PRELIMINARY FINDINGS ON THE SEPTEMBER 16, 2004 DEBRIS FLOW AT PEEKS CREEK, MACON COUNTY, NORTH CAROLINA

摘要

Heavy rains from the remnants of Hurricane Ivan triggered the Peeks Creek debris flow in Macon County, North Carolina at about 10:10 p.m. on September 16, 2004. The debris flow began just below the top of Fishhawk Mountain at elevation 4,420 ft and traveled approximately 2.25 miles dropping 2,200 ft in elevation to the Cullasaja River. Five people were killed, fifteen homes were destroyed, and two people were seriously injured by the slope movement. Since 1901, there have been fourteen recorded, landslide-producing storm or hurricane occurrences (including Hurricanes Frances and Ivan in 2004) in western North Carolina. Several of these have triggered landslides in Macon County including the Fishhawk Mountain area. In 1876, debris flows originated on both the northern and southern sides of Fishhawk Mountain. In 1995, Hurricane Opal triggered a debris flow in the Poplar Cove area of Macon County. Hurricanes Frances and Ivan triggered numerous slope movements in Macon County in September, 2004. Colluvial deposits exposed along Peeks Creek indicate prehistoric slope movements may have occurred in the same area as the recent major debris flow. Remnants of Hurricane Frances produced rainfall totals up to 15 inches in portions of western North Carolina. Eight days later, remnants of Hurricane Ivan dropped 9 inches of rainfall across the region. These heavy rainfalls in combination with the thin ( < 6.5 ft), colluvial soil in sharp contact with the steeply dipping bedrock surface (35 - 55°) created a setting conducive for slope failure. Subparallel striations on the bedrock surface in the initiation zone indicated that the initial movement may have been a debris slide that quickly mobilized into a debris flow. Cross sections measured across the debris flow track provided information to calculate estimates of velocity and discharge. Velocity estimates ranged from 20.3 mi/h to 33.2 mi/h. Discharge approximations ranged from 20,800 cfs up to 45,000 cfs. Velocity and discharge values fluctuated over the length of the debris flow track due to changes in stream gradient and streamflow contribution from side channels. Studies to analyze the cause of the Peeks Creek debris flow will continue. Preliminary GIS-based slope stability assessments indicate other areas in the watershed may be susceptible to debris flows. Soil testing will improve the soil parameters (such as cohesion, angle of internal friction, etc.) used in the slope stability computer models to further refine the delineation of areas of potential slope movement.