THE EFFECTS OF CHANNEL TOPOGRAPHY AND DISCONTINUITIES ON BEDROCK EROSION BY DEBRIS FLOW

Field measurements of bedrock erosion by debris flows are rare, yet are critical for clarifying erosion mechanisms and their influence on patterns and rates of steepland evolution. Herein, we report on bedrock erosion that occurred during a November 19, 2012 debris flow in a monitored-steepland basin in the Elliott State Forest in the Oregon Coast Range. The basin is underlain by the middle Eocene Tyee Formation, a shallow dipping, locally massive sandstone with thin, interbedded mudstones. The debris flow mobilized from a landslide high in the basin, had a minimum sediment volume of 650 m³, contained boulders up to about 1.5 m in diameter, and occurred following 120 mm of rainfall in 65 hours, about 35 minutes after the peak 10-minute rainfall intensity of 39 mm/yr.

Bedrock erosion by the debris flow was measured using 25 erosion bolts in a 40-m channel reach with a mean slope of about 20°. To characterize variability in rock strength in the channel, we used an N-type Schmidt hammer to measure 35 Schmidt R values at each bolt location. The channel reach has a distinct succession of steeply sloping bedrock surfaces (risers) and gently sloping bedrock surfaces (treads). Mudstone beds are located at the base of every riser greater than 30 cm in height. A prominent set of northeast-striking, near-vertical, tectonic fractures determined the locations and azimuths of smaller risers with sharp, steep faces.

Vertical lowering of bedrock by the debris flow preferentially occurred on treads and ranged from 0.2 to 15 cm. Fracturing and plucking of weathering folia on treads resulted in lowering from 0.2 to 4.1 cm, and chipping and plucking of blocks from the downstream ends of treads at the lips of risers resulted in lowering of 7 to 15 cm. Vertical lowering plotted with respect to Schmidt R values and local slope angle indicates that the eroded portions of treads are defined by a curving threshold line bound by R values ≤ 39, and slopes ≤ 35º. This threshold demonstrates that local slope and rock strength (controlled by folia and fractures) can be used to predict patterns of erosion during debris-flow events. The strong control that mudstone beds, folia, and fractures had on channel morphology and consequent debris-flow erosion highlights the need to consider thicknesses and attitudes of discontinuities in channels in studies of steepland evolution.