LANDSLIDE INVENTORY AND SLOPE STABILITY MAPPING OF THE URBAN GROWTH AREAS WITHIN COWLITZ COUNTY, WASHINGTON STATE

In response to the Aldercrest-Banyon (1998) and other recent damaging landslides in Cowlitz County, the WA Div. of Geology & Earth Resources began a field and GIS-based landslide inventory and slope stability mapping project in 2000. 680 km² of urbanizing lands between the Toutle River and the Lewis and Wahkiakum County lines have been mapped for all landslides, regardless of age. Slides were identified from 1951 (1:48,000), 1974, 1984, 1993, 1996, and 1999 (all 1:12,000) aerial photographs. Nearly 600 deep-seated and 260 shallow landslides have been identified, 80% of which were previously unmapped. In total, 59.5 km² or 11% of the sloping lands within the study area are identified as landslide terrain. Of the deep-seated slides identified from aerial photos, 70% were field checked, and of these 20% exhibit evidence of movement within the past 10 years. Deep-seated slides range in size from 0.001 to 1.65 km². Deep-seated slide movement occurs on slopes with gradients as low as 10% (6°).

The study area, characterized by moderate-to-steep slopes that tend to fail via slow-to-moderate rotational-to-translational rock/earth slides, is underlain by high-plasticity clay-rich soils and deeply weathered Tertiary bedrock (saprolites). Irrespective of age, slides occur within all rock units, but have the highest occurrence on steep-to-moderate slopes underlain by deeply-weathered Paleogene volcanic tuffs and volcaniclastic sedimentary rocks, the Cowlitz Fm., the Toutle Fm., and the Neogene Sandy River Mudstone and Troutdale Fm. The majority of slides appear to have moved in response to natural causes, such as above-average annual precipitation. Some of the now-dormant deep-seated slides may have been seismically triggered, and others (below 70 m) may have initiated in response to rapid drawdown of late Pleistocene glacial outburst floodwaters along the Columbia River and tributaries. Human actions, such as the alteration of slope hydrology through development and forestry practices, surface mining operations, and improper placement and design of fill material on slopes, have contributed to the initiation of new, and reactivation of dormant deep-seated slides.

The landslide inventory was combined with geologic unit and slope maps to produce slope-stability susceptibility maps for state and local government growth management planning purposes.