Loading...
HomeMy WebLinkAbout901182006 Geotech AssessmentSTRATUM GROUP' 1451 Grant Street, Bellingham, wA 98225 Phone (360} 714-9409 March 24, 2008 Janice Porter 810109' Street SE Everett, WA 98208 Re: Geology Hazard Assessment ' Tax Pazcel 901182006 and 901182009 ' 147 Orcas Drive . Port Townsend, WA Dear Ms Porter: Stratum Group is pleased to present the results of our geology assessment of the above referenced properiy adjacent on a shoreline bluff. Thepurpose of this geology assessment was to l) evaluate the potential risk of slope;failures impacting proposed development, of the site, 2} assess the potential impact of proposed development of the site. and 3) provide general site development. and maintenance recommendations for-the property adjacent to a potentially . unstable bluff: Based. on my investigation of the site and vicinity, it is my opinion that a home can be constructed on the site and will. not be at risk provided the recommendations presented in this report are followed. This: assessment included a visual inspection of the property, slopes on,and ' in the vicinity of the property, available geologic mapping in the area, and investigation of the shoreline at the base of the bluff 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 thick layer of glacial related and interglacial fluvial sediments were deposited over the underlying bedrock in the vicinity of the subject property. The Surficial Ge©lo is Map of the Port Tov~nsend 30 by 60 Quadrangle, Puget Sound Region, Washington (Pessl, Dethier,l3ooth and Minard, 1989) and the Geolo ig c Map of Northeastern Jefferson County, Washin tg_on (Gayer, 1976) indicate the steep shoreline bluff on the subject property is underlain glacial recession outwash deposits. Observations on the site are consistent with-.the above-described mapping. The bluff is underlain by compact sand .and gravel silt units that are consistent with the bluff ;~ ~`~b}~det~lain by glacial outwash. These deposits are described as consisting of sand, gravel, silt and clay deposited by melt water flowing from the retreating ice mazgin of the Puget lobe of s ~,. ,~ L ~ ~ ~ ~_ t. .March 24, 2008 Parcels 901182006 and 901182009, Orcas Drive, Port Townsend, wA Geology Assessment for Beach Access Trail Vashon age. The shoreline bluffat-the subject property and along the coast at this. location is an erosional bluff and provides sand and gravel for beaches along the bluff and beaches to the south and-north; of the bluff. The bluff is located within Port Discovery. Wave erosion at the. base of the bluff likely only takes place during periods of very high tide combined with strong winds. Some recent erosion from a high tide eventcombined with a storm surge during February 2U06 was apparent along portions of the base of the bluff in the vicinity of the- property. Erosion. undermines the base of the bluff causing slope failures at the base that create an over steep slope. Subsequent failures then take -place fiirther up the slope as the slope reestablishes itself to the angle of repose for'the soils underlying the blur The angle of repose will vary depending on the cohesive strength of thee: soils. and the vegetation cover. Soils with high silt or clay content may. be able to stand nearly vertical for long periods of .time. Slopes with vegetation providing significant root. strength. will. be able to maintain a steep slope aspect than slopes. with. poor vegetation cover. SPECIFIC SITE OBSERVATIONS The steep shoreline slope on the subject property: is approximately 40 feet high. The upper half of ..the bluff has an average slope of approximately 3 8 degrees. The lower half of the bluff has a slope of between 55 and 65 degrees.. The bottom approximately 5' feet: ofthe ,lope. just above the. .beach is steeper due to slope toe erosion. The slope is tree and brush covered. Trees on the slope consist of Douglas fir wind madrone and understory of ferns, moss and evergreen huckleberries. At the time of my site visit, l observed several recent toe erosion areas on adjoining properties that were likely associated with the February 2006 storm event: However, no recent erosion was evident on the subject property. Sand and sandy gravel are the predominant sediments underlying. the bluff slope. Areas of compact sand, silt and gravel are exposed or are just beneath the surface throughout the bluff. The beach at the site consists of a gently sloping sand and gravel beach. Sand. and gravel spits are located north and south of this. shoreline reach, Overall the beach appears to be in good condition. There are some shoreline erosion protection structures located to the south, but overall 'the sediment available to the beach is adequate and the beach appears to be in good condition such that toe of slope erosion should remain at similar levels compared to the .past. The upland area slopes gently towards the blufftop with slopes that are less than lS degrees.: The upland area i.~_primarily tree covered except for an access road on the west portion of the property Stratum Group ~' File: 2.26.08. March 24, 2008 Parcels 901182006 and 901-182009, Orcas Drive, Port Tovmsend, wA Geology Assessment for Beach Access Trail and the access driveway and a small cleared area presently occupied by a camper trailer and shed. A trail with timber. supported steps presently provides access-from the upland area tc~ a wood deck landing above the beach. The trail is stable and has not caused any stability problems on the bluff slope. CONCLUSIONS AND RECOMMENDATIONS Erosion at the base of the shoreline slope will over time lead to the slope becoming steeper and. slides progressing up to the top of the steep shoreline sloe impacting the top of the bluff.. Based on the age of trees on theslope on and off the property and the low frequency of erosion events, it appears that slides impacting the entire slope and the subsequent retreat of the top edge of the bluff takes place over at an interval that is likely greater than l00 years. Erosion and shallow landslides at this site are the result of the bluff being undermined by wave action at the base of the bluff. The slope failures at this site will typically be relatively minor shallow soil failures and slope raveling. Butdding Set$aeks The steep shoreline bluff slope is a geologically hazardous slope due to erosion at the.. toe of the bluff and the steep slope conditions. Any home should be located. at least 3U feet back from the top of the steep slope. This distance will .provide adequate protection to the home over the life of the structure (1:04 years). Shoreline Because the shoreline bluff is an eroding bluff, slope failures should he expected to occur on a periodic basis along. the bluff shoreline. The only way to prevent. continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts: as a feeder bluff (erosion of the bluff provides sediment) for the beaches north and south of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of he subjectproperty should contact Jefferson County officials regarding any proposed shoreline protection projects along. the bluff or any shoreline :protection construction. Storm and Septic Drainage Soils on the upland portion of the property are well drained. The soils appear to consist primarily. of coarse grained deposits. Storm water should be dispersed as much as practical through ~~~~~~~ Stratum Group Filer 2.26.08 ti: ,c. ' fZ .. ~' t ~.. March 24, 2008 Parcels 901182006 and 901182009,. Orcas Drive, Port Townsend, WA , Geology Assessmentfor Beach Access Trail dispersion/infiltration trenches. Due to the gravely and sandy soils underlying the site infiltration' should be readily accomplished. The proposed septic drain field is located on the western portion of the property and should not: cause any problems for. slope stability. Vegetative Site grading soils or debris, landscape debris, or any other material should not be disposed of on - the steep shoreline slope. Overtime debris will break down and-farm an unstable mass that will slide damaging the slopes below. . Trees on the slope and within the 30-foot setback buffer should be maintained along the top of the. steep shoreline bluffslope. Thinning of trees is acceptable as long as at least one tree per 30- foot center is left in place. Limbing for view purposes is also acceptable as long as the limbing is done in a manner that will not kill the tree limbed. If trees are harvested ar thinned or limbed. for view purposes on the slope between the buffer and the. upland area, .all debris should be removed from the slope. Please note that.there are inherent risks associated. with building: on lots near or adjacent to steep; slopes. These are risks that the_ building owner should recognize: and be' willing to accept. If conditions appear different than those described in'this report, or other concerns arise, we request that we be notified so we can: review those: areas and modify our recommendations as required.. Stratum Group appreciates the opportunity to be of service. to you. Should you have any questions regarding please: contact our office at (360} 714-9409.. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc.. Licensed Engineering Geologist ~~~ ~i Stratum Group ~~ ~ ~ ~ 4 File: 2.26.08 c~ i+ 4 p~a T 9 3~ ~3 c+ .... s~. S Cl ~~~' ry'I~ 4 ~: ~~1~ '~" !iI 7 j ~ •p~. 0 ~~ ;~ ~' ~ Q m ~>a'~ ~ N~ ~ N ~ w r+ ^^~ ~..L ~ CD N N ~-~- o~ , ,~\ • ~ ~ -~. ~ ~ ~- ~ t ,1 ~ ,A ~ t .~ ~ ~~ ..- s ~~ ...- r.,. ,....N. ~,,-^` r . o ~ ~. o ~ R 1 - ~~ ~_ fi ~~ a „'`` ~ 3 ~,~ 4-->, -n.--, ~ -,7. ~~ :~ Y