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Physical Geography, 2008, 29, 4, pp. 289-305.Copyright 2008 by Bellwether Publishing, Ltd. All rights reserved. DOI: 10.2747/0272-3646.29.4.289

CLIMATOLOGICAL PERSPECTIVES ON THE RAINFALLCHARACTERISTICS ASSOCIATED WITH LANDSLIDES IN

WESTERN NORTH CAROLINA

Christopher M. Fuhrmann, Charles E. Konrad II, and Lawrence E. Band

Department of GeographyThe University of North Carolina at Chapel Hill

Chapel Hill, North Carolina 27599-3220

Abstract: Landslides are a significant hazard in the mountains of North Carolina.While previous studies have estimated the critical instantaneous rainfall rates that maytrigger a landslide, very little is known about the climatology of rainfall events associated

with landslides. The rainfall climatology of a sample of landslide events in western NorthCarolina from 1950 to 2004 is presented in two parts. First, the two-day concurrent andcumulative antecedent (from 4 to 90 days prior to slope movement) rainfall totals areassessed climatologically by ranking them relative to all heavy precipitation eventsobserved in western North Carolina over a 55-year period. Second, the storm typesresponsible for the rainfall associated with each landslide event are determined using amanual weather map classification scheme. Forty-seven percent (47%) of the landslideevents are connected with concurrent rainfall totals that exceed a one-year return period.In almost half of these cases, the heavy rainfall is associated with a tropical cyclone pass-ing through the region. The other major storm types connected with landslide events (i.e.,synoptic and cyclonic-type events) generally display lower rainfall intensities and longerdurations compared to tropical cyclones. Landslide activity shows the strongest relation-ship with antecedent precipitation totals over a 90-day period, which is the longest timeperiod examined in the study. In many cases, a tropical cyclone produced heavy rainfallover the landslide location between 30 and 90 days before the event. [Key words: land-slide, heavy rainfall, storm types, climatology, western North Carolina.]

INTRODUCTION

Landslides are a significant hazard in mountainous regions. In western NorthCarolina, a region situated in the southern chain of the Appalachian Mountains (Fig.1), over 1000 landslides (i.e., slope movements) have been recorded since the early

1900s (Wooten et al., 2007). Across the southern Appalachians, more than 200fatalities and thousands of acres of destroyed forest and farmland have resulted fromlandslide activity (Wieczorek et al., 2004; Witt, 2005). A combination of thin soils,steep slopes, and orographically enhanced precipitation leaves the mountains ofNorth Carolina highly susceptible to slope failure (Witt, 2005). Further, increaseddevelopment along mountain slopes continues to place additional stress on soilsand roots while changing the natural slope configuration through practices such asundercutting and excavation. Landslides are also a potential hazard to those livingin the flat debris fans located above the floodplain, as slope movement along thenearby hillslopes is more likely to reactivate during periods of heavy rainfall

(Ritter et al., 2002).


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The most common type of landslide in western North Carolina is the debris flow(Witt, 2005). This type of slope movement often originates in mountain hollows(i.e., concavities) where surface and groundwater flows collect. Slope movementgenerally occurs in shallow soils located along steep slopes (at least 20) with theresulting flow often traveling at swift speeds over distances up to several kilometers(Witt, 2005). Debris flows usually consist of high-density and high-viscosity mate-rial and tend to travel along preexisting drainage channels. Other types of land-

slides common in western North Carolina include debris slides, earth slides, rockslides, and rock falls (Wooten et al., 2007). Debris and earth slides typically movemuch more slowly than debris flows because of their higher clay content, requiringmore water for liquification (Varnes, 1978). Persistent wet periods can allow waterto slowly infiltrate existing tension cracks and scarps, thus preconditioning theslope for failure. Approximately two-thirds of all landslides recorded in westernNorth Carolina consist of slope movement deposits, mainly debris fans and otherdebris deposits (Wooten et al., 2007). Rock slides and falls occur most frequentlyalong roadways that have been cut into natural rock slopes. Although heavy rainfallcan trigger a rock slide or fall, most of these landslides have been tied to freeze-thaw cycles, wedging of tree roots, and slope destabilization exacerbated by exca-vation and blasting (Varnes, 1978).

In the absence of sufficiently heavy rainfall, most slopes in western NorthCarolina remain stable due to ample vegetation and strong soil-root cohesion (Witt,

Fig. 1. Study area with shaded relief and COOP station locations.


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RAINFALLANDLANDSLIDES 291

2005). Heavy rainfall that penetrates the bedrock-soil interface or results in signifi-cant increases in soil pore pressure at an interface, however, can induce slope

movement. Past studies have defined a 24-hour rainfall threshold of about 125 mm(5 in) before slope movement can occur (e.g., Eschner and Patric, 1982; Neary andSwift, 1987; Witt, 2005). However, excessive point precipitation totals alone can-not be used to adequately determine the potential for flooding rainfall (or rainfallnecessary to induce a landslide). The timing and spatial distribution of rainfall mustalso be considered to determine the potential for flooding or slope movement(Hirschboeck et al., 2000; Konrad, 2001). This includes examinations of antecedentsoil moisture and rainfall conditions at various time scales (e.g., days, weeks,months) as well as the spatial extent of the rainfall (e.g., local to regional scale dis-tributions). Heavy rainfall events known to activate landslides in western North

Carolina are typically associated with (1) short-lived, intense localized storms, (2)long-lived, regional-scale storms, or (3) multiple short or long duration storms thattrain across the region over a period of days to weeks (Witt, 2005). The returnintervals for these types of storms and how they rank within the context of otherheavy rainfall events in western North Carolina are not known. Moreover, the timescales and intensities of antecedent rainfall required to prime slopes for landslideactivity are not clear, but depend on the soil mass balance of water from rainfallinfiltration, net drainage, and evapotranspiration. In the southern Appalachians,hydrologic conditions promoting landslide activity may be associated with ante-cedent rainfall over a wide range of time periods.

The objectives of this study are as follows. First, the 2-day concurrent and 4- to90-day antecedent rainfall totals associated with landslide events in western NorthCarolina from 1950 to 2004 are determined and ranked within the context of aheavy rainfall climatology for western North Carolina. Second, the predominantstorm types associated with each landslide event are characterized using a classifi-cation scheme (with some variants) adopted from a seminal study of flash floodevents.

DATA AND METHODOLOGY

Identification of Landslide Events

The North Carolina Geological Survey (NCGS), in response to the destructionresulting from major landslides in the fall of 2004, was authorized by the NorthCarolina General Assembly to prepare county-scale slope movement hazard mapswith an emphasis on western North Carolina. A combination of field observations,remote sensing imagery, and digital elevation models were analyzed in a geographicinformation system to identify historical landslide events in the region. At the timethe NCGS database was acquired for this study (August 2006), there were a com-bined 2046 entries for slope movements (i.e., landslides, nearly all post-1940) andslope movement deposits, which are presumed to be mainly prehistoric. Updates tothe NCGS database are made routinely. As of June 2008, it had included 3032 slopemovement processes and 2254 slope movement deposits (R. Wooten, pers. comm.,2008). The reader is directed to Wooten et al. (2007) for details on the NCGS


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landslide hazard mapping project. Although the current NCGS database identifiessome of the major landslide events since the early 1900s, the availability of precipi-

tation data only allowed for analysis of landslides from 1950 to 2004. Therefore, forthe purposes of this research, only those landslides identified in western NorthCarolina were examined for the 1950 to 2004 time period (Fig. 1; see Konrad, 1996,for a detailed description of the study area and its rainfall climatology). Initially, themovement date and location of each landslide were used to aggregate individuallandslides into events. This was done to better relate the occurrence of landslidesto particular storm types. For each event, at least 60% of the individual landslideshad to have unique coordinates (latitude and longitude) and a movement date givento the day (e.g., 17 March 1990) so concurrent rainfall for each event could be deter-mined. Using the above methods, 30 landslide events (encompassing 221 individual

landslides) were identified in western North Carolina from 1950 to 2004 (Table 1).These events included landslides that initiated on modified and unmodified slopes.It is important to note that the landslide events examined in this study only accountfor approximately 25% of all recorded landslides in the NCGS database (as ofAugust 2006) and 58% of recorded landslides during the 1950 to 2004 period. In allother cases there was simply not enough information in the database to determinethe day that slope movement occurred.

Rainfall Data and Methodology

To determine the amount of rainfall associated with each landslide event, dailyrainfall estimates were obtained from the nearest Cooperative Observer Network(COOP) station (Fig. 1). This network of rain gauges provided the most completespatial coverage of rainfall for the study period, but several caveats are noteworthy.First, event rainfall totals typically show much fine-scale variation across complexterrain, thus increasing errors in the extrapolation of COOP rainfall estimates. Sec-ond, there is an inherent bias in the distribution of COOP stations across westernNorth Carolina toward valley locations. This has a large impact on the estimation ofrainfall at local scales (e.g., along isolated peaks or ridge tops) even with statisticallysound interpolation techniques (e.g., co-kriging). Third, there has been a secular

decrease in the number of COOP stations from 1950 to 1996, which may have hada slight negative impact on rainfall estimates in the later years of the study period(Konrad, 2001).

The rainfall thresholds required to initiate landslides are typically expressed interms of a daily or hourly rainfall rate (Neary and Swift, 1987). Although hourlyrainfall amounts are desirable, a sufficiently dense network of hourly weather sta-tions in western North Carolina currently does not exist. The measurement times forthe daily rainfall measurements at COOP stations vary (e.g., many were recordedfor the 24-hour time period ending at 1200 UTC or 0000 UTC), but this informationwas not routinely available at all stations. Thus, it was not possible to provide daily(24-hr) areal estimates of rainfall over the study area. Instead, two-day rainfall totalswere calculated. This subsequently produced a positive bias in rainfall amounts perevent, as most heavy rainfall events generally occur over periods of hours asopposed to days (Giordano and Fritsch, 1991). As such, it was not possible to


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determine rainfall thresholds that could be readily compared to those of other stud-ies. However, major landslide events are typically associated with weather systemslasting at least one day or comprising a series of storms or rainfall events (Witt,2005). Therefore, the use of two-day areal mean rainfall over the entire study areashould capture the cumulative rainfall from a single weather system. Two-day

Table 1. Landslide Events Identified in Western North Carolina from 1950 to 2004

Date No. slides Counties Materials and mechanismsMay 28, 1973 1 Watauga Composite

April 4, 1977 1 Mitchell Debris flow

November 5, 1977 69 Buncombe; Henderson Debris flow; debris slide

March 5, 1985 1 Haywood Rock slide

June 25, 1988 1 Haywood Rock slide; rock fall

April 6, 1989 1 Haywood Rock slide

June 16, 1989 1 Watauga Composite

February 16, 1990 1 Macon Debris flow

March 17, 1990 1 Jackson Debris flow

December 23, 1990 3 Cherokee; Clay; Swain Debris flow

July 9, 1994 2 Haywood Debris flow

August 17, 1994 1 Transylvania Debris flow; debris slide

October 5, 1995 5 Macon; Buncombe; Jackson Debris flow

July 1, 1997 1 Haywood Rock slide

May 11, 1999 1 Swain Rock slide

June 15, 1999 1 McDowell Rock slide

July 6, 1999 2 Madison Debris flow

January 28, 2002 1 Mitchell Rock slide

May 30, 2002 1 Henderson Rock slideApril 10, 2003 1 Caldwell Creep

May 5, 2003 18 Swain; Haywood; Jackson Debris flow; debris slide; weatheredrock

October 3, 2003 1 Mitchell Debris slide

November 1, 2003 1 McDowell Weathered rock

November 19, 2003 1 Macon Debris flow

December 11, 2003 1 Haywood Debris flow; debris slide

June 12, 2004 1 Madison Weathered rock

September 6, 2004 31* 10 counties Debris flow; debris slide; weathered

rockSeptember 16, 2004 68* 13 counties Debris slide; earth flow; blowout

November 24, 2004 2 Graham Earth flow

December 12, 2004 1 Graham Rock slide

*The current landslide count (as of June 2008) for the September 2004 storms triggered by Hurri-canes Frances and Ivan is over 400 based on recent information from the United States Forest Ser-vice (R. Wooten, personal communication).


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rainfall totals from all COOP stations in the study area from 1950 to 2004 wereinterpolated onto a 10 km by 10 km grid using Theissen polygons (Konrad, 2001).This routine was done daily, thus providing temporally overlapping, two-day rain-fall totals for each day over the 55-year period of study. Local rainfall amounts overthe landslide event location were determined from the interpolated grid.

One of the major objectives of this paper was to determine the climatologicalsignificance of rainfall events associated with landslides in western North Carolina.To accomplish this, the two-day rainfall associated with each landslide event (i.e.,the large-scale instantaneous rainfall) and the cumulative antecedent rainfall over arange of temporal scales were ranked within the context of a heavy rainfall clima-

tology for the study area. As in Konrad (2001) and Konrad et al. (2002), the heavyrainfall climatology was defined by events with a recurrence interval of one year orgreater (i.e., the heaviest 55 rainfall events from 1950 to 2004) over the location(i.e., COOP station) with the highest point rainfall total for each event (Table 2). Itshould be noted that the estimation of recurrence intervals did not involve an anal-ysis of the statistical distribution of the rainfall totals. Recurrence intervals were alsoestimated for rainfall totals determined across each landslide area across sevenantecedent time periods: 4 days, 7 days, 14 days, 21 days, 30 days, 60 days, and 90days. These periods represent a broad range of temporal scales over which cumula-tive rainfall may trigger threshold pore pressures, as well as the hydrologic systems

that may induce slope movement (Fig. 2). It is important to note that the rainfalltotals calculated over the antecedent time periods do not include the two-day con-current rainfall totals associated with a given rainfall or landslide event (i.e., rainfallwas calculated over time periods prior to but not including the events).

Classification of Storm Types

The predominant storm types associated with each landslide event were deter-mined using a manual classification scheme developed by Maddox et al. (1979) forflash flood events across the eastern United States. Manual weather type classifica-tion involves simple visual examination of surface (and sometimes upper-level)weather maps to identify predominant weather types or patterns over a given region(Yarnal, 1993). The Maddox et al. scheme identifies three storm types (synoptic,frontal, and mesohigh) based on the location of flooding rainfall relative to the

Table 2. Recurrence Intervals and Ranks of the 55 Heaviest Two-Day RainfallEvents and Antecedent Rainfall Totals (mm) Over Western North Carolina

from 1950 to 2004

Rank

Recurrenceinterval(year)

Antecedent precipitation (days prior to rainfall event) (mm)

2-day 4 7 14 21 30 60 90

1 55 593 439 632 840 925 950 1,312 1,578

10 5 284 260 345 431 449 537 789 1,047

25 2 265 198 249 297 333 379 607 885

55 1 213 114 156 197 223 266 472 635


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location of synoptic and subsynoptic scale features (e.g., frontal boundaries,

cyclones, moisture plumes) and the rain shield (Fig. 3). In the synoptic-type events,

convective cells repeatedly develop and move (i.e., train) over a broad scale in the

warm sector along and ahead of a slow-moving front. Typically, a slow-moving

upper-level trough or cutoff low is present immediately upstream. In the frontal-type events, convective cells, elevated over a stationary frontal surface, train over a

regional-scale area north of a stationary front. In contrast, mesohigh events in the

Maddox et al (1979) scheme result largely from local to mesoscale processes and

produce heavy rainfall at a much more localized scale. In the present study, nomesoscale analysis was undertaken, thus the mesohigh type could not be defini-

tively identified. Instead, events that show very localized precipitation patterns

were classified as isolated. Many of these events occurred near the Blue Ridge

escarpment where localized orographic lifting is common. Surface and 500 hPa

daily (1200 UTC) weather maps were obtained online from the National Oceanic

and Atmospheric Administrations Daily Weather Map Series and used to identify

the synoptic types. Examination of these maps revealed that two additional storm

types were associated with heavy rainfall in the region: cyclonic and tropicalcyclones (Fig. 3). The cyclonic type accounts for events where heavy rainfall

occurred in the cool sector (i.e., north to northwest quadrant) of a mid-latitudecyclone. Additionally, an upstream 500 hPa cyclone or trough was commonly asso-

ciated with these events.

Fig. 2. Time intervals (days) over which rainfall totals are calculated, and the associated hydrologicsystems and conditions that may induce slope movement.


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RESULTS

The spatial distribution of landslide events identified in this study reveals a pref-erence for slope movement along steep escarpments where local precipitation isenhanced by upslope flow. Conversely, the absence of landslide events in the inte-rior valleys may be tied to downslope flow and the suppression of precipitation (i.e.,rain shadow) as well as a general decrease in slope angle. An example of the land-slide distribution is provided for Macon County, which is bounded by the northwestand southeast escarpments (Fig. 4). More than two-thirds of landslide events exam-ined in this study consisted of only one individual landslide. Half of these isolatedevents occurred in the last two years of the study period, likely due in large part to

the better detection of landslides in recent years and possibly other nonmeteorolog-ical and geomorphologic factors (e.g., soil-root cohesion, internal friction; see Godtet al., 2006).

A summary of the rainfall rankings associated with each landslide event is pre-sented in Table 3. All events in which a ranking is provided were connected withrainfall totals that exceeded the one-year return period. Initial inspection of the rain-fall amounts and rankings revealed that none of the rock slide, rock fall, or weath-ered rock events (taken together to be simply rock events) was associated withheavy concurrent rainfall (Table 4). Moreover, four of these events did not exhibitany heavy antecedent rainfall in the 90 days prior to the event. While rainfall can-not be excluded as a contributing factor in these events, data from the NCGS data-base suggests that other processes (e.g., excavation) likely contributed moresignificantly to these types of slope failure (Wooten et al., 2007). Thus, the remain-der of the analysis focuses primarily on the other 19 landslide events (i.e., debris

Fig. 3. Schematic of the surface weather patterns for each storm type. Shaded areas represent com-posite rain shields. Triangles represent mountain peaks associated with isolated rainfall events.


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events), which were composed of debris flows, debris slides, composite material,creep, and earth flows (Table 3). Historically, these types of landslide events havebeen strongly tied to heavy rainfall and high soil moisture. Indeed, the median rank-ing of two-day rainfall totals was more than eight times lower for rock events (x =785) than for debris events (x = 92; Tables 3 and 4).

Nine out of the 19 debris events (i.e., 47%) were associated with heavy concur-rent rainfall, and only four of these 19 events were connected with a tropicalcyclone passing through the study area (Table 3). Tropical cyclones, however, pro-duced an average of 21.4 landslides and were therefore responsible for 48% of allindividual landslides identified in this study. Perhaps the most notable of these wereHurricanes Frances and Ivan, which produced a record number of landslides acrosswestern North Carolina over a two-week period in September 2004 (Table 1). Hur-ricane Frances ranked as the heaviest two-day rainfall event in the climatology,while Hurricane Ivan ranked as the sixth-heaviest event. Two other tropicalcyclones in this study produced heavy (i.e., a recurrence interval of one-year or

Table 3. Ranks of the Concurrent (Two-Day) and Antecedent Rainfall for DebrisFlow, Debris Slide, Composite, Creep, and Earth Flow Eventsa

Date Storm type

Antecedent precipitation (days prior to rainfall event)

2-day 4 7 14 21 30 60 90

May 28, 1973 Synoptic 48 23 25 36 15 27 24 11

April 4, 1977 Synoptic 31 32 42 11 17 36

November 5, 1977 Cyclonic 22 38 49 42 5* 10

June 16, 1989 Synoptic 50 40 48 46 27 28

February 16, 1990 Synoptic 42 24 25 23 17 32 37

March 17, 1990 Synoptic 36 46 52 45 10 8 16

December 23, 1990 Synoptic 53 54 41*

July 9, 1994 TC Alberto 43 47 37 48 49

August 17, 1994 TC Beryl 7 3 4 3 53 45 52*

October 5, 1995 TC Opal 25 * 35 42

July 6, 1999 Isolated 39 28 34 42 40

April 10, 2003 Cyclonic 51 55 54 44 54 47 51

May 5, 2003 Cyclonic 28 22

October 3, 2003 Isolated 35 43 50 12 15*

November 19, 2003 Cyclonic 32 18 26 22 24 21 14 13

December 11, 2003 Cyclonic 38 14 21 18

September 6, 2004 TC Frances 1 2 3 2 6* 8* 2 3September 16, 2004 TC Ivan 6 1 2* 1* 1 1 1 1

November 24, 2004 Synoptic 44 46 41 49 2*

Median ranking 92 53 51 39 43 36 26 20

aItalicized rankings indicate rainfall amounts in the top quartile of all heavy rainfall events. Dashesdenote time periods where the rainfall ranking exceeds 55. Asterisks indicate antecedent periodswhere rainfall from a tropical cyclone fell across the study area.


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greater) concurrent rainfall: Tropical Storm Beryl (seventh-heaviest event) and Hur-

ricane Opal (25th-heaviest event).Five of the 19 debris events were connected with cyclonic-type storms. Three of

these events were also tied to heavy concurrent rainfall. Four out of the fivecyclonic-type storms were associated with a surface cyclone that originated in theGulf of Mexico and tracked eastward across the Florida Peninsula and up the East-ern Seaboard. One exception was the cyclonic-type event associated with theNovember 1977 landslides in Pisgah National Forest. This event was associatedwith the 22nd-heaviest two-day rainfall total in the climatology. In this case, astrong, slow-moving surface cyclone located beneath a 500 hPa cyclone over thecentral Gulf Coast moved to the NNE, placing western North Carolina in the north-east quadrant (i.e., cool sector) of the system. An extremely large rain shield withembedded convective bands of heavy rainfall resulted in the locally intense, long-duration event (Neary and Swift, 1987).

Synoptic-type storms were connected with 7 of the 19 debris events, althoughonly 2 of these events were associated with heavy concurrent rainfall. According toMaddox et al. (1979), synoptic-type storms occur on the warm side of a quasi-stationary frontal boundary, with the mean steering flow oriented parallel to thefront. In this situation, convective cells repeatedly develop and move over the sameregion, resulting in long-duration rainfall events. Nearly half of the synoptic-typestorms in this study were associated with a surface cyclone; heavy rainfall in thesecases was connected with storms that formed ahead of a cold front. These cyclonesmoved slowly across the eastern United States, creating a persistent feed of moistureinto western North Carolina. Also present in nearly half of the synoptic-type events

Table 4. Same as Table 3, but for Rock Slide, Rock Fall, andWeathered Rock Eventsa

Date

Antecedent precipitation (days prior to rainfall event)

2-day 4 7 14 21 30 60 90

March 5, 1985

June 25, 1988

April 6, 1989 38 32

July 1, 1997 30 39 44 35

May 11, 1999 47 50 51 50 38

June 15, 1999

January 28, 2002 43 52 20 29 43

May 30, 2002

November 1, 2003 27

June 12, 2004 52

December 12, 2004 50 18 28 40 21*

Median ranking 785 590 226 317 209 101 59 51

aNote that storm types were not identified for rock events due to the low concurrent rainfall totals. Theasterisk indicates antecedent period where rainfall from a tropical cyclone fell across the study area.


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was a 500-hPa cyclone located to the WSW of the southern Appalachians, whichaided in the middle tropospheric transport of moisture into the region (not shown).

Only 2 out of the 19 debris events were connected with isolated storm types,neither of which produced rainfall exceeding the one-year return period. In bothcases the synoptic-scale circulation regime was characterized by southerly flowaround a subtropical high pressure system off the Atlantic coast (i.e., BermudaHigh). This regime was associated with isolated rainfall across the region, particu-larly along southern and eastern slopes where the regional topography intersectsthe southerly low-level circulation (Konrad, 1994, 1996).

Debris events show a relatively even seasonal distribution, as all months exceptJanuary experienced at least one landslide event (Fig. 5). Landslide frequencies,however, were skewed strongly toward the late summer and fall as many are con-nected with the five tropical cyclones that crossed the area during the study period.Synoptic and cyclonic-type events dominated during the winter and spring seasons,and most were connected with relatively few landslides. Localized landslide eventsoccurred in July and October under relatively benign synoptic conditions (e.g.,Bermuda High circulation regime).

All 19 of the debris events were associated with heavy rainfall over one or moreantecedent time periods (Table 3). While nearly half of the events (44%) were tiedto heavy rainfall in the four to 14-day period prior to the event, all but one event

Fig. 4. Shaded relief map of Macon County, NC and known locations of debris flows (white circles)from the complete NCGS database (as of August 2006). The shaded relief map was constructed from a

6 m pixel resolution light-detecting and ranging (LiDAR) digital elevation model (Wooten et al., 2007,2008).


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was associated with heavy rainfall during the 90-day period prior to the event.Moreover, 6 of the 19 events (32%) displayed exceptionally wet 90-day antecedentrainfall totals (i.e., exceeding the five-year return period or a rank between 1 and

14). Examination of the median ranking for all debris events reveals that the clima-tological significance of the cumulative rainfall over the landslide locationincreased markedly as the antecedent time period became longer (Table 3). In fact,

the 90-day antecedent rainfall displays the highest median ranking for all debrisevents (x = 20), while the two-day concurrent rainfall is tied to the lowest medianranking (x = 92). This underscores the important connection between landslide

activity in western North Carolina and wet soils resulting from high antecedent pre-cipitation totals.

Eight of the 19 debris events (i.e., 42%) were associated with at least one ante-

cedent period in which rainfall from a tropical cyclone fell across western NorthCarolina (Table 3). Most significantly, all five of these tropical cyclones, with theexception of Tropical Storm Alberto, produced two-day rainfall that exceeded the

two-year return period (i.e., rank between 1 and 27). This suggests that, in additionto triggering landslides directly, tropical cyclones that produce locally heavy rainfallare also effective at saturating the soils to the point where additional rainfall mayinduce slope movement. In most cases, these tropical cyclones crossed the study

area between 30 and 90 days prior to the landslide event. A notable exception isHurricane Ivan, which crossed western North Carolina less than two weeks afterHurricane Frances.

Fig. 5. Monthly frequency of debris events by storm type.


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Most of the landslide events identified in this study consisted of isolated slopefailures (Table 1). The degree to which the isolated nature of the event is due to iso-

lated heavy rainfall or some other agent at a local scale is not clear. To investigatethis question, the spatial distribution of rainfall was estimated across the study areaduring each event and the ratio of the local to regional rainfall totals was calculated(i.e., the local rainfall divided by the regional rainfall). A high ratio indicates mark-edly heavier rainfall over the landslide location relative to the mean rainfall over thestudy area (Fig. 6). Only debris events were examined due to the low rainfall totalsassociated with rock events. Regional rainfall totals for western North Carolinawere determined by averaging the two-day rainfall at each recording COOP stationin the region.

The range of ratios for the 19 debris events (stratified by storm type) is presentedin Table 5. In all cases, the ratio of local to regional rainfall was positive, indicatingthat the rainfall over the landslide location was higher than the mean rainfall acrossthe study area. Cyclonic-type events displayed relatively low ratios, with the

Table 5. Mean, Maximum, and Minimum Ratios of Local to Regional Scale

Rainfall for Each Debris Event (Stratified by Storm Type)Storm type N Mean Max Min

Synoptic 7 5.6 9.1 2.3

Cyclonic 5 4.9 16.3 1.4

Tropical cyclone 5 4.3 9.7 1.9

Isolated 2 9.3 10.3 8.2

Fig. 6. Two-day rainfall totals associated with the 5 May 2003 landslide event (cyclonic-type storm).The dark polygon demarcates the study area where the mean regional rainfall amount was calculated,whereas the asterisks denote the locations of known slope movement.


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November 1977 event being a significant exception (i.e., ratio of 16.3). Synoptic-type events showed somewhat higher ratios due to the convergence of air and mois-

ture locally along south-facing slopes. Tropical cyclones also exhibited relativelyhigh ratios; however the broad coverage of heavy rainfall associated with HurricaneOpal (i.e., ratio of 1.9) is reflected in the low mean ratio for all tropical cyclones.Isolated events displayed the highest mean ratio, as expected. It is important tonote, however, that the mean two-day rainfall totals associated with isolated eventswere lower than all other storm types.

DISCUSSION AND CONCLUSIONS

This study has provided a climatological perspective on the rainfall conditions

and storm types associated with landslide events in western North Carolina from1950 to 2004. None of the rock events (i.e., rock slides, rock falls, weathered rockevents) were connected with heavy concurrent rainfall and very few were associ-ated with heavy antecedent rainfall. Thus, the climatological analysis of rainfallcharacteristics focused primarily on the remaining 19 landslide events (i.e., debrisevents: debris flows, debris slides, earth slides). Nine out of the 19 debris events(i.e., 47%) were tied to heavy concurrent rainfall. Tropical cyclones were responsi-ble for the rainfall connected with four of these events, all of which displayed two-day rainfall totals that exceeded the two-year return period. The significance ofcumulative antecedent rainfall, however, suggests that many of the landslides in

western North Carolina are relatively deep and may be associated with monthly toseasonal fluctuations in the height of the groundwater table. In these cases, evenlight to moderate rainfall may supply sufficient water to the debris mass for slopemovement. Having knowledge of the depth of slope movements can help in quan-tifying the contribution of pore pressure capacity thresholds to slope instability,which is further modulated by a combination of geomorphic factors (i.e., limit equi-librium, including shearing force, resisting force, cohesion, and internal friction). Amuch larger sample of landslide events is needed, however, to determine the suffi-ciency of various rainfall thresholds to induce slope movement.

All of the debris events were connected with locally heavier rainfall compared to

the mean rainfall across the entire study area (i.e., local to regional scale rainfallratio > 1). This suggests that landslides in western North Carolina are more likely toinitiate along slopes that are most susceptible to orographic enhancement of rainfall(i.e., locally heavy rainfall embedded in a broader rain shield). Wooten et al. (2008)suggest that this enhancement may be maximized at higher elevations, although amore directed long-term study is needed to address this hypothesis. Such a studywould need to examine this connection in conjunction with the prevailingsynoptic-scale circulation, as the relationship between topography and precipita-tion varies according to the wind direction (Konrad, 1996). The ratios presented inthis study should be interpreted with caution due to the sparse network of COOPstations across the study area. High-resolution (4 km) radar-derived estimates ofrainfall for landslides in the last decade of the study period may be used to betterdiscern the spatial distribution of concurrent and antecedent rainfall connectedwith these events. Although difficulties exist in deriving accurate rainfall amounts in


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mountainous regions using radar data (e.g., the blocking effect of the topography),preliminary analysis of heavy convective rain bands associated with Hurricane Ivan

show good agreement with nearby ground-based rain gauges (Wooten et al., 2008).Cyclonic-type storms, which were not included in the original Maddox et al.

(1979) classification scheme, contributed to one-third of the debris events associ-ated with heavy concurrent rainfall. In these cases, heavy rainfall in the cool sectorof a surface cyclone was likely enhanced through orographic lifting, as the track ofthe cyclone from the SSW generally leads to easterly winds and upslope flow alongthe southeast escarpment. Indeed, many debris events associated with cyclonicstorms initiated along southeast-facing slopes. One exception was the November1977 event in Pisgah National Forest, where a number of debris flows in the BentCreek area initiated on NNE-facing slopes. In some cases, a large fraction of the

rainfall over the southern Appalachians occurred on the backside (i.e., WNW quad-rant) of the surface cyclone. Moreover, cyclonic-type storms accounted for many ofthe landslides that occurred during the cool season, a time when landslide activityin western North Carolina has historically been considered relatively quiescent(Witt, 2005).

Only 17% of all landslides events (i.e., rock and debris events) were associatedwith tropical cyclones; however, the tropical cyclones that passed through the areaproduced numerous landslides and were responsible for 48% of the landslide sam-ple. The attributes of these tropical cyclones may be significant in dictating howmany landslides develop. Konrad et al. (2002) determined the climatological fea-

tures of tropical cyclones that contribute most significantly to rainfall totals overvarious spatial scales. In their study, rainfall totals over the smallest scales weremost highly correlated with the speed of movement of the cyclone. Indeed, three ofthe tropical cyclones in the present study (i.e., Alberto, Beryl, Frances) were slow-moving (i.e., speed of movement below the 50th percentile). On the other hand,although Hurricane Opal was generally fast-moving (i.e., 89th percentile), it was astrong (i.e., peak storm winds in the 96th percentile) and large (i.e., area within theoutermost closed isobar in the 99th percentile) hurricane that interacted with mid-latitude features to produce benchmark rainfall totals over a variety of basin sizes(Konrad, 2001; Konrad et al., 2002).

Hurricane Ivan possessed characteristics similar to Opal, yet it was the record-breaking rainfall from Hurricane Frances less than two weeks earlier that resulted inone of the worst landslide outbreaks in recent history across the southernAppalachian Mountains (Table 1; Witt, 2005; Wooten et al., 2008). Indeed, some ofthe most destructive landslide events in western North Carolina during the 20thcentury (i.e., 1916, 1940, 2004) were connected with major storm systems thatcrossed the region within two weeks of each other (Witt, 2005). In particular, theremnants of a tropical cyclone that crossed western North Carolina in August 1940initiated over 2000 landslides in Watauga County alone, with numerous other largelandslide events in counties along the southeastern escarpment (Wooten et al.,2008). Such numbers underscore the importance of tropical cyclones as potentialtriggering events. Unfortunately, the lack of available precipitation data precludedexamination of the 1940 event in this study. The occurrence of major precipitationevents in short succession has also been noted for major flooding events across the


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304 FUHRMANNETAL.

state of North Carolina during approximately the same time period (i.e., the occur-rence of a precursor and prime event; Robinson, 2003). With regard to the

landslide events identified in this study, antecedent (i.e., precursor) tropicalcyclones generally produced heavy rainfall across the region between 30 and 90days prior to the prime event. In cases not involving an antecedent tropical cyclone,the fact that rainfall was more significant over longer time periods (i.e., 30 to 90days) compared to shorter time periods (i.e., four to 21 days) suggests that anteced-ent rainfall patterns are not tied to one or a short sequence of storms but rather to alarge-scale circulation pattern that promotes a seasonally wet environment. Amore directed study is needed to address this possibility. Moreover, it is likely thatrainfall conditions over longer antecedent time periods (i.e., beyond 90 days) aremost strongly correlated with landslide activity in western North Carolina.

Because there are many factors that influence landslide activity, rainfall charac-teristics alone cannot be used to predict their occurrence. However, the results ofthis study will hopefully aid in identifying the weather conditions and critical timeperiods where rainfall should be closely monitored.

Acknowledgments: We gratefully acknowledge Rick Wooten of the NCGS forassisting us with the landslide data and for providing comments on an earlier ver-sion of the manuscript. We appreciate the efforts of two anonymous reviewers, whoprovided helpful suggestions. NCGS data were acquired through a grant from theU.S. Forest Service awarded to Lawrence Band. Precipitation data were acquiredthrough a grant with the National Science Foundation (BCS-9911315) awarded to

Charles Konrad.

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