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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


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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.


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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


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NC MOUNTAINSNORMAL YEARLY RAINFALL

Highest in the Eastern United States


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TRACKS OF FRANCES AND IVAN



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Hurricane Frances Rainfall




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Frances Rainfall Amounts

Peeks

Creek

NORTH CAROLINA

GEORGIA


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Ivan Rainfall Amounts

Peeks

Creek

NORTH CAROLINA

GEORGIA


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Francis + Ivan Rain Amounts

Peeks

Creek

NORTH CAROLINA

GEORGIA


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Peeks Creek Watershed

Peeks Creek

Fishawk Mountain


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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


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GSP WSR-88D 1-Hr Precip

KGSP One Hour Precipitation 0153-0213 UTC


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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


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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


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Landslide Course ShowingScouring to Bedrock on Steep Slope


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Velocity and Discharge at Selected Points

Courtesy of Rick Wooten

And Rebecca Latham NCGS


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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


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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.


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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


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Natural Dams Hypothesis Frances’ rains.

  • 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


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Wind Damage Hypothesis Frances’ rains.

  • 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


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Progression of Debris Flow Frances’ rains.

From Top to Bottom


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1. Initiation Frances’ rains.


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2. Frances’ rains.

Debris flow probably gained mass and accelerated very quickly in the steep terrain near the top of Fishhawk Mountain.


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3. Frances’ rains.


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4. Scraped Trees Frances’ rains.


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5. Exposed Bedrock Frances’ rains.


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6. Destroyed Cabin Frances’ rains.

  • 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


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7. Wide Point Frances’ rains.

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.


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Huge Boulders Moved Frances’ rains.


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8. Two Small Landslides Frances’ rains.

Photos by Brian Campbell, NWS


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9. Damaged/Destroyed Homes Frances’ rains.

Images courtesy of Brian Campbell(NWS ERH).




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Continued Devastation Frances’ rains.


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House Swept off Foundation Frances’ rains.

Images courtesy of Brian Campbell(NWS ERH).


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Debris Flow Ends as Creek Bed Frances’ rains.becomes Wide and Flat


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Peeks Creek Empties into Cullasaja River Frances’ rains.

  • 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.


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Fire/Rescue searches homes Frances’ rains.


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Elevation Profile Frances’ rains.

  • 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


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Elevation profile Frances’ rains.

  • 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


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Landslides in Frances’ rains.North Carolina

A Historical Perspective

Causes and Triggers

Rick Wooten - Rebecca Latham

North Carolina Geological Survey



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Rock Slide Frances’ rains.

I-40 Pigeon River Gorge

July 1997



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N.C. Geological Survey Frances’ rains.

Slope Movement Database


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Maggie Valley Frances’ rains.

Apartment Complex

On Debris Fan


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  • TRIGGERS Frances’ rains.

  • Volcanic Eruptions

  • Earthquakes - Blasting

  • Freeze - Thaw

  • Precipitation – Water

  • Slope Modifications

CAUSES

Underlying Geology

Geomorphology

Hydrology

Weather-Climate

Slope Modifications



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Blue Ridge Parkway Landslide Frances’ rains.

Landslide that destroyed a section of the Blue Ridge Parkway north of Mount Mitchell in Hurricane Frances.

Mt Mitchell received 23 in. of rain with Frances, 11 in. with Ivan, and 6 in. with Jeanne. The total for September was more than 46 in. (nearly 4 ft. of rain in one month!), the highest monthly total ever in North Carolina.



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High antecedent moisture conditions Frances’ rains.followed by high intensity rainfall.

5” of rain in 24 hours – reasonable threshold for localized slope movement events.

10” of rain in 24 hours:Widespread slope movements.More rainfall gauges would aid in refining these threshold values.


References links l.jpg

North Carolina Geological Survey Frances’ rains.http://www.geology.enr.state.nc.us

U.S. Geological Survey http://www.usgs.gov

National Weather Service http://www.nws.noaa.gov

Coweeta Ecological Laboratory http://coweeta.ecology.uga.edu

References, Links

The End


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