THE AUTHORS 



CONTENTS 



DONALD H. GRAY is Professor of Civil Engineering at the 

 University of Michigan in Ann Arbor, Michigan. He holds a 

 bachelor's degree in geological engineering, a master's 

 degree in petroleum engineering, and a doctoral degree 

 in civil engineering, all from the University of California at 

 Berkeley. 



WALTER F. MEGAHAN is principal research hydrologist and 

 project leader of the Intermountain Station's Nonpoint 

 Source Polfution research work unit in Boise, Idaho. He 

 holds bachelor's and master's degrees in Forestry from 

 the State University of New York, College of Forestry at 

 Syracuse University, and a doctoral degree in watershed 

 resources from Colorado State University. He served as 

 Regional hydrologist for the Intermountain Region of the 

 Forest Service in Ogden, Utah, from 1960 to 1966, and 

 has been in his present position since 1967. 



ACKNOWLEDGMENTS 



Research studies described in this paper were supported in 

 part by Grant No. ENG. 75-22766 from the National Science 

 Foundation. The assistance of Roger Gray in carrying out the 

 field work is also acknowledged. 



RESEARCH SUMMARY 



A study was conducted on two small watersheds in the Boise 

 National Forest to determine the role of forest vegetation in 

 maintaining more secure slopes in shallow, coarse-textured 

 soils typical of the Idaho batholith. Both soil water piezometry 

 and soil shear strength measurements were made in the water- 

 sheds. 



Results of the field studies and supporting analyses indicate 

 that forest vegetation often provides a critical margin of safety. 

 Woody vegetation growing on slopes of the batholith contrib- 

 utes to stability by root reinforcement, by soil moisture depletion 

 from interception and transpiration, by regulation of snow accu- 

 mulation and melt rates, and by soil arching restraint between 

 tree stems. Conversely, removal of vegetation from a slope by 

 timber harvesting or wildfire results in a loss or reduction of 

 effectiveness of these stabilizing mechanisms. Loss of vege- 

 tative stabilization in turn can le^d to increased frequency of 

 landslides as documented in this study. 



Management implications of the study are discussed. Sug- 

 gested measures and approaches include more stringent con- 

 trols on size and location of clearcut units, greater use of 

 "vegetation leave areas" or buffer zones particularly along haul 

 roads and next to streams, and construction of hydraulic struc- 

 tures that divert water away from critical areas. 



Page 



INTRODUCTION 1 



Slope Stability Problems in the Batholith 1 



Potential Effects of Timber Removal 2 



DESCRIPTION OF PINE CREEK STUDY 4 



Physiographic Setting 4 



History of Land Use 4 



Postfire Stability Problems 5 



STUDY DESIGN AND METHODS 6 



Overall Experimental Design 6 



Piezometer Installation and Data Collection 6 



Slope Stability Data Collection 8 



STABILITY MODELS FOR GRANITIC SLOPES 11 



Infinite Slope Analysis — Natural Slopes 11 



RESULTS OF ANALYTICAL AND FIELD STUDIES 14 



Piezometric Responses of Slopes 



to Vegetation Removal 14 



Stability Relationships 15 



Impact of Foliage Loss 20 



Impact of Root Decay 20 



Loss of Buttressing and Soil Arching Action 20 



Loss of Surcharge 20 



MANAGEMENT IMPLICATIONS 20 



Measures to Minimize Mass Erosion Hazard 20 



General Slope Hazard Rating Scheme 21 



CONCLUSIONS 22 



PUBLICATIONS CITED 22 



