HomeMy WebLinkAboutBLD1999-00682 Geotechnical Report G. W. Thorsen, Consulting Geologist
1926 Lincoln Street, Port Townsend, WA 98368
(360)385-6002(also fax)
thorcogw@olypen.com
Geneva Austin and David Carter Aug. 25, 1999
2740 East Marrowstone Rd.
Nordland, WA 98358
Subject: Bank stability of AP #021-332-008, at 2740 East Marrowstone Road, Jefferson
County.
Purpose and scope: The Jefferson County Planning Department requested a geotechnical
report to address slope stability at the above site. The proposal to be addressed was the potential
impact of replacing an existing home with a new home at the same location. (The site already has
approval for a new two-bedroom septic system.)
My investigation included an inspection of the site from both beach and uplands, as well as a
traverse of the bank. Exposure of geologic materials on the bank seemed adequate for purposes
of stability evaluation (see photos). Thus no subsurface exploration(e.g., pits, boreholes, or
geophysical surveys) seems necessary or justified.
This report is also based on a review of published reports and maps (see references). It is also
based on a general understanding of area geology and geologic processes acquired from personal
observations since 1968. Such studies included the mapping of the geology and slope stability of
Whidbey Island for the Coastal Zone Atlas series.
Site description: The property is an approximately 2-acre parcel between the East
Marrowstone Road and Admiralty Inlet. The building site is near-level, with about a 2% slope
toward the shoreline (east) and an average of about 5% slope to the west. The existing home is a
minimum 47 feet from the slightly uneven bank edge.
Bank height is, of course, an important element of any bank analysis. Interpolating between
contours of the best available topographic map (U.S. Geological Survey Nordland quadrangle)
one gets an approximate elevation of 70 feet. Correction for the U.S.G.S. datum (mean lower low
water) and for the toe of the bluff being above the average high tide (more than 6 feet above the
datum) gets us a theoretical bluff height of roughly 60 feet.
Thus, the published map (never intended for such site-specific use) is in reasonable agreement
with my profile (attached) showing the,bank here to be 55 ±2 feet high. In preparing the profile I
used a variety of techniques, including altimeter, clinometer and tape, and hand leveling. It is not
a survey.
Bank slopes vary somewhat across the approximately 240 lineal feet of frontage. Much of the
bank is a grassy surface sloping 40 to 45 degrees. Slopes in scattered bare patches range from
about 60 degrees to vertical. The overall average slope is about 50 degrees. (The irregular aprons
of loose sand along the toe seen in photo A slope about 35 degrees but were not included in the
overall slope average.)
Geology and soils: Published geologic maps (see Selected references) for Marrowstone
Island are in general agreement. This is no surprise as geologic and slope stability maps for this
area are all based largely on field mapping in the mid-1970s by Martin Gayer(1977 M.S. thesis,
North Carolina State University). All show a bank made up almost entirely ofadvance ' Y h
(sand with minor gravel)capped by the glacial till that covers most of the uplands off:
Marrowstone Island.l. My site study in general confirms this earlier work(see photos).
A minor exception to the sandy glacial sediments is a lens of silt about two-thirds of the way up
the bank.This silt extends only across the northern half of the bank (photo A). Where present; t
perches ground water,which may have contributed to the small slide from last winter's
rainstorms. No evidence was found of deep-seated instability associated with this layer.
The soil survey of Jefferson County shows the uplands in this area to be underlain by the Dick
loamy sand, 0-15 percent slopes". This member of the Dick series (mapped as DcC) is described
(McCreary, 1975, p. 18) as
"...somewhat excessively drained. Permeability is rapid. Roots penetrate to a depth of
more than 60 inches. This soil holds about 4 to 6 inches of water available to plants.
Runoff is slow, and the hazard of water erosion is slight."
The same report described the soil on these slopes to have"slight" limitations for"dwellings
without basements".
My traverse of the bank discovered a somewhat oxidized sand 2 to 3 feet thick at the top,
overlying the glacial till. This sand apparently thickens to the west (or the underlying till is
gone?). Either option would explain the soil mappers' description and the tan or brown loose
"loamy sand" described in the logs prepared during drainfield exploration.
Slope stability: This bank, like all Puget Sound shoreline bluffs, is eroding. This stretch of
bank, including adjacent properties, has been mapped as Urs (unstable—recent sliding) in the
Coastal Zone Atlas. Atlas mappers, confined by the scale of their maps, could not show a slide
area much less than 200 feet wide. Thus they showed extensive shoreline areas as Unstable,
recent slide(s) or simple as "Unstable". Such judgements were commonly based on the
proportion of bare bank, unless there was topographic and mappable evidence of an individual
deep-seated landslide.
The straight toe of the bank, the almost straight upper bank edge (photo A), and the lack of
topographic features indicative of mappable landslides confirms the Atlas mapping. Even though
stretches of bank have "grassed over", the lack of maturing trees supports the concept of active
erosion certainly in the long term (50 to 100 years?).
Given that the bank is"unstable" (i.e., eroding) and that the glaciated uplands are stable, we are
then confronted by the question of"buffers" or construction setbacks. This revolves largely
around the question of normal erosion modes and the average erosion rate.
The erosion modes here are superficial rather than deep-seated. Thus, a given slide event such as
is illustrated in photo D may result in no significant short-term (10 to 20 year?) retreat of the
bank "edge". Such slides involve the veneer of colluvial soils that overlie the glacially
compacted parent materials. Slide materials that reach the upper beach are eventually eroded by
storm waves coincident with high tides, thus preventing the formation of a buttress at beach
level.
There is apparently no hard data (actual measurements) of long-term erosion rates along eastern
Marrowstone Island. Keuler's (1988) study of erosion modes and rates probably relied mostly on
studies of aerial photographs taken over an extended period (1942-1985?). This study shows the
subject site to be in a"zone of relatively slow wave erosion—relatively frequent but small
landslides common".
In summaryi available data from several published sources tend to agree with my observations
that erosion here is rather slow and steady,. I estimate that the long-term (20+ year average) rate
of bluff retreat is on the order of 2 inches per year:This may appear on-site and in the short term
as no perceptible erosion for 5 to 10 years and then the loss of a foot or so in an instant. This is
normal for geological conditions such as those at the site.
Discussion: The foregoing is provided as background to this portion of my report. Here, I will
attempt to respond to appropriate parts of Jefferson County's "Critical Areas Ordinance",
specifically Section 9: Geologically Hazardous Areas. (Comments in parentheses are mine.)
Section 9.302.2 describes "landslide hazard areas" as:
Areas potentially subject to mass movement due to a combination of geologic,
topographic, and hydrologic factors including:
a: Areas of historic failures or potentially unstable slopes such as:
(i) areas described and mapped as having severe or very severe building
limitations...
(Soils maps show building limitations here as "slight".)
(ii) areas described and mapped as recent or old landslides within the...Coastal
Zone Atlas.
(The Atlas map shows the proposed upland building site as "stable", fronted by a narrow sliver
of"unstable recent slides" on the bank.)
(iii) areas described and mapped as areas of poor natural stability, former
landslides... (see references: Gayer, 1976).
(See previous discussion of mapping history in the geology and soils section of this report.)
b. Areas potentially unstable as a result of..undercutting by wave action.
(The bank fronting the proposed building site is now unstable—the building site, now stable,
with little potential for seismic liquefaction. Site will likely become unstable or at least
threatened due to a combination of wave action and slope processes within the next 200 years.)
c. Areas with any indications of earth movement, such as:
(i) rockslides; X
(ii) earthflows; X
(iii) mudflows; and X
(iv) landslides
(As previously mentioned, colluvial soils on the bank fronting the property are subject to
superficial landslides. The underlying geologic materials are eroding but not sliding. Re: adjacent
uplands, see b. above.)
Section 9.506 Buffers— Standard Requirements: The following landslide hazard area buffer
provisions shall apply:
(Subsections 9.506.1-9.506.6 describe the purpose and management of buffers and how to
measure them. Subsection 9.506.2 states that:
The appropriate width of the landslide hazard area buffer shall be determined by either;
application of the standard buffer width set forth below(the standard buffer, 30 feet has
• been rejected by the "Permit Center" for this site); or, an individual or firm meeting the
criteria of subsection 11.702. (I apparently meet the criteria or qualifications to prepare a
Geotechnical Report in support of buffer widths.)
Section 9.508 Increasing Buffer Widths: The Administrator may increase the standard landslide
hazard area buffer width...
lagree with the Administrator that the standard buffer of 30 feet would be marginal in this
situation and recommend one of roughly 45 feet to the proposed house foundation.)
Section 9. 509 Geotechnical Report: (sets out the conditions and contents of such a report).
Subsection 9.509.2 states that"the triggering application shall not be approved unless the
geotechnical report certifies all of the following:
a. There is minimal landslide hazard...
(It is my opinion that the proposed home will not be at risk within the next century—
probably well beyond.)
b. An analysis of slope stability indicates that...the landslide hazard area can be
modified so that hazards are eliminated.
(A properly designed and constructed bulkhead would slow the gradual westward
progression of the "landslide hazard area" but would not, in the long term (50+ years)
"eliminate" it without bulkheading adjacent banks. Such a precedent could start a domino
effect that would, in my opinion, be detrimental to beaches in this area.)
c. The proposal will not increase surface water discharge...to adjacent properties.
(The proposal simply replaces an existing old house with a new one of similar size. Local
soils are quite permeable. It is a large (2+ acre) lot, with the house near the middle and
gentle upland slopes. It would require considerable planning and expense to "export"
runoff from the house to adjacent properties.)
d. The proposal will not decrease slope stability on adjacent properties.
(The replacement of the old house with a new one and some of the landscaping
recommendations herein should result in no measurable decrease in local slope stability--
and possibly a slight increase.)
e. All newly created building sites will be stable...
(No new building sites are proposed.)
Conclusions and recommendations: No evidence was found that the proposal to replace
an older home with a new one of similar size will decrease bank stability at the site or on
adjacent properties. Medium-term (20 to 30 years?) stability might be increased somewhat by
appropriate wave protection(not in the proposal), but the potential off-site impacts seem too
great. The proposal should not cause an increase in surface runoff Current plans to preserve
mature conifers on the property warrant encouragement.
Setbacks:
• Set the foundation as far as practical while still preserving the large fir trees to the west.
I suggest a minimum of 45 feet.
Vegetation m ourgent t:
• Preserve existing evergreens (including madrone) wherever practical.
• Re-establish a 10-foot buffer of native brush (e.g., salal, Oregon grape, wild rose) along
the bank edge. This could be trimmed to 2 feet to enhance the view.
• Encourage trees and deep-rooted brush on the bank surface. (Cuttings of native willow
should start readily at the silt layer.)
• Emphasize drought-resistant native evergreens (rather than grass) for landscaping.
Drainage: t
• Emphasize dispersal at the source rather than "collection and control" (e.g., minimize
impermeable surface areas, make patios leaky, crown rather than ditch or berm driveway
surfaces).
• Slopes (and local water supplies) could make this an ideal site for a pond. Any such
retention system would need to be lined and have properly engineered overflow.
Beach access;
• Foot traffic on the bank should be discouraged so that the colluvial soil remains
undisturbed.
• An engineered beach access seems unjustified considering its expense and the
proximity to the public access to the north.
_ / ✓ 1525 ; �gill/
Gerald W. Thorsen, .G. % 5 MPG I « y
C.
References �``,\*to�W "- -
Gayer, M. J., 1976, Geologic map of NE Jefferson County,WA: Washington Division of Geology and
Earth Resources Open File Report 76-21, 1 sheet, scale 1:24,000.
Gayer, M. J., 1977, Quaternary and environmental geology of northeastern Jefferson County,
Washington: North Carolina State University M.S. thesis, 140 p.
Grimstad, Peder; Carson, R. J., 1981,Geology and ground-water resources of eastern Jefferson County,
Washington: Washington Department of Ecology Water Supply Bulletin 54, 125 p., 3 plates.
Keuler, R. F., 1988, Map showing coastal erosion, sediment supply, and longshore transport in the Port
Townsend 30-by 60-minute quadrangle, Puget Sound region, Washington: U.S. Geological Survey
Miscellaneous Investigations Series Map I-1198-E, 1 sheet, scale 1:100,000.
McCreary, F. R., 1975, Soil survey of Jefferson County area, Washington: U.S. Department of
Agriculture, Soil Conservation Service, 100 p., 70 plates [sheets].
Sinclair, K. A.; Garrigues, R. S., 1994, Geology, water resources, and seawater intrusion assessment of
Marrowstone Island, Jefferson County,Washington: Washington State Department of Ecology Water
Supply Bulletin No. 59, 83 p., 5 appendices.
Washington Department of Ecology, 1978, Coastal Zone Atlas of Washington, Volume 11, Jefferson
County: Washington Department of Ecology, 10-p. text, 16 plates, explanatory materials.
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Photo A. Overview of the site's bank. Vertical dart indicates the plane of cross section(see
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silt in the upper bank (see Photos A and B). Note the
concentration of green vegetation, indicating year-round ground water at this elevation. The
shallow slide (see Photo B) apparently originated at this level.
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Photo D. The broad but shallow slide just south of the property. Dart points to a 3- to 4-foot
scarp. Note that the movement has only minimally affected the bank edge (at the top). This
slide might possibly have been triggered by the tree falling from the bank.
RECEIVED
G. W. Thorsen, Consulting Geologist
1926 Lincoln Street, Port Townsend, WA 983iNOV 2
(360)385-6002(also fax) JEFFERSON COUNTY
thorcogw@olypen.com DEPT.Of COMMUNITY DEVELOPMENT
Sue Theiss November 21, 1999
Jefferson County Permit Center
621 Sheridan
Port Townsend, WA 98368
Re: Critical Area Review Case No. CAR 99-0109, October 12, 1999 (2470 E.
Marrowstone Road, Carter/Austin)
Dear Ms. Theiss:
Mr. Carter has asked me to try to clarify a few items in the "Conditions of Approval". (He
is apparently travelling, as I have been unable to reach him regarding the above review.)
Most of his concerns involve the "maintenance"of a 30-foot buffer of native vegetation
along the bank. In order to better place this matter in perspective, I have attached an
enlargement of a section of aerial photo "DOE Jefferson 241 OB, 6-12-77".
Condition#3 cites the Critical Areas Ordinance as requiring "a native vegetative buffer of
30 feet be permanently maintained." Condition#8 states that"associated buffers shall
remain naturally vegetated with native vegetation." The wording of these conditions
suggests that this area is in native vegetation now, although the aerial photo clearly shows
that uplands fronting the house have been clear of trees and brush for decades. (Possibly
the grass is "native" and/or some of the area was a natural meadow.)
Even if we set aside the semantics and the question of whether the Critical Areas
Ordinance was meant to be retroactive, Mr. Carter's concerns about the 30-foot buffer
requirement seem valid. As he suggested to me, efficiently revegetating an area of this size
(30 x 240 feet) might require major soil disturbance,probably including killing the existing
cover of perennial grass. It would take several years,probably including summer watering,
for the new plantings to become established and provide the same level of cover as the
existing grass.
He also asked me to try to get clarification on Condition of Approval#13. His question is
in regard to the applicability of the "small parcel erosion and sediment control plan."
Specifically, the question involves the "triggering mechanism" for this requirement. The
potentially applicable section of the ordinance is the one relating to "construction of a
single family residence or duplex which will create more than 3000 square feet of
impervious surface." The proposed residence is less than 3000 square feet in area; in
addition, it simply replaces/reoccupies an existing impervious surface.
In summary,then, would you review the attached 1977 photo showing the traditional
development patterns of the area. (Tillman Engineering should be able to answer any
questions regarding the specifics of the proposed residence.)Please note the
• Homes closer to the bank than the proposal
• Well-established grass at the site
• Erosion patterns essentially the same as those of today
•No evidence of surface runoff
In general, this appears to be one of the least challenging shoreline sites I have seen in
years. The expansion of my proposed 10-foot bank-edge buffer to 30 feet would at least
temporarily increase the environmental impact of the proposal. Would it enhance "public
health, safety, and welfare"?
I would be happy to meet you on the site to discuss your concerns.
Sincerely,
( \-1
Gerald W. Thorsen, C.P.G. ti- Eft oFh4y
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