Landslides Triggered by Hurricane Hugo in Eastern Puerto Rico, September 1989

Larsen, M. C., and Torres-Sánchez, A. J., 1992, Landslides triggered by Hurricane Hugo in eastern Puerto Rico, September 1989: Caribbean Journal of Science, vol. 28, no. 3-4, p. 113-125.

On the morning of September 18, 1989, a category-four hurricane struck eastern Puerto Rico with a sustained wind speed in excess of 46 m/s. The 24-h rainfall accumulation from the hurricane ranged from 100 to 339 mm. Average rainfall intensities ranging from 34 to 39 mm/h were calculated for 4 and 6 h periods, respectively, at a rain gage equipped with satellite telemetry, and at an observer station. The hurricane rainfall triggered more than 400 landslides in the steeply sloping, highly dissected mountains of eastern Puerto Rico. Of these landslides, 285 were mapped from aerial photography which covered 6474 ha. Many of the mapped landslides were on northeast- and northwest-facing slopes at the eastern terminus of the mountains, nearest the hurricane path. The surface area of individual landslides ranged from 18 m2 to 4500 m2, with a median size of 148 m2. The 285 landslides disturbed 0.11% of the land surface in the area covered by aerial photographs. An approximate denudation rate of 164 mm/1000 y was calculated from the volume of material eroded by landsliding and the 10-y rainfall recurrence interval.

A rainfall intensity duration threshold for landlisdes in a humid-tropical environment, puerto rico

A Rainfall Intensity-Duration Threshold for Landslides in a Humid-Tropical Environment, Puerto Rico
Matthew C. Larsen and Andrew Simon
Geografiska Annaler. Series A, Physical Geography
Vol. 75, No. 1/2 (1993), pp. 13-23

Landslides are triggered by factors such as heavy rainfall, seismic activity, and construction on hill-slopes. The leading cause of landslides in Puerto Rico is intense and/or prolonged rainfall. A rainfall threshold for rainfall-triggered landsliding is delimited by 256 storms that occurred between 1959 and 1991 in the central mountains of Puerto Rico, where mean annual rainfall is close to or in excess of 2,000 mm. Forty one of the 256 storms produced intense and/or prolonged rainfall that resulted in tens to hundreds of landslides. A threshold fitted to the lower boundary of the field defined by landslide-triggering storms is expressed as I = 91.46 D-0.82 where I is rainfall intensity in millimeters per hour, and D is duration in hours. Landslide-producing storms occurred at an average rate of 1.2 per year. In general the landslides triggered by short-duration, high-intensity rainfall events were mainly shallow soil slips and debris flows, while the long-duration, low-intensity rainfall produced larger, deeper debris avalanches and slumps. For storms that had durations of up to 10 h, landsliding did not occur until rainfall intensity was as much as three times as high as the rainfall intensity reported as sufficient to trigger landsliding in temperate regions. As storm durations approach 100 h, the rainfall conditions necessary to initiate landsliding in Puerto Rico converge with those defined for temperate regions. A comparison of the Puerto Rico threshold with rainfall data from other humid-tropical regions suggests that the threshold developed for Puerto Rico may be applicable to other similar environments throughout the world.

Mass Wasting and Sediment Storage in a Small Montane Watershed: an Extreme Case of Anthropogenic Disturbance in the Humid Tropics

LARSEN, M.C. and SANTIAGO-ROMA´ N, A., 2001. Mass wasting and
sediment storage in a small montane watershed: an extreme case
of anthropogenic disturbance in the humid tropics. In: DORAVA,
(eds.), Geomorphic Processes and Riverine Habitat. American Geophysical
Union Monograph, pp. 142–170.

By the peak of land-use conversion for subsistence cropping and plantation agriculture in Puerto Rico in the 1940's, 94 percent of the original forest cover had been eliminated. In a small (26.4 km2) upland watershed that typifies this land-use history, field surveys and examination of aerial photographs indicate that more than 2,000 landslides have occurred since about 1820 when forest clearing began. The landslides are attributable to a combination of three factors: a highly weathered bedrock (Cretaceous granodiorite), episodic heavy rainfall, and almost two centuries of intense land-use practices. On average, landslide scars number 140/km2 in the Cayaguás watershed, equal to 80 landslide scars/km2/100 y. The volume of hillslope material eroded by landsliding is estimated at 660,000 m3/km2 (870,000 Mg/km2). If all colluvium was transported from the catchment, then the volume is equivalent to a mean surface lowering of the entire watershed by 660 mm, or 3.8 mm/y. Soil augering, field observations at construction sites, road cuts and stream banks, mapping from aerial photographs, and GIS-based estimates of the surface area of footslopes, indicate that colluvium may total 149,000 Mg/km2. If mobilized, this would be sufficient stored material to supply the annual average fluvial sediment yield for as long as 129 years. The great availability of colluvial and alluvial sediment on footslopes, floodplains, and in channels will maintain high sediment yield well into the 21st century in spite of government efforts to reforest hillslopes and institute other hillslope soil conservation measures.

Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides

Larsen, M.C., 2001, Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides, in Gruntfest, E., and Handmer, J., eds., Coping with Flash floods: Kluwer Academic Publishers, p. 277-288.


Larsen, M. C., M. T. Va´squez Conde, and R. A. Clark, Landslide
hazards associated with flash-floods, with examples from the December,
1999 disaster in Venezuela, in Coping with Flash Floods,
edited by E. Gruntfest and J. Handmer, NATO ASI Ser., in press,

Landslides and flash floods commonly occur together in response to intense and prolonged rainfall. Although these phenomena may be viewed by the popular media as distinct events, rainfall-triggered landslides and flash floods are part of a continuum of processes that includes debris flows, hyperconcentrated flows, and streamflow. This combination of processes has proven to be highly destructive in populated areas. Without careful planning of human settlements, the impacts of these types of disasters are likely to increase in the future. As stated by the Secretary General of the United Nations, Kofi Annan, “The term ‘natural disaster’ has become an increasingly anachronistic misnomer. In reality, human behavior transforms natural hazards into what should really be called unnatural disasters.”

Geomorphic effects of large debris flows and flash floods, northern Venezuela, 1999

Larsen, M.C. and Wieczorek, G.F., 2006. Geomorphic effects
of large debris flows and flash floods, northern Venezuela,
1999, Z. Geomorph. N.F., Suppl 145:147-175.

A rare, high-magnitude storm in northern Venezuela in December 1999 triggered debris flows and flash floods, and caused one of the worst natural disasters in the recorded history of the Americas. Some 15,000 people were killed. The debris flows and floods inundated coastal communities on alluvial fans at the mouths of a coastal mountain drainage network and destroyed property estimated at more than $2 billion. Landslides were abundant and widespread on steep slopes within areas underlain by schist and gneiss from near the coast to slightly over the crest of the mountain range. Some hillsides were entirely denuded by single or coalescing failures, which formed massive debris flows in river channels flowing out onto densely populated alluvial fans at the coast. The massive amount of sediment derived from 24 watersheds along 50 km of the coast during the storm and deposited on alluvial fans and beaches has been estimated at 15 to 20 million m3. Sediment yield for the 1999 storm from the approximately 200 km2 drainage area of watersheds upstream of the alluvial fans was as much as 100,000 m3/km2. Rapid economic development in this dynamic geomorphic environment close to the capital city of Caracas, in combination with a severe rain storm, resulted in the death of approximately 5% of the population (300,000 total prior to the storm) in the northern Venezuelan state of Vargas.

How wide is a road? the association of roads and mass wsting in a forested montane environment

Larsen, M.C., 1995, How wide is a road? The association of roads and mass wasting in a forested montane environment, Puerto Rico [abs] American Geophysical Union, EOS Supplement, v. 76, no. 17, p. S309.

Mass wasting has confounded road builders for as long as humans have constructed transportation routes through mountainous terrain. The high-intensity rainfall that is typical of mountainous humid-tropical settings results in mass wasting that is exacerbated by construction and maintenance of roads. However, the actual extent of the zone of mass-wasting disturbance associated with roads is not well known. Determination of this zone is important for land use managers, highway engineers, and foresters who must deal with costly and sometimes life-threatening hazards attributed to road-related landsliding. A spatial data base of 1,859 landslides, representing approximately 50 years of landslide activity, was analyzed using a geographic information system to determine landslide frequency in relation to roads. A 268-km-long transportation network in a 276-km2 area of humid-tropical, mountainous, mostly forested terrain in Puerto Rico was used in the analysis by developing a series of 17 buffer zones varying from 5 to 350 m in length, measured perpendicular to road axes. Most of the study area lies within the boundaries of the U.S. Forest Service-administered Luquillo Experimental Forest. Average landslide frequency in the study area was about 7 landslides per km2. In all buffer zones within 100 m of roads, landslide frequency was higher than the average, and in buffer zones beyond 100 m, landslide frequency was about the study area average. The 100-m buffer zone landslide frequency was 2.4 times higher than the average rate, indicating that within 100 m of a road, associated landslide disturbance is significant. The 100-m buffer length is equivalent to a 200-m wide swath along the 268- km road corridor, representing a total of 49 km2 (or 18 percent) of the total surface of the study area. Thi s means that during the 50 years represented by the landslide sample, for every km of road length, 20 hectares of the study area was affected by this high landslide disturbance rate. In addition, the mass of regolith eroded by mass wasting in the 100-m buffer zone is more than 6 times the mass eroded in areas outside the buffer zone. A total of about 94 tonnes per km2 per year is eroded in the buffer zone by mass wasting, compared to about 15 tonnes per km2 per year in areas outside of the 100-m buffer zone. This represents a considerable portion of the approximately 200 to 400 tonnes per km2 per year of fluvial sediment that is exported from Luquillo Experimental Forest watersheds.

Assessing Landslide Hazards

Keefer, D.K., Larsen, M.C., 2007. Assessing landslide hazards. Sciences 316, 1136–1137.

On 31 May 1970, a large earthquake shook the highest part of the Peruvian Andes. Millions of cubic meters of rock dislodged from a mountainside and initiated a rock avalanche that traveled more than 14 km in 3 min, burying a city and killing more than 25,000 people (1, 2). On 17 February 2006, a landslide of 15 million m3 that initiated on a slope weakened by long-term tectonic activity buried more than 1100 people on Leyte Island in the Philippines (3). Landslides such as these are a hazard in almost all countries, causing billions of dollars of damage and many casualties (4). Landslides also contribute to landscape evolution and erosion in mountainous regions (see the first figure). Here we discuss the latest strategies used to assess and mitigate landslide hazards.

Denudation rates determined from the accumulation of in situ-produced 10Be in the luquillo experimental forest, Puerto Rico

Brown, E.T., Stallard, R.F., Larsen, M.C., Raisbeck, G.M., Yiou,
F., 1995b. Denudation rates determined from the accumulation of in
situ-produced Be in the Luquillo Experimental Forest, Puerto
Rico. Earthand Planetary Science Letters 129, 193}202.

We present a simple method for estimation of long-term mean denudation rates using in situ-produced cosmogenic 10Be in fluvial sediments. Procedures are discussed to account for the effects of soil bioturbation, mass wasting and attenuation of cosmic rays by biomass and by local topography. Our analyses of 10Be in quartz from bedrock outcrops, soils, mass-wasting sites and riverine sediment from the Icacos River basin in the Luquillo Experimental Forest, Puerto Rico, are used to characterize denudation for major landform elements in that basin. The 10Be concentration of a discharge-weighted average of size classes of river sediment corresponds to a long-term average denudation of ≈ 43 m Ma−1, consistent with mass balance results.

Determination of predevelopment denudation rates of an agricultural watershed (Cayaguás River, Puerto Rico) using in-situ-produced 10Be in river-borne quartz

Brown, E.T., Stallard, R.F., Larsen, M.C., Bourles, D.L., Raisbeck,
G.M., Yiou, F., 1998b. Determination fo predevelopment denudation
rates of an agricultural watershed (Cayaguas River, Puerto
Rico) using in-situ-produced Be in river-borne quartz. Earthand
Planetary Science Letters 160 (3-4), 723}728.

Accurate estimates of watershed denudation absent anthropogenic effects are required to develop strategies for mitigating accelerated physical erosion resulting from human activities, to model global geochemical cycles, and to examine interactions among climate, weathering, and uplift. We present a simple approach to estimate predevelopment denudation rates using in-situ-produced cosmogenic 10Be in fluvial sediments. Denudation processes in an agricultural watershed (Cayaguás River Basin, Puerto Rico) and a matched undisturbed watershed (Icacos River Basin) were compared using 10Be concentrations in quartz for various size fractions of bed material. The coarse fractions in both watersheds bear the imprint of long subsurface residence times. Fine material from old shallow soils contributes little, however, to the present-day sediment output of the Cayaguás. This confirms the recent and presumably anthropogenic origin of the modern high denudation rate in the Cayaguás Basin and suggests that pre-agricultural erosional conditions were comparable to those of the present-day Icacos.
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