Erosion
and Sedimentation
t h e C o r v a l l i s Symposium,
in the Pacific
August,
1987).
Rim (Proceedings of
IAHS P u b l . n o . 165.
Landslide erosion in central Santa Cruz Mountains,
California, USA
GERALD F. WIECZOREK
United States Geological Survey, Menlo Park, California
9*1025, USA
ABSTRACT Geomorphic work performed by landsliding in an
11.6-km area near La Honda, in the central Santa Cruz
Mountains of northern California, was estimated from an
inventory of 277 active landslides compiled over a period
of 12 years (197^-1986). Landslides were categorized
according to their dominant type of movement, allowing
comparison of erosion caused by different slope processes
and triggering events. Although relatively small debris
flows were the most frequent type of landslide, the
majority of geomorphic work was done by a relatively few
large slumps, block slides, and earth flows. High
seasonal rainfall affected the geomorphic work more than
individual intense storms did. Earthquake-induced
landslides have done only a minor amount of work compared
to climatically induced landslides within the recent
historic past.
INTRODUCTION
Landsliding is a predominant erosional process in the Santa Cruz
Mountains of northern California. The erosional significance of
different landslide processes was evaluated in terms of geomorphic
work (Caine, 1976), which allowed comparison among landslide
processes as well as an assessment of the erosional importance of
different triggering events.
Study area
An 11.6-km area in the lower La Honda and Harrington Creek basins
(Fig. 1 ) was chosen for study because of its high susceptibility to
landsliding (Brabb et_ al_., 1972). La Honda and Harrington Creeks
are the only perennial streams draining the area and have moderate
gradients of 0.019 and 0.033, respectively. The study area ranges
in elevation from 60 to 360 m; it contains both gently sloping
areas, where grasses, chaparral, and oaks predominate, and steep
canyons, where conifers, including California redwoods, are concentrated. The area receives an average rainfall of 791 mm year
with about 85% falling from November through March (Rantz, 1971).
The bedrock geology of the area (Brabb, 1980) (Fig. 1) is shown
as three Tertiary units. The lithology, depth of weathering, and
soil characteristics for the units (from youngest to oldest) are
listed in Table 1. Although the soils are similar, the relatively
shallow depth of weathering in the Mindego Basalt is the principal
difference between these three units. In this area the Mindego
489
490 Gerald F.Wieczorek
Ht, —
Tpt
Tahana Member of the Purisima Fm.
Tmb
Mindego Basalt and related volcanic rocks
^-, 1 VV\
TIs
Lambert Shale and San Lorenzo Fm. (undivided)
L^ALT°É%*i
Landslide volume Cm^)
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EXPLANATION
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Fig. 1 Landslides during 197^-86 in study area with size
of circles representating landslide volume.
Bedrock geology generalized from Brabb (1980).
Drainage pattern from USGS La Honda, Calif. 7.5'
quadrangle. Index map shows location of study
area in central Santa Cruz Mountains of northern
California.
Landslide erosion
491
TABLE 1 Geologic bedrock units, lithology, depth of weathering, and
soil characteristics in study area (Brabb, 1980; Wieczorek, 1982)
Geologic unit
Lithology and depth
of weathering
Soil characteristics3
Tahana Member
of the Purisima
Formation
Very fine grained sandstone and siltstone bedrock overlain by 5 m of
soil and 10 m of moderately weathered bedrock.
Inorganic clays and
silts of medium to high
plasticity (CL-CH, MH)
with clay fraction of
predominantly montmorillonite.
Mindego Basalt
and related
volcanic rocks
Basaltic volcanic rocks
(submarine flow breccia,
pillow lava, and lithic
tuff) overlain by 1.5m
soil and 3.5 m slightly
weathered bedrock.
Inorganic clayey silts
of medium to high
plasticity (ML to MH)
with clay fraction of
montmorillonite and
illite.
Lambert Shale
and San Lorenzo
Formation,
undivided
Mudstone, siltstone, and
shale bedrock overlain by
5 m soil and 9 m moderately weathered bedrock.
Inorganic clays and
silts of medium to high
plasticity (CH-MH) with
clay fraction of montmorillonite and illite.
Soils characterized according to Unified Soil Classification
System.
Basalt tends to support steeper slopes than either of the two
sedimentary units.
METHODOLOGY
An inventory of active landslides was compiled between 197-4 and
1986. On the basis of classification of slope movements by Varnes
(1978), landslides were categorized according to their type of
material (rock, debris, or earth) and type of movement (fall,
topple, slide, lateral spread, flow, or complex, involving more than
one of these). Volume and distance of travel were measured in the
field or estimated from vertical or oblique photographs.
As suggested by Caine (1976), erosion of the landscape by slope
processes that involve the movement of material from higher to lower
elevation can be expressed in terms of the change in potential
energy of the landscape. The change in potential energy of a
landslide, AE (joules) is given by:
AE = Vpg (d sin 3)
where V is the volume of material (m-5), p is its density (kg m ^),
g
492 Gerald F.Wieczorek
is the gravitational acceleration (m s ) , d is the distance of
movement along the slope (m), and g is the slope angle. Geomorphic
work was assessed in terms of the change in potential energy per
rainfall season.
The geomorphic work was calculated by using values of landslide
volume and distance of movement along with several assumptions. The
slope angle had been measured in the field for some of the cases,
but the majority of values were approximately determined from
contour intervals on a 1:4,800 topographic map. For coherent
slides, such as block slides, slumps, and earth flows, the distance
of movement was considered equal to the displacement along lateral
shear surfaces or at toe or scarp locations. Although not typically
equal at all locations, a single value of displacement was used.
For debris flows, debris slides, and rockslides, the distance of
movement was approximated as the distance from the downslope end of
the initial source area to the terminus of the path of travel.
Because some material was retained along the path, this assumption
slightly overestimated the travel distance associated with the total
volume. Likewise, the volume estimates of very large (and deep)
landslides are approximate because only a few landslides were
drilled to determine landslide depth. For simplification, all
materials were assumed to have density of 2.0 x 10^ kg m~ , which
slightly overestimated the density for soil and debris, while
underestimating the value for rock. Even though this assessment of
geomorphic work is only approximate, the values obtained are
probably correct within an order of magnitude and provide a
consistent measure for comparison.
RESULTS
The 277 landslides observed during the 12-year period of study are
depicted symbolically on Figure 1 by dots (circles) representative
of their volumes. The landslides varied greatly in volume, from
less than 1 rrr to more than 10 m^. Their volumes were approxi0
"3
mately normally distributed, with the majority (86?) in the 10—10°
m-> range (Fig. 2 ) . However, a second minor peak of about a dozen
large landslides occurred in the 10-5—10 nP volume range. About
half of these large landslides represent reactivation of the same
landslide during different seasons. Each of these reactivated
landslides, which stop moving during the dry summer season, was
counted separately each season in the inventory, but is shown by a
single symbol in Figure 1.
The distribution of landslide type portrayed in Figure 2 shows
that debris flows are the most common type of landslide, with
slumps, earth flows, debris slides, and block slides occurring in
about equal numbers. The relation between geologic bedrock units
and landslide type is also depicted in Figure 2. The most debris
flows and rock slides occurred in the Mindego Basalt, whereas
slumps, earth flows, debris slides, and block slides were far more
common in the two sedimentary units. These trends reflect the depth
of weathering and slope steepness within the bedrock units (Table
1). Debris flows are common in the shallow soils, generally 1.5 m
thick, on steep slopes developed over slightly weathered or fresh
Landslide erosion
140-
B
493
(45.6%)
L a m b e r t S h a l e and
- San Lorenzo Fm.
(undivided)
120-
100-
GO
w
Q
Q
30 -
Tahana
Mindego B a s a l t
and r e l a t e d
volcanic rocks
Member o f t h e
60 - P u r i s i m a F m .
DC
LU
CD
5
40-.
D
20-
(1.4%)
Slump
3
L O G 1 0 LANDSLIDE VOLUME Cm ]
Earth
flow
Debris
slides
Block
slide
Rockslide
LANDSLIDE TYPE
Fig. 2 Distribution of landslide volume (A) and of
landslide types (B). Numbers in parentheses
indicate percentage of total sample represented by
each landslide type. Proportion of each landslide
type occurring within ^respective bedrock geologic
units is shown to scale.
Mindego Basalt, whereas other types of landslides are less common.
Slumps, earth flows, and block slides are common within the generally thick soil and moderately weathered bedrock on moderately steep
slopes of the two sedimentary units (Wieczorek, 1982).
Very few of the landslides in Figure 1 are distributed along La
Honda or Harrington Creeks. The majority of slumps, block slides,
and earth flows moved short distances, generally less than 2 m, and
remained on hillsides without entering these perennial creek
channels. Debris flows and debris slides travelled from 5 to 190 m
and deposited material on hillsides or in narrow 1st- and 2nd-order
channels. Because only a few landslides in perennial channels were
subject to fluvial processes during periods of high storm runoff,
the majority of material involved in landsliding probably remained
on the hillsides or in low-order drainages in the study area. No
measurements of stream sediment discharge are available to document
the quantity of sediment removed from the basins.
Rainfall
The total amount and distribution of annual rainfall affected the
number and type of landslides, as well as the cumulative volume and
494 Gerald F.Wieczorek
georaorphic work done. Table 2 shows a comparison between the
landslides during the January 3-5, 1982, storm and those that
occurred during the remainder of the 1981-82 season. High-intensity
storms, such as the January 3-5, 1982, storm, triggered a great
number of landslides (90) of relatively small volume, primarily
debris flows. Heavy rainfall that was distributed more evenly in a
series of moderate-intensity storms throughout a season resulted in
fewer landslides overall, but the resulting slumps, earth flows, and
block slides were of generally large volume. The work done by the
relatively few (15) slides during the later part of the 1981-82
season was an order of magnitude greater than that for those slides
that occurred in the January 3 - 5 storm.
The volume of landslides and their geomorphic work increased with
rainfall, when seasonal rainfall exceeded about 600 mm (Fig. 3 ) .
Below this threshold, volume as well as geomorphic work were low,
except for the 1983-84 season when an earthquake triggered one large
landslide (Wieczorek & Keefer, 1986).
Total rainfall values during the 1982-83 and 1981-82 seasons were
the first and third highest locally recorded, based'on records
available at La Honda from 1950 through 1986. Comparison to values
from the longest available rainfall record in northern California at
San Francisco, about 50 km north of La Honda, showed that since
records were begun in 1849, only the 1861-62, 1889-90 and 1867-68
seasons exceeded the 1982-83 seasonal total measured in San
Francisco. The formula:
T = N + 1
r
M
where T = return period in years, N = number of years of record,
and M = rank of the individual item in an array (Dalrymple, 1960)
gave return intervals (T r ) of 33 and 26 years for total seasonal
rainfall for the 1982-83 and 1981-82 seasons, respectively, at San
Francisco. Although these return periods were not exceptionally
high, the amount of geomorphic work during these seasons (Fig. 3) is
as much as two orders of magnitude greater than that during an
average rainfall season.
Two consecutive heavy rainy seasons (1981-82 and 1982-83) did not
result in significant increases of landslide volume or geomorphic
TABLE 2 Comparison of volume and geomorphic work of landslides
from January 3 to June 30, 1982
Time
period
Rain
(mm)
1/3-5/82
153
1/6/826/30/82
531
Number
Predominant
type
Volume
(nH)
Work
(joules)
-90
Debris flows
4.49 x 10 4
1.56 x 10
15
Slumps
Block slides
Earth flows
3-34 x 10 6
1.30 x 10
Landslide erosion
495
work from one year to the next. The slight increases observed in
the 1982-83 season compared to the 1981-82 season (Fig. 3) were
consistent with the slightly higher rainfall of the 1982-83 season.
Earthquakes
At least two earthquakes during the past 80 years have triggered
landslides in this area. Two landslides were noted during the April
18, 1906, earthquake (Youd & Hoose, 1978, p. 96; Wieczorek, 1978, p.
12); although more landslides may have occurred in the more remote
parts of the area, they went unreported. A landslide also occurred
during the April 21, 1981, Morgan Hill earthquake (Wieczorek &
Keefer, 1986). The geomorphic work done by these seismically induced landslides, based on measured displacements and estimated
volumes, is summarized in Table 3.
If the average annual value of geomorphic work from the past 12
seasons of climatically induced landslides is extrapolated to an 80year basis, the work from documented seismically induced landslides
in this area during the past 80 years is equivalent to only about 6%
of that from climatically induced landslides during the same
period. Although earthquakes have a high potential for triggering
landslides in this area, at least during historic time climatically
induced landsliding has been the more dominant erosive force.
DISCUSSION
Comparison of landslide erosion between areas is difficult because
of different methods employed to measure erosion and the limited
number of studies that have addressed individual slope processes.
I
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SEASON
(4)
( 1 0 ) e
(1) 1974-75
(2) 1975-76
(1)
O 6
' (6)
( 3 ) 1976-77
?
• (6)
( 4 ) 1977-78
o
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( 6 ) 1979-80
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( 7 ) 1980-81 O
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( 8 ) 1981-82
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O
(5) i
(9) 1982-83 O 2
(10) 1983-84
o
(11) 1984-85 (12) 1985-86
(21 (3) ( 1 1
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500
1000
500
1000
1500
1500"
30NAL RAINFALL (mm) (JULY 1
SEASONAL
RAINFALL
(mm)
(JULY
1
JUNE
30)
JUNE 30)
5 6-
(1)
(10)o
«
(4>«
^
_
_
•
~
-
/
/
W
(7)
/
—
-
-
Fig. 3 Relations between seasonal rainfall and landslide
volume (A) and geomorphic work performed by
landslides (B). Numbers in parentheses refer to
individual seasons. Average seasonal rainfall
indicated by dashed vertical line. Values for
1983~84 season represent a single seismically
induced landslide. Curved lines show approximate
relation between seasonal rainfall and landslide
volume and geomorphic work for climatically
induced landslides.
496 Gerald F.Wieczorek
TABLE 3
Geomorphio work performed by seismioally induced landslides
Date of
earthquake
Type of
landslide
Displacement
(m)
Volume
(m3)
Work
(joules)
1/18/1906
Block slide
0.9
1.1 x 10'
3.9 x 10 6
4/18/1906
Block slide
0.15
1.0 x 10'
7.6 x 10 6
4/24/1981
Block slide
1.7
1 .1 x 10
11.7 x 10 6
Table 4 shows a comparison of debris flows (debris avalanches) on
forested hillslopes in the Coast and Cascade Ranges of Oregon and
those in the study area near La Honda, in the Santa Cruz Mountains
of California. The data from Oregon are summarized from each of
three sites studied by Swanson and Lienkaemper (1985). Although
more frequent debris flows of smaller size occurred in the Santa
Cruz Mountains, the transfer rates are comparable with those measured in Oregon.
The work performed by active slope processes in this part of the
Santa Cruz Mountains is at least several orders of magnitude less
than that done by slope processes in the San Juan Mountains of
Colorado (Caine, 1976) or in Karkevagge of northern Scandanavia
(Rapp, 1960) (Table 5 ) . The relatively low total relief within the
study area (300 m ) , the relatively small average volume of individual debris flows (23 m 3 ) , the relatively short distances of debrisflow runout (less than 200 m ) , and the lack of rockfalls in the
Santa Cruz Mountains possibly account for some of the difference.
Several studies have examined landslides and sediment movement
associated with large-magnitude earthquakes. Tanaka (1976)
estimated that debris avalanches in the 1923 Kanto earthquake
delivered 10 -10-5 m 3 km
to stream channels in the Tanzawa
Mountains of central Japan; a juantit that was great compared to an
year . Pearce et al.
average erosion rate of 5 x 10 --103 m km
(1985) estimated that 4 x 10-1 m 3 kirf^ of landslide material was
delivered to main stream channels during the 1927 Murchison, New
Zealand, earthquake and found that most of this sediment has not
been transported from the initial deposition site by slope processes
over the subsequent 50-year period. Both of these examples suggest
that the magnitude of erosion from earthquake-induced landsliding
can be quite large. Although the short historic record shows that
erosion in this part of the Santa Cruz Mountains has been due
principally to climate-induced landsliding; a longer period of
records may be necessary to determine the erosive importance of
earthquakes because of possible long return intervals between
earthquakes that cause abundant landsliding.
CONCLUSIONS
1)
Although debris flows were the most frequent type of landslide
Landslide erosion
497
TABLE 4 Comparison of debris-flow frequency, average volume, and
soil transfer rate
Site
Area
sampled
(km )
Santa Cruz
Mountains,
California
Coast Range,
Oregon
Period Frequency
sampled (events km
(years) year )
11.6
14.4-126
3)
Soil transfer
rate (m-^km
year )
12
0.907
23
22
15
0.060-0.58
31-270
16-32
0.012-.023
1460-3120
36-45
Cascade Range, 12.3-50.1 25-34
Oregon
2)
Average
volume
(m^)
in this area, the majority of geomorphic work was done by a
relatively few large landslides—slumps, block slides and earth
flows.
High seasonal rainfall affects the volume of landsliding and
amount of geomorphic work more than single intense storms.
Earthquake-induced landslides have resulted in only a minor
amount of geomorphic work performed by landslides in comparison
to that done by climate induced landslides within the recent
historic past.
TABLE 5
Comparison of annual geomorphic work for slope processes
Geomorphic work
(joules year km
x 10 )
Process
San Juan Mtns.,
Colorado
Rockfalls
Debris flows 3
Slumps, Block slides,
Earth flows, Debris
slides
a
Karkevagge,
Scandanavia
Santa Cruz Mtns.,
California
4.17
11.6
11.44
45.2
0.006
11.9
0.31
Categorized as mudflows by Caine (1976) and as "sheet-slides +
mudflows" and "other mudflows" by Rapp (1960).
Categorized as "bowl-slides or sheet-slides" by Rapp (1960).
498 Gerald F.Wieczorek
ACKNOWLEDGEMENTS
The author is grateful to Robert Zatkin and
Robert Mark for their assistance with computer analyses; Christopher
Alger and William Brown III for their thoughtful reviews of the
manuscript ; Raymond Eis for drafting the illustrations and Jessica
Vasquez for typing the manuscript.
REFERENCES
Brabb, E.E. (1980) Preliminary geologic map of the La Honda and San
Gregorio quadrangles, San Mateo County, California. USGS OpenFile Report 80-245.
Brabb, E.E., Pampeyan, E.H. & Bonilla, M.G. (1972) Landslide
susceptibility in San Mateo County, California. USGS
Miscellaneous Field Studies Map MF-360.
Caine, N. (1976) A uniform measure of subaerial erosion. Geol. Soc.
Amer. Bull. 87, 137-140.
Dalrymple, T. (1960) Flood'frequency analysis. In: Manual of
Hydrology, part 3, Flood Flow Techniques. USGS Water Supply
Paper 1543 A.
Pearce, A.J., O'Loughlin, C.L., & Watson, A.J. (1985) Medium-term
effects of landsliding and related sedimentation evaluated
fifty years after an M 7.7 earthquake. In: International
Symposium on Erosion, Debris Flow and Disaster Prevention.
Tokyo, p. 291-296.
Rantz, S.E. (1971) Mean annual precipitation and precipitation
depth-duration frequency data for the San Francisco Bay
region. USGS Open-file report.
Rapp, A. (1960) Recent developments of mountain slopes in Karkevagge
and surroundings, northern Scandanàvia. Geograf. Annal. 42,
71-200.
Swanson, F.J. & Lienkaemper, G.W. (1985) Geologic zoning of slope
movements in western Oregon, USA. In: IV International
Conference and Field Workshop on Landslides. Tokyo, p. 41-46.
Tanaka, M. (1976) Rate of erosion in the Tanzawa Mountains, central
Japan. Geograf. Annal. 58A (3), 155-163.
Varnes, D.J. (1978) Slope movement types and processes. Chapter 2
of Schuster, R.L. and Krizek, R.S., eds., Landslides: Analysis
and Control. National Academy of Sciences, Washington, D.C.,
National Research Council, Transportation Research Board
Special Report 176, 12-33Youd, T.L. & Hoose, S.N. (1978) Historic Ground Failures in Northern
California Associated with Earthquakes. USGS Prof. Pap. 993.
Wieczorek, G.F. (1978) Landslide susceptibility evaluation in the
Santa Cruz Range, San Mateo County, California. Univ. of
California, Berkeley, unpublished Ph.D. thesis.
Wieczorek, G.F. (1982) Map showing recently active and dormant
landslides near La Honda, central Santa Cruz Mountains,
California. USGS Miscellaneous Field Studies Map MF-1422.
Wieczorek, G.F. & Keefer, D.K. (1986) Earthquake-triggered landslide
at La Honda, California. In: Hoose, S.N., éd., The Morgan
Hill, California, earthquake of April 24, 1984. USGS Bull.
1639.