In the December- January 1964-65 period and in April of 1965, climatic events 

 occurred that resulted in extensive damage to the road system. The probable frequency 

 of these events is not well established. However, it can be concluded from past weather 

 records that such events could happen as often as once in five years. 



During late December and early January of 1964-65, over 6 inches of precipitation 

 (mostly snow) fell in the Zena Creek area. Some road failures occurred below 5,000 feet 

 as a result of this storm. Then, on April 19, 1965, 1.03 inches of rain fell. In the 

 areas below about 4,200-foot elevation, where snowmelt had already been completed, 

 little damage occurred. In areas where elevations were between 4,200 and 5,500 feet and 

 the snowmelt was intense, extensive damage occurred, primarily to road fills. 



The major landslides and erosion on the watersheds in the Zena Creek drainage were 

 judged to have resulted from road construction. The balance of this report will deal 

 primarily with the road systems that were constructed to transport timber from the Zena 

 Creek drainage. 



In figure 4, a planimetric map of the Zena Creek drainage shows the road system 

 used to gain access to the harvest area. The arrows and circled points mark locations 

 of sites that were investigated as part of the Zena Creek study. The Secesh River is a 

 tributary to the South Fork of the Salmon River, as previously noted. 



Failure Investigations 



The major purpose of these investigations was to determine the principal causes of 

 slope failures, both road-associated and nonroad-associated , in the Zena Creek study 

 area. Surface erosion, or the movement of single grains by wind and water, was excluded 

 from consideration except where this type of movement contributed to mass failures. 

 Whether surface erosion by runoff is an important mode of soil detachment is a matter 

 for study. Nevertheless, the importance of surface water as a transporting medium is 

 well recognized, and movement of soil particles in roadside ditches and ephemeral 

 channels takes place readily at Zena Creek. 



Raveling from weathered rock surfaces in cut slopes was also excluded from this 

 investigation. This, too, is recognized as an important contributor of soil grains to 

 surface water in roadside ditches. 



The analysis of surface erosion remains a highly empirical art, and is beyond the 

 scope of this study. Only the movement of soil en masse was considered. 



FIELD AND LABORATORY TESTS 



A detailed discussion of the field and laboratory tests and the results thereof 

 may be found in the original report. 1 The procedures followed were relatively routine 

 for an engineering investigation of this type, and only the more significant findings 

 will be discussed herein. 



l R. B. Gardner, M. J. Gonsior, and G. L. Martin. Zena Creek road and logging sys- 

 tem investigation. September 1969, 173 p., illus. (Unpublished report on file at 

 USDA Forest Serv. , Intermountain Forest and Range Exp. Station, Forestry Sciences Lab., 

 Bozeman, Montana.) 



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