The risk of landslides during intense or prolonged rainfall is high in steeply sloping areas such as the municipality of Ponce, where 56 percent of the 301-square-kilometer municipality has slopes 10 degrees or greater. These are areas where the possibility of landsliding increases when triggering conditions such as heavy rainfall or excavation and construction occur.
Using a 30-meter digital elevation model to classify hillslope angle, a digital map of bedrock geology, and maps showing the locations of landslides associated with a severe storm in October 1985, the municipality was classified into areas of low, moderate, and high susceptibility to landslides triggered by heavy rainfall. Areas defined by geology as having 0-0.1 landslides per square kilometer were mapped as having low landslide susceptibility, areas having 0.1-0.5 landslides per square kilometer were mapped as having moderate susceptibility, and areas having more than 0.5 landslides per square kilometer were mapped as having high landslide susceptibility. Areas with hillslope angles of 5 degrees or less were not classified as they are considered too flat for significant landslide susceptibility. The result of this classification indicates that 34 percent of the municipality has high susceptibility to rainfall-triggered landsliding, 24 percent has moderate susceptibility, and 9 percent has low susceptibility. Approximately 34 percent of the municipality, mainly areas with slopes of 5 degrees or less and water bodies, was not classified.
Because of the uncertainties inherent in the susceptibility classification of extensive landscape areas as well as timing of landslide triggers, landslide susceptibility maps should be used with caution. The results of this study are valid for generalized planning and assessment purposes, but may be less useful at the site-specific scale where local geologic and geographic heterogeneities may occur. Construction in areas of moderate to high landslide susceptibility should proceed only after site evaluation by engineering geologists. Large magnitude earthquakes, which occur rarely in Puerto Rico, are the other major trigger of landslides for Caribbean islands; however, this factor was not considered in the development of this map.