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HomeMy WebLinkAbout965000236 Geotech Assessment~ ~~ 0~~ s STRATUM GROUP 1451 Grant Street, Bellingham, WA 98225 Phone (360) 714-9409 Apri128, 2008 Dave Goldsmith 536 Foster Street Port Townsend; WA 98368 Re: Geology Evaluation Proposed tree removal and tree trimming Baycliff Place, Kala Point Dear Mr. Goldsmith: ~~ - 6 2008 ~~€"i.s e~~ ~~~~E t~ ~~~ Stratum Group visited the proposed tree removal sites along the shoreline bluff at Ka1a Point. The purpose of our visit was to determine if proposed tree removal and proposed tree trimming would have a potential negative impact on slope stability at Kala Point. The trees in question. are located on the slope down slope from the end of Baycliff Place. We visited the. shoreline bluff at Kala Point in March. 2008 to evaluate. bluff conditions, slope failure processes and to assess the role of the trees proposed for removal or trimming on the bluff stability. The locations of the trees in question were provided to us at the time of our. site visit as well as via photographs of the trees provided to us. The photographs were taken from the back yards of the homes located on Baycliff Place. This report provides an overview of the geology conditions of the bluff, erosion and landslide processes at the bluff and an evaluation of the role of trees relative to erosion and slope stability at this bluff site. GENERAL GEOLOGY Northwestern .Washington has been occupied by continental glaciers at least four times during the Pleistocene Epoch (1.6 million to 10,000 years ago). During these glacial and accompanying.. interglacial periods, the underlying bedrock was eroded and a relatively hick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surficial Geolo ig_c Map of the Port Townsend 30- by 60-Minute Ouadran legPu_aet Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) and the Geolo ic~Ma~of Northeastern Jefferson County, Washington (Birdseye, 1976) indicate the shoreline and bluff exposed on the bluff is underlain glacial till and advanced outwash deposits. The glacial till is described as consisting of a mixture of clay, silt, sand and gravel and is in a very compact condition. The advance outwash deposits consist of sand, gravel, silt and clay deposited by meltwater flowing from the advancing Puget lobe of the Fraser glaciation.. The advance outwash April 28, 2008 ~~ - ~ 2fl0~ Kala Point BluffTree Removal and Trimming Geology Evaluation e3L~ ~ ~a~~~l ~~is sediments have been overridden and consolidated by glacial ice. Observations of the bluff face slopes and upland areas are consistent with the mapping described above. Glacial till at the top of the shoreline bluff at the northwest end of the bluff along the Kala Point development and on the bluff at the State Park. The till thins towards the southwest and is not present at the top of the bluff in the area where trees. are proposed to be removed and trimmed. The advance outwash deposits on the bluff consist primarily of sand with occasional lenses of gravel and silt. The sediment generally grades coarser from the lower slopes of the bluff towards the upper slopes. This is consistent with the unit being advance outwash (the coarser material being the result of the glacial ice front getting closer). Much of the sand lacks clay and as-such lacks cohesion even though it has been highly compacted by the over ridding glacial ice. The lower slopes may transition into preglacial deposits; however, no very old pre glacial deposits were observed on the lower slopes of the area below where trees are proposed to be removed or trimmed. The base of the bluff below the proposed tree removal and trimming area is underlain by early advance outwash fine sand. SPECIFIC SITE OBSERVATIONS The shoreline bluff in the-vicinity of the proposed tree removal and trimming area is approximately 160. feet high. However, the upper part of the bluff is this. area is less steep. This less steep slope slopes down towards the shoreline at angles that are on the order of 30 degrees or less and iri part of the area where trees are proposed to be removed and trimmed, the slope is nearly level. A sharp slope break is located water ward of the area where the proposed tree work is located. This slope break represents the true top edge of the shoreline bluff slope. The less steep upper slope sloping down to the bench-area above the sharp slope break likely represents a very old feature and that-may be associated with slope erosion that would have taken place shortly after the glacial ice had retreated out of the area and relative sea-level was higher than at present prior to the region rebounding from the removal of the glacial ice through isostatic rebound. Relative sea level is estimated to have been up to 600 feet above the current sea level shortly after the glacial retreat (Geolo i~ c Map of the Freeland and Northern Part of the. Hansville 7.5-minute Quadrans;le, Island County, Washin on, Polenz, Schasse and Peterson, 2006). Based on the location of the site, relative sea level -may have been on the order l00 to 200 feet in the -area. The former higher relative sea level has formed a variety of elevated shoreline features around Puget Sound and may be the cause of less steep slope and bench area on the.. slope. There is no evidence on this upper slope bench area of recent slope movement with the exception of the steep shoreline bluff itself and there are no landslide deposits anywhere on the Stratum Group File;2.2.08 2 ~'~.t~v ~ April 28, 2008 Kala Point Bluff Tree Removal and Trimming Geology Evaluation 6l~ ~ 2008 slope.. The steep shoreline slope slopes down to the beach at an angle of approximately 55 degrees. Portions of the slope are steeper and some portions are somewhat less steep. Numerous past slope failures are readily apparent all along the steep shoreline bluff. All the slope failures appear to relatively shallow soil failures due to the steepness of the slope. The primary cause of slope failures is from undermining of the slope by wave action at the beach. Soils at the base of the bluff are generally readily erodable due to wave action: As the slope is undermined the lower portions of the slope fail first and the slope failures progress up the slope until the top of the bluff fails as well. The full range of these slope failures at various stages are all along the approximately 2 miles of shoreline bluff in the vicinity of the .subject property. Landslide debris consisting of failed soil and trees is present along most of the base of the bluff. The landslide debris acts as a natural protection for the base of the bluff until, over time, the debris is washed away and the process of progressive slope failures up the shoreline bluff repeats. itself. Landslide debris is not present at a few areas. At these locations, the base of the bluff has been eroded and is steep and the slope above is likely to fail in the next few years. Based on the presence of fairly mature Douglas firs on most of the bluff slope on this stretch of coast line and the consistent geology along this portion of the coast, it appears that the frequency of slope failures that remove all the trees has been less frequent than 100 years. In addition, the top of the bluff and base of the bluff is fairly linear, that is, there is no significant convergent topography. This is consistent with the observations that the slides that have been occurring on this bluff are shallow soil failures. During site visits to the bluff in December 1999, September 2006, and again in March 2008, only very minor limited areas of water seeps were present on the bluff face. A few discontinuous areas of vegetation indicative of seasonally wet areas on the bluff slope are present, but none of these areas appear to be causing .slope problems at this time. AlI of the trees proposed for removal or trimming are. located on the upper less steep section of the shoreline bluff. As such the trees that have been proposed for removal or trimming are located in areas outside the shoreline bluff slope and are located on the upper less. steep slope above the shoreline slope.. CONCLUSIONS All of the trees proposed for removal or trimming are located on the upper less steep section of the shoreline bluff. Based on my observations of the geology of the bluff, the vegetation conditions and the shoreline processes, it is my opinion that the proposed tree removal and File:2.2.08 Stratum Group April 28, 2008 Kala Point Bluff Tree Removal and Trimming Geology Evaluation trimming will not have a negative impact on the long-term slope stability of the bluff. As noted above none of the proposed trees to be removed or trimmed are located on the steep portion of the slope that is related to shoreline erosion. The slope where the trees are located is relatively. gentle and loss of root cohesion will not cauus slope problems on the portion of the slope where the trees are located. The loss of canopy coverage due to tree removal will not have a significant impact as soils are well drained-and perched water zones on the bluff slope causing slope stability problems is not an issue at this site. All of the trees to be removed or trimmed are not located on the steep shoreline slope or near the top edge of the shoreline slope. Hence, none of-the trees are located in area that will potentially fail due to toe erosion at the base of the bluff. Hence none of the trees is question will potentially be part of a landslide and therefore do not have the potential to contribute to large woody debris at the base of the steep shoreline slope. Trimming and removal of trees on the upper portion more gently sloping portion of the bluff is acceptable. Removal of blackberry vines and ivy from this slope area is also acceptable and -will not cause any slope stability problems. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions please contact our office. at (360)- 7 i 4-9409: Sincerely yours; Str tum Group .~.. an McShane, M. c., L.E.G: Licensed Engineering Geologist ' ~ ~1~~ ~ ~ f~~Y~ Stratum Group File:2.2.08 4 U ---~ N (~ ~-. --~' t~ ^.~~~ ..WSJ n .ar ~~ '~ \, ~ ,.~ ~. ~_:i; L,..-• ~~ ~~ `'~ ~~ ~~~~~~~~ M~~° - 6 2Q0~ ~- a+ > , ~~J ~~F ~ E5 ~ii~~~~! U~ ~~ ~~ `. 2o- a~ ~OI a 3< ~ ~ ~a~ tO x c n -ice ~rF~7 tl~ c ^p. U~V am~ ,~ ~' ~' ~rt ~~ ~~ `4s _~~~~ .~ - ~~`~. ~~~ ~ ~ ~ .. -'~ ~~~ fa 9 ~'i"~` - ~ ~~ V' ;~ ~' ~ ~,'. 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