Landslides and pore pressure responses associated with Hurricane Hugo, September 1989, eastern Puerto Rico

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

Abstract

On September 18, 1989 Hurricane Hugo struck eastern Puerto Rico with sustained winds of 225 kilometers per hour. Rainfall associated with the hurricane totaled as much as 340 millimeters during a 48-hour period and resulted in a record peak discharge of 580 cubic meters per second at a gaging station on Río Mameyes. As much as 85 percent of the trees in northeastern sections of the 11,330 hectare Caribbean National Forest were damaged or destroyed. More than 400 landslides occurred in and around the Caribbean National Forest during or shortly after the hurricane. These landslides were mainly shallow soil slips, 0.5-1 meter deep, 3-8 meters wide, and 4-30 meters long. According to observers, the majority of these small landslides occurred during the hurricane. However, the largest landslide, a 9-meter deep, 30,000 cubic meter debris avalanche, occurred approximately 78 hours after the hurricane eye passed the area.

A tipping bucket rain gage and an array of soil-moisture sensors (tensiometers and piezometers) were located on a concave slope in the Río Mameyes drainage basin, one of the hardest-hit sections of the Caribbean National Forest. Nests of soil-moisture sensors were located at six sites along a 40-meter long, steep (25°- 42°) slope, with sensors buried at depths of 0.2-4 meters in the soil. Intense rainfall (approximately 50 millimeters per hour) began 11 hours prior to the passage of the hurricane eye. Rainfall at the instrumented site totaled approximately 250 millimeters. Soil moisture pore-pressure increases recorded during this period of intense rainfall ranged from 7 to 125 centimeters. Initial increases in the soil-moisture pore pressures in response to the intense rainfall occurred within 5 to 9.5 hours after rainfall began; peak responses occurred within 8.5 to 13 hours. Soil moisture p ore pressures returned to pre-hurricane levels within 11.5 to 20 hours. Sensors in the upper slope recorded lower peak pore pressures than those nearer the slope base and shallow sensors responded to the increase in soil moisture more rapidly than sensors at greater depth. Although no landslides occurred on the instrumented slope, several shallow soil slips did occur on nearby comparable slopes. Preliminary analysis of pore-pressure data indicate that the timing of these shallow slope failures probably coincided with periods of peak pore pressures.

• WEBB Project Bibliography
• WEBB Project Chief Home Page
• Caribbean Water Science Center home page