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.