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HomeMy WebLinkAbout977100903 Geotech Assessment and / TECHNICAL ANALYSIS Lots 2 and 3, Block A, Division 3 Ocean Grove Estates, $¢ffcrson County fOr Tobe J'ensen, Ph.D. 2821 Second Avenue//901 Seattle, Wash/ngton 98121 by G. W. Thorsen 8: Associates Consulting Geologists 1926 Lincoln Street · Port Townsend, Washington 98368 October, 199:5 October 14, 1995 Tobe Jensen 2821 Second Ave. #901 Seattle, WA 98121 Re: Geologic recon of shoreline bank property, Lots 2 and 3 Bloc A, Ocean Grove #3 ' Purpose and Scope The primary focus of this geologic recon was to assess the natura. rate and mode(s) of bank erosion at the site. The aim was tc establish a safe building setback and to try to determine i~ development factors such as drainfield effluent might accelerate natural erosion. This report is based in part on a general familiarity with the geology and geologic processes of Puget Sound shoreline bluffs. Published maps and reports on geology, slope stability, soils, and shoreline erosion, as well as site-specific unpublished data were reviewed. In addition, about 2.5 hours were spent examining the bank from the beach and uplands as well as a traverse up the bank. Physical Setting ?opo~raphy: The bank fronting lots 2 and 3 is roughly 210 feet high and averages about 45 degrees in slope. Short pitches of lesser and greater slope exist locally, but there are no continuous benches or cliffs. The upland surface slopes about 10 degrees (18%). Slopes have a general northwesterly aspect, except for the flanks of the north-trending draw along the east edge of the property. Geology: Available recon-level geologic mapping shows bank materials as "Quaternary advance outwash". In plain English this translates as sediments deposited by meltwater from the advancing ice sheet of the last glaciation. ~ Although exposures of in-place glacial materials on the bank range from sparse to none, my observations tended to confirm the mapped interpretation. Bank sediments in general tended to coarsen- upwards - a sequence one would expect from deposition by the meltwater of an advancing ice sheet. The limited on-site exposures of upper bank sediments are predominantly of rather massive (non-layered) silty sand. The silt content, acting as a "binder", enables the sand to stand in near vertical slopes. Minor amounts of gravel, as discontinuous lenses, are also visible in places. Tobe Jensen October 14, 1995 Page 2 of 10 Soils: Local soils are shown on published soils maps as "Cassolar sandy loam . · well-drained soils on upland terraces . . · forme in reworked glacial . . · sediments..." (Soil Survey of Jefferso County). The pebbly, sandy silt exposed on the upland of Lot apparently is fill brought in during dratnfteld construction. appears to be siltier and probably not quite as permeable as on site materials exposed in the bank. The sewage system designer exam/ned on-site soils to a depth of 4. to-6 feet. She reports that soil forming materials occur as discontinuous lenses ranging in texture from silt to gravel and that the texture and amount of cementing is highly variable. The lenticular nature of these soils (in contrast to the massive nature of underlying sediments) is apparently a result of their "reworking" by post-glacial surface runoff. (Both the soil lenses and any cemented layer probably slope roughly parallel to the upland surface.) The discontinuous cemented layers reported in some of the soil logs are a result of chemical action accompanying moisture fronts. The inconsistent depth and distribution of soil texture as well as cementation suggests that the moisture was not necessarily at saturation levels. Also, the cementation may well be a relict of an early cycle of weathering due to conditions that no longer exist (see Climate). Climate: The site is largely protected from the common regional southeasterly winter, and rare northerly windstorms. It is, however, subject to strong westerly and northwesterly storms along the Strait of Juan de Fuca. Strong westerlies occur even in the summer due to regional thermal winds. The site lies near the most protected area of the Olympic "rainshadow". Precipitation is light and mostly as iow-intensity rain and mist. Nearby Port Townsend receives an average of about 18 inches per year. Native cactus thrive on south-facing slopes within a half mile of the site. The north-trending draw or swale (indicated as an "intermittent drainage" on Figure 1) shows no evidence of surface runoff such as disturbance or alignment of forest litter. Topographic features such as this are common on the Qutmper Peninsula. (In inland areas like the ChimacumValley many have prominent alluvial fans at their mouths.) This topography indicates "fossil drainages" from earlier millennia with a wetter climate. Tobe Jensen October 14, 1995 Page 3 of 10 Erosion Banks and bluffs in the glacial and interglacial sediments bordering Washtngton's inland waters were initially formed by wave erosion. As the rate of rise in local relative sea level has slowed during the last 5000 years many erosion patterns have changed. Ail such banks are still eroding, but current rates and modes may no longer be dominated by wave action. Wave erosion: The subject site is well protected from the wave action of winter southerly windstorms by adjacent Beckett' Point. It is, however, exposed directly to prevailing westerlies. Dungeness Spit and nearby Diamond Point and Protection Island offer some shelter from the latter. The best available data (U. S. Geological Survey Map 1-1198-E, Keuler, 1988) indicates that the shoreline here is an "approximately neutral coastal segment" with "little or no net erosion or deposition". Slope stability: Once steepened by wave erosion, all shoreline banks and bluffs are subject to ongoing adjustments due to gravity. Small-scale topography and the pattern of vegetation species and ages on the bank indicate that erosion here is relatively slow. Hard data are not available but I estimate that the average erosion here over recent decades might be less than 2 inches per year, commonly much less. It should be understood that bank erosion, especially in glacially- compacted sediments, is seldom uniform. Bank segments may stand with no perceptible erosion for decades. For example, in places, trees on the bank here indicate little erosion other than soil creep in more than 50 years. Elsewhere, near the north property boundary, young alder suggest shallow debris avalanching in recent decades. Such debris avalanches, involving the surficial 2-3 foot-thick soil-vegetation mat, could occur in the future whether or not these lots are developed. Such events generally involvematerial that is already creeping on the bank. Seldom do they involve significant erosion into the adjacent upland surface. A common trigger for debris avalanching in glacially-compacted but permeable bank sediments is the careless disposal of land clearing or logging debris. "Loading" of shallow-rooted trees by such debris commonly triggers rapid failure of the soil/vegetation mat. Rainstorm-induced avalanching is normally confined to s/it/er (less permeable) subsoils than are found here on the upper bank. Tobe Jensen October 14, 1995 Page 4 of 10 During my bank traverse I observed no evidence of features common2 produced by deep-seated landsliding. Such slides commonly creat seaward bulges of the bank toe, noticeable amphitheaters and/c benches on the bluff face, or pronounced scallops along the edge c the upland surface. Such topographic features persist for decades often centuries after the event and are recognizable even whet forested. ' Seismic Response: No evidence of erosion possibly due t~ earthquake-induced ground failure was found. Two historic earthquakes have been strongly felt in this general area. However~ sparse population during the 1872 North Cascades quake meant tha~ very limited data was collected. The 1946 Vancouver Island quake frightened many people in Port Townsend but caused little damage. Ground failures such as landslides were reported near epicentral areas for both events but no such failures were reported in this area. Evidence of major ground failure if it had occurred, should still be visible, especially for the 1946 event. However, as mentioned earlier, no "landslide topography", split trees, or other indications of deep-seated failures were found. One can only speculate on possible impacts of any future event of historically- unprecedented strength (e.g., 8+ Magnitude). Here, severe ground shaking might be the only response. Discussion Construction Setback: The map (Fig. 1) submitted to Jefferson County Health Department for the sewage disposal permit shows a setback of about 42 feet. The permit (dated 8/29/89) acceptance states that "any change in . . location of house or drainfield invalidates this permit . ." However, the small-scale map submitted with the permit application was, I suspect, more for illustrative purposes than as a recommendation for a house plan or /ts setback. The "Property and Bluff Evaluation . "report by'geologist Norman Dixon (1989) acknowledges the limtte~ setback distance available ("around 45 feet") at the site. Obviously, the size and/or east- west "depth" of a house would influence setback options. (It should be understood that the term "setback" refers to house foundations, not to decks or patios.) The question of reasonable setbacks along shoreline bluffs is difficult to answer in any quantitative manner. This is in part because there is so little hard data - geologists commonly must estimate erosion rates and modes through topographic and vegetative patterns. (Ail bluffs are eroding.) Equally important are individual human priorities. Neither aspect of the question is subject to broad generalization. · Tobe Jensen October 14, 1995 Page 6 of 10 For example, the "human factor" may override technical aspects ( a setback for some individuals. Some people like to crowd the ed¢ to get a better view, knowing that eventually the property val~~ may be diminished by a smaller front yard. Others will want to u~ a maximumpossible setback to help ensure that their grandchildr~ will not inherit property with a home too close to the bank fc ready resale. Thus, setbacks are not merely a geological question, but a serie of personal and technical compromises. For example, some peopl who risk their lives commuting daily on busy freeways may fee threatened by steep eroding slopes. Others will consider suc] slopes and their erosional processes a part of nature they cai accept. Potential Dratnfield Impacts: A modern on-site sewage disposal system to serve future homes on Lots 3 and 4 has already beer installed on Lot 2. The system is designed to eventually receive as much as 360 gallons twice a day. The edge of the dratnfteld is within about 35 feet of the bank. A dratnfield for a single residence has also been installed on Lot 1 to the south. ~ It is almost certain that some drainfield moisture will eventually · "wick", 'via capillary action, to the nearby bank. As saturation by ground water is a common trigger for landslide activity, could this added moisture significantly reduce local bank stability? Various factors suggest that upper bank saturation is highly unlikely to OCCUr. First, it is important to note that drainfields are meant to function as aerobic systems (i.e., under non-saturated Conditions). A state-of-the-art pumped system such as installed here should result in only brief periods of saturation immediately adjacent to distribution lines (unlike old trickle-by-gravity systems, which commonly resulted in localized but long-term saturation). Second, moisture from the drainfield will tend to migrate in all directions, not just towards the bank. Water held as capillary films on fine particles (i.e., clay, silt, fine°sand) essentially does not respond to gravity. In fact, in a semi arid climate such as here, much will move upward and be lost to the atmosphere. As an example, Figure 2 compares annual water budgets for nearby Port Townsend versus an area of more "normal" .Puget Lowland precipitation (Quilcene). Note that the period of soil water surplus in this area, theoretically available for runoff or PORT TOWNSEND !WATE~ HOLD(~ 2" ! 6" Fram= C, eoic~y and ~ {N~AL ~A~{PIRAT~N ~A~UAL ~A~~~ON ~ ~ ~ ~ / ~ ~ WATER ~ ~ECIPITATION ~ ...... ]--: .... ... DEFICIT ~ ,/ .... f '-.;~... ~, ~ , ~E HARG QUILCENE 2 SW _ ~ OFSOiL 2" 6" . PRECIPITATION 50.0 - ACTUAL E VA ~C~.- TRANSPIRATIQN 17.4 ~ WATER SURPLUS :52.6 ! ~.7- ~ ~ ~:.'~i POTENTIAL % ...~i~:-i::~::: ~ · ~ :~:..::::~.'~::~? [~~~ _..,,~'~ ...... . DEFICIT ~... _~~~~ ,~,.."-. dAN i FEB I MAR j APR I MAY ] dUN I JUL I AUG J SEP I OCT I NOV I DEC Figu=e 2. Comparison of "annua[' ~;a~e~ budgets" Townsend and Ouiicene. "Po=en~iaZ ~vapo~=anspt=a~ion'* is ~oun~ of ~a~e~ ~ha~ could be ~oved by evaporation f~om soiz aad ==ans~i=a~ion f=om ~Zan~s le enough avaiZabie. No~e ~he ex~ended "-a~e= deficit" and b~ief of "~a=e~ su~pius" in Po~ To.send. ,,' Tobe Jensen October 14, 1995 Page 8 of 10 recharge, averages only about six weeks per ear and less than one inch. ~ .... ~ ..... .Y the amour ~a~u~ry sanay ~oam holds a inches of water available to -la-~a- ,--~ ..... ~out 7 to p. 14 . ~ -.~ %~=~rson county Soil Su ) rvey Studies in an area of similar rainfall and sandy soils (Spokane exam/ned soils directly beneath dratnftelds for a school, a nurstn~ home, and a small trailer park. These studies tended to conft~ the soil moisture conditions illustrated in Figure 2, generall~ finding dry sediments at depth even at effluent loadings fa~ greater than those of a domestic dratnfteld. The conclusion was that there is "moisture storage in the upper soil layers even during the nongrowtng season, with ultimate loss tc evapotransportation and lateral dispersion". In regard to potential lateral dispersion of moisture towards the bank at this site, I recently examined a residence on the bank overlooking nearby Beckett Point. There, a single family dratnfield about the same distance from the bank face as is this system was covered by a lush growth of green grass. The grass on the nearby bank was brown and dead. This vegetation pattern suggests that any "surplus" moisture from the dratnfteld was still shallow and close by. Without extensive trenching, borings, instrumentation and - te.rm monitoring there is no wa~ t~ ..... ~== ..... ~ long  olsture that m~ght m~grate to the bank from the ratnfiel L · However, available data suggest that it will not be significant to bank stability. It should also be emphasized that moisture below saturation levels is not necessarily destabilizing. Vegetation management: Vegetation plays a multiple role in such a setting. On steep banks it can anchor thin soils to the glacial subsoils, slowing creep rates. On gently sloping uplands it protects soils from erosion by runoff. In addition to the mechanical function of anchoring and binding loose soils, vegetation has an important hydrologic role. A closed canopy of trees and brush, especially evergreen, can intercept a substantial portion of incoming prectp and reevaporate it before reaching the ground. In addition, such trees and brush can transpire soil moisture (natural as well as dratnfteld) into the air almost year around. Thus, bank-edge property owners are urged to maintain and/or reestablish vegetation wherever practical. Trees can be selectively ltmbed (or "windowed") to enhance views but should be ~ cut or topped only as a last resort. Brush can be trimmed to less ' .;:. than 2 feet in height and still perform important functions .. Tobe Jensen October 14, 1995 Page 9 of 10 Bank vegetation should be protected from foot traffic and enhance where feasible. Overly dense tree stands may beneftt from thtnntn, in places, and other areas should beplanted or seeded, preferabl- with native evergreens such as Madrona, fir, or cedar. Trees lean out along the toe of the bank. Limb and top patterns ir places indicate that some such "leaners" have been there for years, some possibly for decades. The root wads of such trees act as buttresses against soil creep as well as wave erosion. Drainage: The dry north-trending draw east of the home site is a fossil drainage. It, and similar features are common on the Qutmper Peninsula. They apparently were cut during periods of wetter climate, possibly during early post-glacial times when vegetation was sparse. Most such features are now dry year around. The draw would be a good place to disperse storm drainage from roofs and other impermeable construction surfaces. However, runoff concentrations should be avoided. Any flows that disturb the natural forest floor litter indicate too much. This includes any future drainage from the road culvert that might occur due upslope development, to A cistern to collect storm water may be worth consideration in such a low prectp area. Saving roof runoff for revegetatton purposes or a small garden could make sense. Such a system would, of course, need to be well engineered to provide proper control and dispersal of overflow. Conclusions - purified drainfield moisture will eventually "wick" to the upper bank, but not in concentrations likely to contribute to instability. - the proposed 40-foot construction setback s~ems adequate from a safety point-of-view. A greater setback might better protect long term (e.g., 50+ year) resale value. - assuring sensitive development along the theme of the following recommendations, I would feel secure and enjoy living here myself. Tobe Jensen October 14, 1995 Page 10 of 10 Recommendations 1. Setback at least 40 feet from the bank edge, the more the better. 2. Revegetate the drainfteld area, in consultation with its designer, emphasizing native evergreen brush species. 3. Maintain at least a 10 foot-wide buffer of native vegetation along the bank edge. 4. Selectively limb ("window") rather than cut or top bank-edge trees where necessary to enhance views. 5. Monitor the county road culvert to ensure that it does not begin to import significantrunoff from new development in the future. ~ 6. Drain roof and other impermeable surfaces to the draw on the ~ east and disperse rather than concentrate flows. 7. Discourage foot traffic on the bank beyond that essential for J vegetation management. 8. Do not cut trees along the beach, no matter how much they lean, until they clearly are dead. GERALD W. THORSEN, C.P.G. Photo A. Wide-angle panorama vegetation extending to beach, not visible but is essentially ~ ~neral bank area fronting Lots 2 and 3. Note dense ~ting low-to-moderate wave erosion locally. Upper bank egetated. Photo B. left. General area of bank fronting Lot 4 and to the north. Bare patches of upper bank and relatively young vegetation of lower bank suggest more recent (more active?) erosion than banks to the south. . ' . . JEFFERSON COUNTY PERMIT CENTER ~ ~UI~ Figure 1. Site map showin~ approximate buildin~ site (Lot 3) and new drainfield (Lot 2). Map from "Sewage Disposal Pem~t" approved 10/3/89 by Jefferson Coun~ Health Depar~ent. -- ~ (SW 1/4 13-,30N-2WO 23 ~1124 002242021 002242023 ~ TAX 15 W/1/17TH TAX 17 T,~ .,,,~,- INT IN COMM TR W/1/17TH INT IN W1/17TH INT IN COMM AREA W/ CC -.t,. ~"' 0(;3 BLK A LOT 16 W/1/17TH INT IN 5.09 e. 0(2.3 BLK A LOT 16 0¢ ..~ r~ 5.23 a. 5.: O(~ %0~' 002242001 6;OV'T LOT 1 (N1/2) <~%d,,~ TAX 15 LESS 36.50 a. (COMMUNi TY TRA C T) lO 002242018 002: ..,,~,,,, ll TAX 12 TAX W/1/17TH INT IN W/l, ~ COMM AREA COM ,... 12 W/1/17TH INT IN W/l, OG3 BLK A LOT 16 OF3 ,4 5.19 a. 5.23 3 1 I HILLCRES' AVE ~ 002242025 ~ TAX 18 LESS R/W '-'" W/1/17TH INT IN CO I.d W/1/17TH INT IN OG Z 3.68 a. GOV'T LOT 2 002242010 \ TAX 5 (S OF ROAD) \ \ W/EASE W/1/17TH INT IN COMM AREA ~ .~, w/,/~ ,~ ,. o~ 3.83 a. W/EASE / ~ INT IN ( D / w/1/~7' ~1~ //~ ~ ' OG3 BL! '~5>> GOV'T002242003 'X./~,X LOT 2 ~. //~ ~9.30 ~. 800X j ~;~%. Ii J { ~'3<- ~ oo~ YAX 1 W/EASE TAX 16 /~ W/1/17~ INT IN W/1/173 /2 FOOT ~/~ C~M AREA COMMA ! FOOT W/1/17~ INT IN W/1/17] OG3 BLK A LOT 16 OF3 B~ 2 FOOT 5.08 .RIES BECKETT POINT (sw 1/4 24-50N-2W) SOUTH BEACH ANNEX NOTE: BECKETT POINT LOTS ARE APPROXIMATE.  LOCATION AND SIZE ..... ' OF lOTS NOT KNOWN. Page: 2 ~ CAR97-0158 .q~k~ ~ W~k~--T~----- CONDITIONS OF APPROVAL /" /~ ~-2--~-3-7 --~='~ 1) A minimum setback (buffer) of ~~ shall be permanently - maintained from the top or edge of the landslide hazard area. ~1 buffers shall be measured perpendicularly from the top or edge of the landslide hazard area. 2) The proposed development shall be located on the subject property exactly as identified on the Universal Plot Plan, or other Site Plan, approved by the County as part of the triggering permit application. 3) ~1 const~ction activities, including the storage and preparation of materials, shall not encroach upon the designated geologically hazardous area, or its associated buffer. 4) Landslide hazard areas and their associated buffers shall remain naturally vegetated. Should buffer disturbance occur during const~ction, replanting with native vegetation shall be required by the Critical ~ea Administrator. No alteration shall be made to the landslide hazard area or its associated buffer without prior autT~the~ Critical ~ea Administrator./~/ . 7 Director of cowry Development 1 I understand that the granting of the above waiver from the provisions of the Jefferson County Interim Critical ~eas Ordinance is made subject to my complying with the above listed conditions, and that any violation of said conditions shall result in revocation of the permit or approval and further review of the project under the Critical ~eas Ordinance. Please sign and return to the Permit Center. Appl i can t ~ Da t e ~~~~Post-it® Fax Note 7671 )a~ # of C~9 7-015 8 ~NDZTION$ OF ~P~~ ~ 1) A ~nim~ so~Da~k (buffer) of ~~ shall be ~rmanently m~1nt~ined from ~e top or edge of the l~dslide hazard area. buffers shall ~ ~asured Berpendi~larly *from the top or ~dge of the l~dslide haz~ 2) The ~o~sed d~velop~t shall b. loca~eW on the ~ubject pru~rty exactly as identified on the Uni~r~al Plot PI~. or other Site Plan, approved by the County as p~t of the triggering permit aRplication. 3) ~I ~ons~c~ion activities, including the storage and preparation of ma=erialm, shall no~ encroach u~n the designated geologically h~a~ous area. or its ~so~iated buffer. 4) ~andslide hez~ area~ ~d =heir ~ssoci~ed buffers shall remain naturally vegetated. Should buffer disturbance occur during cunst~ction, repI~ting with ha=iv, vegetation shall be the Crl=ic~l ~ea ~miuistrator. No alter~ti~ m~all be made to the I~dmlide hazard area or its associated buffer without prior ,u~~:he, c~ : Critical ~.a Administrator./~/~~ - - ~ Data . Director of CoWry ~velc~ent I understand that the granting of the above waiver from the provisions of ~e Jefferson County Interim Critical Ordinate is made subject to my complying with the above listed conditions, and that ~y vioJation of said conditions shall result in revocatio~ of ~he pe~it or approval and further rewiew of the project under the Critical ~eas Ordin~ce. Please sign and return to the ~rmit Center. mpp1 i ca~ O Da te Tobe Jensen ~--'"" . ........ Page 10 of 10 ~ONCOUN~ · " PER . · ~eco~endations · l. Setback at least ~0 feet from the bank edge, the more the better. 2. Revegetate the drainfield area, in consultation with its designer, emphasizing native evergreen brush species. 3. Maintain at least a 10 foot-wide buffer of native vegetation along the bank edge. 4. Selectively limb ("window") rather than cut or top bank-edge trees where necessary to enhance views. 5. Monitor the county road culvert to ensure that it does not begin to import significant runoff from new development in the future. ~ 6. Drain roof and other impermeable surfaces to the draw on the ~ east and disperse rather than concentrate flows. 7. Discourage foot traffic on the bank beyond that essential for vegetation management. i 8. Do not cut trees along the beach, no matter how much they i lean, until they clearly are dead. ? . W. THORSEN, C.P.G. ~/ 1525 _ ~_%~ ~ :.. ,.:'..-