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A SEPTEMBER TO REMEMBER A CATASTROPHIC DEBRIS FLOW TRIGGERED BY HURRICANE IVAN Overview of the Event Torrential rains from Hurricanes Frances and Ivan drenched the mountains of North Carolina.
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A SEPTEMBERTO REMEMBER A CATASTROPHIC DEBRIS FLOW TRIGGERED BY HURRICANE IVAN
Overview of the Event • Torrential rains from Hurricanes Frances and Ivan drenched the mountains of North Carolina. • During heavy rains of Ivan, a landslide was triggered in Macon Co. near the top of Fishhawk Mountain. • A debris flow gathered momentum and raced two miles down Peeks Creek. • The Peeks Creek community was devastated by a mass of water, earth, trees, and boulders up to 15 tons. • Four people were killed and several were seriously injured. • 15 to 20 homes were destroyed or severely damaged. • The debris flow ended where Peeks Creek meets the Cullasaja River.
OUTLINE • Meteorological Setting • What Happened • Geology of Landslides This presentation is based primarily on the work of the following people: Jonathan Lamb and Brian Campbell National Weather Service Richard Wooten and Rebecca Latham North Carolina Geological Survey Warren Cabe Macon County Office of Emergency Management
NC MOUNTAINSNORMAL YEARLY RAINFALL Highest in the Eastern United States
Frances Rainfall Amounts Peeks Creek NORTH CAROLINA GEORGIA
Ivan Rainfall Amounts Peeks Creek NORTH CAROLINA GEORGIA
Francis + Ivan Rain Amounts Peeks Creek NORTH CAROLINA GEORGIA
Peeks Creek Watershed Peeks Creek Fishawk Mountain
Convective Rainfall Bursts Prior to Landslide • Remnants from NE Georgia supercells eventually merged and moved over eastern Macon Co. about 20 min. before debris flow occurred. • Based on radar reflectivities, enhanced rainfall rate occurred over Peeks Creek watershed 20-30 min before landslide started. • The soil’s liquid limit was exceeded, allowing the soil to liquefy and begin to flow. KGSP Composite Reflectivity 0128-0203 UTC
GSP WSR-88D 1-Hr Precip KGSP One Hour Precipitation 0153-0213 UTC
Remnant Supercell’s Track • Remnants of a supercell that spawned a tornado in Northeast Georgia moved directly over Fishhawk Mountain about 20 minutes before the slide occurred. • Broad, weak rotation in mid-levels as storm moved over Macon county. • 0.5° base-velocity data showed no anomalous low-level wind fields. • Convective cell could have brought stronger mid-level winds to the surface. KGSP 0.5° SRM 0143-0203 UTC
Geographic Points of Interest • Fishhawk Mountain is part of a somewhat rare NW-SE ridge • Area is heavily forested • Very steep terrain • Relatively small watershed • Bedrock is near the surface in many locations Photo courtesy of David Phillips
Velocity and Discharge at Selected Points Courtesy of Rick Wooten And Rebecca Latham NCGS
Initiation Point Significance • We noticed a flowing spring at very top of slide area. • Water was trickling down rock sheet from top. • This may have pre-lubricated the rock sheet under the soil and reduced friction. • Soil is very thin and silty. • survey notes by Jonathan Lamb
Hydrologic Factors • Frances’ rains probably fully charged the aquifer. • Surface of confined aquifer rose significantly during Ivan. • Water flow through springs and other outlets increased dramatically. • A gush of water began at headwater of Peeks Creek during evening of 16 Sept. • Topsoil was already saturated from rains.
Natural dams formed in constricted areas of creek during Frances’ rains. Silt, rocks and trees may have clogged narrow creek passages, forming natural ponds in one or two places. Small slides upstream overloaded dams and caused a domino effect. Resulted in a torrent of debris at the bottom of the mountain. A microburst or small tornado felled trees in headwaters area. The falling trees in thin, saturated soil, caused the ground to give way. The landslide then gained momentum on the way down. Natural spring was secondary to falling trees as the trigger. Two Early Hypotheses on Triggering Mechanism
Natural Dams Hypothesis • Maybe, but probably wasn’t crucial in triggering the debris flow • Locations of possible water retention were much farther down the mountain • Burst of water from small retention ponds may have added some momentum to the flow • Possibility of precursor debris dams is still being investigated by the NC Geological Survey
Wind Damage Hypothesis • Possible contributor • Aerial photographs show spotty tree damage • But at initiation point, no adjacent vegetation appeared disturbed • Residents did report very gusty winds during the late evening and wind damage was widespread across the mountains • Even root stress from strong winds could have been enough to overload the soil’s cohesion • But the event can be explained without wind damage Photo courtesy of David Phillips
Progression of Debris Flow From Top to Bottom
2. Debris flow probably gained mass and accelerated very quickly in the steep terrain near the top of Fishhawk Mountain.
6. Destroyed Cabin • Note soil removed off of bedrock • Cabin owner had a road over the creek. Unknown if it was a culvert or bridge. • Area where bridge or culvert was is washed out and missing • Water and debris from farther upstream appears to have destroyed cabin and washed soil off of bedrock
7. Wide Point Slope lessened about 1.2 miles from the top, causing the flow of debris to spread out. Notice the huge pile of trees, rocks and silt in the center.
8. Two Small Landslides Photos by Brian Campbell, NWS
9. Damaged/Destroyed Homes Images courtesy of Brian Campbell(NWS ERH).
House Swept off Foundation Images courtesy of Brian Campbell(NWS ERH).
Peeks Creek Empties into Cullasaja River • Most accumulated debris had already been deposited upstream. • However, some house debris was found down the Cullasaja River. • One of the bodies was also found downstream several weeks later.
Elevation Profile • Valley starts at approximately 3,622 feet • Runs downhill for 2.3 miles • Empties out in the river at 2,142 feet • Approximately 1500 foot drop in 2 miles Courtesy of Brian Campbell
Elevation profile • Valley starts at approximately 3,622 feet • Runs downhill for 2.3 miles • Empties out in the river at 2,142 feet • Approximately 1500 foot drop in 2 miles
Landslides in North Carolina A Historical Perspective Causes and Triggers Rick Wooten - Rebecca Latham North Carolina Geological Survey
Rock Slide I-40 Pigeon River Gorge July 1997