Larsen M.C.


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.”


Larsen, M.C., 2000, Analysis of 20th century rainfall and streamflow to characterize drought and water resources in Puerto Rico: Physical Geography, v. 21, p. 494-521.

During the period from 1990 to 1997, annual rainfall accumulation averaged 87% of normal at the 12 stations with the longest period of record in Puerto Rico, a Caribbean island with a 1999 population of 3.8 million. Streamflow in rivers supplying the La Plata and Loíza reservoirs, the principal water supply of the San Juan metropolitan area, was at or below the 10th flow percentile for 27% to 50% of the time between December 1993 and May 1996. Diminished reservoir levels in 1994 and 1995 affected more than 1 million people in the San Juan metropolitan area. Water rationing was implemented during this period and significant agricultural losses, valued at $165 million, were recorded in 1994. The public endured a year of mandatory water rationing in which sections of the city had their water-distribution networks shut off for 24 to 36 hours on alternate days. During the winter and spring of 1997–1998, water was rationed to more than 200,000 people in northwestern Puerto Rico because water level in the Guajataca reservoir was well below normal for two years because of rainfall deficits. The drought period of 1993– 1996 was comparable in magnitude to a drought in 1966–1968, but water rationing was more severe during the 1993–1996 period, indicating that water management issues such as demand, storage capacity, water production and losses, and per capita consumption are increasingly important as population and development in Puerto Rico expand.

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.

Rainfall-triggered landslides, anthropogenic hazards, and mitigation strategies

Larsen MC (2008) Rainfall-triggered landslides, anthropogenic
hazards, and mitigation strategies. Adv Geosci 14:147–

Rainfall-triggered landslides are part of a natural process of hillslope erosion that can result in catastrophic loss of life and extensive property damage in mountainous, densely populated areas. As global population expansion on or near steep hillslopes continues, the human and economic costs associated with landslides will increase. Landslide hazard mitigation strategies generally involve hazard assessment mapping, warning systems, control structures, and regional landslide planning and policy development. To be sustainable, hazard mitigation requires that management of natural resources is closely connected to local economic and social interests. A successful strategy is dependent on a combination of multi-disciplinary scientific and engineering approaches, and the political will to take action at the local community to national scale.

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.


Larsen, M.C., Torres-Sánchez, A.J., and Concepción, I.M., 1998, Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico [abs] EOS, Transactions American Geophysical Union, vol. 80.

Slopewash, surface runoff, and fine-litter transport in forest and landslide scars in humid-tropical steeplands, Luquillo Experimental Forest, Puerto Rico Rainfall, slopewash (the erosion of soil particles), surface runoff, and fine-litter transport at humid-tropical steepland sites in the Luquillo Experimental Forest, Puerto Rico (18° 20' N, 65° 45' W) were measured from 1991 to 1995. Hillslopes underlain by: 1) Cretaceous tuffaceous sandstone and siltstone in subtropical rain (tabonuco) forest with vegetation recovering from Hurricane Hugo (1989); and underlain by 2) Tertiary quartz diorite in subtropical lower montane wet (colorado and dwarf) forest with undisturbed forest canopy were compared to recent landslide scars. Monthly surface runoff on these very steep hillslopes (24° to 43°) was only 0.2 to 0.5 percent of monthly rainfall. Slopewash was higher in sandy loam soils whose parent material is quartz diorite (averaging 46 g m-2 a-1) than in silty-clay loam soils derived from tuffaceous sandstone and siltstone where the average was 9 g m-2 a-1. Annual slopewash of 100 to 349 g m-2 on the surfaces of two recent, small landslide scars was measured initially but slopewash decreased to only 3 to 4 g m-2 a-1 by the end of the study. The mean annual mass of fine litter (mainly leaves and twigs) transported downslope at the forested sites ranged from 5 to 8 g m-2 and was lower at the tabonuco forest site, where post-Hurricane Hugo recovery is still in progress. Mean annual fine-litter transport was 2.5 g m-2 on the two landslide scars.

Physical Aspects of Hurricane Hugo in Puerto Rico

Scatena, F. N., and Larsen, M. C., 1991, Physical aspects of Hurricane Hugo in Puerto Rico: Biotropica, v. 23, no. 4A, p. 317-323.

On 18 September 1989, the western portion of Hurricane Hugo crossed eastern Puerto Rico and the Luquillo Experimental Forest (LEF). Storm-facing slopes on the northeastern part of the island that were within 15 km of the eye and received greater than 200 mm of rain were most affected by the storm. In the LEF and nearby area, recurrence intervals associated with Hurricane Hugo were 50 yr for wind velocity, 10 to 31 yr for stream discharge, and 5 yr for rainfall intensity. To compare the magnitudes of the six hurricanes to pass over Puerto Rico since 1899, 3 indices were developed using the standardized values of the product of: the maximum sustained wind speed at San Juan squared and storm duration; the square of the product of the maximum sustained wind velocity at San Juan and the ratio of the distance between the hurricane eye and San Juan to the distance between the eye and percentage of average annual rainfall delivered by the storm. Based on these indices, Hurricane Hugo was of moderate intensity. However, because of the path of Hurricane Hugo, only one of these six storms (the 1932 storm) caused more damage to the LEF than Hurricane Hugo. Hurricanes of Hugo's magnitude are estimated to pass over the LEF once every 50-60 years on average.
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