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The Deadly Debris Flow in Macon County NC During Hurricane
Ivan
Jonathan R Lamb
NOAA/National Weather Service
Greer, SC
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I. Introduction
The remnants of
Hurricane Ivan tracked up the spine of the Appalachian Mountains
16-17 September 2004. The track of Ivan was remarkably similar
to that followed by Hurricane Frances ten days earlier, moving
northeast from northern Georgia along the North Carolina -
Tennessee border. The track of both systems along the spine
of the Appalachians concentrated the highest rainfall totals
in western North Carolina. Frances produced widespread rainfall
amounts of 8 to 12 inches, while 6 to 10 inches were common
with Ivan. The southern mountains of North Carolina, including
Macon County, received the highest rainfall totals during
both tropical systems. On the night of Thursday, 16 September,
heavy tropical rains had inundated Macon County for several
hours. At 10:00 p.m. EDT near the peak of Fishhawk Mountain,
approximately 6.5 miles southeast of Franklin, a layer of
soil liquefied and began flowing down Peeks Creek. Due to
the very steep terrain and abundance of loose surface material,
a large debris flow formed and moved down Peeks Creek (Note:
The term for this type of slope movement is "debris flow"
as defined by the U.S. Geological Survey because it better
describes the constituents of this semi-liquid transient mass).
The Peeks Creek housing community sits along Peeks Creek near
the bottom of Fishhawk Mountain, about one-quarter mile above
the Cullasaja River. The massive debris flow destroyed about
15 homes in a matter of seconds, killing four people and seriously
injuring several more. As is the case with most debris flows,
the conglomeration consisted of very little water by the time
it reached the houses.
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Figure 1. Aerial photograph showing the Peeks Creek watershed.
Debris flow began near top of Fishhawk Mountain (blue arrow)
and moved through the Peeks Creek community (yellow arrow).
Photo courtesy of David Phillips.
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Figure 2. Zoomed aerial photograph showing
the portion of the Peeks Creek community that sustained the
most damage. Most of the brown parts are house rubble strewn
about. Photo courtesy of David Phillips.
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II. Meteorological
Background
Hurricane Frances
made landfall near Sewall's Point, Florida, around 1:00 a.m.
EDT, on 5 September 2004, as a Category 2 storm on the Saffir-Simpson
Hurricane Scale. Frances moved west-northwest across central
Florida early Sunday morning and was downgraded to a tropical
storm at 5:00 p.m. EDT that day 20 miles east of Tampa.
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| Figure 3. Hurricane Frances track
map. Note: Most graphics on this page may be enlarged by
left-clicking the image. |
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Figure 4. Regional rainfall from
Hurricane Frances. Data has been smoothed significantly to allow
contouring. |
Frances emerged in the northeastern Gulf of Mexico at around
11:00 p.m. EDT, just north of Tampa. The system regained tropical
storm status as it moved across the northeast Gulf of Mexico
on 6 September, then made a second landfall at Saint Mark's,
Florida, at 2:00 p.m. EDT with maximum sustained winds of 65
mph. Frances weakened rapidly as it moved north-northwest across
southwestern Georgia on Monday evening, 6 September.
The remnants of Frances moved through north-central Georgia,
across the extreme western tip of North Carolina, then up the
spine of the Appalachian Mountains through 8 September. Very
gusty winds and torrential rain buffeted the western Carolinas
and northeast Georgia. Widespread severe flooding occurred,
especially along the French Broad and Swannanoa rivers through
Asheville and Biltmore. Only a few landslides were reported
with Frances, probably because antecedent soil moisture was
low.
Almost exactly
ten days after Frances' landfall, powerful Hurricane Ivan
churned toward the Gulf coast as a Category 4 storm. Shortly
before landfall, Ivan weakened slightly to Category 3 strength,
with maximum sustained winds near 130 mph. The center of Hurricane
Ivan moved onshore near Gulf Shores, Alabama, at around 3:00
a.m. CDT on Thursday, 16 September 2004. Ivan weakened rapidly
as it moved north and then northeast across Alabama on Thursday.
Ivan was downgraded to a tropical storm at 2:00 p.m. CDT on
Thursday when it was located 45 miles west-northwest of Montgomery,
Alabama. Maximum sustained winds had decreased to 70 mph.
Ivan was downgraded to a tropical depression late Thursday
evening while centered 25 miles north-northwest of Gadsden,
Alabama. Maximum sustained winds had dropped to near 35 mph
with a minimum pressure of 986 milibars.
The system remained
a tropical depression as it slowly moved northeast across
northwestern Georgia and eastern Tennessee Thursday night
and Friday morning. By 11:00 a.m. EDT on Friday the 17th,
the depression was located 45 miles east of Knoxville, Tennessee.
Widespread heavy rain affected western North Carolina Thursday
night and Friday. Wind gusts reached between 40 and 60 mph
across the higher elevations of the Appalachians. Numerous
trees were downed, flooding was widespread, and numerous slope
failures occurred. Rainfall amounts reached 8 to 12 inches
across parts of the region. The debris flow was triggered
late Thursday evening as Ivan's rains were at their greatest
intensity. Several geological studies in the past 25 years
have shown that the best primer for widespread slope failures
is high antecedent moisture conditions followed by intense
bursts of rain (Neary and Swift 1987). The amount and duration
of precipitation needed for widespread slope failures varies,
but one geological study in the eastern United States found
that a minimum of only five inches of rain in 24 hours was
necessary (Eschner and Patric 1982). This condition was easily
met in the North Carolina mountains during Ivan.
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| Figure 5. Track of Hurricane Ivan. |
Figure 6. Regional rainfall from
Hurricane Ivan. Data has been smoothed significantly to allow
contouring. |
A tropical system
track just west of the Appalachian Mountain spine is a worst-case
scenario for flooding and landslides from a meteorological perspective.
During September 2004, two storms did just that. Although a
tropical cyclone immediately begins to weaken over land, the
system is still a strong low pressure system well inland. The
flow in a low pressure system is approximately counterclockwise
relative to the center. Therefore, to the right of a northward
moving system, a generally south or southeast wind will occur
until the low center passes a given location. Since the Gulf
of Mexico and Atlantic Ocean are the primary sources of moisture
for this area, a southerly wind will provide the most moisture
transport. Also, precipitation is enhanced when a strong southerly
flow of moist air affects a surface barrier like the Appalachians.
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Local to the Peeks Creek basin, Fishhawk Mountain comprises
a small mountain ridge that runs from northwest to southeast.
In a strong moist, southerly flow, this could slightly enhance
the localized precipitation efficiency by maximizing the upslope
wind component. Although some precipitation reports were received
around Macon County, no residents near Fishhawk Mountain could
be found with an accurate rainfall total for comparison purposes.
The limited rainfall reports shown in Figure 9 do infer
much heavier rain around the landslide initiation point than
to the west near Franklin. Due to terrain blockage in the
southwest North Carolina mountains, the KGSP and KMRX WSR-88D
radars provide only limited data on precipitation intensity
and accumulated rainfall estimates.
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Figure 7. Local topography
of the Peeks Creek watershed. Fishhawk Mountain is in the lower-left
and the yellow trace indicates the path of the debris flow.
The Cullasaja River is just left of US Highway 64 in the upper-right.
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Figure 8a. Composite Reflectivity
image from KGSP WSR-88D at 0143 UTC (9:43 p.m. EDT). Circled
area shows the general region while the home cursor is positioned
at Fishhawk Mountain. Notice enhanced 50-55 dbZ returns approaching
the area.
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Figure 8b. Composite Reflectivity
image from KGSP at 0148 UTC (9:48 p.m. EDT). Notice the enhanced
50-55 dbZ returns over Fishhawk Mountain. The landslide occurred
at approximately 0200 UTC (10:00 p.m. EDT). |
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Figure 9. Rainfall plot for Macon
county from Ivan. Enlarging image will make rainfall totals
legible. The largest rain amounts were in the southeast portion
of the county near Highlands. Circled area shows Peeks Creek
watershed. |
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III. Evolution
of the Event
At approximately 10:00
p.m. EDT, after four to six inches of rain had soaked southeastern
Macon County, a portion of Fishhawk Mountain failed. The sheet
of loamy soil that liquefied was only two to three feet deep,
below which lay solid bedrock. The slope at initiation was about
50 degrees with an elevation of nearly 4450 ft MSL. A natural
spring originated near the pinnacle of Fishhawk Mountain and
likely contributed to this part of the slope giving way. This
spring was trickling two weeks after the event when the author
toured the area. The flow of water through the spring probably
increased drastically during the heavy rains of Ivan. Given
the geological and hydrological setup, it is not surprising
that this particular slope failure occurred.
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| Figure 10. The slope failure occurred
at this point. Scale is difficult to ascertain from this image,
but the soil is one to three feet deep. Woody vegetation is
growing in the thin soil surrounding this. Moisture can be seen
on bedrock and soil. |
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Figure 11. A view of the exposed
bedrock at the top of
the slide area. The steepness of the slope is difficult to see
in pictures, but is about 50 degrees here. Aggregate
organic debris is seen at the bottom of the rock sheet,
with water trickling down the face. Fig. 10 is a close-up of
the area near the top of this photograph. |
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The sheet of soil
rapidly accelerated, behaving as a fluid, constantly gaining
momentum as more material was picked up. Because the terrain
was so steep and the topsoil below the initial landslide was
saturated, a debris flow easily took shape. The flow accumulated
boulders, mud, trees, and anything else in the path as it
accelerated down Peeks Creek. Such a large mass of debris
moving down a narrow creek bed was nearly unstoppable, as
was seen in the aftermath.
In several ways,
trees and vegetation told the story of how this event unfolded.
The debris flow depth was evident from the height that branches
or stems were broken off adjacent bushes. Piles of splintered
trees in several spots on the bank illustrated the momentum
of the flow, as standing trees were swept away and deposited
downstream. A few trees had been snapped in half like toothpicks,
probably because they got trapped in living trees along the
side. An even more vivid demonstration of the flow size was
bark and branches scraped off living trees adjacent to the
ravine, sometimes 80 feet above the ground. These scars were
caused by the abrasive actions of entire trees held erect
in the moving flow.
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Figure 12. A living pine
tree adjacent to the
creek basin. Branches can be seen snapped
off near the top of the tree. Scrape
signatures like this one were common on
trees near the creek basin. |
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Figure 13. A pile of
splintered trees that were deposited
by the moving flow into unaffected adjacent trees. Several
other piles of trees were seen along the edges of the creek
bed.
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About
1.8 miles from the top of Fishhawk Mountain, the slope decreases
considerably. When the flow of debris reached this area, the
slowing material fanned out. The less viscous material split
into two streams along the tree line. As naturally occurs when
a mass of suspended material slows down and spreads, the largest
debris continued straight forward, depositing much of the load
into a natural "island." The contents of the mound
were incredible: car-sized boulders, entire trees with roots
still attached, sticks and a lot of soil. It is not known how
dense the foliage was prior to the flow reaching this area,
but now the cleared area is between 500 and 1000 feet across.
The overall size of the piled soil, trees, rocks and organic
material was impressive. |
Figure 14. Looking upstream at the wide area.
Peeks
Creek flows in two streams around the newly formed
island of rocks and organic debris, then reconvenes at the
lower-left of the image.
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Figure 15. Panoramic photograph of the wide area. Immense
pile of rocks and organic debris can be seen in the center.
The stream flows from right to left in this image.
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Just below the wide area, the creek steepened and narrowed,
accelerating the remaining debris. Two uninhabited vacation
cabins became the first victims of the slide at this point.
They were both total losses. Prior to the event, Fishhawk
Mountain Road crossed the creek near the two cabins then proceeded
up one side of the creek bank to several other undisturbed
vacation cabins. The slide totally scoured away the roadbed,
isolating the remaining cabins for over a month until a temporary
access road could be constructed.
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Figure 16. One of the
destroyed vacation cabins. |
A few tenths of
a mile below the cabins stood the Peeks Creek community, a series
of about 20 houses, most of which were inhabited year-round.
Most of the structures were situated very close to the creek
bed, but as residents noted, in the last several decades the
creek had never risen high enough to threaten the homes. According
to the North Carolina Geological Survey, the flow of debris
barreled through the community at about 30 miles per hour, with
a peak discharge of 45,000 cubic feet of material per second
(Fig. 23). For comparison purposes, this is about three times
the mean discharge of the French Broad River at Asheville, North
Carolina.
In a matter of seconds the torrent crashed through about 15
homes, reducing some to splinters, while pushing others off
their foundations and severely damaging them. Four fatalities
occurred, along with several serious injuries. Just beyond the
houses, the terrain leveled off into a grassy plain. The flow
of debris, including parts of the homes, slowed down and spread
out at this point. Three detached sheds near a large house were
flattened by the remaining flow before it concluded at the Cullasaja
River. This was at about 2250 ft MSL. The entire event took
place over a vertical elevation change of about 2200 feet during
a period of 15-30 minutes.
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| Figure 17. One of the first homes
to be hit by the wall of debris. Although much of the house
remains intact, it was pushed off its foundation. |
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Figure 18. An immense pile of trees,
house debris and several vehicles. |
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| Figure 19. A large pile of felled trees with the
remnants of at least one house. |
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Figure 20. Tree and house debris. Most notable
are the two mangled automobiles. |
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| Figure 21. A house torn apart and
moved from its foundation. Piles of rubble from other destroyed
homes upstream are visible in the center and right. |
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Figure 22. Three detached sheds next
to this house were destroyed; the last damage caused by the
debris flow before entering the Cullasaja River. |
IV. Future Mitigation Efforts
The US Geological
Survey and North Carolina Geological Survey are actively involved
in debris flow research. They are attempting to map regions
with a heightened risk of slope failures for use by municipal
planners and prospective residents of steep terrain. The task,
however, is monumental, due to the sheer quantity of rugged
terrain yet to examine.
Considerable improvements
to short-range landslide prediction and public awareness are
also underway. The U.S.G.S. has, for several years, issued
official Landslide Advisories for broad regions one to three
days in advance of anticipated widespread slope failures.
The North Carolina Geological Survey started issuing Landslide
Advisories for more specific areas of western North Carolina
in 2004. A collaborative effort between the US Geological
Survey, N.C. Geological Survey, and National Weather Service
has recently taken shape. More effective dissemination of
the landslide advisories or the information they contain is
under consideration by the involved agencies. Also, an integral
part of these landslide predictions is accurate quantitative
precipitation forecasts (QPF). The unusually large number
of landslides during the 2004 tropical season has prompted
a new
partnership between the NOAA National Weather Service
(NWS) and the U.S.G.S. The U.S.G.S. plans to directly utilize
precipitation forecasts from the NWS for their landslide products.
This event illustrates
a need for improved landslide awareness by residents of the
higher terrain. Simply because flooding is not common in a
given location does not imply a reduced risk of a slope failure
and/or debris flow. Through the mapping work being done by
geological officials and an ongoing public education campaign,
future fatalities due to debris flows will hopefully be prevented.
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Figure 23. A detailed analysis of
the Peeks Creek debris
flow from the North Carolina Geological Survey. The flow
velocity and estimated discharge were calculated at several
places along the path. |
V. Additional Information
The United States
Geological Survey has a plethora of information about landslides
on their website.
The North Carolina Geological
Survey is tasked with studying the individual events that
affect the state. They played the central role of researching
the Peeks Creek disaster and reporting the findings. Their
website
contains detailed information and pictures from many landslides.
NOAA's
National Weather Service issues a full suite of forecasts
and warning products for the entire U.S. and its territories.
The Hydrometeorological
Prediction Center, a branch of the NOAA National Weather Service,
issues quantitative precipitation forecasts (QPF) for the
entire U.S. Their entire suite of QPF products can be found
here.
The Federal Emergency Management Agency (FEMA) has a website
devoted to landslide safety and preparation.
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ACKNOWLEDGEMENTS.
Joseph Pelissier
made the rainfall contour maps for
Frances and Ivan. Brian Campbell provided the regional topographic
map and
took several of the damage photographs immediately after the event
occurred.
REFERENCES
Eschner, A.R., and Patric, J.H., 1982: Debris avalanches in eastern upland forests.
Journal of Forestry, 80, 343-347.
Neary, D.G., and Swift, L.W., Jr., 1987: Rainfall thresholds for triggering a debris
avalanching event in the southern Appalachian Mountains in Costa, J.E., and
Wieczorek, G.F., eds., Debris flows/avalanches; Process, recognition and
mitigation. Geological Society of America, Reviews in engineering geology,
VII, 81-92.
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