Natural Hazards Maps for Land-Use Planning, San Juan Mountains, Colorado, U.S.A.^∗

Abstract

In response to Colorado State legislation passed in 1974, natural hazard mapping was undertaken for three counties in the northern San Juan mountains. For a base-map scale of 1:24,000, legends for three different map series were developed and mapping was completed for 22 entire or partial 7½-minute quadrangle map sheets, totalling 66 individual maps. The series comprises geomorphic hazards, snow avalanche hazards, and combined hazards assessment (Overall Hazards) maps. The map legends were designed to meet the legal requirements of the 1974 Colorado House Bill 1041 and the maps are intended primarily for the town and county land-use planning officer. Mapping was undertaken using a combination of remote-sensing techniques, especially NASA LANDSAT color IR underflight imagery, and field investigations. This paper provides examples of each of the three map series and describes the individual hazard categories: rockfall, rock glaciers, talus slopes, talus slides, debris fans, debris flows, landslides, expansive soil and rock, colluvial slopes, physiographic floodplains, swamps, mine tailings, and snow avalanches. Recommendations for the land-use planner are added.

The presentation discusses some of the practical problems of mapping and seeks to demonstrate the benefits that the mountain states of the United States can derive from progress that has been made in the Alpine countries. The paper concludes with an outline of future research needs: (1) more precise definition of “degree of hazard” through improved assessment of recurrence intervals and impact pressures, (2) improvement of existing legends, (3) adoption of a state-wide standardized legend, (4) increased public awareness, (5) improved understanding of mechanics of slope failure so that mapping at the site engineering scales (1:1,000 to 1:10,000) can be made more effective, and (6) acquisition of a data bank of historic events and development of a state hazard cadaster. The project was part of a program sponsored by the NASA Office of University Affairs to apply remote-sensing techniques and space technology to the solution of terrestrial problems.