Mass wasting disturbance and denudation in a humid-tropical montane forest, Puerto Rico

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

Abstract

Steep slopes, intense rainfall, and thick regolith in the humid tropics lead to high rates of denudation through mass movement associated with landslides. The steeply sloping, highly dissected Luquillo Mountains of eastern Puerto Rico, which receive from 3,500 to 5,000 mm of rainfall per year, commonly experience landslides during periods of intense rainfall. Much of this rainfall occurs during tropical disturbances or during winter cold fronts. The clayey soil and saprolite on Luquillo mountain slopes, which average 10 m in thickness, become unstable and subject to mass movement when saturated.

Mass movements in a 248 km² area within and adjacent to the Luquillo Experimental Forest were mapped from four sets of aerial photographs from 1951 to 1990. The mass movements during the 40-year period totaled 1,860, had a median surface area of 156 m², and disturbed 0.30 percent of the land-surface area shown by the aerial photographs. The mass movements were most common on concave, forested, east- to northeast-facing hillslopes and showed no unusual trends with regard to slope angle or elevation. Most of the mass movements were debris flows (37 percent) and shallow soil slips (36 percent). The remainder were slumps (26 percent) and debris avalanches (1 percent). Road- and construction-related mass movements (25 percent of total) had a median surface area of 237 m², and accounted for 38 percent of the total surface area affected, attesting to the effect of human disturbance.

Mass movements (705) (38 percent of the total) that occurred on forested hillslopes had a median surface area of 193 m². These mass movements disturbed 0.26 percent of the land surface during the 40-year period represented by the aerial photographs, which is equivalent to a disturbance rate of 0.64 percent of the total land surface area per century. On forested hillslopes, approximately 188 m³ per km² per year or 239 tons per km² per year is eroded by landsliding, based on the estimated thicknesses of the mapped landslides. In comparison, suspended sediment export reported for three streams draining the Luquillo Mountains ranges from 159 to 203 tons per km² per year. Export of both suspended sediment and bedload measured in a fourth stream draining the Luquillo Mountains totaled about 300 tons per km² per year. A denudation rate of 190 mm per 1000 years was estimated from the mass-movement erosion that occurred on forested hillslopes. This rate is comparable to the 227 mm per 1000 years rate of mass-movement denudation reported for a humid-tropical montane environment in New Guinea.

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