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SACRAMENTO – The
California Geological Survey (CGS) today
released its latest landslide inventory
map, to protect public safety and guide
future development on the Palos Verdes
Peninsula.
“People don’t
associate the postcard-perfect weather
we’re enjoying now with landslides,”
said State Geologist Dr. John Parrish,
head of CGS. “But the mechanics of
landslides are complicated, and they
don’t necessarily care whether it’s 90
degrees or pouring rain when they start
to slip. This map is a reminder of the
need to be aware of the potential for
landslides and to take them into account
in the land development process.”
CGS, a branch of the
California Department of Conservation
(DOC), has created several varieties of
landslide maps over the years. The
landslide inventory series is the most
detailed. CGS hopes to release about 100
of the maps over the next several years.
Although the maps are
non-regulatory, they provide important
information to local planners,
decision-makers and geotechnical
consultants: whether a landslide is
considered active or dormant, the
direction of movement, and the type of
movement involved (some landslides are
more destructive than others). CGS
produces the maps by incorporating
previous mapping work with a detailed
review of aerial photography and
geologic fieldwork.
A landslide is any
mass of earth or rock that slides, flows
and/or falls downhill. Landslides can
affect land from a few square yards to
hundreds of acres in area and can be a
few feet to hundreds of feet thick. Many
factors can contribute to the formation
of landslides aside from rainfall,
including improper construction or
grading, earthquakes, weak or loose rock
and soil, and steep slopes.
“While the potential
for landslides sometimes can be
mitigated, many times the best thing to
do is to avoid building on or near
them,” said CGS Supervising Geologist
Chuck Real, who oversees the mapping
program. “The City of Rancho Palos
Verdes has recognized that reality, as
evidenced by its restrictions on
development on the Portuguese Bend
Landslide. What communities need to know
is where the landslides are, how they
move, and how recently they’ve moved,
and that’s what these maps show.”
The Palos Verdes
Peninsula map released today defines 175
landslides of different types, 49 of
them active. The best known is the
Portuguese Bend landslide. The active
movement impacts about 80 acres, and one
section has moved more than 600 feet
since 1956.
“Local agencies and
communities on the Palos Verdes
Peninsula are well aware of their
landslide issues and established
Geologic Hazard Abatement Districts in
the 1980s to mitigate hazards in some of
the areas,” DOC Director Bridgett Luther
said. “Our new map is a ‘one-stop
shopping’ overview of the entire
peninsula at a user-friendly scale (one
inch equals two thousand feet). It also
places the well-known and destructive
landslides in a regional context along
with other lesser-known, but potentially
damaging, landslides.”
Large, slow-moving
landslides composed of bedrock can cause
extensive property damage but usually do
not result in loss of life. A debris
flow, commonly called a mudslide, is a
more dangerous type of slope failure
because it is fast moving and can cause
both property damage and injuries. Mud,
rock and debris caught by these rapid
flows can travel faster than 10 mph and
in rare cases, up to 100 mph.
“Everyone knows that
earthquakes present a high potential for
damage, but landslides occur more
frequently and can be just as
devastating as an earthquake in a given
locale,” Parrish noted. “In the past few
years, the La Conchita and Bluebird
Canyon landslides have been reminders of
the old saying that ‘civilizations exist
by geologic consent.’”
CGS provides
technical information and advice about
landslides, erosion, sedimentation, and
other geologic hazards to the public,
local governments, agencies and
industries that make land-use decisions
in California. More information about
landslides, links to landslide maps, and
mitigation steps can be found
here or from the
United States Geological Survey.
Property owners are
advised to consult a licensed
engineering geologist or geotechnical
engineer before taking any steps
intended to mitigate potential risks or
harm associated with landslides.
In addition to
studying and mapping earthquakes and
other geologic phenomena, the California
Department of Conservation categorizes
mineral resources; administers
agricultural and open-space land
conservation programs, ensures the
reclamation of land used for mining;
promotes beverage container recycling;
and regulates oil, gas and geothermal
wells.
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