Geologic Controls of Basin Denudation from Debris Flows in Rockbridge County,
Virginia
Sas, Robert J. JR and Eaton, L. Scott
I.
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 suggests 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 slopestability by providing failure surfaces and discrete zones of hydraulic pressure head
increases. The denudation for one of these mountainous basins is nearly twice the
magnitude of any other debris flow impacted basin in Virginia.
II.
Storm
A. June 27, 2005?
B. Parameters in Madison County, VA (Smith, et al., 1996)
i. Rapidan River Basin, 14-hr period, more than 775mm of
precipitation
ii. 6-km column of precipitable air mass
iii. “Strong boundary layer winds directed upslope, weak upper level
winds”
iv. Strong topographic controls on storm motion
C. Rockbridge County, VA probably similar results of rainfall and
orographics given location and aspects of flows
D. 213.28 mm precipitation, June 28, 2005, accumulated, estimated (NOAA)
E. 122.17 mm highest antecedent rainfall, June 23, 2005 (NOAA)
F. Water table was already high, soils saturated by antecedent rainfall
III.
Methods
A. Method of reconstructing the pre-erosional slope for volumetric analysis
as described by Shroyer (1997)
B. Cross-sectional area x Length of thalweg = Volume of Material
C. Volume of material (m3) / Area of zero order basin (m2) = Basin
Denudation (m)
D. Joint and bedding plane orientations measured using Brunton geological
compass
E. Joints and bedding planes selected randomly on outcroppings within the
denuded basin
F. Statistical analysis and stereoplotting performed using Rick
Allmendinger’s (Cornell University) computer program “StereoWin v1.2”
IV.
Field observations
A. Bedrock controls evident in debris? flows
i. Regolith deposits are wedges of material that thin towards the
apex, with the thickest deposits in the thalweg.
ii. Bedrock exposures common near lateral escarpments and often
form lateral escarpments
iii. In steepest upper portions of at least 2 flows, bedrock is main
exposure without any regolith
iv. Quartzite jointing main bedrock control on extents of failure
laterally and vertically
v. Vertical incision, especially in thalweg, controlled by resistant
bedding planes with abundant Skolithus
vi. Bedrock ranges in competence from intact to saprolitic
vii. Well-saprolitized bed that has maintained joint structure is the
layer stratigraphically above resistant Skolithus layer
viii. Major strike of main joint on failure plane is 217°-252° which is
strike of mountains, strong correlation to topography
ix. Other joints penetrating exposed faces were 160°-164° which is
approximately 60°-70° difference between strikes
x. One joint system dips at 90° or nearly 90°, which likely allows
expedited infiltration
B. Description of regolith
i. Most evident surficial deposits are orthogonal cobbles of quartzite,
with angular boulders less common
ii. Soil is sandy in upper third of flows and becomes more clay rich
towards the flow base
iii. Lower thirds of sediments often included weathered, rounded
cobbles, sapprolitic rock layers stratified sub-parallel to the dip of
the slope
iv. Knick points are common in the larger flows but are poorly
understood; some contain evidence of previous debris flow
deposits
v. Previous deposits have a thin soil profile in the O-horizon; deposits
directly below are clayey, sandy soils containing matrix supported
cobbles with some boulders
C. Initiation sites
i. Most are small and open into a wider escarpment
ii. Apex failure commonly has rotational and/or slide component
iii. Failures are spoon shaped in map view, shorter width of spoon
facing upslope
iv. Soils are sandy with a smaller amount of clays
v. Depth of failure, order of magnitude 3 m.
vi. Bedrock exposure is consistently resistant layer with abundant
Skolithus
D. Minor failures
i. Primarily cuspate shaped features
ii. Rotational slump, differential stabilities between regolith deposits,
related to colluvial/alluvial fans
iii. Occur near lower reaches of flow
iv. Tree blocks common in larger slumps
v. Best re-vegetation, Eastern White Pine dominant specie
V.
Denudation Volume Results
A. Actual sediment yield (2447.1 m3 and 25185.4 m3) one to two orders of
magnitude smaller than largest yields in Madison and Nelson Counties
(SEE Figure 1)
B. Due to small zero order basins, basin denudation is one to two orders of
magnitude greater in Rockbridge County (.086 m and .130 m)
C. Highest basin denudations measured in VA
i. Micro scale to mesoscopic structures of foliation and
compositional layering found in Nelson County (Gryta and
Bartholmew 1989) (possible foliations in Madison County)
ii. Macroscopic (regional) joint systems in Rockbridge County
VI.
Statistical Analysis of Joint and Bedding Planes
A. Fisher test for uniformity of spherical distribution used to test for the
presence of more than one statistically significant set of joints (Davis
2002)
B. Test utilizes von Mises distribution, which is the circular equivalent to the
normal distribution with a mean direction and a concentration parameter
C. The mean resultant length “R-bar”, concentration parameter “k”, and the
population size “n” are all analyzed
D. Joint plane data results
i. R-bar = .54
ii. Minimum value of R-bar for 95% confidence = .24
iii. k = 2.1
iv. n = 45
E. Bedding plane data results
i. R-bar = .98
ii. Minimum value of R-bar for 95% confidence = .48
iii. k = 50.9
iv. n = 11
VII.
Mechanisms of Slope Failure
A. Rainfall intensity
B. Joint systems
i. Orthogonal sets and conjugate sets
ii. Fissile plane of weakness
iii. Fails easily when pried with shovel
iv. Allow water to quickly infiltrate
C. Steep slope 30-35
VIII.
Discussion (some ideas to ponder for inclusion or exclusion in the manuscript)
A. Neither the quartzite lithology nor clastic lithologies have been reported
to have any sites of debris flow study in Virginia. The greatest basin
denudation reported in a clastic basin in West Virginia is 2.15 * 10-3 m
(Cenderelli and Kite, 1998). In Rockbridge County, the greatest basin
denudation measured is 1.30 * 10-1 m, nearly twice the magnitude of the
basin in West Virginia. Additionally, 5.07 * 10-2 m, the previously greatest
basin denudation reported in Virginia was in a Precambrian granite,
gneiss, and schist lithology (Williams and Guy, 1973) (SEE Figure 2a).
B. Eaton et al. (2003) reported a plot of rainfall total vs. denudation for
mountainous drainage basins in Virginia and calculated a least-squares
regression line for the data. They found that for two basins in the Valley
and Ridge province that experienced similar total rainfall values as
Rockbridge County yielded less than 3.0 * 10-2 m of denudation. A
possible explanation for the great difference in denudation, given total
rainfall, is the role of hydraulic pressure head increase in discrete joint
systems.
C. While the main mechanism in landslide initiation in the mountainous
basins of Virginia is intensity of rainfall (Wieczorek, Williams/Guy,
Hack/Goodlett, Eaton), the structural character of these basins seems to
have a major influence on the denudation. Orthogonal joint systems
provide a flow path for rainwater to infiltrate and pressurize the bedrock.
Due to the unstable nature of the orthogonal joints in quartzite (Lee, 1988;
Panda and Kulatilake, 1995), these basins experience preferential failure
along joint planes. Statistical analysis shows that the joint data for two
joint sets passes the Fisher Test for a spherical distribution within a 95%
confidence interval. The bedding planes have the highest correlation
within the subset shown but the high value of k (SEE Figure 3). However,
the value of k = 2.1 for the joint dataset is also statistically significant
because the null hypothesis is H0: k = 0 and the alternative is H1: k > 1. A
contour analysis of correlation of individual poles to adjacent poles within
a 1% area of the stereoplot suggests a minimum of two subsets of joints
within a population size of n = 45 (SEE Figure 4).
Rock Type
Figure 2a. Graph of Denudation vs. Rock type for the same basins listed in Figure 1.
quartzite
qu
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qu
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ite
300000
quartzite
gneiss/granite/schists
gneiss/granite/schists
gneiss/granite/schists
metabasalt-greenstone
granite/foliated granitoid
gneisses
gneissic granodiorite/mm
greenschist dike
siltstone/shale/sandstone
quartz sandstone
siltstone/shale/sandstone
quartz sandstone
Denudation over drainage basin area (m)
Sediment yield (m^3)
600000
500000
400000
Sediment yield (m^3)
200000
100000
0
Figure 1. Graph of sediment yield for basins in Virginia and West Virginia (Eaton et al
2000) as compared to quartzite basins in Rockbridge County, VA.
Denudation vs. Rock Type
0.140
0.120
0.100
0.080
0.060
0.040
0.020
0.000
Sediment
yield
(m^3)
20900
8500
13900
3300
Drainage
area
(m^2)
9710000
5010000
17480000
1780000
Denudation
(m)
0.00215
0.00170
0.00080
0.00185
13364
2492
544000
88727
173488
87707
25185.4
2447.061
400000
120000
29500000
1750000
4080000
2470000
193858.2
28355.3
0.03341
0.02077
0.01844
0.05070
0.04252
0.03551
0.12992
0.08630
Lithology
quartz sandstone
siltstone/shale/sandstone
quartz sandstone
siltstone/shale/sandstone
gneissic granodiorite/mm greenschist
dike
granite/foliated granitoid gneisses
metabasalt-greenstone
gneiss/granite/schists
gneiss/granite/schists
gneiss/granite/schists
quartzite
quartzite
Figure 2b. Table of values used in Figure 1 and Figure 2a.
Source
Cenderelli and Kite, 1998
Cenderelli and Kite, 1998
Cenderelli and Kite, 1998
Cenderelli and Kite, 1998
Springer et al, 2001
Springer et al, 2001
Eaton, 1999
Williams and Guy, 1973
Williams and Guy, 1973
Williams and Guy, 1973
Sas, In Progress
Sas, In Progress
Figure 3. Stereoplot of bedding planes measured on Lowry Run debris flow.
Figure 4. Stereoplot of joint planes measured on Lowry Run debris flow. Blue contours
show poles within a 1% area of stereoplot hemisphere with a 2% contour interval per 1%
area.