1
Kyle Sleeper
ENVR 325
Mike Valentine
And the Landslide Brought Me Down: a look into the hazards and joys of
living in Tacoma’s Salmon Beach Community
The historical community known as Salmon Beach rests nestled along the shore of Puget
Sound just south of Point Defiance Park in Tacoma, Washington. People have lived here for over
a hundred years, beginning as a fishing village at the turn of the 20th century. The homes there
attract attention locally and from far away for its unique set-up; all of the homes rest upon
large stilts shooting down into the stand. Precariously perched above the community is several
thousand tons worth of feeder bluff prone to slope failure. In fact, Salmon Beach has had
multiple large-scale slides causing extensive damage, and even death. The loose sediment
which the houses rest is thoroughly saturated with water, and is susceptible to liquefaction in
the event of a large enough magnitude Earthquake. From the ocean, quite literally on their
doorstep, one can expect seiches or tsunamis depending on where and when an earthquake
would strike. These hazards are acknowledged and accepted by the community. They are
willing to risk their property and their lives in order to preserve their beautiful and secluded
way of living.
This paper has been written to explore the natural hazards and risks associated with
each major event mentioned above in relationship to Salmon Beach, and the choice residents
have made to continue their existence there. Various figures and maps will be used to clarify
certain sections, and complete references can be found in the footnotes of this document.
LANDSLIDES
Every year several billion dollars are spent on preventing, fixing, and assessing possible
damage caused by mass movements. There are over 2 million slope failures a year, many of
which are preventable. The Puget Sound region is particularly susceptible to failures due to a
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combination of factors involving glacial stratigraphy, sediment types, orientation, precipitation,
and eroding feeder bluffs. Examples of mass movements include creep, shallow slides, slumps,
landslides, and ancient, deep-seated slides.1
Unfortunately for those residents of Salmon Beach, they are prone to each one of the
wasting events mentioned, all of which are gravity-driven. They can be triggered by natural
factors such as excessive rainfall, vibrations, and slope steepening, as well as human factors
changing slope angle like removing vegetation, cutting into slopes, leaks in septic and watering
systems, adding mass to the top of a slope, or removing it from the bottom of a slope (under
cutting).
The Puget Lowland’s glacial stratigraphy has caused extremely steep slopes inundated
with loose sediments, and coastal erosion runs rampant throughout the region. What we find is
Esperance sands resting on top of Lawton clay, causing a prime slip zone for major events. In
the words of Sue Martin, an engineer with the City of Tacoma, about the cliffs threatening
Salmon Beach, “The cliff is made up of layers meant to move: silt and clay at its base, followed
by a layer of sand and gravel, followed by more silt, sand and glacial till. Interwoven through it
all is a series of springs, some of which gush into small waterfalls at the base of the bluff.” 2 The
bluff is constantly eroding, which on one level is essential to the continued equilibrium of the
ecosystem because it provides fresh sediments for beaches and replenishes essential nutrients,
but it also makes for unstable building sites and
dangerous potential energy.
The picture on the right3 is a picture of
Salmon Beach from the water, and provides a prime
example of a slope that has continuously failed,
lacking the anchoring mechanism of vegetation,
with an extremely steep gradient.
1
Information on mass movements provided in this paragraph comes predominately from class lecture given by Mike Valentine on June 17th,
2010
2 Sue Martin via Elaine Porterfield. “Idyllic life at salmon beach outweighs landslide risk for many.” (2001). Seattle Post-Intelligencer
3
Image accessed from Julie Newcombe’s personal website http://www.julienewcombe.com/photos/salmonbeach/index.htm
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Historical Evidence
The most convincing evidence of these
recurring slope failures lies in the historical record.
Major landslides have occurred in the past century
due to both earthquakes and excessive rainfall.
Failures worth noting occurred in 1949 due to the
Olympia earthquake, in 1996 due to excessive
rainfall, and 2001 due to the Nisqually earthquake.
In 1949, 3 days after an earthquake, magnitude 7.6, an “eleven million cubic yard
landslide occurred on Point Defiance at the Tacoma Narrows when a 400 foot high cliff gave
way and slid into the water…there was a row of homes at Salmon Beach at the foot of the cliff
which was narrowly missed…[others] sustained damage.”4
On December 27th, 1996, excessive winter precipitation saturated the bluff and two
slides occurred at Salmon Beach (slumps), and cut off phone lines and electricity to Salmon
Beach. 5 This slide also destroyed 3 homes. Though a slower event which occurred over a period
of days, it exemplified how prone these loose sediments are to slipping due to rainfall, and
danger presented by even slow events.
On February 28th, 2001 there was a magnitude 6.8 Nisqually earthquake which caused
$1,505,428 worth in damages.6 Failures all over the Puget Sound region prompted the
president to appropriate funds via FEMA for what was declared Federal Disaster #1361,
4
Lander, J.F., P.A. Lockridge, and M.J. Kozuch (1993). Tsunamis Affecting the West Coast of the United States 1806-1992, NGDC Key to
Geophysical Record Documentation No. 29, NOAA, NESDIS, NGDC, 242 pp.
5
Rex L. Baum, Alan F. Chleborad, & Robert L. Schuster. (1998). Landslides triggered by the winter 1996-97 storms in the puget lowland,
washington (online edition). Denver, CO: USGS.
6
Lynn M. Highland. (2003). An account of preliminary landslide damage and losses resulting from the February 28, 2001, Nisqually, Washington,
earthquake. USGS.
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“I have determined that the damage in certain areas of the State of Washington, resulting from an earthquake on February 28, 2001
and continuing, is of sufficient severity and magnitude to warrant a major disaster declaration under the Robert T. Stafford Disaster
Relief and Emergency Assistance Act, 42 USC 5121(Stafford Act). I, therefore, declare that such a major disaster exists in the State of
Washington.”7
In Salmon Beach the damages included severed
municipal services and significant damage to at least
8 of the cabins. The rock/soil slide from the bluffs
above the community contained about 1,000 cubic
meters of material, and left 10,000-20,000 cubic
meters of material poised and threatening 6
additional houses.8 The events of the 2001 quake
prompted the City of Tacoma to require residents to
sign a waiver agreeing to the hold the city blameless
for any slides that damage their community,
acknowledging the risks of living there.
What this also means is that if the community wants to enact any mitigation plans and
technologies to prevent slope failure they must assume the financial burden of any such project.
Because they would be multi-million dollar investments, they very well may never commit to
such projects. The Pierce County Hazard Mitigation team determined the recurrence interval
for the landslide hazard in the county to be “100 years or less occurrence.”9 As we can see from
these three examples alone, the “or less” part of that conclusion becomes the dominant train
of thought.
TSUNAMIS and SEICHES
Tsunamis are large and fast waves generated from the displacement of a large volume
of water beneath the surface. These are caused in oceans from seismic activity, landslides, and
impact events from meteorites. Tsunamis can move with tremendous force, and devastate
coastal areas exposed to their power. Seiches are water waves generated in enclosed or partly
7
Federal Emergency Management Agency. (2001). [FEMA-1361-DR]
United States Federal Government.
8 Rex Baum, Al Chleborad, Ed Harp, Randall Jibson, David Keefer, & Scott Miles. Nisqually earthquake, Washington mw 6.8. (2001) USGS.
9
Pierce County. (2003). Region 5 hazard mitigation plan landslide hazard sub-section.
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enclosed bodies of water such as reservoirs, lakes, bays and rivers by the passage of seismic
waves caused by an earthquake.10 They model the waves of the earthquake and are known as
“standing waves,” which appear to superficially raise water levels.
Locally produced tsunamis have happened in
Puget Sound in the past. The 1949 earthquake caused a
6-8 foot tidal wave that damaged boats, dock areas,
boardwalks, and other waterfront installations in
Salmon Beach from landslides going into the water.11
Furthermore, a study was done which found that about
1,000 years ago there was a massive tsunami in the
region which drowned out marshes that once occupied
Figure showing past Tsunamigenic events in Pierce County
(http://www.pmel.noaa.gov/pubs/PDF/vent2981/vent2981.pdf)
downtown Seattle, Whidbey Island, and covered parts of Vashon Island with sand, which was
caused by the Seattle fault.12 This suggests that we can expect future tsunamis in the Puget
Sound area.
Tsunamis generate an enormous amount of momentum, traveling hundreds of
kilometers an hour and they can strike within minutes of a nearby quake. Because of the
evidence of large-scale tsunamis occurring fairly recently by geological standards the NOAA is
now “using a finite difference model based on nonlinear shallow water wave theory and highresolution digital elevation model, we simulate the generation, propagation, and inundation of
tsunamis in Puget Sound. The tsunamis are generated as a result of possible earthquake
scenarios for the Seattle Fault.”13 This investment in modeling should make it clear as to how
serious this hazard is for members living along the coast.
10
Pierce County, REGION 5 HAZARD MITIGATION PLAN TSUNAMI HAZARD SUB-SECTION1
Tacoma News-Tribune, April 18, 1949, p.1
12 Angie J. Venturato, Diego Arcas, Vasily V. Titov, Harold O. Mofjeld, Chris C. Chamberlin, & Frank I. Gonzalez. Tacoma, washington, tsunami
hazard mapping project: Modeling tsunami inundation from tacoma and seattle fault earthquakes. 2007. Seattle, WA: National Oceanic and
Atmospheric Administration.
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13
Koshimura, Mojfeld, “Puget Sound Tsunami Inundation Modeling Preliminary Report : Phase 2,” NOAA.
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In a worst-case scenario sure, because of its
location at sea level, a tsunami approaching the shore
with such force could simply demolishes the Salmon
Beach houses against the cliffs, however, Salmon Beach
is on the south side of the peninsula and is protected
from extremely devastating waves. Tsunamis caused by
subaerial landslides are more likely to damage Salmon
Beach than those triggered directly by earthquakes.
Pierce County deems them secondary in hazard to
This would prove impossible for Salmon Beach Residents
(http://ioc3.unesco.org/itic/images/upload/Washington_TsuH
azZone_big.png)
seiches.14
In the event of seiche, Salmon Beach will experience significant flooding, in part because
“sedimentary basins beneath a body of water can amplify a seiche. Seismic waves…can amplify
water waves by exciting the natural sloshing action in a body of water or focusing water waves
onto a section of shoreline.”15 Already the high tide mark is only a few feet from the bottom of
their houses (which all rest on stilts in the sand) and a 4 or 5 foot increase in the tide will engulf
the first floors, boardwalk, and prevent evacuations over land from the houses. Essentially it
will trap the residents in their houses, cause significant water damage, and could possibly result
in drowning or death.
The Washington State Department of Natural Resources advises its citizens to know
what to expect. They believe the best form of mitigation for tsunami and seiche dangers lie in
the weapon of awareness. As further advances are made in understanding earthquakes and
their effects, so too are the dangers of tsunamis and “information is the foundation for
planning efforts.”16
LIQUEFACTION
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15
Pierce County, REGION 5 HAZARD MITIGATION PLAN TSUNAMI HAZARD SUB-SECTION1
Washington State HIVA Mitigation Plan, 2006/2007
Washington State Department of Natural Resources
http://www.dnr.wa.gov/ResearchScience/Topics/GeologicHazardsMapping/Pages/tsunamis.aspx
16
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Another frightening hazard is that of liquefaction. It is the process of loosely packed,
water-logged sediment, suddenly turning to a heavy-liquid-like state due to intense vibrations, like the
shaking of an earthquake. It can also occur because of tsunami runup and drawdown “due to the
generation of high excess pore pressure and the reduction of the effective over burden pressure during
the drawdown.”17 As with most other hazards dealing with sediment there is a combination of factors
leading up to such an occurrence. In the case of liquefaction the sediment will usually be waterlogged
because of poor drainage, high permeability, and high porosity.
The homes at Salmon Beach are built on stilts driven down into sand. One can assume that the
ground is thoroughly saturated. Drainage is hard to measure, because the in and out flux is a constant in
such an environment, but I picture it as a soup of sand to certain extent. There have been significant
mapping efforts of Pierce County to determine liquefaction-prone areas inland, and in the maps I’ve
looked at, data for Salmon Beach specifically has not been logged. In part this is probably due to the
county realizing that Salmon Beach has so many other risks already associated with living there, and
liquefaction is known to occur on coasts naturally.
In the violent shaking of large magnitude quakes (6.0+) one could expect the homes at Salmon
Beach to shift on their stilt-foundations and/or sink downwards over time due to the constant pressure
of tides. They seem to be holding up so far but one can only guess for how long that will remain.
HAZARD MITIGATION
Acknowledging all these hazards we must then look at ways to mitigate the risks. The
recurrence interval for major quakes is within the timeframe of 100 years, major slides around
50 years, and earthquake-caused tsunamis every 1,000 years. The most prominent hazard to
mitigate against for the residents is by far Landslides, so we will focus on the mitigation
techniques they have employed and could employ, but probably wouldn’t because of the cost
and effort. In this section we can also mention some of the techniques to prevent liquefaction.
Soft approaches to mitigating damages caused by landslides for the Salmon Beach
community haven’t amounted to much. Already the city requires residents to sign a waiver agreeing
17
Stephen P. Palmer, Liquefaction Susceptibility Mapping for Selected Urban Areas in the Central Puget Sound
Region, Washington Final Technical Report, Washington Department of Natural Resources, 2004.
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to hold the city blameless for any slides that damage their community. New building permits aren’t
granted, and some residents complain about additions they want to add to their house but can’t receive
permits for. They can do restoration projects, but this historical community resides in such a precarious
area that the city doesn’t want to be liable for assured damages there anymore. The slopes are tagged
and past slide areas are identifiable by the scars left in the land as well as small flags at the bases of
where the slumps once were.
Some hard approaches have been tried. There are large anchoring cables nailed down into the
cliffs, as well as some nets on the upper vegetation to help hold the sediment back, however it is not
enough. Unfortunately, further efforts will not be pursued for both aesthetic and financial reasons. The
residents wouldn’t want shot crete sprayed on their walls because it would detract from the natural
landscape they appreciate so much. Cellular faces would be extremely difficult to install, and thus would
be extremely expensive. Another way to mitigate slope failure would be to jet-grout the hillside. It
involves drilling cylinders into the sediment and filling it with a cement-like material, and offers
additional stability and possible resistance to liquefaction.
WHY LIVE THERE?
When talking with a Tacoma resident who plans to buy a house in Salmon Beach whenever he
has the funds to, who goes by the name “Bazooka Joe,” he told me, “I’ve been coming to Salmon Beach
since I was a kid. I wouldn’t let landslide scares stop me from moving here. It’s the most beautiful place
on the Sound.”18 That attitude and resiliency displaying a “won’t happen to me” sentiment shows up
frequently when discussing the community, but more on that later.
The risk perception of the community is extremely low. It is probably due to two factors: it is a
familiar hazard, and it is “balanced” by the benefits of remaining there. Everyone living at Salmon Beach
knows about landslides, that they might occur, and the damage it has caused in the past. But one of the
things the community thrives on is just that: community. When a home or boardwalk is damaged by an
event everyone rushes to the aid of the person or structure to fix the problem. When driftwood comes
on to the beach or gets stuck in a foundation they work together to drag it out and chop it up to
distribute as firewood amongst the community.
18
Bazooka Joe, personal communication
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An article published by the Seattle Post-Intelligencer shortly after the 2001 Earthquake captures
the essence of the community and their attitudes about sliding quite well. In it the author interviewed
numerous residents and quoted the president of the homeowner as saying that “We’re more stable now
and not too worried about a hill slide.”19 While some might call this a case of denial the community is
proud to prove its self-reliant ways by staying put under the looming cliffs. There is constant
maintenance that must be done to the boardwalk, stairways, and access road above the community,
probably deforming because of creep, but the work to be done just bonds the community tighter
because they all work together.
A book published in 2006 by Arcadia Publishing was written by Roger
Cushman Edwards, a resident of Salmon Beach, and contains a wealth of
private pictures and images providing a window into the extensive history of
this community. Perusing the history of this stretch of beach allows for a
greater appreciation for the way of life preserved by this haven.
While it wouldn’t be my first choice for a place to own property, I will
continue to visit Salmon Beach from time to time. It certainly offers a brief
respite from the hustle and bustle of life above the cliffs.
19
Elaine Porterfield. “Idyllic life at salmon beach outweighs landslide risk for many.” (2001). Seattle Post-Intelligencer