Figure 2. — Typical soil profile and slope geometry in the Idaho 

 batholith (excluding swales, depressions, and other topographic 

 lows). Arrows indicate the shallow subsurface flow zone. 



DOCUMENTATION OF SLOPE FAILURES 



The Idaho batholith area contains valuable timber reserves 

 as well as other resources important to the regional and nation- 

 al economies. This has led to considerable development activ- 

 ity, particularly road construction and timber harvesting. The 

 batholith is also a critical area with respect to surface and 

 subsurface stability. Surface erosion and landslides are preva- 

 lent. Consequently, much of this development activity has in- 

 creased the occurrence of landslides and provided large ex- 

 posed areas from which sediment can readily be removed by 

 surface erosion processes (Megahan and Kidd 1972). 



Slope stability problems in the batholith have been studied 

 and documented by a number of investigators in the past dec- 

 ade (Croft and Adams 1950; Gonsior and Gardner 1971; 

 fy/legahan and Kidd 1972; and Megahan and others 1978). An 

 inventory and analysis by Megahan and others (1978) of some 

 1 ,400 landslides on two National Forests reveals some interest- 

 ing findings about the nature and cause of these slope failures. 



The survey was carried out on the Cleanwater and Boise 

 National Forests and represents geologic conditions found in 

 the western and central portions of the Northern Rocky Moun- 

 tain physiographic province. Most of the area surveyed in the 

 landslide inventory falls within the Idaho batholith. Portions of 

 the Clearwater National Forest, however, are outside the batho- 

 lith. Here, low grade metasediments (mostly quartzite) of the 

 Belt Series predominate. 



Almost three quarters (72 percent) of the slides inventoried 

 were debris avalanches or debris slides. For a 3-year study 

 period, a total of almost 1 5 acres (6 hectares) of forest land were 

 lost to landslides each year on the Cleanwater National Forest 

 alone. Repair costs to simply clear debris from roads and re- 

 place road fill material averaged $56,000 per year. An average 

 of 56,000 cubic yards/year (43 000 cubic meters/year) of slide 

 material was delivered to active stream channels with adverse 

 impacts on the fisheries resources of the region. 



By far the most likely cause of accelerated landslide activity 

 was road construction. Roads alone accounted for 58 percent of 

 the landslides inventoried. In combination with logging and/or 

 forest fires, roads accounted for a total of 88 percent of all 

 landslides. Vegetation removal alone accounted for 9 percent of 

 the landslide activity. Only 3 percent of the landslides occurred 

 on "natural" or undisturbed slopes. These relationships are 

 shown graphically in figure 3. 



The Idaho batholith is not unique with regard to its high 

 incidence of slope failures. Numerous slides have occurred in 

 granitic areas all around the world (Durgin 1977; Jones 1973). 

 Granitic masses or batholiths commonly form the core of moun- 

 tains that may have extreme topographic relief. Progressive 

 physical, chemical, and biological weathering weakens the 

 granitic rocks and make them susceptible to erosion and mass 

 movement (Clayton and others 1979). Natural events such as 

 intense rainstorms, earthquakes, or other perturbations can 

 then trigger slides at susceptible sites. Developmental activity 

 such as road construction and timber harvesting can accelerate 

 or intensify this process. 



Roaded & Roaded Burned, 

 & Roaded 



TYPE OF DISTURBANCE 

 Figure 3. — Landslide occurrence by type of disturbance, western and 

 central Northern Rocky Mountain physiographic province (from 

 Megahan and others 1978). 



Potential Effects of Timber Removal 



STABILIZING INFLUENCE 

 OF WOODY VEGETATION 



Central to the purpose of this paper is the role of forest 

 vegetation in maintaining more secure slopes. This question 

 has been analyzed in considerable detail by Gray (1970, 1978), 

 who identified four principal mechanisms by which forest 

 vegetation enhances stability, namely: 



(a) Mechanical reinforcement from the root system. 



(b) Regulation of soil moisture content and piezometric 

 levels through transpiration, interception, and by 

 affecting snow accumulation and rate of melting. 



(c) Buttressing or soil arching action between the trunks 

 or stems. 



(d) Surcharging from the weight of trees. 



The effectiveness of these hydromechanical influences de- 

 pends upon soil and slope conditions at particular sites. Root 

 reinforcement and buttressing, for example, would be of little 

 avail in arresting deep-seated, rotational failures in cohesive 

 soils. On the other hand, in shallow, coarse-textured soils which 

 are prone to debris sliding and avalanching along an inclined 

 bedrock surface, the situation is quite different; root reinforce- 

 ment and buttressing may contnbute significantly to stability in 

 this case. Dramatic examples of such slope stabilization by 

 ponderosa pines are shown in figures 4 and 5. The importance 

 of these "hydromechanical" contributions of forest vegetation to 

 the stability of slopes in the Idaho batholith is examined in 

 greater detail later in the paper. 



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