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<br />002799 <br /> <br />3.0 LANDSLIDE HAZARD MAPS <br /> <br />Maps are a useful and convenient method of presenting information about <br />landslide hazards. They can display many kinds and combinations of <br />information and be prepared at various levels of detail. When used in <br />conjunction with land-use maps, they are a valuable planning resource. <br /> <br />. <br /> <br />In order to evaluate the overall landslide hazard of a region in an efficient <br />and cost-effective manner, reconnaissance-level maps can be prepared. <br />Leighton (1976) suggests a three stage approach to landslide hazard mapping. <br />The first stage, or level, of investigation is a regional synthesis of <br />available data and is prepared by a state or federal survey. This is followed <br />by a detailed mapping and subsurface program in complex problem areas and is <br />sponsored by local government. Finally, detailed site-specific investigations <br />are conducted by private consultants. This approach provides a quick initial <br />approximation of the general distribution of hazardous areas. Then, based on <br />this, more detailed studies can be conducted to quantify the hazard processes <br />and extent. If, however, resources are limited it may be more prudent to <br />bypass reconnaissance-level mapping and concentrate on studying known areas of <br />concern. <br /> <br />REGIONAL MAPPING <br /> <br />Regional, or reconnaissance, mapping is used mainly to supply basic data for <br />regional planning, in preparation for detailed studies at the community and <br />site-specific levels, and for setting priorities for future mapping. <br /> <br />These maps are usually simple inventory maps and are directed primarily toward <br />the identification and delineation of broad landslide problem areas and the <br />conditions under which they occur. They concentrate on those geologic units <br />or environments in which landslides have already occurred and in which <br />additional movements are most likely. Such mapping relies heavily on <br />photogeology, reconnaissance field mapping, and the collection and synthesis <br />of all available pertinent geologic data (Leighton, 1976). <br /> <br />Regional mapping is frequently prepared at a scale of 1:24,000, since high <br />quality USGS topographic base maps at this scale are widely available and <br /> <br />- 13 - <br />