Avalanche-Hazard Mapping and Zoning Problems in the Rocky Mountains, with Examples from Colorado, U.S.A.

Abstract

AbstractAvalanche-hazard mapping as a basis for land-use decision-making was not undertaken systematically in Colorado until 1974. Passage of Colorado House Bill 1041 required counties to map areas subject to snow avalanche, landslide, debris flow, and mountain flood at 1 : 24 000, and funds were provided. This legislation induced several approaches: work undertaken directly by the Colorado Geological Survey; private contract work; mapping by the Institute of Arctic and Alpine Research (INSTAAR) funded by a NASA research grant. This latter effort produced 37 individual 1 : 24 000 map sheets of Hinsdale, Ouray, San Juan, and San Miguel counties, San Juan Mountains. This emphasized problems of scale and degree of cartographic accuracy. Swiss, Austrian, and French experience, together with the actual Colorado mapping experience, facilitated further definition of problems facing the Rocky Mountains slates, both in terms of resolution of actual mapping problems, and of using such developing experience to influence the decision-making process.Two mapping attempts are described: (i) development of a combined hazard map for a mountain type area, indicating the difficulties of, and need for, combining hazard assessment of avalanche and other physical processes that frequently overlap; (ii) consideration of avalanche-zoning problems in a wilderness area.Finally, scale limitations and the need to define rigorously “hazard” are discussed. Definition of hazard must include consideration of recurrence interval, impact pressures in the run-out zone, and limitations of displaying this type of data on available topographic maps, all in relation to types of impact, i.e. to moving or stationary objects in relatively sparsely populated terrane. Additional critical needs facing Rocky Mountains states are itemized: (i) standardized mapping legends for different scales; (ii) establishment of an avalanche cadastre; (iii) systematic reporting of climax events; (iv) development of an historical archive; (v) public awareness; (vi) attention to legal aspects of the avalanche equivalent of the hundred-year flood, and its legal testing.