Landslides in Seattle are caused by a
combination of steep slopes (topography), glacial and post-glacial
soils (stratigraphy), and a pronounced wet winter season (typically
November through March). It requires the interaction of all
three to create landsliding in the city. With the exception
of coastal California, Seattle suffers more damage from landslides
than most other large cities in the United States.
Seattle is comprised of a series of linear ridges
and broad plateaus with intervening river valleys and linear
depressions that were shaped by the last glacial ice to reach
this area. To the south of the Lake Washington Ship Canal,
ridges and swales dominate the landscape. The major hills
that crest at about 450 to 500 feet are Magnolia, Queen Anne
Hill, Capitol Hill, Beacon Hill, West Seattle, and Mount Baker
Ridge. They are separated by Interbay, Lake Union, the Duwamish
River Valley, Rainier Valley, and Elliott Bay. Not all of
the swales are water-filled. Some are naturally filled with
glacial and nonglacial sediments and others are modified with
artificial fill. With the exception of Longfellow Creek in
West Seattle, the ground surface is drained by short and steep
In the area north of the Lake Washington Ship
Canal, the ground surface is a broad undulating plain. The
ground rises up to the north gradually from the ship canal,
nearly reaching elevation 500 feet near the north city boundary.
It is broken by depressions, such as Green Lake, Haller Lake,
and Bitter Lake. It has also been incised by Pipers Creek
on the west and Thornton Creek on the east.
As shown on a topographic relief map of Seattle,
Figure A-1 (Appendix
A, Volume 2), the ridges and plateaus are surrounded on all
sides by steep slopes. These slopes range in inclination
from about 25 to 90 degrees with the horizontal. In general,
the steeper slopes are those that border the shoreline of
Puget Sound, particularly the rare ones that are not protected
from wave erosion. The only remaining unprotected bluffs
in the city are in Discovery Park and a short section of shoreline
at the south end of Magnolia. Elsewhere, the shoreline is
armored or otherwise protected by individual short bulkheads
or by long bulkhead/embankments, such as the Burlington Northern
Santa Fe Railroad (BNSF), north of the ship canal.
It is on these steep slopes that surround the
ridges and plateaus of Seattle that Seattle's landslides recur
on a regular basis. This process is particularly evident
in the retreat of bluffs since the disappearance of the last
glacial ice from the Seattle area about 13,500 years ago.
It has been estimated, based on marine charts showing change
in submarine topography, that the Puget Sound bluffs in Seattle
have retreated at a rate of about 75 feet per century (Galster
and Laprade, 1991). The rate was undoubtedly much greater
in the first few millennia following glacial retreat; however,
it is equally obvious that slope instability is still very
Seattle is underlain by bedrock of Tertiary
age, glacial and interglacial soil deposits of the Pleistocene
Epoch (2 million to 10,000 years ago), and nonglacial soil
deposits of the Holocene Epoch (present-day). However, soils
deposited during the most recent glaciation of the central
Puget Lowland dominate the surface and subsurface geologic
conditions in Seattle. These rock and soil deposits are very
completely interwoven by repeated sequences of deposition
and erosion. It is clear that each of the major ridges or
uplands has a unique stratigraphic system.
Bedrock, consisting primarily
of sandstone and siltstone, outcrops sporadically to the
south of the Seattle Fault (see Figure
A-3); however, no bedrock outcrops to the north of this
fault within the city limits because it is buried by 1,000
to 3,000 feet of glacial and nonglacial sediments. The
bedrock does not play a significant role in the landslide
history of Seattle. The only major area of bedrock instability
occurred east of Boeing Field where large excavations were
made for Interstate 5 (I-5) in the 1960s.
Older nonglacial and glacial
soils (pre-Vashon Stade) are present within the downtown
business district and in the cores and flanks of most of
the hillsides. However, these older soils have not produced
much landsliding. Pre-Vashon glaciomarine deposits (Possession
Drift) underlie the downtown business district, Beacon Hill,
and Mount Baker Ridge. They are intermixed and chaotically
stratified clayey till, glaciolacustrine silt/clay, and
sand. Locally throughout the city, these deposits are overlain
by a variety of sediments of the Olympia interglacial period.
These sediments include sand, silt and clay layers with
scattered organic fragments, peat pockets, and thin interbeds
Vashon Glacial Deposits
The primary geologic units
that are involved with landsliding in Seattle are those
that were laid down during the Vashon Stade of Fraser Glaciation,
between about 17,000 and 13,500 years ago (Waldron,
1962; Booth, 1987). Together,
the four members (Lawton Clay, Esperance Sand, Vashon Till,
and Vashon recessional outwash) comprise most of the ridges
and uplands in the Seattle area (Figure
1-1). The lowest three members were overridden by approximately
3,000 feet of Vashon Stade ice. Recessional outwash was
not overridden by this ice.
The Lawton Clay, a glaciolacustrine
deposit, was laid down in a lake that formed as the glacial
ice advanced southward from British Columbia and blocked
the Strait of Juan de Fuca. The unit consists of laminated
and massive silty clay and clayey silt with scattered fine
sand lenses. It is hard, from having been glacially overridden.
Because of its hard condition and fine-grained consistency,
it is relatively impervious such that groundwater tends
to perch on top of its upper surface. The Lawton Clay is
typically interbedded with sands of the overlying Esperance
Sand near the contact of the two units.
The Esperance Sand is a glaciofluvial
advance outwash that was deposited by streams issuing from
the Vashon glacier as it advanced southward. It is comprised
chiefly of fine to medium sand that is locally gravelly.
Locally, it also contains silt layers and pockets and discontinuous
layers of gravel. It is very dense and pervious with groundwater
normally flowing freely through this soil.
Vashon Till (lodgment till)
was deposited beneath the Vashon Stade ice. Also known
locally as "hardpan," it is normally a gravelly,
silty sand or a gravelly, sandy silt with scattered cobbles
and boulders. It is very dense (one of the most compact
soils in the world) and relatively impervious. Water infiltrating
through overlying deposits normally perches on top of the
till; however, locally the till contains pervious water-bearing
Vashon recessional outwash
was deposited by streams issuing from the Vashon glacier
as it receded or wasted. It is a slightly silty to silty
sand with scattered gravel, and is the deposit that is found
at the ground surface on most of the uplands. It is relatively
pervious and loose to medium dense, not having been glacially
overridden. Precipitation commonly infiltrates readily
through this deposit and then perches on top of the Vashon
Holocene (post-glacial) deposits
are ubiquitous throughout Seattle. They include alluvium,
beach deposits, depression fillings, and colluvium. These
soils have not been glacially overridden. As such, they
are typically loose to medium dense or soft to stiff. Alluvium
is deposited along the major rivers and creeks, such as
the Duwamish River and Longfellow and Pipers and Thornton
Creeks. It is comprised of loose sand and gravel that is
normally wet; however, because of its low slope gradient,
it is not normally related to landsliding.
The beach deposits found along
the shorelines are normally not landslide-prone because
of a lack of relief. Landslides do deliver material to
the shoreline that contribute to forming beach deposits.
Depression fillings commonly
consist of soft clay and silt and organic materials, such
as peat. They accumulate in low spots on the ground surface.
They are normally found on the upland ridges and plateaus,
although they can be included in the same areas as river
alluvium. Depression fillings are not normally associated
Colluvium is very commonly
associated with landsliding. Colluvium is the loose to
medium dense or soft to stiff soil that mantles the sides
and toes of slopes throughout the city. Because it was
deposited by gravity processes such as soil creep, surficial
sloughing, landsliding, and slope wash, grain-size can vary
from clay and silt to boulder-size. The mode of accumulation
ranges from slow creep (the imperceptible movement of only
inches per year or less) to catastrophic landslides. Soil
creep in the upper few feet of soil on a slope is commonly
reflected in the bowing of trees on the slope. Colluvium
is normally moist to wet, especially during the rainy season.
Another category of Holocene
soil is fill placed by humans. Fill soils vary widely in
grain-size, location, presence of debris, and size. Although
many new fills have been compacted and engineered, most
older fills were just dumped in place or nominally run over
with the spreading equipment. These fills can be particularly
unstable where they have been placed on or in close proximity
to a steep slope.
Groundwater and Wet Weather
In addition to topography and stratigraphy,
groundwater is the other factor that plays a significant role
in the generation of landslides in Seattle. In spite of loose
soils on steep hillsides, landslides very rarely occur in
the dry summer months, although this sometimes happens under
unusual conditions. It is the water pressures that build
up in the ground, usually during the pronounced wet-weather
season, that nearly always trigger the slide event. The source
of water for any individual landslide can be natural or influenced
in some way by human activity or a combination of these two
factors. This section presents the various ways in which
the groundwater interacts with the geologic units in Seattle.
The groundwater found closest to the ground
surface is that perched atop the Vashon Till (Figure 1-1).
In this case, precipitation or water related to human activities,
such as improper drainage, infiltrates down through recessional
outwash until it encounters the top of the Vashon Till. The
perched water may flow until it emerges in a pond, creek,
or a steep slope, where it forms a spring. Water at this
contact normally dries up in the spring or summer and does
not reestablish itself until the winter months. Because the
contact between the recessional outwash and the Vashon Till
is shallow (normally less than 10 feet deep), it is normally
reachable with a backhoe for dewatering.
On the sides of hills, the undisturbed glacial
soils are covered with a rind of colluvium. Water is commonly
able to penetrate the semi-pervious colluvium because it is
relatively loose and contains some fraction of sand; however,
it cannot infiltrate easily into the very dense or hard underlying
glacial soils. The water, therefore, travels along the inclined
contact between the two materials of different permeability.
Water at this contact also normally dries up in the spring
or summer and does not reestablish itself until the winter
months. Because the contact between the colluvium and the
underlying undisturbed soil is shallow (normally less than
10 feet deep), it is generally reachable with a backhoe for
The most prevalent groundwater aquifer in the
Seattle area is the Esperance Sand. Precipitation infiltrates
through "windows" or cracks in the Vashon Till and
continues vertically down into the Esperance Sand until it
encounters the top of the underlying Lawton Clay. Owing to
the low permeability of the Lawton Clay, the groundwater perches
on the clay and then moves laterally, eventually saturating
near-surface colluvium and/or emerging in a spring on a hillside.
Because of the residual lag travel time of this water, many
of these springs are perennial. They are the most prolific
springs throughout the city. It is fairly easy to trace the
level of the sand/clay contact by locating the springs on
a hillside. The source of water for an individual spring
or group of springs is very difficult to define, as it probably
has a large regional contributing area uphill from the spring.
The key stratigraphic marker for landslide location
is the contact between the Esperance Sand and the Lawton Clay
It is commonly termed "The Contact." In Landslides
in Seattle, Tubbs concluded that "the landslide typically
occurred along the trace of the contact between the Esperance
Sand and either the Lawton Clay or pre-Lawton sediments."
No experiences or collected data in the past 24 years have
changed that conclusion.
Three main sources were used to develop the
historical database for the assessment of landslide hazards
in the City of Seattle (City). The primary source of landslide
information was Seattle Transportation Department (SEATRAN),
which has landslide files dating to 1890. However, good records
were not kept before the 1960s, when the landslide file was
started by Mr. Finney of the Seattle Engineering Department.
These files consist of several types of information including
memorandums by field inspectors, court documents, photos,
maps, subsurface data, geotechnical reports, mitigation plans,
and cost estimates. The accuracy of the City landslide files
is dependent on several factors. These factors include available
staff levels, the amount of damaged or missing information,
and, most importantly, the degree of landslide reporting by
the public. In general, SEATRAN's files record landslides
that primarily affected rights-of-way or utilities; not private
The second source of information was the Department
of Design Construction and Land Use (DCLU), which has maintained
a landslide database since 1986. This database includes the
reported locations of particular landslides with a brief description
regarding the structural integrity of the affected structures.
The DCLU landslide database contains a relatively complete
record of landslides that occurred during severe storm periods.
However, the database is relatively incomplete for other periods.
Furthermore, the landslide dates included in the DCLU database
reflect the time of inspection rather than the initiation
of ground movement. Therefore, when the failure date could
not be determined from the DCLU files, we assumed an approximate
failure date that was close to a previous storm event.
The third source of information for the landslide
database was the Shannon & Wilson, Inc. (Shannon &
Wilson) files. Shannon & Wilson has maintained project
files since 1954. The inventory includes projects that pertain
to ground displacement performed by the company.
The input data needed for the Seattle Landslide
Study is subdivided into six main groups:
and Public Utility Impact
and Repair (Mitigation)
The following is a detailed description of the
data comprising each of the attributes in the landslide database,
which is presented in Appendix
E. The Appendix also contains a legend that defines abbreviations
and provides additional explanations for each data field.
The Record Number field represents
the unique identifying number for each documented landslide
in the database. Each landslide was assigned a sequential
record number when it was entered into the database. After
data processing (refer to Section 3.3),
several landslides were omitted (duplications, etc.) and,
therefore, the record numbers are not in a continuous series.
The Location field consists
of an address representing one or more of the following:
the address of the person or persons reporting the incident,
the address of the closest property to the event, the address
of a property affected by the event, or an approximate address
specifically used for plotting on the Geographic Information
System (GIS). Locations of landslides reported by BNSF
along the railroad right-of-way are referenced by Milepost
number (e.g., MP 8.5).
The Date field contains the
approximate date of initiation of ground displacement.
In cases where the exact date was not known, we estimated
precipitation year. This was accomplished by assigning
the first day of January as the date. A higher percentage
of older landslides were assigned this date because of the
paucity of information in the older files. Note that many
landslides occurred during the New Year's storm of 1997.
The January 1, 1997, dates for these landslides are accurate.
A note in the Comments field or the Date Confidence field
includes the 1997 landslides where the exact date is not
known. A precipitation year is defined for this study as
beginning on July 1st and ending on June 30th of the following
The File Number field contains
the source's file number where the information was obtained.
DCLU file numbers consist of "J#", "J##",
and "J###" (where "#" represents a number,
e.g., J21) for events from 1986 to 1996 and "96-97
storm" for landslides occurring during the 1996-1997
winter. Other file numbers are as follows:
Number or Source Designation
Shannon & Wilson,
Observed During Field
Reported by source
other than the City or Shannon & Wilson, Inc.
All other file numbers
Shannon & Wilson project
files consist of "S&W." Landslide events
discovered during field reconnaissance for this landslide
study consist of "Field." Landslides reported
by sources other than the City or Shannon & Wilson consist
of "Citizen." All other file numbers represent
SEATRAN file locations.
The Consultant Report field
shows a letter code if an engineering consulting company
prepared a report regarding the landslide.
Field Checked shows the confidence
in whether or not the landslide is properly located on the
map. Landslides that are accurately located on the map
and field checked are "True." Landslide locations
shown on the map that were not found during field reconnaissance
are marked "False."
The Date Confidence field is
"True" where the date is believed to be accurate
and "False" when the date may be approximate.
Where no determination could be made, the Date Confidence
field was left blank.
The Slope Height field is an
estimate of the approximate elevation difference, in feet,
between the headscarp and the toe of the slide, as estimated
from historical records and field verification. Differences
between these estimates and actual conditions may exist.
The Landslide Type field consists
of a general classification of each landslide, even though
more than one classification may have been involved at a
specific location. The classification of landslide type
recorded in the database was the predominant type based
on our interpretation of the records and our site visits.
There are four general landslide-type possibilities: high
bluff peeloff (HBP), shallow colluvial (SC), deep-seated
(DS), and groundwater blowout (BO). Please refer to Section
4.0 for detailed descriptions of landslide types.
The Debris Flow field is "Y"
if a debris flow with runout generally longer than 50 feet
occurred and "N" if a debris flow did not occur
or had a short runout. Where no determination could be
made, the field was left blank.
The Size field represents the
approximate aerial extent of the ground displacement. Landslides
covering an area greater than 10,000 ft2 are
denoted with an "L" and those equal to or less
than 10,000 ft2 are denoted with a "S."
Note that some large landslides may cover a small area,
but displace a large volume of material because of their
depth. This type of landslide is not represented because
of the difficulty in estimating depth to the slide plane.
Differences between these estimates and actual conditions
The Vegetation field describes
the vegetative ground cover contained within the landslide
margins based on file pictures and/or field reconnaissance.
There are four vegetation type possibilities: brush (B),
wooded (T), sparse cover or bare ground (S), or grass (G).
If even one tree was contained within the slide margins,
the landslide was designated with a "T." When
there were no pictures in the file and no determination
could be made, the field was left blank.
The Topography field describes
the approximate average slope angle. If the slope angle
is greater than 40 percent, it was described as a steep
slope (SS). Moderate slope (MS) was used for slope angles
less than or equal to 40 percent.
The next four fields of the
attribute table indicate the geologic units involved in
the ground displacement. The units are ordered, as they
would appear in the headscarp, typically from youngest to
oldest (top to bottom). The five designations used in this
study were: fill (HF), colluvium (HC), glacial till (QT),
glacial outwash sand (QS), and lacustrine clay/silt (QC).
The list of geologic units involved in a particular event
is estimated based on the type of landslide, the geographic
and topographic location, and any subsurface information
disclosed in the file. The geologic units were not field
verified and differences between these estimates and actual
conditions may exist.
Landslide Trigger Mechanisms
All four of these fields reflect
documentation in files or reports regarding the trigger
mechanism of ground displacement. These fields do not reflect
the degree of contribution, nor do they necessarily represent
a professional evaluation. Many landslides documented in
the SEATRAN files are claims to the City and may contain
some degree of bias.
The Natural field describes
the trigger mechanism as being natural (Y) or human (N).
For example, precipitation is considered a natural trigger,
whereas pipe breaks and excessive lawn watering are not.
Blank spaces indicate that no determination could be made.
Groundwater and Surface Water
The next two fields, Groundwater
and Surface Water, indicate when groundwater or surface
water may have been the possible triggering mechanism of
the event. A "Y" indicates that groundwater and/or
surface water may have triggered the landslide and an "N"
indicates that these two trigger mechanisms probably were
not involved. Improperly directed surface water by private
parties and naturally occurring surface water were not differentiated
in this field. Blank spaces indicate that no determination
could be made.
Fill and/or Cut
The Fill and/or Cut field indicates
when filling and/or cutting may have triggered the landslide
event. Landslides resulting from inadequate shoring of
an excavation, for example, were denoted by a "Y."
An "N" indicates that filling and/or cutting was
not involved in triggering the landslide. Blank spaces
indicate that no determination could be made.
Roads and Public Utility Impact
All four of these fields reflect
documentation in files or reports regarding the effect of
roads and underground public utilities on ground displacement.
These fields do not reflect the degree of contribution,
nor do they necessarily represent a professional evaluation.
Many landslides documented in the SEATRAN files are claims
to the City and may contain some degree of bias.
Road Cut and/or Fill
The Road Cut and/or Fill field
is similar in nature to the fill and/or cut field, but only
pertains to public roads. Filling at the top of the slope
is denoted by an "F", cutting near the toe of
a slope is denoted by a "C", and in cases where
both filling and cutting were factors in triggering the
event, an "FC" was entered. Blank spaces indicate
that no determination could be made.
The Surface Drainage field
refers to the effect(s) of City-maintained drainage systems
on ground failure. Landslides that may be affected by City-maintained
drainage systems are denoted by a "Y" and landslides
that may not be affected are indicated by an "N."
Blank spaces indicate no determination could be made.
The Pipe Leak field refers
to the presence of additional water introduced to a landslide
as the result of a pipe leak or pipe rupture. A "Y"
indicates the presence of a pipe leak or rupture and an
"N" indicates no involvement. No differentiation
was made between a pipe break resulting from ground displacement
and ground displacement resulting from a pipe break. Blank
spaces indicate no determination could be made.
The Trench Fill field indicates
the presence of trenches serving as conduits for groundwater
that possibly contributed to instability. A "Y"
indicates possible involvement and an "N" indicates
no involvement. Blank spaces indicate no determination
could be made.
Damage and Repair (Mitigation)
This group of fields pertains
to landslide mitigation. It is important to note that the
degree of damage or the type of mitigation does not necessarily
refer to the address listed in the location field.
The Damage field is a numeric
field referring to the degree of damage caused by a particular
event. A value of "3" is equivalent to a "red
tag" or severe damage to the property; a "2"
is equivalent to a "yellow tag" or moderate damage;
a "1" is equivalent to a "green tag"
or some temporary damage; and a "0" indicates
no observable significant damage to property. Blank spaces
indicate no determination could be made.
The Repair Type field reflects
repairs described in the files, reports, and/or observed
in the field during field reconnaissance. Refer to the
Legend in Appendix E for a description of the repair types.
To the extent possible, we included only repairs that were
constructed. However, some landslide records did not include
enough information to determine if the repair was actually
built. These cases may be included. Furthermore, the type
of mitigation described in the table does not distinguish
between structurally engineered repairs and non-engineered
The Repair Effective field
indicates whether the repair was effective in preventing
further ground movement at that particular location thus
far. It is not a warranty that the repair will remain stable.
If repaired and no subsequent ground movement was documented,
a "Y" was entered. If the repair subsequently
failed regardless of design, an "N" was entered.
If the integrity of the repair is questionable regardless
of a failure, an "N?" was entered. If a repair
was not necessary, no repair was accomplished, or if there
was no information regarding repairs, the field was left
Upon completion of the data collection phase
of the study, duplicate addresses with the same failure date
were eliminated. The data was plotted on a GIS using ArcView
3.0a desktop GIS software. Several City ArcInfo GIS coverages
were loaned to Shannon & Wilson for purposes of map reproduction,
discrete address-based geocoding, and analysis of the occurrence
of landslides. These City ArcInfo coverages included, but
were not limited to, orthophoto-derived topography, parcels,
parks, streets, utilities (which includes water mains and
laterals, and drainage and wastewater mains and laterals),
buildings, potential slide areas, and steep slopes.
Each landslide data point was automatically
geocoded using the location field and plotted on the center
of the parcel. Note that each landslide is represented as
a single point, regardless of size. The landslide locations
were then edited based on the comments in the files regarding
location, direction of sliding, type of damage, and/or existing
topography. The point indicates the center of the point of
landslide initiation (middle of the headscarp). Duplicate
landslide events were eliminated based on date and their spatial
relationship. For example, a property owner at the top of
the slope may have reported a landslide to SEATRAN, while
another property owner at the base of the slope may have contacted
the DCLU because the same landslide affected their residence.
Shannon & Wilson initially field checked landslides within
the three study areas (West Seattle, Magnolia/Queen Anne,
and Madrona) for accuracy. Shannon & Wilson subsequently
field checked the balance of the landslides in the database
for the analysis in Part 4. The map
symbol was then moved to the proper location or deleted when
necessary. The maps that portray the landslide locations
are presented in the Map Folio,
This section of the study discusses particular
aspects of landsliding, including the types of landslides
that occur in the city, the timing of landsliding, chronic
landslide areas within the city and their characteristics,
the causes or contributing factors of landsliding, and the
coincidence of landslides with existing potential landslide
areas and steep slope areas. To support this discussion,
a series of figures and maps will be presented. The maps
are presented in a separate volume (Volume
2) because of their size.
In evaluating the landslide data compiled for
this study, most of the landslides were found to fit into
four generalized types. Those types, together with the figure
numbers that illustrate a schematic profile view of each type,
are as follows:
|Generalized Landslide Type
|High bluff peeloff
There are various combinations of these generalized
landslide types, as one type of mechanism may lead to another
during the sliding, or the slide may be complex, exhibiting
different modes of failure in different portions of the slide.
Landslides involving fill material were classified as shallow
colluvial landslides. The following sections describe each
landslide type in greater detail.
High bluff peeloffs (Figure
1-2) occur on the face of near-vertical bluffs where
vegetation is absent or sparse. The soil at and near the
bluff face, which has been loosened by the forces of weathering
(freezing, thawing, root-wedging), slabs off or slides when
it becomes wet during periods of heavy rainfall. This type
of landslide commonly occurs following a period of freezing
weather. Sometimes seepage from more pervious soils, such
as recessional outwash, at the top of the bluff, or runoff
over the edge of the bluff contributes to this type of instability.
Also, water-bearing layers in the steep bluff could contribute
to saturation of the face soils. Normally, the thickness
of soil that slides off the face is only a few feet. The
soil that comes off the bluff may or may not slide for a
considerable distance, depending on the water content of
the soil and the angle of the slope below the bluff. Alternative
names for this type of landslide are earth fall and blockfall.
A profile of a groundwater
blowout landslide is shown on Figure
1-3. This type of slide occurs where a pervious soil
(sand) overlies a lower permeability soil (clay or silt).
Groundwater collects in the pervious soil and becomes perched
on the underlying, relatively impervious soil. The lower
permeability soil could be either a relatively thin silt
or clay layer or a thick stratum of silt and clay. Seepage
travels to the slope face immediately above the contact
with the underlying, relatively impervious zone and causes
instability where the sand essentially blows out and flows
downslope (runout). Because of this blowout, the upper
portion of the slope becomes undermined and also fails.
Groundwater is more important in the development of this
slide type than direct infiltration of precipitation and
is commonly found at "The Contact," Figure
A-4. Nevertheless, this type of slide normally takes
place during or shortly after periods of heavy precipitation
because of the added water near the spring exit. It should
be noted that this mechanism for causing landslides (seepage
at pervious/impervious contact) was probably involved in
a number of slides that were categorized as shallow colluvial
landslides in the database table and landslide maps. This
categorization would occur where there was a lack of detailed
data on a landslide, particularly in the older records.
In the database table, those
landslides that were identified or estimated to involve
a depth of movement greater than an estimated 6 to 10 feet
were categorized as deep-seated (Figure
1-4). These landslides may involve higher density,
in-place soil as well as colluvial soil. This type of slide
normally consists of the block movement of soils where a
mass of soil slides downhill on a failure surface that is
often arc-shaped. Sometimes the surface of rupture parallels
the ground surface. As blocks of soil move downhill, a
setdown of the ground surface occurs at the upper edge of
the blocks, thus forming a slide scarp. Such movement is
commonly progressive; that is, a lower block of soil moves
first, which takes away lateral restraint for higher blocks
that, in turn, slide.
The deep-seated landslide is
initiated by water coming into the slide mass, which takes
place either from rising groundwater levels, direct infiltration
of heavy precipitation, surface runoff, saturation by the
discharge or leaking of pipes into or onto slope soils,
or a combination of these sources of water. Where the soils
subject to movement are relatively pervious, such as sand
and/or gravel, the movement normally occurs rather abruptly
(within minutes or hours). Where the soils are silt or
clay, movements usually occur gradually, over days, weeks,
or even months.
Shallow Colluvial (Skin Slide)
Shallow colluvial landslides
occur when loose, heterogeneous soils on a steep slope become
saturated and slide (Figure
1-5). The term "skin slide" is sometimes
applied to this slide type because a relatively thin depth
of soil is normally involved. They generally consist of
rapid movements of the saturated soils, and commonly act
like a thick fluid, flowing or running out over a considerable
distance. In the database, they are noted as "debris
flows" when the runout generally exceeded 50 feet.
The saturation of soils that causes shallow colluvial landslides
takes place by infiltration of surface runoff, direct infiltration
of precipitation, groundwater seepage, discharge from pipes,
or a combination of these sources of water.
1-6 illustrates a relatively shallow slide involving
fill material. This type of landslide was categorized as
shallow colluvial landslide in the database table and on
landslide maps. If fill is placed at the top or the side
of a slope without compaction and suitable drainage provisions
(surface and subsurface), instability is likely inevitable.
The timing of landslides is dependent on precipitation
at three different scales. These are total annual rainfall,
monthly rainfall, and a single storm event. The longest scale
is that of annual rainfall. Although heavier rainfall can
occur in years widely spaced or consecutively, the citywide
pattern of landsliding, as shown on Figure
1-7, indicates that about every decade a higher than average
amount of mass wasting (landsliding) occurs. For purposes
of this study, the rainfall (landslide) year has been designated
from July to June. In this way, all of the rainy winter season
is tied together statistically.
The most notable landslide winters were 1933/34,
1955/56, 1959/60, 1960/61, 1966/67, 1968/69, 1971/72, 1973/74,
1985/86, 1995/96, and 1996/97. Of these eleven winters, three
produced particularly large numbers of landslides: 1933/34,
1985/86, and 1996/97. The damage incurred during the winter
of 1933/34 was responsible for the formation of the Works
Progress Administration (WPA) drainage program in Seattle,
administrated through the Seattle Engineering Department.
The number of landslides during that winter may have been
comparable to the 1985/86 or 1996/97 winters had there been
comparable development in the city.
A map showing the distribution of landslides
by decade is presented on Figure
A-5 (refer to Figure
A-3 for area locations described below). Figure
A-5 indicates some interesting trends in the incidence
of landsliding and/or changes in the recording of the landsliding.
Many of the landslides on the west side of the Beacon Hill
(east of Interstate 5 [I-5]) are older, probably because many
of the larger landslides were stabilized by the construction
of the freeway in the 1960s. The newer slides in this area
are mostly smaller shallow colluvial landslides. This reduction
in severity of landsliding illustrates how the construction
of a major public works project can increase the stability
of a hillside; the reason being that buttressing and drainage
were widely incorporated into the project.
Two areas that appear to be dominated by new
(post-1960) landslides are the Burke-Gilman Trail and Inverness.
These locations may be prone to increasing numbers of landslides
because the area had been only sparsely developed prior to
1950. However, it is also possible that older slides in this
area were not reported.
The areas that have large numbers of landslides
dispersed through time are chronic slide areas and they include:
Beach Drive S.W., Alki, Pigeon Point, Madrona, Rainier Avenue
S.E., Interlaken, Lakeview Boulevard, North Capitol Hill,
Laurelhurst, East Queen Anne, Southwest Queen Anne, Southwest
Magnolia, and Northwest Seattle.
An above-average winter rainfall punctuated
by a large heavy storm on January 18, 1986, led to a rash
of shallow colluvial slides throughout the city (refer to
Figure A-6 for
severe storm events). The most disastrous storm was the Holiday
Storm of December 29, 1996, through January 2, 1997, during
which heavy and prolonged rain melted an accumulation of about
12 inches of snow in a two-day period. The water equivalent
of the snow and direct precipitation caused a total of 8.35
inches of water (as measured at SeaTac Airport) to run off
and infiltrate the ground from December 29 to January 2.
Two other time periods were important, but did
not produce landsliding on the level of 1933/34, 1986/87,
and 1996/97. First, the landslides that occurred in the winter
of 1971/72 were the basis for the statistical and geologic
conclusions drawn by Tubbs in Landslides in Seattle.
This publication was one of the major factors used as the
basis for establishing the boundaries of the landslide-prone
areas in the city. Then during the winter of 1995/96, the
Northwest experienced a record winter-long rainfall; a four-month
period from November through February. This exceptionally
wet winter was a major contributing factor of numerous deep-seated
landslides throughout the region, including Seattle.
A recent study (U.S.
Army Corps of Engineers, 1997) of the relationship between
landslide frequency and precipitation indicates that the most
extensive landslide activity is related most closely to a
3-day storm event (lower bound of about 3.8 inches of precipitation)
with an 8-year return interval. This statistic approximates
a rule of thumb that has been used in the Seattle geotechnical
professional community for many years. That rule of thumb
says that landslides are likely to initiate whenever there
is more than 2 inches of rain in one day or 3 inches in 2
The time of year in which landslides occur is
very closely related to the precipitation regime in Seattle,
as shown on Figure 1-8 (sheet1
2). The overwhelming number of slides occurs in January
(45 percent); however, the landslide season typically encompasses
a four-month interval: December through March (86 percent).
Although November normally has more precipitation than March,
it is likely that a certain threshold of antecedent groundwater
is necessary to trigger landslides. In summary, although
landslides are most likely to happen in January and February,
it is not uncommon for landsliding to occur in December and
March. However, for planning purposes, the landslide season
could begin as early as November and end as late as April.
Although slides can occur in the other months, the probability
is low. Only 7 percent of the landslides in the database
occurred during May through October, outside of the normal
6‑month landslide season. These landslides are often
not related to the normal factors that contribute to landsliding
(precipitation, steep slope, high groundwater table). Examples
include such things as overwatering, pipe breaks, and excavation
The following sections discuss the distribution
of landslides that have occurred in Seattle. These discussions
and associated maps show the historical aspects of landsliding
by decade, landslides that occurred without human influence,
and the different types. The 1,326 landslides contained in
the City database are presented on Figure
A-5, where they are shown by decade. Figure
A-7 shows the same events by type of landslide. The citywide
map (Figure A-3)
shows 22 specific areas in the city that have experienced
landslides. They are as follows:
- Northwest Seattle
- Burke-Gilman Trail
- Southwest Magnolia
- Southwest Queen Anne
- East Queen Anne
- Northwest Queen Anne
- North Capitol Hill
- Lakeview Boulevard
- Rainier Avenue S.E.
- West Beacon Hill (I-5)
- West Marginal Way
- Admiral Way
- Beach Drive S.W.
- 47th Avenue S.W.
- Seola Beach
- Pigeon Point
- Cheasty Boulevard S.
There are many other scattered areas of landsliding
and singular landslides in the city; however, the areas listed
above are those where densities and frequencies are the greatest.
Five areas in particular appear to have the highest density
of landslides: Southwest Magnolia, Southwest Queen Anne,
Madrona, Interlaken, and Alki. The pattern for natural landslides
(slide records that did not indicate human influence), as
shown on Figure A-8,
mimics the general map of landslide locations. Except for
Southwest Queen Anne, the most dense areas of natural landslides
appear to be the same as those that have the highest density,
considering all landslides. This is no coincidence because
those areas that are naturally unstable are more likely to
continue unstable behavior when human activity disturbs the
area than relatively stable areas. It has long been recognized
that disturbance of the ground surface and improper drainage
increases the frequency of landsliding.
The chart on Figure
1-9 indicates that only about 13 percent of the landslides
recorded citywide were totally natural. For three percent
of the events, there was not enough data to categorize if
the slide was natural or influenced by human activity. About
84 percent of the landslides were determined to have some
factor of human influence. This is consistent with other
studies and estimates, including the estimated 80 percent
in Landslides in Seattle, 1974.
Four maps (Figures A-9 through A-12) present
the distribution of each of the four landslide types. In
addition, a chart presented on Figure
1-10 indicates the percentage of each of the types of
slides. The majority citywide (68 percent) were shallow colluvial
slides, followed by deep-seated landslides at 20 percent.
High bluff peeloff (3 percent) and groundwater blowout (6 percent)
landslides were small percentages. A combination of shallow
colluvial and deep-seated landslides accounted for 88 percent
of the total landslides. Three percent of the landslides
could not be categorized because of insufficient information
in the records.
The distribution of high bluff peeloff landslides
is shown on Figure
A-9. This type of slope instability (illustrated on Figure
1-2) only occurs on precipitous cliffs, which are normally
comprised of till or sand. Many, but not all, of these landslides
are naturally triggered. They are either associated with
the headscarps of deep-seated landslides or old sea bluffs
that formed prior to the construction of shoreline protection.
Such high bluffs are found in only a few locations: Perkins
Lane, Northwest Seattle, and Southwest Queen Anne.
Groundwater blowout landslides (illustrated
1-3) are spread around the city, as shown on Figure
A-10. They are a direct indicator of the sand/clay contact,
where high groundwater pressures commonly exist. This landslide
type may be more common than indicated by this database, but
they are difficult to discern from the older records. Only
those slides where an engineer or geologist noted the characteristics
of this type of slide were placed in this category. Some
of these slides are natural because they are related to groundwater,
which is more likely than not from natural sources. Groundwater
blowout landslides occurred in Northwest Seattle, Southwest
Magnolia, Southwest Queen Anne, Alki, and West Beacon Hill
The locations of deep-seated landslides are
shown on Figure
A-11. They are located in significant numbers in the
following areas: Southwest Magnolia, Northwest and Southwest
Queen Anne, East Queen Anne, Alki, Admiral Way, West Beacon
Hill (I-5), Interlaken, Madrona, and Pigeon Point. Because
deep-seated landslides (illustrated on Figure
1-4) are dependent on regionally recharged groundwater,
these slides are mostly natural. This type of slide commonly
encompasses several properties and sometimes one or more city
As shown on Figure
1-10 and Figure
A-12, shallow colluvial landsliding is the most prevalent
and widespread type in Seattle. The areas with the highest
densities of shallow colluvial landslides include Burke-Gilman
Trail, Laurelhurst, Madrona, Rainier Avenue S.E., Alki, Beach
Drive S.W., East Queen Anne, Southwest Queen Anne, Southwest
Magnolia, and 47th Avenue S.W. Although the distribution
of this type of slide (illustrated on Figures 1-5
indicates that they follow the overall pattern of landslides
in the city, they often occur outside of the areas where natural
slides occur because a shallow colluvial slide is the type
of landslide most likely to be caused by human activity.
In its two most basic elements, a landslide
can be categorized as natural or human influenced. Virtually
all landslides in Seattle occur where natural factors are
conducive to landsliding, but many are also influenced by
human activity. It is normally difficult to discern the percentages
of contribution between these two elements to landsliding.
Likewise, it is very difficult to assign percentage of contribution
among the many human-influenced contributing factors in a
landslide. The natural factors that contribute to landsliding
(geologic conditions, topography, freezing and thawing, heavy
or prolonged precipitation, and natural groundwater seepage,
among others) are conditions to be accepted. For sites or
areas located on or near slopes, there is always a risk that
instability can occur. Engineering solutions are generally
available to reduce the risks to acceptable levels of safety;
however, there will always be risks. As shown on Figure
1-9, totally natural slides only comprise about 13 percent
of the total number of slides in the records reviewed. Their
distribution is shown on Figure
A-8. In general, totally natural slides are most likely
deep-seated, groundwater blowout, or high bluff peeloff landslides.
Deep-seated slides and groundwater blowout are influenced
most often by regional groundwater sources. High bluff peeloffs
are in mostly inaccessible locations that are not susceptible
to human disturbance.
Some factor of human influence was reported
in 84 percent of the landslides citywide in Seattle. The
implications of this are that there are measures that can
be implemented by the City and by private property owners
to reduce the risk of damage to public and private properties.
Some of the human reasons that contribute to landsliding in
Seattle include improper drainage/subdrainage, broken or leaking
pipes, excavation at the toe of a slope, fill placement at
the top or side of a slope, imprudent cutting of vegetation,
and the lack of maintenance of drainage facilities or vegetative
cover. All of these factors have been chronicled in the files
that were reviewed for this landslide inventory and study.
The percentage of reported landslides with some
factor of human influence noted in this study may be high
with respect to the total number of landslides that have taken
place within the City. The actual percentage of landslides
with such human influence may actually be lower. The reasons
for this are as follow:
the reported slides were included in the database that provided
the basis for this study.
of the reported landslides were those where people were
making claims usually against the City.
reported landslides were generally in developed areas, and
totally natural landslides in other areas may not have
Nevertheless, property owners and developers
need to realize that human influences can be significant contributing
factors to instability. It is thus imperative that competent
professional advice be obtained to reduce the risks of landsliding
and damage. On the other hand, the influence of geology in
this area must also be recognized as a significant contributing
factor in landslides since, with or without some factor of
human influence, most of the instability occurs on the steep
slopes that surround Seattle's ridges, near the sand-clay
contact, or at other locations where adverse soil layering
and groundwater conditions are present.
Potential Slide and Steep Slope Areas
The City of Seattle presently regulates public
and private development in environmentally-critical areas
by requiring special standards for design and construction
in potential slide areas (and known slide areas) and steep
slope areas. Potential and known slide areas are defined
by historical landslides and by a zone encircling many of
the hills and ridges based on the sand/clay contact as shown
in Tubbs' Landslides in Seattle, 1974. Steep slopes
are defined as slopes steeper than 40 percent, with a rise
exceeding 10 vertical feet. These restricted areas are shown
on maps prepared by the DCLU. If a proposed new development
is within one of these zones, geotechnical evaluations must
be completed to obtain a permit for construction.
Some of the benefits of accurately delineated
potential slide areas in Seattle are for zoning, administration
of construction permits, notification for landslide education
and public meetings, and emergency notification.
Some of the pitfalls for such accurate mapping
may be complaints from property owners who did not want to
be included within a restrictive zone, and the variation in
accuracy of the data and geologic conditions on an individual
A common theme of homeowners who object to being
included in a potential slide area is that their property
values would be diminished. It has been our experience that
property values can be reduced temporarily for one or two
years when landsliding is active on a property; however, upon
remediation of the instability, the property values revert
again to the same or higher value as before the landslide
occurred. Most of the property that is in a potential slide
area is also view property that has risen steadily in value,
unless an individual property is impacted by a landslide without
suitable remediation. Therefore, it is beneficial for a public
agency to accurately map and regulate construction in such
sensitive lands to assist the public to prevent unwarranted
reduction of property values. Knowledge of the potential
landslide risk and education of property owners (including
public owners) are the most effective methods to maintain
property value in the long-term.
Of the total number of landslides in the database,
58 percent were within existing mapped potential slide areas
and 76 percent were within the existing mapped steep slope
areas. Note that 24 percent of the landslides occurred on
slopes flatter than 40 percent. This shows the need for improving
the potential slide area mapping. The percentage of landslides
within either the steep slope or existing potential slide
areas was 88. Nine areas in Seattle were identified where
clusters of slides were outside of the potential slide areas.
is a map showing the landslides in Seattle in relationship
to the potential slide areas. Those areas where significant
groups of slides were outside the designated potential slide
areas include: Interlaken, North Capitol Hill, Laurelhurst,
Shilshole (south end of Northwest Seattle), the hillside west
of Burke-Gilman Trail, Seola Beach, Rainier Avenue S.E., Mount
Baker Ridge (south of I-90, at south end of Madrona), Admiral
Way S.W. (southern end), Alki (high elevations), and 47th
Avenue S.W. As shown on a map of landslides in relationship
with steep slope areas (Figure
A-14), the outliers are primarily scattered, isolated
occurrences, but five areas have concentrations of slides
outside the steep slope areas: Burke-Gilman Trail, Northwest
Queen Anne, Admiral Way, east of Lincoln Park (in Beach Drive
S.W. area), and the east side of Beacon Hill (Cheasty Boulevard
A subset of shallow colluvial and groundwater
blowout landslides is debris flows, those landslides defined
herein as those that flowed more than 50 feet beyond the toe
of the steep slope on which they fail. These slides are significant
in that their runout zones do not normally confine themselves
to the current potential slide areas, as presently mapped.
As shown on Figure
A-15, they are scattered around the city in the chronic
In our opinion, the above discussion points
out the need for further definition of the hazard zones based
on landslide prone characteristics and on runout zones that
could impact downstream properties. Please refer to Part
5 for an evaluation of the existing Potential Slide Areas.