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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 september to remember

A SEPTEMBERTO REMEMBER

A CATASTROPHIC DEBRIS FLOW

TRIGGERED BY HURRICANE IVAN

overview of the event
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
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 mountains normal yearly rainfall

NC MOUNTAINSNORMAL YEARLY RAINFALL

Highest in the Eastern United States

frances rainfall amounts
Frances Rainfall Amounts

Peeks

Creek

NORTH CAROLINA

GEORGIA

ivan rainfall amounts
Ivan Rainfall Amounts

Peeks

Creek

NORTH CAROLINA

GEORGIA

francis ivan rain amounts
Francis + Ivan Rain Amounts

Peeks

Creek

NORTH CAROLINA

GEORGIA

peeks creek watershed
Peeks Creek Watershed

Peeks Creek

Fishawk Mountain

convective rainfall bursts prior to landslide
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
GSP WSR-88D 1-Hr Precip

KGSP One Hour Precipitation 0153-0213 UTC

remnant supercell s track
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
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
Velocity and Discharge at Selected Points

Courtesy of Rick Wooten

And Rebecca Latham NCGS

initiation point significance
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
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.
two early hypotheses on triggering mechanism
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
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
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

Progression of Debris Flow

From Top to Bottom

slide27
2.

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

6 destroyed cabin
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
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
8. Two Small Landslides

Photos by Brian Campbell, NWS

9 damaged destroyed homes
9. Damaged/Destroyed Homes

Images courtesy of Brian Campbell(NWS ERH).

house swept off foundation
House Swept off Foundation

Images courtesy of Brian Campbell(NWS ERH).

peeks creek empties into cullasaja river
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
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 profile44
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

Landslides in North Carolina

A Historical Perspective

Causes and Triggers

Rick Wooten - Rebecca Latham

North Carolina Geological Survey

slide47

Rock Slide

I-40 Pigeon River Gorge

July 1997

slide51

N.C. Geological Survey

Slope Movement Database

slide52

Maggie Valley

Apartment Complex

On Debris Fan

causes underlying geology geomorphology hydrology weather climate slope modifications

TRIGGERS

  • Volcanic Eruptions
  • Earthquakes - Blasting
  • Freeze - Thaw
  • Precipitation – Water
  • Slope Modifications

CAUSES

Underlying Geology

Geomorphology

Hydrology

Weather-Climate

Slope Modifications

blue ridge parkway landslide
Blue Ridge Parkway Landslide

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.

slide61

High antecedent moisture conditions 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
North Carolina Geological Survey 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