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Light Detection and Ranging System (LIDAR)
 State
geologists are now mapping the state with
LIDAR − light detection and ranging
− a system that gathers high-resolution images
of the surface of the earth. It employs lasers that let scientists see landslides
that occurred decades, even centuries, before.
State and federal scientists are combining LIDAR maps with
geologic data and records of past landslides to identify levels of hazards
throughout Portland.
Portland Metro Area LIDAR Project
As part of its mission to assess geological hazards and
natural resources to protect the life, safety and welfare of Oregonians,
the Oregon Department of Geology & Mineral Industries, in partnership with
the United States Geological Survey, has acquired 441.5 square miles of
digital Light Detection and Ranging (LIDAR) data for portions of the Portland
metro area and the Columbia River. Other government authorities, such as
the City of Oregon City, also have acquired and provided LIDAR data.
The purposes of this project are to make images of the
“Bare Earth” digital elevation model (DEM) derived from these LIDAR data
available and searchable on the web by street address and to be able to
compare and contrast these against aerial photographs, topographic maps,
and 10-m DEM derived from the topographic maps, in order to educate Oregonians
as to the practical and superior utility of this form of elevation information.
Where earthquake and landslide hazard data are available, these hazard layers
may be turned on and off.
How to Use LIDAR to Access Slide and Earthquake Information
for Properties in the Portland Metro Area
Here is the link to LIDAR:
http://www.oregongeology.org. You can access the interactive maps
in one of three ways:
You can zoom directly to your address, if it is in the
mapped area. You can also access the "Type a street address . . ."
function from the "Tools" tab − click on the
"GO TO FULL MAP EXTEND" icon and then click on "Zoom to Address/Scale"
− a window will display in which you can type
an address.
The
Help text is very useful to fully understand how the LIDAR system works
and it explains many of the terms.
New Maps Released in Late 2010
New maps outline bigger landslide-prone areas in the Portland
area than previously known. The maps, released in late 2010 by the
Oregon
Department of Geology and Mineral Industries, identify 257 existing
slides in 48 square miles. Those areas include zones of the West Hills and
riverside cliffs in North Portland populated with people and homes. The
largest landslide is north of U.S. 26 and the Vista Ridge Tunnel. It's 8
million square feet − roughly the size of 90
football fields and includes parts of Washington Park.
The collection of maps highlights vulnerable portions of
Multnomah and Washington counties. Landslides hit these areas hard in 1996
and 1997 with more than 300. As a result, the Legislature asked state geologists
to assess landslide-prone places. The maps can help prepare people and governments
for costly slides.
State geologists will create more maps of other populated
urban regions. They will map the rest of the Portland area over the next
several years while Astoria and Silverton maps are near completion.
You can view the new maps online by clicking
here. You can also purchase a CD for $15 or for $25 you can obtain
printed maps. Call 971-673-2331 or order online at
Nature of the Northwest.
Homes in Peril
 Destructive
slides hit the region almost every winter, but the state of Oregon doesn't
require that residents at risk be warned that their homes sit in the middle
of an "extremely high hazard" site. Oregon's standard disclosure statement
for residential real estate sales includes a question about whether the
property is in a designated slide zone. But sellers only violate the disclosure
standard if they are aware of the designation and don't disclose it.
Homeowners who recognize landslide risk can take steps
to protect their homes and family. Typical homeowners insurance policies
don't cover landslides, mudslides, flooding or earthquakes. Homeowners who
know they face landslide risks can try to get a separate rider to cover
landslide damage.
Scott Burns, a professor of geology at Portland State University
and an authority on slides states that the three ingredients of landslides
are:
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Steep slopes
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Loose soil
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Water
The Portland metro area as well as most of Western Oregon,
has all three.
Landslide Hazard Zones Have Been Mapped But Never Adopted
by Cities/Counties
After landslides killed five people in 1996 in western
Oregon, the state launched a safety initiative that called for landslide
hazard maps. State geologists developed taxpayer-funded maps of extreme
hazard zones. But the state never formally adopted those maps after some
city and county officials complained that the maps represented a regulatory
headache that labeled too much area as hazardous and might restrict development.
State building codes have standards for setbacks from slopes,
but they give engineers leeway to be more −
or less − conservative than state standards
at a specific site.
Definition of a Landslide
A landslide is a mass movement of rock, soil, and debris
down a slope under the influence of gravity. There are various types of
landslides, including rock falls, creep, earthflows, slumps, block slides,
debris avalanches and debris flows. The size and rate of movement can vary
tremendously, and as a result so can the risks to life and property. In
steep terrain, landslides may transform into dangerous, rapidly moving events
often referred to as debris flows, mudslides, mudflows, or debris torrents.
Determining if Your Property is Susceptible to Slides
If you are concerned about potential landslides on your
property you may want to hire a geotechnical consulting company to evaluate
the property. Contact the
Oregon Board of Geologist Examiners (phone 503-566-2837) or the
Oregon State Board
of Examiners for Engineering and Land Surveying (phone 503-362-2666)
for lists of registered professional consultants available for site-specific
evaluations. Here are some factors to consider in determining if your
property is susceptible to slides:
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Does the slope consist of loose sediment, such as
sand or gravel?
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Is development in the area making the slope more vulnerable
to erosion?
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Does the property have structural deformation such
as large foundation cracks, misaligned doors and windows, tilted floors,
or sagging decks?
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Do you have large, open cracks in driveways, curbs,
and roads?
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Does the property have any failing retaining walls?
An aerial photographs of the property may reveal areas
of little vegetation. This is indicative of mass wasting because after being
destroyed by a landslide, the growth of vegetation is slow to resume.
Another thing you may want to do is to make a landslide inventory of the
area and record where past landslides have occurred.
Preventing Slides
The most important preventative step is to route
runoff away from homes so it doesn't infiltrate the ground and weaken the
soil. Here are some suggestions to prevent slides:
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Make certain that your gutters and downspouts are
connected to the sewer system (hopefully a separated system) or routed
to a street drain basin. Alternatively, you can install a natural
catch basin, which help keep storm-water runoff from flowing into rivers
and streams.
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Vegetation reduces soil erosion and increases slope
stability by providing different types of root systems which can help
strengthen and binding the soil together. Carefully selected plants
can also intercept precipitation before it hits the ground, thereby
reducing runoff and excessive infiltration.
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Plant trees to stabilize hills. Geologists know
that a tree is like a pump as it pulls water out of the soil as part
of tree respiration. Conifers are better than deciduous since
they pump water all year long.
 In
2000 the Portland City Council established a reward system for ratepayers
who keep stormwater from leaving their property. This program, called
Clean River Rewards, came into effect in October 2006 after the city
launched a new utility billing system. Clean River Rewards offers residential
ratepayers up to a 30 percent discount based on the extent to which they
can manage runoff from roof areas. Commercial customers can claim a discount
for managing runoff from both roof and paved areas. Credits are offered
for having a small impervious footprint (less than 1,000 square feet), creating
or maintaining tree coverage, disconnecting downspouts, installing rain
gardens or drywells, and other low impact development BMPs. The City processes
applications without site visits and conducts oversight via spot checks
to ensure that BMPs are in effect and maintained properly.
The photo above is the Mt. Tabor Middle School rain garden.
Neighboring residents had experienced sewer back-ups during heavy rains
as a result of excess stormwater, so a rain garden, landscaped curb extension,
and sump were installed on and surrounding the school grounds. The rain
garden drains three-quarters of an acre of roof area and asphalt, and it
contains a variety of shrubs, grasses, and trees to slow runoff and promote
infiltration. The design was intended to appear open while at the same time
providing shade. A fence and shrubs were used to limit access to the feature,
and the garden was designed to drain within a day with a maximum ponding
depth of 6 inches. The design incorporated a variety of playful elements
that allow passersby to watch the water flow through the system during rainstorms.
Portland State University Study: 1996 Slides
A comprehensive study by PSU geologists following the 630-some
landslides that hit the region after the heavy rains of early 1996 found
that "roughly 76 percent ... were linked to poorly planned land-use practices
on steep or unstable slopes." They were often triggered by "uncontrolled
storm-water runoff coming from roofs and streets that contributed to adjacent
slope instability."
The report is entitled, "Landslides in the Portland, Oregon
Metropolitan Area Resulting from the Storm of February 1996: Inventory Map,
Database and Evaluation" and the authors are Dr. Scott F. Burns, William
J. Burns, David H. James, and Jason C. Hinkle. The report was completed
by the Department of Geology at Portland State University. You can
download the report by clicking
here.
The U.S. Geological Survey in 2006 also installed
monitors at Mount Cavalry Cemetery in Northwest Portland to track rainfall
and soil moisture, looking for a correlation between the two. That may allow
them to better predict when the conditions are right for slides to occur.
West Hill Slides of 1996-97
During the winters of 1996-97, when the Portland area received
record rainfall, the West Hills had over 350 slides (the state of Oregon
had approximately 9,500 slides). The majority of the slides caused minor
damages, but a few wiped out homes and a some caused structural problems
to homes. Many people were shocked to learn that their standard insurance
policy didn't cover slides.
Most slides are small and shallow, the exceptions are undated
prehistoric slides under what is now the Oregon Zoo and the Washington Park
Rose Garden.
Because of the '96 slides, the City of Portland requires
geotechnical engineers in slide hazard zones to drill soil samples, evaluate
the firmness of underlying soils and measure depths to bedrock. The
city requires geotechnical reviews at new building sites or for major remodeling
projects; existing homes and buildings do not get the same scrutiny.
As a result of the renewed study, what became better known
is that most of the homes in the West Hills are built on glacial loess
− windblown glacial particles. The windblown
silt ranges from a few feet deep in some places to 100 feet in others over
basalt. Although most have survived just fine through decades of Oregon
rainstorms, there can still be problems. Portland State University Bill
Burns states that "The particles are not as packed together as they could
be, which makes them more moisture sensitive. Combine that with the steep
slopes and the risk for landslides goes way up."
The windblown Portland Hills Silt ranges from a few feet
deep in some places to 100 feet in others over basalt. On steep slopes,
water provides the weight that pulls soil down. Humans make the hills
less stable by unwittingly cutting into toes of old slides, or dumping new
fill at the top of unstable areas. Trees act to stabilize hills.
October 2008: Southwest Home Slide Caused by Leaky
Irrigation Pipes
The remains of the Hendrickson home lie at the bottom of
a slope below Southwest Burlingame Place. The house slid down the hill it
early October 2008. The Portland Water Bureau shut the water off two days
later on a 250-foot section of pipe that serves the Southwest Burlingame
Place neighborhood where the slide happened.
In an October 10 edition of The Oregonian, Scott
Burns, chairman of Portland State University's geology department stated:
"More than ever, I'm convinced it was a leaky pipe" that caused the slide.
Later investigation revealed that a new irrigation system had recently been
installed and the system had developed some leakage of water into the ground.
Lake Oswego Slides of January 2009
Like thousands of other homeowners across Oregon, residents
on Lake Oswego's Woodhurst Place live along slopes that state geologists
have mapped as landslide hazard zones. In early January 2009, three
different slides occurred in the neighborhood from a recent storm.
All were relatively small.
But one destroyed a million dollar home that slid down
a roughly 200-foot hillside. The slide destroyed the home and forced the
evacuation of 20 houses nearby. Another slide stopped against the wall of
a house under construction down the street. The third covered a portion
of Greenbluff Road just above Woodhurst Place.
Lake Oswego recognized the potential for landslides in
the area years ago according to the geotechnical engineering firm Shannon
and Wilson. The state has identified past slides on the hill and has been
mapping the area for landslide hazards. Lake Oswego suggests that any homeowners
concerned about potential landslides hire their own geotechnical consulting
company to evaluate their home.
Washington Park Reservoirs and Landslides
 During
1894 the City of Portland Water Board built two small reservoirs, each having
a capacity of about 16,000,000 - 17,000,000 gallons, situated in a small
ravine in Washington Park (called City Park at the time). It was designed
to supply water to the west side to include the business section. These
were named Reservoir 3 and Reservoir 4 and both are still in service today
having undergone a number of modifications and repairs.
A short time before these reservoirs were completed a movement
of the adjacent hillside was detected, which, at first, was thought to be
entirely local and of minor importance. The work of construction the
reservoirs was pushed to completion, but the reservoir basins had scarcely
been filled before the real magnitude of the movement became apparent. The
reservoirs were emptied at once, and instrumental surveys were promptly
commenced in order to determine the extent of the slide. The surveys
were continued at regular intervals for a number of years and a series of
test borings and open shafts were dug (33 drill borings and 22 open-shaft
excavations reaching to the bed rock). The dimensions of the moving
ground were at length determined to be approximately 1,700 feet from east
to west, and 1,100 feet from north to south along the reservoir front (an
area of approximately 30 acres). The depth ranges from 46 to 112 feet, the
average being 77.8 feet. The approximate volume was 3,400,000 cubic yards
and the approximate weight 4,600,000 tons.
After a study of all the observed conditions, it became
clear to the Water Board engineers and the experts called into consultation,
that the probable remedy was the construction of a system of drainage tunnels
along the surface of the bed-rock, ant that these should be located so as
to tap the underground reservoirs which had been developed by the borings
and open wells. A total of 2,507 linear feet of such drainage tunnels, with
timber supports, was constructed between June, 1900, and December, 1901,
at at total cost of $14,161.14, or an average cost of $5.65 per linear foot
for materials and labor. The volume of drainage from the tunnels was
observed for the two years following their completion, and was found to
range from 10,000 to 15,000 gallons per day in summer, and from 25,000 to
75,000 gallons per day in winter. At the end of two years it was decided
that the drains were doing effective work and that it would be safe to proceed
with the work of reservoir repairs.
Source: "A Phenomenal Land
Slide" by D.D. Clarke. The paper (No. 1415) appeared in the American
Society of Civil Engineers Volume LIII (1904) publication. The paper
is 34 pages in length with numerous drawings.
 On
the evening of January 1, 2009, a landslide was discovered completely blocking
SW Sherwood Boulevard in Washington Park. Sherwood Boulevard is directly
uphill (west about 100 feet) of the Water Bureau's Washington Park Reservoirs
3 and 4 and just below the International Rose Garden's amphitheatre.
The landslide was about 40 cubic yards in diameter and
about two feet deep. It completely covered Sherwood Road and blocked normal
rain drainage. Portland rainfall was very heavy the night of January 1st
and the newly diverted rain ran across the road, along the slide, down the
hillside above Reservoir 3 until finally crossing SW Sherwood Boulevard
at Washington Park Dam 3. The rainfall eroded a small amount of the hillside
abutment near Reservoir 3. The water continued down the hillside and slightly
flooded a Water Bureau pump house.
Crew covered the slide in plastic and sand bags later when
the forecast called for 2-3 inches of rain. About 300 sandbags and yards
and yards of plastic were keeping more rain from reaching into the slope
and percolating down to create further slide problems.
The slide occurred in the same area that drainage tunnels
were installed in the early 1900s after it was discovered that the hill
was moving. It was pointed out by a city employee that the area where
the slide happened was filled in years ago so to facilitate the construction
of a service road that is accessed from SW Sacajawea Boulevard/Lewis and
SW Clark Way. The service road is used by park crews to bring equipment
into the Rose Garden area.
Resources
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Earthquake Tech Earthquake Tech provides customized anchoring
of old Portland homes built before 1976. Most insurance companies deem
this year as the cut-off for requiring proof of proper foundation connections
before approving earthquake coverage.
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National Landslide
Information Center This center was created by the United States
Geological Survey (USGS) to collect and distribute all sorts of information
pertaining to mass wasting. It is designed to assist researchers, scientists,
as well as the general public.
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Nature of the Northwest Information Center The center is operated
by the Oregon Department of Geology and Mineral Industries, carries
earthquake and landslide hazard maps and other reports. Address:
800 NE Oregon St., Suite 965, Portland, OR 97232, phone 971-673-2331.
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Oregon Coastal
Atlas You can see how your home is rated for landslide risk
by zooming to your property's location. The site covers Oregon
coastal counties.
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Oregon Department
of Geology The state site has outdoor recreation and natural
resource information, plus the largest selection of maps in the Northwest.
Check out their
Landslide Fact Sheet.
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Pilot LINAR Project for the Portland Metro Area The interactive
map lets you compare the resolution of imagery generated with LIDAR
with other kinds of mapping techniques such as 10-m shaded topographic
relief (DEM), orthophoto, and topographic (DRG) maps. You can display
additional overlays, such as tax lot outlines, streets, and five geohazard
study maps. You can zoom directly to your address, if it is in the mapped
area.
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Statewide Landslide Information Database for Oregon (SLIDO)
A compilation of landslides in Oregon that have been identified on published
maps. The database contains only landslides that have been located on
these maps. The interactive map lets you view information on location,
type, and other attributes related to identified landslides in Oregon.
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Water
Environmental Research Foundation (WERF) The goal of
WERF is to provide content that is appropriate for anyone interested
or involved in stormwater management and development projects.
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