Landslides caused by the intense precipitation of Hurricane Hugo, September 1989, Eastern Puerto Rico

U.S. Geological Survey GSA Center 651 Federal Drive Guaynabo, Puerto Rico 00965-5703, USA

Abstract

On the morning of September 18, 1989, a category four hurricane struck eastern Puerto Rico with maximum sustained winds of over 225 kilometers per hour. The 3-day rainfall total associated with the hurricane ranged from 165 to 344 millimeters. Rainfall totals varied considerably in affected areas as a result of rain shadows behind topographic highs. An average rainfall intensity of 39 millimeters per hour was recorded during a 6 hour period at one observer station in a peak-rainfall area. Before it was destroyed by high winds, a rain gage equipped with satellite telemetry measured an average intensity of 34 millimeters per hour for a 7 hour period which included periods with rainfall intensities as high as 42 millimeters per hour. Because of the high winds, much rainfall data in the area were lost when rain gages were blown over or destroyed. In addition, rainfall measurements were difficult to assess because high winds blew rain across the tops of rain gages.

As a result of the hurricane, more than 200 landslides occurred in the steeply sloping, highly dissected mountains of eastern Puerto Rico. These landslides were surveyed using low-flying aircraft, four-wheel drive vehicles, and standard ground-survey techniques. Half of the landslides were associated with construction and road cuts; the rest occurred on slopes unmodified man. About 50 percent of all the landslides were earth slides; 25 percent were debris flows and debris slides; the remainder were slumps and rockfalls. The largest observed landslide was 130 meters long, 21 meters wide, and moved approximately 40,000 cubic meters of soil and rock 600 meters downslope into a river.

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