Mass Wasting in Forested 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 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 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 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 slopes. 18 Processes leading to 19 Relative 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 . 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, and forestdebris) of varying , 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 , 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 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 -rich cohesive soil materials.

2. Slides Slides are defined as mass movements resulting

from finite failure of a soil mass along -

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 , 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 types and classifications was presented by Sheng (1966). Ladd's ROTATIONAL FAILURES 5

FALLS

SLUMP 'r' 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 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 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 stress provides an index to the stability of a slope.

Slope Stability Analysis

The complexities of slope stability analysis were 11- lustrated by Baker and Yoder (1958):

"The analyses of stability cannot be made for every type of landslides and for any type... assumptions based on idealized con- ditions and materials will be required. It Is impossible to treat mathematically all of the variables imposed by nautre... In 10

application, the results are dependent on the validity of the assumptions and simpli.- ficat ions",

Certain factors and assuniptions required for conventional

stability analysis are listed by them, including:(1) a

shear failure must have occurred or be a threatfor slides,

(2) the average shearing resistance along the slip-surface

at the time of failure must be known, (3) nolateral shear- ing resistance exists along the sides of the slice,(4)

an assumption must be madeconcerning the location of the piezometric or the ground water surface at the instant of

failure, and (5) a factor of safety of one is assumed for a failed slope, Similarly, Swanston (1971a, 1974b) commented that:

"Direct application of theory to analysis of mass movement processes is dif- ficult because of the heterogenous nature of soil materials, the extreme variability of soil water conditions, and the related varia- tions in stress-strain relationships with time. It does, however, provide a convenient means of expressing the general mechanism and com- plex interrelationships of the various factors active in development of soil mass movements on timbered slopes".

General Medhanism

The forces acting upon a soil mass are clearly il- lustrated in Figure 4 (Swanston 1971a, 1974b), The dia- gram shows ttthe geometrical relationship of the various factors, Any increases in the tangential F,F11E,E = Equal and opposite shear forces actingW1 a= = on the soil mass= Equal and opposite WeightnormalShearInclination of forcesstress the soil oacting= W niasssinct on the soil mass the sliding surface aCT == Cohesion,Normal stress a soil on= propertyAnglethe sliding of internal surface=W , cosc a soil property £ shearIn the strength presence (5) of =pore C + water(W cosa-p)tan pressure (p),Ss = FrictionalShear strength resistance = C+W cosctan= W cosatan4 Figure 4, Diagram o forces acting on a mass of soil on a slope (after Swanston, 171a). \ \ \ 'V 7Wco. normalfrictionalshearthenopposite"For consist stress. simplicity,component and forces thereforeof theof andthe theweight frictionallateralcancel. soil of weight theand resistance soilshear (a) mass throughforces (s)(W) actingthewhichand angleits ison tangential proportionallytheof internalmass arecomponent frictionassumed related equal to the and Resisting forces consist of cohesion (C) which is independent of the The driving forces tending to cause downslope movement CT) or ir) component of gravitational stress will in- crease the tendency for the soil to move downslope. Increases in shear stress result from increasing inclination of the sliding surface (cL) or increases in the effective weight of the soil mass (W). Increases in slope angle may result from epeirogenetic and orogenic rejuvenation or local glacial and stream modification. Increases in weight of the soil may result from increased water content or surface loading. Shear stress can also be augmented by application of wind stresses transfered to the soil through the root systems of trees, the local build-up of internal stresses in the soil by progressive creep, frictional "drag" pro- duced by seepage pressures, and removal of downslope support by undercutting or progres- sive failure. is governed by a more complex interrelationship in the soil and slope characteristics. These fact- ors include: (a) cohesion, the capacity of soil particles to stick or adhere together. This is a distinct soil property independent of gravitational stresses, produced by cemen- tation, capillary tension, or weak electrical bonding of organic colloids and clay parti- cles; (b) the angle of internal friction (), which is an expression of the degree of inter- locking of individual grains; and(c) the ef- fective weight or normal component of gravita- tional stress (WcoscL), which includes both weight of the soil mass and any additional sur- face loading plus the effect of slope gradient (a). The tangent of the angle of internal friction () times the effective weight (Wcosa) constitute a mathematical expression of fric- tional resistance (s=Wcoscttanp). Moisture content and active (pres- sure produced by the head of water in a sat- urated soil and transferred to the base of the soil through the pore water) act to modify the component of frictional resistance by reducing the value of the normal component of gravita- tionalstress. This is frequently expressed by the modified equation for effective weight (Wcosct-), where (p) is pore pressure". 13

Method of Slices

The method of slices, based on a circular mode of de- formation, is adopted for stability analyses of on which pore-water pressure has an effect (Swanston, 1970).

The slope is assumed to be infinite and with a constant angle of inclination. Consolidation or swelling of soil during creep are negligible. With this method, the soil mass delineated by field measurements is "sliced" into segments (Figure 5). The factor of safety is given by the formula:

Fs= E CtL + (iWn + Qn - L\L) Cosa tan EC(L,Wn + Qn) Sina

where eWn = weight of the soil Qn = surface loading p = pore-water pressure L,L = slope width of each section a = slope angle = effective angle of internal friction, and C effective cohesion of the soil

All- parameters in the formula need to be obtained by meas urements and/or estimates. Therefore, the reliability of stability analysis depends on several basic factors: (1) accurate description of geometry (topography, cross sec- tion); (2) accurate knowledge of soil properties (shear strength, unit weight); (3) correct definition of external loads, if any, (vehicle and structural loads, rain or snow or perhaps sonic booms); (4) correct 14

SOIL ZONE

AC 0 CIRCLE WITH RADtUS 55FEET C0D 0 ARC 60 FEET

'__...'..' DEPTH TO SLIDE SURFACE 3 FEET ANGLE OF SLOPE:a c 0 1OFEET

-

o

-. o

0 QQ cz, : 00 0 BEDR0CPc

0 _e - a 0 UNWEATHERED° GLACIAL TILLoco o O .-. -

Figure 5, Method of slices with circular arc constructed from the measured dimen- sions of the initial failure zone (after Swanston, 1970). 15

description of ground water or seepage conditions (pore water pressure); (5) correct method of analysis (valid as- sumptions and simplifications)3 and (6) considerable field

checking and on-the-ground experience (Gonsior and Gardner,

1971; Rice and Krammes, 1971). Apparently, conventional analyses of slope stability may not adequately appraise

natural slopes due to simplified assumptions (Rice and Krammes, 1971).

Progressive Failure Model

Rice and Krarnmes (1971) present a different concept- ual model of progressive failure. A progressive failure starts at the weakest point on the failure surfaces. The stresses transferred to surrounding surfaces may result in a chain reaction of failure which culminates witha rapid downslope movement of the soil mass. Progressive failure assumes the two masses divided by the failuresur- face are not held together primarily by friction but mainly by dowelling effects at various points of strength. With this model, external loading to the potential failuremass does not necessarily increase the strength of the dowels.

Thus, the additional downslope component due to overloading may contribute to the instability of slopes.

In summary, the processes and mechanisms involved in. individual mass movements on forest landsmay vary 16

considerably from site to site. Though the soil mechanics

approach to the slope stability analysis is not likely to be adequate to comprehend the conditions of forested moun-

tain slopes, it is still an important tool evaluating the general mechanisms and the principal factors involved.

Ideally, all possible parameters functioning at a given site should be included in the slope stability analysis

to obtain reliable results. However, due to cost and priority considerations, only a few detailed studies on

the analyses of slope stability on forest lands have been accomplished. For a better understanding of mass movement processes and mechanisms, additional research should be undertaken to study the appropriate methods of slope sta- bility analysis on forested mountain terrain.

Variables Affecting Slope Stability

Mass movement events are the result of an interaction of numerous interacting factors. As indicated by Ritchie

(1958):

'The basic conditions that favor slides de- pend on the character, , and structure of the underlying rocks and , on the topography, climate and vegetation, and on surface and underground waters. All of these factors vary widely from place to place; their variations are reflected in differences in the rate and kind of land- slide movements that result from their interaction".

Leighton (1969) also referred to the complexity and number 17 of variables affecting slope stability:

"Controlling factors in the evolution of natural landslides in the Far West (U.S.A.) include differences in topography, climate, vegetation, , internal geo- metry, processes, and the sequence and stages of landform developmentt'.

The principle factors contributing to the instability of earth materials (Table 1) have been outlined by Varnes

(1958). Earlier, Terzaghi (1950) grouped causes of land- slides into (1) external causes: steepening or heightening of the slope by river erosion or man-made excavation, depositio.n of materal along the upper edge of slopes, and shocks, and (2) internal causes: increase of the pore-water pressure and progressive decrease of the cohesion of the slope-forming material. A review of land- slide-producing processes was presented by Terzaghi (1950) to show the agents that may contribute to the instability of a slope and their action and effects (Table 2).

For the convenience of discussion, the variables af- fecting mass wasting are separated as site, climatic, land use, and combined factors.

Site Factors a. Gravity: The force of gravity acts directly or in-

directly in all types of mass movements. Every soil

mass on a slope is under a shear stress resulting from

downslope component of its own weight. Rice and FactorsTable 1. that Contribute to High Shear Stress Factors contributing to instability of earth slopes (after Varnes, 1958). Factors that Contribute to Low Shear Strength A. Removal of Lateral Support pits,HumanandErosion-bank rivers etc.agencies-cuts, cutting canals,by streams A, Initial State GrossTexturematerialsComposition-inherentlygrain structure-faults, structures- loose soils, metastable weakjointing, B. Surcharge Naturalings,Humanice and etc. agencies-fills,agencies-wt. rainwater of build-snow, B, Changes Due to Physico-Chemicaland Other Reactions beddingFrost actionplanes, and varying, thermal etc. expansion D.C. Regional Transitory Tilting Earth Stresses - C. Changes in Intergranular Forcesto PoreDue Water LeachingDryingHydration and ofcracking clay minerals E. Removal of Underlying Support HumanSubterraneaningSubaerial by agencies-miningground weathering-solution.- erosion water - piping groundSeepagesaturationLossBuoyancy in water pressurecapillaryin saturated of tension percolating state upon F. Lateral Pressures RootSwellingFreezingWater wedging in watervertical in cracks D. Changes in Structure 2,1. Grain Fissuring structure of preconsolidated collapse upon turbanceduedis- clays to release of lateral restraint Table 2 Processes leading to landslides (after Terzaghi,, 1950). agentportingTrans-Nat-ne of Agent ferosionConstructionoperationsAgent intoEvent Actionor Bringing ofheightAction the1. Increases slopeor of angle Agent EverySensitive material to ActionMaterials Most ofChangesPhysical stressslope Naturestatematerialsin of Actions IncreasesEffectsstresses on shearingEquilibrium Conditions TectonicTectonicstresses EarthquakesmovementsTectonic of2.3. Deformationearth's High-fre- crust Everyclay,Stiff,Every shalefissured material material angleIncreasesopensofChangesTransitory stress fissures stateand slope IncreasesIncreasesproces.sstressesstresses 8shearingand shearing initiates orexplosivesstresses or blasting vibrationsquency andcementedMediumLoess, slightly ,or fine InitiateschangesgranularDamagesstress of re- bonds inter- SpontaneousDecreasesstressesand increases cohesion shearing ofWeightslope Processslopecreated which the 4.slope Creep on slidesclay,Stiff,sandloose shale,fissuredsaturated old producesclosedOpensofarrangement grains upjoints, new Reducesliquefaction cohesion, materialWater meltincRains or snow weakvoidsment6.below5. Displace-Creep stratum of slope air in intoe MoistonesRigidresting materialssand on plastic jointwaterIncreases pressures pore ProcessacceleratesresistanceDecreases 8 frictional Water (Cont ) of7. Displace-air in open shaleJointed rock, jointswithsurecapillary8. Reduction swellingassociated pres- of Stiff,shalesclay fissuredand some Causes swelling Decreases cohesion Frost duemeltingand9.10. Expansionto subsequent freezingFormation of ice SiltJointed and rocksilty sand contentIncreasesnewandOpens producesones old of waterjointssoil Decreasesresistance cohesion.frictional RiseRapidDry of spelldrawdown water towardslopelayers12.11.13. SeepageShrinkagefoot Causes of a FineSiltClay sand,or sand ProducesporeProducessure water cracks excess pres- Decreases cohesion Seepagetanttablereservoir aquifer in dis from -or materialmetrictowardinrise14. slope of Seepagesurface piezo-slope Saturatedbelowlayers betweenclay silt layers or Increaseswater pressure pore frictionalDecrease ofresistance canal airerosionsoluble 17.16.15.in the RemovesSubsurfaceDisplaces bindervoids FineMoist, sand fine or sand silt faceEliminatesslopeUnderminesgranularDestroys tension bond inter-sur- the IncreasesDecreasesstresses shearingcohesion 21

Krammes (1971) indicate the role of gravity is 10

to 20 times greater in mountainous terrain than on

agricultural lands and emphasize gravity is the

principal source of energy working to strip soil

from the land surface on mountain slopes.

Geology: Ladd (1935) tabulated and discussed twenty-

three causes of landslides with emphasis on attri-

butes of geological structures. Dyrness (1967a)

found that mass soil movement occurred more frequent-

ly in areas of tuff and which weather readily

to clay-rich soils than in those of basalt and ande-

site in the H.J. Andrews Experimental Forest of

Oregon. Swanson and Jaiiies (1975) conclude both deep- seated and shallow mass wasting events tend to occur

in the relatively unstable volcaniclastic rocks of

ash flow and mudf low origin in the western Cascades

of Oregon. Chemical weathering of clay stratum, channeling of runoff water to weak strata through

cracks, and formations parallel to slope surface are

destabilizing forces related to geological character-

istics (Rice and Kramrnes, 1971).

Soil properties: Excessive soil-water content is a principal factor causing mass wastage in southeast

Alaska (Swanston, 1969). In the study of four soil series in western Cascades of Oregon, Paeth, et al. 22

(1971) found the slide-prone soils, derived from

greenish tuff and breccia, were characterized by high

amounts of smectite clay, absence of kaoline, and

moderate amounts of free iron oxide, Based on the

results of former researchers, they explained that shear strength had an inverse relation to cation ex-

change capacity (kaolinite has the smallest shear ii 44iL 4e* strength); smectites reduced shearing resistance as

a result of internal swelling, and iron oxides acted

as cementing agents providing strength to soil.

Particle size distribution, angle of internal fric.-

tion, and parent material structure and type are im- portant site characteristics involved in the occurence

of mass movements on steep lands (Swanston and

Dyrness, 1973). Liquefaction due to high and soil moisture and the presence of 1og.and debris

in fills are related to -associated slope fail-

ures in the Idaho Batholith (Gonsior and Gardner,

1971). d. Slope: A positive correlation exists between slope

steepness and slide frequency. In southern California

slope was the most iiiportant site factor associated

with soil slip events where the soils are shallow in

depth and coarse in texture (Rice, et al. 1969).

Gonsior and Gardner (1971) recommended350as the 23

critical man-made slope in the Idaho Batholith where

shallow and cohesiveless soils overlie weathered gran-

ite. Studies in the Maybeso Valley of southeastern Alaska, characterized by steep, shallow, permeable

till soils, show that majority of debris avalanches

and flows develop on slopes greater than340and are

especially frequent around a critical angle of 37°

(Swanston, 1970). e Aspect: In brush areas on the San Dimas Experimental Forest in southern California, northerly aspects with

growing larger plants were more stable than southerly

ones (Rice, et al. 1969). But Dyrness (1967a) found

almost no mass movements occurred on south or south-

west slopes in the H.J, Andrews Experimental Forest

of Oregon after the winter of 1964-65 possibly

because weathering proceeds much more slowly on the

drier aspects. f. Vegetation: In the studies of soil slips in the San Dimas Experimental Forest, Rice et al. (1969) conclud-

ed the rooting habit and density of the vegetation

were linked to the occurrence of soil slips, the fre-

quency was inversely related to the size and density of vegetation and they also found conversion from

brush to grass resulted in increased soil slip erosion,

The root system of brush apparently had an important 24

function in stabilizing surface soils. Swanston (1974a) concluded the decay of root systems 3 to 5

years after logging contributed massmovements to the

till soils of southeast Alaska and that tree roots

accelerated slope failure in these shallow soils

through the action of swaying trees buffeted by strong

winds. In his aerial photo study of landslides in Japan, Fujiwara (1970) also found about tenfold in-

crease in landslides 5 to 8 yearsafter cutting due

to the loss of anchoring root structures,

Location: Proximity to stream channel was an import ant variable in slippage occurrence in southernCali-

fornia (Rice, et al. 1969). This is due to the stream cutting or scouring action at the toe of the slope.

Drainage: Dyrness (1967a) found that road drainage water was responsible for about one-quarter of road-

related mass movements on the H.J, Andrews Experiment-

al Forest. These mass movement events were caused by concentration of road drainage water and/or by fail-

ures of some portion of the drainagesystem.

1. Subsurface water movement: Swanston (1967) explained saturation and subsequent free water movementthrough

the soil effectively reduce shear resistance. The

increased weight of the water-saturated soil contri-

butes to the effective gravitational force actingto 25

pull the soil downslope. At the same time, the

bouyancy of the vertical component of free water

movement or the pore water pressure tends to reduce

the frictional resistance and adhesion between parti-

cles and along the surface. The forces in-

volved in reducing shear strength are illustrated in

Figure 4.

Climatic Factors

Of the various climatic factors that might be associ- ated with mass movements only precipitation and snow cover factors will be reviewed here. a. Precipitation: Rainfall plays an important role in

the mechanics of mass movements. Sheng (1960) report- ed that numerous landslides occurred in an extensive

area of central Taiwan following the storms of August

7, 1959. In the Idaho Batholith, Gonsior and Gardner

(1971) indicated precipitation events in the winter

and spring of 1965 caused numerous mass movements.

Rice and Foggin (1971) reported that the intense

winter storms of 1969 deposited 802 mm of precipita-

tion in 8 days and unusual amounts of soil slippage

occurred on mountainous viatersheds in southern Cali-

fornia. Swanston (1969) showed that mass wasting in

southeast Alaska, regardless of soil type, is 26

concentrated in areas of recurring highprecipitation.

Later, Swanston (1974a) observedrainfall in excess

of 5 inches in 24 hours produces completesaturation

and maximum pore-water pressure andis believed to

be the main initiating device for themassive debris avalanching in till soils at Hollis in coastalAlaska.

Swanston (1970) explained: "During high rainfall periods, the soil becomes saturated.. .The increasingvol- ume of water causes a risein the piezo- metric surface with two important conse- quences: (1) increasing shearstress along potential sliding surfaces caused by ris- ing seepage pressures and increasingunit weight of the soil materials, and (2) de- creasing shear resistance resulting from increased pore-water pressure in the soil.,. high pore pressure stands out as the most effective triggering force for debrisaval- anches with maximum effect at complete sat- uration of the soil profile in the areaof initial sliding" But Harr and Yee (1975) found the porous ,permeable,

and well aggregated forestsoils of the Oregon Coast

Range seldom reached saturation. The low intensity,

long duration rainfall distributedin this region is dispersed predominantly by unsaturatedflow. They

attribute the possible mechanism for someslope

failures to aggregate destructionby direct wetting.

of the b. Snow cover: Snow cover increased unit weight soil and delays release of large amountsof water un-

til spring melt. Such delay is identified as the 27

principal initiating factor of many landslideson the east flank of the Cascade Range in Washington and

Oregon (Swanston and Dyrness, 1973).

Land Use Factors

Man's activities may cause mass movements by changing the slope or other conditions of the mantle materials and by influencing the distribution of water (Dyrness, 1967).

Sheng (1966) indicated road construction is the major land- use factor in triggering landslides in the steep mountain- ous watersheds of Taiwan. Dyrness (1967a) also pointed out roads are so often an important factor causing mass movements and man-caused disturbance influences the occur- ence of mass soil movements. Forest operations, such as roadbuilding, cutting of trees, fire, and accumulation and flow of debris are factors contributing instability

(Swanston and Dyrness, 1973). Rice and Krammes (1971) concluded mass movements are especially sensitive to man's disturbance such as road building, logging, and vegetative manipulation. In the Idaho Batholith, Gonsior and Gardner

(1971) observed most slope failures were associated with roads. Man's activities will undoubtedly aggravate natur- ally unstable slope conditions. 2

Combined Factors

In his landslide studies of Taiwan, Sheng (1966)

listed several factors which are responsible formass movements; namely, (1) rain and runoff, (2) stream cuttin

(3) groundwater and seepage, (4) earthquake, (5) wind, (6)

road excavation, and (7) man's activities (mining, logging

farming, grazing and fire). Also, in the aerial-photo study of landslides in Lo-tong watershed in Taiwan, Kuo

and Chien (1970) identified heavy rainfall, runoffconcen- tration, stream scouring, and slope as the main factors

responsible for the landslide events.

The causes of mass movements cannot be generalized owing to the complexity of combined contributing factors.

Sowers and Sowers (1951) point out:

"In most cases a number of causes exist simul- taneously, and so attempting to decide which one finally produced failure is not only dif- ficult but alsoincorrect, Often the final factor is nothing more than a trigger that set in motion an earth mass that was already on the verge of failure.

In the study of mass wastin.g in coastal Alaska, Swan- ston (1969) explained tt.superimposed on this rough cor- relation of rainfall and slide occurrence is the effect of regional bedrock and glacial geology which modifies the effect of rainfall on landslide initiation through changing slope and soil characteristics,"

Rice and Foggin (1971).. studied the effect of storms or 29 slides and concluded the thresholds of several environment al factors (slope, intense rainfall, aspect, decay of root system) are not linear, and the 1969 storms crosses the thresholds of several factors responsible for more wide- spread slippage in the San Dimas Experimental Forest.

Furthermore, Rice and Krammes (1971) concluded

.in practically all cases, the final trig- gering factor causing a landslide is an extend- ed period of intense rainfall. Preceding it, however, there is usually rather complicated interaction between site conditions and meteor- ological conditions which set the stage".

As a result of these and other studies it is obvious that the factors contributing to mass wasting are numerous, com- plex, and interacting. Any attempt to isolate which one caused a slide will be impracticable. EFFECTS OF FOREST OPERATIONS ON MASS MOVEMENTS

Forest Cover and Slope Stability

Swanston (1974a) generalized that vegetation helps control the amount of water reaching the soil and the quantity of water stored in the soil, largely througha

combination of interception and evapotranspiration . Rice and Foggin (1971) concluded water consumption androot anchoring are the most important effects of plantcover on soil slips in southern California. Rice and Krammes (1971 also pointed out that vegetation can serve to increase slope stability in two ways: (1) anchoring effect of roots and (2) transpiration. Removal of forest cover leath to: (1) decay of roots which lace the soil together and stabilize it, (2) reduced transpiration, and (3) more free movement of groundwater. Many oversteepened slopes in mountainous terrain are maintained by roots anchoring the soil to the bedrock (Patric and Swanston, 1968), Removal of this mechanical support generally results in landslides or slumps. The potential effects of forest vegetative cover on slope stability also depends upon site conditions such as topography, geology, soil type and depth, soil moisture content, upon climatic factors including rainfall distribution and wind patterns, and also upon conditions of vegetative cover such as rooting characteristics, plant ages, single or mixed species, and percentage or degree o

forest cover altered.

Positive Effects

Swanston (1974a) suggested tree roots can probablyix

crease shear strength in unstable soils as a result of: (

roots anchoring the soil mass by permeating the seams and

fractures in the parent material, (2) roots providing a

continuous long fiber adhesive binder to the entire soil

mass, (3) roots tying the slope together across zones of

weakness and instability to more stable soil masses, and

(4) roots providing downslope supp.ort to unstable soil maE

through buttressing, Therefore, the depth of both root

distribution and soil mantle are very important factors

when the potential of tree roots to increase shear strengt

is considered. Deep rooted vegetation species usually pro

vide greater potential for slope stability. It should be pointed out that tree roots may fail to serve as stabilize

if the potential sliding surface is below the root mass.

Negative Effects

Growing roots may wedge out pieces and blocks of bed-

rock and loosen the underlying bedrock surface;In addi-

tion, tree blowdowns resulting from gravity fall may be a principal initiating cause of sliding on unlogged slopes 32

(Swanston, 1967). Destruction of penetrating roots, by windthrow or decay following cutting, may substantially increase the susceptibility of these slope to sliding

(Swanston, 1969). Rice and Krammes (1971), cited from

Swanst.on (1969), indicated trees may transmit the force of strong winds to an incipiently unstable and there- by trigger sliding. Furthermore, the rupture of log dams, created by uprooted streambank trees, greatly increases channel scour and may further destablize the adjacent slopes.

Vegetation Manipulation

Kesseli, as cited by Rice and Krames (1971), found abundant soil slips on grassy slopes but none on slopes covered with oak chapparral or coniferous forest. Apparent ly, dense deep rooted vegetation tends to prevent and mini- mize the extent of slips more than grass or similar light shallow-rooted plants. Rice and Foggin (1971) found that the conversion of brush areas to grass lands for increasing forage production and water yield also increased soil slip- page on mountainous watersheds in southern California dur- ing intense winter storms of 1969. The conversion both re.- duces the mechanical reinforcement due to roots and increas es soil water pressures and the weight of soil mass due to lower transpiration rates (Rice and Krarnmes, 1971). The 3

importance0the transpiration factor depends on climate. It is probably negligible where precipitation far exceeds

potential evapotranspiratiOn and very likely significant

where substantial moisture deficits develop in arid areas.

Indicator of Slope Stability

A more or less permanent cover of vegetation species

on the older slide areas provides avisual tool for indi- cating present stability and, perhaps, past sliding histor

of a slope and for appraising landslide hazards. For ex-

ample, in the Northern Rocky , aspen (Populus spp.) are commonly found on areas disturbed by mass move- ments; in south coastal Alaska, new slides are invadedby willow (Salix spp.) and alder (Alnus spp.), while older ones can be recognized by even-agestands of Sitka spruce (Picea sitchenis (bong.) Carr.) (Swanston, 1969; Riceand

Krammes, 1971). Hydrophytic plant species frequently in- dicate high moisture sites most susceptible to mass move- ments. Other features of vegetation including jackstrawed tipped and pistol-butted trees may also indicateinstabil- ity. But it should be kept in mind that the deformation. of trees may also be related to mechanical andphysiolog- ical causes. As a result of various studies it is probablethat the anchoring effect of plant roots may contributeto the stability of a slope under certain conditions. But it 34 should be also mentioned that other important stability- bearing factors (amount of rainfall, harvesting method adopted, soil type and depth, size of land logged, and windthrow) need to be concurrently considered in any eval- uation of the merits of forest cover in terms of slope stability.

Land Use Activity and Mass Movements

Land use activities such as roadbuilding, timber - ting, yarding and slash removal can cause slope disturbanc which may initiate and accelerate mass movements. As

Swanston (1971b) indicates the overall effect of forest operations is to disrupt the delicate balance of forces exerting on the soil mass forming the slopes, often result- ing in further initiation and acceleration of mass wasting.

Rice, et al. (1972) concluded mants activities associated with timber harvesting constitute the most important impact on forest lands in mountainous topography, Because dis- turbance is an unavoidable result of logging, these activi- ties have been of major concern to forest managers. The extent of soil disturbance from a timber harvest depends on numerous factors including, but not limited to, logging methods, road systems, slope gradient, soil stability, and the size of logged areas. 35

Roadbuilding

Roadbuilding related to timber cutting has long been identified as an initiator of mass movements in unstable areas. Rice, et al. (1972) indicated road excavations undercut upslope soil masses and may alter the drainage patterns from the hillside. They may also decrease the strength of the slope by exposing formerly underlain mat- erial to weathering. Side casting and road fills place additional load on the underlying soil mass. The fills with over-steepened slopes are prone to sliding. Poor drainage and culvert systems may also cause slope failures.

As a result, roads are frequently associated with mass movements.

In the air-photo study of landslide occurrence along forest roads (railroads, wide truck roads and narrow truck roads) in Taiwan, Bethlahmy and Koh (1968) showed that road construction activities induced land damage; widths and volume of road cuts determined the relative severity of damages. Fredriksen (1970) found landslide events are most frequent where roads crossed stream channels in steep, unstable terrain of western Oregon and suggested disturbance from road construction may be minimized by re- duction of midslope road mileage. Dyrness (1967a), after surveying of mass movements in the H.J. Andrews Experimenta

Forest, found that slumps and caused by road 36 fill failures, road backsiope failures, and failures due to road drainage waters were the most frequently occurring events with 31 out of the total 47 mass soil movements in the winter of 1964-1965. In Idaho, Megahan, as cited by Swanston (1971b), reported 90 percent of the mass movements were road-associated, In all these cases, roads had been built on steep topography and failed during stormE

Logging

Swanston (1971b) suggested timber harvesting adverse- ly affects soil stability through changes in soil hydro1og and mechanical support provided by vegetation, Rice, et al.(1972) also pointed out anchoring effect of tree roots contributes part of a soil mass's strength. Therefore, th susceptibility of landslide would gradually increase as tree roots decay after logging. In the study of landslides in the Wasatch Mountains of Utah, Croft and Adams (1950) related the increased mass movements to the loss of the stabilizing effects of the roots resulting from timber harvesting. Similar findings, as cited by Rice, et al. (1972), are reportedby Cappuccini and Bernardini (1957) in Italy, by Bishop and Stevens(1964 in Alaska, Swanston (1969, 1970) in Alaska, Kawaguchi, et al. (1959) and Fujiwara (1970) in Japan, and Zaruba and

Mend (1969) in Czechoslovakia. 37

Bishop and Stevens (1964) attributed the increased

occurrence of debris avalanches and debris flows to log-

ging activities in southeast Alaska following the storms

in the fall of 1961 (Figure 6). Dyrness (1967a) found a greater frequency of mass movements related to logging dis-

turbance on the H.J. Andrews Experimental Forest. Kojan,

as cited by Swanston (1971a) believed that soil mass move- ments in the northern California Coast Range are "..pro-

foundly and extensively affected by activities such as

logging". Swanston (1974a) discussed the direct effect of

timber harvesting in southeastern Alaska: ,timber har-

vesting operations have a major impact on initiation and

acceleration of these movements. The cutting of timber itself has been directly linked with accelerated mass movements, and the accumulating of debris in gullies and

canyons has been identified as a major contributor to the

formation of large-scale or debris torrents".

If the forest cover provides root structures which serve as a significant component of slide resistance,logging may

result in an increase of occurrence of mass wasting. How- ever, it is cautioned that the effects of logging on slope

failure can not be objectively evaluated without consider- ation of climatic and geological conditions in the problem areas. Logging alone may not increase mass movements. However, road construction and logging activities 0

ON UNIFORM SLOPES IN CONCENTRATED 60- DRAINAGE BEFORE (I) 50.- LOGGING DURING AND AFTER LOGGING (2) ELAPSED TIME PERIOD I (2) 2O (7) (I) -j (I) 10 0 (2) cs (100) #4) (7) 20 1 L i F7 1943 1952 1959 1931 19S21948 1952 1959 1961 1962

YEAROFSTUDY

160

(fll40 Iii

C-) 'I2O Li 1-100 LiC-) LL. LL. LOGGING co

e0 >Li 40 -j

20

1948 1952 I95 1961 1962 YEAR OF STUDY Figure 6, Frequency and acreage of slides before and after logging in Maybeso Creek Vally, Hollis,Alaska (after Bishop and Stevens, 1964). 39 associated with timber harvesting are often responsible for the occurrence of mass erosion.

Fire

Fire is also an influential factor affecting soil mass movements on unstable slopes. In drier regions, fire can devastate large watershed areas by causing the des- truction of vegetation cover. Destruction of plant cover by fire can lead to gradual deterioration of root systems and resulted increase in soil mass movements will be ex- pected (Croft and Adams, 1950; Swanston, 1971b), In the California Coast Ranges, the conversion by fire of brush to grass for producing forage and yielding water caused significant increases in soil mass movements. Accelerated dry creep rates were observed immediately after fire by

Kramrnes (1960). Mersereau and Dyrness (1972) recommended slash disposal methods, such as clean yarding or chipping and returning slash to the slope to replace broadcast burn- ing in order to preserve vegetation during logging opera- tions. A new slash disposal method developed by Tornbom may be promising for lessening disturbance to soil and understory vegetation (Edgerton, et al., 1975). This method utilizes equipment consisting o a rubber-tired, front-end loader with a hydraulic grapple. The equipment is probably subject to topographical limitations andmay not be useful in steep mountainous terrain. 40

In the appraisal of erosional impactsof timber har- vesting, Rice, et al. (1972) summarizedthe present state of knowledge about mass wastingassociated with land use activities. They indicated land use activities willin- crease erosion rate (bothsurface and mass erosion); the cutting of trees does not generally increaseerosion, but clearcutting on steep unstable slopes may lead toincreased mass erosion. Accelerated erosion is a possible and unde- sirable side effect of use of fire following atimber har- vest. However, the road system installed tofacilitate timber harvest far overshadows logging orfire as a cause of accelerated erosion and they triggerlandslides more frequently than any other disturbances by man, 41

METHODS OF PREDICTION, PREVENTION, AND CONTROL

Current research on the effects of forest practices on soil stability tends to: (1) anticipate hazardous sites

avoid disturbances conducive to slope instability, and

reduce landslide incidence after disturbances (Swanstot

1974b).

Identification and qualitative rating system of hazarc areas is perhaps the best approach to prescribe measures to cope with mass movement problems in mountainous water- sheds. These procedures require mapping unstable slopes, analyzing factors affecting slope instability, and classi- fying unstable areas according to acceptable levels of operation (Swanston, 1974a). To predict future slides is difficult due to the large number of interrelated variableE

Basic information on rock character and structure, soil, topography, climate, vegetation, and ground water regime is usually required, Recognition of past failures as po- tential areas for future failures and a detailed analysis of individual slides can be used as indicators for predict- ing the behavior of adjacent slopes (Rice and Krammes,

1971). Swanston (1974b) also suggested many techniques are available to identify hazardous sites in the western

U.S., assuming adequate knowledge of site conditions and failure mechanisms. Interpretations of topographic fea- tures can be based on: (1) aerial photos used to spot 42 unstable areas, (2) maps of geology and seismic refraction surfaces, (3) vertical color-infrared photos, and (4) iso- line map of slope angle. For example, debris slide inci- dence was predicted using geomorphic criteria and aerial photos in the Santa Ynez-San Rafael Mountains of Californi

(Kojan, et al,, 1972).

Hazard Rating System

Stratification of terrain represents one promising approach to reduce mass movements associated with forest operations and minimize environmental impacts. Delineatior of strata is based on some or all of the following criteria geology, landform, soils, precipitation, slope, and vege- tation (Rice, et al., 1972). McRorey, et al, (1954) pre- sented a rating of erosion (both surface and mass erosion) hazard of logging areas in relation to site factors (par- ent material, soil, slope, precipitation, and vegetation or other organic matter) for forested lands. From the

McRorey et al (1959) rating system, another erosion (both surface and mass erosion) hazard table with mantle stabilit as an added site factor was developed by U.S. Department of Interior (1970) (Table 3). A preliminary scheme on slope stability classification was developed in Taiwan

(Sheng, 1966). Five factors (site, slope, scars, soil tex- ture, land use or cover) were considered and rock, aspect, Table 3: Relative erosion hazard of logging areas in r lation to site factors -- USD1 (after U,S. De- Department of Interior, 1970).

High 11oderate Low Eros on Erosion Erosion Site Factors Hazard Hazard Hazard Sedimentary and Parent rock Acid Igneous Metamorphic Basic Igneo

Granite,diorite, Sandstone, schist (Lava rocks , shale; slate, Basalt, pumice, some conglomerates, andesite, schists chert serpentine

Sofi Light textured, Medium textured Heavy textu with little or with considerable largely cla. no clay clay adobe

Mantle Unstable mantles Mantles of ques- Stable rnant Stability (cutbank stabili- tionable stabili- (classes I, ity Class V) ity (Cutbank sta- and III) bility Class IV)

Slope Steep Moderate Gentle (Over 50%) (20- 50%) (0 - 20%)

Precipitation Heavy winter Mainly snow Heavy snow rains or intense with some rain light rain summer storms

Vegetation None to very Moderate Large and other little amounts amounts organic matter on and in the soil

Soil texture refers to the size and distribution of themi particles n the soil ,the range extending from sand (ligh texture) to clay (heavy texture). 44 and groundwater excluded (Table 4). Bailey (1971) strat- ified the lands of the Teton National Forest in northwest

Wyoming into: (1) highly unstable and unsuitable for logging, road construction or other soil disturbance,

(2) questionable stability, and (3) stable under present conditions. Swanston (1969) recommended slide-prone areas be identified on aerial photos and from a slope-gradient map which can be used to (1) delineate potential slide areas, (2) determine percentage of slide-prone ground in the total area, and (3) establish cutting patterns which will best utilize these areas with minimum disturbance.

Swanston (1970) also pointed out construction of an iso- sine map (mapping of slope angles) is a practical method to locate potential debris areas for watersheds associated with the till soils of southeast Alaska. If a critical area lies within timber stands within planned har vest units, careful consideration should be given to har- vesting techniques and road construction. In judging land slide potential in glaciated valleys of southeastern Alask

Swanston (1973) discussed the land stratification techniqu

"Land stratification and qualitative stability rating using slope as a basic parameter is an effective approach to identify adequately pre- sently or potentially unstabloterrain and determine practical, usable methods for occu- pancy and use. . .For the purpose of adequate land-use planning, stability stratification of an area must first of all identify as closely as possible all areas of potential landslides and major variations in slope 45

Table 4. A preliminary scheme on slope stability classification (after Sheng, 1966).

A B Factor C D (4 points) (3 points) (2 points) (1 point)

Site Sites does High Along Along stream not belong ridge road or reservoir to B,C, orD

Slope <200 >46° 20°- 26°-45°

Presence None Few Some Many of slid- ing scars

Soils Medium Coarse MediumFine texture Texture Texture stony or coarse soil stony soil

Land Use Dense Sparse Culti- Use with or Cover cover cover vation severe soil without disturbance conser- vation

Area ha. Total Points

Stability Class: Stable (20-18 points) Relatively Stable (12-17 points) Unstable (10-11 points) Highly Unstable (5-9 points) 46

stability characteristics from one location to the next, a practical hazard rating sys- tern defining the principal zones of strat- ification can be developed...Using the slope values... .stratify landforms into three broad zones of low, medium, and high hazard. Such basic stratification can then be followed by a careful analysis of the factors contribu- ting to the unstable conditions and substrat- ification of the medium and low hazardareas by activities and operations thatcan be safetly performed within them",

In the study of geology and of the H.J.

Andrews Experimental Forest, Swanson and James (1975)sug- gested potentially unstable areas may be recognized by the

(1) presence of altered volcanicalstic rocks, (2) proximity to contact zones where lava flows or other relatively com- petent rocks overlie volcaniclastic rocks, and (3) evi- dence of past history of instability in thearea. Unstable areas should be identified from aerial photos, field in- spection of topography and vegetation, and topographic maps.

With the predictive techniques presently available the prediction of landslides is still difficult, if not impossible. Each mass movement event represents a unique case showing a particular combination of interacting factorE which culminates in mass wasting. The great natural vari- ability and lack of appropriate basic data on forested mountain terrain make the rating of erosion hazard diffi- cult. 47

Prevention and Control

Root (1958) suggested prevention of all types of landslides may be accomplished by one or more of the three methods: (1) reduction of activating forces, (2) increas- ing the forces resisting movement, and (3) avoiding or detouring the slide. Prevention and control methods for minimizing mass movements on steep lands have also been proposed by Swan- ston and Dyrness (1973) as follows: Avoidance of all operations in highly unstable areas

identified or areas of questionable economic valued

Swanson and James (1975) pointed out bedrock geology

showing slide proneness should be considered in for-

est management decision-makings in western Cascades.

Potentially hazardous areas can be identified in ad-

vance of the timber harvest (Rice, etal., 1972). Areas above the maximum angle ot internal friction or

showing evidence of active mass movement should be

entirely withdrawn from the proposed timber sale

(Swanston, 1971b). Controlling operational effects in high-value areas

of questionable soil stability or where other consid-

erations prevail over maintaining stability.

Reduction in forest road construction in unstable

areas. The use of skyline and balloon logging methoth 48

are a few of the obvious means for reducing road

mileage to an absolute minimum (Dyrness, 1967a).

Reduction of slope disturbance by alternative timber

harvesting methods include balloon, helicopter and

skyline systems. The choice of a logging system has a significant impact on the amount and type oferosion which may accompany a timber harvest. Skyline, bal-

loon, or helicopter systems are especially effective

4 for reducing logging impacts (Rice, et al., 1972)

Dyrness (1972) found balloon logging causes substan- tially less soil disturbance than tractor, high-lead, and skyline logging methodse In the evaluation of the effects of logging systems on erosion in the

Idaho Batholith, Megahan and Kidd (1972) concluded jammer logging systems in steep mountain lands are unacceptable in many areas and even skyline logging should be rejected in areas of extreme erosion hazard.

Improved road design and construction. The criteria for reducing mass erosion in the Idaho

Batholith by careful road location, design, con- struction, and maintenance measures were recommended by Gonsior and Gardner (1971). Based on literature review and field experience, Rothwell (1971) presented an excellent report concerning the guidelines for log- ging practices, road construction, and road maintenanc 49

to minimize erosion problems. Burroughs, et al., (1973) also prepared an excellent guide toreduce

slope failures along roads in westernOregon

Dyrness (1967a), in the massmovement study on the

H.J. Andrews Experimental Forest,pointed out that modification of waste handling to avoidside casting

on steep slopes andprovision for adequate road drainage are important means tominimize mass move-

'i... ment hazard, However, it should be noted that a decision to build aroad in an area of unstable topography constitutes a calculatedrisk no matter

how well the road is designed andconstructed to

minimize damage" (Fredriksen, 1963).

6, Planting vegetation to stabilizedisturbed areas. Swanston (1971b) indicated stabilityof road cuts and

fills has been substantiallyincreased by planting of

grass and legume (Bethlahmyand Kidd, 1966; Wollum,

1962; Dyrness, 1967b, 1970). Deep-rooted vegetation,

including trees and shrubs, shouldincrease the mass

stability of fill slopes. In the Idaho Batholith, ponderosa pine was planted on sidecast slopes to

control erosion (Megahan, 1974). In Taiwan, hydro-

seeding for stabilizing cut slopes(Sheng, 1966; Lee,

1973) and staking cuttings of treespecies and bamboo

on fill slopes (Kraebel,1936; Yen, 1970; Lee, 1970; 50

Chang, et al. ,1971; Hu, 1972) were recommended to

apply along newly-excavated roads, A report of the

slope stabilization works in Japan was briefly pre-

sented with emphasis on vegetal methods by Lin (1971)

Other control measures such as artificial drainage, removal of overburden, improvement of shear strength char- acteristics of the soil by maintenance of living root sys- tems, and check das are suggested by Swanston (1969) and

Sheng (1966).

In summary, prescriptions to cope with the mass move- ments on steep terrain should meet three principles: (1) acquiring necessary materials locally, if possible, (2) time-saving and economical, (3) simply and easy. The sol- ution chosen for a particular problem depends on exper- ience and judgment. Sometimes environmental impacts, mainly esthetic value, need to be considered. 51

RESEARCh NEEDS

Mass wasting caused byland use activities on forested

mountain topography can beminimized through increased

knowledge of mass movement processes. To accomplish this

objective, additional researchis needed in the following

subject areas: (1) hydrologic characteristicsof soil masE

on the slope,(2) variables affecting slopestabili'ty, (3) influence of forestmanagement practices on slope move- ment, (4) innovationin road construction andtimber har- vesting methods, and (5)improvement and development of

control measures. Therefore, the following six typesof studies on slope stabilityproblems are recommended;

Basic data compilation

Basic data on topography,precipitation, geology, vege- tation, underground waterand properties Of soilsand rocks on regionalbasis need to be compiled. This information is essential to facilitateapplication of research results of a given in land use planningand to minimize the impact management activity on other usesor values. It would als provide researchers withimportant informationrelative to their research programs.

Physical processes involvedin slope failures

1. The role of subsurface water inslope stability. Little is knownabout thoinfluence ui ubsurt ace 52

water on slope stability on forest lands character-

ized by heavy rainfall, steep slopes, weak geologic

structure and slide-prone soil properties. The im-

pact of hydrostatic pressure on slope stability and

quantitative effects of water on the mechanics of

slope failure are of fundamental importance to the

full understanding of the nature of slope instability.

2. Processes of slope development on steep mountain ter-

rain. More intensive studies should focus on the physical

and chemical weathering of rocks and on changes in

soil characteristics on regional basis. The estimate

of erosion rate of forested slopes and the impacts of

increased erosion on the channel system in a given

drainage should also be attempted.

C. Effects of forest cover on slope stability

1, Clarification df the relationships relative to the

vegetation-stability problem, The net effects of forest cover on slope stability are

not always clear More efforts are needed to. adequate- ly evaluate the advantages and disadvantages offorest

cover under various site conditions.

2. Effectiveness of slope stabilizing function of root system of major plant species under differentsite

conditions in a given watershed. 53

The rooting characteristics including depth and extent of distribution, root size and strength, and decaying after logging should be studied, The effectiveness of root anchoring should be examined under different stands with various amounts of vegetative cover. The relative importance of various species and age classes in regards to root strength properties needs to be identified.

Hydrologic modification of soil water regime through interception and transpiration.

Answers should be found as to what extent the forest stands with different species and ages and on various slope-aspect combinations can modify soil moisture con ditions and its significance to mass wasting processes

Potential critical period of increased landslides afte: logging.

If peak landslide activity typically occurrs a certain number of years after logging or road building, this can provide a reference point for carrying out other land use activities thereafter.

Effect of windthrow on slope disturbance.

Windthrow may be important triggering mechanism in steep areas frequently subject to strong winds. Its relative role in mass wasting activities needs to be identified. 54

D. Land use activity and mass wasting

Evaluating the effects of timber harvest and slash

burning on accelerated mass erosion.

Additional quantitative information relating tonatura

and accelerated rates of mass erosion is needed, How

does forest harvesting affect these rates?This in- formation is basic to developing management guidelies

or making sound land use decisions.

Development of improved methods for road building,

timber harvesting, and other forest managementprac- tices to prevent and reduce mass movements.

Prevailing logging methods and road design andcon- struction have been evaluated in terms of their environ

mental impacts. But how can they be improved to appre-

ciably reduce disturbances conducive tomass movements' And then, what are the effectiveness and feasibility of those improved methods?

E. Slope stability analysis

If the contribution of each factor active on the soil mass of a slope can be accurately quantified, then mor

reliable methods of slope stability analysis can be ob-

tamed. With better understanding of the physical pro- cesses, more reasonable assumptions and simplifications

can be made and improved methods of slope stability

analysis should result. These methods should adequatel 55

appraise the stability of natural slopes on forest

lands and provide important insight relating to pro-.

cesses which are altered by land useactivities. The

results of slope stability analysis will provide rigi

a basis for setting uphazard rating systems and

guidelines to reduce slope failures.

F. Prediction and control methods

1. Developing methods of identifying forest areas sus-

ceptible to landslides. It is essential to broaden and refine the criteria fo

the recognition of potentially. unstable areas to set

up hazard rating system throughland stratification

techniques and air-photo studies.

2, Assessment of the probability of the occurrenceof slides of a given range of size as a result of cer-

tain extent of man's disturbances. With this information, damages can be estimatedand

preventive methods can be tentatively proposed and

justified.

3. Useful plant species and techniques forvegetative rehabilitation on landslide areas.

Techniques for applying vegetative treatments onprob-

lem areas should be improved and useful plantspecies

identified. Vegetative methods of landslide rehabili- tation offer the advantages of relatively low cost an( 56

usually not requiring heavy equipment,

In summary, the state of our knowledge about mass wasting processes on forested mountain topography is still at a preliminary stage. Research programs and efforts need to be strengthened and expanded. The above-mentioned areas of study may be subject to priority or facility con- siderations, but all have a bearing on understanding the processes of mass wasting on mountainous forest lands.

With improved knowledge, the evaluation and alleviation of environmental impacts resulting from forest land use acti- vities can be achieved, 57

BIBLIOGRAPHY

Bailey, R.G. 1971, Landslide hazards related to land use planning in Teton National Forest, Northwest Wyoming. U.S. Forest Serv. Intermountain Region, 131 p.

Baker, R.F. and E.J. Yoder. 1958. "Stability analyses anc design of control methods". In Landslides and Engin- eering Practices. E.B. Eckel, ed. National Research Council Pub. 544. Highway Res. Board Spec, Report 29, pp. 189-216.

Bethlahmy, N. and W.J. Kidd. 1966. Controlling soil move- ment from steep road fills. U.S. Forest Serv, Res. Note INT-45, 4 p.

and C.C. Koh. 1968. Road and landslides. Jour. of Chinese Forestry 1(3): 161-169. Taipei, Taiwan.

Bishop, D. and M.E. Stevens. 1964. Landslides on logged areas in southeast Alaska. U,S Forest Serv. Res, Pap. NOR-i, 18 p.

Brown, G.W. 1973. The impact of timber harvest on soil and water resources. Extension Bull. 827. Oregon State University Extension Service. 16 p.

Burroughs, E.R., G.R. Chalfant, and M.A. Townsend. 1973, Guide to Reduce Road Failures in Western Oregon. Bureau of Land Management Portland, Oregon. 111 p.

Chang, T.T., C.P. Yen, and C.Z, Lee. 1971. Sprouting ex- periment on vegetative stakes for slope stabilization in Taiwan. Jour. of Chinese Soil & Water Cons. 2(1): 124-137. Taipei, Taiwan.

Croft, A.R. and J.A. Adams. 1950. Landslides and sedi- mentation in the North Fork of Ogden River, May 1949. U.S. Forest Serv. Intermountain Forest and Range Exp. Sta. Res. Pap. 21, 4p.

Dyrness, C.T. 1967a. Mass soil movements in the H.J. Andrews Experimental Forest. U.S. Forest Serv. Res. Pap. PWN-42, 12 p.

1967b. Grass-legume mixtures for roadside soil stabilization. U.S. Forest Serv. Res. Note PNW-71, 19 p. 58

1970. Stabilization of newly constructed road backslopes by mulch and grass-legume treatments, U.S. Forest Serv. Res. Note PNW-123, 5 p.

1972. Soil surface conditions following balloon logging. U.S. Forest Serv, Res, Note PWN-182, 7 p. Edgerton, P.J., BR. McConnell, and J.G. Smith. 1975. Initial response of bitterbrush to disturbance by logging and slash disposal in a lodgepole pine for- est. Jour, of Range Management 28(2): 112-114.

Fredriksen, R.L. 1963. A case history of a mud and rock slide on an experimental watershed. U.S. Forest Serv. Res. Note PNW-1, 4 p.

1970. Erosion and sedimentation following road construction and timber harvest on unstable soils in three small western Oregon watersheds. U.S. Forest Serv. Res. Pap. PNW-104, 15 p.

Fujiwara, K. 1970. A study on the landslides by aerial photographs. Hokkaido Univ., College Exp. For. Res. Bull. 27:297-346.

Gonsior, M.J. and R.B. Gardner. 1971, Investigation of slope failures in the Idaho Batholith. U.S. Forest Serv. Res. Pap. INT-97, 34 p.

Harr, R.D. and C.S, Yee. 1975. Soil and hydrologic f act- ors affecting the stability of natural slopes in the Oregon Coast Range. Water Resources Research Insti- tute, Oregon State University. WRRI-33, 204 p.

Hu, S.C. 1972. Staking method for Subcostate Crape Myrtle. Jour, of Soil & Water Cons. 5: 44-47. National Chung Hsing University, Taichung, Taiwan.

Hurtubise, J.E. and P.A. Rochette. 1957. The Nicolet slide. Tech. Memo. No. 48, Associate Committee on Soil & Snow Mechanics. National Research Council, Canada. 11 p.

Kojan, E., G.T. Foggin III, and RIM. Rice. 1972. Pre- diction and analysis of debris slides incidence by photogranimetry, Santa Ynez-San Rafael Mountains, California. 24th IGC, 1972, Section 13, pp. 124-131.

Kraebel, C.J. 1936. on mountain roads. USDA Circular No. 380, pp. 15, 21-23. 5

Kramrnes, J.S, 1960, Erosion from mountain side slopes after fire in southern California. Pacific Southwes Forest and Range Exp. StaU.S. ForestServ, Res. Note 171. 8 p.

Kuo, P.C. and W.T. Chien. 1970. Air-photo study of land slides in Lo-tOng watersheds Experimental Forest of National Taiwan University Tech. Bull.78, 38 p. Taipei, Taiwan.

Ladd, G.E. 1935. Landslides, subsidences and rock-falls as problems for therailroad engineers. Bull. Am. R. Eng. Assoc. 37(377), 72 p.

Lee, C.Z. 1970. Experiment on the sprouting of bamboo stakes for slope stabilization inTaiwan, Jour. of Chinese Soil & Water Cons. 1(1):60-66 Taipei, Taiwan.

1973. Hydroseeding for road cut slope stabili zation with grass seed and asphaltemulsion. Jour. of Chinese Soil & Water Cons 4(2): 14-32. Taipei, Taiwan.

Leighton, F.B. 1969. "Landslidest'. Geologic Hazards an Public Problems Conference Proc, R.A.Olson and M.M Wallace, ed. Office Santa Rosa,California. pp. 97- 132.

Lin, Y.L. 1971. Studies on the watershed managementand slope stabilization in Japan. (In Chinese) Jour. of Chinese Soil & Water Cons. 2(2):18-29. Taipei, Taiwan.

McRorey, R.P., N.F. Meadowcroft,and C.J. Kraebel. 1954. A Guide to Erosion Reduction onNational Forest Timb Sale Areas. U.S. Forest Serv. California Region. 78 p Megahan, W.F. 1974. Deep-rooted plants for erosion con- trol on granitic road fills in theIdaho Batholith. U.S. Forest Serv. Res. Pap. INT-161,18 p.

and W.J. Kidd. 1972. Effects of logging and logging roads on erosion and sedimentdeposition frc steep terrain. Jour. o Forestry 70(3): 136-141.

Mersereau, R.C. and C.T. Dyrness. 1972. Accelerated mas wasting after logging and slash burning. Jour, of Soil & Water Cons. 27(3): 112-114, 60

Patric, J,H, and D,NSwanston. 1968 of a slide-prone glacial till soil in southeast Alaska. Jour, of Forestry 66(1): 62-66,

Popov, I,V. 1963. "The pattern of origin and development of landsliding processestt. In the Stability of SlopeE I.V. Popov and F.V. Kotlov, ed, Transactions of the F,P. Savarenskii Hydrology Lab. Consultants Bureau, New York. pp. 1-4,

Rice, R.M., E.S. Corbett and RG, Bailey. 1969. Soil slips related to vegetation, topography and soil in southern California. Water Res. Research 5(3): 647- 659.

and G.T. Foggin. 1971. Effect of high inten- sity storms on soil slippage on mountainous watershed in southern California. Water Res. Research 7(6): 1485-1496.

and J.S. Kramrnes. 1971. I!Mass_wasting pro- cesses in watershed management". In Proc. of the Symp. on Interdisciplinary Aspects of Watershed Man- agement. Montana State University. Amer. Soc. Civ. Eng. pp. 231-259.

J.S, Rothacher, and W.F. Megahan. 1972. Ero- sional consequences of timber harvesting: an apprai- sal. In Proc. Natl, Symp. Watersheds in Transition. Amer. Water Resources Asso. pp. 321-329.

Ritchie, A.M. 1958. "Recognition and identification of landslides". In Landslides and Engineering Practice. E.B. Eckel, ed. National Research Council Pub. 544. Highway Res. Board Spec. Report 29, pp. 48-68.

Root, A.W. 1958. 'Prevention of landslides". In Land- slides and Engineering Practice. E.B. Eckel, ed. National Research Council Pub. 544. Highway Res. Board Spec. Report 29, pp. 113-149.

Rothacher, J.S. and T.B. Glazebrook. 1968, damage in the National Forests of Region 6. U.S. Forest Ser Pacific Northwest Forest and Range Exp. Sta. 20 p.

Rothwell, R.L. 1971. Watershed management guidelines for logging and road construction, Forest Res. Lab, Edmonton. Alberta. Information Rep. A-X-42. Dept. of Fisheries and Forestry, Canadian Forestry Serv. 78 p. 61

Sharpe, C.F.S, 1938. Landslides and Related Phenomena. Columbia University Press, New York. 137 p.

Sheng, T.C. 1960. Soil conservation and August 7, 1959 flood. (In Chinese) In Special Issue on August7, 1959 Flood, Bank of Taiwan Quarterly 11(2): 86-111.

1966. Landslide classification and studies of Taiwan. Chinese-American Joint Commission on Rural Reconstruction. Forestry Series No. 10, 97 p. Sowers, G.B. and G.F. Sowers. 1951. Introductory Soil Mechanics and Foundations. New York: Macmillan Co. 284 p. Spangler, M.G. and R.L. Handy. 1973. Soil Engineering 3rd ed. New York: Intext EducationalPublishers. 748 p. Swanson, F.J. and M.E. James. 1975. Geology and geomor- phology of the H.J. Andrews ExperimentalForest, west- ern Cascades, Oregon. U.S. Forest Serv. Res. Pap. PNW-188, 14 p.

Swanston, D.N. 1967. Debris avalanching in the thin soils derived from bedrock. U.S. Forest Serv Res. Note PNW-64, 7 p.

1969. Mass wasting in coastal Alaska. U.S, Forest Serv. Res. Pap. PNW-83, 15 p.

1970; Mechanics of debris avalanching in shal- low till soils of southeast Alaska. U.S. Forest Serv. Res. Pap. PNW-103, 17 p.

1971a. "Principal mass movement processes in- fluenced by logging, road building and fire". In Proc. Sym. Forest Land Uses andStream Environment. Oct. 19-21, 1970. Oregon State University. pp. 29- 39.

1971b. "Judging impact and damage of timber harvesting to frest soils in mountainousregions of western north America". In Western Reforestation. West Ref or. Coord. Comm. Proc. 1971, pp.14-19.

1973, Judging landslide potential in glaciated valleys of southeastern Alaska. Explorers Jour. 51(4) 214-217. 62

1974a. 5, Soil mass movement1 The Forest Eco- system of Southeast Alaska. U.S. Forest Serv. Genera Tech. Report PNW-17, 22 p.

1974b, Slope stability problems associated wit timber harvesting in mountainous regions of the west- ern United States. U.S. Forest Serv. Gen. Tech. Rep. PNW-21, 14 p.

and C.T. Dyrness. 1973. Stability of steep land. Jour, of Forestry 71(5): 264-269.

Terzaghi, K. 1950. "Mechanism of Landslides". In App. of Geology to Eng. Practice. The Geological Society of America. Berkey vol. pp. 83-123.

U.S. Department of the Interior. 1970. Industrial waste guide on logging practices. Federal Water Pollution Control Administration. 80 p.

Varnes, D.J. 1958. "Landslide types and processes't. In Landslides and Engineering Practice, E.B. Eckel, ed. National Research Council Pub. 544, Highway Res. Board Spec. Report 29, pp. 20-47.

Wollum, A.G. II. 1962. Grass seeding as a control for roadbank erosion. U.S. Forest Serv. Pacific North- west Res, Note 218, 5 p.

Yen, C,P. 1970. Sprouting experiment on applicable vege- tative stakes for slope stabilization at medium alti- tudes of Taiwan. Jour. of Chinese Soil & Water Cons. 1(1): 47-59. Taipai, Taiwan.