| OCR Text |
Show will allow accurate estimates of the avalanche slab thickness. Prominent painted marks on existing structures can serve the same purpose. A useful adjunct to estimating avalanche size is a table of distances compiled by measuring the width and length of the active avalanche paths, especially those which are clearly defined by terrain features. If a large- scale map is available, this can be done from the map. Otherwise it is more convenient- and sometimes more accurate-- to tape distances on the ground in the summer. Selection of sites for these measurements, as well as for the fracture zone markers described above, presupposes some experience with avalanche behavior in a given area. In a new development, it may take a minimum of two or three years of experience to know where to mark or measure. The accurate recording of avalanche size and shape is greatly facilitated by a set of good photographs of all the principal avalanche paths in an area. Ground photos taken from good vantage points or low- level oblique aerial photos can be used. They should be printed to at least 8" xl0" size and mounted on stiff cardboard or matt board. The boards can be punched for filing in a large, 3~ ring notebook. Principal avalanche paths should be clearly outlined and named on the photos. Current avalanche activity for each season, or even for each storm, can be entered on transparent acetate overlays with grease pencil, or on tracing paper overlays with marker pen. It is important to label each overlay clearly according to date and associated photo. Ideally, each major area where systematic avalanche observations are to be conducted should establish a master file of avalanche path photos annotated to show details of typical avalanche activity, response to different storm winds, and typical control measures. This serves as a permanent record by which hard- earned experience can readily be transmitted from each snow ranger to his successor, and provides a basis for analyzing each season's reported avalanche occurrence. Areas contemplating preparing such an annotated master photo file should request as an example a copy of the " Alta Avalanche Control and Safety Plan" ( November 1969) from the Supervisor, Wasatch National Forest, Federal Building, Salt Lake City, Utah Mill. 33 |
