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Home My WebLink About989600017 Geotech AssessmentG. W. Thorsen, Co~asulting Geologi. 1926 Lincoln Street, Port Townsend, WA 98368 (3G0) 385~i002 Jcff Ingman May l8, 1999 23611 Glen AI(en Pl. Mt. Vernon, WA 98274 Subject: Lots 17 - 21 Prospect View Estates just south of Prospect Avenue and Kala Point in Jefferson County. Also, a single building site composed of lots 20-24 of the old Irondale Block Ibl, adjacent to and north of the end of Prospect Avenue (see accompanying plat map). Purpose and scope: This report is to help provide guidance for the residential development of the lots described above. It is also in support of the 50-foot setback for construction on those lots. Lots 17-21 were given such a variance by the county commissioners on April 24, 1989. (County staff have asked for an update on the geologic report upon which that variance was based.) Geologic input for the report consisted of a review of the old report and available maps and literature (see Selected References). New field work consisted of examination of exploratory pits (not available in 1989), another recon of the bank (see Profile), and photographing the site from the uplands, beach, and the air. Site description: The subject lots include about 800 feet of uplands along an easterly facing bank just south of Kala Point (see area map). In general, the uplands are planar, with average slopes of about 3-4 degrees (5-7 percent) to the southeast. The bank averages 40-43 degrees, including anear-vertical segment as high as 1 ~ feet fronting lot 21. This steeper segment is lower or missing in places to the north. Total bank height ranges from about 100 feet at the south lot (#21) to about 110 feet on the north. The bank is largely underlain by sand and pebbly sand of glacial outwash origin (Grimstad and Carson, 1981). This unit was compacted by the last ice sheet, here about 4,000 feet thick. (Thus, it is quite resistant to root penetration.) This lower 80 feet (or so) or bank height is largely obscured by a wedge of colluvium that tapers from near zero in places at the top to as much as 8-10 feet thick along the upper beach. Exploratory pits and, in places, the upper bank expose a glacial drift made up of gravelly, silty sand. It appears to be of late glacial origin as it is not as compact as a "true" till (hardpan'. Nevertheless, its well-graded nature permits it to stand in vertical banks. This description and interpretation is in general agreement with soils maps showing "Hoypus gravelly sandy loam" in this area. McCreary (1975, p. 25) further describes this soil as: ...somewhat excessively drained. Permeability is rapid. Roots penetrate to a depth of more than 60 inches. The soils holds 2 to 4 inches of water available to plants. Runoff is slow to medium, and the hazard of water erosion is slight to moderate. Slope stability: The bank here was cut by wave action since sea level stabilized about x,000 years ago. Longshore drift later forn~ed the spit at Kala Point. The spit and the easterly aspect of the bank both serve to limit wave action at the site. Wave erosion from winter southeasterly storms is also limited by the rather short (2 mile) fetch in that direction. LAG !`~E[1~ # ~- ~ ~~~~ ~ ~€ f~': Natural erosion at the site now consists largely ol'crecp and localized dcbcis avalanching {aerial-photos A and B} of the colluviai soils that overlie the glacial-sands. The-role of wave action in recent centuries is merely to remove creep and avalanche deposits from the upper beach where they would otherwise accumulate and buttress the toe of the bank. The natural triggering mechanism for debris avalanching is generally an unusually intense rain storm or rain-on-snow event. The age of the alder in one slide scar suggests that it probably occurred during the New Year's Day 1997 rain and snowmeit that triggered shallow avalanching throughout Puget Sound shoreline banks. Little topographic evidence ofdeep-seated landsliding was seen. Such erosional forms tend to leave telltale signs, such as step-like topography, a deeply scalloped bank edge, or a sloping bowl-shaped "amphitheatre" on the bank. A broad scallopwith a chord of about _15 feet was seen at lot.l8,~but the lack of a scarp and the presence of fir to. l2-inch diameter on the bank suggest that this is either a very old feature or developed gradually. Seismic impacts on such materials are not apt to change current modes of erosion. Neither dired~ (springs, horizons of seepage) nor indirect (concentrations of water-loving vegetation) evidence of~~ perched ground water were found. Thus, the potential for quake-induced ground failure, such as soil liquefaction, seems remote. Both the 1946 Vancouver Island and the 1965 Seattle earthquakes were strongly felt in the Port Townsend area, but there were no reports of soil failures.-This was in spite of the fact that there are miles of shoreline banks in the area-that are more susceptible to such triggering than here. Discussion: We have been asked to address the addition of septic waste water on building sites. (It should be noted that this wilt be "imported" water, not on-site welt water simply being transferred from one depth to another.) This-water will be in part offset by the street/ditch-storm drainage system. That system, upslope-from the subject lots, should-intercept and "export" some of the surface: runoff that might otherwise reach the lots during extreme rain andLor snowme[t events. In regard to long-term day-today septic infiltration, it is important to consider potential impacts in the conte~.~t of local soils and precipitation patterns: Grirnstad and Carson (1981, p. 31, attached) show an annual "water budget" for nearby Port Townsend. Note that here there is a theoretical water surplus (available for recharge or runoff only 5 weeks of the year (in contrast to nearly 6 months at Quilcene). The rest of the year there is a soil water deficit and/or soil moisture is being "recharged". Research in the Spokane Valley (Crosby and others, 1971), where mean annual precipitation is slightly less (17.5 inches) than at Port Townsend (18.3 inches) produced some interesting results in regard to possible ground-water recharge by drainfields. (Glacial outwash subsoils there are similar to soils at the subject site.) The researchers drilled through drainfields at a nursing home, atrailer-park, schools, and a commercial dairy, all sites receiving effluent at.-much higher rates than asingle-family residence. In each situation, drill holes encountereddry soils at depths.of 40-feet or less below the drainfelds. The interpretation of the researchers was that there is a "capacity of the outwash deposits to retain much more water than. is characteristically present" (Crosby and others, 197.1). They explain the lack of long-term downward progression of saturation on such retention and subsequern evaporation to the atmosphere. They also speculate regarding possible "lateral dispersion" of soil moisture. This potential would be enhanced by well-stratified subsoils (not present at this site). In summary, there seems little chance that drainfeeld effluent could reach the bank and have a _ destabilizing influence thereon. LOG ITEM PagB ~ ~/ . Conclusions: The setback variance of SO feet granted by the Jefferson County Board of Commissioners (Apri124, 1989) should be ample to protect homes well beyond their useful Iifetime. The same setback distance seems appropriate for the nearby lot north of Prospect Avenue (see plat map). Recommendations: • Maintain and/or restore a I O-foot buffer strip of native vegetation such as salal along the bank edge. This can be trimmed low (2 feet) where necessary to enhance the view. • Minimize the disturbance of bank vegetation. Selectively limb (rather than top) trees where necessary for view enhancement. Control the blackberry fronting lot 21 as it will compete with desirable species. • Minimize the total area of impervious surfaces (e.g., decks, patios, and paths). All such areas should be designed to "leak". • Design drives and parking areas to disperse rather than collect runoff by employing techniques such as crowning, outsloping, grooving, or water bars. • Disperse roof nanoffas far from the bank as possible. • Emphasize low-maintenance, drought-resistant species for landscaping to minimize the need for watering. • Monitor the maintenance of street ditches to ensure that runoff is not imported onto bank lots from upslope areas. ~~~~ ~~ y Gerald W. Thorsen, C. P. G. Selected References .~`~,~`~~`~ ufar'N~f~~~~ y r ~ I525 ~''• ~~, a AIPG ~y ..-~°'s ~ Crosby, J. W., III; Johnstone, D. L.; Fenton, .''~ ~`1~g~1 ~~ - ` on of pollutants in a glacial outwash fs3`o 713-720. environment, [Part] 3: Water Resources Research, , p. Grimstad, Peder; Carson, R. J., i 981, Geology and ground-water resources of eastern Jefferson County, Washington: Washington Department of Ecology Water Supply Bulletin 54, 125 p., 3 plates. McCreary, F. R., 1975, Soil survey of Jefferson County area, Washington: U.S. Department of Agriculture, Soil Conservation Service, 100 p., 70 plates [sheets]. Washington Department of Ecology, 1978, Coastal Zone Atlas of Washington, Volume 1 1, Jefferson County: Washington Department of Ecology, 10-p. text, 16 plates, explanatory materials. ~ c~ l~ago ~ of~ y ~ ~ _ •...~0..~. 53 pro ~ `!„ ~ :}Sf:. a7 O` ••\~. ~ ,+ . • ' !a g 2 ~ ~ ( 1 3~ ++ /''~~ ~~ 1 ~ V~ + q p~ ~~~~ r I O D; O N E ~• =~ 1i M a I ~ ~ O r .•I I a l P R ~~ ~ c -i .' ~1 ~ ~~ o ,'~ 3~ `2'° •~`.%~&•.. ~ X24 - - ~~:$°• . --mss ~ :oo Piles Ljl °a r '~" b: • ~: t~ ~ s~ ..\ r r ~ M ~'• '` ``, `~ =gym ``6 `\` \ ,' `S° • xQ~ _ ~~ : i ~ 2 ~~~ `! i \1 b 2 3 ' ~ a 4 ~ aqp `~: 11 t ~ h s i ` \ t ~°m ~ r ,_ ~ + ~ a ~_~sa 'os ~ 334 ::~~~~~ ~ .. w s r~ ~::. \\ - . + `~ a `` ° ~•\\ r s ax-~~ a~ 'Lh7`~~\` 4~~~ 3M~ c°= ~ 24r f 1.K.at Point 2 I - - c o i.. ,: •~~ ZOO. ra~Y 'ferson County B I a s ;i`!~ ,~ motional Airport ~ sxe r r .b;r ~~ ' ~6 I ~ ~ ~ •~: M G= 120 •°:= a a a ~ 5- ~ • • `~` ~ . 100 r r •:~:. ~ ~~` 2 I _ ~ 3 :;a"~~~ Hers 33 I =_ : K: ~:~~ --. .~ ~~~`~~~~~~d~~ I 100 '<::::^ 1 ~ ~ \, ~ I Prea Substation N I ~ •~1•TOnda a '•.• J y c'b" '' i ~ !!--~~ ~ ~ er Perk ~J•' ~::. / ! ,~~ I _~~~ o 'o o~ " ~ I ~ ° ~ s'21 j , ~ • +i. T. 3C 00 I xx~ e ~c;~l o~ ~~ - - - ----- r22`~a --~'~ --- i :•~ 2'30 i a i N + ~ ~ ~B 3 ~, I • .. - -- _ ~~ ~ ~ _~ 4 N • 3 ~ ~ _~ Gravel '!~ 5326 a ~ ~ ~ t ndin ~StriP ~ Pit I _~ ,r Trails ark i Had -•~ r ~• ~ of 2' y Y. ' •i~. \ ~ "'o° yy,~ ~ p• Gravel Pits ro. 3= -r-->_ _ ~ _ 00 N ~ ]Q ' ~ + a :: Vicinity map from the U.S. Geological Survey ~`~` ~~ --- a ---- • - ~~` 0 0 4`~=:.-- r '_ ~ Port Townsend South quadrangle (1 in. = 2,000 fl; ~ _ 4 • ~ ~ contour interva120 feet}. Yellow dart points to the ~ ~. --- ~ L' ~ ~T ~ ~'' end of Prospect Avenue (see plat ma~0e .~f ®~~ ` i o •. , __L__ r .~ ~ .. Adapted from plat map of ~ ~ Prospect View Estates --- . ---r- 1----- --- ~ Subject lots enclosed by green line. ` I i i ~ ~ ~ ~ .,. _ ~ .. ' _ ~ Approx. top of bank ~ ,1 . ~' ~ ___r_-_ ~- i- ~. i ~ ~~- Approx. line of profile Prospect Ave. , ~~ o ~, 3 ..rv, . _.~ `1 -~-- ~7 - -~- i ~, ~~. ~ in. _ goo tt Cascade Ave. I- . , Slope angles are averages. ..L~~ I ~ E~ O ~. i~ ; , t; - ~ 4 ~~ : ,-I . ~ ~ . ~ -~ - °~ ~~ v , ~ L ~-`. ~ ,~ ~ ~ L ~, y i n ~ Z +I 11 - ~ ~ ~~3 _ V ~ '`` `~ ~ 0 ~' a • `o`tt (( ei . ~ h ~\ _ s i., ~ ~~ i ~ z . 3 ~ J o' ~~ ; _ ~ ~ ~~ 1 ~~ ~ a S ~ ~ ~a~~ ~~ PORT TOW"'SEND _ WATER HOLDIh_ 2++ 6++ CAPACITYOFSOIL PRECIPITATION 18.3 POTENTIAL 25 2 TRANSPIRATION . A RA A 14.4 17.7 WATER SURPWS 3.9 0.6 s From: .,rimsted and Carson POTENTIAL EVAPOTRANSPIRATION v~ 4 ACTUAL EVAPOTRANSPIRATION ~~ ~ ~~~ 1~~ _ ~ WATER ~ ~ ~ PRECIPITATION i~•-•'• DEFICIT ~ ~ •• ~ Z 2 ~~' ~r'~• ..• •• ~ IL MOISTURtn •, ••.•.•••••• ._ `"~~"~WATER SURPLUS `~_~~1 ~ -" SOtL MOISTURE UTILIZATION 0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC QUILCENE 2 SW WATER HOLDING CAFAgTY OF S01 L 2+~ 6++ PRECIPITATION 50.0 POTENTIAL EV ANSPIRATION 25 I ACTUAL V PO- TRANSPIRATION 17.4 20.3 WATER SURPWS 32.6 29.7 10 ~f'M~"~~•••:. PRECIPITATION '•••'~``''~~~_~~~+~`' W ''`~:J~~`~:~'.`,~`:.:-~:-`•.. EVAPOTRANSPIRATION •°.~"-''.~;'~WATEFt':; c.~ -}.•,<~~f;;..r;;.;:--::}}::::;'-}s:-~:. ACTUAL x~?SURPI.U; ;,.::~ n:::. . ~.::.:: z ~';;~;-'>'};~:::..:::.r;: -. EVAPOTRANSPIRATION ~ ~~:- n~~~.k•-~>: F~~:' yr ~ J'r •:j}i$in:i:%' Vii-`}:::-:v ' \ - ~ f•~'•:' ~, , bss.;r<f: k<;:_:> !~•• WATER ~ .;},.; ,:;.: ,--.~.~-.-e' :..~~.:,., :WATER .:,;<.< `~' r. DEFICIT }~.. ~r~~~-::SURPLUS:`::: `s:~~~__ •~~ ::;u;~;vz>x:-,.>.:~:: ;~':~-:y::-,::~;_fsr;.:;::%~ ?<.': 2 RECkIARGE .,,~„M:;;.'~fi,-,•,.... Q ''/'$.1-~r.} Ni~•'~.;'u`,s:i~.._.:<f::?}v~ C?'.%::~ ;ri~' ~~:al' kac~i ,x,.k:, `.::•Y•::.. ,wt~~~[r~~,.c'~fr!~y:,Niy Y%:.~%'Si>.':<,• =:/ ~~ 4~. 'aF2.C"=~:': J - .r Yom:{}>'f:;}Yr ~iij~?: "k.v.~,~41:, ~.i•?'~1~i:::: ".f}:`''}"'`;::`r_'":':•}`+r SOIL MOISTURE UTILIZATION }-{lam;-}JlY:'~jv.::ii _r;.::.;.~.y O JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV D LAG iTE~ Figure 19. MEAN ANNUAL WATER BUDGETS . 3i --- f- ._ ._. -_-_.__... _ -A .. Photo B. Ti?c 7al:k rontii?~_ ::?::~?t~r;.:cticn ,t P-espcc: and 41~~mpic A~~cnues (left to riglzt):- ~~'±± ~otc tl?:tt :I:~ souti:;;~n .a?e ~i Baia ?;~~s? $vIt in ti?;: lo~:cr ri~._*ht is marked b.~ drift lads. Lots ~.~V I~/~~~'~~ ~ X114 i. ~ ar:, ~n :::~ :art ~'... _..::.:a ar,:.. °~~`iit '? ;h~ end Jt PCOSp~Ct AV~nllc !S [h~ ~j•~j !Inilllli?C':;r~i :~~t ,'.~:.•.i_~:. ?_. _.. ...~ .'•!:.: :?•.::_'i ~ ~~ v ~t~rs Photo A. The banl: fronting lots ? 1 (left) to l8 (right). The shallo~~~ debris avalanche in the center n~as reportedl}~ triggered accidt:ntall~~ during land-clearing operations. Older avalanche scars to the Left apparent(}- ~~~ere natural (triggered b~~ rain and/or sno~vmelt). Yello«- darts mark common reference point in panorama. Photo= ~ 'v iCt~ _~; a,..:~~;r,h~ast -. ~!:: _L~ _..... _e~..~~:~ o* ?rospcct and Cascade _~~~=„nucs. Sicec ._ : cct:.. :,er_...._ ._~ .~:......a::~ _._ .,.~ :e:..~ ';:,, ~~orth boundan of this unnwnbcrcd Photo C. Vie~~~ to the southeast from the north«~est corner of lot 17. Oh~mpic Avenue is on the right. Maximum upland slops (lots 18-21) a~~erage about 3 degrees (~ percent). The forest in upper right is the south boundan~ of Prospect Vie~~- Estates. LOG IT ~ ~~. Photo E. Note the-asymmetry_of. the trunk and root system. Had this-fir matured on a flat upland surface its trunk would be straight and there would now be roots jutting horizontally into the air. The tree's shape and root form indicate that it seeded and grew near the bank edge, suggesting rather slow erosion. Photo F. Note the large "flying bumess" root in this bank-edge t~- inch fir {more than ~0 years old). This root developed in colluvial soil {nocv largely eroded} along the face of the bank. Its exposure now suggests along-term erosion rate on the order of 1 inch per year. Paele ~~ G. W. Thorsen, Consulting Geologist 1926 Lincoln Street, Port Townsend, WA 98368 (360) 385-6002 (also fax) thorcogw@olypen.com Al Scalf, Director of Community Development Attn: Jefferson County Permit Center 621 Sheridan St. Port Townsend, WA 98368 ~qF? ~~ I!~ f ~ u `rt :~ ~~ ~ AUG ~ g ~ggg ;August 19, 1999 r _~. Subject: Addendum to geologic report of May 18, 1999, regarding Prospect View estates, lots 17-21 plus a single building site adjacent to the end of Prospect Avenue on the north. Background: It appears that my earlier report was rejected by Jefferson County due largely to its format. However, I believe that the report and the accompanying engineer's report covered the subjects of slope stability and drainage in general. Here I attempt to rephrase those findings to better follow the layout of country regulations. {A copy of what I perceive as the relevant regulations is attached.} Discussion: First, it can be assumed that the shoreline bank fronting the above described property is unstable and falls within Ordinance 11-0822-94's section 9 definition of a "geologically hazardous area". (It should be a given that all Puget Sound shoreline bluffs are unstable and subject to varying landslide potential during the "rainy season".) Second, it also seems obvious that glaciated uplands in this area, with their gentle slopes, are by themselves naturally stable. Thus the essence of section 9 seems to revolve around the_ subject of appropriate buffers between these contrasting topographic surfaces and the geologic processes that shape(d) them. Subsection 9.506-6 states that "a standard buffer of 30 feet shall be established from the top, toe, and all edges of landslide hazard areas." The Jefferson County Department of Community Development has determined that "a larger buffer is necessary to protect the proposed project and the landslide hazard area" (subsection 9.508). The Department has further decreed that a Geotechnical Report is necessary (subsection 9.509). The Geotechnical Report, as defined, must certify to all of the following: " a. There is minimal landslide hazard as proven by a lack of evidence of landslide activity in the vicinity in the past." Response: My initial report concludes (see Slope Stability) that the subject bank has been cut by various geologic processes including wave action, soil creep, and landslides over at least the last 5,000 years. Such processes have been active in various combinations throughout Puget Sound's hundreds of miles of eroding "view property" shorelines. My conclusion that "there is minimal landslide hazard" here (i.e., to life or structure) is based on a recognition of these processes and an estimate of their rates. "b. An analysis of slope stability indicates that the proposal .will not. be subject to.risk :of._ . landslide, or the proposal or the Iandslide hazard area can be modified so that hazards are ~~ eliminated." Response: I assume here that the term "risk" is based on the generally accepted equation Hazard [type +degree] X Duration of exposure [time] =Risk It was my conclusion that, based on available evidence of erosion rates, that the "proposal" (i.e., construction of a house) "will not be subject to risk of landslide" during its useful lifetime {50 years?). I cannot certify that the eventual loss of, for example 10 feet of front yard through normal erosional processes would not influence the resale value of a house in ihe.year 2050. This could be a risk for some people. "c. The proposal will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions." Response: It is assumed here that "predevelopment conditions".refer to those existing now, prior to the "proposal" (construction of homes). Although I do not consider myself an expert in storm runoff, I agree with the N.T.I engineer, Mr. Leach; that-the recently completed street ditch system (see his map) should decrease runoff onto the subject lots. In addition, existing covenants direct that "all runoff from roof, yard drains and other impervious surfaces must be directed or detained such that water runoff from the lot does- not increase the quantities of water leaving the lot beyond the quantity that would have naturally occurred prior to improvements on the lot:" The recommendations in my earlier report outline some of the methods by which this can be accomplished. "d. The proposal-will not decrease slope stability on adjacent properties." Response: It should be recognized that the existing situation is-far from "natural". Following County ordinances, local covenants, common sense, and my recommendations should result in a small but possibly significant increase in the stability of the adjacent properties over existing conditions. "e. All newly created building sites will ~be stable under normal geologic conditions (if applicable)." Response: We have experienced two recent winters ('9? and '99} with precip patterns that have caused bank instability throughout Puget Sound shorelines. The proposed building sites were (and are) still "stable". {Those precip events were probably "normal geolagic conditions" in the long term.) Nor would i consider a normal (historic?} earthquake apt to create instability at the building sites. Gerald W. Thorsen, C.PG. G. W. Thorsen, Consulting Geologist 1926 Lincoln Street, Port Townsend, WA 98368 (360) 385-6002 (also fax) thorcogw@olypen.com ~~~i ~' ~~Y~~Z~ ~~ !i Fc.S f ,- Fr-- Jefferson County Permit Center October 4, 1999 621 Sheridan St. Port Townsend, WA 98368 Critical Area Review CAR99-0091/SEP95-0180 Parcels No. 96900020, 989600021 Elston Ave., Port Townsend, WA Prospect View East Block 192 ,Combined lots 20 and 21 (Sec. 35, T30-RO1 W) Applicant: Gene Seton Project Description: Future single-family residence to be located no closer than 50 feet from a steep shoreline bluff (pending shoreline variance approval from the Jefferson County Hearing Examiner and Washington State Department of Ecology). Residence to be served by an existing conventional trench sewage disposal system (system and reserve drain field on lot 20 that have already been approved by Jefferson County). Site Summary: The bank fronting lots 20 and 2l, totaling more than 2501ineal feet, is about 95 feet high. The bank consists of an upper 15- to 25-foot-high segment composed of silt-rich glacial till and drift that slopes from 60 degrees to near-vertical. This upper bank is largely bare, but in places where colluvial soils are intact, supports isolated Douglas fir to 15 inches in diameter. The lower, roughly three-quarters of the bank slopes about 42 degrees, with a 3- to 8-foot near-vertical pitch at the beach. This bank segment, largely vegetated, is underlain by glacial outwash sand covered largely by brushy colluvium. The bank slope from upper edge to high tide averages Iess than 45 degrees. Discussion: Potential development impacts due to the nature of upland soils here can be largely offset by the nearly two-thirds acre area of the combined lots. The 50-foot "buffer" to the north, between lots 20 and 19, should also serve to mitigate any hydrologic impacts of either lot. Ditching along Elston Avenue should intercept most runoff from developed slopes to the west. Most of lot 21 is covered by fill that tapers in thickness from 0 on the west to about 5 feet near the southeast corner. The evaluation of site preparation prior to emplacement or fill quality is beyond the scope of my assignment. However, a fill as thin as this should present no challenge to a competent contractor. Certification: The Geotechnical Report, as defined, must certify to all of the following (italic emphasis is none): " a. There is minimal landslide hazard as proven by a lack of evidence of landslide activity in the vicinity in the past." Response: My initial report concludes (see Slope Stability) that the subject bank has been cut by various geologic processes including wave action, soil creep, and landslides over at least the last 5,000 years. Such processes have been active in various combinations throughout Puget Sound's hundreds of miles of eroding "view property" shorelines. My conclusion that ::. , . "there is minimal landslide hazard" here (i.e., to life or structure) is based on a recognition of these processes and an estimate of their rates. There certainly is evidence of past shallow landslide activity on banks fronting both lots 17 and 20121. No geologist would deny that, but few would consider this a "hazard" to lives or homes beyond the proposed 50-foot setback. "b. An analysis of slope stability indicates that the proposal will not be subject to risk of landslide, or the proposal or the landslide hazard area can be modified so that hazards are eliminated." Response: It was my conclusion that, based on available evidence of erosion rates, that the "proposal" (i.e., construction of a house and drain field). "will not be subject to risk of landslide" during its useful lifetime (100 years?), I cannot certify that the eventual loss of, for example 10 feet of front yard through normal erosional processes would not influence the resale value of a house decades in the future. This could be an economic risk for some people, but probably not a hazard. I do not recommend any attempt to "eliminate" hazards (geologic- processes} by structural methods such as bulkheads or rock buttresses at beach level, but a carefully planned revegetation effort for bare. areas of the upper bank should be considered. "c. The proposal will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions." Response: It is assumed here that "predevelopment conditions" refer to those existing now, prior to the "proposal" (construction of homes). Although I do not consider myself an expert in storm runoff, I agree with the N.T.I engineer, Mr. Leach, that the recently completed street ditch system (see his map) should decrease runoff onto bank edge lots such as 20/21. In addition, existing covenants direct that "all runoff-from roof, yard drains and"oflier impervious surfaces must be directed or detained such that water runoff from the lot does not increase the quantities of water leaving the lot beyond the quantity that would have naturally occurred prior to improvements on the lot "The recommendations in my earlier report outline some of the methods by which this can be accomplished "d. The proposal will not decrease slope stability on adjacent properties." Response: It should be recognized that the existing situation is far from "natural" (i.e., old- growth forest?). Following County ordinances, local covenants, common sense, some revegetation and my earlier recommendations should result in a small but possibly significant increase in the stability of the adjacent properties over existing conditions. "e. All newly created building sites will be stable under normal geologic conditions (if applicable}." Response: We have experienced two recent winters ('97 and '99) with precip patterns that have caused bank instability throughout Puget Sound shorelines. The potential building sites on lot 20/21 were (and are) still "stable". There has been no significant retreat of the bank edge here since my 1989 report. Nor would I consider a normal (historic?) earthquake apt to create instability at the building sites. /-~~X/~y[~/I v y .. Gerald W. Thorsen, C.P.G. Copies to: Gene Seton Jeff Ingman - ~ EcF ProFfss~o •~4~a1 vBf~ :, coo ,~ ; 1525 ~, ~: a AIPG ..y ~,_ ~'_,~ nFgF.Q,9~0 1J~ T1~+~~5~ ~\5 NTI May 19, 1999 NORTHWESTERN TERRITORIES, tNC. Engineers ^ Land Surveyors ^ Planners Construction Coordination ^ Materials Testing Sue Theiss Jefferson County Permit Center 624 Sheridan Street Port Townsend WA 98368 Subject: Prospect View Estates -Drainage Dear Ms. Theiss: ~~ 11 ~ j;. -_~ ~: MAY 2 4 1999. i -~' f; The developers of Prospect View Estates have made roadway drainage modifications to various streets as discussed with Jefferson County Public Works Department personnel earlier this year. These activities, along with similar work to be performed by another land owner to the west, will substantially redirect the street drainage south of Prospect Avenue and east of Elston Avenue. The net result is to reduce the water currently reaching the bluff area by an estimated 75%. This work is consistent with the recommendations contained in the May 18, 1999, Geologic Report prepared by G. W. Thorsen, Consulting Geologist, which is attached hereto as an appendix. Storm water had been flowing southeasterly to Elston Avenue south of Cascade Avenue and onward to the bluff where it had caused some erosion south of the Prospect View Estates project. The installation of Jefferson County standard ditches and culverts will redirect the runoff to a more natural southerly direction for sheet flow disbursal within an unopened county road right-of--way along the more gentle slope towards Chimacum Creek. The attached drawing illustrates the flow routes. This will allow natural biologic water treatmen# such as a bio-swale and, ultimately, the water will be disposed of by absorption. It is anticipated that this drainage revision will not only reduce the potential for bluff erosion, but also improve the vicinity's water quality by providing biological filtering. However, as a "safety net" if erosion is experienced, there is sufficienf area in the un- opened right of way of Main Street between Elston and Olympic Avenues to install a. sufficiently sized infiltration gallery (dry-well) for the disposal of the roadway runoff. 717 SOUTH PEABODY, PORT ANGELES, WA 98362 (360) 452-8491 1-800-654-5545 FAX 452-8498 ~: 1 ' ~ Sue Theiss Jefferson County Permit Center Prospect View Estates May 20, 1999 Page 2 of 2 Infiltration of storm water in ditches, ponds, dry wells or infiltration galleries is the preferred method of disposal identfied in~the Department of Ecology's Storm Water Management Manual as this method. provides local recharge, of aquifer systems.. It.is . anticipated that each new home constructed will have its own on-site storm water disposal system (most likely infiltration) and that the roadway ditches will cant' only the roadway runoff. - . It is, therefore, my recommendation tha# time be given to observe the operation of the ditching over time and as homes are developed in the region to ensure that the system is operating as expected before installing an infiltration gallery that may not be needed. If you have any questions or comments on this issue, please don't hesi#ate to call me. Sincerely, NORTHWESTERN TERRITORIES, INC: , _GaT A• [ ~. II 1i Robert A. Leach, P.E., MBA Senior Civil Engineer RAL:eas c: Gene Seton Jeff Ingman s Zp-99 Q- .• • 1 •• . ~ ~ m N ~~ U ~ "~ • s ~ n 1 /! 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Also, a single building site composed of lots 20-24 of the oId Irondale Block 16I, adjacent to and north of the end of Prospect Avenue (see accompanying plat map): Purpose and scope: This report is to help provide guidance for the residential development of the lots described above. It is also in support of the 50-foot setback. for construction on those lots. Lots 17-21 were given such a variance by the county commissioners on April 24, 1989. (County staffhave asked for an update on the geologic report upon which that variance was based.) Geologic input for the report consisted of a review of the old report and available maps and literature (see Selected References). New field work consisted of examination of exploratory pits (not available in 1989}, another recon of the bank (see Profile}, and photographing the site from the uplands, beach, and the air. Site. description: The subject lots include about 800 feet of uplands along an easterly. facing bank just south of Kala Point (see area map). In general, the uplands are planar, with average slopes of about 3-4 degrees (5-7 percent) to the southeast. The hank averages-40-43 degrees, including anear-vertical __ segment as high as 15 feet fronting lot 21. This steeper segment is lower or missing in places to the north. Total bank height ranges from about I00 feet at the south lot (#Z 1) to about 110 feet on the north. The bank is largely underlain by sand and pebbly sand of glacial outwash origin (Grimstad and - Carson, 1981): This unit was compacted by the last ice sheet, here about 4,000 feet thick. ('Thus, it is quite resistant to root penetration.) This lower 80 feet (or so) or bank height is largely obscured by a wedge.. of colluvium that tapers from near zero in places at the top to as much as 8-10 feet thick along the upper beach. Exploratory pits and, in places, the upper bank expose a glacial`drift made up of gravelly, silty-sand. It appears to be-of late glacial origin as it is not as compact as a "true"-till (hardpan'. Nevertheless, its well-graded nature permits it to stand in vertical banks. This description and interpretation is in general agreement with soils maps showing_"Hoypus gravelly sandy loam" in this area. McCreary_ (1975, p. 25) further describes this soil as: ...somewhat excessively drained. Permeability is rapid. Roots penetrate to a depth of more than 60 inches. The soils holds 2 to 4 inches of water available to plants. Runoff is slow to medium, and the hazard of water erosion is slight to moderate. Slope stability: The bank here was cut by wave action since sea level stabilized-about 5,000 years ago. Longshore drift later formed the spit at I{ala Point. The spit and the easterly aspect of the bank both serve to Limit -vave action at the site. Wave erosion from winter southeasterly storms is also limited by the rather-short (2 mile}.fetch in that direction. j. Natural erosion at the site now consists largely of creep and localized debris avalanching (aerial photos A and B) of the colluvial soils that overlie the glacial sands. The role of wave action in recent centuries is merely to remove creep and avalanche deposits from the upper beach where they would otherwise accumulate and buttress the toe of the bank. The natural triggering mechanism for debris avalanching is generally an unusually intense rain storm or rain-on-snow event. The age of the alder in one slide scar suggests that it probably occurred during the New Year's Day 1997 rain and sno~vmelt that triggered shallow avalanching throughout Puget Sound shoreline banks. Little topographic evidence ofdeep-seated landsliding was seen. S~ech erosional forms tend to leave telltale signs, such as step-like topography, a deeply scalloped bank edge, or a sloping bowl-shaped "amphitheatre" on the bank. A broad scallop with a chord of about 1 ~ feet was seen at lot 18, but the lack of a scarp and the presence of fir to 12-inch diameter on the bank suggest that this is either a very old feature or developed gradually. Seismic impacts on such materials are not apt to change current modes of erosion. Neither dreet~ (springs, horizons of seepage) nor indirect (concentrations of water-loving vegetation) eviden~ceof perched ground water were found. 'T'hus, the potential for quake-induced ground failure, such asassoil liquefaction, seems remote. Both the 1946 Vancouver Island and the 1965 Seattle earthquakes were strongly felt in the Port Townsend area, but there were no reports of soil failures. This was in spite of the fact that there are miles of shoreline banks in the area that are more susceptible to such triggering than here. Discussion: We have been asked to address the addition of septic waste water on building sites. (It should be noted that this will be "imported" water, not on-site well water simply being transferred from one depth to another.) This water will be in part offset by the street/ditch storm drainage system. That system, upslope from the subject lots, should intercept and "export" some of the surface runoff that might otherwise reach the lots during extreme rain and/or snowmelt events. In regard to long-term day-to-day septic infiltration, it is important to consider potential impacts in the context of local soils and precipitation patterns. Grimstad and Carson (1981, p. 31, attached) show an annual "water budget" for nearby Port Townsend. Note that here there is a theoretical water surplus (available for recharge or runoff) only 5 weeks of the year (in contrast to nearly 6 months at Quilcene}. The rest of the year there is a soil water deficit and/or soil moisture is being "recharged". Research in the Spokane Valley (Crosby and others, 1971), where mean annual precipitation is slightly less (I7.5 inches) than at Port Townsend (18.3 inches) produced some interesting results in regard to possible ground-water recharge by drainfields. (Glacial outwash subsoils there are similar to soils at the subject site.) The researchers drilled through drainfields at a nursing home, a trailer park, schools, and a commercial dairy, all sites receiving effluent at much higher rates than asingle-family residence. In each situation, drill holes encountered dry soils at depths of 40 feet or less below the drai~elds. The interpretation of the researchers was that there is a "capacity of the outwash deposits to retain much more water than is characteristically present" (Crosby and others, 1971). They explain the lack of long-term downward progression of saturation on such retention and subsequent evaporation to the atmosphere. They also speculate regarding possible "lateral dispersion" of soil moisture. This potential would be enhanced by well-stratified subsoils (not present at this site). In summary, there seems little chance that drainfield effluent could reach the bank and have a ~.- ~ destabilizing influence thereon.. r Conclusions: The setback variance of 50 feet granted by the Jefferson County Board of Commissioners (April 24, 1989) should be ample to protect homes well beyond their useful lifetime. The same setback distance seems appropriate for the nearby lot north of Prospect Avenue (see plat map). Recommendations: • Maintain and/or restore a 10-foot buffer strip of native vegetation such as satal-along the bank edge. This can be trimmed low (2 feet) where necessary to enhance the view. • Minimize the disturbance of bank vegetation. Selectively limb (rather than top) trees where necessary for view enhancement. Control the blackberry fronting lot 21 as it will compete with desirable species. • Minimize the total area of impervious surfaces (e.g., decks, patios, and paths). All such areas should be designed to "leak". • Design drives and parking areas to disperse rather than collect runoff by employing techniques such as crowning, outsloping, grooving, or water bars. • Disperse roof runoff as far from the bank as possible. • Emphasize low-maintenance, drought-resistant species for landscaping to minimize the need for watering. • Monitor the maintenance of street ditches to ensure that runoff is not imported onto bank lots from upslope areas. . Gerald W. Thorseq C. P. G. Selected References Crosby, J. W., III; Johnstone, D. L.; Fentoq environment, [Part] 3: Water Resources Res S1 ~ ~ ~ ~ t ~ i c„ l ~ . r ~s s i o ' ~ t t ic~ ~~P tc~o ._ ~,' I525 A1PG N ~n of pollutants in a glacial outwash p. 713-720. Grimstad, Peder, Cazsoq 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. McCreary, F. R, 1975, Soil survey of Jefferson County azea, Washington: U.S. Department of Agriculture, Soil Conservation Service, 100 p., 70 plates [sheets]. Washington Department of Ecology, 1.978, Coastal Zone Atlas of Washington, Volume 11, Jefferson County: Washington Department of Ecology, 10-p. text, 16 plates, explanatory materials. it i " Gr I Py s ~^::-> ,I ', :::: V ~~T /O 1 J` 1 h . i `; - - 3 `~ I $ 2 J i ~f 3~ ```~ I a y'c/\,\-~N1~1J `111 I i V ' I I- 0 D~ O N E n I A p g ;°. I~ °II I --+------- 111 -- - - - `~o ,~ 0 1 '_ .~ . I ~~` 11 _ °~~ o = Ile I II u I n u I (lo. 11 11 ~ ~ III \ ~ II 11 ~o-_`\ II 11 :S 11 ~ _ ll 1 I ='~ u II d Q n ° ° Q. 11 II n ~~ ___ ~_ u II u "~ n ii ~~" ,n 4 0 - V ~ n II 30p ~ ~ ~ ` 11 n In ` II ///~ ~ h h a o h ,% ,, t; ~: ,_-. BM ~ / "~~ zar 2 I coax. - fferson County t3 I :national Airport ~~~ ~ / ~ i l 11~;;~ 1 . `J ii ~~ Hers 2 i °~' 3 33 I ~~ ~ I ire 3ra I' ~It: •. I ~ 100: I ~ - Substation N I • Iron n i sq ~~ s y Lf2 d Point w s325 s3P4 23 Q h J O' ~o 0 ~ ~~ 5322 J ((( s \ 0 ,\ a ~~ \` .. • Ga ~~. Pte ua b\e ~~// ~4.: per P~rk ?~ Q• •~ ~•~~~ ~~ ~~ ~ ' 0 n I io ~`~ o, ii I ~ ~ r raveF-R-i I I~ . oo I ----- •--~ >- - --i-~ --- ~• I. ~~ ., a _ _________________ „ ~ „ ~, L - 300 // r~ y i ~ N ~ • • ~ ~° ~ I ` _ ~~ `~ + . r -"'__ , 4- III F~ n ", •3 _---____-__ Gravel i . 11 p ndin Stiip ~ Trailer ark Plt u n ~ .-~, Had I~ II 1 _ Gravel) itX •,~ • II ~.. q C - II '11• i - 11 I J• ° I~ II I1 II x • •I~• ~! ~' p N ° • •• Gravel Pits / II t/ - - ~ u - - -- ` I- ____~_ ____ . _ . +1~ _ -= Vicinity map from the U.S. Geological Survey __"._ ~' ~ Port Townsend South quadrangle (1 in. = 2,000 ft; - y' " '~ G I~ = contour interval 20 feet). Yellow dart points to the ~ " ~. 1 i =___--" ". I ~`' °° end of Prospect Avenue (see plat map}. I u n O I • n ~h ri .,~ I s'21 T. 3C 2r30 T. ?c \~ _ Y~ ~~ -.. -, ` _ •.. D •~: i1::-0. mn ~.. O Z ~:` ~ °a ~::: :•, ~F 1 .4 O O a~ {~ - ~ ~ Adapted from plat map of I ~ I E Prospect View Estates . I ;~ i --- ~ - ~ ~ _-- Subject lots enclosed by green line. ---~" i '~ 1 ~ •, ~ j -, ... ~ ~ Approx. top of bank ~ i i j ~ ~. ~ 3' ~ _ ~ ~ i~ ~ i T - -...._-~. ~ ~ _ , Prospect Ave. ~ ~ ~ Approx. line of profile i : - ,~ i ~ ` ~, ~ ~ ~ ~ ~- ~ i ~ 18 -- - - - - ;: ~ - ,, 3 o i I i Q i ~ Q ( ~ ~ } ~ ~ ~ ~ ~ ' O f ~~° ~ ~ ~ 1 In. = 100 ft ~ ~ ~ :~ 20 ~ ~ ~. i ~ ~ i ~ j ~ ~ I . ~ ~Oo - ~ i .Cascade Ave. ~ ~ 2T -- ~ I i F ~ E ~ f Slope angles are- averages. . ~ _; ~o ~ ono - ~ ~ ~ 4 ~ = ~' g ~~ I a ~~ ~Q ~( "~: ~~ ~ w \ -~ QO ` ji V L ,4 ~~ Y ~~ J Y' i i Z r. ~ r ~' .-v ~3 ;J _,~ ~O 4~ ` ~~ ~ ~~ `' 0 ~ ~ } e o ' o ct` (l ~ ~ h ~\ c ,~ ~ v ~ \ ~~ ~ ~ 3 a ~ ~ °' ~ z ~ 2 J a~ 0 PORT TOWNSEND WATER HOLDING CAPACITYOFSOIL 2„ 6+~ PRECiP1TATI0N 18.3 POTENTIAL TRANSPIRATION 25.2 A R~TA i4.4 i7.7 WATER SURPWS 3.9 0.6 From: GrimstEd and Carson 6 POTENTIAL EVAPOTRANSPERATION H 4 ACTUAL EVAPOTRANSPIRATION r ~~~ ~ ~~' '~.~ _ ~~ WATER ~ ~ z • ••. DEFICIT ~ • PRECIPITATION ~ •"' ~- 2 ~ ~/', •;~iP• •'•~ ~••• • ~ •••. ~ 6;, • ~ •••~'• IL MOISTURE •••• ~ .FtECHARGE {6 ~~WATER SURPLUS ~._. - SOIL MOISTURE UTILIZATION O JAN FE8 MAR APR MAY JUN JUL AUG SEP OCT NOV DEC QUILCENE 2 SW WATER HOLDING 2+~ 6+~ CAPACITY OF SOIL PRECIPITATION 50.0 POTEN RA O 25 1 TI N . ACTUAL V PO- ANSPIRATION 17.4 -20.3 WATER SURPWS 32.6 29.7 t0 8 PRECIPITATION ~- . N ~~~:-- ~ w - -~;, POTENTIAL - , .. - _ ;`- - - ~-;;~-.;~>. EVAPOTRANSPIRATION w--'-s''- ` ~: WATE ~ _ '- ~,~"~~~ ACTUAL .~ _ - ,~'~'~~~-`'>.~- ~ ~ SURF U; ~``-:~'~~=:~~ EVAPOTRANSPIRATION ~~ ~~1 ~~-,'--j~ - ~.m>~ °~• ~~ ~~-~.~- _ i ~ • 1.~~, WATER ~~ SOtL ~~ •• ` WATER x , ~ SURPLUS~~~~ - ~ ~~ 2..' • • •N DEFICIT •~` OISTURE - : s,~ ,- y~ 4 . 4 `v. _ ~' y. ~i• RECHARGE ` :-.~` dWL-S~ \- ~.~,~ ~'n >~,,;_:.:;.=~,..+:~•• IL MOISTURE UTILIZATION 0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Figure t9. MEAN ANNUAL WATER BUDGETS . 31 Photo B. Tire bank fronting the intersection of Prospect and Olympic Avenues (left to right). ~iotc that die southern end of e~ala PoInI Splt In dle lo~~cr right is marked b~~ drift lags. i,ots 1? and [~ arc on the icft. Tltc c?eared ar;;a right of the end of Prospect Avenue is the annumb:;red iot inc'.u~1ed 'l~rci;: 'sc:; ulat :nap) Photo A. The bank fronting lots 21 (left) to 18 (right). The shallow debris avalanche in the center vas reportedly triggered accidentally during [and-clearing operations. Older avalanche scars to the left apparently were natural (triggered by rain and/or snowmelt). Yellow darts mark common reference point in panorama. Photo C. View to the southeast from the northwest comer of lot 17. Olympic Avenue is on the right. Maximum upland slopes (lots i3-2I) average about 3 degrees {5 percent). The forest in upper right is the south boundan• of Prospect View Estates. Photo D. Vic«• to tho .~orth~:ast from the i:ltcrs~ction of Prospect and-Cascade. Avenues.- Siope is Zbout ~ ~crc:;at. Tl:c «oodland ~n the left is the north boundan• of this unnumbered lot 1ort21 Jt P'OS~c;Ct :~'-:;:.U.: iS:::. ~~~C .11. U~. Photo E. Note the asymmetry of the trunk and root system. Had this fir matured on a flat upland surface its trunk would be straight and there would now be roots jutting horizontally into the air. The tree's shape and root form indicate that it seeded and grew near the bank edge, suggesting rather slow erosion. Photo F. Note the large "flying buttress" root in this bank-edge 15- inch fir (more than 50 years old). This root developed in colluvial soil (now largely eroded) along the face of the bank. Its exposure now suggests a long term erosion rate on the order of 1 inch per year.