I Kinking about
salmon, drinking
water, forests and
the future?
" hen the Xssociation brought watershed ecologist Rich \ a » a to «peak in Seaside,
this |M«t spring, he mentioned this article by Robert /ie m e r as the best there is on
steepland erosion. H e hope sou read it carefully in order to appreciate the
importance of this issue. Robert Z.iemer works for the Forest Sers.ce at the Pacific
Southwest Forest X Range I tperim ent Station, \rc a ta . ( V
Think........
STEEPLAND
EROSION
Steepland erosion is
eon tro lle d most effectively,
both in phy sical and economic
terms, by preventive land-use
practices ra th e r than
corrective action.
By Robert Ziemer
D e b ris a v a la n c h e s are rapid sh a llo w h ills lo p e
'steepland erosion is the result o f comj»|ieated
in teractio n s betw een clim ate, sod. getdogy, to p o g ra p h y
and vegetation A change in anv o f these co m p on e nts
mav re su lt in an a d ju s tm e n t o f the d riv in g or resisting
forces and lead to increased o r decreased erosion
process.
I his paper describes h o w the in teractio n s of
clim ate , soil, geology, to p o g ra p h y, and vegetation affect
the m anagem ent o f steepland erosion. Ihrc-x- types of
erosion are considered surface, channel, and mass
erosion.
In surface e ro sion , in d iv id u a l soil particle's an
rem oved by ra in d ro p s, th in film flo w , and concentrated
surface ru n o ff in the fo rm o f sheet and n il erosion Sur
face erosion is characterized by the lack o f p erm anent
channels. In u n d is tu rb e d steepland ton-sis surface
erosion is g enerally in s ig n ific a n t because in filtra tio n rales
u sua lly exceed ra in fa ll in ten sitie s
failures gen e ra lly fo u n d in shallow noncohesive* soils on
steep slopes where- subsurface wate r becomes con
centrated I L int roots can reduce the frequency o f the-s.
sh allow failu res Roots can anchor th ro u g h the soil mass
in to fractures in bedrock Ih e y can also d e v e lo p lateral
support by crossing zones ot Weakness to m o re stable
soil as w e ll as p ro v id in g lo n g fib ro us h in de rs w ith in a
weak soil mass
I "K H 'n K- road co n stru ctio n , w ild fire s , o r mass
erosion, h o w e ve r, can expose m in e ra l soil w here the
n a tu ra lly h igh p o ro sity of forest soils m av be severely
reduced by ra in d ro p im pact and com paction by heavy
e q u ip m e n t. I ire can also pro du ce w a ter repel,ency in
steepland soils If the re su ltan t flo w o f w a te r o ver these
bare areas is not c o n tro lle d , surface erosion mav progress
fro m sheet to rill to g u lly erosion as channels are fom nrd
Channel erosion is the d eta chm en t and m ove m e nt
o f m a teria l from a g u lly o r stream channel I he m aterial
m ay be in d iv id u a l particles d e rive d fro m the channel
skin , p er se; or it m ay be se dim en t eroded by surface or
mass erosion that was deposited in the channel, for
exam ple, w hen an u n d e rc u t stream bank collapses in to
the channel
I j n d m anagem ent a ctivities in flu e n ce channel
erosion p rin c ip a lly by plac in g e ro d ib le m a teria l in existing
channels; by in tro d u c in g large organic debris in to sm all
p eren n ial channels; by increasing surface ru n o ff from bare
and com pacted soils, bv m o d ify in g the surface m icro
d rainage n e tw o rk by roads, tra cto r trails, and ditches,
and by c o n v e rtin g subsurface drainage to surface ru n o ff
(i e., bv intersectin g subsurface flo w w ith road cuts)
t revp is the s lo w d o w n slo p e move m e n t o f the soil
m a n tle w h e re the long term g ra vita tio n a l shear stress is
large enough to p ro d u ce p erm anent d e fo rm a tio n b ut too
sm all to cause discrete failure. ( reep is the m ost co m m on
and W’idespread mass erosion process m steeplands, but
is the least und e rstoo d and d ocu m e n te d
A lth o u g h d ire ct m easurem ent o f m anagem ent
in duced changes in creep rate mav be nearly im possible,
the q u a n tity of m aterial d e live re d to the n um e ro u s stream
channels in the area can be large I o r exam ple, it tim b e r
c u ttin g increased the average- creep rate in a catchm ent
fro m i to 10 m m y r, the- change w o u ld p ro ba b ly n ot be
noticed even by detailed h iliso p e o bservation. But the
q u a n tity o f soil added to stream channels w o u ld be
trebled, and the change in sedim ent tra n sp o rt m av be
easily detected
I a rth tlo w can be- considered accelerated creep
w h e re shear stress exec-eds the stre ng th o f the soil m antle
and results m discrete failures I he rate o f m o ve m e n t of
e a rth flo w s . as w ith creep, mav Is- imperceptibly slo w , but
can exceed a m etre per day
M o ve m e n t m av be co n tin u o u s, seasonal, or
episodic. I ike creep, deep seated e a rth flo w s may be
affected little bv tim b e r c u ttin g o r road b u ild in g unless
the d is trib u tio n o f mass o r the w a te r re la tio n sh ip s w ith in
the slide changes su bsta ntia lly The d is trib u tio n of mass
can Is- changed by excavations w h ic h u n d e rcu t the lex- o f
the e a rth flo w , re m o vin g d o w n slo p e s u p p o rt
Koad till can add mass to the head o f an
e a rth flo w , a d d in g to the g ra vita tio n a l forces c o n trib u tin g
to slope failu re Koad« can also m o d ify the w a ter relations
w ith in the e arth flo w Koad cuts can in te rce p t subsurface
flo w . If this w a ter or surface road drainage is d ive rte d
aw ay fro m the e a rth flo w , (he slide below the road m ay
become m ore stable It w a ter is d iv e rte d o n to the slide,
d o rm a n t e a rth flo w s may Is- reactivated
Tim ber c u ttin g can also m o d ify the in te rn a l w a ter
relations o f the e a rth flo w 1 v a p o tra n s p ira tio n by forests
may deplete SO ft» 'S cm o f soil m o istu re p e r year
I here are interactions and feedback m echanism s
between erosion types In some cases, channel incision
u nd e rcu ts the toes of e a rth flo w s, u p s e ttin g the balance of
forces on the h ills lo p e In o th e r cases, a ggradation w ith
ac- com pan led increases in bank erosion u n d e rc u t the
tc*es o f e a rth flo w s In sm all steep stream s, in cision is
m ore co m m o n than a ggradation, w h ile in large lo w -
g ra d ie n t stream s the reverse is true A ccelerated e arth -
flo w erosion, in tu rn , can m o d ify o th e r types o f erosion
U h c rc- and how land m anagem ent is conducted
.in tin tw o p rim a ry " ’!’'>idi r a l i o i i s i n e t t o j |s t ,.r e d u c e
-t. > ¡ 'la in ! «
I he how c o n s id e ra tio n is o llc n
tin .lig h t t.» h. c o m p le t,d W ith p la n n in g . A lth o u g h good
p la n n in g is a m a jo r and necessary step in m in im iz in g
' '
th* carrying
h of the plan is ..Il ............
m id. rp la v e d
I he on the g ro u n d o p e ra to r is the key to
success or ta ilu i. ot a plan C o m m o n ly little e ffo rt is
« xpended to in c lu d , operators m the p la n n in g process
In g. n, ial th e ir skills have been developed th ro u g h
personal , x js -rie n ,,- o f w hat seems to w o rk
I n to rtu n a t, lv w h a t w orks best to r d ra g g in g a log
or co n s tru c tin g a stream crossing may not be bc-st tor
m a naging ,-rosion \n im p o rta n t part in m anaging
st. . pla nd erosion is succe ssful inte ractions betwe en
p la nn , ;s am ! operators Success is often based as m uch
on personalities .is on technical abilitie s
Manv manage m e n t rules approach p re ve n tio n as
th o u g h there was an equal p ro b a b ility o f erosion
o c c u rrin g .it any given location It is becom ing
uh r. astnglv clear that most steepland erosion « c u r s in a
lim ite d number of areas and that m ost ,»f the- area
produces o n ly a sm all a m o un t ot e rosion
I.» ,-tteitivelv m an ag e erosion in steeplands, it is
m o te im p o rta n t to sjiecttv w h ere land is to be treated
than to I», concerned w ith how m uch land is to be
treated
111» key to successful m anagem ent of erosion is
th, a b ility to I) id e n tify pote n tia lly e ro d ib le sites 2)
c o rre c tly asse ss a p p ro p ria i, activités on those sites, and 1)
has. a p o litic a l re g u la to ry system that a llo w s to r the
ex. lusion ot hazardous sites fro m land tre atm e n t In some-
eases th. most a pp ro p ria te ac tiv ity on a sit,- m ay be no
activity lit . cost ie»|uiied to c o rn . I m anagem ent
in d uce d erosion is , .ft« n tar beyond the benefits obtained
fro m th.- land m anagem ent a, tiv ity or the costs required
to fo llo w a m ore sensitive a lterna tive plan,
A lth o u g h m any studies have d ocu m e n te d debris
avalanche e rosion fo llo w in g lo g g in g roads appear to
increase- the- fre q u e n cy of d e b ris avalanche m uch m ore
than does tim b e r c u ttin g In a d d itio n to p ro fo u n d ly
a ffe ctin g the soil w a te r regim e, road cuts can intersect
and u n d e rcu t the sh a llo w fa ilu re surface, and road fills
can add a su bsta ntia l mass surcharge to the slope.
D e b ris to rre n ts are the failure and rapid
move-ment o f w a ter-sa tu ra ted soil, r«x k and organic
debris in sm all, steep stream channels Debris torre nts
m ig h t be co nsidered a tra n s itio n a l lin k between debris
avalanche mass erosion and channel erosion I hey
typ ic a lly « c u r d u rin g ,». r i« |s o f h ig h ra in fa ll and
stre a m tlo w I hey m av be started bv a debris avalanche
w h ich enters the channe l o r they m av result from an
in itia l fa ilu re o f accum ulated debris w ith in the channel
typically, as debris fro m the fa ilu re m oves d o w n slo p e . it
traps large q u a n titie s of a d d itio n a l m a teria l obtained fro m
the channel banks and bed. I he re s u ltin g channel m av be
scoured to bedrock for a great distance
L an d m anagem ent activites m av increase the
fre qu e ncy o f debris torre nts bv increasing the q u a n tity ot
w a ter d e live re d to a channel o r by increasing the q u a n tity
o f de bris m a channe l C h a n n e l flo w can be d ra m a tically
changed by roads in te rc e p tin g subsurface How. re ro u tin g
o f sm all drainage n e tw o rks, a n d the co nce n tra tin g ot
surface ru n o ff fro m com pacted road o r tra ctor trails
Mate-rial fro m accelerated h ills lo p e erosion can increase
the a m o u n t o f debris a ccum ulated in channels
Koad tills at stream crossings place a large- mass of
rock and so il in channels It is co m m o n fo r road culverts
m sm all steep stream channels to p lu g w ith soil and
organic de bris, re su ltin g in sa turation and fa ilu re of the-
road till, f a ilu re o t road crossings is a p rin c ip a l cause- o f
accelerated channel erosion and de bris to rre n ts in m any
fe»rested stee p la nd are as
I he key to successful management of
erosion is the ability to:
I ) Identify potentially erodible sites.
2) Correctly assess appropriate
activities at those sites, and
3) Have a political/regulatory system
that allows for the exclusion of
hazardous sites from land treatment.
—
*
I hannel erosion and mass erosion are u sually
a s s « ia t,-d w ith rare storm s G uid e lin e s tend to address
th, c o n tro l ot erosion on the basis o f the typical event
(sto rm ) It is howe ver n o rm a lly tin unusual 'e v e n t
w h ic h produces tin erosjonal charac (eristics that are-
generally considère el to In- unacceptable I ffo rls to
co n tro l erosion fro m tin- tvpie.il ru n o ff event could le ad Io
m ore erosion d u rin g tin- large- storm
( ’ tie com m on m e tho d te» m in im iz e road-related
debris torre n ts is ,.» in stall oversized " cu lverts or to
bridge the- w ater cours. I his m e tho d is often discounted
Is c a u s e o f Its h igh in itia l expense ( o n s tru c tio n costs are
fre q u e n tly v ie w e d in the sh ort term and fail to in clude
subsequent costs ot m aint, nance- and replacem ent It the
a cco u n tin g system in cluded tin- total costs required
d u rin g the design lit. of the project m any c u rre n t
co nstru ctio n practices w o u ld probably be changed.
M anagem ent activités ta n m o d ify the- s ta b ility of
debris w ith in the channel I he | « a l g ra d ie n t ot a
ste-e j»l.inil e lia n n e l, as w e ll as its s ta b ility , is often
co n tro lle d by l» e d r« k H o w e ver, large w o o d y debris, a
n atu ra l c o m p o n e n t o t fore sted steepland channels, can
also co n tro l ch an n el g ra die n t I he residence- tim e o f large
decay resistant logs, such as large logs of D o u g la s-fir,
mav rem ain in a .h a n n e l fo r several h u n d re d years
W hen this organic de bris decays, accum ulated m a teria l is
subje c t to i hannel erosion and. fu rth e r, is available fo r
ra p id m o b iliz a tio n in to a de bris torrent
It efi.inne I s ta b ility is co ntro lle d bv the long term
s u p p ly ot larg. organic debris, am) large trees adjacent to
channels are e lim in a te d by co n tin u e d forest
m anagem ent, active ch an n el erosion mav fo llo w the
decay of e xistin g logs because new larger logs arc- no
longer available for replacem ent In intermittent
channels, live resits fro m s u rro u n d in g tree s p ro vid e
su bsta ntia l stre ng th and re in forcem e n t to the channel
bed If the se tree s arc cut, the Strength of the debris
Com posing the bed w ill p ro g re ssive ly w eaken as the resits
decay I his c o n d itio n may re s u lt in accelerated channel
erosion o r increased risk of a d eb ris to rre n t
I ’ru d e n t m anagem ent s h o u ld id e n tify the values
•it risk and dirc-e t c-nwion c o n tro l activités tow ard
proce sses m ost like ly to affect those value s Steepland
erosion is co n tro lle d most e ffe ctive ly, both in physical
a m i econom ic term s, bv p re ve n tive land-use practices
rath, r than corrective action M anagem ent of steepland
« rosion is m erely the a p p ro p ria te a p p lica tio n of v a ry in g
levels o f ta re and caution w hen d e a lin g w ith terrain of
v a ry in g erosional s e n sitivity.
There is a great tendency to fix past m istakes.
H o w e v e r, unless m ore e ffo rt is d evo te d t<» lo o k in g
fo rw a rd to w a rd p re v e n tio n rather than backw ard to w a rd
co rre ctio n, w e w ill c o n tin u a lly be try in g to catch up.
I he successful m anagem ent o f erosion is as m ilc h a
p h ilo s o p h ic a l and , »ohfie.il p ro b le m as a technical one.
VTO in t to&t s m m i l l