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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
745
DOI: 10.4408/IJEGE.2011-03.B-081
THE 2005 LA CONCHITA LANDSLIDE, CALIFORNIA:
PART 1 - GEOLOGY
P
HiliP
J. SHALLER
(*)
, P
aRmesHwaR
L. s
HRestHa
(**)
, m
aCan
DOROUDIAN
(***)
,
d
avid
W. SYKORA,
(****)
& d
ouGlas
L. HAMILTON
(*****)
(*)
Exponent, Inc., 320 Goddard Way Suite 200, Irvine, California; pshaller@exponent.com; Phone: (949) 242-6006;
(**)
Exponent, Inc., 320 Goddard Way Suite 200, Irvine, California; pshrestha@exponent.com; Phone: (949) 242-6037
(***)
Exponent, Inc., 320 Goddard Way Suite 200, Irvine, California; mdoroudian@exponent.com; Phone: (949) 242-6025
(****)
Exponent, Inc., 9 Strathmore Road, Natick, MA 01760; dsykora@exponent.com; Phone: (508) 652-8503
(*****)
Exponent, Inc., 320 Goddard Way Suite 200, Irvine, California; dhamilton@exponent.com; Phone: (949) 242-6016
INTRODUCTION
The community of La Conchita is located along
the sparsely developed coastline between Los Angeles
and Santa Barbara, California, USA at latitude 34.36°
N, longitude 119.45° W. Although the region is known
for its mild climate, powerful winter storm sequences
occasionally pummel the region. In late 2004 and early
2005, an intense series of rainstorms impacted the re-
gion, causing flooding and triggering numerous land-
slides. The January 10, 2005 La Conchita landslide
was the deadliest single event triggered by the 2004-
2005 storm sequence in southern California.
The 2004-2005 winter rainfall season was marked
by a series of major Pacific storms that brought heavy
precipitation to California. The first major Pacific
storm of the season occurred in October. Heavy rain-
fall then returned on December 27
th
. The storms took
a consistent track colloquially known as the “Pineap-
ple Express.” During this storm sequence, Los Ange-
les had its wettest 15-day period on record. Between
January 6th and 11
th
, over 500 mm of rainfall was re-
corded at mountain weather stations in Santa Barbara,
Ventura, and Los Angeles Counties (NCDC, 2005).
Approximately 378 mm of rain fell in the City of
Ventura between December 27
th
and January 10
th
, only
slightly less than the 390 mm average yearly rainfall
for the region (s
CHiek
& H
uRtado
, 2005).
The January 10, 2005 La Conchita landslide was
initiated at the head of a shallow swale located along
the eastern lateral margin of a prior slump failure that
ABSTRACT
This is Part I of a two-part causation analysis
of the January 10, 2005 La Conchita, California,
USA landslide. This paper describes the geology and
geomorphology of the event, which killed 10 per-
sons and damaged or destroyed 36 residences. The
landslide is located in a complex and active tectonic
setting. It was triggered by two weeks of heavy rain-
fall, which initiated a failure in the backscarp of a
large slump that had occurred ten years earlier. The
2005 landslide displaced older landslide deposits de-
rived from Tertiary sedimentary rocks. Over 30,000
m
3
of wet debris was mobilized in the event, which
formed two distinct lobes of debris. The main lobe,
comprising 90% of the deposit, rapidly transformed
into a large scale debris flow, eroded and entrained
over 4,000 m
3
of material along its path, overran a
temporary wall, then flowed into the residential com-
munity. This lobe exhibits characteristic debris flow
textures, including raised lateral levees and a surface
pattern of ridges and troughs. The minor lobe, com-
prising about 10% of the total landslide volume, was
deposited in pulses that arrived from different direc-
tions at different times. This material impacted and
locally breached the temporary wall, but did not flow
into the community below.
K
ey
words
: Debris flow, La Conchita, geology, geomorpho-
logy, landslide
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P.J. SHALLER, P.L. SHRESTHA, M. DOROUDIAN, D.w. SYkORA & D.L. HAMILTON
746
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
tracks were already impacted by landslide debris by
1889. The tracks were moved in 1909 after a work
train was buried in a landslide. That year the railroad
leveled the area between the tracks and the slope to act
as a catchment for landslide debris and thus reduce the
hazard posed by the steep bluff face. The original use
of the leveled area was soon forgotten, however, and
the La Conchita residential development was estab-
lished on the leveled area in 1924 (H
emPHill
, 2001).
The bluff and hilltop terrace areas upslope from La
Conchita formed a portion of the La Conchita Ranch
property (Ranch). The Ranch originally practiced graz-
ing and dry agriculture, then switched to irrigated agri-
culture in the mid-1970s (H
emPHill
, 2001). In March
1995, a large slump failure occurred on the bluff fol-
lowing a period of heavy rainfall. This event damaged
or destroyed seven residences, destroyed a portion of
Ranch Road, an access road that traversed the bluff
face, and covered a major street in the community, Vis-
ta Del Rincon Avenue, with up to about 6 m of debris
(Fig. 1). Despite its large volume, estimated at between
200,000 and 460,000 m
3
, the 1995 landslide was an-
ticipated and slow moving, allowing residents time to
evacuate (o’t
ousa
, 1995; H
aRP
et alii, 1995).
The slump failure was never remediated. By early
1996, the Ranch had performed minor earthwork to
convey stormwater from the truncated uphill portion
of Ranch Road across the slump to a point along the
toe of the 1995 slump at the northern end of Fillmore
Avenue. These drainage improvements (DI) are indi-
cated on Fig. 1. Also shown in the figure is a steep-sid-
ed channel incised by surface runoff draining toward
the western side of the slump by 2002.
In 2000, the County of Ventura constructed a
temporary soldier pile wall consisting of steel H-piles
and a wood lagging wall along the northern margin of
Vista del Rincon Avenue to allow the removal of de-
bris from the street. The wall was 82 m long, stood be-
tween 1.5 and 7.2 m above the road surface (including
a guardrail at the top), and had a freeboard of about
1.2 to 2.7 m, including the guardrail.
GEOLOGICAL CONDITIONS
The 2005 slope failure originated in old landslide
deposits near the crest of a 180 m high, southwest-
facing coastal bluff. The bluff represents a modified
Holocene sea cliff that is capped by the 45,000 yr BP
Punta Gorda marine terrace. The Punta Gorda ter-
affected a much larger portion of the slope in 1995
(Fig. 1). Material from the initial slope failure rapidly
transformed into a large-scale debris flow. The major-
ity of the debris flow (main lobe) traveled down the
swale between the 1995 slump and the adjacent in-
tact slope area, and entered the residential community
of La Conchita. This lobe of the debris flow traveled
about 100 m through the community, destroying ap-
proximately 30 homes and resulting in the deaths of
10 persons (G
uRRola
, 2005). A second lobe of the
debris flow, containing about 10% of the total mobi-
lized volume of debris (minor lobe), traveled down
the heavily vegetated slope to the west of the main
lobe. The minor lobe impacted and partially breached
the central part of a temporary retaining wall located
along Vista del Rincon Avenue (see Fig. 1), but caused
no additional personal property damage or loss of life.
Remarkably, a television news crew that was in the
area to cover earlier rain-related highway and rail line
closures videotaped the incipient failure and much of
the debris flow as it descended the slope.
The La Conchita area has a long history of land-
slide activity. The Southern Pacific Railroad laid rail
lines through the area in 1887, but sections of the
Fig. 1 -
Comparison of aerial photos of the La Conchita
area taken in 2002 (left) and 2005 (right). The
photo at left shows the approximate limits of
the 1995 slump failure, the location of drainage
improvements (DI) to convey water across the
slump (yellow dotted line), and the location of a
temporary wall (Tw) constructed in 2000 to al-
low reopening of Vista del Rincon Avenue (solid
yellow line). The photo at right defines the main
(eastern) and minor (western) lobes, shows the
inferred movement direction of the debris in these
lobes, the location of a breach (B) in the wall
caused by impact of the debris, and locations af-
fected by emergency grading after the event (E)
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THE 2005 LA CONCHITA LANDSLIDE, CALIFORNIA:
PART 1 - GEOLOGY
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
747
mm/yr. Figure 3 shows an interpretation of the subsur-
face conditions underlying the bluff above the commu-
nity of La Conchita, including the inferred location and
orientation of the Red Mountain fault, offset bedrock
units, pre-existing landslide deposits, and the Punta
Gorda marine terrace (R
oGeRs
et alii, 2007)
. SEDIMENTOLOGY
Test pits excavated into the minor lobe of the La
Conchita landslide behind the temporary wall encoun-
tered pale yellow-brown, low plasticity silt exhibiting
a massive, ungraded texture (Fig. 4). The test pits also
exposed slope-parallel layers of pulverized vegetation
located at approximately 1.2 m vertical intervals. This
evidence indicates that the minor lobe was deposited
in discrete pulses.
VEGETATION
At the time of the 2005 landslide, thick chaparral
vegetation mantled most of the surface of the 1995
slump. Somewhat lighter vegetative cover was present
in the area of the drainage improvements made by the
Ranch, which included regrading the Ranch Road
race formed when sea level stood about 38 m below
present-day sea level, indicating that the terrace is
rising at a long-term geologic rate of over 4 mm/yr
(H
uftile
et alii, 1997).
As shown on Fig. 2, most of the bluff above
the communty of La Conchita is mantled by land-
slide deposits. These landslides occurred within a
sequence of poorly indurated sedimentary rocks of
Tertiary age. The principal geologic units in the vi-
cinity include the Upper Middle Miocene Monterey
Formation (shale, siltstone, and sandstone), the Mi-
ocene-Pliocene Sisquoc Shale (silty shale and clay-
stone) and the Pliocene Pico Formation (sandstone
and conglomerate). The distribution of bedrock units
in the bluff is obscured by the thick mantle of land-
slide deposits and complicated by the presence of the
Red Mountain fault, which is mapped intersecting
the slope face in the source area of both the 1995 and
2005 landslides (Fig. 2).
The Red Mountain fault is a major, active reverse
fault. H
uftile
et alii (1997) observed 34 m of vertical
separation of the Punta Gorda wave-cut platform along
the fault, corresponding to a dip-slip rate of about 1.5
Fig 2 - Geologic map of the 2005 La Conchita landslide
and vicinity. The Red Mountain fault extends
through the headscarp of the 1995 and 2005
slope failures. Geologic unit designations: Qls–
Landslide deposits; Qhf–Undivided alluvial and
colluvial deposits ; Qhpr-s–Terrace deposits as-
sociated with 1,800-5,800 BP Sea Cliff marine
terrace; Qppr-p–Terrace deposits associated with
40,000-60,000 BP Punta Gorda marine terrace;
Qpmw–Undivided Pleistocene talus, colluvium,
and landslide deposits ; Tp–Pliocene Pico For-
mation; Tsq–Miocene-Pliocene Sisquoc Shale.
Dash-dot line indicates inferred shoreline angle
of Punta Gorda terrace. Modified from Geologic
Map of the Pitas Point 7.5-Minute Quadrangle
(USGS, 2005)
Fig 3 - Conceptual geologic cross section through the
bluff face in the vicinity of the 1995 and 2005 La
Conchita landslides (modified from r
oGerS
et alii,
2007)
Fig 4 - View of stratigraphy exposed in test pit excavated
into minor lobe of the landslide. Arrows indicate
layer of vegetation sandwiched between pulses of
debris
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P.J. SHALLER, P.L. SHRESTHA, M. DOROUDIAN, D.w. SYkORA & D.L. HAMILTON
748
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
superelevation as the debris flow banked to the left in
the mid-slope area (Fig. 1). The overflow appears to
have occurred before the main lobe incised its bed,
which subsequently lowered its level relative to the
surrounding slope area.
No morphological evidence was observed indicat-
ing that the temporary wall erected by the County in
2000 altered the direction of movement of the debris
flow. As shown on Fig. 5, the levee bounding the west-
ern (right) side of the main lobe shows no significant
deflections where it crosses the eastern margin of the
temporary wall. This observation is consistent with
the behavior of large, rapid landslides elsewhere. Due
to their substantial thickness and momentum, these
landslides rarely demonstrate significant deflections
unless the object encountered cannot be overridden,
crushed, or displaced by the onrushing debris. Exam-
ples of the interaction of large-scale landslides with
obstacles in their path are available in a series of pho-
tos of rapid landslides triggered by the 2002 Denali
earthquake that traveled across Black Glacier, Alaska
(USGS, 2008). Many of these images show the land-
slide debris draped over ~10 m high medial moraines
located across their path.
PHYSICAL DIMENSIONS
Key physical measurements of the January 10,
2005 La Conchita landslide are reported in Table
1 and illustrated in Fig. 1 of the companion article
(s
HRestHa
et alii, 2011). The volume of the initial
slope failure was approximately 27,190 m
3
, corre-
sponding to the upper area of depletion located be-
tween elevation 76 and 162 m (Tab. 1). Geomorphic
evidence and the available video footage indicate that,
once initiated, the landslide rapidly transformed into
where it had been displaced in the 1995 event. The
terraced area at the top of the bluff was developed by
the Ranch as an avocado orchard
MORPHOLOGY
The principal morphological elements of the
2005 La Conchita landslide are illustrated on Fig.
5, a ground photo taken on the day of the event. The
main lobe of the deposit exhibits raised lateral levees,
a common feature of small scale mudflows and de-
bris flows (s
HaRP
& n
obles
, 1953; J
oHnson
, 1970),
as well as a ropy pattern of ridges and troughs aligned
transverse to the direction of movement. This ropy
pattern is similar in appearance, though not scale, to
the morphology that commonly develops on fluid lava
flows (pahoehoe). A similar surface texture has also
been described from a very large debris flow deposit
in central Idaho (s
HalleR
, 1991a) and from several
giant martian landslides (s
HalleR
, 1991b).
The minor lobe of the La Conchita deposit lacks
these morphological characteristics, and instead ex-
hibits a hummocky, irregular surface texture. The
contrast in surface morphologies between the two
lobes implies differences in their emplacement
mechanisms. The geometry and morphology of the
minor lobe suggest it formed by way of: 1) fluid de-
bris entering the Ranch Road drainage channel, then
overflowing onto the slope face; and 2) overflow of
debris from the western edge of the main lobe due to
Fig. 5 - Oblique ground photo of the La Conchita land-
slide taken shortly after the 2005 event. The
lateral margins of the main lobe are marked by
moderately well developed levees (circled). The
interior of the deposit exhibits a ropey pattern of
ridges and troughs aligned transverse to the travel
direction. The location of the temporary wall con-
structed to reopen Vista del Rincon Avenue is indi-
cated in right center of image. The dashed portion
of the line indicates portions of the wall that were
breached or otherwise covered with debris follow-
ing the event
Tab. 1 - key physical measurements of the January 10,
2005 La Conchita landslide
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THE 2005 LA CONCHITA LANDSLIDE, CALIFORNIA:
PART 1 - GEOLOGY
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
749
VELOCITY
Based on a review of the video coverage of the
event, the main lobe locally appears to have been
moving at a velocity of around 6 m/s where its western
margin was filmed just downslope from Ranch Road.
Likely, the central portion of the flow was moving
somewhat faster (~10 m/s). Upon entering the com-
munity the velocity was substantially lower, probably
5 m/s or less, based on eyewitness accounts of some
residents outrunning the advancing flow.
An independent check on these velocity estimates
was made using a method set out by P
RoCHaska
et
alii (2008). This method involves back-calculation
of debris flow velocity using superelevation. In this
method, fluid pressure is equated to centrifugal force
using the forced vortex equation (m
C
C
lunG
, 2001):
where v is the mean flow velocity (m/s), Rc is the
channel’s radius of curvature (m), g is the acceleration
of gravity (m/s
2
), is the superelevation height (m), k
is a correction factor for viscosity and vertical sorting,
and b is the flow width (m).
The velocity was estimated for a location near the
toe of slope (see profile a-a’ on Fig. 4 of the compan-
ion article, s
HRestHa
et alii, 2011). Key input values
were Rc = 107 m, g = 9.8 m/s
2
, = 3.2 m, b= 38 m and k
= 1. These input values yield a mean flow velocity, v,
of approximately 9 m/s, generally consistent with the
velocities estimated from the video coverage.
Notably, similar conditions should have prevailed
at profile location b-b’ (see Fig. 4 of the companion
article, s
HRestHa
et alii, 2011), i.e., ~3 m, though
only ~1 m of elevation difference occurs between the
paired levees at this location. This finding appears to
support the conclusion that debris was shed from the
outer (western) edge of the debris flow as it rounded
the curve near profile b-b’ because the swale was not
initially deep enough to contain the entire flow.
CONCLUSIONS
Intense rainfall coupled with unfavorable geologic
conditions triggered the 2005 La Conchita landslide,
which transformed into a giant debris flow containing
over 30,000 m
3
of wet soil and rock. A combination
of poorly cemented bedrock, active tectonics, rapid
a large-scale debris flow. As it traveled downslope,
the main lobe eroded and entrained material along its
path, creating the lower zone of depletion shown on
Fig. 1 in s
HRestHa
et alii (2011). This conclusion is
based on the following observations:
The lower depletion zone was located di-
rectly in the path of the main lobe;
The material occupying the swale was like-
ly in a wet and easily erodible condition due to the
heavy antecedent rainfall; and
The elongated shape and U-shaped pro-
file of the lower depletion zone mimics the shape of
glacially-carved valleys, consistent with theory that
debris flows should exhibit glacier-like erosional be-
havior (J
oHnson
, 1970).
Scouring added approximately 4,140 m
3
of mate-
rial to the debris flow, corresponding to about 13% of
the total depletion volume (Tab. 1). Comparison of the
total depletion and accumulation volumes indicates a
deficit of approximately 4,240 m
3
in the accumulation
figure. This difference appears to correspond to debris
removed from Santa Barbara Avenue during the initial
emergency response. The average thickness of debris
in the accumulation zone is estimated at 1.8 m, though
the thickness exhibited significant local variation. The
thickest accumulations of debris, locally exceeding 4.5
m, occurred near the toe of the main lobe.
As indicated on Table 1, the maximum (horizon-
tal) length of the 2005 La Conchita landslide was 407
m between the crown of the headscarp and the toe of
the main lobe, corresponding with an elevation drop of
152 m. The corresponding average travel path slope or
“fahrböschung” (H
eim
, 1932; H
, 1975) was 152/407
= 0.37 or tan (20.5
o
). The latter inclination (20.5
o
) rep-
resents the angle between the toe of the debris flow and
the crown of the headscarp. By comparison, the angle
of repose of loose rock typically varies between about
32
o
and 45
o.
Hence, the debris flow traveled much far-
ther from the mountain front than would be anticipated
from a “normal” dry rock landslide and is one likely
reason for the high casualty and damage figures re-
sulting from the event. Such “long runout” behavior
is characteristic of both wet and dry landslides with
volumes exceeding ~106 m
3
(~1.3 million yd
3
), and is
pronounced in landslides arising from weak or highly
fragmented source materials (s
HalleR
, 1991b). No
scientific consensus yet exists as to the origin of this
behavior (s
HalleR
& s
HalleR
, 1996).
(1)
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P.J. SHALLER, P.L. SHRESTHA, M. DOROUDIAN, D.w. SYkORA & D.L. HAMILTON
750
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
uplift, steep slopes, and pre-existing landslides all
helped set the stage for the event. The landslide debris
separated into two lobes during its emplacement. The
main lobe of the debris flow, containing 90% of the
mobilized debris, flowed downslope at velocities up
to 10 m/s, scoured over 4,000 m
3
of debris along its
path, and entered a residential neighbourhood, killing
10 persons and damaging or destroying 36 residences.
This lobe exhibited many common morphological
characteristics of small scale debris flows, and even
of fluid lava flows (pahoehoe). The minor lobe was
formed by the deposition of multiple waves of debris.
This debris impacted and breached a temporary wall
at the base of the slope but inflicted no additional cas-
ualties within the subdivision.
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CHiek
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uRtado
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