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
October 25, 2004
Introduction
The frequency of debris flows associated with catastrophic flooding in the Blue Ridge
of central Virginia occurs approximately once every two to three thousand years for
upland mountainous basins. These low frequency, high magnitude events have impacted
Albemarle, Greene Madison, Nelson, and Rockbridge, Counties, Virginia (Williams and
Guy, 1973, Morgan and Wieczorek, 1996; Wieczorek and others 1996; Wieczorek and
others, 2002).
Several researchers have concluded that these episodic events are important forces in
mobilizing sediment, and modifying the landscape (e.g., Kochel, 1988; Jacobson and
others, 1989; Eaton, 2003a, 2003b). Although a few studies have examined sediment
transport and denudation of mountainous basins underlain by crystalline rocks in the Blue
Ridge, no research to date has focused on these processes in sedimentary and
metasedimentary lithologies in this geologic province.
The hypothesis for this study compares the denudation of upland basins that differ in
bedrock lithologies. Specifically, denudation from zero- first- and second order basins
situated in Quartzite terrains in Rockbridge County will be compared to previous studies
situated in the crystalline rocks of the Blue Ridge. 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 in 2004 has shown that many salient
features from the debris flows are still present, and should be documented soon before the
evidence of geomorphic processes from the storm are masked by time.
Objectives
The primary goal of the proposed study is to compare and contrast the geomorphic
processes and landforms associated with the June 1995 storm near Buena Vista and
Glasgow, with respect to previously documented events in Madison, Greene, and
Albemarle counties. Specifically, the study will examine the role of debris flows in
eroding steep mountainous channels, primarily first and second-order streams, and the
deposition of this sediment on fans and floodplains.
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
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
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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
The widespread flooding and subsequent debris flow activity was the result of
multiple storms occurring on June 27,1995. The region had received ample antecedent
rainfall several days prior to the event, saturating the soils in the region. On June 27, the
region received 30.5 in of rainfall in 14 h, thereby exceeding soil infiltration capacity.
Increased pore pressures along the water saturated soil-bedrock boundary triggered over
one thousand debris flows. Rainfall was intense, measured up to 11.8 in/hr for 35
minutes over a short period of time (Eaton, 1999). Peak storm discharge from the event
approached the flood envelope curve, making this event one of the largest for its basin
size. The topography of the Blue Ridge coupled with a stalled cold front to the north
allowed for the storm center to stall over the region. Although rainfall data was collected
for impacted sites in Albemarle, Greene, and Madison Counties, none exists for
Rockbridge County. 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. The gage station on the Maury River near Buena Vista shows a daily
average flow between 200 and 500 ft3 s-1. In comparison, the peak discharge occurring a
day after the storm measured 19,600 ft3 s-1 for the daily average flow. Other gage stations
in the region show similar magnitudes of increases in stream flow on June 28.
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 more aggressive
flooding. It is unknown what exactly contributed to greater flooding in Buena Vista, but
major factors could include drainage density, proximity to third and fourth order
channels, and elevation relief between the town and the flood channels. 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, and though the memory of
Camille continues to linger in the local history for years to come.
In 1972, the area was better able to prepared 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, 1992).
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
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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 South and
Maury Rivers, major tributaries of the James River basin, drain upland streams along the
base of the Blue Ridge. Alluvial and debris fan deposits along the base of the mountains
have formed through a combination of catastrophic debris/hyperconcentrated flows and
uniform fluvial processes. The flow source areas are confined within the George
Washington National Forest. Minor impacts to private property were observed, such as
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 third-order channels.
The underlying bedrock in the study area is primarily classified in the Chilhowee
group, primarily siliclastics (Spencer, 2000). 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
competent bed is a heavily jointed and fractured unit that serves as the major source of
debris in colluvial deposits.
Methods
Aerial photos will be georeferenced and entered into a Geographic Information
System (GIS) to map source areas, flows paths, and depositional areas. A volumetric
analysis will also be conducted using GIS and field verification. Field data collection will
be used to analyze flow velocities and for hazards mapping. Understanding the energy
with which these landslides exit mountain hollows can help to determine the magnitude
of damage that may result, as well as, what mitigations can be employed. 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
magnitude of these events, including the level of denudation and sediment transport.
Once these factors are established, data on colluvium, channel morphology, and flow
initiation can be incorporated to establish the level of the potential hazards of future
events.
Previous studies by Dietrich and Dunne (1978), 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
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
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studies. Since, the morphology of the debris flows discussed in these aforementioned
studies is similar to the morphology at the current site, similar methods for calculating
sediment budgets will be employed. [it is not clear what methods will be similar, a more
thorough discussion is necessary for the reader to thoroughly evaluate the adequacy of
the methods for achieving the study goals].
One of the tasks to calculate sediment loss will be establishing the boundaries of the preflood 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 stadia 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 cross-sectional 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 pre-erosional 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 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.
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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 measured, and the
distribution of sizes established (Wolman, 1954). 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
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.
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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
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.
Conclusion
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
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
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Ridge province is not complete. The purpose of our study is to contribute to the
knowledge base of catastrophic geomorphic processes in the region.
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References
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,
Geological Society of America, Reviews in Engineering Geology 7, pp. 125-138,
1987.
Dietrich, W.E. and Dunne, T. Sediment budget for a small catchments in mountainous
terrain. Zeitschrift fur Geomorphologie N.F. 29:191-206. 1978.
Ellen, S.D., 1988, “Description and mechanics of soil slip/debris flows in the storm.” In
Ellen, S.D. and Wieczorek, G.F. (Editors), Landslides, Floods, and Marine
Effects of the Storm of January 3-5, 1982, in the San Francisco Bay Region,
California,U.S. Geological Survey Professional Paper 1434: U.S. Geological
Survey, Denver, CO, pp. 31-40.
Miller, L.M.N. Glasgow, Virginia: One Hundred Years of Dreams. Rockbridge
Publishing Company: Natural Bridge Station, VA. 1992.
Morgan, B.A., et al. “Inventory of Debris Flows and Floods in the Lovingston and
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,
1999.
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.
Reneau, S.L. and Dietrich, W.E., 1987, “Size and location of colluvial landslides in a
steep forested landscape”: International Association of Hydrological Sciences,
Publication No. 165, pp. 39-48.
Ritter, D.F., Kochel, R.C., and Miller, J.R. Process Geomorphology, fourth ed. McGrawHill: Boston, MA. 2002
Shroyer, H. 1997. Unpublished Manuscript. Selected Sedimentologic Aspects of the June,
1995 Flood Event of Madison County, Virginia. James Madison University
Department of Geology and Environmental Studies, Harrisonburg, Virginia. pp.
1-90.
Sitar, N.; Anderson, S.A.; and Johnson, K.A., 1992. Conditions for Initiation of RainfallInduced Debris Flows, Stability and Performance of Slopes and Embankments II:
ASCE Geotechnical Special Publication 31, Vol. 1, 834-849.
U.S. Geological Survey. “Gage Station: Daily Streamflow.”
<http://waterdata.usgs.gov/va/nwis/nwis>. 26 July 2004
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Whittecar, G.R. and Duffy, D.F. Geomorphology and Stratigraphy of Late Cenozoic
Alluvial Fans, Augusta County, Virginia. Department of Geologic Sciences, Old
Dominion Univ.: Norfolk, VA, 2000.
Wieczorek, G.F., et al. “Debris-flow hazards in the Blue Ridge of central Virginia.”
Environmental and Engineering Geoscience 6, 3-23.
Wieczorek, G.F., et al. “Preliminary Inventory of Debris-Flow and Flooding Effects of
the June 27, 1995, Storm in Madison County, Virginia Showing Time Sequence
of Positions of Storm-Cell Center.” Open-File Report 96-1.3 U.S. Geological
Survey, 1996.
Williams, G.P. and Guy, H.P. “Erosional and Depositional Aspects of Hurricane Camille
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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
October
Volumetric analysis of one flow in Belle Cove drainage. Collect cobble size distribution
from same channel and make superelevation measurements.
November
Volumetric analysis of flow south of Battle Run. Collect cobble size distribution from
same channel and make superelevation measurements.
November
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/Meteorological interpretations.
Completed by October 30, 2004.
Build layers for GIS. Digitize new layers for area measurements and DEM analyses.
Completed by December 3, 2004.
Sas Jr., R.J.; Eaton, L.S. Landslides and Debris Flows of Rockbridge County, VA
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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
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