HomeMy WebLinkAboutDundee Place Block 27 - Report of Geologically Hazardous Area Investigation 2003.01.23G. W. Thorsen, Consulting Geologist
927 56th Street, Port Townsend, WA 98368
(360) 385-6002 (also fax)
thorcogw@olypen.com
. M (ltr,-TL
fa"rcll4'tVg-o clz-Zo cl
I u.,*Jr. P l^re
Bl,t*L 27
Lo* U*7
Dobson Construction Inc.
42?Discovery Ridge Road
Port Townsend, WA 98368
January 23,2003
Report of my investigation of January 10, 2003. This investigation, a review of literature, and my 15
years of experience in the Port Townsend area together convince me that this site is not a 'geologically
hazardous area' as defined in Section 19.05.100 of the Port Tovmsend Municipal Code. The following
description summarizes my reasons for this conclusion.
Site location and description: The subject property consists of two lots (nos. 6 andT,block 27) south of
the T intersection of Silver Street (at the north end of Sheridan) and Umatilla Avenue. The combined lots
result in 100 feet in frontage along Umatilla and 125 feet along Silver (total 12,500 sqrxre feet). The
planned home (two story) would have a 1,000-square-foot 'footprint', which, combined with the 500-
square-foot of surface of the driveway and tumaround, would occupy 12 percent of the lot surface.
Slopes: The aspect of local slopes is northeasterly with elevations ranging from about 170 to 195 feet.
When Silver Street was cut, soil from the cut was all deposited on the subject properly, leaving a berm
along this side of the street. The steepest slopes on the property (38 degrees) are near the west comer
(along Silver) on a short pitch (about 30 feet) from the top ofthe berm to the natural surface to the east.
The steepest natural slopes, mostly southwest of the proposed homesite average about 31 percent. Slopes
below the site are 11 percent, and the average slope of the site (excluding the nalrow berm) is about 17
percent.
Slope stability: Douglas-fir to about 14 inches diameter on the cut slope of Silver Street as well as the
berm indicate the age and stability of these man-made slopes. Neither these not the adjacent natural slopes
had topographic or vegetative evidence of past slope failure. No water-loving plants, suggesting
potentially destabilizing concentrations of ground water, were found on the property.
Soils: The property is at a mappedboundary between Hoypus and Clallam soils. The scale of the soils
maps is such (i in. : 1,800 ft) that they cannot be relied on for l00-foot-level precision, nor did the soil
scientists who did the mapping pretend such accuracy, especially near unit boundaries. The soil exposed
in the Silver Sheet cut, in a pit near the center of the property, and in the root ball of a blow-dovm fir all
indicates glacial till rather than outwash as parent rnaterial. The 3-foot granite boulder near the center of
the properly is firrther evidence of a till rather than outwash orign.Thus the exposed soil on the site
matches the description of the Clallam soils. Further soil exploration (digging more prts) does not seem
walranted considering site characteristics, the consistency of existing soil exposures, and the proposed
land use.
The 'topsoil' on the Clallam series is described in the U.S. Deparhent of Agricultrne's "Soil Survey of
Jefferson County"as well drained but has a 'very slowly permeable cemented layer at a depth of 20 to 40
inches'. This range coincides with the depth of root penetration (in contrast to the Hoypus soils where
Page 2 of 3
'roots penetrate to a depth of more than 60 inches'). This shallow depth to a cemented layer can also
resultin seasonal zones ofsaturation. Shallowroots and saturationbothtendto increase potential
blowdown hazards. Thus, current plans to cut all of the larger conifers here are probably a good idea.
Vegetation: Most of the site is covered by a canopy of Douglas-fir and madrone as much as 2 feet in
diameter. Smaller willow are sparsely scattered but abundant along the south property bomdary. The
understory consists largely ofocean spray. A natural ground cover ofsalal exists onmuch ofthe property
but is in places being overwhelmed by ivy. The rvy is also climbing many trees on the property.
Seismic response: This site, like the rest of Port Townsend, will be severely shaken by an e4rthquake of
historically unprecedented intensity. Quakes even shonger or closer than the 1946 Vancouver Island
quake or the 1965 Seattle quake (both shongly felt in Port Tovmsend) are unlikely to result in significant
damage to a frame house built to current codes. Soils here are not subject to liquefaction. Other forms of
ground failure (e.g., compaction or landslides) are not a problem in the natual soils and slopes that exist
on the site.
Discussion: As mentioned earlier, the 1,000-square-foot footprint of the proposed house and the short
drive and turnaround (total 500 sq ft) occupy only I2percent ofthe 12,500 square feet ofthe two lots.
Assuming an additional 500 square feet of house due to possible changes in plans and a total of 500
square feet in patio and paths, the exha footage would mean 20 percent of the lot would be covered by
impervious surfaees. (This would still be significantly less than the 25 percent limitation cited in Section
19.05.060-5a for a slope averaging 17 percent.) Thus, there should be no problems in keeping the total
area of slope disturbance (site clearing, grading, plus work area for construction equipment) to less than
the 60 percent recommended (19.05.100-D.2.b.v.). That fact, the dense vegetation, and the gentle (llyr)
slope toward Umatilla from the developed site all suggestthat there is little chance of sediment moving
off the site. Nevertheless, there are obvious reasons for avoiding construction during the rainy season if
possible. Also, disturbed till can be very difEcult to compact as backfill when too wet. .
The mix of native vegetation at the site and the well-drained soils can simplifi disposal of storm drainage.
Evergreens such as salal and madrone can moderate precipitation events by interception as well as
evapotranspiration of moisture that reaches the ground. (Ivy can serve the same roles but is hard to control
and tends to choke out native species, even trees.) Plans to minimize the area of clearing are an added
plus. Use of native species (e.g., rhododendrons, evergreen huckleberry, kinnikinnik) for landscaping can
minimize watering needs as well as disease and deerproblems.
ln such a setting it would seem that dispersal (rather than collection and control) of storm drainage should
be considered. The problem with concentrating runoffis'lvhat to do with it then?" Structures such as
French drains require more land disturbance, add to costs, and my not be necessary. Careful thought to
design and construction might preclude the need for such solutions. Two obvious technologies to
minimi2s runoffconcentration are to design surfaces (e.g., driveways, patios, decks) to be 'leaky' or with
a slight outslope. For example, a patio of flagstone in sand rather rhan tile in concrete or a driveway and
tumaround uniformly 'outsloped' slightly rather than 'insloped' with drains (to urhere?) might be used.
$imilarly, a deck of spaced lumber will not present the runoff concentration of one based on plywood.
Page 3 of 3
Conclusions and recommendations: The site is not an "Erosionally Sensitive Area" as defined in the
Municipal Code Section 19.05. Development following best managementpractices and applicable City
codes should not significantly change this.
Recommendations for site teatnent include:
o Confine the area of disturbance of native vegetation to necessary site clearing, grading, and
. work access.
. Control ground iny and cut it on trees. (It need not be removed, simply killed.)
. Disperse (rather than concentrate) runoff from impermeable surfaces.
. Minimize area of impermeable surfaces.
. Use native vegetation, preferably evergreen, for landscaping.
Should you or City personnel wish to discuss any part of this letter or meet onsite, please let me know.
Gerald W. Thorsen
GERALD W.
14
9
1l
,+
4 3
2
4 2
'+
7
11 10 11 14
2 1 44 3 1
o ?Jr,
*
Jr>(
.ru
I
I
IIYDRANT €Ld,W'= o+Olg1n
IIYDRANT (FLOVtr = fo0-999g
I{YDRANT (FtOTy = l000g1m
\rALrvrE )"( cLoSI
wa,ruRft,ffiTER.
AIRREI,reF
BI,OW.OF'F
WATER I!S{E (SlZq, MATERIT
WA.TER. IIISE (LATECOIvER I
SSRV,ICE L.BIE
PRO.PCISEP F"IIPELINE N@RO](FRo'h4 1,99.e wATEn systpM
ruqfIZO}$E E€EJT,TDARY
FUTURE }fiGH ZONE BOI,ND,
WATER LINE EASEMENI
'-:-.!+---
.{'-iFD
k
v-Tz
rol-34
3t
00l.}li
t2
00r-3-sE
I4
ui
5
l0I-33-S lOl-3ILSW
35 40
00I.+N8 001-aNw
l6 l3
001-3-SCr
l5
oota-si
l8
CIFY O,F'
PORTT€}.\TTI\T*$E}:{E
181 QsiBgy Srreer-
Port Townsead, UtAgS6g
(3.60),3813000
SIffiET
}IIATERMAP
13 0F 48