Introduction
Landslide activity in the Eastern United States is normally associated with heavy
rainfall due to large NW and NE tracking storms. Rainfall intensity is considered the
main mechanism of mountainous slope failure. However, debris flow activity on the
western flank of the Blue Ridge Mountains in Virginia suggest that some slope failure
processes are heavily dependent upon structural deformation of the ridge forming rocks.
Orthogonal joint sets in the Antietam quartzite act to reduce slope-stability by providing
failure surfaces and discrete zones of hydraulic pressure head increases.
Storm Effects
On June 27, 2005, an intense storm passed over a major portion of the Blue Ridge
Mountains of Virginia. The storm was characterized by high intensity precipitation that
tended to track into zero-, first- and second-order mountainous basins. A thorough
analysis of the storm was performed by Smith et. al. (1996) for the Rapidan River basin
in Madison County, Virginia; however, no such site-specific study exists for Rockbridge
County, Virginia, located 160 km. southwest of Madison County.
In a 14 hr. period, the storm in Madison County produced 775 mm. of rainfall in the
region. Topography controlled a majority of the motion of the 6 km. column of
precipitable air mass. Storm cells tended to concentrate in zero-order basins because
“strong boundary layer winds directed upslope” were impacted by weaker winds near
mountain ridges activating the cells of the strongest rainfall (Smith et. al. 1996). More
than 500 slope failures resulted from the deluge in the Rapidan basin.
Since, flood damage and landsliding was not as extensive in Rockbridge County as
Madison County, it is assumed that the storm did not reside for a comparably extensive
period of time nor did it have as great a control from topography. Slope failures are
confined mostly to zero- and first-order basins located on the western flank of the range.
Orography is assumed to have contributed to rainfall intensity since the greatest density
of landsliding occurred on slopes draining directly into the Maury and James Rivers.
Antecedent rainfall of 122.17 mm. from June 23, 1995 likely contributed to failure
initiation. The highest estimated value of accumulated rainfall on June 28 was 213.28
mm. (NOAA 1995). No weather or stream gaging station was located directly in the
impacted basins. However, the stations from which data was collected are proximal to
within 6 km. of the area of greatest debris flow activity, thus actual rainfall values within
the failed basins in likely greater than these estimated values.
Description of Debris Flows and Basin Morphometry
A debris flow is a viscous slope failure containing colluvium, sand, clay, and organic
debris mobilized when regolith is sufficiently saturated. The slope failures studied here
are best described as debris flows in their general morphology; however, some specific
distinctions should be made to clarify the exact nature of failure. Well-defined layers of
saprolite are found in upper and middle reaches of the debris path. Resistant bedrock
planes are observed at each initiation site. Due to the age of the failures when the study
began it is unclear the extent of bedrock exposure due to continued erosion of the head
escarpment. However, the depth of saprolite, intact bedrock, and regolith exposure along
lateral escarpments suggest that bedrock exposure is relatively continuous at the initiation
sites. As the initial debris slide continues downslope bedrock exposure becomes
increasingly discontinuous.
The bedrock exposed at the failure apices have similar sedimentary structures
including the fossil Skolithus linearis and grain size was observed to be consistent. It is
likely that this is a stratigraphically-continuous unit based on the depth of saprolite
developed on top of the resistant failure surface. The high density of Skolithus in the
resistant layer and lack in the saprolite suggests that the trace fossils provide a greater
rheologic strength and the tightly spaced sediments within the wormholes decreases pore
space and infiltration capacity.
Methods
Denudation volumes were calculated by reconstructing the pre-erosional slopes
adjacent to the debris flow path and measuring a cross-sectional area as described by
Shroyer (1997). Cross-sectional depths were measured from a string line spanning across
the width of the failure and measuring to the eroded surface at width intervals of 1 m.
Measurement of the debris flow in Bennetts Run included 16 cross-sections at intervals
of 60 m. through the thalweg. The smaller flow located on Poplar Cove required a total of
6 cross-sections at 30 m. intervals through the thalweg.
The volume of material removed is calculated by averaging each value of depth with
the previous value of depth measured across the width of the failure. These averages are
then summed and multiplied by the distance between each cross-section. The sum of
these values yields the total volume of material removed by denudation. Basin
denudation is then calculated by dividing the total volume by the area of the basin. A
planimeter was used to measure the basin area on 7.5 minute U. S. Geological Survey
topographic quadrangles. The extent of basin area was precisely located using aerial
photo and field reconnaissance.
The orientation of joints and bedding planes were measured using a Brunton
compass. Selection of planes measured depended upon available outcrop within the
debris flow. On the outcrop planes were selected systematically based upon the geometric
relationship between the thalweg, relatively unweathered surfaces, and the aspect of
failure. Bedding planes were differentiated from joint surfaces using primary sedimentary
structures such as cross-bedding and Skolithus linearis. Orthogonal jointing was observed
in outcrop and in colluvial deposits so there was no necessity to establish this geometry
using stereonet analysis.
The Fisher test was used to determine the statistical correlation between the joint data.
The steronet analysis program StereoWin v. 1.2 includes a Fisher test so its validity and
accuracy were confirmed in personal communication with the program developer
(Allmendinger, 2006). The Fisher test utilizes the von Mises distribution for spherical
data to determine the significance (within a 95 percent confidence interval) of variance
within the sample. Essentially, if the initial test for variance results in a critically high
value of R/n, then there is more than one subset of data within the total sample. A Post
hoc test of variance is then performed on each sample to determine the statistical
significance of each subset. Subset selection depended upon 3-dimensional contouring of
the data and visual estimation to include or exclude poles within subset.
Results and Discussion
A comparison of basin denudation values reported for 12 basins in Virginia and West
Virginia showed that the two basins measured in this study have significantly higher
values. The previously greatest value reported for a clastic basin in West Virginia was
2.15 * 10-3 m. (Cenderelli and Kite, 1998). The highest value measured in this study was
1.30 * 10-1 m. Patterns of erosion and deposition are similar for both of these regions
(Cenderelli and Kite, 1994). In the upper two-thirds of the debris flow track erosion is
dominant, and in the lower third deposition is dominant. Cenderelli and Kite (1994)
describe four zones within impacted basins: “an upper failure zone, a middle transport
zone, a lower deposition zone, and a scour zone immediately beyond the debris-flow
terminus.” Additionally, they described the presence of hyperconcentrated flow deposits
in stream channels. Stream channels in Rockbridge County show similar deposits from
ancient activity. Well-weathered, poorly to moderately imbricated cobbles are supported
in a red, clay rich sediment matrix. Hyperconcentrated stream deposits are likely to have
existed in this region directly following recent activity, however, their record was not
preserved within the ten years between activity and the time of this work.
Sediment yields in West Virginia range from 3300 m3 to 20900 m3. Basin areas
range from 1.78 km2 to 17.48 km2. Comparable sediment yields of 2447 m3 and 25185
m3 measured in Rockbridge County. However, basin areas are 0.28 km2 and 1.94 km2.
These relatively small basin areas may provide a mathematical explanation for such high
values of basin denudation where sediment yields are comparable. However, mathematics
only account for the calculation, they do not necessarily account for the geologic process
allowing such extensive erosion in a relatively small basin.


Discuss what they described in this region.
Compare values for volume loss.
The denudation for one of these mountainous basins is nearly twice the magnitude of
any other debris flow impacted basin in Virginia.