HomeMy WebLinkAboutIsland Vista Lot 13 - Geologic Reconnaissance - 2000.09.09Clet2 K-)dszr
G" . Thorsen, Consulting Geologist
927 56th Street, Port Townsend, WA 98368
360) 385-6002 (also fax)
thorcogw@olypen.com
Rev. and lvlrs. Jim Finkbeiner
620 North lTft Place
Monnt Vernon, WA98273
September 9,2000
Subject
Geologic recon, Lot 13, plat of Island Vista.
Purpose and Scope
Although the proposed building site is dry and nearly level (4%o slope), the adjacent bank to the southeast
averages about 33 degrees (-66yA. The slope to the west is as much as 407oto 45% nplaces and is mapped
as a Sensitive Area. Thus, the City of Port Townsend has requested a geologic recon to detennine an
appropriate bufler (i. e., builrlin g setback).
The subject report is based on a visual examination of the site from the upland as well as from below. This
included a traverse of the bank except for the near-vertical upper 20 to.25 feet (see attached profile). I also
reviewed available soils information and geologic mapping. Because the upper bank soils and geologic
parent materials are well exposed and consistent across the front of lot 13 as well as adjacent property, soil
pits seem unwarranted. My conclusions are also based on more than 30 years of experience studying the
geology and geologic processes ofPuget Sound's shoreline bluffs and forested hills.
Site Description
Soils: This area has been mapped as "Townpend fine sandy loam, 0 to 15 percent slopes (TlC). This soil
occurs on uplands fringing shoreline bluffs. Strong upslope winds are especially erosive in drier rain-shadow
areas such as here and the west sides of Protection and Whidbey Islands (where bluFtop dunes may exceed
20 feet thicD. Such erosion (and deposition) is greatly diminished when slopes become yegetated. This
wind-deposited soil is about 10 feet thick here (see profile) at the bluffedge but no doubt thins inland. ds
mentioned, this member of the Townsend Series is nol derived from in-situ weathering processes.
From this depositional history it is obvious that both the soil and its zubstrate are quite different from
Townsend gravelly loam" (TnC) which'Tormed from very compact glacial till". Possibly the reason these
soils are gpuped in the "Townsend Series" (McCreary ,1975,p. 44,45) is because of their close proximity
and the fact that TnC is also veneered by a wind-bloum deposrq in that case silt rather than sand. Other than
that, the general description in the soil survey of a "cemented layet'' of very low permeability at a depth of 24
to 36 inches is misleading and does not apply to TlC, at least at this location I qrrestion whether these two
soils belong in the same series.
Geologt: Published geologic mapping for the area (Gdmstad and Carson, l98l) is based largely on field
work done by one of Carson"s graduate stude,nts (Gayer, 1977). Gayer shows the bank in this area as Qva
sediments deposited by the meltwaters from the advance of the last continental glaciation). The practical
implication of this work is that except for the uppe,most sand, the sediments making up this bank have been
compacted by the weight of about 4,000 feet of ice and are quite cohesive.
These sediments are more exposed o* rho banks to the northeast of lot 13 than he- fhis is in part because of
a layer of silt in mid-bank that tends. to form a vertical section about 5 to I 5 feet high. This silt perches
ground water and creates a horizon of seepage during the "rainy season". Neither the vertical bluff section
nor a horizon of green vegetation indicating the presence of this silt and perched ground water can be seen
fronting lot 13 ornearby banks.
Topograplry: The bank fronting (SE) lot 13 was cut after locAl sea level stabilized about 6,000 years ago.
Since then waves have been able to focus at a consistent elevation and, over millsryf4, create the bluffs we
have today. Here, zuch wave action has been stopped by industial development (i.e., fills for the paper mill
and the railroad transportation system). Thus, wave action is no longer an erosional factor.
The steep draw to the west of the property is a "fossil" remnant of the last glaciation. I could find no
evidence of recent or even historic erosion. For example, we have had at least three major rain storms and/or
snowmelt episodes since the fall of 1996. There is no evidence of erosion or disturbance of forest litter in the
floor of this draw. Sloppy development'trpstream" or careless disposal of runofffrom streets could create
erosion in the draw, but the potential for runoff from lot 13 is negligible due to the insignificant portion of
the "drainage basirr', permeable upland soils, and dense slope vegetation.
Vegetation: The vegetation along the flanks of the steep (4045 %) draw to the west of the site (a "sensitive
Area") consists mainly of dense Douglas fir (to 2* feetin diameter) and ocean spray. Neither shows evidence
of soil movement. Transversesof the bank found no evidence of water loving plants that could be an
indication of potential instability (nor were there topographic signs of previous instability).
The southeast-facing bank above the abandoned railroad grade is grass-covered here. Banks of similar slopes
and geolory to the north zupport extensive patches of Oregon gape. Such deeper rooted woody vegetation
can anchoi surficial loose soils to the glacially compacted subsoils much better than can grasses.
Even though there is no evidence of shallow sliding here. The existence of such to the north suggests that
some vegetative enhancement on this seaward slope might be wa:ranted in the long term. Any higlr-tech
approaches, such as mini-terracing and plantings, would be expensive and possibly disturb the delicate
equilibrium of the bank and its grass cover, even if you.could get the land owner's pennission to do such
work. Possibly a low-tech approach, such as simply tossing Oregon grape and salal berries or Douglas fir
cones ontd the grassy slope, wouldbe worth aby.
Drainage: A goal of development should be to minimize any hydrologic impacts of development. This
should not be too difficult, grven the nature of the soils. The ken in my opinion, is to emphasize dispersal
rather than "collection and concentration''. (The problem with the latter is: what to do with the concentrated
storm or rare snovrmelt runofi?) Another facet of drainage planning is to minimize or avoid importing runoff
from the sheet and cul de sac upslope.
Some approaches to the dispersal option are:
Intercept (via water bm?) and disperse any imported water as far from the westem slope as possible.
Croum driVeway or pave with a chewon surface to disperse direct precipitation..
Outslope parking areas or turnarounds
Minimize long roof gutterruns; provide for adequato dispersal of downspout water.
Make patio and path surfaces "lealqr" (e.g., use flagstone, bark, gravel, etc.) rather than paving them.
Seismicity: Both the 1965 "Seattle earthquake" (actual epicenter in the Federal Way area) and the 1946
Vancouver Island quake were strongly felt ,in Port Townsend. I fomd no evidence of ground failure, either
landsliding or soil liquefaction" from these events. Historically unprecedented ground motion, zuch as from a
futwe subduction zone event along the coast or a shallow Puget Sormd evenl night cause "slabbing off'of
some local nem-vertical bank segments along pre-existing cracks in zuch glacially compacted sediments. For
exanple, the upper bank silt here and the till banks downtown migbt "shed" 3 to 5 feet, but vibration damage
should be ofgreater concern lhan ground failtne here.
o
Discussion
Whereas the bank fronting Vista Boulevard and lot 13 was largely formed by wave action, it is obviously no
longer a shoreline bank. The construction of the railroad and its sprn to the paper mill (in the mid-'2Os?)
created a buttress about 60 feet thick along the lower bank. The toe of this buthess is protected by a blanket
ofjetty rock. Thus, the bank has been isolated from the shoreline for more than 70 years by a broad strip of
indushial development
For all practical puqroses, the bank here is about 85 feet (+1 foot) high (above the railroad grade). The toe
protection (i.e., the grade fill and its rock buthess) is essentially "bullet proof in such protected waters. The
proposed location for the single-family residence is more than 250 feet from the shoreline. I suggest a
setback of at least 50 feet, more for long-term practical reasons (e.g., vegetation buffer, patio,landscaping)
than for structural safety. (It should be noted that some homes to the northeast with a higher bank and less toe
protection are within 35 feet (or so) of the bank edge.)
In regard to the "sensitive area" of steep slope to the west of lot 13, I could find no evidence of slope failure.
There tue no topographic signs zuch as bowl-shaped amphitheatres. There are no benches such as might be
caused by perched ground water. I found no "stripes" of same-age vegetation zuggesting debris avalanch'es
within the age of the conifer cover (50 yr + ?). Nor did I find "levels" of water -loving vegetation that might
indicate a focus of potentially destabilizing ground water for the future. A few moderate "leaners" in the
conifer forest can be explained by wind gusts drning periods of wet soils - a common feature on even flat
uplands.
In summary, althougfu slopes in the area may be steep enough to fall within a "sensitive Area" category,
there is no evidence that they have a potential stability problem. In my opinion, one could safely construct a
daylight basemenf'home at the top of the west-facing slope (i.e., a home with zero bank setback).
However, even if the City would permit it, I do no think it would be a good idea as it would limit options for
stonn water dispersal. Obviously, focused storm runoffcould cause erosional problems, but common sense
and the retention of existing native vegetation can avoid this potential.
Conclusions and Recommendations
Slopes on and adjacent to the properly are naturally stable except for the bare upper 20 feet (or so) ofthe
seaward bluff. That section will eventually erode until it reaches the same angle of repose as the grassy bank
below. Thus, with present erosion rates and modes, in 200 years or so the bank edge might be 20 to 25 feet
farther inland than now. Even so, the lot size and local geologic and soil factors suggest that a safe and
environmentally sensitive home could be built here with minimal "shetching" of city buffer concems
regarding either bank. bn regard to the wooded west bank, note that in the *guidelines" for Consulting
Geologists, item 5, it states that "geologically hazardous areas include those "10 feet from the top of the
slope", not._25 feet.
The following recommendations are not in any particular priority, nor are the numbers suggested exact.
Reestablish a buffer of native vegetation , zuch as Oregon gape, salal, or wild rose, at least 10 feet wide
along the bank edge.
IJse a "low-tech" (see Vegetation) approach to established deeper-rooted native woody vegetation on the
curently grass-covered slope to the southeast.
Minimize distmbance of natiye vegetation on the upland surface. Cut trees only as necessary for
conskuction or to remove potential blow-doum hazards.
I Get rid ef Himalayan blackberry a"-.-g the street access, It will spread and chok rt desirable native
specres.
Adopt construction design and practices that encouage dispersal of storm runoff(rather than collection and
conhol - see Drainage).
Set the house fotrndation (not the deck) back at least 50 feet from the seaward bank.
Remind your architect/engineer/builder that the nonstratified sands over most (all?) ofthe lot are not
glacially compacted -- in contrast to most subsoils in Port Tovmsend. The uncompacted nature of the soils
may require consideration in foundation and/or drainage design.
Gerald W. Thorsen, C. P. G.
References cited:
Gayer, M.J., 1977, Quatemary and envifonmental of northeastem Jefferson County, Washington;
North Carolina State University M.S. thesis, 140 p.
Grimstad, Peder; Carson, R. J., 1981, Geolory and ground-water resources of eastern Jefferson County,
Washington: Washington Deparhent of Ecolory Water Supply Bulletin 54,125 p., 3 plates.
McCreary, F. R., 19?5, Soil survey of Jefferson County area, Washington: U.S. Deparhent of Agriculture,
Soil Conservation Service, 100 p., 70 plates [sheets].
AItrG
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