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 agricultural 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 subjects 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 required to develop a comprehensive plan of action, the county was required to establish a critical areas protection ordinance (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 urbanising 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 project 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 vegetation) as well as areas in which the combination of geologic and topographic factors favour 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 management 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 corporations, and land developers.
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 reactivated in the Alder crest neighbourhood 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 Alder crest–Banyon landslide and numerous 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 recognised the need for improved slope-stability mapping within the urbanising 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)
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 Cowlitz 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 together various geologic mapping projects 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 portions 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, colour) ae- rial photographs.
Previous landslide inventories 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, commenced 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 publication and presentation of results in late 2002 to early 2003.
Landslides verified by field evidence will be digitised into Arc View coverage using 1:12,000 DNR digital orthophotos. Our goal is to release published 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, dormant, or ancient), certainty of geologist
Figure 3. View northwest along the main scarp of the deep-seated reactivated Alder crest–Banyon (Kelso, WA) earth slide–earth flow as it appeared in August 2000. Landslide motion initiated 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.
Figure 2. Stereo photo pair of Alder crest–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.
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 erosion in the recent geologic past means that, in places, the ground has been subjected to weathering processes for millions of years (Thorsen, 1989).
This has resulted in deeply weathered clay-rich soils (saprolites) formed by the weathering of Tertiary sedimentary and volcanic rocks as well as unconsolidated upper Tertiary to Quaternary fluvial and eolian deposits. Extensive portions of the study area are underlain by Tertiary sedimentary and volcanic rocks containing inherent weaknesses, such as dipping bedding planes, joints, brecciation and shear zones, paleo weathering (paleosol) surfaces, and clay-rich inter-beds.
Many bedrock-dominated landslides initiate along such inhomogeneities. Upper Tertiary to Quaternary fluvial deposits of the ancestral Columbia River form dissected terraces along the lower slopes of the study area, filling in paleo topography developed upon the underlying Tertiary bedrock. Many of these superficial 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 land sliding within the study area is the rotational to translational earth and (or) rock slide, composed of extensively weathered bed- rock and (or) superficial deposits (Figs. 3–6). Faster-moving rock falls and topples are limited to the steep bluffs along the Columbia River west of Long view, the inner gorges of the Kalama and Coweeman Rivers, and the rocky head scarps of some of the larger rock slide complexes.
Many of the larger landslides appear to have multi-part movement histories (Fig. 7), as exhibited by recently active deep-seated failures such as the Alder crest–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 areas are underlain by thick deposits of high-plasticity (and potentially swelling) clay derived from the weathering of both the underlying bed- rock and superficial deposits.
Figure 4. View to the southeast across the middle section of the Alder crest–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 private 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 failing 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, land sliding might result.
A third triggering mechanism 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 modifications can alter the amount and flow direction of surface and ground water on slopes, which in turn may trigger slope failure.
The undercutting of slopes for roads, building foundations, 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 humans and an increase in annual and regional precipitation levels (such as occurred during the late 1990’s) 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 evidence 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.
Landslides such as the Alder crest–Banyon slide serve as stark reminders of the potentially devastating consequences of human 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 project 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.
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Buss, K. G.; Benson, B. E.; Koloski, J. W., 2000, Alder crest–Banyon landslide—Technical and social considerations [abstract]: AEG News, v. 43, no. 4, p. 78.
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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-central Olympic Peninsula: Washington Division of Geology and Earth Resources Open File Report 99-2, 36 p., 2 plates.
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