Mass Wasting in Forested Mountain Topography by Su Cherng Hu A PAPER submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Forestry Completed March 1976 Commencement June 1976 ACKNOWLEDGEMENTS Deep appreciation is extended to Drs, RobertL. Beschta, George W. Brown, and Henry A. Froehlichfor their continued advice, guidance, and assistanceduring the preparation of this paper, Grateful thanks are also due to my wife,Cheng-li, for her encouragement and patience whilethe writer was away from home and studyingin the United States. TABLE OF CONTENTS Page INTRODUCTION 1 CHARACTERISTICS OF THE MASS MOVEMENT PROCESS 3 MECHANISM OF MASS MOVEMENTS 7 Concept of Slope Stability 7 Slope Stability Analysis 9 General Mechanism 10 Method of Slices 13 Progressive Failure Model 15 Variables Affecting Slope Stability 16 Site Factors 17 Climatic Factors 25 Land Use Factors 27 Combined Factors 28 EFFECTS OF FOREST OPERATIONS ON MASS MOVEMENTS 30 ForestCover and Slope Stability 30 Positive Effects 31 Negative Effects 31 Vegetation Manipulation 32 Indicator of Slope Stability 33 Land Use Activity and Mass Movements 34 Roadbuilding 35 Logging 36 Fire 39 METHODS OF PREDICTION, PREVENTION, AND CONTROL 41 Hazard Rating System 42 Prevention and Control 47 RESEARCH NEEDS 51 BIBLIOGRAPHY 57 LIST OF FIGURES Figure Page 1 Diagram of the several important mass movement types found on mountain for- est land. 5 2. The principal forces acting on a sloping soil mass. 8 3 Natural reduction of slopestability with time. 9 4 Diagram of forces acting on a massof soil on a slope. 11 5 Method of slices with circular arc constructed from the measureddimen- sions of the initial failure zone, 14 6 Frequency and acreage of slidesbe- fore and after logging inMaybeso Creek Valley, Hollis, Alaska. 38 LIST OF TABLES Table Page 1 Factors contributing to instabilityof earth slopes. 18 Processes leading to landslides 19 Relative erosion hazard oflogging areas 3 in relation to site factors--USDI. 43 4 A preliminary scherie on slopestability classification. 45 MASS WASTING IN FORESTEDMOUNTAIN TOPOGRAPHY INTRODUCTION As the demands for forestproducts increase, addition- al timber harvestingoperations can be expected on steep mountainous terrain. The resulting disruptionof natural slope stability by man'sdisturbances (roadbuilding, log- ging and vegetativemanipulation, etc.) may also acceler- ate mass movement processesin this terrain. Swanston (1969) defines massmovement as ". .the slow to rapid downsloPemovement of large masses ofearth material (soil, rock and forestdebris) of varying water content, primarily underthe force of gravity". Earlier, Popov (1963) quotedPogrebov5 definition of masswasting as .movement of a rock massdownward under the pressure of gravity, commonlyassociated with the activity of surface and ground water". As a dominant form oferosion on mountainous lands in the Pacific Northwest, massmovement may reduce site productivity by removingsoil material and lowering the nutrient capital; cause damageto roads, other improvements and scenic values; lead toserious channel degradation and scoured channel banks;contaminate water qualitywith increased sediment loads,turbidities and dissolved chem- ical content; shorten thelife span of reservoirs due to 2 excessive siltation, and impair fish habitat through in- creased sediment in spawning gravels and blockage of fish passage by landslides (Brown, 1973; Swanston and Dyrness, 1973). Accelerated mass movement is probably the most serious problem facing land managers in areas character- ized by steep slopes and heavy rainfall. The objective of this paper is to review and summar- ize the present knowledge about mass movement processes on mountainous forest lands, This review will emphasize problems associated with man's activities and may provide professional hydrologists and land managers with informa- tion that can be applied in making effective management decisions, In addition, several types of studies will be identified from which an improved understanding ofmass movement processes may be obtained. 3 CHARACTERISTICS OF THE MASS MOVEMENT PROCESS A classification of mass movement processes, based on the mechanics of failure and variables affecting slope in- stability on mountainous forest lands, consists of four categories differentiated by movement speed and process, type of failure at the point in initiation, and surface configuration (Swanston, 1974a). These four important types of mass movements are: 1, Creep Creep is the slow and somewhat continuous down- slope movement of soil and rock material due to gravitational stress sufficient to cause perma- nent deformation. Though creep is usually im- perceptible, it dominates as a major process on deep clay-rich cohesive soil materials. 2. Slides Slides are defined as mass movements resulting from finite failure of a soil mass along well- defined planes or surfaces. They can be further divided into: (a) rotational failures character- ized by backward rotation of a land mass along a circular plane, and (b) planar failures (some- times referred to as translational slides) characterized by movement of a block of soil or rock along straight or planar surfaces. 4 Flows Flows are mass movements of unconsolidated material showing a continuity of movement and semifluid behavior. They depend greatly on the degree of cohesiveness of the disintegrated material and the total water content. The move- ment is the result of either a rotational or planar failure. Falls Falls are very rapid movements of rock or soil, mostly through the air, by free falling, leaping, bounding, and rolling. They are initiated by rotational or planar failures. The mode of failure and resultant downslope movement de- pend greatly on soil depth, degree of cohesion, and soil water content. The moving soil mass may proceed by any one of the above types of movement or in combination. In the western United States, slides and flows are the most important and frequent types of mass movements, while falls are rela- tively uncommon (Swanston, 1974b). A diagram illustrating the characteristics of the more important mass movement processes is presented in Figure 1, An excellent review of landslide types and classifications was presented by Sheng (1966). Ladd's ROTATIONAL FAILURES 5 FALLS SLUMP 'r' ROCKFALL EARTH FL extremely rap4d I \'' PLANAR FAILURES Weathered bedrodi, soil, etc. very ropld to DEBRIS AVAL .4NCH extremely rapid Figure 1, Diagram of several important mass movement types found on mountainous forest land (after Swanston, 1974a). 6 (1935), Sharp's (1938), and Varnes' (1958) are among the most outstanding classification schemes. It should be mentioned that all of the prevailing landslide classification systems are designed for apar- ticular purpose. In coping with mass wasting events, additional effort is required toward understanding the mechanisms causing the slides instead of only categorizing form or type of movements, 7 MECHANISM OF MASS MOVEMENTS Concept of Slope Stability Popov (1963) indicates a disturbance of equilibrium of the mass on a slope is the principal cause of sliding. Spangler and Handy (1973) tersely described the concept of slope stability: ?Every mass of soil which is bounded by a sloping surface is subject to shearing stresses on nearly all its internal sur- faces because of the gravitational force which tends to pull the upper portions of the mass downward toward a more near.- ly level surface. If the shearing strength of the soil is at all times greater than the stress of the most severely stressed internal surface, the slope will remain stable. On the other hand, if the strength at any time should become less than the stress the soil will slump or slide down the slope until a position is reached such that the stress is reduced to a value less than the strength". Swanston and Dyrness (1973) depict the stability of a soil mass as shown in Figure 2. Slope stability depends on a balance between forces expressed in terms of a safety factor (F5): Shear strength along the potential failure F5 surface Shear stress promoting sliding along the potential failure surface Theoretically, a slope failure will occur when Fs is less than unity. Hurtubise and Rochette (1957) indicated that ". .the 8 / SLIDE RESISTANCE GRAViTATIONAL STRESS Figure 2. The principal forcec acting on a sloping soil mass (after Swanston and Dyrness, 1973). "Gravitational stress represents the downslope component of gravity acting on the soil mass. Sliding resistance is the sum of cohesion of. the soil particles and the frictional resistance between parti- des and between the soil mass and the sliding surface. As long as slide resistance is greater than gravitational stress, the soil mass will remain relatively stable, Adding water to the soil decreases stability by increasing weight and decreasing frictional resistance. The rooting structures of trees and other vegetation can serve as external stabilizers, binding soil material and anchoring the soil mass to a more stable substratum". stability decreases with time at a geological scale, the general trend being accelerated or retarded by accidental phenomena until a factor of safety of one is reached't (Figure 3). General trend of de- creasing stability Artificial causes Elapsed time after deposition Figure 3. Natural reduction of slope stability with time (after Hurtubise and Rochette, 1957). It is clear that two forces are exerting on a dynamic- equilibrium soil mass and the ratio of slide resistance to gravitational