landslides, Surface Erosion, and Forest Operations in the Oregon Coast Range

Arne E. Skaugset, Gordon H. Reeaes, nnd Richard F. Ketm

I ntroduction Accelerated erosion from forest management The last section of the chapter presents the activities, particularly timber harvest, has long been practices currently used to mitigate the effects of a focus of environmental concern. Associated with forest management activities. The regulation of the accelerated loss of soil is an accelerated loss of forestpractices to mitigate erosion has evolved over soil nutrients, which ultimately may reduce the the last several decades from the application long-term productivity of of intensively managed water-quality standards to the use of best manage_ forest soils. As erosion processes in foresied mentpractices (BMPs). Currently, these practices are landscapes of western Oregon became better rn a bansition from site-specific BMps to watershed_ understood, especially the patchiness of erosion by or landscape-scale practices that fit into a construct shallow, translational landslides, the focus changei of ecosystem management. This section presents to effects on downstream values. With the tisting of site-specific BMPs that have been developed to salmonids as threatened or endangered species, reduce accelerated erosion, with emphasis on those concern has become more sharply focused on practices usecl to locate high-risk, landslide-prone aquatic habitat in headwater streams draining terrain and to mitigate against the occurrence of managed forest landscapes. shallow, translational landslides. The section After a briefsection providing the physical setting _ concludes with a discussion of BMps that could be of the Oregon Coast Range, we discuss the erosion added to the current prescriptions, and how these processes found in forested landscapes of western should be used on the landscape to ensure that Oregon, with an emphasis on the factors and watersheds function optimally for aquatic habitat environmental conditions that initiate shallow, while allowing for forest management and the translational landslides. This section concludes with production of solid wood proclucts. a synopsis of what is known about the relationship between these landslides and aquatic habitat. The next section discusses how contemporary Physical Setting of the and legacy forestpractices in westem Oregon forests Oregon Coast Range have affected accelerated erosion. The emphasis is Geology on the observed relationship between forest management practices and the increased occurrence The Oregon Coast Range is composed of rocks of shallow, translational landslides. Tlris section uplifted by the collision of two plates. The ocean concludes with a discussion of how forest manage- floor, which is the Juar.r de Fuca plate, is subducting ment activities might affect the occurrence, size, and under the terrestrial NorthAmerican plate, causing composition of debris-flow deposits. the Coast Range to rise by as much as 5 millimeteri per year (Kelsey et al. 7994, 7996). The presence of

213 211 Forest and Stream Managenrer'rt ir'r tl're Oregor.r Coast Rang,e

small fatrlts and folds throughout the overriding color section following page 212). Extended plate results in spatially variable uplift; the late of droughts in the summer can reduce streamflow to uplift varies by as much as an order of magnitude extremelv low levels. (Kelsey ancl Bockheim 1994). At the same time, Precipitation falls predominantly as rain in the erosion is lol'ering the range by an avelage of 0.03 contributing areas for most streams, although some to 0.11 nillimetels per year (Rencau et al. 1989; mountain peaks in the Coast Range can receive Reneau and Dietrich 1991). frequent sno',r,fall during the winter. Most rain falls The rocks of the Coast Range are predominantly cluring frontal storms with intensitics of 0.5 to 1.0 layered ancl interbcddcd sandstones and muclstones centimeters per hour (Figure c)-4 in color section formed from ocean scdimcnts bcfore uplift, ancl follou'ing page 212). It is the extended periods of there are rnany intlr-rsions of basalt into this matrix rainfall that result in peak stlcamflon's during the (Kelsey ct al. i996). Some of these basalts r'r'ere $'ettest months; hor,r'ever, peak flows are largest already in placc rvhen the seclimentarv platform when an intcnsc rainfall occnrs during such periods formed on thc former ocean floor and the two rock of extended rainfall orn4ren rain fa Lls on sno\\,. typcs uplified togetl-rer (Orr et al. 1992). Most of the basalt in thc northem Coast R;rng;e ancl some of the Landslides and Surface Erosion in coastal headlands, hon'ever', originatecl from Unmanaged Coast Range Forests terrestrial volcanoes east of the Cascade Range (C)rr et al. 1c,lcl2). Because basalt is less susceptiblc to Erosion processes can be divided into thrcc general erosion than sedimentarv rock, many of these areas groups: slrrface erosion, channel scour, and mass of intrusive basalt are nou' thc highcst elevations in movement (Brown 1980). Surfnce erosion is defined the Coast Range. This differential susccptibility to as the movement of individr.ral soil particles. erosion also means that sedimentary areas are morc Sr'r'anson and others (1982) list the surfacc-erosion highly dissected than the basalt areas. The pro- processes that occur in unmanaged, forested portions of weaker, intcrbedded rnuclstones, as r,r,'elJ watersheds as dry ravcl, snow- and ice-indr.rced as variations in strike and dip, affect tlre stabilitr,r rnoVement, and soil mor,'ement acconrpanving root and erosivitv of the bedrock. throw. Scdiment btrdgets show that surface erosion Much of thc eastern bor.rndary of the Coast Range is negligiblc in unmanaged, forested watelsheds of is the Willamcttc Vallev n'lriclr is a fault-bJock valley western Oregon (Dietrich and Dunne 1978; Slvanson of alluvial and llrcustrine sediments. The volcanic ei al. 1982). Cascade Rarrge borders the Coast Range to the Channel scour occurs within the stream channel, southeast, ancl tl.re volcanic ancl metamolphic rocks as soil and rock material from the streambed and of the Siskiyou Mountains form the southern stream bat-tks becomes suspendcd scdiment and bed bouncl;rry (Figure 9-1 in color section folkrwing page load, which are available to the stream to transport 212). The Coast Range is bounded on the wcst by (Brown 1980; Swanson et al. 1982). In gcntly sloping the Pacific Ocean. watersheds in the absence of debris flows, char.rnel scour is the dominant proccss responsible for eroding stream banks and transporting streambed Hy drology and bank material or.rt of a watershed and further The Coast Range has a Medjterranean clirnate, in downstream. n'hich most of the precipitation faJls as rain between Mnss soll tltore rctrt is the tyPe of erosion that October and April (Figure 9 2 in color section dominates secliment budgets for unmanaged, following page 212). During these months, pre- humid, temperate, forested watersheds (Swanston cipitation exceeds er.apotranspiration, causing 1978). Mass soil-movement processes that occur in rttnoff as streamflow. Flon May through September, lrruranaged, forested watersheds of the Coast Range \4,armer temperatures and long clays prornote include: soil creep, earthflows, deep-seated land- \rigorous Plant growth, and eVapotranspiration slides, ernd shallow-translational landslides exceecls lainfall. This seasonal shift in weather (Dietrich and Dunle 1978; Swanston 1978; Swanson results in a signature annual hydrograph for rir.ers et al. 1982). of the Coast Range, in r'r'hich nearlv all streamflon Soil creep (Figure 9-5a) is a slon (millimeters per occurs in the fall through the spring (Figr-rre 9-3 in year) downslopc movement of the entire soil profile - Landslicles, Surface Erosion, and Forest Operations in the Oregon Coast Range 215

(,r) in response to gravity. It is very small soil r.nor.e- ments over a very large scale, and can involve entire forested slopes or a small r,r'atershed. Hor,r'ever, tlre soil deformations are barely notlceable and difficult to Ineasure; tor the most part, thev do not represent discrete failures (Sr,r'anston 1928). For gcntle slopes (< 50%), soil creep is tlre domjnant erosion process responsible for transporting soil fron hillslopes to streambanks in nnmanaged, forested rt'atershcds. In contrast, carf//7orls (Figure 9-5b) are large u.ith Local slump - eadhflow distinguishable margins and a discrcte failure surface. Although their. speed may be faster than that of soil creep, it is still slotr, (cen timcters to rneters per- year). Tlre soil florvs or g;lides or.er the underlying bedrock as a ser.ies of distjnct blocks. The movement is sufficient to produce discrete failurcs (Swanston 1 978). Datp scntcd lnnclslidt:s can be eit]rer rotational shunps (Figure 9-5c) or bktck-glidc-tvpe faih-rres. These landslicles are often large but locally discrete lailures in which a block of soil ancl regolitl.r either moves by rotation over a broadly concave failure surface or slides along beddinp; planes betr,r,,ecn layers of beddecl deposits. Deep-seated landslides k) occur over periods lasting from nrinutcs or hours to several days. They are often characterized by a steep scarp at the head of the failure and earthflon, Clay fcatures;rt thc toe. Slttllou, trnnslttiotrLtI lottdslitles rn the Co;rst Range Slump consist primarily of debris slides and flovr,s Clay clebris (Figure 9-6). Tl.re primary difference between a CIay debris slide and a debris florv is the moisture content of the slidc rnass. A di,lrls s/ldc is on the drv side of Earthflow the continuum and consists of soil ancl rock that is clisplaced outward and dou,nr,r,ard along a failure surface, sliding out or.er the original ground sur{ace Figlure 9 5. Thrce cJii%rent types of nrass soil (Cruden ancl Varnes 1996). Debris slides deposit at movement processes tl.rat ()ccur in the Oregon Co.rst the foot of the failure scarp and do r.rot mobilize and Range inclucling (a) (b) soij creelr, earlhflows, and (c) move {arther downslope. Additional \ .,ater cor.rterlt rolational landslides (Brown 1 980). creates a slurrv bf soil, rock, r,r,'ater, and olganic material called a tltbris floio. Debris flows follor,r, a stream channel until they set up in deblis-flow deposits (Cmden and Varnes 1996). (Although the terms iTcil'is atnlanclu,and r/cllis /orl,c/?l ar.e often usecl interclrangcably with the morc general term "debris fkrw" [Cruclen and Vames 1996], ,,debris avalanche" is a dated term ;rnd is currentlv not nsed In lhisc,)rle\t.) In thi'chaptcr "drbrisslide re[cr, to the larldslide-ini tiation sites.rnd "debris flow,, to teatures that move down stream channels ancl ultimately deposit lower in tlle system. 216 Forest and Stream Management in the Oregon Coast Range

size and ubiquitous distribution across the land- scape, debris flows are most prone to being influenced by forest management activities, particularly forest roads and timber harvest.

Geomorphic contefi for debris fTouts Debris flows occur on only a small proportion of the landscape (Ice 1985). Nearly all initiate on slopes steeper than 50 percent, and most occur on slopes between 80 and 99 percent (Ketchcson and Froehlich 1978). Debris flows are hypotl.resized to initiate in unchanneled bedrock hollows (Dietrich et al. 1987; Reneau and Dietrich 1987; Montgomery and Dietricl-r 1994), n'hich are almost always thouliht of as being synonymous with topographic con- vergence or topographic swales (Montgomery and Dietrich 1994). In fact, unchanneled bedrock hollou's may exist on relatively planar slopes that exhibit mete rs 05 little or very subtle topographic expression as well as slopes with distinctive topographic sr,r'ales. Figure 9 6. A dcbris flow with associatccl downslopc Landslide inventories have reported that as many scour (Humphrcy 1981). as half of the inventoried landslides initiate on either planar slopes or slopes with very little topographic Shallow, translational landslides are initially expression (Ketchcson and Froehlich 1978; May small, locally discrete failures that can move very 1998; Robison et al. 1999). rapidly. They occur on steep slopes (> 50%) and Bedrock hollows are characterized by bedrock involve thc movenrcnt of shallow layers of soil surfaces that are concave both across-slope and dor'r'nslope. Within seconds or minutcs, they can downslope (Sidle et al. 1985) and parabolic in cross move tens to hundreds of meters. Thev flon' down section (Burroughs 1984), witlr the long axes stream channels and deposit in lower-graclient, extending downslope (Swanson et al. 1982). This higher-order stream channels. shape causes subsurface groundwater to converge Debris flows are the mass movement processes toward the hollow (O'Loughlin 1986; Dietrich et al. of most interest and concern relative to both public 1982; Reneau and Dietrich 1987; Montgomery and safety and impacts on aquatic resources (Pyles et Dietrich 1994), as does the accumulation of al. 1998). Landslide inventories show them to be colluvium (Sidle ei al. 1985). The bedlock de- ubiquitous across the landscape in the Coast Range pressions are themselves created by repeated (Robison et al. 1999; see Table 9-1). Although landsliding (Alger and Ellen 1987), and tl.rey earthflows and rotational slurnps may dominate undergo cycles of failtrre followed by filling wiih sedimentbudgets within a givcn watershcd or along colluvium (Sidle et al. 1985; Benda and Dunne a particular stream reach (Swanson and Swanston 1997a). The material that refills the hollows comes lq77r, dcbris flow' domin.rlc qspsipll p19qsss6's from hillside erosion that is primarily due to soil across the Coast Range. As the dominant erosion creep (Swanston and Sn'anson 1926), to the biogenic proccss in western Oregon, they or,.erwhelm the activities of animals and plants (Swanson et al. 1982), sediment budget of unmanaged watersheds and to surface erosion (Swanson et al. 1987). The (Dietrich and Dunne 1928; Swanson et al. 1982). susceptibility of the holkrws to landsliding during Debris flor,r's are critical to public safety because of rainstorms increases over time because of this their ubiquity, size, speed, and extent (Pyles et al. colluvial in-filling (Dietrich et a1. 1995; Benda and 1998), ancl they dominate concerns about aquatic Dunne 1992a; Dunne 1998). habitat quality (Sullivan et al. 1987; Swanson et al. Bedrock hollows filled with collur.ium arc found 1987; Reeves et al. 1995; see Chapter 4). Given their all over the world (Hack and Coodlett 1960; Marron Landslides, Surface Erosion, and Forest Operations in the Oregon Coast Range 217

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1985; Tstrkamoto and Minematsu 1987; Crozier et methods can involve onJy a simple balance of folces al. 1990). Their density rraries from 0.1 to 2.0 hollor'r's or moment equilibrium, or botb could be required. per hectale dependir-Lg upon geology, topography, The slide mass involved can be analyzed as a onc-, and climate (Benda et al. undated). Colluvial two-, or three-dimensional body. Finally, the failule hollows are predictably the locations of landslides, surface can be modeled as;r simple circular surface especially where shallor,r' soils or.erlie bcdrock or as a more complicated ilregular surfacc. The (Montgomery et al. 1998), as in the Coast Range. sir.nprlest ana.lysis is a stress balance on a one- Estimatecl return intervals of landslides for an dirnensional slicle mass with a reglrlal surface; much individual hollow vary from a feu' hundrecl years more complex is a rigorous force-and-moment to 15,000 years or more (Sr,r'anson et al. 1987; Benda analysis on an irregulal three-dimensional surface. ancl Dunne 1992a), although the effect o{ fluc- To learn more about slope-stability analysis tuations in climate on failurc rates is ur.rknown. methods, see Nash (1987). Locallv, differences in surficial rocks, such as a The sirnplest and most straightforward method concentration of mudstone laycrs or differences ir.r of slope stability analysis is the infinite-slope ihe physical intcgritv of sandstone, may affect the method. It is used most often to quantitatively rate of accumulation of colluvium (Reneau et aJ. investi€iate the stability o{ shallor'r', landslicle-prone 1989) and the conccntration of subsurface water forest soils (Consior and Gardncr 1971; O'Loughlin (Andcrson et al. 1997). 1972,7974;Wu at al. 1979; Sidle ancl Srvanston 1962). The infinite-slopc mcthod is a ratio of tl.re soil shear strength and thc a representative Mechanistic context debris shcar stress of slice for flotos of soil that has a unit rvicltl.r ancl Jength; thr-rs only Using the geomorphic context for cleblis fkrn's soil depth varies, making the analysls one- clcscribcd above, we can identify the locations that dimensional. Thc gcncralized eqrration for the are n'rost likeJy to be initiation sites. In any given infirrite-slope method is pro,,,icled belon' for a soil lar.rclslide-producing storm, less than 10 percent of in rvhich strength is represented by both cohesion sites prone to clebris slides r'r'ill fail (Dunne 1998; and friction ancl grotrndwater flor,r'is represented Robison et al. 1999). Ho,,r'evcr, r,^"'ith thc crrrrent level by steady state seepalle parallel to thc slope of knor'r'ledge, it is not possible to prcdict r'r'hich sites (Abran.rson et al. 1996). prone to debris slides r.r,ill fail during the ncrt c' + tt coi Ff(l nt)y ,,, + m y'lnn Q' landslide-prod ucing storm. Howcver, slope- FOS = stability analysis can bc uscd to gain some insight ft sinBcosB[(l nl)y,,, + my.,,,] into the likelihood that a sitc will ol r'r'ill not fail. In this cquation, FOS is the factor of safety; and

c' and @'are soil-strength parameters; y,,,,y'and SIop e -st ability analrl sis /.,,/ are the moist, buoyant, and saturated unit Slope-stabilitv analysis js a wav to assign a weights of the soil; /r is soil depth; and lr is a numerical rrerhre to the stabil:ity of a slide prone site. dimensionless dcpth of water ralrging from zero to It js a qulrntitative treatment of the stresses acting onc. The other terms in tl.re eciuatjon are ilh-istrated .)-7. on a hillslope or vallev and the strength of the soil irr f igure lr't $er'terJl. the c,'ntr ibufiorr of I'lanl involved. In carrying out a skrpe-stability analvsis, roots to soil strength takes the forn of an apparent a factor of safety for the slide-prone site is calculatcd. cohesiolr, crl, or root cohesior-r, cr, in the numelator The factor of safety is thc ratio of thc forces drivinS; of the equation that is summed with thc soil failure and those resisting it for a given slide mass. cohesion term, (Siclle et al. 1985). The forces drir-ing failure ale duc to thc donmslope All sJope-stability .inalysis methods, including component of tlre r'r'eight of tlre soil within the slide the infinite-slopc mcthod, require the following mass. The forces resisting failure comprise soil infolmation: (1) the location of the failrrre surface, strcngth and any additional strenfltlr due to plant (2) the mode of failure, n'hich dictates tlre ap- roots. propriate analysis rnethod, (3) the loading on the The methods avaiLabie for analyzing slope critical failure sr-rrface, (4) the soil strer.rgth p;rra stability ranse frorr farirly simple and straight- meters, including the effect of roots, and(5) the pore foru'ard to exceedingly complex. Thc analysis \ rater pressure on the faihue surface at failure. Landslides, Surface Erosion, ancl Forest Operatictns in the Oregon Coast Range 219

granular soils. These are the estimates of soil strength and pore water pressure at failure resulting from the hydrologic response ofhillslopes to rainfall.

SoiI strength

The strength of the forest soils for steep, landslide- prone terrain ir.r the Coast Range (or for any soil) is not a fixed quantity. It is a function of the ef{ective normal stress on a failure surface wher.r failure occurs. These soils are generally considered to be cohesionless or granular soils and are classified as fine sands or fine sandy loams. They are shallow, Figure 9-7. Definition sketch i,or the infinite slope 1.0-meter-deep (1 0.5 meter), loose soils with loW .l.rbilit) melhod for 'lope ,tn.ily.is. dry densities and high void ratios; thus the confining and normal stresses at typical soil depths are low. The relationship between soil strength and the Slope-stability analysis has been carried out for effective normal stress is called the Mohr-Coulomb naturally occurring debris flows il shallow, residual strength envelope. Figure 9-8 shows a typical Mohr- soils (Swansion 1970; Consior and Gardner 1971; Coulomb strength envelope. This relationship takes O'Loughlin 1972, 7974; Wu et al. 1979; Sidle and the following form: Swanston 1982; Buchanan and Savigny 1990). The 'r=/+oi,tanQ' infinite-slope method was used for most of these where 7is the soil strength (kPa); is the effective analyses. The exceptions were those of Swanston 6.' (1920) who used the "ordinary method of slices," soil cohesion (kPa); oj is the effective normal stress (1990), and Buchanan and Savigny who used on the failure surface (kPa); and @' is the effective Bishop's modified method. Both analyses combined internal angle of friction ('). conventional soil mechanics with limit-equilibrium The effective normal stress on a failure surface is methods. Soil strength was determined using the total normal stress minus the pore water standard engineering tests. Positive pore pressures pressure, which is a function of the depth of the were estimated based on the direct response of a groundwater table over the failure surface. The total groundwater table to rainfall. normal stress is that component of the total soil Although a standard analysis of the stability of a weight above the failure surface that is per- site prone to debris slides using the infinite-slope pendicular to the failure surface. Normal stress is method provides a general framework for con- sidering mechanisms offailure, it is of little practical value in determining absolute stability. Its value is Iimited because some of the critical parameters in a slope-stability analysis can be measured or known only with a very low degree of precision. These parameters include soil strength, root strength, and pore water pressures at failure. The lack of precision for these critical parameters results in a less precise I estimate of the absolute stabillty of an individual site. Therefore, slope-stability analysis has a very limited application for identifying stable versus unstable sites. f Considerable research has been carried out in recent years on two of the critical components of l slope-stability analysis for forested sites prone to Normalstress 0 debris slides that have shallow, coarse-textured or Figure 9-8. A Mohr Coulomb strength envelope. 220 Forest and Strcam Management in thc Oregon Coast Range

expressed per unit area. Thercfore, the Mohr- Savigny 1990). These conventional tests are carried Coulomb strcngth enYelope is prcsented in terms out usir.rg standard gcoteclrnical enginccring o1 total stress as: pracLices, which include the triaxial test as well as others. Unfortunately, the resulting estimatcs of soll r =.+(o- - l-L)tan4 strength reflect stress states .rrd test conditions that are i1[)t telir?se tnfl-oc of shallor,r' soils on steep, wherc c is the total soil cohesior.r (kPa); o- is thc landslide prone forestecl slopes at the time of failure. total nornal stress on the failure surface (kPa); and The conventional tests tor soil strength are not p is the pore tr,ater pressure, lr.hich is the prcduct representative because they are carriccl out at high of the depth of tlre I\'ater times the dcnsitv of I,r'ater. corfining pressures using a standard stress path. (kI'a). AJtl.rough these conditions are representative of the Knowing the loc.rtion of the gxrundwater table stress statcs and the tvpe of loading commoniy at failure is critical to the stability analysis of steep, found in geotechnical enginecring, the i/t slil landslide-plone slopes, becausc it is the only l,r'ay conditions of a higlr confining stress (i.e., cleep soils) to calculatc the effective stress on the failure surface. and an increase in weight or total load (i.e., standard The location of the groundwatcr table anct the pore stress path) do not occur on {orcstecl slopes. The pressures on the faihrre surfacc at failure are doubly failure surface for most debris slides is shallow, critical bccause the groundwater response to rainfall roughly 1.0 meter (1 0.5 meter). In addition, the soils is the Lrltimate triggering mechanisrn for almost all are loose and cohesionless, with lott' bulk densitics landslides on forestlands irr Oregon. (1.1 to 1.4 megagrams per cubic meter). As a result, Soil strength is not a theorctical cluantity; values the lr sllri confining stresses for these soils at these of soil strength must be cletermined empirically. depths are Jow, in the range of 6 kPa t 3 kPa. Another There are a variety o{ soil-strengtl-r tests, r'anging factor making conventional estimates inappropriatc frorn relatively simple lir sltri fjeld tests to conplex for predicting lainfall-inducec{ lanclslides is that the and time-consunling laboratory tests. The accuracy stress path is not an increase in total load, but rather and precision of the estimates of soil strength a decrease in ttre effective stress. Shallow, trans- derived from thcse tests are generally proportional iational landslides occrlr when the effective stress to the time ancl effort put into the tcst. Tlre most is reduced by the water table that occurs in response common and accurate soil strength test is a to rair.rfall. laboratorv test called the triaxial test. It uses a Soil strength at low confining stresses (i.e., at the cvlindrical soi) sample that is nominally 5 centi- ll .si/ir confining stresses frtr loose, shallow, meters in diameter and 15 centimeters long. The cohesionless soils) is r.rot a linear function of effectir:e sample is placed in a cell whcrc different pressures stress. For standard soil-strcngth testir.rg at con- or stresses can be applicd to determine the strength ventionai stress states, the intcrnal angle of friction of the soil sample at dif{ercnt clepths. Once the for granular soils increases with clecreasing appropriate str"esses or confining pressures have confining stress (Lambe and Whihran 1969; Mitchell been applied, a forcc or an increase in load is applied L976; Hctltz ancl Kovacs 1981). Therefore, triaxial to the long axes of the sample until it fails. The soil-strength tests must bc carried out using lor,r' tri;rxial test has scveral aclvantages: (1) it allor,r.s a confining stresses for shallow, cohesioniess soils failure surface to develop along the wcakest failure from steep, landslide-prone forested slopes. Morgarr plane in the soil sample instead of forcing an (1995) has compiled published soil strength values arbitrary failure surface through the sample, (2) a tor Coast Range soils; Figtrre 9-9 shorvs a non-lineaL wide array of stress states can be imposed on the strength envelopc fitted tlrrough the data points. As -,'il \,rmplc .rnJ (l) the pr,*itirt' pore pres,Lrle in shown ir-r the figurc, the slope of the line or the the sample can be modeled and mcasured. ir.rternal angle of frictior.r, q)', decreases arrd the Estirna tes of soil strength that have been used for intercept of the linc, c', increases with increasing skrpc-stability analysis of stcep, lanclslicle prone avcrage normai effectivc stress, which is lepresented forcstcd slopes with shalkx'r' soils have generally by p'in the figure. Lincar regressions of the daia come frorn conventional tests of soil stlength slron' r'alues as high as 45" for an effectir-c stress of (Swanston 1970; O'Loughlin 1972; Sidle and less than 35 kPa, rvhich corresponds roughly to a Swanston 1982; Wu et al. 1988; Buchanan and soil depth of approximately 4 to 5 meters, ;rnd as Landslides, Surface Erosion, and Forest Operations in the Oregon Coast Range 221

envelope. Values for the apparent cohesion due to roots have been derir.ed using various research methods for a variety of soils and plant species. These methods include determining the tensile strength of roots (Burroughs and Thomas 1977), carrying out both laboratory and ln slil direct shear tests (Endo and Tsuruta 1969; Waldron and Dakessian 1987,7987; O'Loughlin et al. 1982; Waldron et al. 1983), and landslide back-analysis (Swanston 1920; O'Loughlin 1974; Sidle and oro20304050 Swanston 1982). Values of apparent cohesion due to roots vary from 1.0 to 17.5 kPa (Sidle et al. 1985). Fisure 9-9. A sraph ofthe ri'l'rln ,r'n, u,, "nu",oo" the information available for soil strength test data Groundw ater resp onse to from shallory cohesionless forest soils on steep, ffiinf dll on hillslopes landslide-prone sites in the Oregon Coast Range. The index p' is the normal stress on the soil sample and q Using slope-stability analysis to distinguish is an index of the shear stress or strength (Collins unstable sites prone to debris slides from stable ones 1995). requires knowing the location of the groundwater table at failure, because its location at the point of low as 29'for effective stresses greater than 200 kPa. incipient instability defines the degree of stability The corresponding values of c' range from nearly or instability. The response of the groundwater zero to approximately 25 kPa. After conducting regime in a given debris-slide-prone site is the key triaxial tests of loose, cohesionless soils from the to predicting the point of incipient instability as a Coast Range, Collins (1995) also reported a function of low-frequency, long-return-period nonlinear strength envelope and recommended that storms. strength tests for these soils be carried oltt using Our understanding of the relationship between triaxial tests at in sifr effective stress levels. Similarly, rainfall and the development ofa groundwater table testing a shallow, landslide-prone soil from the San on steep, landslide-prone forested slopes has Francisco Bay area, Anderson and Sitar (1995) found der.eloped over the last several decades (Campbell the strength envelopes to be nonlinear and steeper 1925). The current model consists of a two-plrase at lower confining pressures. system representing a residual soil, or colluvium, The effect of the stress path on soil strength has overlying bedrock (Figure 9-10). The soil has high also been investigated. Collins (1995) conducted permeability and is quite porous compared to the strength tests using a field stress path, which impervious bedrock. Rainfall infiltrates the soil and initiates failure by reducing effective stress without moves vertically until it encounters bedrock (or increasing total load, and compared the results with other less permeable layers); the subsurface flow those for the same soils using a standard stress path. then becomes parallel to the underlying slope. Differences in the two stress paths for cohesion and According to this model, groundwater will ac- the internal angle of friction were negligible cumulate and the transient groundwater table will compared with the differences that occurred simply get thicker toward the base of slopes (Figure 9-10). as a result of variability between test specimens and Based on this model and the relationships observed tests. So although testing at ln slfl confining between rainfall and piezometric head, predictive pressures is recommended, it is not necessary to use models have been developed to locate the position a non-standard stress path in the test. of groundwater tables for a given amount of rainfall or return periods of storms (Swanston 1967; Root strength Burroughs 1984). These relationships also have been included in slope stability models to help predict The effect of roots on soil strength is modeled by the location of landslide-prone terrain (Schroeder adding an apparent-cohesion term, cr, or a root- and Swanston 1987; Hammond et al. 7992). cohesion tern, cr, to the Mohr-Coulomb strength 222 Forest and Stream Management in the OreEjon Coasl RanSe

/ /^(/l / l.kiltr,.,, /,,' / ZK Lt -J=ct'b--u*o* ^^;;,ffi'U/-n 4'1*40"."".

FiSure 9-10. A conceptual model for a two phase systenr of groundwater response to rainf.rll for steep, {orested h illslopcs (Carnpbell 1975).

However, research has shown that this conceptual runoff generation for a small bedrock hollow and model is or.ersimplified. I1alr (1977), Johnson and to thc generation of positir.e pore pressure. Zones Sitar (1990), and Montgomery ei al. (1997) observed of positive pole pressure were cliscontinuous and that the occurrence of positive pore pressule during higlrll, localized, and their occlrrlence coincided large storms was highly localized on a hillslope. with areas of grounclwater exfiltration from the Zones of positive pore pressure did not occur at the beclrock into the ovellying soil. Subsurface flow base ofslopes as expected, but at mid-slopc locations tended to infiltrate into ancl erfiltrate out of the associated with con\.ergent topography. Therefore, fractured bedrock layer ser-eral timcs, r'hich Johnson and Sitar (.1990) hypothesized that topo- accounted for the discontinuous pattern ofposjtive graphic convergcnce causes flow to move laterally pore pressure. When thc bcdrock hollow sub- to swales or hollows, and that subsr.rrface flon'in sequently failed during a natural storm, that faiJure these locations does not come from the surface soil coincided h'ith the location of subsr,rrface flovr' layers directly upslope. Thev further hypothesized exfiltration from the bedrock to the or,'erlyirrg soil that subsurfacc flow may also occur through the (Dietrich and Sitar 1997). bedrock laver. More sophisticatecl physically based models of ln the Oregon Coast Range, Anderson et al. (1997) subsurface flort' that predict the location of and Montgomery et al. (1997) have observed a three- groundwater tables mav heJp to differentiate pl.rase system, with a highly pervious layer of unstable and stable landforms on forested hillskrpcs; fractured bedrock separating the soil and im- for example, TOPMODEL (Bevin et al. 1995) or pervious bedrock. Subsurface fkrw ihrough the CLAWS (Duan 1996). However, these models fractured bedrock contributcd significantly both to currently cannot deal with the sitc-specific com- Lanclslicles, Surface Erosion, and Forest Operations in the Oregon Coast Ran9e 223 plexity illustrated by Montgomery et al. (1997) or response has been observed in physical modeling camot parameterize it. But even though preferential .-rf Reid ;rnd others ( lOaT); lhe ma r.imum e\ce5: Pore flow paths and pipes or macropores in soil are pressure during debris-flow initiation and notoriously difficult to measure and to represent in mobilization was much greater than the static pore models (Bonell 1998), they govern the behavior of Pressure. subsurface flow in hollows during landslide- producing storms. Debfis and aquatic habitat An alternative that avoids this problem is the use fTozos of simple empirical models to simulate subsurface Landslides initiate on steep, forested hillskrpes in fJow (Beschta 1998). They can be used to simulate response to the presence of a groundwater table, the location of groundwater tables in sites prone to which means that the soils have high water contents debris slides in response to rainfall (Blansom 1992). and high positir.e pore pressures. At such times the The Antecedent Precipitation Index or API model saturated-water content of forest soils in steep, sites has been used to simulate runoff from small, prone to debris slides is higher than their liquid limit forested watersheds in response to rainfall (Fedora (Bransom 1990); once initial failule occurs, the soils and Beschta 1989). The greatest benefit of the API tend to behave as liquids and quickly mobilize into model is that the parameterization of the complex debris flows. sub s urface-drainage environment can be ac- Debris flou's initiate on steep, soil mantled complished using a single empirical coefficient hillslopes at the upstream end of the drainage (Bransom 1997). However, this model approach system and run dou.'n through steep zero-order does require that local piezometric data be dev- swales and sometimes first- and second-order eloped to generate the empirical coefficient, which stream channels. Eventtrally they deposit in lower- can be a formidable task. gradient first- and second-order streams while some So far this discussion of the initiation oflandslides reach third- and higher-order stream channels has dealt only with static pore pressures (those (Swanson and Lienkaemper 1978). Debris flows can resulting from precipitation and the aggregation of remove all the sediment and large wood from the subsurface flow). Another consideration is dynamic steep slvales and first-order stream channels, pore pressures, specifically the excess pore pressures leaving a bedrock channel (Benda 1990). Hor'r'eve4 that are generated as a result of the incipient the stream channel upstream of a deblis-flow movement of the larrdslide. The pore pressures deposit is generally neither totally erosional nor during mobilization of the debris flow are termed totally depositional; it retains some sediment and excess pore L)ressures because they cannot be channei substrate material although the charrrrel is predicted or accounted for by traditional means, highly disturbed and the large wood is removed. u'hich consider the drained initiation state and not Debris-flow deposits r.ary in size and type, the undrained mc-rbilizatjon state (Anderson and depencling on the size of the flow ancl the gradient Sitar 1995). and width of the stream channel (Benda 1990; May As a part of the research on the influence of 1998). Atone end of the continuum rs Ihe debris jnnt, corrfining pressures and stress path on soil strength, a large, valley-spanning accumulation tl1 or€ianic the response of the porc pressure in the soil sample debris that dams the stream, forcing the ac- to undrained loading was investigatecl (Bransom cumulation of a lar€ie wedge of sediment. However, 1990; Keough 1994; Molgan 1995). The pro- debris-flow deposits can also form fans of large portionality constant between axial undrained wood and sediment that force a larger stream krading and pore-pressure rcsponse, called Skemp- against the opposite bank, thus increasing channel ton's "A" pore-pressure parameter, was positive lor sinuosity without blockir-rg the stream. At the other these strength tests, meaning that these soils are end of the continuum, debris fans of sediment and compressive (Morgan 1995). This means that their large wood may deposit across floodplains and not volume decreases with incipient axial kradinp; and interact with the receiving stream channel except slrear'. Althorrgh we lack information about how a at very high flon's (Benda 1990; May 1998). Of the compressive soil responcls to incipient shear strair.I 53 debris-flow deposits in the central Coast Range as a result of reducecl effective stress, it is likely that that occurred following the February 1996 storm, such a situation would increase pore pressures. This only 12 (23 percent) were valley-spanning debris Landslides, Surface Frosion, and Forest Operations in the Oregon Coast Range 223 plexity illustrated by Montgomery et al. (1992) or response has been observed in physical modeling q97); cannot parameterize it. But even though preferential ol' Reid .r nd others ( I lhe ma \im u m e\ccs\ pore flou/ paths and pipes or macropores in soil are pressure during debris-flow initiation and notoriously difficult to measure and to represent in mobilization was much greater than the static pore models (Bonell 1998), they govern the behavior of Pressure. subsurface flow in hollows during landslide- producing storms. Debris and aquatic habitat An alternative that avoids this problem is the use flozas of simple empirical models to simulate subsurface Landslides initiate on steep, forested hillslopes in flow (Beschta 1998). They can be used to simulate response to the presence of a groundwater table, tl-Le location of groundwater tables in sites prone to which means that the soils have high water contents debris slides in response to rainfall (Bransom 1997). and high positive pore pressures. At such times the The Antecedent Precipitation Index or API model saturated-water content of forest soils in steep, sites has been used to simulate runoff from small, prone to debris slides is higher than their liquid limit forested watersheds in response to rainfall (Fedora (Bransom I990); once initial failure occurs, the soils and Beschta 1989). The greatest benefit of the API tend to beha\.e as liquids and quickly mobilize into model is that the parameterization of tlre complex debris flolvs. subsurface-drainage environment can be ac- Debris flows initiate on steep, soil-mantled complished using a single empirical coefficient hillslopes at the upstream end of the drainage (Bransom 1997). However, this model approach system and run down through steep zcro-otdet does require that local piezometric data be der- swales and sometimes first- and second-order eloped to generate the empirical coefficient, which stream channels. Eventually they deposit in lower'- can be a formidable task. gradient first- and second-order streams while some So far this discussion of the initiatiou of landslides reach third- and l-righer-order stream channels has dealt only r,r,'ith static pore pressures (those (Swanson and Lienkaemper 1978). Debris flows can resulting fron'L precipitation and the aggregation of remor.e all the sediment and large wood from the subsurface flow). Another consicleration is dynamic steep swales and first-order stream channels, pore pressures, specifically the excess pore pressures leaving a bedrock channel (Benda 1990). Hon'ever, that are generated as a result of the incipient the stream channel upstream of a debtis-flow movement of the landslide. The pore pressures deposit is generally neither totally erosional nor during mobilization of the debris flow are termed totally depositional; it retains some sediment and excess pore pressurcs because they cannot be channel substrate material although the chamrel is predicted or accounted for by traditional means, highly disturbed and the large wood is removed. which consider the drained initiation state and not Debris-flow deposits vary in size and type, the undrained mobilization state (Anderson and depending on the size of the flow and the gradient Sitar 1995). and width of the stream channel (Benda 1990; May As a part of the research on the influence of 1998). At one end of the continuum is the tlabris jnnt, confining pressures and stress path on soil stret-Lgth, a large, valley-spanning accumulation of organic the response of the pore pressure in the soil samPle debris that dams tl.re stream, forcing the ac- to undrained loading was investigated (Blansom cumulation of a large wedge of sediment. However, 1990; Keough 1994; Morgarr 1995). The pro- debris-flow deposits can also form fans of large portionality constant between axial undrained wood and sediment tl.rat force a larger stream loading and pore-pressLrre response, called Skemp- against the opposite bank, thus increasing cl.rannel ton's "A" pore-pressure parametet was positive lor sinuosity without blocking the stream. At the other thesc strength tests, meaning that these soils are end of the continuum, debris fans of sedimer-Lt and compressive (Morgan 1995). This means that their large wood may deposit across floodplains and trot volume decreases with incipient axial loading and interact with the receiving stream channel except shear'. Although we lack information about how a at very high flows (Benda 1990; May 1998). Of the compressive soil responds to inciPient slrear strain 53 debris-flow deposits in the central Coast Range as a result of reduced effective stress, it is likely that that occurred folkrwing the February 1996 storm, such a situation would increase pore pressures. This only 12 (23 percent) were valley-spanning debris 224 Foresl and Stream Management in the OreBon Coast Ran51e

dams. Of the remainder, 36 (66 percent) interacted disturbance, specifically stand-replacement wild- with the receiving channel to various degrees fires followed by intense, landslide-producing without completely blocking it, and 5 (11 percer-rt) storms that cause the widespreacl occurrence of were washed away during the storm ancl not founcl debris flows (Benda and Dunne 1997a, b; Benc{a et (May 1998). al. 1998). The aquatic habitat is of highcst cluality Debris flows can be disastrous to fish and aqr-ratic and the fish communities are most diverse ,,r,hen habitat. When the dcbris flow occurs, there is the the disturbances are of intermediate age, neither likclihood of direct mortality to fish and other recently disturbed nor senescent (Reeves et al. 1995; aquatic species. Later, the form of the stream may see also Chapter 3). This disturbance-based thcory be greatly altered and simplified. The large wood of the succession of aquatic habitat recognizes debris will be gonc, the stream will have low hydraulic flows as the mechanism that brings critical habitat retention, the or"-erhanging canopy will be more ingredients, nlrmely large wood, boulders, and open, and the surrounding riparian area will be sediment, from the hillslopes and into the streams. highly disturbed (Everest and Mcehan 1981; Lamberti et al. 1991). Downstream crosion may be landslides and Surface Erosion in accelerated; large, abrupt inptrts of sediment may Managed Coast Range Forests mean rnore fine sed jment, which affects spawning, bed quallty, and more transport-resistant sediment, Surface erosion n'hich can cause aggradation of strcam channels and Bccause of the Iow intensities of precipitation in the filling in of pools (Everest et al. 1987; Sullivan et al. Coast Rangc ancl the high infiltration capacities of 1987). forest soils, surface erosion resulting from overland ,evct Some research results suggest, ho that the flow is virtually nonexistent. However, rninor relationship between debris flou's and aquatic amounts ofsurface erosion occur, especiallv on steep habitat may not be cntirely ncgative. Dcspite short- slopes because of rar.el from exposed soils resulting term localized damage, Ever.est and Meehan (1981) from local disturbances, srrch as windthrou' and {or,rnd that debris-flow deposits createcl resting burrowing animals. Managed forests offer more habitat for spawning adult coho sah.non and rearing opportunitv for surface erosion because soil habitat juvenile for coho witl.rout interfering with clisturbance is more drastic, widespread, and upstream spawning migr.ation. It-r this case, the net chronic. Surface erosion resulting from {oresi effect of debris flows on fish ancl aquatic habitat ft,as management activities can be dir.ided into two positive. ln thc Elk Rir.er watershed in southwest general categories: surface erosion from within Oregon, the most productive areas for both rearing han est units, where the predomir.rant manil€iement and spawning habitat were associated \.ith large activities include thc removal of trees and site "flats" or areas of relatively low gradient. These preparation, and surface erosion from drastically areas were persistent features at tributary junctions disturbed and compacted soil surfaces, such as skid and apparently resulted from debris-fktw deposits trails for ground-based harvestir.rg systcms, roads, that occurrcd in conjunction wjth catastrophic and landings. wildfires over 100 years ago (G. Reeves, personal communication). This scen;rrio would link high- Surface erosion harztest quality aquatic habita t with the occurrence of dcbris from units flows. Surface erosion coming from within harvest units Recently, Reeves et al. (1995) offered a long-term takes tl'r'o main forms. The first, dry ravel, is view that portrays aqr-ratic habitat as dynamic over predominantly associated with steep slopes and the geographical scale of watersheds and landscapes cohesionless or skeletal soils in wester.n Oregon. The and over the temporal scale of clecades and second form, infiltration-limited surface erosion centuries. In responsc to disturbance, aquatic (wet erosion) is associated with rainfall and habitat, like terrestrial habitat, undergoes suc- overland flow. cessional stages and its quality at a given point in a watershed will succeed from young to old forms Dry ravel over time. The mechanism for r.esetting the age of Timber harvestirrg can accelerate the rate of dry aquatic habitat has historically been catastrophic ravel, especially if broadcast burning is used as a Landslides, Surface Erosion, and Forest Operations in the Oreg,on Coast Range 225 site-preparation treatment. Swanson et al. (1989) Even moderate soil disturbance, compaction, and summarized the research findings on dry ravel. broadcast burning do not reduce infiltration Increases in either slope or slash burning cause the capacities of forest soils to a point where they are rate of dry ravel to increase greatly. On steep, less than rainfall rates. Johnson and Beschta (1978) broadcast-burned harvested areas in the Oregon found that infiltration capacities on areas harvested Coast Range, the surface-erosion rate was ap- using various methods, as well as infiltration proximately eight times the rate on similar but capacities on tractor skid roads, were several times gentler terrain and approximately 13 times the rate higher than expected rainfall rates. For areas that for steep, unburned slopes. On steep, burned slopes, had been harvested and broadcast bumed, McNabb two-thirds of the first year's erosion occurred within and others (1989) found that the single lowest 24 hours after the burn (Berurett 1982). In the Oregon measurement of infiltration capacity was still twice Cascades, dry ravel on a steep, unburned slope the expected 100-year-return-period ra infa ll increased markedly after harvest, then within 3 to 5 intensity. years settled to a level approaching that of an Research results from paired watershed studies undisturbed forest (Swanson et a1. 1989). Therefore, reinforce these findings. During the Alsea Water- even with marked increases in dry ravel on shed study, one small sub-watershed was 90 percent harvested sites, the window of accelerated erosion clearcut and harvested using a highJead cable is very small compared to expected rotation lengths, system; there was no increase in suspended and the total amount of erosion produced is sediment as a result of the clearcut krgging alone minuscule compared to other erosion processes. (Brown and Krygier 1971). In a paired watershed study in the H. J. Andrews Experimental Forest, the I nf i ltration-l im ited surface erosron rate of soil loss attributed to surface erosion lrom Surface erosion associated with infiltrationlimited clearcut harvested areas was very small (Fredriksen overland flow is virtually nonexistent for harvested 1970). areas in western Oregon. The simple removal of Therefore, erosion associated with infiltration- trees does not change the infiltration capacity of limited overland flow in western Oregon forested forest soils sufficiently to cause infiltrationlimited watersheds occurs only on sites that are drastically surface erosion. While the absence of trees does not disturbed and compacted. These sites include roads, increase surface erosion, the manner in whicl-r they landings, and the portions of skid trails that have were removed and the site-preparation techrtques experienced multiple passes (six turns or more) with used after harvest may increase its risk. Aerial- ground-based harvesting equipment. On such sites, Iogging systems, such as skyline systems or the soils are bare and have been compacted helicopter logging, expose and disturb less surface sufficiently to make the infiltration capacity less than soil than ground-based harvest systems, such as expected rainfall intensities. tractors and skidders (Chamberlin et al. 1991; see Chapter 6). Surface erosion from roads and landings Chemical site preparation leaves the duff, litter, Forest roads have long been a focus of concern and slash intact and causes the least increase in the regarding accelerated erosion. The construction ol risk of surface erosion. In contrast, site preparation a forest road, especially one with any degree of side by broadcast burning removes logging slash, may slope, results in a high degree of disturbance. When remove litter and duff layers, and may create the road is completed, the remaining structure has hydrophobic conditions, all factors that increase the two locally or.ersteepened slopes (the cut slope and risk of surface erosion. The risk is increased most the fill slope), a surface that must support loaded by mechanical site preparation, in which logging log trucks, and a roadside ditch. All of these surfaces slash is pushed into windrows using a bulldozer or are bare soil until r.egetation becomes reestablished. put into piles using a hydraulic excavator or a The forest-road surface is intentionally compacted bulldozer. In such cases, not only is the slash layer to develop its strength and is a virtually impervious removed, but the surface soil is also disturbed and surface. The roadside ditch is designed to carry both compacted. runoff from the road surface and subsurface flow Regardless of these factors, it is still difficult to intercepted by the cut slope to a cross-drain culvert generate surface erosion in Coast Range forests. or other drainage structure. Moreover, the en- 226 Fofest and Streanr Managenrent in the Oregon Coast Range vironmentarl effects of forest roads are much more In the central Coast Range, Luce and Black (1999) persisteni than the effects of harvesting. Because investigated how road attributes affected strrface forest roads are constructed, maintained, and used erosion in the absence of traffic. They found that over an extended period oi time, they generate secliment production was best predicted using a surface erosiolr over this period. term that is the product of the length and the square The effects of construction in accelerating erosion of the skrpe of the road segment. Total sediment have been studied extensively. These research production from road segments was independent sources are paired n'atersheci stuclies for the Alsea of the height of the roacl cutslc-rpe. Flowever, \,\41en Watershed (Brown ancl Krygier 1971), the H. J. the road cutslope and roadside ditch wele veg- Andrews Experimental Forest (Fredriksen 1920), etated, the amount of surface erosion was reduced and Caspar Creek in northern Californi;r (Krammes bv a factor of seven. Similarll,, Foltz (1996) found ancl Burns 1923). Horvever', the information obtained 'vt'ater vclocitics in unvcgctatcd roadsiclc ditches to is of rnarginal use, for a number of reasons. The be tr,r.'o to three times fastcr than thosc in vcgetatcd stlldies u'ere not specific to erosion processes, and or grass-lined ditches. the researchers lumped erosion proccsscs togcthcr Modcling the process of surface erosion from and measured overall effects at the mouth of the roads has shown that it is influcncccl bv thc wt-rtershed. Also, the three studies were carried out frrrmation of ruts (Foltz and Burroughs 7990, 1991.; in landslide-prone tcrrain, and landslicles un- Foltz 1993). Well-definecl, deep, contilruous ruts doubtedlv contributcd a significant amouni to the cause long, concentrated flow paths, in contrast to reported erosion. Thercforc, thc rcsults say littlc the short flolv paths across the roacl. Maintenance about snrface erosion from forest roads. FinalJy, the of the road snrface to prevent;rnd fjx mt formation cxisting roads werc constructed according to lor,r,'erecl the erosiorr rate drastica]Jr,., because the methods used 30 to,10 vears ago, and road-location lcngth of the flon' paths rvas recluced. Although and -cor.rstruction plactices har,'e impror.ed ereatly grading and shaping the roacl surface made material during the inten'ening years. ar';rjlable to erode in the next storm, overlancl flor.v In contemporary managecl folests, it is the \'\'as not;rble to remove the material efficiently environmental effect of this legacy of forest roacis because of the shorter flo\ / path. However, traffic that is of most conceln. ln westen.r C)regon, ther"e on a road increased sr-rrface erosion long; before rr,rts are literally tens of thousands of kilometers of forest form. Even a slight increase in the flow patl-L caused roacl, and aJi represent some potential to produce an increase in surface erosion from the road. chronic surface erosion. Several stuclies have investigated the environmental factors that affect Effect of forest fll&nagement &ctipities on surface erosion fron forest roads. One of the most debris slides and important predictors of surface enrsion fxrm these fJoTos roads is the level of traffic (Reicl and Dr"rnne 198,1; Mass movement processes (most debris slides and Bilby et a1. 1989). Reid and Dunne (1984) r'cport ihat fkx,r's) dominatc thc scdiment buclgets of pristine a heavily tlaveled mad rvill yield, by an order of watersheds in u'estern Oregon (Dietrich and Dumre magnitude, more fine sediment than a sin-Lilar road 1978; Sr'r'anson ct al. 1982). It is thesc s.tmc processes that has only light traffic. The thickness ;rnd quality that h;rve the greatest Potential to be adYersely of thc sr,rrfacing matcrial of the road also affect affected by forest management activities and to sedimcni production. Surfacc crosion is lcss lrdrcrc result in the largest net increases in acceleratecl a thick la1'er of higlrer-qrralitv erosion-resistant rock erosion. Landslides associated $'ith forest man;rge- '\,\'as used for the surface course than r'r'here a thinner ment activities are dividecl into t\\'o groups: in-Llnit lavcr of lcsscr-quality rock r'r'as uscd (Bilbv et al. lanclslides (landslides that occur in hirn'est r.uriis and 1989). The thick sr-rrface seems io rninimize the are not associated $'ith forest roads) ancl road- amonnt of fines that is pumped fi"om the subgrtrde reltrtecl landslicles. The mar-Lagement related canses to the snrface, and the high-clr,rality rock seems to of landslicles ancl tl.reir mitigation are different for be less pror-Le to erode. However, both surfacing each group. For in unit landslicles, the causes relate thickness;rnd qualitv were secondary to traffic to Veget;rtion mana€iement; for road related lntensl$r l;rnclslides, ihe causes are relaiecl to site clisturbance and changes in hillslope hydrobgy. Lanclslides, Surface Erosion, and Forest Operations in the C)regon Coast Range 227

In-u n it landslides conclusions on inventories that are systematic and Since the earliest research in this area, inr.estigators ground based, and that give every landslide an har.e belier,,ed that there is a basic cause-and-effect equal opportunity to be sampled. relationship between timber harvesting and The ODF landslide inventory was intended to landslide occurrence (Croft andAdams.1950; Bishop monitor the effect of forest practices on landslides and Stevens 1964). The early research tool used to (Robison et al. 1999). The inventory covered eight investigate this relationship was landslide in- 26-square-kilometer study areas in the Cascade ventories. In particular, in\rentories fo| harvested Mountains and Coast Range of western Oregon. terrain were comparecl with inventories for similar Five of the str-ldy areas were labeled "red zones" but unmanaged terrain. Landslide inventories have because they had a high incider.rce of lanclslides been carried out for much of the Pacific Rim, resulting from the 1996 storms. Table 9-2 presents including the Pacific Northwest, coastal British the landslide densities for four of the red zones Columbia, southeast Alaska, Japan, and New stratified by the age class of the forest stand (data Zealand. The results have been compiled to show for one red zone, the Tillamook study area, is background landslide and erosion rates and the omitted because only one forest age class was effect of timber harvesting on these rates (Sidle et Present). al. 1985). Table 9-1 contains such a compilation. In Although this study represents only the response every landslide inventory but one, the erosion rate of steep, landslide-prone, forested landscapes to a attributed to debris flows increased after harvest, single landslid e-producing storm, its results by 1.9 to 21.2 times. Similarly, all but two studies encapsulate the trends shown by a compilation of documented an increase in the rate of occurrence of systematic, ground based landslide inventories landslides after harvest. (Pyles et al. 1998). Therefore, the trends seen in the However, care must be used when interpreting ODF data seem to be consistent across many types these data. Most of the studies used landslide data of landslide inventories from a variety of terrain collected from aerjal photographs. Unfortunately, types. given differential visibility in aerial photographs of The ODF study shows that there is, on average, debris flows in forests versus clearcuts, photo-based an increased incidence of landslides in the first inventories could bias the results of the research decade after clearcut harvest compared with (Pyles and Froehlich 1982). The Oregon Department adjacent late.seral fore:ls. The.rver;ge increase in of Forestry (ODF) conducted a systematic, ground- landslide density was 42 percent, based on a based landslide inventory after a series of storms in background rate of 5.2 lanclslides per square in 1996 in western Oregon. The survey showed kilometer and a post-harvest landslide rate ol 7.4 conclusively that landslide-inventory data from landslides per square kilometer. This average aerial photographs und errepresented both the increase is less than the average increase indicatecl number of landslides and total landslide erosion in Table 9-1, but it is consistent with the increase (Robison et al. 1999). Tlrerefore, it is best to base observed in ground based landslide inventories (Pyles et al. 1998).

Table 9-2. Landslide density for forest age classes in four "red zone" study areas in the Coast and Cascade Ranges after the February 1996 storm (Robison et al. I999). I Sil/dy:,r?a 0g ),rs (#/kn:,i) I0-29 ),rs (#/kt1tt ) 3A-100 yrs (#/k:r") >100 yrs (#/knl ) Ch.rrrgr i ,)

Etk 22.2 20.5 1 5.,1 26.1 -15 Vida 13.7 1.3 2.7 8.8 56 Mapleton 21 .1 1.c) 6.I 13.5 5B

ScottsburB )0.1 1 t).5 7.3 5.7 25 i) Average 19.2 9.1 7.9 l 3.5 ,+2

This column is the percent ch;lfBc in the landslide clensit), for the 0-9 year age class conrpared with the >100 yedr .18e c lass. 228 Forest and Stream ManaBement in the Oregon Coast Range

The increased incidence o{ landslicles in the fhst intact forest in areas with steep, convergent decade after han cst is followed by a decrease in topography within a clearcut harvest area (head- landslicle occurrence in the 10- to 30-year anc{ 30- to wall-lear,'e aleas) in mitigating the occurrence of 100-year forest age classes. Becatrse the ODF study landslides (Martil 1997). The incidence of landslides was the first to sample multiple forest age classes, in forested areas, clearcut areas, and headn'all-leave there is no other da tabase for comparison. Howevet areas was found to be statistically insignificant this result r,r'as previouslv hypothesized (Froehlich because of high variabiljty. The results from the ODF 1978; Swanson and Fredriksen 1982), and the merit landslide inventory for erosion volume correspond of the hypothesis has been discussed vigorously to those for landslide clensity, including the lack of rt'ithout resoh,rtion. statistical slgnificance. The average increase in landslide density Although landslide density and erosion r.olume attributed to timber harvest represents widely are appropriatc variables given the monitoring varying r"'alues for individual study areas, ranging questiol.ls that were asked in the ODF study, they {rom a 15 pcrcent decrcase in the inciclence of provide little insight into what the response would landslides in one study area to a 250 percent increase mcan for a specific managed forest. In otlrer words, in another. These two cxtremely different results what cloes a 42 percent increase in landslide density come from study areas that are within 10 kilometers krok like? An approach that will yield more insight of each other and have the same geology, landforms, is to present the landslide-density data as the and management regime; the landslides occurred number of failed slide-plone sites pcr unit area or during the same storm. Such variability in lar.rdslide the percentage of slide-prone sites that failed. In response has been noted when only ground-based calculating these numbers, some implied as- inventories were considered (Pyles et al. 1998); sumptions are made. First, the tcrm "bedrock tlrerefore, it seems to be normal. hollow" will be used synonymously for a clebris- These results indicate the importance of naturally slide-prone site. Since bedrock hollows can exist on occurring factors that influencc landslide occurrence planal slopes with little or very subtle topographic as compared with tir.nber harvest. One of these expression, the assumption is made that all the factors is the variability in the total amount and landslides inventoried by the ODF initiaied in intensity of prccipitation that trigger landslides. bedrock hollows. Second, even thotrgh the reported lnsufficient precipitation data were collected to value o{ the average spatial density of 100 hollows satisfactorily determine whether the cause of the per square kilomcter is for the central Oregon Coast extrerne variability in landslide occurrence was Rangc (Benda 1990), it is assumed that this vah-re driven by precipitation. However, sufficient will adeguately represent the ODF study areas. It is precipitation-intensity data were collected during recognized that both of these assumptions are morc the Febluary 1996 storm to show that precipitation than Jikely in error; howevel the consecluence of amounts and intensities that trigger landslides do the error will result in the numbers bcing mor.e vary significantly across distanccs represented by conservatrve. the ODF landslide inventory (M. Clark, personal Using the value of 100 hollows per square communication). kilometer', it is possible to convert landslide density For the four multi-age-class study areas, the values to the number and percentage of failed results of the ODF landslide inventory show an hollows per square kilometer (Table 9-3, for 0- to 9- average increase in landslicle density as a result of and >100-year forest age classes). The highest timber harvest. However, thcre are no statistically landslide density, for the greater-than-10O-year age significant differences in landslide density among class in the Elk study area in the centr.al Coast tl-Le four age classes (Robison et al. 1999). The smatl Range 26 per squa.re ki lometer-represents a sample size (only four trcatments and four study tailure rate of only about 10 pcrcent of the hollows. .rr(,r.) Jnd lhe lrigh \afi.rbilit) in r(.\ponse un- Therefore, on a landscape basis, fewer than 10 doubtedly contribute to this lack of significance. The percent of the hollows failed during the landslide rcsult is consistent with those from the only other producing storm, wl.rether thc forest was harvested landslide study in which the effect of timber harvest or not. The average increase in landslide density,42 was tested statisticallv (Martin 1997). This study percent, means an increase of two landslides per investigated the effectir,'cness of leaving patches of square kilometer if the entire square kilometer were Landslicles, Surface Erosion, and Forest Operations in the C)rcgon Coast Range 229

Table 9-3. The percent of debris slide-prone sites that of road-related landslides is much greater than failed in two forest age classes in four "red zone" background variability, it is easier to infer that a study areas in the Coast and Cascade Ranges of problem exists. Oregon after the February | 996 storm. During ihe last two to three decades, considerable Study area O-9 t rs (''") >1OO yrs (%) Ch.rngrr /n,) effort and expense have been put into improving

E ll< lr.6 10.2 -1.6 the environmental performance of forest roads, especially with regard to landslides. Improvements Vida 5.1 3.4 1 .9 include wholesale Mapleton 8.2 5.3 2.9 changes in the way steep, landslide-prone forested terrain is harvested and Scottsburg 7.B 2.2 5.5 how forest roads are located, constructed, and Average 7.5 5.-l ).1 maintained, including (1) widespread use of long- ' This column is the change in the percent of debris slide span, high-lift cable systems, like skyline systems, prone sites that failecl for the 0-9 year age class compared to reduce the amount of road needed and increase r/ith the >100 year age class. Data based on an assumecl density of debrjs slide prone sites of 1o(/kmr. the flexibility of road location; (2) use of ridge-top locations for forest road systemsi (3) use of high- harvested. That is, the number of {ailed hollows gradient roads to reach and stay on ridge-tops; (4) u ould increare by approrimately 2.2 percent in the use of full-bench, end-haul road-construction harvested area. The change in the percentage of practices; and (5) improved road-drainage and road, hollows that failed ranged from a low of -1.5 percent, maintenance practices. which means fewer hollows failed after harvest, to The degree to which these improved practices a high of 5.6 percent. These values represent an have reduced the negative environmental effects of extreme condition: a rare, landslide producing roads is not fully known. However, an inventory of storm on a 100-percent-clearcut harvest on highly road-related landslides in the central Coast Range unstable terrain. These values are actually high for compared the effects of forest roads constructed an entire managed forest because a whole forest is using old practices with those constructed using rarely in a 0- to 9-year age class. For areas with older impror.ed ones (Sessions et al. 1987). The results age classes interspersed, the increase in the showed that the improved practices reduced both percentage of hollows that failed would be lower. the number and the size of road-related landslides. Unfortunately, the validity of these results remained Road-related landsl ides in doubt because, at the time of the study, the roads Forest roads have long been considered the constructed using the improved practices had not dominant source of accelerated erosion caused by been tested by a landslide-producing storm. The forest management activities. The early paired February 1996 storm provided the opportunity to watershed studies first showed the importance of evaluate the effects of contemporary forest-road road-related landslides in harr.est-related ac- systems. celerated erosion (Fredriksen 1970; Brown and Two inventories of road-related landslides were Krygier 797L; Beschta 1978). Inventories of road- carried out following the February 1996 storm. As a related landslides further documented the part ofthe ODF study discussed previously, all road- importance of forest roads to the increase in related landslides within the eight 26-square- landslide erosion associated with forest manage- kikrmeter stucly areas were inventoried (Robison et ment (O'Loughlin 1972; Fiksdal 1974; Morrison al. 1999). The other road-related landslide inventory 1975; Swanson and Dyrness 1975; Swanson et al. was carried out on Blue River and Lookout Creek 1977; Amaranthus et al. 1985). As shown in Table 9- in the central Oregon Cascades (Wemple 1998). 1, the erosion rate from the rights-of-way of forest The results from both inventories show that roads is 12 to 343 times the erosion rate from similar contemporary forest-road systems remain a unmanaged forested terrain. As discussed above, significant source of erosion from debris slides/ many of the iandslide in\.entories represented in flows (Skaugset and Wemple 1999). Road drainage Table 9-1 were photo-based; the same concerns was associated with approximately half of the debris about bias in the data holcl for road-related flows that initiated at road fills, confirming the landslides as for in-unit landslides. However, importance of road drainage to accelerated erosion. because the difference indicated by the inventories The cutslope height of the roads was correlated with 230 Forest and Stream ManaBement in the C)re8on Coast Range

Table 9-4. Comparisons of inventories of road-related landslides for the Oregon Coast and Cascade Ranges from the literature with the post-.l996 ODF flood monitoring study and the PNW inventory of Blue River and Lookout Creek watersheds. Oregon Coasl Range Comparisons Road length NLtnber ol Average landslide Landslide Erosion t ate (k'n) lanclslides volLllt1e (r,') .lensit)/ (#/kn) (nt'/knt) Swanson ct .rl. 1977 U0.0 89 lB6 t.t0 711 93 179 0.:19 9 Mapleton (Robison et al. 1999) 29.6 29 t20 l l0 201 Tillamook (Robison et al. 1999) 28.6 10 t24 0.3 5 74

Oregon Cascades Comparisons Ro.td RAr\/ area Nunber of Averay,e landsliclt' Landslide Etosion nle (k|j) landslicles volune (nt) deDsity (#/ktn) (ni/knt)

Swnnson & Dyrness I915 2.1 0 73 1,351 6. (l 610

Morrison 1975 0.6 l 75 1q4) 2 0.0 3,200 Vicia (llobison ct al. 19!l!l) 0.5 l )2 173 7.0 B0 Bl!e R./Lookout Cr. 11.20 482 0.8 13 (Wemt)le 1998)

Ro;1d R,A / arc,1 Nunber ol Average landslide Eft)siot1 tale density (#/km) (rn'/kn)

Swanson & Dyrness I975 171r o.41 7 .90 Morrison I975 49 L50 40.00 Vida (Robison et al. 1999) 12 0.5 2 4.20 Blue R./LookoLrt Cf. B9B 0.05 0.54 (Wemp e 1 998)

' Assumecl a I2.5 mctcr road prisnr r,viclth hased on aver.rge road widlh for "red zone" st!dy.rreas in the Ol)F stLr(lv (Robison ct al. 1!1991. both the occurrence and the volume of road-related related landslicles still occur; thc total accelerated landslides. erosion from roads should be lcss (Skaugset and On a lanclscape level, road age and topographic Wemple 1999). position were factors in erosion-related damage to roads. Olcler roads (those constructed during the The effect of on debris 1960s or earlier) \,\'ere the source of significantly forestry IToLus more erosion by landslides than more recent roads. Since the presentlrtion of a disturbance-based Midslope roads produced more erosion than eithcr approach to the management and recor.ery of ridge or valley-bottom roads. aquatic ecosystcms (Reeves et al. 1995; see Chapter Despite improved road standards, the density of 3), incrcasing attention has been paicl to the effects landslides from logp;ing roads in 1996 was roughly of forestry on the size and composition of debris the same as in older inventories of roads with lower flor,r's and their deposits. There is sorne evidence that construction and drainage standards (Table 9-.1). the prirnary cffect of forest manaEiement is on lrorv This demonstrates that, regardless of the standards, iar d( bri* f low' rur ,rnd r,r here they Jepo*:t if roads are constructed in steep, landslide-prone Ketcheson ancl Froehlich (1978) rcport that debris terrain, ihere will be landslides. However, the data flows from clearcuts run farthcr and set up in larger also show that the avelage volume of the road- deposits than dcbris flou's from forests. May (1998) related landslidcs in 1996 decreased compared lvith forurd that, of53 debris flow deposits il anadLontous rates from the older inventories (Table 9 4), in some fish-bearing streams, a disploportionate share came cases markedly. These results, and the lower road from ancl ran through harvest units. density that is now standard, mean that r.r'hile road- Landslides, Surface Erosion, and Forest Operations in the Ore€lon Coast Range 231

There is very little information regarding the research results from paired watershed studies, the direci effect of timber harvest on the size and development of alternative practices to mitigate composition of debris flows. Empirical models that accelerated erosion has intensified. describe where debris flows will run and deposit Three management or regulatory tools can drive show that deposition is governed by stream-charurel changes in forest practices to prevent or mitigate geometry, stream gradient, and stream-valley width accelerated erosion: water-quality standards, BMPs, (Benda and Cundy 1990; Fannin and Rollerson and watershed management. Water-quality stan- 1993). However, the models describe only the runout dards, which are the regulatory tool of choice for and deposition of debris flows and not the com- point sources of pollution, are not practical for non- position of the deposit. May (1998) and Robison and point sources (Brown 1980). Nevertheless, debate others (1999) suggest that although the size of trees continues regarding their efficacy for non-point along the periphery of the debris flow may affect sources oi pollulion from forestry pracfices. the length of runout, the primary de-terminants are Most progress made with regard to the pre- channel geometry and gradient vention and mitigation of accelerated erosion has May (1998) investigated 53 deposits from debris come through the establishment of BMPs. They are flows in the central Coast Range resulting from the forest practices that are prescribed for a specific February 1996 storm. Forest management appeared operation and are effective; that is, they reduce to have no effect on the distribution of large wood accelerated erosion from forestry activities and are in debris-flow deposits, and the size of the wood technically feasible and economically viable. did not correspond to the size of the trees on the The last tool is watershed management, or the hillslope at the time of the debris flow. This result management of forest practices on a scale that is reflects a legacy of large wood stored in low-order larfler than an individual forest stand or operation stream channels. Forest management did affect the (the scale for BMPs). Watershed management lengths of large wood pieces in the debris-flow prescribes the distribution of forest practices on a deposits. Although short pieces of wood dominated watershed bulh:patially and through time. the length distribution for all management classes, Contemporary forest management is currently in pieces were longer in debris {lows that ran through transition bet\,veen using BMPs and adopting water- mature forests. shed management. Best management practices have Roads were found to be the one forest man- been implemented for virtually all forest manage- agement activity that undeniably affected the ment activities, as evidenced by the robust sets of composition, size, and run-out length of debris forest-practice rules adopted by all of the western flows. Debris flows that initiated from roads had timber-producing states. Some changes in BMPs and the largest initial slide volumes, traveled the a few aspects of forest practices continue, but by farthest, and contained the most sediment (May and large BMPs are firmly in place. Conversely, 1998). In the ODF study areas, road-related although some rudimentary watershed-sca le landslides were, by count, the minority of all management practices are required, for example, landslides, yet they accounted for a disproportionate harvest unit size and "green up" or adjacency amount of the length of highly impacted channels. constraints for clearcuts, watershed management as This may be a result of the larger initial slide a regulatory tool is in its infancy. volumes of road-related landslides, which would cause them to travel farther (Robison et al. 1999). Best management prectices Ltnd accelerated etosion Prevention and Mitigation of Accelerated Erosion in a Managed Coast Many BMPs have been developed to mitigate the effect of forest management activities on accelerated Range Forest erosion. Their breadth of coverage in the forest- The recognition of the link between lorest manage- practice rules of western timber-producing states is ment and accelerated erosion has prompted the a testament to the sheer number of site-specific and development of alternative forest management activity-specific BMPs that have been developed. It practices that prevent or mitigate erosion. Since the is beyond the scope of this chapter to review all initiation of Forest Practice Rules in 1972, and the BMPs developed to prevent or mitigate accelerated 232 Forest and Stream Management in the Oregon Coast Range erosion. Howcver, their efficacy at the n'atershed was harvested, Lew:is (1998) reported an increase scale can be illustrated by reviewin!! r'esearch results in suspended sediment of 188 kilograms per hectare from the Caspar Creek Watershed study. This per year for the six-year period after harvest began watershed study is an cxcellent example of the (1990 to 1996). When cijfferences in sampling advances made to prevcnt and mitigate accelerated methods and streamflow are accounted for, thc erosion from timber han esting by using BMPs. relatir.c increases in excess suspended sediment load The Caspar Creek study is unique becaLrse it pairs are 212 percent to 331 percent for the South Fork a study carried out in the 1960s, before thc advcnt study and 89 percent for the North Fork stucly of forest practice rules, with anotlrer carried out ir.l (Ler,,"is 1998). Thcse data suggest that harvesting the 1990s nsing contemporary BMPs. The ex- using BMPs reduced the potential increase in perimer.rtal r,r'atersheds are the North Fork (473 suspended sediment load by 2.4 to 3.2 times. hectares) and the South Fork (424 hectares) ofCaspar The difference in the rate of accclcrated erosion Creek, krcated in northern California in the redwood between the South Fork and North Fork watersheds region approximately 7 kilometers from the Pacific is attributerl to a diffcrcnce in the han,esting Ocean. The watersheds, originally krgged between practices or BMPs used. In both cascs, ror,rghly the 1860 and 1904, grew back to young-growth redwood same volume of timber was rcmcx'ed. The South (Str1tLoin surperairc;ls) and Douglas fir (Pscudotsugo Fork was selectiveiy harvested to rcmove 65 percent nt anzirci i) before contemporary treatments began. of the volume while the North Fork was 48 pcrcent Both lcccivc approximately 1,200 millimeters o1 clearcut. Three kinds of BMI's were used in the precipitation annually coming as rain dr-rring the North Fork watershed, associated with thc har- \,\'inter months, and both arc underlain by marine vesting systen, the road system, and strean sedimentary rock (Wright ct al. 1990; Lewis 1998). plotection. During the original studv, the North Fork was The South Fork n'as harr.ested using a ground- usecl as a control and the South Fork was harvested. bascd tractor svstem where location of the skid Iioads were cor.rstrlrcted in the Sorrth Fork in 1967 roads for the tractors was at the discretion of the ancl harvestir.rg took place between 797L and 7973. tractor operators, meaning that a tractor \^'as driVen Approximately 65 percent of the timber volume n'as to r.irtually everv log that u'as harvested. This relectirel) logHed Lrsing lr.)cli\r:; 75 percenl of lltc han'esting method r'r'ould have lelt a quarter to a roads n'ere within 60 meters of the stream. As a third of the n'atershed disturbed (Froehlich l976; result, 15 percent of the r'r'atershed r,r'as compacted Froehlich et al. 1981) and 15 pcrcent o{ the South (Wright et al. 1990). Fork was compacted by roads, landings, and skid Harvesting for the seconcl study was carried out trails (Wright et al. 1990, Lcu.'is 1998). The North between 19B9 and 1992. In this study, the South Fork Fork was harvested using skvJine-yarding systcms, was the control and the North Fork was harvested. which are hlgh-lift, long-span, cablc-logging Because of limits on the size of clearcuts and systems that result in less soil disturbance (3 to 4 adjaccncy constraints, only about 48 percent ol the percent; Rice et al. 1972), and virtually no North Fork area was harvested. The harvesting r'r'as compaction. clcarcut sih.iculture, using, for the most palt, The second BMP that is credited with reducing skylineJogging systems. Half of the han-est units accelerated erosion is improved road systems. The u'ere broadcast burncd and half were not. Ground tractorlogginti system for the South Fork requirecl disturbance for roads, landings, skid trails, and fire that roads be constructed low in the u'atershed ancl lines was limited to 3.2 percent of thc watcrshed. close to the stleam. Over 70 percent of tl.re roacl Streams were buffered by lcaving a protcction zone system in the South Fork was built r,r,ithin 60 meters from 24 to 60 mctcrs in width (Lewis 1998). of the stream. The skyline systems used in tlre North Rice and others (1979) four.rd increases in the Fork watelshcd requirc fcwer roads, and these roads suspended sediment load in the South Fork are constructed on ridges or high in the watershed watershed of 1,403 kilograms per hectare pcr ycar away from streams. Only 3.2 percent of the North after road construction in 1962 and 3,25,1 kilograrns Fork was in roads, landings, and skid trails and was per hectare per year for the fir.e years after timber considcred compacted (Lewis 1998). harvest began (1971 to 1976). When the roles of the Thc third BMP used to reduce accelerated erosion two watersheds were reVersed and tl-Le North Fork was stream protection. The S()uth Fork n'as Landslides, Surface Erosion, and Forest Operations in the Oregon Coast Range 233 harvested without any formal buffer strips for the Mapleton headwall risk-rating system that is stream protection and with no equipment-exclusion used to rate the risk associated with debris-flow- zones around streams. In contrast, when the North initiation sites within the Mapleton District of the Fork was logged, selectively logged buffer strips Siuslaw National Forest (Swanson and Roach 1987). from 24 to 60 meters in width were left adiacent to The system stratifies headwalls into hazard classes the streams, and heavy equipment was excluded that describe their likelihood offailure (Martin 1992). from these streamside areas. The ODF has used a definition of a high-risk site as Manuals are available for BMPs for forest a geomorphic hazard rating for potential initiation practices and erosion. The most comprehensive are sites of debris flows. Their criteria for defining a those associated with a given state's forest-Practice high-risk site include: (1) active landslides, (2) rules (see Oregon Department of Forestry 1995). uniform slopes steeper than 80 percent, (3) head- Even states that lack regulatory programs for walls (convergent slopes) steeper than 70 percent, forestry have manuals that describe BMPs for (4) abrupt slope breaks where the steeper slope forestry. In addition, Weaver and Hagans (1994) exceeds 70 percent (mid-slope benches), and (5) have authored a good manual on BMPs to reduce inner gorges with side slopes steeper than 60 erosion from forest roads. percent. Recognizing slope steepness and convergence ratings must be Best management practices dnd and applying geomorphic hazard done in the field, standing on the site of interest and ilebris slides dnd flou)s evaluating iis potential to be a debris-slide or -flow- With recognition of the link between forest initiation site. However, computer algorithms can management activities, especially timber harvesting, use digital elevation models (DEMs) with one- and debris flows (Bishop and Stevens 1964; dimensional stability-analysis and rainfall-runoff Swanston and Swanson 7976) carne the need to relationships to predict the locations of collur.ial prevent or mitigate harvest-related debris flows. hollows (Montgomery and Dietrich 1994; Wu and Therefore, BMPs have been developed over the Sidle 1995). Evaluation of the efficacy of these years to prevent or mitigate the occurrence of debris models has shown that model output is generally flows associated with timber halvesting. consistent with empirical findings (Dietrich and The first step in applying BMPs is recognition of Sltar 1,997; Montgomery et al. 1998; Sidle and Wu where landslides are most likely to occur. Iden- 1999). The efficacy of these models illuminates the tification of the attributes of ilandslide-prone terrain importance of topography to the initiation sites for has been an output of most landslide inventories debris-flow failures, and modeling allows the (Bishop and Stevens 1964; O'Loughlin L972; locations of potential high-risk sites to be evaluated Megahan et al. 1978; Cresswell et al. 1929). over a large land area with a minimum investment Repeatedly, landslide inventories have shown that in time and resources. debris flows initiate on steep slopes and in Of course, the efficacy of the computer algorithms convergent topography. In general, debris slides do to predict the location of potential debris-slide- not occur on slopes less ihan 50 percent, and most initialion sites is only as good as the data they use occur on slopes greater than 80 percent. In addition, or the DEMs being interrogated. Large scale DEMs, debris flows are identified with topographic features such as 3O-meter DEMs, can obscure micro toPo- or landforms that are convergent or concave, such graphy and underestimate slope steepness, both as incipient stream channels, headwalls, hollows, characteristics that are important to landslide- swales, and deep, v-notched gullies. hazard assessment. Robison and others (1999) found The identification of key landscape attributes that a poor correlation between site-specific slope are correlated with initiation sites has allowed the measurements and those derived from 30-meter development of formal risk ratings for debris flow DEMs and they also found that, on an area basis, failures. These risk ratings are determined on the 3o-meter DEMs overrepresented moderate sloPe basis of aggregations of geomorphic and topo- terrain (< 50%) and underrepresented very steep graphic features that are numerically weighted and terrain (> 70%). summed. One such risk-rating system (Environ- Once initiation sites of potential debris-flow mental Protection Agency 1980) was the basis for failures have been identified, a number of BMPs can 2\A lorp.t ancl 5l.e.rm M,rnJ6enprl in rho O'cgon { u.t.l R,r-}-e be prescribed to mitigate the increase in failure rate During the ODF study, invcstigators checked for' associated with timber harvcsting. These BMPs are compliance r'r'ith tlre Oregon Forest Practice Rules designed to maintain more root biomass aftel timber at sites rvhere debris-flow failures initiatecl. The harvest than woulcl be present after a typical clearcut specific forest practices considcred included ljnear silr, icullrrre prercriptiorr. Tlrc cile-\pt-(iIic Pre- gouging during cable yarding, construction of scriptions range from a ban on harvesting to clearcut tractor skid trails, and accumulations of krgging silviculture that accelerates the growth of young slash. Irr no case had the forest practicc rules been trees. At one cnd of the continuum is a complete violated, and none of these factors was assocrated ban on timber harvest on large areas that contain with the irritiation of debris flon's (Robison et al. very steep slopes. This is follc'xt'ed by headwall- 19e9). ieave areas, which are patches of uncut trees that A final concern is the effect of timber harvesting are lcft on the high-risk sites or hollows and on very on deblis-flor,r, composition, especially that timber steep skrpes lr,ithin a harvest unit. These leave areas han-esting would result in decreases in the alnount can range in size from scr.eral hectares to several and size (cliameter and length) of )arge wood in tens of hcctares. Other possible prcscriptions inclucle debris fkrws. What little rescarch has been done on minimizing site disturbance during harvest to this subject does rot support that hypothesis. ln firct, maintain understory shrubs or brush to provicle harvested terraln exl.ribits a lcgacy of large wood in reinforcement after harvest. For the same reason, debris flor,r's (May 1998). Howevct if iarge wood is site preparation by either broadcast burning or not allowcd to glow adjacent to dcbris-florv paths, silvicultural chemicals u'ould be cliscottraged alter therr thosc paths that fail will not fill r'vith large han'est. Drastic distr,ubance and gotrging of the soil wood. Civen that timber harvest has generally on these sites during harvest are also discor.rraged rednced the amount of large wood in and available to minimize effects on subsurface prefercntial-flon, to streams and that large n'ood is an integral paths and soil strength. On some sites, an in- component of ;rquatic habitat ar-rd the storage ancl termediate brnsh-releasc spray also would bc routing of sediment, it is imlrortant tbat debris flou's discouraged. associated with tirnbcr harvest incJude largc woocl At the opposite encl of the prescription con- (see Chapter 3). Therefore, although site-specific tinuun is \.ery intensive treatment of the site, BMPs have not been dcveloped for debris-flow including clearcut harvest, site prcparation, planting composition, it r,r'ould scem reasonable to reqllire superior stock, and brush treatment. A window of both an increase in the size and extent of protected vnlneraL.rility after harvest is recognized as a part of riparian zones and the establishment Lrr non- this tleatment (Ziemer 1981). Holvevet instead ot harvesting of conifers aJong selected debris-flow providing more reinforcement during that period paths. of vulnerabilitt this prescription narroh's the r'r'indorl.' by encor.rraging rapid of vulncrability Watershed nlanagement and debris rcforestation of the site and fast growth of the stock flows through intensive forest management. A growing body of researcl-L ancl tlrought proposes The efficacy of only one of these BMPs for a new paradigm for the managemerrt of landslide- treatment of initiation sites has been tested. Martin prone forestecl terrain. Thc new paradigm does not (1997) found numerical differenccs fol the fai.h-rre prcscribe BMPs alone to mitigate the effect of timbcr' rates of hcadr'r'alls in r-trur.ranaged foresi (36 percent), han'esting on debris flows and aquatic habitat. clearcut (38 percent), and headwall-leave area Ilather, it is basecl on the prescription of forest within a clearcut (41 percent). Thcsc numerical managcment activities on a larger scale in both sprace differences u'ele not statistically significant. The and time. This watershed-scale prescription of forest efficacies of other BMPs have not been tested. That mana[Jement activities is designed to make timber is, the effects of differcnces in site clisturbance, site harvest morc nearlv mimic a natural disturbance preparatjon, planting-stock qualit)., and inter- regime. Thc theoretical framer,r'ork and the basis for mcdiate treatments have not been tested; all havc this neu' paradigm is presented by Reeves et al. been lumped into thc category of "clearcut" in (1995) and in Chapter 4. landslide inventories. Anadromous salmonids evolved in ecosystems that were dvnamic in both space and timc. The Landslides, Surface Erosion, and Forest Operations in the Oregon Coast Range 235

dynamic nature of aquatic habitat is the result of a including the episodic delivery of large amounts natural disturbance of regime that is, in turn, the result sediment and large wood. of stand-replacing wildfires, which leave the terrain Several prescriptions at the landscape scale might r.ulnerable to landslides, whose frequency, . size, and bring a managed disturbance regime to more closely spatial distribution on a landscape scale create a resemble a natural disturbance regime: (1) lengthen range of channel conditions. When a landslide- the rotation ages to t 150 years; (2) reduce the producing stolm occurs subsequent to a wildfire, proportion of the landscape in young stands the resulting widespread occurrence of debris flows (Robison et al.'1999); (3) aggregate harvest units transports large qr-rantities of wood and sediment lnto one area in a watershed instead of scattering mto the stream channel. The episodic delivery of them across the landscape; and (4) leave debris_flow sediment to stream channels causes them to ihift paths with a biological legacy of large wood. The between periods of aggradation and degradation. debris-flow paths that have the strongest likelihood Because the quality of aquatic habitat is i function of delivering large wood to sheams can be identified of channel conditions, it will also shift, with the (Benda 1990) and conifers should be established, optimum_in complex aquatic habitat occurring allowed to become large, and left to enhance the between the extremes of aggraded and degradeJ biological legacy of subsequent debris flows. states. Several attributes characterize a natural dis_ turbance regime in the Oregon Coast Range (Benda Summary 1994). First is disturbance, such as a stand_ Erosion is a natural process that occurs even in replacement wildfire, which has an average size of pristine forested terrain in western Oregon. The approximately 30 square kilometers and a return dominant erosional form is mass movement and, rate of 200 to 300 years. Using these figures, Benda in particular, debris flows. Surface erosion is (1994) reports that in a natural regime, about 15 to virtually nonexistent. Forest management activities, 25 percent of the forested landscape will be in an especially timber harvest, cause accelerated erosion early-successional state at any gir.en time. The in the forms of surface erosion from forest roads and wildfires leave standing ancl down lar.ge wood, a an increased occurrence of landslides from clearcut biological legacy that becomes incorporated into harvest units and forest roads. debris flows after the fire. Together these attributes The BMPs used over the Iast three decades have lead to the pattern of channel conditions and reduced the amount of accelerated erosion that associated aquatic habitat conditions in which occurs as a result offorest management. These BMps salmonids evolved. include protection zones around streams or buffer The disturbance regime created by contemporary . strips that are used to limit the amount of dis_ forest management practices differs in thesl turbance to streambed and banks. Contemporary attributes. First, the size of the disturbance made harvest systems iriclude more tong-span, highJiit by a typical harvest unit is much smaller, and its cable systems that require fewer roads and leave recurrence is much more frequent, such as every 50 less ground disturbance than older systems. Finally, to 100 years. As a result,35 percent or more of the density of forest roads has been reduced, ani contemporary managed forests may be in an early_ the roads are better located, built, drained, and successional state at any given time. Second, gir.en maintained. Even so, the opportlrnity for accelerated tire purpose of timber harvest, managed forests eloslon rematns. historically have not left the amount or size of One objectir.e of BMPs for surface erosion and standing and down wood that wildfires have. clebris flows from roads is to limit accelerated The landrc.tpe corrdition, i1 a conlemporary eroslon to the extent practicable and feasible. In managed forest may not be capable of providing contrast, a different perspective is being taken for the range of channel and habitat condiiions tha"t debris flows from within harvest units. According ultimately result in high-quality aquatic habitat. For to new data and a new line of thinking regarding these conditions to develop, the forested landscape the role of natural disturbances in the formation o] must be managed in such a manner that tire aquatic habitat, debris flows are the vectors that disturbance regime more closely resembles the bring large woocl and sediment, the basic building natural disturbance reg;ime of the Coast Range, blocks for aquatic habitat, into the stream. Rathei 236 Forest and Stream Managcment in the Oregon Coast Ran8e than trying to prevent all debris flon's from harvest Benda, L. E.. and T. Dunne. 1997b. Stochastic forcinli of units, it may be more important and realistic to sediment routing and storagc in chamrel netu,orks. institute forest management schemes in which l{dl./' Rcsorr'..s llcscd,'./l 33: 2E65 28E0. Bcncla, L. E., D. Miller, T. Dunne, Ll. H. Reeves, i,rrrd K. resulting debris flows mimic natural processes lvith J. J. Agee. 1998. Dynamic landsc.rpe svstems, pp.261-288 in re6;ards to timing and large-wood composition as Ritter Ecolo.qv ancl Mnnngentc t: L.ssotls.fftint t'lt Pncifit much as possible. Codsfa/ Eco/'r'.qio,?, Il. J. Niriman and R. E. Bilby, ed. Forest management can affect aquatic habitat by Springer-Verlag, Nerv York. cl-ranging the timing and composition of debris Benda, L., C. Veldhrrisen, D. Millcr, and L.R. Miller flows. Timber harvest changcs the disturbance Undated. S/op. i'sinbilitv 0 Ll Forcst LdnLl Mnnngrrc, n pattern of a forested landscape by introducing Printr ntrtl Ftcltl Cuidd. Earih Systems lnstiiute, Seattle rotation ages that are shorter than the interval Bennett, K. A. 1982. Etlects of Slash Bru'ttittg ott Surfact SLtil between naturally occurring wildfilcs and by ErosiLn RnLcs it1 tht Oregotl Const l

Benda, L.8., ancl T. W Cund,v. 1990. Prcdicting dcposition debris avalanche initiation. Cnrndia' C.o t rclltl i c n l

of deLTris flo*'s irr mountain ch.rnncls. Corddin, J ot | | nal I9(2): 1 67'17 4. Ccatuclt i(al laun 1127:1l)9-11,7. Benda, L. E-, ancl T. DLrnnc. 1997a. Stochastic forcing of sedimcnt supply to channel netwolks from landslicling ancl dc'bris flow. lv4l./ il.so/r/.cs .i{.s.rvch 33:2849-2863. Landslides, Surface Erosion, and Forest Operations in the Oregon Coast Range 237

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P o c e s s c Debds initiation experiments using divcrsc Hr1 tlrol Lt 11ic r s 12: 943-955. flow Morgan, D. J. 1995. Stt olgth Pnmtlettrc inll Tf ilrinl Strct1gtll irydrologic triggcrs, pp. 1-11 it.t DcDris-r{aw Hnzads Tcsting o;f Soils of the O,'.gor? CodsF Rdr?9.. Engineerjng Mitigntk)u: Mrchnni.s, PreLliction, nud Asscs-sri.rl, C. Report, Dcpartnent of Civil Engineering, Oregon Stntc Chen, ed. Procecdings of First InternatLonal University, Corvallis. Confercnce, San Francisco CA. Morrison, P H. 1975. Ecological and geomorphological Reneau, S. L., and W. E. Dietrich. 1987. Sizc and location of corscquences of mass movements in the Alder Crcek colluvial landslides in a steep forested landscape, Pp. watershecl and jnlplications for forcst lancl 38-48 in E/osior/ dr'l,7 S inrcntafit)]t it1 thc Ptlcific Ril1t,11.L. management. BA thcsis, Universi5/ of Oregon, Eugene. Beschia, T. Blimr, G E. Grant, G. G. Icc, and F..J. Nash, D. 1987. Chaptcr 2: A comparative revicw of limit Sh'anson, ed. IAHS pub)ication No. 165, Wallingtord equilibrium metl.rocls of stability analvsis, pp. 1l-75 in U,K. Shp( Stnbilit!1.M. G. Anderson and K. S. Ilich;rrds, ed. Reneau, S. L., and W. E. Dietrich. 1991. Erosion rates in tllc John Wiiey & Sons lnc., Nen'York NY southefll Oregon Coast Range: evidence for an O'Louglrlin, C. L. 1972. An ln'ocstigntittll of tlrc Stabilitlt of Illt equilibrium between hillslope erosion and sediment SfeL:plnnd Forest Soils it1 tltr C00sl Mo fiLnils, Soufl].L]est yieId. Earth Strfacc Protsscs nnLl Lanclfontrs 16: 307-322. British Colrunbia. PllD dissertation, University of British Reneau, S. L., W E. Dieirich, M. llubin, D J. Donahue, and Coiumbia, Vancor-rver A. T. Ju11. 1989. Analysis of hillslope erosi(n rates using O'Loughlin, C. L. 197:1. The effect of timber rcmoval on the dated colluvial deposits. /our,rd1 ofC.?/ogv 97: 45-63. stalrility of forest sorIs. Journal of Hydrology (NZ) 13(2): Rice, R. M., L. S. Rothacher, and W F. Mcgahan. 1972. r, F i' I hn r\ an 1.21. r34. I 'na aon:eq.rcn. e- of timber c.iir]g: O'Loughlin, C. L., and A. J. Pearce. 1976. Tnfluence of appraisal, pp. 321 329 in Procccdirgs rf a St/nlpasitnt on Cenozoic geology on mass lllove1alent and scdinent "WntercheLls ifi Tt'tltlslfio'," S. D. Csallany, T. G. yield response to forest removal, Nodh Westl;rnd, Nen' McLaughlin, ancl W D. Striffler, ed. Fort Collins CO. Zedand. Brrllefin of tllc Int?nnfit lttl Association of Rice, R. M., F B. Tilley, and P A. Datzma]],.1979. A F,ngitrerittg and Ccology 11: 41-46. Wnfersftcd's Rcsporrs-e to Loggitlg and l

Schroeder, W L., and D. N. Swanston. 1987. Apltlicotion of S\,vanson, -F-. ]., M. M. Swanson, and C. Woods.1977. Geotechnicnl DLita ta R?sourct Pldltning i Southcnst Itlocntorll of Mnss Etosion in lhe Mdplefott Ranget Dish'ict, ,4lnskn. General Technical I{eport PNW-198, USDA Siuslnio National FLn'csf. Final liePort, Siusla!v National Forest Service, Pacific North\a'est Rcsource Station, Forest and Pacific Northu'est Forest antl Range Portland OR. Experiment Station, Foresiry Sciences Laboratory, Sessions, J., J. C. Balcom, and K. Boston. 1987. I{oad Corvallis OR. location and construction practices: cffects on landslidc Swanson. F. J., Ii. L. Fredriksen, and F M. McCorison. frequency and sizc in the f)regon Coast Range. Wcsil/,l 1982. Material transfer in a western Oregon forested \,\'aiershed, Chapter 8, pp. 233-266 in AtralL7s-is I ot L r nal Ltf ApTt I i c d F ot c st I v 2(1) : 11'9'1.21. { Sidle, R. C., and D. N. Swanston. 1982. 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Waldron, L. J., S. Dakessian, and J. A. Nemson. 1983. Shear resistance eniancement of 1.22-meter diameter soil cross sections by pine and alfalfa roots. Soil Science Saciety of America ]ournal 47:9-14 Weaver, W. E., and D. K. Hagans. 1994. Halldbook for Forcst anrl Ranch Roads.Mendocino County Resource Conservation District, Ukiah CA Wemple, B. C. 1998. Inoestigdtions of Runo]f;f prodttction and Sedine tation an Forcsf Roads. PhD dissertation, Orel;on State University, Corvallis. Wright, K. A., K. H. Sendek, R. M. Rice, and R. B. Thomas. 1990. Logging effects on sheamflow: storm runoff at Caspar Creek in northwestern California. Wafcl Reso rces Research 26(7): 7657-1667. Wu, T. H., W P McKinr.rell III, and D N. Swanston. 1979. Strength of tree roots and landslides on Prince of Wales Island, Alaska. Canadian Geotechnical lournal l,6: 19 33. Wu, T. H., P E. Beale, and C. Lan. 1988. In-situ shear test of soil-root systems. A SCE lournal of Geotechnical E n gin e e t in g 114(12) : 737 6 -139 4. Wu, W, and R. C. Sidle. 1995. A distribured slope stability model for steep forested basins. Water Rlources Research 31 2097 2110. Ziemer, R. R. 1981. Roots and the stability of forested slopes, pp. 343-361 in Erosion and Sedhnent Transport i Paci.fic Rit Steeplands. IAHS Publication No. 132, Wallingford U.K. Forest and Stream Management in the Oregon Coast Range

edited by

Stephen D. Hobbs,lohn P. Hayes, Rebecca L. lohnson, Gordon H. Reeaes, Thomas A. Spies,lohn C. Tappeiner II, and Gail E. Wells

Oregon State University Press Corvallis The paper in this book meets the guidelines for permanence and durability of the Committee on Production Cuidelines for Book Longevity of the Council on Library Resources and the minimum requirements of the American National Standard for Permanence of Paper for Printed Library Materials 239.48-198+

Library of Congress Cataloging-in-Publication Data Forest and stream management in the Oregon coast range / edited by Stephen D. Hobbs ... let al.1. P. cm. Includes bibliographical references. ISBN 0-87071-544-5 (alk. paper) 1. Ecosystem management--Oregon. 2. Forest ecology-- Oregon.3. Stream ecology--Oregon. I. Hobbs, Stephen D QH76.5.O7 F67 2002 333 .75'09795-dc77 2002002680

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