landslides, Surface Erosion, and Forest Operations
in the Oregon Coast Range
Arne E. Skaugset, Gordon H.
I
Reeaes, nnd Richard F. Ketm
ntroduction
Accelerated erosion from forest management
activities, particularly timber harvest, has long been
a focus of environmental concern. Associated with
the accelerated loss of soil is an accelerated loss of
soil nutrients, which ultimately may reduce the
long-term productivity of intensively managed
forest soils. As erosion processes in foresied
landscapes of western Oregon became better
understood, especially the patchiness of erosion by
shallow, translational landslides, the focus changei
to effects on downstream values. With the tisting of
salmonids as threatened or endangered species,
concern has become more sharply focused on
aquatic habitat in headwater streams draining
managed forest landscapes.
After a briefsection providing the physical setting
_
of the Oregon Coast Range, we discuss the erosion
processes found in forested landscapes of western
Oregon, with an emphasis on the factors and
environmental conditions that initiate shallow,
translational landslides. This section concludes with
a synopsis of what is known about the relationship
between these landslides and aquatic habitat.
The next section discusses how contemporary
and legacy forestpractices in westem Oregon forests
have affected accelerated erosion. The emphasis is
The last section of the chapter presents the
practices currently used to mitigate the effects of
forest management activities. The regulation of
forestpractices to mitigate erosion has evolved over
the last several decades from the application of
water-quality standards to the use of best manage_
mentpractices (BMPs). Currently, these practices are
rn a bansition from site-specific BMps to watershed_
or landscape-scale practices that fit into a construct
of ecosystem management. This section presents
site-specific BMPs that have been developed to
reduce accelerated erosion, with emphasis on those
practices usecl to locate high-risk, landslide-prone
terrain and to mitigate against the occurrence of
shallow, translational landslides. The section
concludes with a discussion of BMps that could be
added to the current prescriptions, and how these
should be used on the landscape to ensure that
watersheds function optimally for aquatic habitat
while allowing for forest management and the
production of solid wood proclucts.
Physical Setting of the
Oregon Coast Range
on the observed relationship between forest
Geology
management practices and the increased occurrence
The Oregon Coast Range is composed of rocks
uplifted by the collision of two plates. The ocean
floor, which is the Juar.r de Fuca plate, is subducting
under the terrestrial NorthAmerican plate, causing
the Coast Range to rise by as much as 5 millimeteri
per year (Kelsey et al. 7994, 7996). The presence of
of shallow, translational landslides. Tlris section
concludes with a discussion of how forest management activities might affect the occurrence, size, and
composition of debris-flow deposits.
213
211
Forest and Stream Managenrer'rt ir'r tl're Oregor.r Coast Rang,e
small fatrlts and folds throughout the overriding
plate results in spatially variable uplift; the late of
uplift varies by as much as an order of magnitude
(Kelsey ancl Bockheim 1994). At the same time,
erosion is lol'ering the range by an avelage of 0.03
to 0.11 nillimetels per year (Rencau et al. 1989;
Reneau and Dietrich 1991).
The rocks of the Coast Range are predominantly
layered ancl interbcddcd sandstones and muclstones
formed from ocean scdimcnts bcfore uplift, ancl
there are rnany intlr-rsions of basalt into this matrix
(Kelsey ct al. i996). Some of these basalts r'r'ere
already in placc rvhen the seclimentarv platform
formed on thc former ocean floor and the two rock
typcs uplified togetl-rer (Orr et al. 1992). Most of the
basalt in thc northem Coast R;rng;e ancl some of the
coastal headlands, hon'ever', originatecl from
terrestrial volcanoes east of the Cascade Range (C)rr
et al. 1c,lcl2). Because basalt is less susceptiblc to
erosion than sedimentarv rock, many of these areas
of intrusive basalt are nou' thc highcst elevations in
the Coast Range. This differential susccptibility to
erosion also means that sedimentary areas are morc
highly dissected than the basalt areas. The pro-
portions of weaker, intcrbedded rnuclstones, as r,r,'elJ
as variations in strike and dip, affect tlre stabilitr,r
and erosivitv of the bedrock.
Much of thc eastern bor.rndary of the Coast Range
is the Willamcttc Vallev n'lriclr is a fault-bJock valley
of alluvial and llrcustrine sediments. The volcanic
Cascade Rarrge borders the Coast Range to the
southeast, ancl tl.re volcanic ancl metamolphic rocks
of the Siskiyou Mountains form the southern
bouncl;rry (Figure 9-1 in color section folkrwing page
212). The Coast Range is bounded on the wcst by
the Pacific Ocean.
Hy drology
The Coast Range has a Medjterranean clirnate, in
n'hich most of the precipitation faJls as rain between
October and April (Figure 9 2 in color section
following page 212). During these months, precipitation exceeds er.apotranspiration, causing
rttnoff as streamflow. Flon May through September,
\4,armer temperatures and long clays prornote
\rigorous Plant growth, and eVapotranspiration
exceecls lainfall. This seasonal shift in weather
results in a signature annual hydrograph for rir.ers
of the Coast Range, in r'r'hich nearlv all streamflon
occurs in the fall through the spring (Figr-rre 9-3 in
color section following page 212). Extended
droughts in the summer can reduce streamflow to
extremelv low levels.
Precipitation falls predominantly as rain in the
contributing areas for most streams, although some
mountain peaks in the Coast Range can receive
frequent sno',r,fall during the winter. Most rain falls
cluring frontal storms with intensitics of 0.5 to 1.0
centimeters per hour (Figure c)-4 in color section
follou'ing page 212). It is the extended periods of
rainfall that result in peak stlcamflon's during the
$'ettest months; hor,r'ever, peak flows are largest
when an intcnsc rainfall occnrs during such periods
of extended rainfall orn4ren rain fa Lls on sno\\,.
Landslides and Surface Erosion in
Unmanaged Coast Range Forests
Erosion processes can be divided into thrcc general
groups: slrrface erosion, channel scour, and mass
movement (Brown 1980). Surfnce erosion is defined
as the movement of individr.ral soil particles.
Sr'r'anson and others (1982) list the surfacc-erosion
processes that occur in unmanaged, forested
watersheds as dry ravcl, snow- and ice-indr.rced
rnoVement, and soil mor,'ement acconrpanving root
throw. Scdiment btrdgets show that surface erosion
is negligiblc in unmanaged, forested watelsheds of
western Oregon (Dietrich and Dunne 1978; Slvanson
ei al. 1982).
Channel scour occurs within the stream channel,
as soil and rock material from the streambed and
stream bat-tks becomes suspendcd scdiment and bed
load, which are available to the stream to transport
(Brown 1980; Swanson et al. 1982). In gcntly sloping
watersheds in the absence of debris flows, char.rnel
scour is the dominant proccss responsible for
eroding stream banks and transporting streambed
and bank material or.rt of a watershed and further
downstream.
Mnss soll tltore rctrt is the tyPe of erosion that
dominates secliment budgets for unmanaged,
humid, temperate, forested watersheds (Swanston
1978). Mass soil-movement processes that occur in
lrruranaged, forested watersheds of the Coast Range
include: soil creep, earthflows, deep-seated land-
slides, ernd shallow-translational landslides
(Dietrich and Dunle 1978; Swanston 1978; Swanson
et al. 1982).
Soil creep (Figure 9-5a) is a slon (millimeters per
year) downslopc movement of the entire soil profile
Landslicles, Surface Erosion, and Forest Operations
(,r)
in
the Oregon Coast
Range 215
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
k)
Clay
Slump
Clay
CIay
Earthflow
Figlure 9 5. Thrce cJii%rent types of nrass soil
movement processes tl.rat ()ccur in the Oregon Co.rst
Range inclucling (a) soij creelr, (b) earlhflows, and (c)
rolational landslides (Brown 1 980).
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
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,
fcatures;rt thc toe.
Slttllou, trnnslttiotrLtI lottdslitles rn the Co;rst Range
consist primarily of debris slides and clebris flovr,s
(Figure 9-6). Tl.re primary difference between a
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
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
(Cruden ancl Varnes 1996). Debris slides deposit at
the foot of the failure scarp and do r.rot mobilize and
move {arther downslope. Additional \ .,ater cor.rterlt
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 landscape, 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.
05
mete rs
Figure 9 6. A dcbris flow with associatccl downslopc
scour (Humphrcy 1981).
Shallow, translational landslides are initially
small, locally discrete failures that can move very
rapidly. They occur on steep slopes (> 50%) and
involve thc movenrcnt of shallow layers of soil
dor'r'nslope. Within seconds or minutcs, they can
move tens to hundreds of meters. Thev flon' down
stream channels and deposit in lower-graclient,
higher-order stream channels.
Debris flows are the mass movement processes
of most interest and concern relative to both public
safety and impacts on aquatic resources (Pyles et
al. 1998). Landslide inventories show them to be
ubiquitous across the landscape in the Coast Range
(Robison et al. 1999; see Table 9-1). Although
earthflows and rotational slurnps may dominate
sedimentbudgets within a givcn watershcd or along
a particular stream reach (Swanson and Swanston
lq77r, dcbris flow' domin.rlc qspsipll p19qsss6's
across the Coast Range. As the dominant erosion
proccss in western Oregon, they or,.erwhelm the
sediment budget of unmanaged watersheds
(Dietrich and Dunne 1928; Swanson et al. 1982).
Debris flor,r's are critical to public safety because of
their ubiquity, size, speed, and extent (Pyles et al.
1998), ancl they dominate concerns about aquatic
habitat quality (Sullivan et al. 1987; Swanson et al.
1987; Reeves et al. 1995; see Chapter 4). Given their
1987;
Reneau and Dietrich 1987; Montgomery and
Dietricl-r 1994), n'hich are almost always thouliht of
as being synonymous with topographic convergence or topographic swales (Montgomery and
Dietrich 1994). In fact, unchanneled bedrock hollou's
may exist on relatively planar slopes that exhibit
little or very subtle topographic expression as well
as slopes with distinctive topographic sr,r'ales.
Landslide inventories have reported that as many
as half of the inventoried landslides initiate on either
planar slopes or slopes with very little topographic
expression (Ketchcson and Froehlich 1978; May
1998; Robison et al. 1999).
Bedrock hollows are characterized by bedrock
surfaces that are concave both across-slope and
downslope (Sidle et al. 1985) and parabolic in cross
section (Burroughs 1984), witlr the long axes
extending downslope (Swanson et al. 1982). This
shape causes subsurface groundwater to converge
toward the hollow (O'Loughlin 1986; Dietrich et al.
1982; Reneau and Dietrich 1987; Montgomery and
Dietrich 1994), as does the accumulation of
colluvium (Sidle ei al. 1985). The bedlock depressions are themselves created by repeated
landsliding (Alger and Ellen 1987), and tl.rey
undergo cycles of failtrre followed by filling wiih
colluvium (Sidle et al. 1985; Benda and Dunne
1997a). The material that refills the hollows comes
from hillside erosion that is primarily due to soil
creep (Swanston and Sn'anson 1926), to the biogenic
activities of animals and plants (Swanson et al. 1982),
and to surface erosion (Swanson et al. 1987). The
susceptibility of the holkrws to landsliding during
rainstorms increases over time because of this
colluvial in-filling (Dietrich et a1. 1995; Benda and
Dunne 1992a; Dunne 1998).
Bedrock hollows filled with collur.ium arc found
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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218
Forest and Streanr M.rnaglcmenl in the Oregon Coast Range
1985; Tstrkamoto and Minematsu 1987; Crozier et
al. 1990). Their density rraries from 0.1 to 2.0 hollor'r's
per hectale dependir-Lg upon geology, topography,
and climate (Benda et al. undated). Colluvial
hollows are predictably the locations of landslides,
especially where shallor,r' soils or.erlie bcdrock
(Montgomery et al. 1998), as in the Coast Range.
Estimatecl return intervals of landslides for an
individual hollow vary from a feu' hundrecl years
to 15,000 years or more (Sr,r'anson et al. 1987; Benda
ancl Dunne 1992a), although the effect o{ fluctuations in climate on failurc rates is ur.rknown.
Locallv, differences in surficial rocks, such as a
concentration of mudstone laycrs or differences ir.r
ihe physical intcgritv of sandstone, may affect the
rate of accumulation of colluvium (Reneau et aJ.
1989) and the conccntration of subsurface water
(Andcrson et al. 1997).
Mechanistic context for debris flotos
Using the geomorphic context for cleblis fkrn's
clcscribcd above, we can identify the locations that
are n'rost likeJy to be initiation sites. In any given
lar.rclslide-producing storm, less than 10 percent of
sites prone to clebris slides r'r'ill fail (Dunne 1998;
Robison et al. 1999). Ho,,r'evcr, r,^"'ith thc crrrrent level
of knor'r'ledge, it is not possible to prcdict r'r'hich sites
prone to debris slides r.r,ill fail during the ncrt
landslide-prod ucing storm. Howcver, slopestability analysis can bc uscd to gain some insight
into the likelihood that a sitc will ol r'r'ill not fail.
SIop
e
-st ability analrl sis
Slope-stabilitv analysis js a wav to assign
methods can involve onJy a simple balance of folces
or moment equilibrium, or botb could be required.
The slide mass involved can be analyzed as a onc-,
two-, or three-dimensional body. Finally, the failule
surface can be modeled as;r simple circular surface
or as a more complicated ilregular surfacc. The
sir.nprlest ana.lysis is a stress balance on a onedirnensional slicle mass with a reglrlal surface; much
more complex is a rigorous force-and-moment
analysis on an irregulal three-dimensional surface.
To learn more about slope-stability analysis
methods, see Nash (1987).
The sirnplest and most straightforward method
of slope stability analysis is the infinite-slope
method. It is used most often to quantitatively
investi€iate the stability o{ shallor'r', landslicle-prone
forest soils (Consior and Gardncr 1971; O'Loughlin
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 shcar stress of a representative slice
of soil that has a unit rvicltl.r ancl Jength; thr-rs only
soil depth varies, making the analysls onedimensional. Thc gcncralized eqrration for the
infirrite-slope method is pro,,,icled belon' for a soil
in rvhich strength is represented by both cohesion
and friction ancl grotrndwater flor,r'is represented
by steady state seepalle parallel to thc slope
(Abran.rson et al. 1996).
FOS =
c' + tt
ft
coi Ff(l
+ m y'lnn Q'
nl)y,,, + my.,,,]
nt)y
sinBcosB[(l
,,,
In this cquation, FOS is the factor of safety; and
c' and @'are soil-strength parameters; y,,,,y'and
the moist, buoyant, and saturated unit
weights of the soil; /r is soil depth; and lr is a
/.,,/ are
a
numerical rrerhre to the stabil:ity of a slide prone site.
It js a qulrntitative treatment of the stresses acting
on a hillslope or vallev and the strength of the soil
involved. In carrying out a skrpe-stability analvsis,
a factor of safety for the slide-prone site is calculatcd.
The factor of safety is thc ratio of thc forces drivinS;
failure and those resisting it for a given slide mass.
The forces drir-ing failure ale duc to thc donmslope
component of tlre r'r'eight of tlre soil within the slide
mass. The forces resisting failure comprise soil
strcngth and any additional strenfltlr due to plant
roots.
The methods avaiLabie for analyzing slope
stability ranse frorr farirly simple and straightforu'ard to exceedingly complex. Thc analysis
dimensionless dcpth of water ralrging from zero to
onc. The other terms in tl.re eciuatjon are ilh-istrated
irr f igure .)-7. lr't $er'terJl. the c,'ntr ibufiorr of I'lanl
roots to soil strength takes the forn of an apparent
cohesiolr, crl, or root cohesior-r, cr, in the numelator
of the equation that is summed with thc soil
cohesion term, (Siclle et al. 1985).
All sJope-stability .inalysis methods, including
the infinite-slopc mcthod, require the following
infolmation: (1) the location of the failrrre surface,
(2) the mode of failure, n'hich dictates tlre appropriate analysis rnethod, (3) the loading on the
critical failure sr-rrface, (4) the soil strer.rgth p;rra
meters, including the effect of roots, and(5) the pore
\ 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
Figure 9-7. Definition sketch i,or the infinite slope
melhod for 'lope .l.rbilit) ,tn.ily.is.
Slope-stability analysis has been carried out for
naturally occurring debris flows il shallow, residual
soils (Swansion 1970; Consior and Gardner 1971;
O'Loughlin 1972, 7974; Wu et al. 1979; Sidle and
Swanston 1982; Buchanan and Savigny 1990). The
infinite-slope method was used for most of these
analyses. The exceptions were those of Swanston
(1920) who used the "ordinary method of slices,"
and Buchanan and Savigny (1990), who used
Bishop's modified method. Both analyses combined
conventional soil mechanics with limit-equilibrium
methods. Soil strength was determined using
standard engineering tests. Positive pore pressures
were estimated based on the direct response of a
groundwater table to rainfall.
Although a standard analysis of the stability of a
site prone to debris slides using the infinite-slope
method provides a general framework for considering 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
estimate of the absolute stabillty of an individual
site. Therefore, slope-stability analysis has a very
limited application for identifying stable versus
unstable sites.
Considerable research has been carried out in
recent years on two of the critical components of
slope-stability analysis for forested sites prone to
debris slides that have shallow, coarse-textured or
The strength of the forest soils for steep, landslideprone 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,
1.0-meter-deep (1 0.5 meter), loose soils with loW
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
effective normal stress is called the Mohr-Coulomb
strength envelope. Figure 9-8 shows a typical MohrCoulomb strength envelope. This relationship takes
the following form:
'r=/+oi,tanQ'
where 7is the soil strength (kPa); 6.' is the effective
soil cohesion (kPa); oj is the effective normal stress
on the failure surface (kPa); and @' is the effective
internal angle of friction (').
The effective normal stress on a failure surface is
the total normal stress minus the pore water
pressure, which is a function of the depth of the
groundwater table over the failure surface. The total
normal stress is that component of the total soil
weight above the failure surface that is perpendicular to the failure surface. Normal stress is
I
f
l
Normalstress 0
Figure 9-8. A Mohr Coulomb strength envelope.
220
Forest and Strcam Management in thc Oregon Coast Range
expressed per unit area. Thercfore, the MohrCoulomb strcngth enYelope is prcsented in terms
o1 total stress as:
r =.+(o-
-
l-L)tan4
wherc c is the total soil cohesior.r (kPa); o- is thc
total nornal stress on the failure surface (kPa); and
p is the pore tr,ater pressure, lr.hich is the prcduct
of the depth of tlre I\'ater times the dcnsitv of I,r'ater.
(kI'a).
Knowing the loc.rtion of the gxrundwater table
at failure is critical to the stability analysis of steep,
landslide-plone slopes, becausc it is the only l,r'ay
to calculatc the effective stress on the failure surface.
The location of the groundwatcr table anct the pore
pressures on the faihrre surfacc at failure are doubly
critical bccause the groundwater response to rainfall
is the Lrltimate triggering mechanisrn for almost all
landslides on forestlands irr Oregon.
Soil strength is not a theorctical cluantity; values
of soil strength must be cletermined empirically.
There are a variety o{ soil-strengtl-r tests, r'anging
frorn relatively simple lir sltri fjeld tests to conplex
and time-consunling laboratory tests. The accuracy
and precision of the estimates of soil strength
derived from thcse tests are generally proportional
to the time ancl effort put into the tcst. Tlre most
common and accurate soil strength test is a
laboratorv test called the triaxial test. It uses a
cvlindrical soi) sample that is nominally 5 centimeters in diameter and 15 centimeters long. The
sample is placed in a cell whcrc different pressures
or stresses can be applicd to determine the strength
of the soil sample at dif{ercnt clepths. Once the
appropriate str"esses or confining pressures have
been applied, a forcc or an increase in load is applied
to the long axes of the sample until it fails. The
tri;rxial test has scveral aclvantages: (1) it allor,r.s a
failure surface to develop along the wcakest failure
plane in the soil sample instead of forcing an
arbitrary failure surface through the sample, (2) a
wide array of stress states can be imposed on the
-,'il \,rmplc .rnJ (l) the pr,*itirt' pore pres,Lrle in
the sample can be modeled and mcasured.
Estirna tes of soil strength that have been used for
skrpc-stability analysis of stcep, lanclslicle prone
forcstcd slopes with shalkx'r' soils have generally
come frorn conventional tests of soil stlength
(Swanston 1970; O'Loughlin 1972; Sidle and
Swanston 1982; Wu et al. 1988; Buchanan and
Savigny 1990). These conventional tests are carried
out usir.rg standard gcoteclrnical enginccring
pracLices, which include the triaxial test as well as
others. Unfortunately, the resulting estimatcs of soll
strength reflect stress states .rrd test conditions that
are i1[)t telir?se tnfl-oc of shallor,r' soils on steep,
landslide prone forestecl slopes at the time of failure.
The conventional tests tor soil strength are not
representative because they are carriccl out at high
corfining pressures using a standard stress path.
AJtl.rough these conditions are representative of the
stress statcs and the tvpe of loading commoniy
found in geotechnical enginecring, the i/t slil
conditions of a higlr confining stress (i.e., cleep soils)
and an increase in weight or total load (i.e., standard
stress path) do not occur on {orcstecl slopes. The
failure surface for most debris slides is shallow,
roughly 1.0 meter (1 0.5 meter). In addition, the soils
are loose and cohesionless, with lott' bulk densitics
(1.1 to 1.4 megagrams per cubic meter). As a result,
the lr sllri confining stresses for these soils at these
depths are Jow, in the range of 6 kPa t 3 kPa. Another
factor making conventional estimates inappropriatc
for predicting lainfall-inducec{ lanclslides is that the
stress path is not an increase in total load, but rather
a decrease in ttre effective stress. Shallow, transiational landslides occrlr when the effective stress
is reduced by the water table that occurs in response
to rair.rfall.
Soil strength at low confining stresses (i.e., at the
ll .si/ir confining stresses frtr loose, shallow,
cohesionless soils) is r.rot a linear function of effectir:e
stress. For standard soil-strcngth testir.rg at conventionai stress states, the intcrnal angle of friction
for granular soils increases with clecreasing
confining stress (Lambe and Whihran 1969; Mitchell
L976; Hctltz ancl Kovacs 1981). Therefore, triaxial
soil-strength tests must bc carried out using lor,r'
confining stresses for shallow, cohesioniess soils
from steep, landslide-prone forested slopes. Morgarr
(1995) has compiled published soil strength values
tor Coast Range soils; Figtrre 9-9 shorvs a non-lineaL
strength envelopc fitted tlrrough the data points. As
shown ir-r the figurc, the slope of the line or the
ir.rternal angle of frictior.r, q)', decreases arrd the
intercept of the linc, c', increases with increasing
avcrage normai effectivc stress, which is lepresented
by p'in the figure. Lincar regressions of the daia
slron' r'alues as high as 45" for an effectir-c stress of
less than 35 kPa, rvhich corresponds roughly to a
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
Swanston 1982). Values of apparent cohesion due
to roots vary from 1.0 to 17.5 kPa (Sidle et al. 1985).
oro20304050
Fisure 9-9. A sraph ofthe ri'l'rln
,r'n, u,,
the information available for soil "nu",oo"
strength test data
from shallory cohesionless forest soils on steep,
landslide-prone sites in the Oregon Coast Range. The
index p' is the normal stress on the soil sample and q
is an index of the shear stress or strength (Collins
1995).
29'for effective stresses greater than 200 kPa.
The corresponding values of c' range from nearly
zero to approximately 25 kPa. After conducting
triaxial tests of loose, cohesionless soils from the
Coast Range, Collins (1995) also reported a
nonlinear strength envelope and recommended that
strength tests for these soils be carried oltt using
triaxial tests at in sifr effective stress levels. Similarly,
testing a shallow, landslide-prone soil from the San
Francisco Bay area, Anderson and Sitar (1995) found
the strength envelopes to be nonlinear and steeper
at lower confining pressures.
The effect of the stress path on soil strength has
also been investigated. Collins (1995) conducted
strength tests using a field stress path, which
initiates failure by reducing effective stress without
increasing total load, and compared the results with
low
as
those for the same soils using a standard stress path.
Differences in the two stress paths for cohesion and
the internal angle of friction were negligible
compared with the differences that occurred simply
as a result of variability between test specimens and
tests. So although testing at ln slfl confining
pressures is recommended, it is not necessary to use
a non-standard stress path in the test.
Groundw ater resp onse to
ffiinf dll on hillslopes
Using slope-stability analysis to distinguish
unstable sites prone to debris slides from stable ones
requires knowing the location of the groundwater
table at failure, because its location at the point of
incipient instability defines the degree of stability
or instability. The response of the groundwater
regime in a given debris-slide-prone site is the key
to predicting the point of incipient instability as a
function of low-frequency, long-return-period
storms.
Our understanding of the relationship between
rainfall and the development ofa groundwater table
on steep, landslide-prone forested slopes has
der.eloped over the last several decades (Campbell
1925). The current model consists of a two-plrase
system representing a residual soil, or colluvium,
overlying bedrock (Figure 9-10). The soil has high
permeability and is quite porous compared to the
impervious bedrock. Rainfall infiltrates the soil and
moves vertically until it encounters bedrock (or
other less permeable layers); the subsurface flow
then becomes parallel to the underlying slope.
According to this model, groundwater will accumulate and the transient groundwater table will
get thicker toward the base of slopes (Figure 9-10).
Based on this model and the relationships observed
between rainfall and piezometric head, predictive
models have been developed to locate the position
of groundwater tables for a given amount of rainfall
or return periods of storms (Swanston
Root strength
The effect of roots on soil strength is modeled by
adding an apparent-cohesion term, cr, or a rootcohesion tern, cr, to the Mohr-Coulomb strength
1967;
Burroughs 1984). These relationships also have been
included in slope stability models to help predict
the location of landslide-prone terrain (Schroeder
and Swanston 1987; Hammond et al. 7992).
222
Forest and Stream Management in the OreEjon Coasl RanSe
/^(/l
/
l.kiltr,.,,
/,,'
/
/
ZK
Lt
-J=ct'b--u*o*
n
^^;;,ffi'U/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
model is or.ersimplified. I1alr (1977), Johnson and
Sitar (1990), and Montgomery ei al. (1997) observed
that the occurrence of positive pore pressule during
large storms was highly localized on a hillslope.
Zones of positive pore pressure did not occur at the
base ofslopes as expected, but at mid-slopc locations
associated with con\.ergent topography. Therefore,
Johnson and Sitar (.1990) hypothesized that topographic convergcnce causes flow to move laterally
to swales or hollows, and that subsr.rrface flon'in
these locations does not come from the surface soil
layers directly upslope. Thev further hypothesized
that subsurfacc flow may also occur through the
bedrock laver.
ln the Oregon Coast Range, Anderson et al. (1997)
and Montgomery et al. (1997) have observed a threepl.rase system, with a highly pervious layer of
runoff generation for a small bedrock hollow and
to thc generation of positir.e pore pressure. Zones
of positive pole pressure were cliscontinuous and
higlrll, localized, and their occlrrlence coincided
with areas of grounclwater exfiltration from the
beclrock into the ovellying soil. Subsurface flow
tended to infiltrate into ancl erfiltrate out of the
fractured bedrock layer ser-eral timcs, r'hich
accounted for the discontinuous pattern ofposjtive
pore pressure. When thc bcdrock hollow subsequently failed during a natural storm, that faiJure
coincided h'ith the location of subsr,rrface flovr'
exfiltration from the bedrock to the or,'erlyirrg soil
(Dietrich and Sitar 1997).
More sophisticatecl physically based models of
subsurface flort' that predict the location of
groundwater tables mav heJp to differentiate
fractured bedrock separating the soil and impervious bedrock. Subsurface fkrw ihrough the
for example, TOPMODEL (Bevin et al. 1995) or
CLAWS (Duan 1996). However, these models
currently cannot deal with the sitc-specific com-
fractured bedrock contributcd significantly both to
unstable and stable landforms on forested hillskrpcs;
Lanclslicles, Surface Erosion, and Forest Operations in the Oregon Coast
plexity illustrated by Montgomery et al. (1997) or
camot parameterize it. But even though preferential
flow paths and pipes or macropores in soil are
notoriously difficult to measure and to represent in
models (Bonell 1998), they govern the behavior of
subsurface flow in hollows during landslideproducing storms.
An alternative that avoids this problem is the use
of simple empirical models to simulate subsurface
fJow (Beschta 1998). They can be used to simulate
the location of groundwater tables in sites prone to
debris slides in response to rainfall (Blansom 1992).
The Antecedent Precipitation Index or API model
has been used to simulate runoff from small,
forested watersheds in response to rainfall (Fedora
and Beschta 1989). The greatest benefit of the API
model is that the parameterization of the complex
sub s urface-drainage environment can be accomplished using a single empirical coefficient
(Bransom 1997). However, this model approach
does require that local piezometric data be developed to generate the empirical coefficient, which
can be a formidable task.
So far this discussion of the initiation oflandslides
has dealt only with static pore pressures (those
resulting from precipitation and the aggregation of
subsurface flow). Another consideration is dynamic
pore pressures, specifically the excess pore pressures
that are generated as a result of the incipient
movement of the larrdslide. The pore pressures
during mobilization of the debris flow are termed
excess pore L)ressures because they cannot be
predicted or accounted for by traditional means,
u'hich consider the drained initiation state and not
the undrained mc-rbilizatjon state (Anderson and
Sitar 1995).
As a part of the research on the influence of
corrfining pressures and stress path on soil strength,
the response of the porc pressure in the soil sample
to undrained loading was investigatecl (Bransom
1990; Keough 1994; Molgan 1995). The proportionality constant between axial undrained
krading and pore-pressure rcsponse, called Skempton's "A" pore-pressure parameter, was positive lor
these strength tests, meaning that these soils are
compressive (Morgan 1995). This means that their
volume decreases with incipient axial kradinp; and
slrear'. Althorrgh we lack information about how a
compressive soil responcls to incipient shear strair.I
as a result of reducecl effective stress, it is likely that
such a situation would increase pore pressures. This
Ran9e 223
response has been observed in physical modeling
.-rf Reid ;rnd others ( lOaT); lhe ma r.imum e\ce5: Pore
pressure during debris-flow initiation and
mobilization was much greater than the static pore
Pressure.
Debfis fTozos and aquatic habitat
Landslides initiate on steep, forested hillskrpes in
response to the presence of a groundwater table,
which means that the soils have high water contents
and high positir.e pore pressures. At such times the
saturated-water content of forest soils in steep, sites
prone to debris slides is higher than their liquid limit
(Bransom 1990); once initial failule occurs, the soils
tend to behave as liquids and quickly mobilize into
debris flows.
Debris flou's initiate on steep, soil mantled
hillslopes at the upstream end of the drainage
system and run dou.'n through steep zero-order
swales and sometimes first- and second-order
stream channels. Eventtrally they deposit in lowergradient first- and second-order streams while some
reach third- and higher-order stream channels
(Swanson and Lienkaemper 1978). Debris flows can
remove all the sediment and large wood from the
steep slvales and first-order stream channels,
leaving a bedrock channel (Benda 1990). Hor'r'eve4
the stream channel upstream of a deblis-flow
deposit is generally neither totally erosional nor
totally depositional; it retains some sediment and
channei substrate material although the charrrrel is
highly disturbed and the large wood is removed.
Debris-flow deposits r.ary in size and type,
depencling on the size of the flow ancl the gradient
and width of the stream channel (Benda 1990; May
1998). Atone end of the continuum rs Ihe debris jnnt,
a large, valley-spanning accumulation tl1 or€ianic
debris that dams the stream, forcing the accumulation of a lar€ie wedge of sediment. However,
debris-flow deposits can also form fans of large
wood and sediment that force a larger stream
against the opposite bank, thus increasing channel
sinuosity without blockir-rg the stream. At the other
end of the continuum, debris fans of sediment and
large wood may deposit across floodplains and not
interact with the receiving stream channel except
at very high flon's (Benda 1990; May 1998). Of the
53 debris-flow deposits in the central Coast Range
that occurred following the February 1996 storm,
only 12 (23 percent) were valley-spanning debris
Landslides, Surface Frosion, and Forest Operations in the Oregon Coast
plexity illustrated by Montgomery et al. (1992) or
cannot parameterize it. But even though preferential
flou/ paths and pipes or macropores in soil are
notoriously difficult to measure and to represent in
models (Bonell 1998), they govern the behavior of
subsurface flow in hollows during landslideproducing storms.
An alternative that avoids this problem is the use
of simple empirical models to simulate subsurface
flow (Beschta 1998). They can be used to simulate
tl-Le location of groundwater tables in sites prone to
debris slides in response to rainfall (Bransom 1997).
The Antecedent Precipitation Index or API model
has been used to simulate runoff from small,
forested watersheds in response to rainfall (Fedora
and Beschta 1989). The greatest benefit of the API
model is that the parameterization of tlre complex
subsurface-drainage environment can be accomplished using a single empirical coefficient
(Bransom 1997). However, this model approach
does require that local piezometric data be dereloped to generate the empirical coefficient, which
can be a formidable task.
So far this discussion of the initiatiou of landslides
has dealt only r,r,'ith static pore pressures (those
resulting fron'L precipitation and the aggregation of
subsurface flow). Another consicleration is dynamic
pore pressures, specifically the excess pore pressures
that are generated as a result of the incipient
movement of the landslide. The pore pressures
during mobilization of the debris flow are termed
excess pore pressurcs because they cannot be
predicted or accounted for by traditional means,
which consider the drained initiation state and not
the undrained mobilization state (Anderson and
Sitar 1995).
As a part of the research on the influence of
confining pressures and stress path on soil stret-Lgth,
the response of the pore pressure in the soil samPle
to undrained loading was investigated (Blansom
1990; Keough 1994; Morgarr 1995). The proportionality constant between axial undrained
loading and pore-pressLrre response, called Skempton's "A" pore-pressure parametet was positive lor
thesc strength tests, meaning that these soils are
compressive (Morgan 1995). This means that their
volume decreases with incipient axial loading and
shear'. Although we lack information about how a
compressive soil responds to inciPient slrear strain
as a result of reduced effective stress, it is likely that
such a situation would increase pore pressures. This
Range 223
response has been observed in physical modeling
q97); lhe ma \im u m e\ccs\ pore
ol' Reid .r nd others ( I
pressure during debris-flow initiation and
mobilization was much greater than the static pore
Pressure.
Debris flozas and aquatic habitat
Landslides initiate on steep, forested hillslopes in
response to the presence of a groundwater table,
which means that the soils have high water contents
and high positive pore pressures. At such times the
saturated-water content of forest soils in steep, sites
prone to debris slides is higher than their liquid limit
(Bransom I990); once initial failure occurs, the soils
tend to beha\.e as liquids and quickly mobilize into
debris flolvs.
Debris flows initiate on steep, soil-mantled
hillslopes at the upstream end of the drainage
system and run down through steep zcro-otdet
swales and sometimes first- and second-order
stream channels. Eventually they deposit in lower'gradient first- and second-order streams while some
reach third- and l-righer-order stream channels
(Swanson and Lienkaemper 1978). Debris flows can
remor.e all the sediment and large wood from the
steep swales and first-order stream channels,
leaving a bedrock channel (Benda 1990). Hon'ever,
the stream channel upstream of a debtis-flow
deposit is generally neither totally erosional nor
totally depositional; it retains some sediment and
channel substrate material although the chamrel is
highly disturbed and the large wood is removed.
Debris-flow deposits vary in size and type,
depending on the size of the flow and the gradient
and width of the stream channel (Benda 1990; May
1998). At one end of the continuum is the tlabris jnnt,
a large, valley-spanning accumulation of organic
debris that dams tl.re stream, forcing the accumulation of a large wedge of sediment. However,
debris-flow deposits can also form fans of large
wood and sediment tl.rat force a larger stream
against the opposite bank, thus increasing cl.rannel
sinuosity without blocking the stream. At the other
end of the continuum, debris fans of sedimer-Lt and
large wood may deposit across floodplains and trot
interact with the receiving stream channel except
at very high flows (Benda 1990; May 1998). Of the
53 debris-flow deposits in the central Coast Range
that occurred folkrwing the February 1996 storm,
only 12 (23 percent) were valley-spanning debris
224
Foresl and Stream Management in the OreBon Coast
dams. Of the remainder, 36 (66 percent) interacted
degrees
with the receiving channel to various
without completely blocking it, and 5 (11 percer-rt)
were washed away during the storm ancl not founcl
(May 1998).
Debris flows can be disastrous to fish and aqr-ratic
habitat. When the dcbris flow occurs, there is the
likclihood of direct mortality to fish and other
aquatic species. Later, the form of the stream may
be greatly altered and simplified. The large wood
will be gonc, the stream will have low hydraulic
retention, the or"-erhanging canopy will be more
open, and the surrounding riparian area will be
highly disturbed (Everest and Mcehan 1981;
Lamberti et al. 1991). Downstream crosion may be
accelerated; large, abrupt inptrts of sediment may
mean rnore fine sed jment, which affects spawning,
bed quallty, and more transport-resistant sediment,
n'hich can cause aggradation of strcam channels and
filling in of pools (Everest et al. 1987; Sullivan et al.
1987).
Some research results suggest,
Ran51e
disturbance, specifically stand-replacement wildfires followed by intense, landslide-producing
storms that cause the widespreacl occurrence of
debris flows (Benda and Dunne 1997a, b; Benc{a et
al. 1998). The aquatic habitat is of highcst cluality
and the fish communities are most diverse ,,r,hen
the disturbances are of intermediate age, neither
recently disturbed nor senescent (Reeves et al. 1995;
see also Chapter 3). This disturbance-based thcory
of the succession of aquatic habitat recognizes debris
flows as the mechanism that brings critical habitat
ingredients, nlrmely large wood, boulders, and
sediment, from the hillslopes and into the streams.
landslides and Surface Erosion in
Managed Coast Range Forests
Surface erosion
Bccause of the Iow intensities of precipitation in the
Coast Rangc ancl the high infiltration capacities of
forest soils, surface erosion resulting from overland
ho ,evct that the
relationship between debris flou's and aquatic
flow is virtually nonexistent. However, rninor
habitat may not be cntirely ncgative. Dcspite shortterm localized damage, Ever.est and Meehan (1981)
{or,rnd that debris-flow deposits createcl resting
habitat for spawning adult coho sah.non and rearing
habitat for juvenile coho witl.rout interfering with
upstream spawning migr.ation. It-r this case, the net
effect of debris flows on fish ancl aquatic habitat ft,as
positive. ln thc Elk Rir.er watershed in southwest
Oregon, the most productive areas for both rearing
and spawning habitat were associated \.ith large
"flats" or areas of relatively low gradient. These
areas were persistent features at tributary junctions
and apparently resulted from debris-fktw deposits
slopes because of rar.el from exposed soils resulting
that occurrcd in conjunction wjth catastrophic
wildfires over 100 years ago (G. Reeves, personal
communication). This scen;rrio would link highquality aquatic habita t with the occurrence of dcbris
flows.
Recently, Reeves et al. (1995) offered a long-term
view that portrays aqr-ratic habitat as dynamic over
the geographical scale of watersheds and landscapes
and over the temporal scale of clecades and
centuries. In responsc to disturbance, aquatic
habitat, like terrestrial habitat, undergoes successional stages and its quality at a given point in a
watershed will succeed from young to old forms
over time. The mechanism for r.esetting the age of
aquatic habitat has historically been catastrophic
amounts ofsurface erosion occur, especiallv on steep
from local disturbances, srrch as windthrou' and
burrowing animals. Managed forests offer more
opportunitv for surface erosion because soil
clisturbance is more drastic, widespread, and
chronic. Surface erosion resulting from {oresi
management activities can be dir.ided into two
general categories: surface erosion from within
han est units, where the predomir.rant manil€iement
activities include thc removal of trees and site
preparation, and surface erosion from drastically
disturbed and compacted soil surfaces, such as skid
trails for ground-based harvestir.rg systcms, roads,
and landings.
Surface erosion from harztest units
Surface erosion coming from within harvest units
takes tl'r'o main forms. The first, dry ravel, is
predominantly associated with steep slopes and
cohesionless or skeletal soils in wester.n Oregon. The
second form, infiltration-limited surface erosion
(wet erosion) is associated with rainfall and
overland flow.
Dry ravel
Timber harvestirrg can accelerate the rate of dry
ravel, especially if broadcast burning is used as a
Landslides, Surface Erosion, and Forest Operations in the Oreg,on Coast
site-preparation treatment. Swanson et al. (1989)
summarized the research findings on dry ravel.
Increases in either slope or slash burning cause the
rate of dry ravel to increase greatly. On steep,
broadcast-burned harvested areas in the Oregon
Coast Range, the surface-erosion rate was approximately eight times the rate on similar but
gentler terrain and approximately 13 times the rate
for steep, unburned slopes. On steep, burned slopes,
two-thirds of the first year's erosion occurred within
24 hours after the burn (Berurett 1982). In the Oregon
Cascades, dry ravel on a steep, unburned slope
increased markedly after harvest, then within 3 to 5
years settled to a level approaching that of an
undisturbed forest (Swanson et a1. 1989). Therefore,
even with marked increases in dry ravel on
harvested sites, the window of accelerated erosion
is very small compared to expected rotation lengths,
and the total amount of erosion produced is
minuscule compared to other erosion processes.
ltration-l im ited surface erosron
Surface erosion associated with infiltrationlimited
overland flow is virtually nonexistent for harvested
areas in western Oregon. The simple removal of
trees does not change the infiltration capacity of
forest soils sufficiently to cause infiltrationlimited
surface erosion. While the absence of trees does not
increase surface erosion, the manner in whicl-r they
were removed and the site-preparation techrtques
used after harvest may increase its risk. AerialIogging systems, such as skyline systems or
helicopter logging, expose and disturb less surface
soil than ground-based harvest systems, such as
tractors and skidders (Chamberlin et al. 1991; see
Chapter 6).
Chemical site preparation leaves the duff, litter,
and slash intact and causes the least increase in the
risk of surface erosion. In contrast, site preparation
by broadcast burning removes logging slash, may
remove litter and duff layers, and may create
hydrophobic conditions, all factors that increase the
risk of surface erosion. The risk is increased most
by mechanical site preparation, in which logging
slash is pushed into windrows using a bulldozer or
put into piles using a hydraulic excavator or a
bulldozer. In such cases, not only is the slash layer
removed, but the surface soil is also disturbed and
compacted.
Regardless of these factors, it is still difficult to
generate surface erosion in Coast Range forests.
I
nf
i
Range 225
Even moderate soil disturbance, compaction, and
broadcast burning do not reduce infiltration
capacities of forest soils to a point where they are
less than rainfall rates. Johnson and Beschta (1978)
found that infiltration capacities on areas harvested
using various methods, as well as infiltration
capacities on tractor skid roads, were several times
higher than expected rainfall rates. For areas that
had been harvested and broadcast bumed, McNabb
and others (1989) found that the single lowest
measurement of infiltration capacity was still twice
the expected 100-year-return-period
ra
infa
ll
intensity.
Research results from paired watershed studies
reinforce these findings. During the Alsea Watershed study, one small sub-watershed was 90 percent
clearcut and harvested using a highJead cable
system; there was no increase in suspended
sediment as a result of the clearcut krgging alone
(Brown and Krygier 1971). In a paired watershed
study in the H. J. Andrews Experimental Forest, the
rate of soil loss attributed to surface erosion lrom
clearcut harvested areas was very small (Fredriksen
1970).
Therefore, erosion associated with infiltrationlimited overland flow in western Oregon forested
watersheds occurs only on sites that are drastically
disturbed and compacted. These sites include roads,
landings, and the portions of skid trails that have
experienced multiple passes (six turns or more) with
ground-based harvesting equipment. On such sites,
the soils are bare and have been compacted
sufficiently to make the infiltration capacity less than
expected rainfall intensities.
Surface erosion from roads and landings
Forest roads have long been a focus of concern
regarding accelerated erosion. The construction ol
a forest road, especially one with any degree of side
slope, results in a high degree of disturbance. When
the road is completed, the remaining structure has
two locally or.ersteepened slopes (the cut slope and
the fill slope), a surface that must support loaded
log trucks, and a roadside ditch. All of these surfaces
are bare soil until r.egetation becomes reestablished.
The forest-road surface is intentionally compacted
to develop its strength and is a virtually impervious
surface. The roadside ditch is designed to carry both
runoff from the road surface and subsurface flow
intercepted by the cut slope to a cross-drain culvert
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
persisteni than the effects of harvesting. Because
forest roads are constructed, maintained, and used
over an extended period oi time, they generate
surface erosiolr over this period.
The effects of construction in accelerating erosion
have been studied extensively. These research
sources are paired n'atersheci stuclies for the Alsea
Watershed (Brown ancl Krygier 1971), the H. J.
Andrews Experimental Forest (Fredriksen 1920),
and Caspar Creek in northern Californi;r (Krammes
ancl Burns 1923). Horvever', the information obtained
is of rnarginal use, for a number of reasons. The
stlldies u'ere not specific to erosion processes, and
the researchers lumped erosion proccsscs togcthcr
and measured overall effects at the mouth of the
wt-rtershed. Also, the three studies were carried out
in landslide-prone tcrrain, and landslicles undoubtedlv contributcd a significant amouni to the
reported erosion. Thercforc, thc rcsults say littlc
about snrface erosion from forest roads. FinalJy, the
cxisting roads werc constructed according to
methods used 30 to,10 vears ago, and road-location
and -cor.rstruction plactices har,'e impror.ed ereatly
during the inten'ening years.
In contemporary managecl folests, it is the
environmental effect of this legacy of forest roacis
that is of most conceln. ln westen.r C)regon, ther"e
are literally tens of thousands of kilometers of forest
roacl, and aJi represent some potential to produce
chronic surface erosion. Several stuclies have
investigated the environmental factors that affect
surface erosion fron forest roads. One of the most
important predictors of surface enrsion fxrm these
roads is the level of traffic (Reicl and Dr"rnne 198,1;
Bilby et a1. 1989). Reid and Dunne (1984) r'cport ihat
a heavily tlaveled mad rvill yield, by an order of
magnitude, more fine sediment than a sin-Lilar road
that has only light traffic. The thickness ;rnd quality
of thc sr,rrfacing matcrial of the road also affect
sedimcni production. Surfacc crosion is lcss lrdrcrc
a thick la1'er of higlrer-qrralitv erosion-resistant rock
'\,\'as used for the surface course than r'r'here a thinner
lavcr of lcsscr-quality rock r'r'as uscd (Bilbv et al.
1989). The thick sr-rrface seems io rninimize the
amonnt of fines that is pumped fi"om the subgrtrde
to the snrface, and the high-clr,rality rock seems to
be less pror-Le to erode. However, both surfacing
thickness;rnd qualitv were secondary to traffic
lntensl$r
In the central Coast Range, Luce and Black (1999)
investigated how road attributes affected strrface
erosion in the absence of traffic. They found that
secliment production was best predicted using a
term that is the product of the length and the square
of the skrpe of the road segment. Total sediment
production from road segments was independent
of the height of the roacl cutslc-rpe. Flowever, \,\41en
the road cutslope and roadside ditch wele vegetated, the amount of surface erosion was reduced
bv a factor of seven. Similarll,, Foltz (1996) found
'vt'ater vclocitics in unvcgctatcd roadsiclc ditches to
be tr,r.'o to three times fastcr than thosc in vcgetatcd
or grass-lined ditches.
Modcling the process of surface erosion from
roads has shown that it is influcncccl bv thc
frrrmation of ruts (Foltz and Burroughs 7990, 1991.;
Foltz 1993). Well-definecl, deep, contilruous ruts
cause long, concentrated flow paths, in contrast to
the short flolv paths across the roacl. Maintenance
of the road snrface to prevent;rnd fjx mt formation
lor,r,'erecl the erosiorr rate drastica]Jr,., because the
lcngth of the flon' paths rvas recluced. Although
grading and shaping the roacl surface made material
ar';rjlable to erode in the next storm, overlancl flor.v
\'\'as not;rble to remove the material efficiently
because of the shorter flo\ / path. However, traffic
on a road increased sr-rrface erosion long; before rr,rts
form. Even a slight increase in the flow patl-L caused
an increase in surface erosion from the road.
Effect of forest fll&nagement &ctipities on
debris slides and fJoTos
Mass movement processes (most debris slides and
fkx,r's) dominatc thc scdiment buclgets of pristine
watersheds in u'estern Oregon (Dietrich and Dumre
1978; Sr'r'anson ct al. 1982). It is thesc s.tmc processes
that h;rve the greatest Potential to be adYersely
affected by forest management activities and to
result in the largest net increases in acceleratecl
erosion. Landslides associated $'ith forest man;rgement activities are dividecl into t\\'o groups: in-Llnit
lanclslides (landslides that occur in hirn'est r.uriis and
are not associated $'ith forest roads) ancl roadreltrtecl landslicles. The mar-Lagement related canses
of landslicles ancl tl.reir mitigation are different for
each group. For in unit landslicles, the causes relate
to Veget;rtion mana€iement; for road related
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
it landslides
Since the earliest research in this area, inr.estigators
har.e belier,,ed that there is a basic cause-and-effect
relationship between timber harvesting and
In-u
n
landslide occurrence (Croft andAdams.1950; Bishop
and Stevens 1964). The early research tool used to
investigate this relationship was landslide inventories. In particular, in\rentories fo| harvested
terrain were comparecl with inventories for similar
but unmanaged terrain. Landslide inventories have
been carried out for much of the Pacific Rim,
including the Pacific Northwest, coastal British
Columbia, southeast Alaska, Japan, and New
Zealand. The results have been compiled to show
background landslide and erosion rates and the
effect of timber harvesting on these rates (Sidle et
al. 1985). Table 9-1 contains such a compilation. In
every landslide inventory but one, the erosion rate
attributed to debris flows increased after harvest,
by 1.9 to 21.2 times. Similarly, all but two studies
documented an increase in the rate of occurrence of
landslides after harvest.
However, care must be used when interpreting
these data. Most of the studies used landslide data
collected from aerjal photographs. Unfortunately,
given differential visibility in aerial photographs of
debris flows in forests versus clearcuts, photo-based
inventories could bias the results of the research
(Pyles and Froehlich 1982). The Oregon Department
of Forestry (ODF) conducted a systematic, groundbased landslide inventory after a series of storms in
in 1996 in western Oregon. The survey showed
conclusively that landslide-inventory data from
aerial photographs und errepresented both the
number of landslides and total landslide erosion
(Robison et al. 1999). Tlrerefore, it is best to base
Range 227
conclusions on inventories that are systematic and
ground based, and that give every landslide an
equal opportunity to be sampled.
The ODF landslide inventory was intended to
monitor the effect of forest practices on landslides
(Robison et al. 1999). The inventory covered eight
26-square-kilometer study areas in the Cascade
Mountains and Coast Range of western Oregon.
Five of the str-ldy areas were labeled "red zones"
because they had a high incider.rce of lanclslides
resulting from the 1996 storms. Table 9-2 presents
the landslide densities for four of the red zones
stratified by the age class of the forest stand (data
for one red zone, the Tillamook study area, is
omitted because only one forest age class was
Present).
Although this study represents only the response
of steep, landslide-prone, forested landscapes to a
single landslid e-producing storm, its results
encapsulate the trends shown by a compilation of
systematic, ground based landslide inventories
(Pyles et al. 1998). Therefore, the trends seen in the
ODF data seem to be consistent across many types
of landslide inventories from a variety of terrain
types.
The ODF study shows that there is, on average,
an increased incidence of landslides in the first
decade after clearcut harvest compared with
adjacent late.seral fore:ls. The.rver;ge increase in
landslide density was 42 percent, based on a
background rate of 5.2 lanclslides per square
kilometer and a post-harvest landslide rate ol 7.4
landslides per square kilometer. This average
increase is less than the average increase indicatecl
in Table 9-1, but it is consistent with the increase
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.
Sil/dy:,r?a
0g
),rs (#/kn:,i)
I0-29 ),rs
(#/kt1tt
)
I999).
3A-100 yrs
(#/k:r")
>100 yrs (#/knl )
Ch.rrrgr I
Etk
22.2
20.5
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
t).5
7.3
5.7
25 i)
Average
19.2
9.1
7.9
l 3.5
,+2
1
1
i
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
decade after han cst is followed by a decrease in
landslicle occurrence in the 10- to 30-year anc{ 30- to
100-year forest age classes. Becatrse the ODF study
was the first to sample multiple forest age classes,
there is no other da tabase for comparison. Howevet
this result r,r'as previouslv hypothesized (Froehlich
1978; Swanson and Fredriksen 1982), and the merit
of the hypothesis has been discussed vigorously
rt'ithout resoh,rtion.
The average increase in landslide density
attributed to timber harvest represents widely
varying r"'alues for individual study areas, ranging
{rom a 15 pcrcent decrcase in the inciclence of
landslides in one study area to a 250 percent increase
in another. These two cxtremely different results
come from study areas that are within 10 kilometers
of each other and have the same geology, landforms,
and management regime; the landslides occurred
during the same storm. Such variability in lar.rdslide
response has been noted when only ground-based
inventories were considered (Pyles et al. 1998);
tlrerefore, it seems to be normal.
These results indicate the importance of naturally
occurring factors that influencc landslide occurrence
as compared with tir.nber harvest. One of these
factors is the variability in the total amount and
intensity of prccipitation that trigger landslides.
lnsufficient precipitation data were collected to
satisfactorily determine whether the cause of the
extrerne variability in landslide occurrence was
driven by precipitation. However, sufficient
precipitation-intensity data were collected during
the Febluary 1996 storm to show that precipitation
amounts and intensities that trigger landslides do
vary significantly across distanccs represented by
the ODF landslide inventory (M. Clark, personal
communication).
For the four multi-age-class study areas, the
results of the ODF landslide inventory show an
average increase in landslicle density as a result of
timber harvest. However, thcre are no statistically
significant differences in landslide density among
tl-Le four age classes (Robison et al. 1999). The smatl
sample size (only four trcatments and four study
.rr(,r.) Jnd lhe lrigh \afi.rbilit) in r(.\ponse undoubtedly contribute to this lack of significance. The
rcsult is consistent with those from the only other
landslide study in which the effect of timber harvest
was tested statisticallv (Martin 1997). This study
investigated the effectir,'cness of leaving patches of
intact forest in areas with steep, convergent
topography within a clearcut harvest area (headwall-lear,'e aleas) in mitigating the occurrence of
landslides (Martil 1997). The incidence of landslides
in forested areas, clearcut areas, and headn'all-leave
areas was found to be statistically insignificant
because of high variabiljty. The results from the ODF
landslide inventory for erosion volume correspond
to those for landslide clensity, including the lack of
statistical slgnificance.
Although landslide density and erosion r.olume
are appropriatc variables given the monitoring
questiol.ls that were asked in the ODF study, they
provide little insight into what the response would
mcan for a specific managed forest. In otlrer words,
what cloes a 42 percent increase in landslide density
krok like? An approach that will yield more insight
is to present the landslide-density data as the
number of failed slide-plone sites pcr unit area or
the percentage of slide-prone sites that failed. In
calculating these numbers, some implied assumptions are made. First, the tcrm "bedrock
hollow" will be used synonymously for a clebrisslide-prone site. Since bedrock hollows can exist on
planal slopes with little or very subtle topographic
expression, the assumption is made that all the
landslides inventoried by the ODF initiaied in
bedrock hollows. Second, even thotrgh the reported
value o{ the average spatial density of 100 hollows
per square kilomcter is for the central Oregon Coast
Rangc (Benda 1990), it is assumed that this vah-re
will adeguately represent the ODF study areas. It is
recognized that both of these assumptions are morc
than Jikely in error; howevel the consecluence of
the error will result in the numbers bcing mor.e
conservatrve.
Using the value of 100 hollows per square
kilometer', it is possible to convert landslide density
values to the number and percentage of failed
hollows per square kilometer (Table 9-3, for 0- to 9and >100-year forest age classes). The highest
landslide density, for the greater-than-10O-year age
class in the Elk study area in the centr.al Coast
Range 26 per squa.re ki lometer-represents a
tailure rate of only about 10 pcrcent of the hollows.
Therefore, on a landscape basis, fewer than 10
percent of the hollows failed during the landslide
producing storm, wl.rether thc forest was harvested
or not. The average increase in landslide density,42
percent, means an increase of two landslides per
square kilometer if the entire square kilometer were
Landslicles, Surface Erosion, and Forest Operations in the C)rcgon Coast
Table 9-3. The percent of debris slide-prone sites that
failed in two forest age classes in four "red zone"
study areas in the Coast and Cascade Ranges of
Oregon after the February | 996 storm.
Study area
E
O-9 t rs (''")
lr.6
ll<
Vida
Mapleton
Scottsburg
Average
5.1
8.2
7.B
7.5
>1OO yrs (%)
10.2
3.4
5.3
2.2
5.-l
Ch.rngrr /n,)
-1.6
1
.9
2.9
5.5
).1
' This column is the change in the percent of debris slide
prone sites that failecl for the 0-9 year age class compared
r/ith the >100 year age class. Data based on an assumecl
density of debrjs slide prone sites of 1o(/kmr.
harvested. That is, the number of {ailed hollows
u ould increare by approrimately 2.2 percent in the
harvested area. The change in the percentage of
hollows that failed ranged from a low of -1.5 percent,
which means fewer hollows failed after harvest, to
a high of 5.6 percent. These values represent an
extreme condition: a rare, landslide producing
storm on a 100-percent-clearcut harvest on highly
unstable terrain. These values are actually high for
an entire managed forest because a whole forest is
rarely in a 0- to 9-year age class. For areas with older
age classes interspersed, the increase in the
percentage of hollows that failed would be lower.
Road-related landsl ides
Forest roads have long been considered the
dominant source of accelerated erosion caused by
forest management activities. The early paired
watershed studies first showed the importance of
road-related landslides in harr.est-related accelerated erosion (Fredriksen 1970; Brown and
Krygier 797L; Beschta 1978). Inventories of roadrelated landslides further documented the
importance of forest roads to the increase in
landslide erosion associated with forest management (O'Loughlin 1972; Fiksdal 1974; Morrison
1975; Swanson and Dyrness 1975; Swanson et al.
Amaranthus et al. 1985). As shown in Table 91, the erosion rate from the rights-of-way of forest
roads is 12 to 343 times the erosion rate from similar
1977;
unmanaged forested terrain. As discussed above,
many of the iandslide in\.entories represented in
Table 9-1 were photo-based; the same concerns
about bias in the data holcl for road-related
landslides as for in-unit landslides. However,
because the difference indicated by the inventories
Range 229
of road-related landslides is much greater than
background variability, it is easier to infer that a
problem exists.
During ihe last two to three decades, considerable
effort and expense have been put into improving
the environmental performance of forest roads,
especially with regard to landslides. Improvements
include wholesale changes in the way steep,
landslide-prone forested terrain is harvested and
how forest roads are located, constructed, and
maintained, including (1) widespread use of longspan, high-lift cable systems, like skyline systems,
to reduce the amount of road needed and increase
the flexibility of road location; (2) use of ridge-top
locations for forest road systemsi (3) use of highgradient roads to reach and stay on ridge-tops; (4)
use of full-bench, end-haul road-construction
practices; and (5) improved road-drainage and road,
maintenance practices.
The degree to which these improved practices
have reduced the negative environmental effects of
roads is not fully known. However, an inventory of
road-related landslides in the central Coast Range
compared the effects of forest roads constructed
using old practices with those constructed using
impror.ed ones (Sessions et al. 1987). The results
showed that the improved practices reduced both
the number and the size of road-related landslides.
Unfortunately, the validity of these results remained
in doubt because, at the time of the study, the roads
constructed using the improved practices had not
been tested by a landslide-producing storm. The
February 1996 storm provided the opportunity to
evaluate the effects of contemporary forest-road
systems.
Two inventories of road-related landslides were
carried out following the February 1996 storm. As a
part ofthe ODF study discussed previously, all road-
related landslides within the eight 26-squarekikrmeter stucly areas were inventoried (Robison et
al. 1999). The other road-related landslide inventory
was carried out on Blue River and Lookout Creek
in the central Oregon Cascades (Wemple 1998).
The results from both inventories show that
contemporary forest-road systems remain a
significant source of erosion from debris slides/
flows (Skaugset and Wemple 1999). Road drainage
was associated with approximately half of the debris
flows that initiated at road fills, confirming the
importance of road drainage to accelerated erosion.
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
length
(k'n)
ol Average landslide
lanclslides volLllt1e (r,')
NLtnber
Road
Swanson ct .rl.
1977
Mapleton (Robison et al.
Tillamook (Robison et al.
89
lB6
93
179
29.6
29
28.6
10
U0.0
1999)
1999)
Landslide
(#/kn)
t.t0
.lensit)/
Erosion t ate
(nt'/knt)
711
0.:19
9
t20
l l0
201
t24
0.3
5
74
Oregon Cascades Comparisons
Ro.td RAr\/
(k|j)
area
of Averay,e landsliclt' Landslide
landslicles volune (nt) deDsity (#/ktn)
Nunber
Swnnson & Dyrness I915
2.1 0
73
1,351
Morrison 1975
0.6
75
1q4)
Vicia (llobison ct al. 19!l!l)
0.5 l
)2
173
482
Bl!e R./Lookout
Cr.
l
11.20
nle
(ni/knt)
(l
610
0.0
3,200
7.0
B0
0.8
13
6.
2
Etosion
(Wemt)le 1998)
Ro;1d R,A
/
arc,1
Nunber ol
Eft)siot1 tale
Average landslide
density (#/km)
Swanson & Dyrness I975
Morrison I975
Vida (Robison et al. 1999)
Blue R./LookoLrt Cf.
(Wemp e 1 998)
(rn'/kn)
.90
171r
o.41
49
L50
40.00
12
0.5 2
4.20
B9B
0.05
0.54
7
' 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
landslides.
On a lanclscape level, road age and topographic
position were factors in erosion-related damage to
roads. Olcler roads (those constructed during the
1960s
or earlier) \,\'ere the source of significantly
more erosion by landslides than more recent roads.
Midslope roads produced more erosion than eithcr
ridge or valley-bottom roads.
Despite improved road standards, the density of
landslides from logp;ing roads in 1996 was roughly
the same as in older inventories of roads with lower
construction and drainage standards (Table 9-.1).
This demonstrates that, regardless of the standards,
if roads are constructed in steep, landslide-prone
terrain, ihere will be landslides. However, the data
also show that the avelage volume of the roadrelated landslidcs in 1996 decreased compared lvith
rates from the older inventories (Table 9 4), in some
cases markedly. These results, and the lower road
density that is now standard, mean that r.r'hile road-
related landslicles still occur; thc total accelerated
erosion from roads should be lcss (Skaugset and
Wemple 1999).
The effect of forestry on debris IToLus
Since the presentlrtion of a disturbance-based
approach to the management and recor.ery of
aquatic ecosystcms (Reeves et al. 1995; see Chapter
3), incrcasing attention has been paicl to the effects
of forestry on the size and composition of debris
flor,r's and their deposits. There is sorne evidence that
the prirnary cffect of forest manaEiement is on lrorv
iar d( bri* f low' rur ,rnd r,r here they Jepo*:t
Ketcheson ancl Froehlich (1978) rcport that debris
flows from clearcuts run farthcr and set up in larger
deposits than dcbris flou's from forests. May (1998)
forurd that, of53 debris flow deposits il anadLontous
fish-bearing streams, a disploportionate share came
from ancl ran through harvest units.
Landslides, Surface Erosion, and Forest Operations in the Ore€lon Coast
There is very little information regarding the
direci effect of timber harvest on the size and
composition of debris flows. Empirical models that
describe where debris flows will run and deposit
show that deposition is governed by stream-charurel
geometry, stream gradient, and stream-valley width
(Benda and Cundy 1990; Fannin and Rollerson
1993). However, the models describe only the runout
and deposition of debris flows and not the composition of the deposit. May (1998) and Robison and
others (1999) suggest that although the size of trees
along the periphery of the debris flow may affect
the length of runout, the primary de-terminants are
channel geometry and gradient
May (1998) investigated 53 deposits from debris
flows in the central Coast Range resulting from the
February 1996 storm. Forest management appeared
to have no effect on the distribution of large wood
in debris-flow deposits, and the size of the wood
did not correspond to the size of the trees on the
hillslope at the time of the debris flow. This result
reflects a legacy of large wood stored in low-order
stream channels. Forest management did affect the
lengths of large wood pieces in the debris-flow
deposits. Although short pieces of wood dominated
the length distribution for all management classes,
pieces were longer in debris {lows that ran through
mature forests.
Roads were found to be the one forest management activity that undeniably affected the
composition, size, and run-out length of debris
flows. Debris flows that initiated from roads had
the largest initial slide volumes, traveled the
farthest, and contained the most sediment (May
1998). In the ODF study areas, road-related
landslides were, by count, the minority of all
landslides, yet they accounted for a disproportionate
amount of the length of highly impacted channels.
This may be a result of the larger initial slide
volumes of road-related landslides, which would
cause them to travel farther (Robison et al. 1999).
Prevention and Mitigation of
Accelerated Erosion in a Managed Coast
Range Forest
The recognition of the link between lorest manage-
ment and accelerated erosion has prompted the
development of alternative forest management
practices that prevent or mitigate erosion. Since the
initiation of Forest Practice Rules in 1972, and the
Range 231
research results from paired watershed studies, the
development of alternative practices to mitigate
accelerated erosion has intensified.
Three management or regulatory tools can drive
changes in forest practices to prevent or mitigate
accelerated erosion: water-quality standards, BMPs,
and watershed management. Water-quality standards, which are the regulatory tool of choice for
point sources of pollution, are not practical for nonpoint sources (Brown 1980). Nevertheless, debate
continues regarding their efficacy for non-point
sources oi pollulion from forestry pracfices.
Most progress made with regard to the prevention and mitigation of accelerated erosion has
come through the establishment of BMPs. They are
forest practices that are prescribed for a specific
operation and are effective; that is, they reduce
accelerated erosion from forestry activities and are
technically feasible and economically viable.
The last tool is watershed management, or the
management of forest practices on a scale that is
larfler than an individual forest stand or operation
(the scale for BMPs). Watershed management
prescribes the distribution of forest practices on a
watershed bulh:patially and through time.
Contemporary forest management is currently in
transition bet\,veen using BMPs and adopting watershed management. Best management practices have
been implemented for virtually all forest management activities, as evidenced by the robust sets of
forest-practice rules adopted by all of the western
timber-producing states. Some changes in BMPs and
a few aspects of forest practices continue, but by
and large BMPs are firmly in place. Conversely,
although some rudimentary watershed-sca le
management practices are required, for example,
harvest unit size and "green up" or adjacency
constraints for clearcuts, watershed management as
a regulatory tool is in its infancy.
Best management prectices Ltnd
accelerated etosion
Many BMPs have been developed to mitigate the
effect of forest management activities on accelerated
erosion. Their breadth of coverage in the forestpractice rules of western timber-producing states is
a testament to the sheer number of site-specific and
activity-specific BMPs that have been developed. It
is beyond the scope of this chapter to review all
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
scale can be illustrated by reviewin!! r'esearch results
from the Caspar Creek Watershed study. This
watershed study is an cxcellent example of the
advances made to prevcnt and mitigate accelerated
erosion from timber han esting by using BMPs.
The Caspar Creek study is unique becaLrse it pairs
a study carried out in the 1960s, before thc advcnt
of forest practice rules, with anotlrer carried out ir.l
the 1990s nsing contemporary BMPs. The experimer.rtal r,r'atersheds are the North Fork (473
hectares) and the South Fork (424 hectares) ofCaspar
Creek, krcated in northern California in the redwood
region approximately 7 kilometers from the Pacific
Ocean. The watersheds, originally krgged between
1860 and 1904, grew back to young-growth redwood
(Str1tLoin surperairc;ls) and Douglas fir (Pscudotsugo
nt anzirci i) before contemporary treatments began.
Both lcccivc approximately 1,200 millimeters o1
precipitation annually coming as rain dr-rring the
\,\'inter months, and both arc underlain by marine
sedimentary rock (Wright ct al. 1990; Lewis 1998).
During the original studv, the North Fork was
usecl as a control and the South Fork was harvested.
Iioads were cor.rstrlrcted in the Sorrth Fork in 1967
ancl harvestir.rg took place between 797L and 7973.
Approximately 65 percent of the timber volume n'as
relectirel) logHed Lrsing lr.)cli\r:; 75 percenl of lltc
roads n'ere within 60 meters of the stream. As a
result, 15 percent of the r'r'atershed r,r'as compacted
(Wright et al. 1990).
Harvesting for the seconcl study was carried out
between 19B9 and 1992. In this study, the South Fork
was the control and the North Fork was harvested.
Because of limits on the size of clearcuts and
adjaccncy constraints, only about 48 percent ol the
North Fork area was harvested. The harvesting r'r'as
clcarcut sih.iculture, using, for the most palt,
skylineJogging systems. Half of the han-est units
u'ere broadcast burncd and half were not. Ground
disturbance for roads, landings, skid trails, and fire
lines was limited to 3.2 percent of thc watcrshed.
Streams were buffered by lcaving a protcction zone
from 24 to 60 mctcrs in width (Lewis 1998).
Rice and others (1979) four.rd increases in the
suspended sediment load in the South Fork
watershed of 1,403 kilograms per hectare pcr ycar
after road construction in 1962 and 3,25,1 kilograrns
per hectare per year for the fir.e years after timber
harvest began (1971 to 1976). When the roles of the
two watersheds were reVersed and tl-Le North Fork
was harvested, Lew:is (1998) reported an increase
in suspended sediment of 188 kilograms per hectare
per year for the six-year period after harvest began
(1990 to 1996). When cijfferences in sampling
methods and streamflow are accounted for, thc
relatir.c increases in excess suspended sediment load
are 212 percent to 331 percent for the South Fork
study and 89 percent for the North Fork stucly
(Ler,,"is 1998). Thcse data suggest that harvesting
using BMPs reduced the potential increase in
suspended sediment load by 2.4 to 3.2 times.
The difference in the rate of accclcrated erosion
between the South Fork and North Fork watersheds
is attributerl to a diffcrcnce in the han,esting
practices or BMPs used. In both cascs, ror,rghly the
same volume of timber was rcmcx'ed. The South
Fork was selectiveiy harvested to rcmove 65 percent
of the volume while the North Fork was 48 pcrcent
clearcut. Three kinds of BMI's were used in the
North Fork watershed, associated with thc har-
vesting systen, the road system, and strean
plotection.
The South Fork n'as harr.ested using a groundbascd tractor svstem where location of the skid
roads for the tractors was at the discretion of the
tractor operators, meaning that a tractor \^'as driVen
to r.irtually everv log that u'as harvested. This
han'esting method r'r'ould have lelt a quarter to a
third of the n'atershed disturbed (Froehlich l976;
Froehlich et al. 1981) and 15 pcrcent o{ the South
Fork was compacted by roads, landings, and skid
trails (Wright et al. 1990, Lcu.'is 1998). The North
Fork was harvested using skvJine-yarding systcms,
which are hlgh-lift, long-span, cablc-logging
systems that result in less soil disturbance (3 to 4
percent; Rice et al. 1972), and virtually no
compaction.
The second BMP that is credited with reducing
accelerated erosion is improved road systems. The
tractorlogginti system for the South Fork requirecl
that roads be constructed low in the u'atershed ancl
close to the stleam. Over 70 percent of tl.re roacl
system in the South Fork was built r,r,ithin 60 meters
of the stream. The skyline systems used in tlre North
Fork watelshcd requirc fcwer roads, and these roads
are constructed on ridges or high in the watershed
away from streams. Only 3.2 percent of the North
Fork was in roads, landings, and skid trails and was
considcred compacted (Lewis 1998).
Thc third BMP used to reduce accelerated erosion
was stream protection. The S()uth Fork n'as
Landslides, Surface Erosion, and Forest Operations in the Oregon Coast
harvested without any formal buffer strips for
stream protection and with no equipment-exclusion
zones around streams. In contrast, when the North
Fork was logged, selectively logged buffer strips
from 24 to 60 meters in width were left adiacent to
the streams, and heavy equipment was excluded
from these streamside areas.
Manuals are available for BMPs for forest
practices and erosion. The most comprehensive are
those associated with a given state's forest-Practice
rules (see Oregon Department of Forestry 1995).
Even states that lack regulatory programs for
forestry have manuals that describe BMPs for
forestry. In addition, Weaver and Hagans (1994)
have authored a good manual on BMPs to reduce
erosion from forest roads.
Best management practices dnd
ilebris slides dnd flou)s
With recognition of the link between forest
management activities, especially timber harvesting,
and debris flows (Bishop and Stevens 1964;
Swanston and Swanson 7976) carne the need to
prevent or mitigate harvest-related debris flows.
Therefore, BMPs have been developed over the
years to prevent or mitigate the occurrence of debris
flows associated with timber halvesting.
The first step in applying BMPs is recognition of
where landslides are most likely to occur. Identification of the attributes of ilandslide-prone terrain
has been an output of most landslide inventories
(Bishop and Stevens 1964; O'Loughlin L972;
Megahan et al. 1978; Cresswell et al. 1929).
Repeatedly, landslide inventories have shown that
debris flows initiate on steep slopes and in
convergent topography. In general, debris slides do
not occur on slopes less ihan 50 percent, and most
occur on slopes greater than 80 percent. In addition,
debris flows are identified with topographic features
or landforms that are convergent or concave, such
as incipient stream channels, headwalls, hollows,
swales, and deep, v-notched gullies.
The identification of key landscape attributes that
are correlated with initiation sites has allowed the
development of formal risk ratings for debris flow
failures. These risk ratings are determined on the
basis of aggregations of geomorphic and topographic features that are numerically weighted and
summed. One such risk-rating system (Environmental Protection Agency 1980) was the basis for
Range 233
the Mapleton headwall risk-rating system that is
used to rate the risk associated with debris-flowinitiation sites within the Mapleton District of the
Siuslaw National Forest (Swanson and Roach 1987).
The system stratifies headwalls into hazard classes
that describe their likelihood offailure (Martin 1992).
The ODF has used a definition of a high-risk site as
a geomorphic hazard rating for potential initiation
sites of debris flows. Their criteria for defining a
high-risk site include: (1) active landslides, (2)
uniform slopes steeper than 80 percent, (3) headwalls (convergent slopes) steeper than 70 percent,
(4) abrupt slope breaks where the steeper slope
exceeds 70 percent (mid-slope benches), and (5)
inner gorges with side slopes steeper than 60
percent.
Recognizing slope steepness and convergence
and applying geomorphic hazard ratings must be
done in the field, standing on the site of interest and
evaluating iis potential to be a debris-slide or -flowinitiation site. However, computer algorithms can
use digital elevation models (DEMs) with onedimensional stability-analysis and rainfall-runoff
relationships to predict the locations of collur.ial
hollows (Montgomery and Dietrich 1994; Wu and
Sidle 1995). Evaluation of the efficacy of these
models has shown that model output is generally
consistent with empirical findings (Dietrich and
Sltar 1,997; Montgomery et al. 1998; Sidle and Wu
1999). The efficacy of these models illuminates the
importance of topography to the initiation sites for
debris-flow failures, and modeling allows the
locations of potential high-risk sites to be evaluated
over a large land area with a minimum investment
in time and resources.
Of course, the efficacy of the computer algorithms
to predict the location of potential debris-slideinitialion sites is only as good as the data they use
or the DEMs being interrogated. Large scale DEMs,
such as 3O-meter DEMs, can obscure micro toPography and underestimate slope steepness, both
characteristics that are important to landslidehazard assessment. Robison and others (1999) found
a poor correlation between site-specific slope
measurements and those derived from 30-meter
DEMs and they also found that, on an area basis,
3o-meter DEMs overrepresented moderate sloPe
terrain (< 50%) and underrepresented very steep
terrain (> 70%).
Once initiation sites of potential debris-flow
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
associated with timber harvcsting. These BMPs are
designed to maintain more root biomass aftel timber
harvest than woulcl be present after a typical clearcut
silr, icullrrre prercriptiorr. Tlrc cile-\pt-(iIic Prescriptions range from a ban on harvesting to clearcut
silviculture that accelerates the growth of young
trees. At one cnd of the continuum is a complete
ban on timber harvest on large areas that contain
very steep slopes. This is follc'xt'ed by headwallieave areas, which are patches of uncut trees that
are lcft on the high-risk sites or hollows and on very
steep skrpes lr,ithin a harvest unit. These leave areas
can range in size from scr.eral hectares to several
tens of hcctares. Other possible prcscriptions inclucle
minimizing site disturbance during harvest to
maintain understory shrubs or brush to provicle
reinforcement after harvest. For the same reason,
site preparation by either broadcast burning or
silvicultural chemicals u'ould be cliscottraged alter
han'est. Drastic distr,ubance and gotrging of the soil
on these sites during harvest are also discor.rraged
to minimize effects on subsurface prefercntial-flon,
paths and soil strength. On some sites, an intermediate brnsh-releasc spray also would bc
discouraged.
At the opposite encl of the prescription continuun is \.ery intensive treatment of the site,
including clearcut harvest, site prcparation, planting
superior stock, and brush treatment. A window of
vnlneraL.rility after harvest is recognized as a part of
this tleatment (Ziemer 1981). Holvevet instead ot
providing more reinforcement during that period
of vulnerabilitt this prescription narroh's the
r'r'indorl.' of vulncrability by encor.rraging rapid
rcforestation of the site and fast growth of the stock
through intensive forest management.
The efficacy of only one of these BMPs for
treatment of initiation sites has been tested. Martin
(1997) found numerical differenccs fol the fai.h-rre
rates of hcadr'r'alls in r-trur.ranaged foresi (36 percent),
clearcut (38 percent), and headwall-leave area
within a clearcut (41 percent). Thcsc numerical
differences u'ele not statistically significant. The
efficacies of other BMPs have not been tested. That
is, the effects of differcnces in site clisturbance, site
preparatjon, planting-stock qualit)., and intermcdiate treatments have not been tested; all havc
been lumped into thc category of "clearcut" in
landslide inventories.
During the ODF study, invcstigators checked for'
compliance r'r'ith tlre Oregon Forest Practice Rules
at sites rvhere debris-flow failures initiatecl. The
specific forest practices considcred included ljnear
gouging during cable yarding, construction of
tractor skid trails, and accumulations of krgging
slash. Irr no case had the forest practicc rules been
violated, and none of these factors was assocrated
with the irritiation of debris flon's (Robison et al.
19e9).
A final concern is the effect of timber harvesting
on deblis-flor,r, composition, especially that timber
han-esting would result in decreases in the alnount
and size (cliameter and length) of )arge wood in
debris fkrws. What little rescarch has been done on
this subject does rot support that hypothesis. ln firct,
harvested terraln exl.ribits a lcgacy of large wood in
debris flor,r's (May 1998). Howevct if iarge wood is
not allowcd to glow adjacent to dcbris-florv paths,
therr thosc paths that fail will not fill r'vith large
wood. Civen that timber harvest has generally
rednced the amount of large wood in and available
to streams and that large n'ood is an integral
component of ;rquatic habitat ar-rd the storage ancl
routing of sediment, it is imlrortant tbat debris flou's
associated with tirnbcr harvest incJude largc woocl
(see Chapter 3). Therefore, although site-specific
BMPs have not been dcveloped for debris-flow
composition, it r,r'ould scem reasonable to reqllire
both an increase in the size and extent of protected
riparian zones and the establishment Lrr nonharvesting of conifers aJong selected debris-flow
paths.
Watershed nlanagement and debris
flows
A growing body of researcl-L ancl tlrought proposes
a new paradigm for the managemerrt of landslideprone forestecl terrain. Thc new paradigm does not
prcscribe BMPs alone to mitigate the effect of timbcr'
han'esting on debris flows and aquatic habitat.
Ilather, it is basecl on the prescription of forest
managcment activities on a larger scale in both sprace
and time. This watershed-scale prescription of forest
mana[Jement activities is designed to make timber
harvest morc nearlv mimic a natural disturbance
regime. Thc theoretical framer,r'ork and the basis for
this neu' paradigm is presented by Reeves et al.
(1995) and in Chapter 4.
Anadromous salmonids evolved in ecosystems
that were dvnamic in both space and timc. The
Landslides, Surface Erosion, and Forest Operations
dynamic nature of aquatic habitat is the result of a
natural disturbance regime that is, in turn, the result
of stand-replacing wildfires, which leave the terrain
r.ulnerable to landslides, whose frequency, size, and
spatial distribution on a landscape scale create a
range of channel conditions. When a landslideproducing stolm occurs subsequent to a wildfire,
the resulting widespread occurrence of debris flows
transports large qr-rantities of wood and sediment
mto the stream channel. The episodic delivery of
sediment to stream channels causes them to ihift
between periods of aggradation and degradation.
Because the quality of aquatic habitat is i function
of channel conditions, it will also shift, with the
optimum_in complex aquatic habitat occurring
between the extremes of aggraded and degradeJ
states.
Several attributes characterize a natural dis_
turbance regime in the Oregon Coast Range (Benda
in
the Oregon Coast
Range 235
including the episodic delivery of large amounts of
sediment and large wood.
Several prescriptions at the landscape scale might
.bring
a managed disturbance regime to more closely
resemble a natural disturbance regime: (1) lengthen
the rotation ages to t 150 years; (2) reduce the
proportion of the landscape in young stands
(Robison et al.'1999); (3) aggregate harvest units
lnto one area in a watershed instead of scattering
them across the landscape; and (4) leave debris_flow
paths with a biological legacy of large wood. The
debris-flow paths that have the strongest likelihood
of delivering large wood to sheams can be identified
(Benda 1990) and conifers should be established,
allowed to become large, and left to enhance the
biological legacy of subsequent debris flows.
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
units, it may be more important and realistic to
institute forest management schemes in which
resulting debris flows mimic natural processes lvith
re6;ards to timing and large-wood composition as
much as possible.
Forest management can affect aquatic habitat by
cl-ranging the timing and composition of debris
flows. Timber harvest changcs the disturbance
pattern of a forested landscape by introducing
rotation ages that are shorter than the interval
between naturally occurring wildfilcs and by
spreading timber harr.est out in space and time.
Fulthermore, timber harvest removcs more large
u'ood and leaves less standing and down large
woocl than wildfires. lt is possible to develop BMPs
that will ultimately result jn debris-flow paths
retaining sufficient large woocl. However, landscape
management, not BMPs, is neecled to provide a more
rratural timing of the occurrence of debris flows.
Benda, L. E.. and T. Dunne. 1997b. Stochastic forcinli of
sediment routing and storagc in chamrel netu,orks.
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IAHS Publication No. 132,
Range 241
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
O 2002 Oregon State University Press
All rights reserved. First edition
2002
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