Mass-wasting risk assessment and management
in the Santa Cruz area
Graeme Bell, Ian Borrows and Alan Kilroe
Abstract
This investigation has identified areas in Santa Cruz, Wilder Ranch
State Park and Love Creek which are susceptible to mass wasting. The
properties of soils on the slopes were used as an indicator of potential
failure. It was found that within each area, there was a significant risk
from a range of mass wasting processes, and that although measures
were being implemented to counter the threat, the risk was still
present. The key findings of the project are that anthropogenic factors
and land-use influence the degree of risk from mass wasting, and that
this risk can be reduced by effective planning, management and
engineering practices.
Introduction and Objectives
“As the human population increases and cities and roads expand
across the landscape, mass-wasting processes become increasingly
likely to affect people” (Murck et al., 1997).
Landslide or mass wasting can result in a great number of casualties
and huge economic losses in mountainous regions (Dai et al., 2002).
The Santa Cruz area has experienced mass-movement events on both
the large and small scale. Slope failure, mass-movements and soil
erosion are significant natural hazards in the area. One such event was
the Love Creek landslide of 1982 that impacted an entire
neighbourhood (after Sharp, 2003). In order to mitigate landslide
hazards effectively, methodologies are required to develop a better
understanding of landslide hazard and to make rational decisions on
the allocation of funds for management of landslide risk. In this
respect, the city of Santa Cruz has regulations and ordinances that
govern building development on slopes (after Sharp, 2003).
This study focuses on the city of Santa Cruz and its surrounding area,
including Wilder Ranch State Park and Love Creek. The project
examines the potential for mass wasting in this area, and also
examines what responses have been taken to deal with this hazard.
The main aims of the project are to: identify the areas at risk from
mass wasting; identify the type of mass-wasting the area is at risk
from; and examine and identify the methods used to manage the
hazards posed.
Research methodology
Physical analysis of soil properties
This investigation used soil properties as a proxy to determine the risk
of areas to shallow-seated landslides. We originally intended to visit
three areas to evaluate the risk of mass wasting; these were Santa Cruz
city (urban location), Wilder Ranch State Park (rural location) and
UCSC campus (rural-urban location). However, after a meeting with
Gerry Weber, an expert in site investigation and engineering geology,
it was decided to use Love Creek as our rural-urban location as this
was considered to be more appropriate (Love Creek was also visited
to see what responses had been taken since the 1982 landslide). When
evidence of mass wasting was found, we determined the type of mass
wasting that was present in the area. This was later verified with
reference to literature-based sources. In order to determine the
potential for mass wasting at a site, characteristics of the site were
recorded - in particular faults in rock faces and the soil properties.
Annual and antecedent rainfall was also taken into account, and
rainfall data were obtained from the internet
(http://www.weather.com).
The approach for the soil analysis is a modified version of the
approach taken by Baker et al. (1979). The following components
were measured: vegetation cover; slope angle; soil erodibility, based
on texture; present erosion; current land use; potential and actual
rainfall erosivity; and infiltration capacity. The site was located using
the relevant USGS map of the area, the site on the map was then used
as a starting position and a transect line was taken upslope of this
position, and the aforementioned readings taken at equal 10m
intervals. Vegetation cover was determined by measuring out a 5m2
quadrat, the percentage ground cover of vegetation was estimated and
then classified for a value between 1 and 5. This classification scheme
was used for each erosion risk criterion. The slope angle was
measured upslope using an abney level by using two equal height
points (these makers were used at all sites): one marker was used as a
reference marker and the other was used as a constant height level for
the abney level. These markers were placed 5m apart in a straight line
along the transect, the slope angle was recorded and classified. To
measure soil erodibility based on texture, a small amount of soil was
removed from beneath the organic layer (if present) and was analysed
by adding a small amount of water until the soil sample reached the
‘sticky point’. This then allowed the soil type to be determined and
classified using the field assessment guide given by Nortcliff et al.
(1994). Present erosion in the area was determined and classified by
analysing the site visually for effects of fluvial and erosional features,
and also any evidence of previous or current mass wasting processes.
The land use at the site was also classified and noted by taking into
account the activity in the area. Potential rainfall erosivity was
determined by analysing the area in the quadrat and omitting the effect
that vegetation would have on the area. This value was then classified
and noted. Actual erosion was determined by adding a value to the
potential score that corresponds with the vegetation score and, thus,
takes into account the effect of vegetation in the area. The final
erosion risk factor to be measured was infiltration capacity. This is
done by inserting a cylinder of 5cm diameter into the ground, after
clearing the organic layer (if present). 150ml of water was then be
poured into the cylinder and the time for the water to drain out of the
cylinder was measured, recorded and classified. This process was then
repeated for every point along each transects and for each site
determined to be ‘at risk’.
Interviews and analysis of management strategies
This investigation used visual analysis, secondary data, and interviews
to determine what is done to manage mass wasting hazard. During our
investigation, after identifying a site, the first aspect of management
we looked for was risk reduction measures. They included, terraces,
retaining walls and tiebacks. Many areas of the city were inaccessible,
and therefore we were limited to using visual analysis to determine
management approaches. During our interviews, we managed to ask
appropriate, searching questions to gain information as to what we
were to look for when we were out in the field.
Our investigation explored many routes of enquiry, and one which
proved extremely fruitful was that of the use of secondary data. We
gained newspaper reports regarding sites such as Love Creek, which
had seen past instances of mass wasting. However, our search for
secondary data regarding management approaches in Santa Cruz
County proved to be a disappointment as there was little information
held in public records. To counter this, we used qualitative methods
such as interviews to gain information of management strategies
employed by the City and County councils. Our interviews were
conducted with representatives from, the construction industry, city
council, and the county council. Throughout the investigation we met
with Jesse Nickell III (Barry Swenson Builders), Joe Hannah (The
County Geologist), Jessica Degrassi (Resource Planner), Mark
Johnsson (California Coastal Commission) and Tom Sharp
(Engineering Associate for the City of Santa Cruz), and we also had a
meeting in the Park Rangers office. Joe Hannah and Jessica Degrassi
gave us information which proved extremely useful in determining the
strategies used by the County in managing mass wasting
The use of interviews was employed because they allowed us much
more flexibility. We were able to adapt the interview to different
situations and gain relevant information even when the questions
asked failed to gain a satisfactory response. We did not approach each
interview with the same questions, and thus did not employ the use of
a formal question sheet. The use of a Dictaphone was limited, but in
the instances it was used we later transcribed the conversation to
13
retrieve relevant information. This flexible interview method was
employed by Burns (2000).
Results
Physical Analysis
Escalor Drive on the second marine terrace. Here the slope leading to
the third terrace showed evidence of solifluction in the form of soil
lobes towards the base of the slope. As a result of this movement the
area had been cordoned off and detour signs placed to keep traffic
away from the area. The only physical response noted was the
spreading of wood-chippings over the slope to reduce the effect of
rainsplash erosion.
Wilder Ranch:
The investigation at Wilder Ranch State Park revealed that the area
has a history of mass wasting. From our analysis we found that the
area is at moderate to high risk of experiencing shallow seated
landslides. It was not possible to take readings along a transect line at
every site due to the inaccessible nature of some areas. We had to,
therefore, make observations and gauge an erosion risk based on the
factors that we could measure remotely. Figure 1 shows the results
from transects taken from Wilder Ranch. Wilder Ranch transect one
shows that this 160m slope leading to the second marine terrace is at
risk from slope failure. Overall the risk of failure along this slope
decreases with increased distance upslope. However, due to variations
in the factors affecting erosion (e.g. slope angle, vegetation cover)
there is considerable variation in the total erosion risk along the
transect line. At 90m there was a rocky outcrop that reduces the
erosion risk because the rock is more stable and less likely to be
eroded under “normal” rainfall conditions. There was evidence on this
slope of previous landslides, and, as a result, the soils in the area have
been disturbed. This slightly hindered our project as we where unable
to determine the exact soil type at each location due to a mixing of the
soil by previous mass-wasting events. This was seen by the variation
in the soil type along this transect. However, the deeper soils do not
have an effect on surface processes and, thus, the shallow seated
landslides which are the subject of this study.
The second transect in Wilder Ranch was located near to a footpath,
close to the main entrance of the park. Therefore, if a landslide of
significant size occurred in this area, access to the park would be
severely affected. When the graph of this area is analysed (Figure 1,
transect two) the risk of erosion is constant overall. However, there
are some variations, for example at 10m where risk decreases, and at
Notes
Escolar Drive –
2nd Terrace
Solifluction
Moderate erosion risk of
erosion. Slow movement, but
could cause damage to structures
upslope.
High Street –
2nd Terrace
Non-active, as
management is
present, but potential
for block fall.
Low erosion risk, as
management present, but must
be properly maintained.
Junction of
Mission Street
and Centre
Street – 1st
Terrace
Blockfall
High erosion risk. Evidence of
recent fall. Management
urgently required, or structure
may be damaged. Especially
during a seismic event.
Car Park, North
Pacific – 3rd
Terrace
Solifluction /
Slumping
Moderate erosion risk, and
evidence of management
drainage channel to carry runoff
to drains and terracing to reduce
slope angle.
Table 1: Results from mass wasting risk analysis in Santa Cruz
Our second site was located at the base of a shear rock face on the
second terrace leading to the third on High Street. Although
there was no active movement present, there was evidence of
management in the form of a retaining wall that was holding the top of
the rock face in place and protecting the road beneath. Overall the risk
of erosion at this site is low, as the retaining wall has stabilised the
slope. Our third analysis site was located on the first marine terrace at
the junction of Mission Street and Center Street. It was determined
Overall Erosion Risk - North Pacific Car park
25
20
15
Transect 1
10
Transect 2
5
0
0
50
100
150
200
Distance along transect measured upslope(m)
Figure 1 Overall erosion risk at Wilder Ranch
40m were risk is increased. The construction of a footpath at 60m
along the transect has an effect on the total erosion risk as the risk
increases at this site. This is due to the potential for more runoff from
the footpath. This is compounded by the fact that there was no
drainage system in place in this area.
Overall the two transects are at risk from landslides. However, due to
lack of building structures in the area, the likely response to landsliding events is not to interfere with them, unless it is likely to affect
an important infrastructural component, such as footpaths or fences.
Santa Cruz:
The investigation in Santa Cruz revealed that mass wasting is a major
problem in the area, and many different responses are taken to
mitigate the hazard. In our analysis of the area, we identified four sites
that were at risk from, or have experienced, mass-wasting. The results
of the analysis are shown in Table 1. The first site we visited was at
Erosion risk
Erosion Risk
Mass wasting
Process
Overall Erosion Risk -Wilder Ranch
30
14
Site
30
25
20
15
10
5
0
0
10
20
30
40
50
Distance along transect measured upslope(m)
that the risk of mass-movement at this site was high as there was
evidence of recent movement in the form of small to medium sized
rocks at the base of the slope. Management at this site is required as
Figure 2. Overall erosion risk at North Pacific
the building at the top of the slope is in serious danger of failure
should the slope erode further. The final site we visited in our analysis
of Santa Cruz allowed us to apply the physical methodology. This was
not possible at other sites due to inaccessibility. The results from this
analysis are shown in Figure 2.
This site was located on the third marine terrace above a car park on
North Pacific. There was evidence of solifluction and slumping at this
site, and management techniques were employed to stabilise the slope.
These included the use of terracing to hold the slope and also the use
of a drainage channel to carry away excess runoff. The results in
figure 2 show that with increasing distance upslope, the risk of erosion
decreases. This is due to the effect of decreasing runoff potential.
There are variations in the erosion potential at 20m and 30m along the
transect; this is due to the effect of a drainage channel which stabilises
the slope. Overall this site is at moderate risk from further erosion as
the management techniques are in operation and the slope is relatively
stable as a result. However, the protection measures must be
maintained if slope stability is to be ensured.
Love Creek:
Love Creek was the scene of a devastating landslide in 1982 that
killed many people and devastated the area. The Love Creek area is
located within a steep sided V-shaped river valley just north of
Boulder Creek. The main soil type in the area was determined to be a
sandy-loam, with a 5cm layer of clay before the bedrock which is
sandstone. The general vegetation cover in the area is deciduous
woodland composed of Redwoods and Scots Pine, with some smaller
shrubs and bushes.
During our investigation of the area we were unable to run a transect
line for two reasons: first, the slopes in the area were steep and
possibly unstable alongside the road; and secondly, most of the land in
the area was private property and we did not want to disturb the
residents, especially as they, or their friends or families may have
experienced losses in the 1982 landslide.
This area is at risk from two different types of landslide. The first, and
most common, is shallow seated landslides, which are mainly caused
by heavy rainfall and the natural movement of material downslope as
the slope stabilises itself. Anthropogenic processes can also cause
shallow seated landslides due to undercutting by road construction for
example. This area is most at risk from shallow seated landslides,
which are much less devastating and are smaller in magnitude than
deep-seated slides. However, they can still cause serious problems.
The main causes of these slides, apart from the natural movement of
material under the force of gravity, are anthropogenic causes. There is
evidence that this has occurred in Love Creek as areas where
telegraph poles have been located seem to be prone to landslides as a
result of the removal of vegetation. Similarly, where the road has
undercut the slope by quite a large way there is evidence of erosion.
Again this is due to the removal of vegetation from the slope. When
the vegetation cover is minimal, there is evidence of debris flows
occurring. These have not been properly stabilised (the only response
we noted was the spreading of hay over the slide) and are, therefore, at
risk from further movement should a high intensity rainfall event
occur. Many small-scale slides could be triggered by high intensity
rainfall or seismic events. An example of the road undercutting a slope
was noted at one point along Love Creek Road. Here the slope had
failed and taken a large tree and half of the road with it into the river
on the valley floor. The road had been repaired, but only a type of
mortar had been spread over the slope to stabilise it. It was noted on
our second visit that this was undergoing erosion.
The second type of landslide the area is at risk from is the deep-seated
type, as occurred in 1982. The 1982 landslide occurred after a “500
year storm” (Cotton et al., 1982) hit the area, which caused the water
table to rise above the layer of clay, thus producing a sliding plane. As
the friction between the two soils was reduced, the land was free to
move under the influence of gravity and the excess weight of the land
due to saturation of the soil by the heavy rainfall. The entire slope
came away from the side of the mountain and travelled into the
settlement below killing 10 people. Cotton et al. (1982) state that the
landslide was 10m high, 600m long and 250m wide, so there was no
chance of anybody out-running the slide. The main slide was
compounded by the fact that a secondary mudslide followed the main
landslide which killed 3 people and hampered rescue attempts.
From our analysis, the effect of the slide on the area could be
identified, and we were able to locate the site of the landslide from a
scar on a slope. The road that runs through the area changes in height
by approximately 10m below the scar, and the shape of the valley also
changes from a steep-sided V-shape to an asymmetrical valley with a
flat floor, due to the influx of material from the landslide.
Management Approaches in Santa Cruz
Information was obtained from the Santa Cruz County Council
Offices during our interviews. It is clear that within the city of Santa
Cruz there are many approaches towards managing mass wasting as
regards to building on new sites. Indeed, “All development activities
shall be located away from potentially unstable areas, as identified
through the geologic hazards assessment, full geologic report, soils
report or other environmental or technical assessment”. Other forms of
management in the Santa Cruz area concerning mass wasting are as
follows: “The prevention of the building of new roads, building sites
or driveways across slopes exceeding thirty percent grade”. Similarly,
“A full geological report and any other appropriate report shall
demonstrate, that each proposed parcel contains at least one building
site and access which are not subject to significant slope instability
hazards, and that public utilities and facilities such as sewer, gas,
electrical and water systems can be located and constructed to
minimise landslide damage and not cause a health hazard”
“New building sites shall not be permitted which would require the
construction of engineered protective walls, diversion walls, debris
walls or slough walls designed to mitigate potential slope instability
problems such as debris flows, slumps or other types of landslides”.
Within Santa Cruz, the assessment of slope stability depends on an
evaluation of the site, which should include a brief site description
including the slope gradient of the site, the gradient of hillsides above
and below the site, and the topographic position of the site. This
requires noting such features as ridge top, nose of ridge, hillside, and
drainage.
Evidence of Management in Wilder Ranch State Park:
The two sites where we took transects are at risk from landslides.
However, within Wilder Ranch and the surrounding area of the sites,
there is an absence of structures. This means that the likely response
to landslides would be to leave the slide unless it affects an important
part of the park, such as a footpath. In an interview, we were told that:
“landslides are fairly common in Wilder Ranch, but are usually only
small scale and only a few centimetres in length”. However, there is a
history of larger events occurring within the park. The response to any
slide in the park was to leave it alone, unless it affects people’s safety
in the park. This is done as the rangers do not want to be seen to be
disturbing the natural landscape and as landslides are a natural
occurrence, it is only managed if it threatens accessibility, property or
lives. One response to reduce the effect of rainfall, and more
specifically rain splash erosion, is to place gravel and small stones at
highly eroded areas to reduce the effect of rainfall.
Evidence of Management in Santa Cruz:
As previously mentioned in the management approaches in Santa
Cruz, much of the management employed in the city is to avoid
building on areas of landslide risk. However, we saw several
responses to the risk as we investigated the city. Within the city we
saw many different approaches to landslides. Sites we visited
displayed many different responses to the threat. They included,
retaining walls, tiebacks and even wood chippings. We also
discovered that there is no one method that can be applied to
managing landslides in a built environment, and that responses in the
city are based on the specific circumstances of the possible slide and
the surrounding area. These management methods, such as tiebacks,
are designed to hold the cliff face and prevent it falling off. However,
this can fail under the stress of a seismic event. Retaining walls act in
a similar fashion, but look to hold back the landslide and block debris
that might fall during periods of heavy rain.
Evidence of Management in Love Creek:
From our observations, it seems unlikely that another deep-seated
landslide will occur in the Love Creek area, as the slopes are now
relatively stable, apart from smaller scale slides. However, there is
pressure on the area to stop people living in vulnerable area. In this
area the main type of management with regards to roads is not
preventative but reactionary. This was confirmed to us by the Public
Works Department of the County who are responsible for the
management of roads. This Department has only a very limited
budget, and are constantly being sued. Therefore, when repairs are
required, it is generally the cheapest option that is taken, which is to
react to an event rather to plan to prevent it. In the case of the damage
done to a road by undercutting in Love Creek, as previously
mentioned, the road was rebuilt using sandbags, rocks and mortar.
15
This technique is applied to every site where this type of repair is
required; it may work in one place but not in others. In areas where
there is no actual damage to the road, the response is to simply clear
the debris off the road and do very little or nothing at all to prevent the
area being affected by slides again. One response we noted was to
construct drainage ditches to carry away excess runoff from the slopes
and also retaining walls being constructed. However, we believe that
these measures were constructed privately as not all vulnerable areas
had protection measures in place, and the measures were of varying
quality and design.
and increasing costs of development within the city, slopes may be the
site of major future development.
•
Slope failure is a major problem in Santa Cruz County, more so
than in the City of Santa Cruz. Even though there is evidence of
mass-wasting in the City of Santa Cruz, this area is most at risk
from block-fall, slumping and solifluction.
In general terms, the County is very much like the City of Santa Cruz,
in that there is no real development on the slopes as it is too expensive
to construct in such areas (one estimate we were given was that up to
50 per cent of construction costs can be spent on foundations alone if
building on a slope). However, with increasing pressure on the land,
we were informed that there are approximately only 500 plots
currently available in Santa Cruz County. Therefore, there is likely to
be future development on the slopes of the marine terraces.
•
Love Creek is unlikely to experience a major landslide within the
near future, as the slopes in the area are stable following the 1982
landslide. However, the area is at risk from shallow seated
landslides that may restrict access to the area. The principle
mass-wasting process here is mudslides.
•
Wilder Ranch is at risk from slides, but unless a slide threatens
the operation of the park there is unlikely to be a response. The
major types of mass movement operating in this area are
slumping and solifluction.
Discussion
•
A review of management policy is required in Santa Cruz
County. Road management is under-funded, and as a result when
slides occur, the only response is reactionary where it should be
preventative in order to manage the hazard effectively.
•
Construction techniques must be effective and environmentally
sensitive as there was evidence of slope failure at several sites as
a result of insensitive construction methods. For example at Love
Creek, telegraph poles where installed without adequate
replanting of vegetation which resulted in movement of material.
•
Older properties are more at risk from mass wasting than newer
structures. This is due to changes in construction and, more
importantly, more stringent construction guidelines that are now
thoroughly enforced.
•
There was no real evidence of any major technological response
to the Love Creek landslide. The structures damaged by the
landslide had either been demolished or abandoned.
At Wilder Ranch State Park there is a history of landslides. When
looking at our graphs of our transects, they show a risk of moderate to
high risk of shallow seated landslides. The first transect shows an
increasing landslide risk downslope. This is because the materials in
the slope are stable, as they have already failed before and in doing so
they have stabilised the top of the slope. The second transect showed
varying risk from mass wasting, but the overall risk is the same at the
top of the slope. Typically landslides in Wilder Ranch are small, and
the response is to leave-be unless it is likely to affect an area which is
deemed important, such as a footpath. Rangers don’t want to disturb
the natural environment, so they only manage the environment if the
landslide affects accessibility. Responses common to the park are:
clear the path of mud; fence off the area; or put gravel down to reduce
risk of rainfall causing mass wasting.
It is unlikely that a deep-seated landslide this would happen again at
Love Creek, as shallow landslides are much more common. We found
evidence of shallow landslides around telegraph poles and near to the
side of the road. These could be a result of anthropogenic activity,
such as undercutting which was evident in numerous areas along the
Love Creek Road. We believe that the undercutting displayed here can
lead to further instability. Responses to the hazard were varied. We
also observed several retaining walls and drainage ditches to divert
water away from slopes. Within the Love Creek, we observed many
different instances where the road was falling into the river. As the
County Works only have a limited budget, they cannot afford to
implement preventative measures. Therefore, only reactionary
methods were undertaken, which seemed to be uniform. These
methods consisted of the reconstruction of slopes with sandbags,
mortar and rocks. This approach showed varying degrees of success.
Within the County if you wished to construct a building, it is essential
to take out a survey likely to cost between ten to fifteen thousand
dollars. Then the construction would be approved, or denied, and if
denied it would require further additional methods to prevent the
threat of landslides. They could include adding deep foundations,
tiebacks, and are usually expensive.
Within Santa Cruz, mass wasting is an issue. There is a history of
buildings within the city being damaged by mass-wasting processes.
Areas on the first and second marine terraces are more at risk than
areas on the third terrace as these areas are more densely populated.
Undercutting by construction of road and buildings is a cause of mass
wasting as this leads to instability. Therefore, if poor construction
techniques are used then mass wasting may occur as slopes may fail.
There was evidence at all the sites we visited that mass wasting is
evident. The processes observed include: soilifluction; soil creep
(observed at car park site at N. Pacific on the third terrace and
Escolar); and block fall (observed during the investigation on Mission
Street, and High Street, on the second marine terrace). Management
techniques on slopes are not currently present as slopes tend to be
avoided for construction. However, with increased pressure on land
16
Conclusion
Acknowledgements
Throughout our investigation we have received help from many
people. We would like to thank them all for their time, specialist
knowledge and skills. We are grateful for the help given to us by
Gerry Weber, Jesse Nickell III, Joe Hanna, Jessica Degrassi, Tom
Sharpe, The Park Rangers Office and Mark Johnsson. Thank you.
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