**Recent landslide activity on the alluvial fan along Meadow Run,
Shenandoah Valley, Virginia**

**Wieczorek, G.F., Eaton, L.S., Yanosky, T.M., and Turner, E.J.**

**Abstract**

Although intense rainfall and localized flooding occurred as Hurricane
Isabel went northerly inland towards and along the Blue Ridge mountains
of the Eastern United States on September 18-19, 2003, very few
landslides occurred. However, due to Hurricane Isabel, a dormant
landslide along a bluff of an alluvial fan along Meadow Run on the
western side of the Blue Ridge mountains in central Virginia was
reactivated by erosional flooding along the toe of the landslide.
Subsequent monitoring of the landslide showed retrogressive movement
involving several separate landslide blocks that continued moving slowly
for the next several months during continuing periods of precipitation.
Previous examination of this and another adjacent dormant landslide
using dendrochronology had revealed periods of landslide activity
similarly related to times of high stream runoff. Although the relative
height and steepness of alluvial fans does not usually suggest high
degrees of landslide hazards, the migration of stream channels and their
strong erosional force during flood stages, indicates that landslides in
this terrain are widespread along the channel banks and are reactivated
on the average roughly once per decade, locally posing a relatively high
landslide hazard.

**Key Words:** Landslides, Hurricane Isabel, dendrochronology,
retrogressive, stream erosion

**Introduction**

Historic and recent landslide activity was examined along Meadow Run, a
stream that emerges from the western flank of the Blue Ridge Mountains
of Virginia into the Shenandoah Valley, northeast of the town of Crimora
(Fig. 1). Meadow Run flows across an alluvial fan primarily during
periods of high runoff and enters the South Fork of the Shenandoah
River. The Pleistocene-Quaternary alluvial fan is composed of fresh and
weathered silt, sand, gravel and cobbles derived from quartzites and
sandstones of the Antietam and Harpers Formations of the Chilhowee Group
(Morgan et al., 2003). The largely forested alluvial fan is underlain by
the Shady Dolomite, which has limited exposure as saprolite along banks
of the stream channel.

Stream migration and undercutting of topographically higher fan surfaces
has created bluffs up to about 20 m in height that in some places show
signs of instability along Meadow Run. The recent historic stream
channel paths of Meadow Run towards the South Fork of the Shenandoah
River are confined within a zone of about 250 m in width on the northern
edge of the alluvial fan. Examination of aerial photography taken since
1937 shows that the channels of Meadow Run within this northern zone of
the alluvial fan have periodically fluctuated their routes (Fig. 2)
(Peebles and Eaton, 2004).

Observations of several landslides along the bluffs of Meadow Run raised
the question of whether the ages and possible triggers of these
landslides could be accurately determined. Dendrochronologic techniques
were used to determine if the documentation of tree tilting and
sprouting due to ground rotation of the landslides would correlate with
severe regional storms. The hypothesis that high stream flow in Meadow
Run caused erosion at the toe of the alluvial bluffs initiating slope
movement was examined in conjunction with dendrochronologic evidence of
previous landslide activity, recent landslide activity, and stream flow
records of the South Fork of the Shenandoah River.

**Landslide Activity along Meadow Run**

Two dormant adjacent landslides of apparently different ages were
roughly distinguishable on the basis of revegetation and slope
modification along the bluffs on the northeastern side of Meadow Run.
Beginning in 2002, we examined these two adjacent landslides, the
apparently younger upstream landslide (UL) had a steep fresh looking
main scarp (Fig. 3a), whereas an older, more revegetated downstream
landslide (DL) had a slightly gentler slope with more well established
vegetation (Fig. 3b). In March of 2003, a topographic map of the two
adjacent landslides (Fig. 4) was compiled from field surveys.

These two landslides are composed of predominantly earth (soil) from
alluvial deposits, although a small exposure of degraded bedrock
(saprolite) was visible near the stream bed of the lower western scarp
of UL. Geotechnical laboratory soil tests (Table 1) found that the
landslide materials were slightly plastic, clayey sands and clayey
gravelly sands, classified as SP-SC, according to the Unified Soil
Classification.

Tilting of trees and slope surfaces on these two contiguous landslides
indicates that rotation was the dominant type of slope movement, hence
these two landslides are termed earth slumps, according to the landslide
classification system of Cruden and Varnes (1996). The time(s) of
movement of these two earth slumps were investigated by examining the
timing of tree rotation, trunk resprouting and growth of new trees
determined from dendrochronology.

**Dendrochronology**

Trees were either cored or sliced to obtain samples for determining the
absolute age or variation in annual tree ring thickness which could be
correlated with the approximate time when landslide movement caused
disruptions of tree growth or initiation of new growth. Close
examination of the tree rings showed the general year when the trees
were modified; however, depending upon the type of tree, these changes
may occur the same year or up to several years after the slide movement.
Locations of the sampled trees within the two landslides are identified
on Figure 4.

The core of a tilted white oak near the main scarp of DL (Fig. 4), which
dated back to about 1850, showed a distinct "root pruning" effect dating
to about June of 1937. A new sprout from the same tilted oak (Fig. 5)
with a date of 1940 and a nearby sprout from a black gum of 1941 lends
credence to the general dating of the initial recognized slide event
occurring around 1937. The historic record of annual floods along the
South Fork of the Shenandoah River at Harriston, located about 4.8 km
north of Meadow Run, is shown in Fig. 6. At Harriston, floods were
recorded in March 1936 (12,600 cfs) and April 1937 (11,700 cfs),
suggesting that rainstorms generating high stream flows in Meadow Run
could have temporally coincided with the slope instability. Landslides
could have been initiated by heavy rainfall causing saturation of soils
and buildup of pore-water pressure or removal of the toe of the slopes
associated with erosion and slope instability associated with these
flood events.

Further examination of the rings of the white oak, black gum, and of a
nearby Virginia pine showed additional further rotation of the trees in
1972, highly suggestive of slope movement triggered by peak flow (21,300
cfs) recorded on June 21, 1972 during Hurricane Agnes. Elsewhere on the
DL, a maple resprouted and a new white pine started growing in 1982;
although not definitive, these vegetative changes that could normally
take several years, could possibly be associated with instability caused
by large runoff (16,200 cfs) in September 1979.

The upstream landslide (UL) showed fresh scarp exposures and several
blocks of recent landslide movement. The tree core data of sassafras,
mountain laurel, red oak, maple and Virginia pines on UL (Fig. 4a)
showed similar evidence of disruptive events dating around 1993, 1997,
and 1999. Specifically, one Virginia pine shows a dramatic increased
growth reaction beginning in the 1993 ring, followed by a virtual
cessation of growth beginning in 1998 (Fig. 7). These changes are
possibly associated with a clearing of vegetation by a slide, resulting
in an increased growth rate, followed by root shearing with subsequent
slope movement, which almost terminated growth.

The most distinctive historic flood event (1924-2002) in terms of
maximum peak flow (28,900 cfs) along the South River occurred during
Hurricane Fran (September 9, 1996). The three other peak flows during
the period of 1990-2002 occurred February 1998 (10,000 cfs), April 1992
(9,840 cfs), and March 1993 (7,360 cfs). In addition to the correlation
between the dendrochronlogy of trees on UL and the peak discharge flood
events, the geomorphic evidence that paleo channels in Meadow Run have
realigned in many locations during floods and eroded the base of the
slope further substantiates these dendrochronologic dates as the general
times of landslide triggering events.

**Hurricane Isabel**

On September 18-19, 2003, Hurricane Isabel struck the mid-Atlantic
coast, came inland across North Carolina and then went northerly through
Virginia along the Blue Ridge mountains into Maryland and Pennsylvania.
The heaviest recorded rainfall reported on the NOAA-NWS IFLOWS network
was along the Blue Ridge in central Virginia (Fig. 1). The maximum
rainfall reported by the IFLOWS system network was 20.2 in (513 mm)
during 24 hours at the Upper Sherando station of Sherando Lake (20 km
south of Waynesboro, Virginia). Elsewhere along the central Blue Ridge,
up to 11 in (279 mm) of rainfall was recorded by the IFLOWS network.
Major flooding occurred in the Sherando region and the larger Back Creek
basin. Preliminary reports by the National Park Service indicate no
landslide activity to the north of Sherando, along the Blue Ridge in
Shenandoah National Park. Imagery in the Sherando-St Mary's River region
revealed a very limited number of landslides triggered by Hurricane
Isabel, with as few as five relatively small landslides detected in this
very large area, although no debris flows were detected.

Hurricane Isabel was expected to trigger landslides as it came ashore
and passed through mountainous areas; however, the observed landslide
impacts were much less severe than anticipated. The speed of Hurricane
Isabel as it passed from south to north from North Carolina through
Virginia accelerated slightly so that rainfall totals at most locations
were less than expected. Rainfall totals of 11 in (279 mm) over 24 hours
from Hurricane Isabel would not have reached the critical thresholds of
rainfall necessary for triggering debris flows previously identified in
a region of central Virginia (Wieczorek et al., 2000).

Hurricane Isabel dropped 174 mm of rain within 24 hours at Paine Run
(Fig. 1), about 3.5 km to the northeast of Meadow Run. Flooding along
the South Fork of the Shenandoah River was higher than previously
recorded since Hurricane Fran in September 1996 (Fig. 6). This heavy
rainfall caused flooding along two separate tributary stream channels of
Meadow Run starting upstream of these two adjacent landslides. When
these two adjacent landslides along Meadow Run were subsequently
examined on September 21, a portion of the upstream landslide (UL) had
been reactivated (Fig. 8). The direction of stream flow along the
southern tributary of the Meadow Run was directed towards the toe of UL,
whereas the other tributary channel was directed toward the toe of the
DL, where the two channels joined. During or shortly after Hurricane
Isabel, the toe of UL had been removed and a new scarp had formed midway
up the slope. The portion of UL reactivated by Hurricane Isabel in
relation to the direction of the tributary channel of Meadow Run is
shown in Figure 9. Although there was no reactivation of DL during
Hurricane Isabel, the flow in the other tributary channel did suggest
the process that could have previously triggered DL when the Meadow Run
channel followed this tributary channel recognized on 1937 aerial
photography (Peebles and Eaton, 2004).

Following Hurricane Isabel, the main scarp was recognized on September
20, halfway upslope within UL. The sliding appeared to be confined to
weak soil materials, although some stronger saprolite (or very weathered
bedrock) was observed at a few locations along the flanks of the slide
(Fig. 10). A portion of the toe of the landslide including soil, trees,
and previously fallen tree trunks had been removed by the erosive
flooding along Meadow Run. Some fragments or blocks of the slightly
cohesive red clayey sand removed from the toe were noted up to about
half a kilometer downstream of UL along the main downstream channel of
Meadow Run.

**Rates of Landslide Movement**

As observed periodically following Hurricane Isabel, the reactivated UL
continued to move and expand in the upslope direction during the next
several months. New cracks formed and in some areas these cracks became
minor escarpments which generally indicated an upslope retrogressive
form of slope movement (Fig. 11).

The cracks and escarpments separated the moving mass into several
general individual blocks (Fig. 12). Measurements of displacement were
made along scarps and within the blocks with flag points installed as
stakes within parts of the separate blocks (Fig. 13). The rates of
movement of three main blocks varied over the period from Hurricane
Isabel (Sept. 18-19) until the end of January 13, 2004, when movement of
all blocks within UL had ceased. The rates of movement of the three main
blocks are shown in terms of meters per week (Fig. 14). The block on the
lower part of the slope on the downstream side (Lower Slope Downstream
Side) (Fig. 11) had moved 1.3 m within the first four days (rate of 2.3
m/wk) immediately following Hurricane Isabel. Subsequently, the rate of
movement decreased to 0.9 m/wk from Sept. 23 through October 7, when it
temporarily ceased moving.

In the upper part of UL, the initiation of retrogressive movement above
the main scarp was recognized by October 7, as tension cracks started to
develop upslope. An intact block with a tree on the top upstream side
(Top Tree Block), began to show slight movement of 5 cm between October
18 and October 28, following 0.76 in of rainfall on October 14. During
the next four weeks, this block continued to move at a steady rate of
6.4 cm/wk after a storm of 1.13 in. on November 5-6, and another storm
of 1.87 in on November 12-16. During the period from November 18 to 25,
with no additional precipitation, this same block moved another 80 cm.
Two winter storms in early December each brought 4 to 5 inches of snow
and ice. Additional movement was observed after subsequent freeze-thaw
processes. The block continued moving over the next three weeks at a
steady rate of about 35 cm/wk until December 30, after which movement
had ceased.

During this same period of snow and ice in December, the Lower Slope
Upstream Side reactivated and moved rapidly (1.6 m/wk) for two weeks
between November 30 and December 13, before slowing down greatly and
ceasing movement by January 13, 2004. By this date, additional fresh
tension cracks were observed upslope of the Top Tree Block, reaching the
uppermost dormant scarp of UL (Fig. 13). However, these small tension
cracks quickly filled with crumbled alluvium and no displacement was
subsequently observed.

**Landslide Morphology**

The displacement of UL following Hurricane Isabel showed little evidence
of rotation of the earth blocks or of vegetation as they moved
downslope. Although the rupture surface could not be observed beneath
the ground surface and its depth was unknown, it is likely to be
relatively shallow, perhaps only several meters deep, based on the
relative short height of the scarps and the thickness of the individual
blocks. Because the slope angle below the main scarp became steeper,
e.g. the downstream part of UL had developed a steep slope angle of 39
degrees, there was no indication that rotation had made the lower part
of the slope flatter near the toe. This increase in the degree of slope
along the lower part of UL was likely the cause for the development of
upslope instability that lasted for the several months following
Hurricane Isabel. Although previous movement of UL before Hurricane
Isabel with evidence of rotation of trees suggested rotational slumping,
the recent landslide movement is more planar resembling an earth slide.
Presence of stronger material at slightly greater depth, e.g. saprolite
observed along the flanks of UL, could be limiting the depth of the
rupture surface beneath the center of UL.

The changes in the alignment between the upper tributary of Meadow Run
and the toe of UL from Hurricane Isabel are hard to recognize. Following
Hurricane Isabel, more material was removed from the center and
downstream part of UL than from the upstream section. Using cross
sections of UL from the March 2003 topographic map and resurveying along
the same profiles in March 2004 (Fig. 15), the volume of removed
landslide material was estimated at approximately 900 m^3^. From
immediately after Hurricane Isabel to the time that the retrogressive
activity of UL had ceased expanding the size of the reactivated
landslide, the length had about doubled from about 15 to 29 m from the
toe along the stream to the bottom of upper unaffected scarp. With a
width of about 24 m and assuming a median depth of about 2 m remaining
to the rupture surface, the remaining landslide volume on the slope
would be about 1400 m^3^. The width, depth, direction and velocity of
the tributary of Meadow Run that impacted the toe of UL during the flood
stage of Hurricane Isabel were not observed or measured. Likewise,
whether the initial movement of UL was gradual or sudden is unknown. If
the movement of UL was sudden, it could have resulted in a temporary
blockage of Meadow Run.

In the months following Hurricane Isabel, two springs with (small or
slow) seepage were observed in the lower parts of UL. These springs
located at elevations about 4 m above the base of the stream channel
indicated that a perched (?) groundwater table existed in the lower part
of UL, influenced by infiltration of rainfall rather than flow from
Meadow Run. These springs affected the stability of the landslide after
Hurricane Isabel. The springs were roughly located near parts of the UL
where the blocks of soil had become sufficiently crumbled and moistened
so that their strength was greatly reduced and the remobilized material
flowed downhill resembling minor-sized earthflows.

**Discussion**

Although landslides, e.g. debris flows, frequently occur during heavy
storms and hurricanes in steep parts of the Blue Ridge mountains of
central Virginia (Wieczorek et al., 2000), landslides initiating on
alluvial fans are not as frequent. The height of bluffs within the
alluvial fan incised by the streams of Meadow Run is higher than most
observed regionally and accounts for potential destabilization. Although
the flow along Meadow Run is intermittent, the high flows during flood
stages not only causes migration of flow down different and multiple
stream channels, but as suggested by dendrochronologic dating of two
adjacent landslides, is a frequent trigger for slope instability.

The observations of continuing instability of the two adjacent
landslides and association with climatic events following Hurricane
Isabel suggests that multiple triggering events occurred within the last
century. Including the Hurricane Isabel event, there have been seven
landslide events identified (1937, 1972, 1979, 1993, 1997, 1999 and
2003). Within the 66-year period from 1937 to 2003, there have been
roughly one event per decade at this specific site, a relatively high
landslide recurrence rate for central Virginia. Whereas stream erosion
was the preliminary trigger, following Hurricane Isabel, subsequent
movement was extended by infiltration of rainfall increasing pore water
pressure during several severe storms and later snow and ice affecting
freeze and thawing with the landslide mass.

Although stream flow during Hurricane Isabel on the South Fork of the
Shenandoah River was not as great as during several other recent storms,
e.g. Hurricanes Agnes, Juan and Fran (Fig. 6), the direction and impact
of stream flow on Meadow Run was sufficient to reactivate UL. Of
additional interest is the fact that landslide movement of UL continued
well beyond the time of Hurricane Isabel. This suggests that using
dendrochronolgy for dating the time of landslide activation can be
stretched temporally because not all impacts on vegetation occur
simultaneously.

If future severe stream-flow events adopt the predominant route of the
northern tributary of Meadow Run, the potential reactivation of the DL
is more likely than continued reactivation of UL, which has shown
frequent dendrochronologic evidence of recent movement since 1993.
Possible reactivation of DL would correspond to the previously
photographically identified period (1937-1957) when the stream tributary
direction of Meadow Run directly impacted DL (Fig. 2).

**Acknowledgements**

The preparation of this report was significantly assisted by Boros
Barrios, who prepared the figures showing the mapped representations of
the landslides. We also acknowledge Harry Hibbitts and Sargent Bray of
James Madison University who conducted a field survey in March 2003 to
prepare the original topographic map of two landslides along Meadow Run.
Jonathan McKenna aided us by conducting the geotechnical laboratory
tests of the soil samples.

**References**

Cruden, D.M., and Varnes, D.J., 1996, Landslide types and processes:
Chapter 3 [in]{.underline} Landslides: Investigations and Mitigation,
Turner, A.K., and Schuster, R.L. eds., Transportation Research Board,
Special Report 247, p. 36-75.

Morgan, B.A., Eaton, L.S., and Wieczorek, G.F., 2003, Pleistocene and
Holocene colluvial fans and terraces in the Blue Ridge region of
Shenandoah National Park: U.S. Geological Survey Open-File Report
03-410, 25 p., plate 1:100,000.
(<http://pubs.usgs.gov/of/2003/of03-410/>).

Peebles, E.K., and Eaton, L.S., 2004, The past, present, and future
locations of the stream of Meadow Run, Blue Ridge mountains, Virginia:
Abstract, Geological Society of America NE-SW Section Meeting, March
26-29, 2004.

Wieczorek, G.F., Morgan, B.A., and Campbell, R.H., 2000, Debris-flow
hazards in the Blue Ridge of central Virginia: Environmental &
Engineering Geoscience, v. VI, n. 1, p. 3-23.

**List of Figures:**