A. J. Torres-Sanchez | Search USGS

USGS1999

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 (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 per cent 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.

Earth Surface Processes and Landforms↗

USGS1996

Geographic relations of landslide distribution and assessment of landslide hazards in the Blanco, Cibuco, and Coamo basins, Puerto Rico

Landslide occurrence is common in mountainous areas of Puerto Rico where mean annual rainfall and the frequency of intense storms are high and hillslopes are steep. Each year, landslides cause extensive damage to property and occasionally result in loss of life. Landslide maps developed from 1:20,000 scale aerial photographs in combination with a computerized geographic information system were used to evaluate the landslide potential in the Blanco, Cibuco, and Coamo Basins of Puerto Rico. These basins, ranging in surface area from 276 to 350 square kilometers, are described in this report. The basins represent a broad range of the climatologic, geographic, and geologic conditions that occur in Puerto Rico. In addition, a variety of landslide types were documented. Rainfall-triggered debris flows, shallow soil slips, and slumps were most abundant. The most important temporal control on landslide occurrence in Puerto Rico is storm rainfall. Forty-one storms triggered widespread landsliding about 1 to 2 times per year during the last three decades. These storms were frequently of 1 to 2 days duration in which, on average, several hundred millimeters of rainfall triggered tens to hundreds of landslides in the central mountains. Most of these storms were tropical disturbances that occurred during the hurricane season of June through November. Land use and the topographic characteristics of hillslope angle, elevation, and aspect are the most important spatial controls governing landslide frequency. Hillslopes in the study area that have been anthropogenically modified, exceed 12 degrees in gradient and about 350 meters in elevation, and face the east-northeast are most prone to landsliding. Bedrock geology and soil order seem less important in the determination of landslide frequency, at least when considered at a generalized level. A rainfall accumulation-duration relation for the triggering of numerous landslides throughout the central mountains, and a set of simplified matrices representing geographic conditions in the three river basins were developed and are described in this report. These two elements provide a basis for the estimation of the temporal and spatial controls on landslide occurrence in Puerto Rico. Finally, this approach is an example of a relatively inexpensive technique for landslide hazard analysis that may be applicable to other settings.

Water-Resources Investigations Report↗