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Landslides and Landslide Hazards
Landslides are the result of natural geologic processes that have worked to shape the landscape among the mountains of North Carolina, and are hazards endemic to mountainous regions all throughout the world. The frequency and severity of landslides is considerably less here than in many other areas, such as in California, primarily due to specific factors guiding our geology, climate, and vegetation, with damaging landslides occurring nearly every year. Of the 3,290 landslides documented since before June 30, 2011, debris flows, also known as mudslides, were determined to be the most common and widespread type. Rockslides and embankment failures along major transportation routes are also common, often resulting in significant direct and indirect costs throughout the region. Similarly, large, multi-acre and slow-moving landslides have resulted in homes being condemned in Haywood County, and have caused significant property damage in Macon and Jackson Counties. Such losses can be reduced by implementing the proper location design, construction, and maintenance of developments on steep mountainside slopes, and by siting buildings and other structures away from the paths of landslides where they may travel on the flatter slopes, often along stream valleys. The map shows the locations of known landslide events as of June 30, 2011. Landslide hazard mapping was completed in Macon, Watauga, Buncombe, and Henderson Counties. The high concentration of landslides in Watauga County is from those triggered by the August 13-14, 1940, storm.
Aside from the losses incurred by fatalities and injuries, landslides can also have a significant economic impact to the region, especially for those who have experienced losses from the path of their destruction, as homeowner’s insurance does not cover damage from landslides. Some examples of incurred costs include $1.4 million to stabilize slopes after the February 2, 2010, retaining wall failure from debris flow in Ghost Town in Maggie Valley, and the buyout of property after the Peeks Creek debris flow, costing $3.2 million. Direct contract costs to repair the October 10, 2009, rockslide on I-40 was $10.2 million. Estimates of the loss in commercial revenues were about $1 million per day while I-40 was closed. The direct contract cost to repair the July 1997 rockslide on I-40 was about $4.8 million. Similarly, rockslides and embankment failures that close the Blue Ridge Parkway result in economic losses to the communities reliant on Parkway tourism.
Recurring weather patterns over the last century give evidence that major landslide events will affect Western North Carolina in the future. At least seven major storms and sequences of storms that triggered numerous landslides and caused flooding across Western North Carolina have occurred since 1916, in 1940, 1977, and 2004. Including the storms of 1876 and 1901, at least 17 other rainfall events or periods of wet weather have triggered damaging landslides in Western North Carolina, with two of these being major rockslides that caused the closing of I-40 in Haywood County for months.
Rainfall from the remnants of a hurricane that hit the area during July 15-16, 1916, produced a storm of record for the French Broad watershed. During this storm, 22 inches of rain fell at Altapass in Mitchell County over a 24-hour period. Landslides set off by this storm killed at least 24 people in Henderson, Transylvania, Rutherford, Wilkes, and Alexander Counties. The severity of this mid-July storm was likely due to an earlier storm that preceded it, which lead to a significant increase in soil moisture within the region. The remnants of a hurricane that tracked over much of the Southeast from August 10-17, 1940, dropped heavy rainfall over Watauga County on August 13 and 14. In Watauga County alone, the rainfall triggered nearly 2,100 landslides that killed 14 people, destroyed 32 structures, and cut off road and rail access to many parts of the county. Additionally, heavy rains during August 28-31, 1940, storms triggered at least 600 landslides and severe flooding, killing four people in Jackson and Haywood Counties. The most recent sequence of major storms occurred in September 2004, when intense rainfall from the remnants of Hurricanes Frances and Ivan triggered at least 400 landslides in the region that caused five deaths and destroyed or resulted in the condemnation of 27 homes, and also disrupted transportation throughout Western North Carolina.
Since 1990, 57 documented landslides have resulted in six fatalities, including an unborn child, five injuries, 40 destroyed or condemned structures, damage to 24 structures and 56 other private properties, and serious damage to 32 private and North Carolina Department of Transportation (NCDOT) roads. The destroyed, condemned, and damaged structures were mainly homes. With the exception of the Peeks Creek debris flow of September 16, 2004, in Macon County that resulted in five fatalities and 16 destroyed homes, all of the other documented landslides mentioned above occurred where slope modifications by human activity were likely contributing factors to the landslide events. These numbers do not include landslides originating along NCDOT-maintained roads.
Some progress has been made toward understanding the factors that contribute to when and where landslides are most likely to occur. Rainfall and the subsequent increase in groundwater is a common trigger for landslides in this region, with the causal factors being the soil types and shape of the land surface, all of which relate to the underlying bedrock geology, and in many cases to modifications to slopes by human activity. Widespread occurrences of landslides coincide with major rainfall events, especially when the remnants of tropical cyclones track over the mountains. Localized landslides can be expected where there is about five inches of rainfall within a 24-hour period. More widespread landslides can be expected when there is around 10 inches or more of rainfall within a 24-hour period. The chart shows the dates and rainfall amounts of some of the known damaging debris flow events from 1876 through 2010 in Western North Carolina.
Information collected to date indicates that less rainfall is needed to trigger landslides on slopes where modifications to the hillside, such as cut slopes or fill slopes, have contributed to the instability. In cases studied so far, three inches or less of rainfall within a 24-hour period have triggered landslides on modified slopes where some had pre-existing signs of instability. In all, at least 380 slope failures from embankments built for road, house pads, or waste areas have mobilized into damaging landslides. Lack of compaction, inadequate ground surface preparation, large woody debris, and untreated acid-producing rock incorporated into embankments contribute to their instability.
For landslides that have happened since 1990, most occurred on slopes of 30 degrees (~60 percent) or steeper; however, the number of landslides begins to increase on slopes of 20-25 degrees (22 degrees = 40 percent). By comparison, 237 of 1,617 landslides in Watauga County occurred on slopes of 20-24 degrees. The large number of landslides on slopes in this range was likely due to more intense rainfall in Watauga County during the August 13-14, 1940, storm than that experienced over widespread areas since 1990, including Frances and Ivan in 2004.
Landslides usually happen in the same general areas affected by previous landslides, as indicated by the presence of pre-existing landslide deposits in those areas affected by recent landslides. The recurrence intervals between landslide events at the same location can be on the order of hundreds or thousands of years. Past landslide deposits are readily identified where significant volumes of unconsolidated soil and rock fragments have accumulated over geologic time. These deposits frequently occur at the toes of slopes below steep mountain coves, and are commonly deep, productive forest soils. Such landslide deposits are generally stable; however, they can be destabilized by over-steeping from excavations or stream erosion.
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January 2009 Maggie Valley Debris Flow: North Carolina Geological Survey.
August 1940 Boone Landslide: P. Weston.
February 2010 Maggie Valley Debris Flow: P. Parton.