Landslides and Debris Flows of Rockbridge County

Landslides and Debris Flows of Rockbridge County, Virginia
Research Plan for Investigation of Debris Flow and Flooding Effects of the June 27,
1995 Storm, near Buena Vista and Glasgow, Virginia
By Sas Jr., R.J. and Eaton, L.S.
Department of Geology and Environmental Sciences, James Madison University,
Harrisonburg, VA 22807
September 28, 2004
The frequency of debris flow and landslide activity is low in the central Appalachians
(Blue Ridge), and have a recurrence interval of a few thousand years. However, data
from storm effects from the June 27, 1995 storm collected from Madison and Albemarle
Counties, Virginia show that these low frequency events occur with a high intensity
(Wieczorek et al., 1996; Wieczorek et al, 2002). The remnants of Hurricane Camille in
1969 created a storm of near equal magnitude, striking Nelson County and triggering
thousands of debris flows. Both Williams and Guy (1973) and Morgan, et al. (1999) also
concluded that low frequency, high magnitude events are important forces in mobilizing
sediment, and modifying the landscape. To date, scant work has commenced in
investigating the effects of the June 1995 storm in eastern Rockbridge County near Buena
Vista and Glasgow. Fortunately, reconnaissance has shown that many of the salient
features from the debris flows are still present, and should be documented soon.
The primary goal of this proposed study is to compare and contrast the events
associated with the June 1995 storm near Buena Vista and Glasgow, with respect to
documented events in Madison and Albemarle Counties. Specifically, the study will
examine the role of debris flows in eroding steep mountainous stream channels, primarily
first and second-order systems, and the deposition of this sediment on fans and
A secondary goal will be to investigate factors contributing to the stability of this
region during Hurricane Isabel’s impact upon the region in September 2003, where no
documented landslides or debris flows occurred. The landslides and debris flows of
interest are located approximately between the communities of Buena Vista and Glasgow
along the western flank of the Blue Ridge Mountains in Rockbridge County, Virginia.
There are an estimated 40 discrete failures that are situated in first order drainages. These
drainages will constitute the main focus region of the study, as this is where most of the
slides and flows began.
Meteorology of the Storm
A series of rainstorms in late June 1995 saturated the surface soils in the region and
increased pore pressures within jointed bedrock. On June 27, the most devastating storm
effects occurred, as the groundwater infiltration capacity was reached. This particular
storm resulted in a relatively short period of time between bankfull flow and flood stage,
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
in the affected streams. Rainfall was intense, measured at several inches per hour, over a
short period of time, which precipitated flooding and landslides.
The topography of the Blue Ridge coupled with the unique nature of the pressure
systems allowed for the storm center to stall over the region. Unfortunately, official
rainfall data does not exist. Tributaries of the James River Basin situated in eastern
Rockbridge County show the tremendous increase in discharge on June 27 compared to
previous years of record. U.S.G.S. streamflow gage station 02024000 on the Maury River
near Buena Vista normally shows a daily average flow between 200 and 500 ft3 s-1.
However, on June 28, 1995 (the storm occurred on June 27, with one day lag time before
rainwater reached the gage station) this gage measured 19600 ft3 s-1 for the daily average
flow. Other stations in the region show similar increases in streamflow on June 28 (U.S.
Geological Survey).
Historical Perspective
The region of eastern Rockbridge County, specifically Buena Vista and Glasgow, has
a history of flooding. Events of flooding are officially recorded as early as 1877 (Miller,
1992). Though not official, well-documented floods occurred in March 1836 and
September 1950 (Miller, 1992). The most devastating flooding experienced in
Rockbridge County to date occurred on August 18, 1969 due to the intense rainfall
caused by Hurricane Camille. The effects of Camille caused approximately 155 deaths
throughout the Blue Ridge, with dozens in Rockbridge County. Glasgow received heavier
loss of property than Buena Vista (upstream on the Maury River) due to the higher
drainage density. More than 25 percent of the homes in Glasgow were heavily damaged.
The townspeople worked for several months to restore the community to its pre-flood
state, though the memory of Camille will remain in the local history for years to come.
In 1972, the area was able to prepare for the storm effects of Hurricane Agnes.
Though flood mitigations were employed to reduce the damage to private and public
property, there was still $150,000 of damage caused by floodwaters. In 1985, however,
the region was much less fortunate. Glasgow suffered loss of 40 percent of residences
and 70 percent of businesses were severely affected from the remnants of Hurricane Juan
(Miller, L.M.N., 1992).
Geologic Setting
Rolling hills and mountains peaking at a range from ~2000 ft. to 4500 ft. elevation
characterize the topography of the Blue Ridge Physiographic Province. The areas of
interest are the South and Maury Rivers, which serve as major tributaries of the James
River basin in Rockbridge and Amherst Counties.
Alluvial and debris fan deposits along the base of the mountains have formed through
catastrophic debris/hyperconcentrated flows and uniform fluvial processes. The flow
source areas are contained in the George Washington National Forest. Minor impacts to
private property have been observed in field study based on minor cobble deposits,
dendochronology (tree scars and age differentiation along flow paths), and broken/shifted
fences. Landslides in this region flow from zero-order basins (most ephemeral stream
valleys) into first-order streams and deposit larger fan surfaces along second and thirdorder channels.
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
The Chilhowee group, primarily siliclastics, is the most prominent rock type in the
study area (Spencer, 2000). The Antietam quartzite is the primary formation providing
sediment in the debris flows. The quartzite is composed of gray, well-sorted, gradational
sediments with numerous Skolithos trace fossils. The Harpers and Waynesboro
Formations are also mapped in this region.
The structure of the beds includes major orogenic folds represented by exposed
inclined bedding. Field observations yielded a preliminary stereographic analysis, which
suggests a preferred bed of failure, with a strike, dip of S 225 W, 54. Overlying a more
cohesive bed is a heavily jointed and fractured unit that serves as the major source of
debris in colluvial deposits.
Aerial photos will be georeferenced and entered into a Geographic Information
System (GIS) to map the flow source areas, paths, and depositional areas. A volumetric
analysis will also be conducted using GIS. Field data collection will be used to analyze
flow velocities and for hazards mapping. Additional observations will be made on
vegetation regeneration and influence of root depth on slope stability.
Personal interviews with area residents will provide further meteorological data, in
addition to local impacts of flooding and debris/hyperconcentrated flows.
Field Data Collection
The majority of data collected in the field will be used to facilitate volumetric
analyses. Knowing the volume of sediments released is necessary for understanding the
intensity of these events, including the level of denudation and sediment transport. Once
the intensity (the correlation between velocity and volume of debris) is established, data
on colluvium, channel morphology, and flow initiation can be incorporated to establish
the level of the potential hazards and the damage capacity of future events.
Previous studies by Pierson (1980), Reneau and Dietrich (1987), Ellen (1988), Sitar
and others (1992), and Wieczorek and others (2000), have shown that debris flows
initiate preferentially at sites of concave slopes. Preliminary field observations in the
study area show trends similar to those found in these previous studies. Therefore, this
project will employ similar methods of volumetric data collection.
One of the tasks to calculate sediment loss will be establishing the boundaries of the
pre-flood channel. The intact hill slope surface and its extension into the upland stream
channels will give a close approximation of pre-failure steepness, and thus the pre-failure
channel morphology.
Within the failure and scour areas, cross-sectional measurements will be taken using a
stadio-rod and measuring tape to determine the width, depth, and length of the erosional
channel. The method of reconstructing the pre-erosional slope will be done as described
by Shroyer (1997). Slope surface cross-sections will be reconstructed by projecting crosssectional tangent lines, which estimate the pre-failure curvature of the slope. The tangent
will cross the point where eroded and uneroded surfaces intersect; the trajectory of
tangent lines will be decided according to the line that best approximates the preerosional curvature of slope. Measurements of depth will be taken along the tangent at
meter and fractions-of-a-meter increments. The location of individual cross-sections will
be determined in the field. Selecting representative sites for measurement will be based
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
upon the overall morphology of the pre-erosional surfaces and sites showing the sequence
of the flow migration (See bottom of Sheet 1).
For example, the relatively flat pre-erosional surfaces in the upper middle sections of
the landslide area will have a string line running from the tops of each escarpment.
Whereas, in the scoured stream channel, lines tangent to the pre-erosion slopes should be
used because the pre-erosion channel’s concavity is augmented by the scouring due to
landslide effects (See Sheet 1). Similar methods of volume data collection will be
employed to determine volume of landslide-borne sediments in stream channels.
Additional volume measurements will be made on alluvial/debris fans. Area
measurements for fans can be made using a GIS query of polygon areas. Fan depth
measurements will be made in the field along areas of incision, either from fluvial
processes or human alteration. Volume measurements from all areas of field data
collection will be entered into the GIS and used in spatial analyses.
When considering the overall characteristics of debris flow events, volumetric data is
complimented by velocity estimates. At sites of superelevation, velocity is approximated
by the equation:
V=[g * rc* sin(S) * tan(θ)]1/2
Where V is the velocity at peak discharge; g is the gravitational constant; rc is the
radius of curvature; S is the channel slope in percent; θ is the degree angle (in degrees)
between the high water marks on opposite banks in the area of superelevation. The radius
of curvature measured by anchoring three measuring tapes at the point of maximum
curvature on the point bar bank. Two tapes will be positioned so that they form a right
angle at the vertex (where the tapes are anchored), and so that their lengths from point bar
bank to cut bank are equivalent. The third tape should also be of equal length and pass
through the 45 degree angle between the two outer tapes. The angle of superelevation
will be measured using a clinometer.
The size of particles in debris flows is controlled by the differential velocity
necessary for entrainment determined by the types of source material and the viscosity of
the debris and water. Particle size distributions will be measured in depositional areas,
including largest boulder and average cobble sizes. Across a 30-meter section of
alluvium, at 1-meter intervals, the medium axis length of the particle is measured, and the
distribution of sizes is established. The ten largest boulders within a hundred meter radius
of the size distribution measurements will be measured on their medium axis length.
These measurements will be taken at selected sites throughout the depositional fans and
stream channels.
To understand the genesis of debris flows, the storm effects must be taken into
account. The drainage basin area can be mapped and measured using GIS techniques.
Drainage density values, as well as rainfall per unit area of the drainage basin can be
extracted from GIS analysis. Actual rainfall accumulation, for June 1995 and September
2003, will be collected through personal interviews with local landowners.
Since some instances of bedrock sapprolites and colluvial/alluvial sapprolites were
observed in the field, the extent of weathering of these rock materials will be measured to
semi quantitatively and qualitatively categorize the age of events and the rates of
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
weathering. The sapprolitization of the source materials will be classified according the
Clast Weathering Scale developed by Whittecar and Duffy, (2000).
Generalized field observations will be noted on the depth of root penetration into soil,
vegetation regeneration in scoured areas, and the effect of woody debris on channel
morphology and sediment accumulation.
Mapping Techniques
Stereoscopic interpretation of aerial photos provided by the US Geological Survey
taken on March 30, 1998 will be used to accurately locate the positions and morphology
of flow paths. Interpretations of topography and slope stability differences will also be
interpreted from aerial photos. Viewing stereopairs through a stereoscope will allow the
use of Geographic Information Systems (GIS) to record, in greater detail, the actual areas
of deposition and erosion from landslide events.
GIS techniques will be used to map debris flow source areas, paths, and depositional
areas. Volumetric analysis will be conducted using GIS. Remote sensing data, and field
data will be used in map-imbedded databases to precisely map the landslide events.
Aerial photos will be input into ArcGIS through high resolution scanning. Using
ArcMap, photos will be georeferenced using a minimum of 5 locations precisely
measured from the USGS 7.5 minute quadrangles for Buena Vista and Glasgow, VA.
Polylines will be digitized onto aerial photos in AutoCAD to establish area polygons over
debris flows. These polygons will serve as map indicators of flow and source areas, as
well as aid in the volumetric analysis. In MS Excel databases on the crossectional depth
of escarpment will be built and linked to area polygon databases using ArcToolbox and
ArcMap. A spatial analysis will be performed to interpret the volume of material
removed from debris flow source areas. Additional data pertaining to vegetation
regeneration may also be input into the GIS databases and linked to map tips.
Preliminary Work: Description of Landslides, Debris Flows, and Floods
Preliminary field observations of debris flow source areas show escarpments initiated
near ridge tops along bedrock failure planes. Some of the main scarps show evidence of a
rotational slump component. The failure planes are located along deformed beds of
Antietam quartzite inclined down slope, failing parallel to the bank of the Maury River.
Bedrock failures range from planar to wedge type. In general, these failures are described
as having a soil-bedrock interface, the most common type of debris flow failure interface
(Clark, 1987).
Exposed surficial outcrops show a colluvial deposition in the upper half of the
failures, whereas the basal units exhibit deposits that suggest either hyperconcentrated or
water flow processes. Just downstream from superelevation sites show fining upward
gradational sequences with the long axis of boulders, cobbles, and gravel perpendicular to
flow direction, indicative of tractive force flows.
Within the stream channel, flow velocity and debris composition was sufficient in
many areas to mobilize and entrain channel fill; this is evidenced by extensive exposure
of channel bedrock. In areas of channel constriction and tributary confluence, superelevation sites were subjected to erosion. Larger boulders are typically deposited in low
velocity zones at superelevation sites. Woody debris is often deposited with larger
boulders, thus providing a catchement for finer sediments. Higher clay content and
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
friable soils are found near the bottom of flow paths, while sandier sediments are found at
higher elevations.
Preliminary measurements of slope properties found failure surfaces are 30 +/- 4,
with the preferred bedrock failure planes striking at S 225 W, 54 (average of 3
measurements). One site has poorly developed bedrock saprolite at the base of the flow
path, as well as in the initial failure surface. Other saprolites are composed of rounded
cobbles located along flow scours, and could represent previous debris flow activity. The
saprolitization ranges from category 1 to 5 according the Clast Weathering Scale
developed by Whittecar and Duffy, (2000).
Moving towards the distal ends of the debris flow chute-fan system, minor landslides
initiated at escarpments along the riverbanks are found downstream from larger debris
flow source areas. Floodwaters and hyperconcentrated deposits impacted land near debris
flows and throughout the lower elevations of the drainage basins. Cobbles and a few
small boulders were located on private land with some evidence of erosion and
deposition from incipient channels.
The numerous studies conducted in Madison and Albemarle Counties widened the
understanding of the geomorphic processes operating in the Blue Ridge province of
Virginia. The conclusions of these studies provide new insight into the frequency and
intensity of landslide events. Hazards associated with landslide and flood processes need
to be continually studied in the region in order to more effectively define the potential
impact of catastrophic events on human developments.
Landslide and debris flow events are highly variable between localities within a
single region; therefore the sites of debris flow studies must be numerous to gain a full
geomorphic spectrum. The areas of Glasgow and Buena Vista, Virginia have not yet been
studied for debris flow impacts, so the understanding of landslide activities in the Blue
Ridge province is not complete. The purpose of our study is to contribute to the
knowledge base of catastrophic geomorphic processes in the region.
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
Clark, G.M. “Debris slide and debris flow historical events in the Appalachian’s south of
the glacial border.” Debris flows/avalanches: Process recognition and mitigation,
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Miller, L.M.N. Glasgow, Virginia: One Hundred Years of Dreams. Rockbridge
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Horseshoe Mountain, Va., 7.5' Quadrangles, from the August 19/20, 1969, Storm
in Nelson County, Virginia.” Open-File Report 99-518 U.S. Geological Survey,
Pierson, T.C., 1980, Piezometric response to rainstorms in forested hillslope drainage
depressions”: Journal of Hydrology, New Zealand, Vol. 19, No. 1, pp. 1-10.
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steep forested landscape”: International Association of Hydrological Sciences,
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1995 Flood Event of Madison County, Virginia. James Madison University
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<>. 26 July 2004
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Alluvial Fans, Augusta County, Virginia. Department of Geologic Sciences, Old
Dominion Univ.: Norfolk, VA, 2000.
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
Wieczorek, G.F., et al. “Debris-flow hazards in the Blue Ridge of central Virginia.”
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Tentative Research Schedule
Field Work
Note: In addition to the specifics below these activities will also take place:
- Tabulate number and density distribution of vegetation
- Take notes on flow morphology, soils, woody debris dams
- Photograph important features
Volumetric analysis of one flow in Belle Cove drainage. Collect cobble size distribution
from same channel and make superelevation measurements.
Volumetric analysis of flow south of Battle Run. Collect cobble size distribution from
same channel and make superelevation measurements.
Volumetric analysis of flow on North face of James River. Collect cobble size
distribution from same channel and make superelevation measurements. Study major
alluvial fans on Maury River.
Lab Work
Hydrologic/Meterologic interpretations.
Completed by October 30, 2004.
Build layers for GIS. Digitize new layers for area measurements and DEM analyses.
Completed by December 3, 2004.
First Final Draft of paper due by January 14, 2005.
Second Final Draft of paper due by February 4, 2005.
Final Paper due March 30, 2005.
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA