Paper No. 5
Presentation Time: 9:00 AM

CHARACTERISTICS OF RECENT DEBRIS-FLOW SOURCE AREAS IN THE OREGON COAST RANGE


BAUM, Rex L., U.S. Geological Survey, Box 25046 MS 966, Denver, CO 80225-0046 and COE, Jeffrey A., U.S. Geological Survey, Denver Federal Center, P.O. Box 25046, MS 966, Denver, CO 80225-0046, baum@usgs.gov

During early May 2012, we surveyed debris-flow source areas in the Elliott State Forest, in the Coast Range of Oregon. Our objective was to characterize the geometric properties of the source areas to aid in understanding and modeling the initiation processes. Our observations indicate that the initial slides can be modeled as tabular slabs resting on steep slopes and partly supported by irregular bedrock surfaces. The area is underlain by gently dipping sandstone with siltstone interbeds, which are overlain by a 1 – 3-m-thick mantle of colluvial and residual soil. All of the debris flows mobilized from shallow slides induced by heavy rainfall during the winter and early spring of 2012. Of the 15–20 source areas studied, all were located in recently logged areas or in fill at the edge of roads. The source areas were typically steep, shallow, narrow, and near the heads of small drainage basins (convergent). In plan view, the source areas are roughly rectangular, with rounded corners at the head. Topographic slope of individual source areas ranged from 35° to 46°. Most were concave upward, with the head scarp sloping 45° – 47° on average. Widths of source areas ranged from 5 m to 14 m and slope-normal depths along the edges ranged from about 0.2 m to 2 m. Depth differed by as much as 1 m along individual scarps, indicating that the slabs of soil that failed were asymmetrical in cross section. Lateral scarps sloped 45° – 75° toward the basal shear surface, which exposed bedrock and patches of soil. Downhill edges of the slides were indistinct because the basal shear was continuous with the channel and debris-flow track. The bedrock surfaces exposed in the source areas were commonly rough and either sub-planar or slightly concave parallel to the topographic contours. Asperities on the bedrock surface were commonly rounded with amplitudes of 1 – 5 cm. However, depending on the orientation of bedding and jointing, asperities at some sites were jagged with amplitudes of 5 – 15 cm. The presence of seepage zones suggests that at the time of slope failure, positive pore-water pressures were present at the contact between bedrock and the overlying colluvium. We observed water seeping from the basal contact of soil resting on bedrock at many of the source areas. Damp soil exposed in scarps was also consistent with the presence of seepage zones.