Instruction and article attached 

30 Washington Geology, vol. 29, no. 1/2, September 2001

Landslide Hazard Mapping in Cowlitz County—
A Progress Report

Karl W. Wegmann and Timothy J. Walsh

Washington Division of Geology and Earth Resources

PO Box 47007, Olympia, WA 98504-7007

INTRODUCTIO

N

The need for mapping of potential geologic hazards such as
landslides, volcanic lahar inundation zones, and areas of earth-
quake-induced liquefaction susceptibility is increasing in step
with regional population growth and expansion of the urban
fringe into once sparsely populated rural forest and agricul-
tural lands. This article discusses in-progress landslide hazard
mapping for the urban growth areas of Cowlitz County (Fig. 1)

.

With passage of the Washington Growth Management Act
(GMA) and amendments in 1990 and 1991, counties and cities
were directed to delineate critical areas (including those sub-
ject to geologic and hydrologic hazards) to aid in formulating
regulations governing development in such areas (Brunengo,
1994). Although Cowlitz County did not meet the population
threshold for inclusion in the GMA and therefore was not re-
quired to develop a comprehensive plan of action, the county
was required to establish a critical areas protection ordinanc

e

(CAO), which was adopted in 1996 (Cowlitz Co. Ordinance
96-104). Section 19.15.150 of this CAO pertains to geologic
hazard areas, including landslide hazard areas. Identification
of potential slope-stability hazard areas within the rapidly ur-
banizing areas of Cowlitz County is an important first step to-
ward effective implementation of the geologic hazards section
of the county’s CAO.

The purpose of the current landslide hazard mapping pro-
ject in Cowlitz County is to update and expand previous slope
stability studies for the Longview–Kelso urban area (Fiksdal,
1973) and to extend slope-stability mapping to include the
high-growth areas adjacent to the Interstate 5 corridor from the
Clark County line in the south to the Toutle River in the north
(Fig. 1). The intended outcome of this mapping project is the
production of landslide hazard maps and an associated data-
base delineating the distribution of identified deep-seated
landslides (landslides that fail below the rooting depth of vege-
tation) as well as areas in which the combination of geologic
and topographic factors favor the likelihood of future slope in-
stability. Deep-seated landslides are often large in areal extent
and once reactivated, by either natural causes or land manage-
ment practices, often prove to be expensive and difficult
(sometimes impossible) to mitigate. Updating and extending
landslide hazard mapping for Cowlitz County will allow
county officials to make better-informed decisions regarding
implementation of slope-stability provisions in their CAO. In-
tended benefactors from this hazard mapping project include
county and city governments, private citizens, state and federal
agencies, geologic consultants, public and private utility cor-
porations, and land developers.

PROJECT HISTORY

Significantly higher than normal annual precipitation was re-
corded for most of western Washington State, including Cow-
litz County and the Longview–Kelso urban area, beginning in

the 1995/96 water year (October 1 to September 30) and lasting
through the 1998/99 water year. The several-year increase in
annual precipitation resulted in elevated ground-water levels
that, in turn, likely triggered reactivation of numerous dormant
deep-seated landslides throughout southwestern Washington.
In February of 1998, a deep-seated earth slide–earth flow reac-
tivated in the Aldercrest neighborhood of Kelso (Figs. 2–4). In
October of 1998, President Clinton issued a federal disaster
declaration for the 138 homes affected by the landslide (Burns,
1999; Buss and others, 2000).

In response to the Aldercrest–Banyon landslide and numer-
ous other recent landslides in Cowlitz County, geologists from
the Washington Division of Geology and Earth Resources
(DGER), Cowlitz County officials, and members of the state
legislature representing southwestern Washington recognized
the need for improved slope-stability mapping within the ur-
banizing Interstate 5 corridor. During the second half of 1998,
in preparation for the 1999–2001 biennial state budget, the
Washington Department of Natural Resources (DNR) re-
quested and received funding from the state legislature for geo-

study area
(approximate)

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(Figs. 2, 3, and 4)

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location of Fig.

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EXPLANATION

map
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Figure 1. Location of the study area.

Washington Geology, vol. 29, no. 1/2, September 2001 31

l o g i c h a z a r d m a p p i n g t o e v a l u a t e
ground stability in high-growth areas
and to provide geologic expertise to
small communities.

DGER began the Cowlitz County
Landslide Hazard Mapping Project in
February of 2000. Approximately 200
square miles were identified by Cow-
litz County GIS Department staff as
critical to the urban growth needs of the
county and in need of improved slope-
stability mapping (Fig. 1). Partnerships
were established between geologists
from Oregon State University and the
U.S. Geological Survey to bring to-
gether various geologic mapping pro-
jects to provide coverage for the entire
study area at a scale of 1:24,000. To fill
g a p s i n t h e c o v e r a g e a t t h i s s c a l e ,
DGER geologists will also map por-
tions of the Kalama and Mount Brynion
7.5-minute quadrangles.

PROJECT TIMELINE
AND METHODS

The project timeline calls for all work
to be completed within three years of
initiation, by early 2003. During the
winter and spring of 2000, potential
deep-seated landslides were delineated
using DNR 1993 (1:12,000, black &
white) and 1999 (1:12,000, color) ae-
rial photographs. Previous landslide in-
ventories in western Washington State
have shown that the combination of ae-
rial photograph interpretation and in-
the-field verification is an effective
method for properly identifying deep-
seated landslides (for example, Drago-
v i c h a n d B r u n e n g o , 1 9 9 5 ; G e r s t e l ,
1999). Field verification of individual
landslides identified during the initial
aerial photographic analysis, as well as
the mapping of geologic conditions
conducive to slope instability, com-
menced in the summer of 2000 and is
planned to continue through the fall of
2 0 0 1 . T h e c o m p i l a t i o n o f g e o l o g i c
mapping and identified landslides and
the construction of a landslide database
will be completed in 2002, with publi-
cation and presentation of results in late
2002 to early 2003.

Landslides verified by field evi-
dence will be digitized into ArcView
coverages using 1:12,000 DNR digital
orthophotos. Our goal is to release pub-
lished maps as both digital (ArcView
coverages) and paper products along
with a landslide database in Microsoft
Access. Database fields will include: a
unique identification number, location,
state of activity (active, recent, dor-
mant, or ancient), certainty of geologist

Figure 3. View northwest along the main scarp of the deep-seated reactivated Aldercrest–

Banyon (Kelso, WA) earth slide–earth flow as it appeared in August 2000. Landslide motion initi-

ated in February of 1998 and by October of the same year had affected 138 homes, causing Presi-

dent Clinton to declare it a federal disaster area. Damage to public facilities and private property is

estimated in excess of 30 million dollars (Buss and others, 2000). The landslide is about 3,000 feet

wide by 1,500 feet in length, and the main scarp is over 100 feet high in places. Note the destroyed

houses and tilting trees at the base of the scarp. Prior to the landslide, these houses were slightly

above the elevation of the top of the scarp. This photo was taken in the former basement (light gray

area on the left) of a house now at the bottom of the hill outside the photo area. The scarp exposes

Pliocene to Pleistocene fluvial gravels and sands of the Troutdale Formation.

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pre-existing dormant scarp reactivated Aldercrest–Banyon slide

0 400 ft

Figure 2. Stereophoto pair of Aldercrest–Banyon Landslide from 1999 DNR aerial photographs.

Note that the reactivated portion of the slide is interior to a larger landslide feature, as defined by

the pre-existing dormant scarp. To view this photo in 3D, focus your eyes on the far distance and

bring this figure up in front of your face at your normal reading distance.

32 Washington Geology, vol. 29, no. 1/2, September 2001

that feature is a landslide (definite, highly prob-
able, probable, or questionable), cause of land-
s l i d e i f d e t e r m i n a b l e ( n a t u r a l , h u m a n – i n-
f l u e n c e d ) , l a n d s l i d e d i m e n s i o n s , g e o l o g i c
unit(s) involved in failure, type of impacted in-
frastructure, and previously reported identifica-
tion and (or) mitigation work conducted on indi-
vidual landslides if any.

LANDSLIDE TYPES
IN THE STUDY AREA

Much of southwestern Washington, and the
study area specifically, was not glaciated during
the Pleistocene Epoch. The lack of glacial ero-
sion in the recent geologic past means that, in
places, the ground has been subjected to weath-
ering processes for millions of years (Thorsen,
1989). This has resulted in deeply weathered
clay-rich soils (saprolites) formed by the weath-
ering of Tertiary sedimentary and volcanic
rocks as well as unconsolidated upper Tertiary
to Quaternary fluvial and eolian deposits. Ex-
tensive portions of the study area are underlain
by Tertiary sedimentary and volcanic rocks con-
taining inherent weaknesses, such as dipping
bedding planes, joints, brecciation and shear
zones, paleoweathering (paleosol) surfaces, and
clay-rich interbeds. Many bedrock-dominated
landslides initiate along such inhomogeneities.
Upper Tertiary to Quaternary fluvial deposits of
the ancestral Columbia River form dissected ter-
races along the lower slopes of the study area,
filling in paleotopography developed upon the
underlying Tertiary bedrock. Many of these sur-
ficial deposits have weathered almost entirely to
high-plasticity clays.

L a n d s l i d e s w i t h i n t h e s t u d y a r e a o c c u r
within Tertiary sedimentary and volcanic units
(Fig. 5), at the interface between Tertiary bed-
rock and overlying younger unconsolidated flu-
vial units (Fig. 3), and within the younger un-
consolidated deposits (Fig. 6). The dominant
form of landsliding within the study area is the
rotational to translational earth and (or) rock
slide, composed of extensively weathered bed-
rock and (or) surficial deposits (Figs. 3–6).
Faster-moving rock falls and topples are limited
to the steep bluffs along the Columbia River
west of Longview, the inner gorges of the Ka-
lama and Coweeman Rivers, and the rocky
headscarps of some of the larger rock slide com-
plexes. Many of the larger landslides appear to
have multi-part movement histories (Fig. 7), as
exhibited by recently active deep-seated fail-
ures such as the Aldercrest–Banyon slide that
have reactivated only a portion of the larger
overall landslide feature (Fig. 2). Also within
the study area are gently to moderately sloping
regions that are not distinct landslides, but
rather areas of prominent slope creep. These ar-
eas are underlain by thick deposits of high-plas-
ticity (and potentially swelling) clay derived
from the weathering of both the underlying bed-
rock and surficial deposits. Such areas of accel-

Figure 4. View to the southeast across the middle section of the Aldercrest–Banyon

landslide. Two uninhabitable houses are present in this view. Note the internal rotation

within the landslide body as evidenced by the back-tilting of the distant house.

Figure 6. Human-influenced, small deep-seated rotational earth slide–earth flow north

of Kalama. The slide is about 75 feet wide by 40 feet long by 15 feet deep and is failing in

a clay-rich diamicton (older landslide debris). This landslide initiated after a period of

heavy rain in the spring of 2000. The slope had recently been cut back to enlarge a pri-

vate yard, resulting in a lack of lateral support for the lower portion of the slope.

landslide scarp pipeline right-of-way

Figure 5. Large deep-seated rock slide along the north side of the Kalama River. View

is to the north, across the Kalama River valley. This slow moving 90-acre landslide is fail-

ing in Tertiary volcanic and volcaniclastic rocks. In 1996, movement on this landslide

ruptured and ignited a natural gas pipeline that is routed across the landslide.

Washington Geology, vol. 29, no. 1/2, September 2001 33

erated slope creep can be damaging to structures and utilities
over time.

CAUSES OF LANDSLIDES

A majority of the deep-seated landslides so far identified in
this study seem to have been triggered by natural causes. The
primary initiating factor behind many of the landslides appears
to have been climatically driven increases in ground-water lev-
els and soil pore-water pressures. Some of the inactive deep-
seated landslides may have been seismically induced. During
the 1949 Olympia earthquake, for example, rock falls and earth
slides were reported within the study area (Chleborad and
Schuster, 1998). It stands to reason that if a moderate to large
earthquake occurred close to the study area, especially during
the wet season when ground-water and soil moisture levels are
elevated, landsliding might result. A third triggering mecha-
nism for landslides in lower elevations (below approximately
250 feet above mean sea level) may have been the rapid
drawdown of late-Pleistocene glacial outburst floodwaters
(Missoula floods) along the Columbia River and tributaries.

A significant minority of landslides appear to have been
influenced by human activities (Fig. 6). Land-use modifica-
tions can alter the amount and flow direction of surface and
ground water on slopes, which in turn may trigger slope fail-
ure. The undercutting of slopes for roads, building founda-
tions, pipelines, and other construction projects has also been
observed to contribute to slope failure. In a fair number of
cases, it may be the combination of slope modification by hu-
mans and an increase in annual and regional precipitation lev-
els (such as occurred during the late 1990s) that triggers slope
failure.

RESULTS TO DATE

To date, approximately 350 individual deep-seated landslides
have been field-verified in the southern half of the study area.
Of these landslides, about 20 percent exhibit demonstrable evi-
dence of movement within approximately the past 5 years.
Field verification of landslides and areas of potential slope in-
stability will continue throughout the summer and fall of 2001.

CONCLUSIONS

Landslides such as the Aldercrest–Banyon slide serve as stark
reminders of the potentially devastating consequences of hu-

man development on unstable slopes. As our population in-
creases outward from established urban areas, the need for new
and updated geologic hazard mapping increases in step. It is
with this in mind that the intended and ultimate goal of this pro-
ject is to provide the citizens of Cowlitz County and Washing-
ton State with socially relevant slope-stability maps based
upon the identification of areas of potential geologic instability
and individual deep-seated landslides.

REFERENCES

Brunengo, M. J., 1994, Geologic hazards and the Growth Manage-
ment Act: Washington Geology, v. 22, no. 2, p. 4-10.

Burns, S. F., 1999, Aldercrest landslide, Kelso, Washington, engulfs
subdivision [abstract]: Geological Society of America Abstracts
with Programs, v. 31, no. 6, p. A-41.

Buss, K. G.; Benson, B. E.; Koloski, J. W., 2000, Aldercrest–Banyon
landslide—Technical and social considerations [abstract]: AEG
News, v. 43, no. 4, p. 78.

Chleborad, A. F.; Schuster, R. L., 1998, Ground failure associated
with the Puget Sound region earthquakes of April 13, 1949, and
April 29, 1965. In Rogers, A. M.; Walsh, T. J.; Kockelman, W. J.;
Priest, G. R., editors, Assessing earthquake hazards and reducing
risk in the Pacific Northwest: U.S. Geological Survey Profes-
sional Paper 1560, v. 2, p. 373-440.

Cruden, D. M.; Varnes, D. J., 1996, Landslide types and processes. In
Turner, A. K.; Schuster, R. L., editors, Landslides—Investigation
and mitigation: Transportation Research Board Special Report
247, p. 36-75.

Dragovich, J. D.; Brunengo, M. J., 1995, Landslide map and inven-
tory, Tilton River–Mineral Creek area, Lewis County, Washing-
ton: Washington Division of Geology and Earth Resources Open
File Report 95-1, 165 p., 3 plates.

Fiksdal, A. J., 1973, Slope stability of the Longview–Kelso urban
area, Cowlitz County: Washington Division of Geology and Earth
Resources Open File Report 73-2, 4 p., 2 plates.

Gerstel, W. J., 1999, Deep-seated landslide inventory of the west-cen-
tral Olympic Peninsula: Washington Division of Geology and
Earth Resources Open File Report 99-2, 36 p., 2 plates.

Thorsen, G. W., 1989, Landslide provinces in Washington. In Galster,
R. W., chairman, Engineering geology in Washington: Washing-
ton Division of Geology and Earth Resources Bulletin 78, v. I,
p. 71-89. �

ANATOMY OF AN EARTH SLIDE–EARTH FL

OW

FO
OT

SU
R

F

AC
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F

SE
PA

RA
TIO

N

MAIN BODY

ORIGINAL GROUND
SURFACE

RI
G

H
T

FL
A
N
K

CROWN

MINOR SCARP

HEAD

transverse

cracks

transverse
ridges

radial
cracks

surfa
ce

of
ru

pt
ur

e

toe of
surface of

rupture
TIP

L

TOE

crown
cracks

MAIN SCARP

EA
RT

H
FL

OW
EA
RT

H
SL

ID
E

DI
SP

LA
CE

D
M

AT
ER

IA
L

longitudinal
fault zone

TOPtransverse

cracks

Figure 7. Anatomy of an idealized complex landslide, a deep-seated

earth slide–earth flow. Labeled components apply to most landslides.

From Cruden and Varnes (1996).

IDENTIFYING UNSTABLE SLOPE CONDITIONS

Landslides can often be identified in the field through careful
observation. Tension cracks, hummocky topography, springs
and seeps, bowed and jackstrawed trees, abrupt scarps, and toe
bulges are all readily observable indicators (Fig. 7, p. 33).

Tension Cracks—Tension cracks, also known as transverse
cracks, are openings that can extend deep below the ground
surface. Tension cracks near the crest of an embankment or
hillside can indicate mass movement. However, cracks may oc-
cur anywhere on the slide. They are perpendicular to the direc-
tion of movement and are typically continuous in a pattern
across the width of the landslide. Tension cracks can fill with
water, which lubricates the slide mass and may cause addi-
tional movement.

Hummocky Ground—Hummocky ground can indicate past or
active slide movement. A slide mass has an irregular, undulat-
ing surface.

Continued on next page.

CE112 Homework 5 – Article Review Winter 2020

1/2

Due: TUESDAY 02/25/2020

• Turn in single-sided, printed and timestamped copy to Box in EB200 CEE office by 4PM
• Turn in PDF to D2L HW5 Submission Folder by 4PM
• No assignments are accepted after 4PM on the due date.

Objective:

There are six articles related to engineering failures or unintended design consequences posted on the
HW5 folder on D2L. The first week of class you will be assigned one of the articles to review. To prepare
you for a discussion of the articles on March 11, 2020 your review should address the following points:

1. Summary of Incident (~ ¾ page): Briefly summarize the history surrounding the project or
event discussed in the article.

2. Discussion (~ ¾ page): Comment on the following:
a. Was the failure or unintended consequence preventable? If so, how?
b. What impact does the failure or unintended consequence have on society and/or the

environment?
c. What responsibility does the professional engineer with respect to the event discussed?

3. Recommending Article to Others (~ 1 paragraph): Would you recommend the instructor use
this article for this assignment in the future? Why or why not? Give at least 2 reasons.

Essay Grading: (see rubric on page 2)

• Grammar, spelling and punctuation (10 pts) – Use spell-check and proofread for proper
punctuation. Try to use Word’s grammar check but do not always believe it. Consider using the
Grammarly add in for Word or other online tool. Having a classmate review your essay is highly
encouraged.

• Content (25 pts) – See points to address in the objectives section above.
• Class Discussion (10 pts) – Actively listen to discussion and volunteer feedback.
•

Requirements

(5 pts) – Please don’t lose any points on the basics.

Requirements
Page order:

1. CE 112 Cover sheet completed with your name and title of articles review. Cover sheet template
is provided on D2L.

2. Text of your review (~2 pages) (do not include a copy of the article).

The required format elements are:
• Double spaced text.
• Times New Roman eleven (11) point font.
• One inch page margins.
• Numbered pages (on the bottom).
• It is highly suggested that you use the Word template provided. (Do not change the styles).

https://www.grammarly.com/

CE112 Homework 5 – Article Review Winter 2020

2/2

Grammar, Spelling, and Punctuation (10 pts)

Below Avg. (5 pts) Average (7 pts) Good (8 pts) Excellent (10 pts) Points

Many errors and a large
number of the corrections
are major errors.

Several corrections but
most of the corrections
are minor errors.

Few corrections,
mostly minor.

Almost no
corrections.

Content (25 pts):

Summary of Incident:

Below Avg. (4 pts) Average (6 pts) Excellent (10 pts) Points
Writing style is hard to follow or
summary is very brief.

Followable writing and
organization.

Clear, concise writing and
organization.

Discussion:

Below Avg. (4pts) Average (6 pts) Excellent (10 pts) Points
Discussion is brief and/or
address only one discussion
prompt.

Acceptable discussion which
addresses at least two
discussion prompts.

Clear, concise discussion
which addresses all three
discussion prompts.

Recommending Article to Others:

Below Avg. (0 pts) Average (2.5 pts) Excellent (5 pts) Points
Recommendation not
provided

A least 1 solid reason provided
to support student’s opinion.

At least 2 solid reasons provided
to support student’s opinion.

Class Discussion (10 pts):

Unacceptable (0 pts) Average (5 pts) Excellent (10 pts) Points
Absent. Present, but many not be actively

contributing to the discussion.
Present and actively contributing
to the discussion.

Requirements (10 pts):

Requirement Unacceptable (5 pts) Expected (10 pts) Points
Use CE 112 Cover Sheet template with

name, number of article reviewed

If any of the
requirements are
missing or do not meet
specification.

All format
requirements met.

Correct page order and limitations met.
Double spaced text.
Single sided print copy.
Times New Roman 11 pt font.
One inch page margins.
Numbered pages (on the bottom).
One staple in the upper left corner.
PDF submitted to D2L

Total

/50

Due: TUESDAY 02/25/2020
Objective:
Essay Grading: (see rubric on page 2)
Requirements
Grammar, Spelling, and Punctuation (10 pts)
Content (25 pts):
Class Discussion (10 pts):
Requirements (10 pts):