the 74-km² municipality has slopes of 21 percent (12°) or more. A set of simplified matrices representing geographic conditions was developed from earlier studies in three other regions of the central mountains of Puerto Rico and provides a basis for the estimation of the spatial controls on the frequency of rainfall-triggered landslides. The approach integrates four geographic characteristics to categorize hillslope types: slope, elevation, aspect, and land use. In general, hillslopes with a slope angle of 12 percent (7°) or less have a relatively low landslide frequency. A slope angle in excess of 21 percent (12) is a threshold above which the frequency of landslides is relatively high. The frequency of landslides is higher in the wetter regions of the island, or generally at elevations greater than 300 m. In Puerto Rico slope aspect is also relevant because hillslopes which generally face the prevailing winds (between 22.5° and 112.5°) which deliver much of the rainfall to the island have the highest frequency of landslides. Conversely, hillslopes facing away from the prevailing winds (between 202.5° and 292.5°) have the lowest frequency of landslides in the study areas. Hillslopes in the remaining quadrants (between 112.5° and 202.5°, between 292.5° and 360°, and between 0° and 22.5°) generally have an intermediate landslide frequency. The greatest variation in the frequency of landslides exists among land use categories. The average ratio of landslide frequency among various hillslope types modified for roads or structures as compared to similar hillslopes in forested land use ranges from 2:1 to 8:1. Using the approach described above, a map showing landslide susceptibility in the Comerío municipality was developed. The municipality was divided into generalized regions of low, moderate, and high landslide susceptibility. These susceptibility zones were amended by including additional areas defined by the locations of recent and historic debris-flow tracks and runout zones (mapped from digital topographic maps and 1:20,000-scale aerial photographs) and areas of ancient mass movement deposits taken from geologic maps.

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Last update: Jul 30, 2004, 17:09
URL: http://pr.water.usgs.gov/public/webb/bibliography/abstract130.html