THE SLUMGULLION LANDSLIDE (SOUTH

THE SLUMGULLION LANDSLIDE (SOUTH-WESTERN
COLORADO, USA): INVESTIGATION AND MONITORING
MARIO PARISE
CNR-IRPI, National Research Council of Italy, Bari, Italy
JEFFREY A. COE, WILLIAM Z. SAVAGE, DAVID J. VARNES*
U.S. Geological Survey, Geologic Hazards Team, Golden, Colorado, USA
*deceased
ABSTRACT: This paper summarizes the research performed at the Slumgullion landslide. The landslide consists of an active slope
movement currently moving on the upper-middle part of an older, larger, and inactive landslide. After the first specific study carried
out in 1958, the landslide has been the object of detailed investigation and monitoring starting from 1990. These, which will be briefly
described here, consisted of: high-scale and multi-temporal mapping of the features produced by the landslide movement at the
ground surface; geological characterization of the materials involved in landsliding; evaluation of the hazards, with particular
reference to deformation of the area ahead of the active toe; geophysical investigation; monitoring of the landslide by means of
surveys, photogrammetric methods, and GPS methods; comparison of landslide movement with meteorological data, and assessment
of the relationships between movement and rainfall and snow melting.
Keywords: landslide, monitoring, earth flow, hazard
1 INTRODUCTION
The Slumgullion landslide (Fig. 1), in the San Juan Mountains
of south-western Colorado, is one of the most famous slope
movements in North America. The landslide has attracted the
attention of many investigators since the XIX century beginning
with Endlich (1876). The name Slumgullion, already ascribed to
the landslide by miners at the time of Endlich’s description, is an
old term referring to a meat stew containing many different
ingredients of various colors and having a variegated
appearance. The term accurately describes the variegated
landslide deposits, that consist of bright yellow clay-rich
materials, reddish brown volcanic rocks, and local
concentrations of red and purple clay-rich materials. In addition
to a wide variety of colors, the landslide deposits show a strong
lateral variability, with individual units typically elongated in the
direction of movement which results in a striped appearance.
The Slumgullion landslide is a complex feature consisting of
an active landslide currently moving on the upper-middle part of
an older, larger, and inactive landslide (Fig. 2). After Endlich
(1876), who described it in a short note in the report of the
Hayden Survey of 1874, it was cited by numerous other
investigators in the early part of the XX century (Cross, 1909;
Howe, 1909; Atwood, Mather, 1932; Burbank, 1947). The
earliest photographs of the landslide were published at the
beginning of last century by Whitman Cross (plate XXB in
Howe, 1909). In 1958, Crandell and Varnes made the first
measurements of movement on the Slumgullion landslide
(Crandell, Varnes, 1960; 1961). Observations of changes in
survey lines crossing the slide and in advances of the active toe
were then made several times between 1958 and 1973.
In 1990, the U.S. Geological Survey (USGS), with the
assistance of scientists provided by a cooperative agreement
between the USGS and the Italian National Research Council
(CNR), began a new study of the landslide. The preliminary
results of this new phase of study have been published in several
USGS open-file reports and in a Slumgullion-dedicated USGS
Bulletin (Varnes, Savage, 1996).
Figure 1 - Aerial view of the Slumgullion landslide. Lake San
Cristobal is at lower right. The inactive landslide extends from
the lake uphill to the active toe. Active toe is prominent lightcolored bulge uphill from where Colorado State Highway 149
crosses the landslide.
In 1998, Brigham Young University (BYU) and the USGS
began work on a National Aeronautics and Space Administration
(NASA) funded study to monitor hydrologic and meteorologic
fluctuations on and in the landslide and attempt to relate these to
movement measured using interferometric capable Synthetic
Aperture Radar (INSAR) and Global Positioning System (GPS)
surveys. Results from this study are presented in Coe et al.
(2003).
This paper is a summary of research that has been carried out
since 1990 at Slumgullion. Even though some studies are still
on-going (e.g. periodic campaigns for GPS and leveling
measurements), many interesting results have been obtained so
far. These results are briefly described in this paper.
Observations dealing with kinematics of the movement at
Slumgullion site showed that most of the movement of the active
part of the Slumgullion earth flow takes place along the shear
surfaces which bound the landslide (Crandell, Varnes, 1961;
Guzzi, Parise, 1992; Baum, Fleming, 1996; Fleming et al. 1996,
1999); therefore, it is most correct to refer to it as an earth slide
or landslide (Cruden, Varnes, 1996). However, since it has often
been described as an earth flow (and sometimes also used as
typical example for this category of slope movement), the term
earth flow has become, and still remains, traditionally popular
for the Slumgullion. In this paper, we use the term landslide to
describe the Slumgullion slope movement.
Meteorological data collected between October, 1980 and
March, 2002 at a Snotel weather station (Western Regional
Climate Center, U.S. Department of Agriculture, unpublished
data, 1999) located about 4 km southeast of the head of the slide
at an elevation of 3487 m, reveal the following observations
about precipitation and temperature conditions in the area (Coe
et al. 2000b, 2003). Annual precipitation totals ranged from 500900 mm, about two-thirds of which came in the form of snow
between October and June. Extreme temperatures ranged from 33˚C on February 1, 1985 to 27˚C on July 4, 1989. Average
daily temperatures ranged from -28˚C on February 1, 1985 to
16˚C on July 4, 1989.
Figure 2 - The Slumgullion landslide, in southwestern Colorado, showing boundaries of the active and inactive portions.
2 GEOLOGIC AND GEOMORPHIC SETTING
The Slumgullion landslide is located in the San Juan Mountains,
a large Tertiary volcanic center in south-western Colorado. The
landslide occupies the valley formed by Slumgullion Creek, a
tributary of the Lake Fork of the Gunnison River. It formed as a
result of collapse of hydrothermally altered volcanic materials in
the rim of the Lake City caldera (Lipman, 1976), on the southern
end of Mesa Seco. The detached materials slid and flowed
downhill to block the Lake Fork of the Gunnison River, creating
Lake San Cristobal. Originally the lake was 4.3 km long, with an
area of 1.8 km2, but sediment deposited in deltas of Slumgullion
Creek and Lake Fork have reduced its length to 3.3 km and its
area to 1.34 km2 (Schuster, 1985; Parise, Moscariello, 1997); it
has a maximum depth of 27 m and a volume of about 14 million
m3. Except for an average of 10 m of erosional downcutting of
the natural outlet channel across the landslide dam, little change
has occurred in the geometry or character of the landslide dam
since it was formed. Headward erosion of the downstream
channel has created the 25-m high Argenta Falls at a resistent
bedrock ledge. The continued stability of this natural dam is of
great importance to Lake City and the surrounding area. Because
the channel now appears to be stable in regard to erosion, there is
no reason to expect failure of this broad, fairly flat dam
(Schuster, 1996).
The materials exposed in the scarp are hydrothermally altered
and unaltered volcanic rocks including tuffs and intrusive and
extrusive rocks of variable compositions (Diehl, Schuster, 1996).
At the top of the 250-m high scarp is the Miocene Hinsdale
Formation, an unaltered vesicular basalt, which is underlain by
the Miocene Sunshine Peak Tuff, a welded ash flow tuff (Fig. 3).
The Miocene units unconformably overlie the highly altered
Oligocene Uncompahgre Peak volcanics, which consist mainly
of andesite flows (Lipman, 1976). Alunite, smectite, and opal,
are the main products of hydrothermal alteration, and can be
found in the main scarp as well as in the landslide debris
(Larsen, 1913; Diehl, Schuster, 1996). The hydrothermal
brecciation and alteration, combined with the weakening of the
rock mass by intersections of numerous faults, was apparently a
prime factor in the origin of the Slumgullion landslide. The
bedrock surrounding the landslide consists of volcanic rocks
ranging in composition from rhyolite to basalt. A few glacial
erratics of granite and related crystalline rocks occur in glacial
deposits south of the landslide.
Figure 3 - View of the main scarp.
In the Slumgullion area, as well as in many other sites of the
San Juan Mountains, the topography is generally controlled by
the erosional resistance of different types of volcanic rocks. The
steep cliffs are composed of lava flows, and gentler topography
is formed on unconsolidated and altered tuffs. The pre-landslide
topography of the valley of Slumgullion Creek was the object of
a study by Parise and Guzzi (1992): on the basis of analysis and
comparison of shape and morphology of nearby valleys in this
sector of the San Juan Mountains to the valley of Slumgullion
Creek, they reconstructed the original topography, and estimated
an average thickness of the displaced material at about 40 m
(Parise, Guzzi, 1992). In this reconstruction, the maximum
thickness of landslide deposits is about 140 m; it occurs where
the toe of the active landslide has piled material on top of the
inactive part of the landslide. This point is about 250 m uphill
from where Colorado State Highway 149 crosses the inactive
landslide (Figs. 1 and 2). Based on the results of seismic
reflection and refraction profiles, Williams and Pratt (1996)
estimated a maximum thickness of about 95 m for the landslide,
along a profile below the active toe and east of State Highway
149. In the same area, Parise and Guzzi (1992) suggested a
thickness of about 120 m. The difference can partly be explained
when considering that the pre-landslide topography
reconstructed by Parise and Guzzi assumed a V-shape prelandslide valley, and, on the other hand, Williams and Pratt
(1996) interpreted the pre-landslide valley topography to be Ushaped, so they obtained a lesser value for the thickness of the
deposit. The seismic surveys were locally limited because steep
topography prevented acquisition of data across the boundaries
of the earth flow and along the stable slopes (Williams, Pratt,
1996). The shape of the valley underlying the inactive landslide
could best be resolved by drilling a few boreholes in the area
downhill from the active toe.
much larger, inactive part. The active part of the landslide is 3.9
km long, covers an area of 1.46 km2, and has an estimated
volume of about 20 x 106 m3 (Parise, Guzzi, 1992). Total
elevation difference from the toe of the inactive part at the Lake
Fork of the Gunnison River to the top of the 250-m high head
scarp is about 1000 m (Table I). The length from the top of the
scarp to the inactive toe at the level of the Lake Fork is about 7
km. Part of the inactive toe is concealed under the water of Lake
San Cristobal.
Table I – Morphometry of inactive and active parts of the Slumgullion
landslide
Feature
Dimensions of
Dimensions of
the inactive landslide
the active landslide
Area of deposit *
4.74 km2
1.46 km2
Length
6.8 km
3.9 km
Width
- head
1,130 m
280 m
- narrowest part
290 m
150 m
- toe
530 m
430 m
Relief
- elevation of top
3,700 m
3,500 m
- elevation of tip
2,700 m
2,960 m
Average slope
- deposit only
7° (12%)
---- including main scarp
8° (14%)
11° (19%)
Thickness
- average
40 m
13 m
- average on thalweg
of buried valley
90-100 m
--- maximum
140 m
48 m
Volume
6
3
19.5 x 106 m3
- earth flow deposit
168 x 10 m
6
3
---- detached mass **
142 x 10 m
Length : width
> 6:1
9:1
* Including the deposit identified by Chleborad (1993).
** Including a 30% bulking factor; for further details, see Parise, Guzzi
(1992).
Based on radiocarbon dating, differences in degree of
weathering and soil formation, and morphologic observations to
separate ages of deposits, several episodes of movement have
been identified at the Slumgullion landslide (Crandell, Varnes,
1960, 1961; Chleborad, 1993; Madole, 1996; Fleming et al.
1999): the first episode blocked Slumgullion Creek about 10001,300 years ago; the second episode blocked the Lake Fork and
caused the impoundment of Lake San Cristobal about 800-900
years ago; the third episode was collapse of the north part of the
headscarp about 300 years ago; the fourth episode is the
reactivation of old landslide deposits, and is represented by the
current episode of movement. This sequence is based on field
evidence and measurements at several single points at the
landslide surface. Fleming et al. (1999), however, suggested that
there is also the possibility that the above sequence might have
occurred as a continuum, rather than in separate episodes.
The landslide deposits are fine sand, silt, and clay derived
from hydrothermally altered volcanic rocks. Locally, large
assemblages of boulders are present; in plan view they have an
elongated lenticular shape with the long axis parallel to the
movement direction of the earth flow. Overall, these materials
are elongated in the direction of sliding and have a strong lateral
variability.
3 LANDSLIDE DESCRIPTION
3.1 The active landslide
The Slumgullion landslide (Fig. 2; Table I) is 6.8 km long,
covers an area of 4.74 km2, and has an estimated volume of
about 170 x 106 m3 (Parise, Guzzi, 1992). It consists of a
younger, active, upper part that moves on and over an older,
Observations of the surface features produced by differential
movement of the active part of the Slumgullion landslide have
been used to understand the behavior of different parts of the
landslide, and to estimate a zonation of landslide elements
(Guzzi, Parise, 1992; Baum, Fleming, 1996; Fleming et al.
1999): among the many different types of features which can be
observed at the surface of the landslide, the most diffuse and
important were scarps, fractures, flank ridges, and basins. Each
of these features is related to the style of deformation acting at
that moment in that particular part of the landslide (Fleming,
Johnson, 1989). Even though many of them are ephemeral, since
weathering, erosion, and continued movement tend to erase prior
traces of movement, their periodic observation may result in
interesting information about the landslide evolution.
At the Slumgullion landslide, additional observations were
made by investigating both the presence and conditions of trees
and vegetative cover. In particular, vegetation on the landslide
surface, and its degree of disturbance, provided information to
distinguish between active and inactive sectors, and to identify
the areas more affected by movement. The continuously active
sectors are generally characterized by bare ground, or by the
presence of few trees; clear evidence of disturbance can be
recognized on trees in the areas of recent movement. Trees have
been tilted during the movement, and then grow vertically, so
that the relative age of movement can be inferred by the
curvature of trunks and by their position. Further information
may be drawn from the many places on the landslide where trees
have been split by differential displacement across a fracture: the
direction of differential movement required to produce the split
can be used, together with stretching or buckling of tree roots, as
an indicator of local deformation (Fleming et al. 1999).
The base of the main scarp is mantled with continuous belt of
detrital material consisting of talus cones and rock-fall deposits.
The talus cones are built by successive and discontinuous
accumulations of debris falling from the near-vertical to
overhanging rock walls above. Rock-fall deposits are widespread
at the foot of the main scarp and form a large accumulation of
clast-supported angular blocks and coarse boulders, which
typically show an irregular hummocky topography.
The direction of sliding at the head is toward the south and is
in the same direction as the inferred collapse of the Mesa Seco.
In this area, there is a mixture of compressional and extensional
features: the latter are produced by normal faults and tension
cracks in the active landslide, while the compressional features
are related to the collapse of part of the main scarp (Fleming et
al. 1999). A prominent flank ridge is present at the left boundary
of the active landslide (Fig. 4); east of it, the landslide head
enlarges.
Figure 4 - View to southwest from main scarp region showing
the upper reaches of active slide with prominent levee on left
flank.
Downhill from the active source area, a change in the main
direction of movement of the landslide material (from a N-S to
an about E-W strike) is evidenced by a bend toward the right in
the active as well as the inactive landslide. This stretch, which
has many normal scarps and tension cracks, is located just before
the so-called neck, where width of the landslide reaches its
minimum value, and displacement rate is at its maximum (Table
I). Narrowing of the landslide is in fact accommodated in an area
where the width decreases from the 300 m at the head, to 230 m
and then to 150 m. The narrowest part of the landslide is where
Crandell and Varnes (1961) measured the maximum
displacement rate and established a survey line (Fig. 5).
Figure 5 - View from camera station of narrow neck region,
where velocities are up to 7 m/yr.
The materials entering the narrowest part of the landslide are
constrained laterally, and are limited by shear zones on both the
sides of the landslide. The narrowing is also accompanied by an
abrupt change in slope.
The most striking features of the Slumgullion landslide are its
flank ridges, that form on both its flanks, and adjacent to shear
zones within the landslide as well (Fig. 6). Even though data on
formation of the ridges are available from many studies (e.g.
Fleming, Johnson, 1989; Baum, Fleming, 1991; Baum et al.
1993), a coherent explanation of the process of formation is still
lacking. Many of the proposed mechanisms for formation of
flank ridges have been reviewed by Keefer, Johnson (1983),
Fleming, Johnson (1989), and Corominas (1995). Growth of
flank ridges and the presence of relict flank ridges in both the
active and inactive deposits of the slide have been documented at
the Slumgullion earth flow. Flank ridges extend continuously for
several hundred meters, following the landslide boundaries.
Observation of the main geomorphic, pedologic, and
sedimentologic characters of the several generations of flank
ridges helped in reconstructing the chronology of their formation
and to relate them to the landslide history of movement (Parise
et al. 1997; Fleming et al. 1999). The great majority of the flank
ridges at the Slumgullion landslide appear to have formed along
the boundary between active and inactive landslide movement as
a result of displacement on the bounding strike-slip shear
surface. Flank ridges that have formed along the flanks of a
landslide provide a basis to evaluate the history of movement in
much the same way as moraines are useful indicators of
movement history of a glacier. Analysis of flank ridges
performed at the Slumgullion landslide provide indications of
past boundaries and height of the landslide; in addition, the
distribution of ridges shows evidence for gradual thinning and
narrowing of the earth flow (Parise et al. 1997).
Figure 6 - Flank ridges at the left boundary of the active
Slumgullion landslide. Note the difference in height between the
presently forming flank ridge and the older one.
An area characterized by the presence of ponds is
recognizable further downhill: pond deposits show uniform color
and silty texture. About 0.6 km uphill from the active toe, there
is a wide area of pond deposits, extending 80 m by 80 m.
Apparently, the location of the pond has remained fixed while
the landslide material has been displaced across the pond site.
Parise and Guzzi (1992) suggested that the pond sediments and
thrust features mark the initial position of the toe of the
reactivated Slumgullion landslide. Figure 7 is a sketch showing
the process of formation and preservation of the pond: after the
collapse of part of the scarp, and after that loading in the head of
the inactive landslide triggered reactivation, fractures propagated
downslope through the inactive landslide material to the
approximate position of the pond (Fig. 7A). Then, the fracture
along the basal failure surface emerged to the ground surface
(Fig. 7B). From this point on, the reactivated landslide advanced
over the surface of the old, inactive landslide (Fig. 7C).
The active toe has a rounded shape in plan view; it has steep
slopes, up to 40 m high, with an average slope angle of 18°. The
overall difference in elevation between active and inactive
deposits is accommodated through a series of scarps, whose
height ranges from 0.5 to 8 meters. The active toe is advancing
across the surface of the old, inactive landslide, pushing trees
over and engulfing them under the advancing landslide. The toe
has an irregular trace along its northern half, which is probably a
result of a velocity discontinuity (higher displacement rates have
actually been observed at the southern half, south of the major
right lateral shear surface of the landslide; Fleming et al. 1996).
Observations of the displacement vector of the active toe
relative to nonmoving ground farther downhill indicate
continuing thickening of the toe (Fleming et al. 1999); therefore,
displacement of the landslide tends to steepen the front of the
active toe. This could lead deformation in the area ahead of the
active toe (see below), as well as favoring minor slope
movements at the steep and unstable active front. As a matter of
fact, several debris-flow episodes occurred at this site in 1999;
debris-flow material deposited ahead of the active toe, covering
some measurement points of the level circuits at the inactive
landslide surface. The debris-flow deposit consists of subrounded to sub-angular boulders, cobbles and pebbles in a finer
matrix. Several small flank ridges, up to 0.4-m high, are also
present, showing sub-angular clasts aligned at their top.
On the basis of the above described observations, the active
part of the Slumgullion landslide can be partitioned into three
regions (Guzzi, Parise, 1992; Fleming et al. 1999), characterized
by distinct styles of deformation: i) the head of the slide, with
prevailing extensional features; ii) the central part, where the
width reaches the smallest value (150 m) and surface features
related to the strike-slip shear surfaces which bound the landslide
predominate; iii) the active toe, showing a series of thrusts and
internal toes, which are morphologically expressed by scarps of
variable height.
3.2 The inactive landslide
Figure 7 - Sketch of pond formation in relation to emergence of
the toe of the reactivated landslide (after Fleming et al. 1996).
See text for explanation.
Downhill from the narrowest part, the landslide widens
through lateral steps, where basins (pull-apart like) are formed.
One of the best expressed basin is present on the north side,
along the right flank of the active landslide; its formation and
evolution have been described in detail by Fleming et al. (1999).
Downhill from the active toe, the deposits of the inactive
landslide are mostly hummocky, but the hummocks are smooth
and rounded. Gentle slopes hundreds of meters across,
containing closed depressions and ponds, are joined by
intervening steep ramps as much as tens of meters high. Along
the longitudinal profile extending from the active to the inactive
toe, four separate zones can be distinguished (Parise,
Moscariello, 1997): the first one is a pond-rich region, with
abundance of water, and perennial and ephemeral streams
flowing off and out of the active landslide. Then, there is a main
bulge, containing several internal ridges; it is comparable in size
to the active toe and perhaps represents an old internal toe in the
landslide, that has been remodeled by erosion and weathering to
Figure 8 - Map of the area near the active toe of the Slumgullion landslide, showing vertical movements at the leveling stations (after
Varnes et al. 1996). All movements are downward unless marked “+”.
its present rounded shape. Further downhill, an area of steps is
present. The surface of the steps consists of rounded, curved
downslope, hummocks a few meters high. Finally, the inactive
toe occupies the valley of the Lake Fork of the Gunnison River,
and its materials extend both upstream and downstream from the
point where the landslide deposits entered the valley. The
materials in the inactive toe are different from the rest of the
landslide: most of the deposit is fine-grained materials, and
boulders are rare to absent.
3.3 Hydrology
Very few quantitative hydrologic data exist so far for the
Slumgullion landslide. It appears, however, that the active
landslide, and even the inactive landslide debris, are largely
water saturated. Many ephemeral streams flow down the
landslide and perennial streams flow down each flank (e.g. the
Slumgullion Creek at the left flank of the active landslide).
Perennial and ephemeral springs, with discharge of several litres
per second, exist at several places on the surface of the landslide.
Changes in location and discharge of springs, and diversions in
pattern of drainage ways, are controlled by the overall movement
of the landslide, and by secondary, shallow slope movements at
its surface. Water flowing from the springs can thus flow
downhill hundreds of meters before it either soaks into the
ground, flows into a sink, or joins a stream. Abundance of
surface water on the Slumgullion landslide, and complex
relationships between topography, structure, and hydrology of
the landslide are yet to be completely understood.
In 1998, as part of the NASA-funded study, the USGS began
to conduct systematic observations of surface water, landslide
movement, and pore pressure data from pressure transducers
installed in a shallow (~2.5 m deep) auguer hole. These
observations
and
measurements
indicated
a
close
correspondence between available surface water, landslide
movement, and pore pressure (Coe et al. 2003). The work also
indicated that the pattern of continual, but seasonally variable
movement observed, closely fits the bathtub model for landslide
movement (Baum, Reid, 2000). In this model, the landslide is
isolated both mechanically and hydrologically from adjacent
materials by low permeable clays. These clays cause the
landslide to retain water and thus be responsive to seasonal
changes in infiltration of surface water.
(Varnes et al. 1996). The most general deformation is
depression, in the range 5-20 mm/yr, which seems to decrease
away from the active front. However, some points close to the
toe rose significantly between 1991 and 1993 (20-80 mm in 2
years).
Continuing movement of the active part of the landslide,
which might reach Colorado State Highway 149, was further
testified by debris-flow activity in 1999 at the active toe and in
the area just ahead of it. Moreover, it must be remembered that
several additional hazardous areas are present at Slumgullion.
Other than the possibility of a repeat of a major collapse of a
significant part of the main scarp, the principal concerns in terms
of hazards are rock-fall and talus accumulation processes at the
head scarp, debris-flow and flooding processes in Slumgullion
Creek, deformation of the inactive landslide deposit downhill
from the active toe, and secondary slope movements within the
main landslide body. In particular, at the mouth of Slumgullion
Creek, an alluvial fan consisting of several debris-flow and flood
deposits is identifiable. Due to the presence of several resort
facilities close to Slumgullion Creek and within the boundaries
of the depositional area of debris-flow materials, the alluvial fan
at the mouth of the creek poses one of the most significant
hazards related to the landslide (Parise, Moscariello, 1997).
3.4 Geological hazards
4 MONITORING
Several geological hazards are present at Slumgullion (Parise,
Moscariello, 1997). The older, inactive part of the landslide is
crossed by Colorado State Highway 149 which connects Lake
City and Creede, Colorado. The inactive landslide is increasingly
occupied at its lower reach by condominiums and other
recreational structures near Lake San Cristobal.
State Highway 149, in particular, is only 250 m downhill from
the actively deforming area and is in possible jeopardy from a
continuation of the current movement pattern of the active toe.
To determine if loading exerted by the currently moving material
causes deformation of the inactive deposits, a party led by David
J. Varnes began surveying this part of the landslide in 1991 (Fig.
8). The surveys carried out to date indicate that the inactive part
of the Slumgullion landslide is responding to the load caused by
the advancing toe, and is not as stable as previously assumed
Crandell and Varnes (1960, 1961) began the first investigation
of the rates and history of movement of the Slumgullion
landslide in 1958. Observations of changes in survey lines
crossing the slide and in advances of the active toe were made
several times between 1958 and 1973. Aerial photographs
(1:12,000 scale) were obtained in 1985 by the Colorado
Geological Survey and in 1990 and 2000 by the USGS (1:6,000
scale). A photogrammetric study of surface displacements
performed on the active part of the landslide identified and
measured over 300 natural points (trees and bushes) on the 1985
and 1990 photographs (Smith, 1993, 1996); displacements from
1985 to 1990 ranged from less than 0.25 m to more than 25 m.
Figure 9 - Active part of the Slumgullion landslide with structural elements, contours of mean annual movement, GPS base stations,
control points, and monitoring points.
An alternative study was performed by using Geographic
Information System to determine the movements of visually
identifiable objects, such as trees and rocks, on the surface of
the active landslide (Powers, Chiarle, 1996). More than 800
horizontal displacement vectors were measured directly from
ortho-images that were produced by digital photogrammetric
techniques using the same sets of aerial photographs. Overall,
the results obtained are consistent with those derived from the
standard photogrammetric study.
The comprehensive study begun in 1990 by USGS
included geophysical investigations, precise surveying and
leveling, history of movement, displacement rates, landslide
kinematics, material properties, and landslide dams. Since
June 1993, GPS has also been used to measure movements on
the landslide (Jackson et al. 1996). In 1993, the GPS method
was tested with two objectives: to determine if satellite
geodesy could be used to map the short-term spatial and
temporal distribution of velocity on an active slide; and to
determine possible very slow movements of the inactive part
of the landslide. Beginning in 1998, GPS observations have
been made at 11 control points and 19 monitoring points
(MP1 to MP19; Figs. 9 and 10) installed around and on the
active landslide, respectively. Control points were distributed
in nonmoving areas around the periphery of the landslide,
while the monitoring points were evenly distributed on the
active landslide, but also placed strategically in relation to the
main structural features of the landslide, and with a clear view
to the sky for reliable acquisition of the GPS signal (Coe et al.
2003).
Displacement data collected during the past 40 years by
surveying, photogrammetric methods, and GPS methods
reveal that surface velocities are, in general, inversely
proportional to the width of the active earth flow (Fig. 9).
Maximum surface velocities of 6-7 m/yr occur in the
narrowest (150 m-wide) region of the flow, and surface
velocities of 1 m/yr occur in the head and toe regions of the
active flow (300- to 450-m wide). The rate of movement
appears to be fairly constant from year to year and season to
season, but generally corresponds with annual precipitation.
In a 3.5-year monitoring period (from July 1998 to March
2002) twenty 1- to 3-day field campaigns of GPS surveys
were performed, with a mean time between campaigns of 69
days (Coe et al. 2000a, 2003). This monitoring (also done
with extensometers) showed that the Slumgullion landslide
moved throughout the period, but that daily velocities varied
on a seasonal basis (Fig. 11). Landslide velocity increased in
response to snowmelt and rainfall and decreased during dry
periods. Annual movements and average daily velocities were
smallest at the head and toe of the landslide and largest in the
central, narrowest part of the landslide (Figs. 12 and 13);
movements and velocities deviated from this distribution in
areas where they were affected by major structural elements
within the landslide (also see Gomberg et al. 1995; Baum,
Fleming, 1996).
Figure 10 - Station consisting of radar corner reflector and
GPS monitoring antenna, providing ground truth for INSAR
measurements.
Figure 11 - Diagram showing average daily velocities of GPS monitoring points. Dates shown are the mid-points between GPS
observations.
Figure 12 - Bar graph showing the ratio of minimum velocity
to maximum velocity at each monitoring point. See Figure 9
for location of monitoring points.
Figure 13 - Bar graph showing the annual horizontal
movement of GPS monitoring points. See Figure 9 for
location of monitoring points.
Velocities were slowest in mid-winter when air and soil
temperatures were coldest and precipitation was generally
low and/or in the form of snow with a low water content. The
seasonal variability in velocities has been interpreted (Coe et
al. 2003) as due to ground-water levels and corresponding
pore pressures that decrease when surface water is
unavailable or cannot infiltrate frozen landslide material, and
increase when surface water from melting snow or rainfall
infiltrates unfrozen landslide material (Fig. 14). This
hypothesis is supported by data showing seasonal changes in
soil-water content and by a correspondence between velocity
variability and air- and soil-temperature data from a
meteorological station on the slide. In addition, patches of
bouldery debris and fractures (created by continuous
movement of the landslide) have been considered likely
conduits through which surface water can infiltrate, regardless
of the frozen or unfrozen state of the landslide matrix
material. Such high permeability zones exist at Slumgullion
as evidenced by sinks and perennial springs on the landslide
(Guzzi, Parise, 1992; Fleming et al. 1999). Therefore, the
availability of surface water is more important than landslide
temperature in controlling the rate of landslide movement.
This hypothesis is supported by field instrumentation data that
show (1) landslide velocities coinciding with precipitation
amounts regardless of the depth of freezing of landslide
material, (2) spring and annual landslide velocities that were
greatest when the depth of freezing was also the greatest, and
(3) a rapid (several weeks or less) velocity and pore pressure
response to rainfall.
Figure 14 - Diagram showing landslide velocity, soil temperature, and pore pressure recorded at station IS2 and daily precipitation
recorded at IS1 (after Coe et al. 2003). Pore pressure is shown as pressure head in meters of water above the piezometer, which is 2.0
m below the ground surface.
In the summer and fall of 2001, five sets of INSAR data
were collected by BYU over the active part of the landslide.
These data are currently being processed and interpreted by
BYU and the USGS.
5 CONCLUSIONS AND FUTURE PERSPECTIVES
Nearly constant movement makes the Slumgullion
landslide an excellent, large-scale natural laboratory. The
active part of the Slumgullion landslide has been moving
continuously for the past 300 years, transporting material
hundreds of meters and repeatedly creating and destroying
surface features. Mapping and interpretation of these features
along the active margins and within the body of the active
landslide show that the Slumgullion landslide can be
partitioned into three separate regions based on the style of
deformation.
The main scarp and the head of the slide exhibit listric
normal faulting in a region of extending flow. Deformation in
the narrow, central part of the landslide is characterized by
nearly vertical, strike-slip shear surfaces that bound discrete
blocks; in this part of the slide, individual blocks move
downslope as rigid bodies (region of plug flow). The toe of
the slide is characterized by listric thrusts developed in a
region of compressive flow. The Slumgullion landslide fits a
Coulomb plastic model for landslide flow with extending,
plug, and compressive flow in the upper, middle, and lower
reaches respectively (Savage, Smith, 1986).
The pattern of the overall movement on the landslide
surface is consistent with the above model; it shows in fact
that the highest surface-flow velocity, averaging 6-7 m/yr,
occurs halfway down the active portion where the earth flow
is confined to a width of 150 m. Flow velocities in the upper,
wider part of the flow are estimated to be 2 m/yr. Flow
velocities in the active toe (about 430-m wide) are estimated
to be 1.3 m/yr.
Surveys indicate that the inactive part of the Slumgullion
landslide is responding to changing loads caused by the
advancing toe and is not as stable as previously assumed. In
general, ground is depressed in front of the advancing toe, the
amount decreasing with distance from the toe. Current
evaluations of hazards related to the landslide include
potential damage to Colorado State Highway 149 and to
residences in recreational areas in the vicinity of Lake San
Cristobal.
The continual but seasonally variable movement observed
at Slumgullion closely fits the bathtub model for landslide
movement proposed by Baum and Reid (2000). In this model,
the presence of low permeable clays impede drainage of water
from the landslide, keeping the shear zones perennially
saturated and intermittently allowing pore pressures to
increase sufficiently to destabilize the slide (Baum, Reid,
2000).
Eventually, the most recently collected data (Coe et al.
2003) have shown (i) that movement was measured on
landslide deposits near the head of the landslide that were
previously identified as inactive, thus implying that the active
landslide is larger and more complex than previously
recognized; and (ii) that the driving force responsible for
moving the upper part of the landslide may be less than it has
been in the past, due to slower annual movements measured
on the upper part of the landslide.
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