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HomeMy WebLinkAboutBLD2021-00757-09A-Geotech ReportStratum Group File: 6.1.20 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 December 8, 2020 Michael McIntyre & PY McIntyre Mm@mmcontrols.com Re: Geologic Hazard Assessment 61 Drew Lane, Jefferson County Parcel 990400528 Port Ludlow, WA Dear Mr. and Ms. McIntyre: This geologic hazard evaluation was conducted to assess the risk of landslide and erosion at the property. Jefferson County Public Land Records landslide hazard map indicates that property is located in a slight landslide hazard area and the shoreline stability map indicates that property is located in an area that the shoreline stability map indicates are unstable. Based on my geology hazard assessment, it is my opinion that the subject property is not at risk of landslides or erosion. Furthermore, development of the site will not increase the risk of landslides or erosion on or off the site as long as stormwater for development is managed in a manner consistent with the Western Washington Stormwater Manual. The moderate slopes on the property are underlain very compact gravely sand with gravel glacial till with some reworked sediment in the upper soil associated with a former shoreline on the slope. The subject property is not subject to failure or erosion given the moderate slopes. This geologic hazard assessment included a field inspection of the site and vicinity including. The evaluation also included review of available geologic mapping, lidar (light detecting and ranging) imagery and our own notes and observations we have made in the vicinity of the site and at locations with similar geologic conditions. GENERAL GEOLOGY The Geologic Map of the Port Ludlow and Southern Half of the Hansville 7.5-minute Quadrangles, Kitsap and Jefferson County, Washington (Polenz and others, 2015) indicates that the property is underlain by Vashon stade lodgment till (Figure 1). Vashon till is a very poorly sorted mix of clay, silt, sand, and gravel with scattered cobbles and boulders that was deposited by glacial ice during the last glacial period between 18,000 and RECEIVED BY DCD 1/3/2022 December 8, 2020 Jefferson County Parcel 990400528 Geologic Hazard Assessment Stratum Group File: 11.25.20 2 14,000 years ago. The unit is highly compacted to the point of resembling concrete and commonly forms steep, nearly vertical coastal bluffs in the Salish Sea region. The highly compact nature of the deposit is from compaction by the glacial ice that had a maximum ice thickness in the area of 3,000 feet. Figure 1. Clipped and annotated geologic mapped (Polenz and others, 2015) showing the subject property location indicated with red arrow. Qgt = Vashon till, Qgas = Vashon advance outwash, Qgod = Vashon glacial recessional delta deposits, ml = modified land, Qls = landslide, Qmw = mass wasting. Blue line demarks an estimated former shoreline. My observations of the area are consistent with the geology map described above. The till consists of gravel and sandy glacial sediment that is poorly sorted and fairly compact. The slide feature indicated on the map, appears to be an indistinct, very old, relict slide feature. It likely formed during the late stages of the last glacial period and is likely associated with the former shore line. The former shoreline marks the sea level when ice initially retreated out of the Puget Sound region. The mass of glacial ice that covered the area had pushed the local land surface downward hundreds of feet. When the ice retreated the area was inundated by seawater to the approximate level indicated on the geology map. This is consistent with shoreline strands in the vicinity as well as the delta deposits found just below the elevation of the subject property, Qgod on the geology map. SITE SPECIFIC OBSERVATIONS The property slopes gently down to the south-southeast (Figures 2 and 3). The site is tree covered with a mix forest stand that includes western red cedar and Douglas fir. The understory includes sword fern, braken and salal as well as other brush. Soils underlying the site consist of compact sand and gravel that is consistent with coarse-grained glacial till. The slopes throughout RECEIVED BY DCD 1/3/2022 December 8, 2020 Jefferson County Parcel 990400528 Geologic Hazard Assessment Stratum Group File: 11.25.20 3 the site appear to be well drained. No indications of slope instability was observed anywhere on the site of in the vicinity. Figure 2. View of site from Drew Lane Figure 3. View of site from the mid section of property looking down slope GEOLOGIC HAZARD ASSESSMENT The property is underlain by compact and well sandy and gravel Vashon till. No evidence of previous, ongoing, or incipient landsliding is present on the property. There is no surface water on the site and therefore no risk of erosion. RECEIVED BY DCD 1/3/2022 December 8, 2020 Jefferson County Parcel 990400528 Geologic Hazard Assessment Stratum Group File: 11.25.20 4 Jefferson County GIS map indicates the site and the vicinity is located in an area of Shoreline Slope Stability Unstable Slope. This designation is from the Washington Coastal Atlas. The Coastal Atlas is identical to the Slope Stability Map of the Uncas-Port Ludlow Area Jefferson County (Hansen, 1976) designation of the area as a "Class 3: Areas of inferred to be unstable". Hansen (1976) also designated Class 5 areas as areas of recent landslides and Class 4 areas as areas of former landslides. Hansen’s Class 3 designation appears to include all areas that were mapped as being underlain by glacial outwash sediments on the Geologic Map of Uncas—Port Ludlow Area Jefferson County (Hansen, 1976). I speculate that Hansen (1976) made this designation based on the potential that the Vashon outwash sediments could become saturated and destabilized by perched water as well as areas where the outwash sediments were located on moderate to steep slopes. Perched groundwater and associated unstable slopes are not present at the subject property or in the vicinity. CONCLUSIONS AND RECOMMENDATIONS Based on my geologic hazard assessment, the property is not a landslide or erosion hazard area and will not be at risk of landslides or erosion. Furthermore, development will not increase the risk of landslides or erosion as grading and drainage follow standard best practices. Stormwater management: The site stormwater can be infiltrated or dispersed consistent with the Department of Ecology’s Stormwater Management Manual for Western Washington. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding this assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist RECEIVED BY DCD 1/3/2022