Geologic hazards in North Carolina — Landslides

This section provides rock slope stability information about landslides.

Landslide contents

The landslide section consists of several pages to facilitate on-line user viewing. This page is the third of five that are based on a recent staff geologist presentations at numerous public meetings. The pages are:

Landslide presentation

The following images were included in a MS PowerPoint presentation used by North Carolina Geological Survey geologists from the Swananoa office at many public landslide outreach meetings. The presentation has been adapted to the Internet for broader distribution. This page is on "Rock Slope Stabilty." Links to other topics appear in the contents shown above.

Slide numbers correspond to those of the original MS PowerPoint presentation. Slide numbers "missing" are slides that were turned into text. Captions are from the original presentation.

Slide 30 - Landslides and landslide related fatalities from the mid-July 1916 hurricane in Transylvania County, NC alone. Damage from landslides and flooding occurred over much of south-central mountain area. The July 15-16, 1916 flood is considered the ‘flood of record’ in western North Carolina.
Landslides and landslide related fatalities from the mid-July 1916 hurricane in Transylvania County, NC alone. Damage from landslides and flooding occurred over much of south-central mountain area. The July 15-16, 1916 flood is considered the ‘flood of record’ in western North Carolina.

 

Slide 31 - Color-infrared aerial photograph of the Gorges State Park area, Transylvania County showing mapped locations of deposits left by the catastrophic failure of Lake Toxaway Dam on August 13, 1916. The dam failure triggered a debris flow along the Toxaway River that traveled over 7 miles and into South Carolina. Lake Jocassee is underlain by cobble, gravel and sand deposits from the flood. The original dam was in about the same location as the current dam. Quote from S.W. McCallie, State Geologist of Georgia at the time. Studies by the NCGS estimate that the outflow just below the dam was on the order of 293,938 cfs (discharge) and 50 mi/hr (velocity). Information from Geology of Gorges State Park, N.C. Geological Survey Information Circular 31.

 

Slide 32 - Top: View looking downstream along the Toxaway River below the dam showing the assumed scour lines and the location of cross section D (bottom) used to reconstruct the superelevation angle of the dam failure torrent. This information goes into computing an estimated velocity and discharge of the outflow.

 

Slide 33 - Top Left: 60-foot long boulder weighing nearly 900 tons transported by the flood waters along the Toxaway River from the August 13, 1916 Lake Toxaway Dam failure. Photograph taken about 0.5 miles downstream from Toxaway Falls. Top Right. Imbricated boulders at the crest of the boulder levee shown in red in the cross section at Bottom Left. Bottom Right. Photograph of the contact (shown by arrow) of the boulder flood deposits overlying pre-existing flood plain deposits along the Toxaway River.

 

Slide 34 - Left: Detailed map of a ~4 acre active weathered-rock slide along the Toxaway River in Gorges State Park. This slow-moving landslide was probably triggered by the 1916 dam failure torrent that eroded and over-steepened the slope along the river. Right: Tree ring studies of trees on and off the slide indicate a period of slide movement during the 1965-1974 timeframe corresponding to a period of above average rainfall. Tree ring studies were done cooperatively with the U.S.F.S. Coweeta Hydrologic Laboratory, Otto, N.C. Information contained in Geology of Gorges State Park, N.C. Geological Survey Information Circular 31.

 

Slide 35 - Trees growing on an active landslide are commonly curved. The tree tilts with the moving slide, and over time attempts to regain vertical growth resulting in the curved trunk. A and B. Curved trees showing the effects of movement on the Toxaway River weathered-rock slide. White arrow points to person for scale in photo A (left).

 

Slide 36 - Field developed cross section view of the Toxaway River weathered-rock slide in Gorges State Park. Information contained in Geology of Gorges State Park, N.C. Geological Survey Information Circular 31.

 

Slide 37 - Left: Example of landslide hazard mapping. Slope movements (landslides), flood and other surficial deposits mapped in Gorges State Park by the N.C. Geological Survey. Right: Bedrock geologic map of Gorges State Park. Mapping landslides and surficial units along with bedrock provides the best geologic framework for constructing landslide hazard maps. Information contained in Geology of Gorges State Park, N.C. Geological Survey Information Circular 31. Center: Schematic block diagram showing the relationships between bedrock structure, streams, and slope movements for Gorges State Park, Transylvania County, N.C.

 

Slide 38 - Bottom Left. Digital shaded relief map of Watauga County – slopes greater than 30 degrees are shown in red. Bottom Right. Debris flows and debris slides (shown in red) triggered by the Aug. 10-17, 1940 hurricane. Base map is a georegistered Sept. 1940 aerial photograph of the Blue Ridge Escarpment area near Deep Gap in Watauga County (unregistered photograph courtesy of U.S.G.S.). Light colors in the main stream and river channels are sediment from the debris flows and flooding. The flooding and landslides from this event killed 26 people in North Carolina alone. At least two fatalities resulted from a landslide along the Watauga River.

 

Slide 39 - Boulders line the surface of a small debris fan deposit in Watauga County believed to be deposited by a debris flow triggered by one of the the August 1940 storms. A ‘for sale’ sign is posted on the tree.

 

Slide 40 - Upper Right: Light Detecting and Ranging Radar (LiDAR) elevation data and digital imagery collected in conjunction with the floodplain mapping program will greatly aid in landslide hazard mapping. LiDAR image of the Seven Devils-Foscoe area along the Watauga River southwest of Boone showing some debris fans and colluvial deposits (indicated by red arrows). Location shown by inset on Watuaga County 10 meter shaded DEM map bottom left.

 

Slide 41 - The November 2-6, 1977 tropical depression triggered flooding and landslides across western North Carolina. The map shows the location of numerous debris flows triggered by this storm (red) in the Bent Creek watershed near Asheville.

 

Slide 42 - Map and charts showing location, velocity, rainfall and soil data for the Lands Creek Debris Flow I. The velocity of a debris flow can be estimated by determining the banking, or super-elevation, angle it makes as it rounds channel bends, the radius of curvature of the channel, and the channel gradient. A velocity of 23 mi/hr is about 33 ft/sec. Debris flows are particularly dangerous because they often happen without warning and move very rapidly downslope. The destroyed mobile home and chlorinator were built on pre-existing debris fan and flood deposits.

 

Slide 43 - Upper Left: Initiation zone of the Dec. 23, 1990 Lands Creek Debris Flow I near Bryson City. Red arrow points to person for scale standing above failed road embankment constructed across a hillslope hollow. White dashed line shows profile of hillslope hollow. Bottom Right: Mud line in a tree left by the debris flow next to new foundation pads for the chlorinator building for the Bryson City municipal water system. The debris flow destroyed the original chlorinator building.

 

Slide 44 - Left: Block diagram and cross section views of initiation zone of the Lands Creek Debris Flow I where a private logging road crossed a hillslope hollow. Block diagram illustrates a conceptual model of a hillslope hollow where the Lands Creek debris flow began. Soil and groundwater accumulate in a subtle hillslope depression overlying a concavity in the bedrock surface. Right: Cross section shows approximate configuration of the road profile and the underlying geology. The failure appears to have occurred where the slope transitions from convex to concave.

 

Slide 45 - Rainfall vs. elevation chart for rain gauge stations in the region of the Lands Creek Debris Flow I. The elevation of the initiation point for the Lands Creek debris flow is approximately 3000 ft. Rainfall amounts are generally greater at higher elevations for a given storm event. Rainfall data courtesy of U.S.F.S. Coweeta Hydrologic Laboratory and WHBN in Bryson City.

 

Slide 46 - Rainfall intensity as much as rainfall amount is an important in triggering debris flows. Graph shows plots of 1 hr and 24 hr rainfall intensity readings from the USFS Coweeta Hydrologic Laboratory for the Nov. 2-6, 1977 storms and the Dec. 23, 1990 storm that triggered the Land Creek I debris flow. The graph shows relative rainfall intensity and return periods for the two events. Note that for the Dec. 1990 event the rainfall intensity and return period is greater for the higher elevation rainfall gage (Coweeta-31).

 

Slide 47 - Views of the Lands Creek Debris Flow II track and debris (left side of both images) that went into the Bryson City reservoir. Reservoir was no longer a water supply source at the time of the debris flow, and has since been drained (lower photo). This debris flow originated from the same road as Lands Creek Debris Flow I.

 

Slide 48 - Hydrologic and rainfall data, radar map, and map showing the locations of the Charley Branch debris flows triggered by the 3-day rain event in Swain County in May 2003 that caused extensive flooding and landslides. Rainfall and streamflow data from U.S.G.S. Radar image from National Weather Service.

 

Slide 49 - Top: Cross section through the path of the the Charley Branch 5 debris flow track showing the debris flow superelevation angle and cross sectional area used to compute the estimates of velocity and discharge. Bottom Left: Track of CB5 debris flow on CIR DOQQ base – track length is ~1000 ft. Bottom Center: Initiation zone of CB5 in road embankment. Red dashed line indicates roadway. Bottom Right: Mudline on tree from debris flow at point A-15 on cross section.

 

Slide 50 - Left: Scarp in driveway embankment constructed with excavated sulfidic-graphitic bedrock that failed and mobilized into a debris flow during heavy rains in Swain County during May 5-7, 2003. Upper Right: Tension crack in road embankment containing a water line hook-up. Leaning trees (red arrow) indicate down slope creep of the embankment material. Bottom Right: Sulfidic-graphitic bedrock with characteristic iron oxide stained weathering surface and silver-gray fresh surface exposed in an excavation near home in photo left.

 

Slide 51 - Top Right: This is a portion of the geologic map of southwestern North Carolina (NCGS, 1992) near Bryson City in Swain County. The red dots show locations of slope failures that occurred with summer thunderstorms in May of 2003. From this perspective it is hard to see any correlation between the slope failures that coincide with areas underlain by sulfidic-graphitic bedrock. Studies so far indicate that steep slopes underlain by sulfidic-graphitic rock are more susceptible to landslides.

 

Slide 52 - Generalized geologic map showing locations of debris flows coinciding with areas underlain by sulfidic-graphitic bedrock. Red dots indicate the number and dates of debris flows. Easternmost debris flow is in sulfidic rock. Studies so far indicate that steep slopes underlain by sulfidic-graphitic rock are more susceptible to landslides.

 

Slide 53 - Tracks of debris flows (yellow arrows) triggered by Hurricane Opal, October, 1995 that damaged the Blue Ridge Parkway northeast of Asheville. (1998 CIR image).

 

Slide 54 - Color-infrared aerial photograph showing reconnaissance map of debris fan, debris fan source areas, and alluvial deposits in the Maggie Valley area, and locations of recent debris flows. The 12/11/03 debris flow resulted in one fatality and a destroyed house – legal action is pending.

 

Slide 55 - Left: Green dashed line outlines debris fan with an apartment complex; yellow dashed line outlines source area. Right: Schematic block diagram showing typical debris flow track and deposit. Over thousands of years debris fans accumulate at the toes of slopes from multiple debris flow and flood deposits. Debris fans are attractive building sites because of moderate slopes above the main flood plains, and the general lack of bedrock excavation required for roads and foundations. Renewed debris flow activity originating in the source area can put developments on debris fans at risk.

 

Slide 56 - Prehistoric debris fan deposits near Maggie Valley, N.C. Inset: Completely decomposed rock clasts suspended in a silty sand soil matrix.

 

Slide 57 - Steep, high excavations in debris fan deposits can be unstable. This cut slope failed during the May 5-7, 2003 rains in Swain County. Although the log cabin remained intact, the failed debris pushed it 3-5 feet off its foundation.

 

Slide 58 - Demolished remains of a residence at the location of a fatal debris flow on Dec. 11, 2003 near Maggie Valley. Much of the demolition took place during the effort to rescue the victim buried in the back of the house. The embankment failure that originated in the scarp in the background mobilized into a debris flow. A broken water supply line, the road embankment, and a buried dark line marking the location the original ground surface can be seen in the scarp. A lawsuit pending against the Maggie Valley Sanitary District and the N.C. Department of Transportation claims that the leaking waterline caused the embankment failure. At this time it is not known if the water line was leaking, and if it was, what caused the leak.

For additional information

The contact for additional information about geologic hazards in North Carolina is Mr. Richard Wooten, P.G.; his e-mail is Rick.Wooten@ncmail.net. He is located in the Swannanoa, North Carolina office (western North Carolina) and can be reached by telephone at 828.296.4500. His mailing address is: 2090 U. S. Highway 70, Swannanoa, North Carolina 28778. An alternate North Carolina Survey staff geologist contact is Dr. Jeff Reid, P.G., 512 North Salisbury Street, Raleigh, North Carolina, 27699-1612. His telephone number is 919.733.2423 x403. His e-mail is Jeff.Reid@ncmail.net.