Torres-Sanchez A.J.

The Frequency and Distribution of Recent Landslides in three MontaneTropical Regions of Puerto Rico

Larsen, Matthew C.; Torres-Sanchez, Angel J. 1998. The frequency and distribution of recent landslides in three montane tropical regions of Puerto Rico. Geomorphology 24 :309-331.

Abstract: 
Landslides are common in steep mountainous areas of Puerto Rico where mean annual rainfall and the frequency of intense storms are high. Each year, landslides cause extensive damage to property and occasionally result in loss of life. Average population density is high, 422 peoplerkm2, and is increasing. This increase in population density is accompanied by growing stress on the natural environment and physical infrastructure. As a result, human populations are more vulnerable to landslide hazards. The Blanco, Cibuco, and Coamo study areas range in surface area from 276 to 350 km2 and represent the climatologic, geographic, and geologic conditions that typify Puerto Rico. Maps of recent landslides developed from 1:20,000-scale aerial photographs, in combination with a computerized geographic information system, were used to evaluate the frequency and distribution of shallow landslides in these areas. Several types of landslides were documented— rainfall-triggered debris flows, shallow soil slips, and slumps were most abundant. Hillslopes in the study area that have been anthropogenically modified, exceed 128 in gradient, are greater than 300 m in elevation, and face the east-northeast, are most prone to landsliding. A set of simplified matrices representing geographic conditions in the three study areas was developed and provides a basis for the estimation of the spatial controls on the frequency of landslides in Puerto Rico. This approach is an example of an analysis of the frequency of landslides that is computationally simple, and therefore, may be easily transferable to other settings.

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.

Abstract: 
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.

SLOPEWASH, SURFACE RUNOFF, AND FINE-LITTER TRANSPORT IN FOREST AND LANDSLIDE SCARS IN HUMID-TROPICAL STEEPLANDS, LUQUILLO EXPERIMENTAL FOREST, PUERTO RICO

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.

Abstract: 
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.

SLOPEWASH, SURFACE RUNOFF AND FINE-LITTER TRANSPORT IN FOREST AND LANDSLIDE SCARS IN HUMID- TROPICAL STEEPLANDS, LUQUILLO EXPERIMENTAL FOREST, PUERTO RICO

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.

Abstract: 
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.
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