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Home My WebLink AboutBLD2021-00701-09A-Geotech Report Stratum Group File: 12.6.21 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 February 21, 2022 Cathleen Bingaman c/o Studio STL mave@studio-stl.com Re: Geologic Hazard Assessment Jefferson County Parcel 021202009 633 Fort Gate Road, Nordland, WA Dear Cathleen Bingaman: This geologic hazard assessment was conducted to assess the risk of landslide and erosion hazards on the subject property to inform development of the property. It is our understanding that you are planning to construct a metal building on the site. The Jefferson County shoreline slope stability map indicates that the western third of the subject property is potentially unstable. The Jefferson County Public Land Records landslide hazard map indicates that slopes on the western portion of the property are slight landslide hazard areas. Based on our geologic hazard assessment, it is our opinion that the subject property is not in a landslide or erosion hazard area and can be developed without risk of landslides or erosion. Furthermore, development of the site will not increase the risk of landslides or erosion on or off the site. This assessment included a visual inspection of the property and vicinity, a visual inspection of all slopes, review of available geologic mapping in the area, review of lidar (light distance and ranging) imagery of the vicinity 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 Northeastern Jefferson County, Washington (Gayer, 1976) and the Surficial Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region, Washington (Pessl, Dethier, Booth, and Minard, 1989) indicate that the western edge of the subject property is underlain by Vashon advance outwash (Figure 1). The remainder of the property is mapped as Vashon till (Figure 1). RECEIVED BY DCD 2/22/2022 February 21, 2022 Jefferson County Parcel 021202009 Geologic Hazard Assessment Stratum Group File: 12.6.21 2 Figure 1. Clipped and annotated geologic map (Pessl, Dethier, Booth, and Minard, 1989) showing the subject property location. Vashon advance outwash is composed of compact sand and pebble to cobble gravel with minor silts and clays. The unit is typically parallel bedded can also be locally cross-bedded. The unit is well sorted and generally coarsens upwards. Advance outwash was deposited by meltwater streams as glacial ice advanced into the area during the early stages of the last major glacial period, the Vashon Stade of the Fraser Glaciation approximately 20,000 to 18,000 years ago. Vashon till is described as compact diamicton consisting of clay, silt, sand, and gravel with scattered cobbles and boulders. Vashon till was deposited directly by overlying glacial ice between 18,000 and 14,000 years ago. The maximum ice thickness in the area has been estimated to be approximately 4,000 feet thick during the last major regional glaciation (Thorsen, 1981). Geologic observations were made on the site using hand dug test pits and are consistent with the above-described mapping. Dense, medium sand with minor fine sand and scattered pebbles and small cobbles consistent with Vashon advance outwash was observed in hand dug test pit on the western third of the property (Figure 2). Compact, poorly sorted clay, silt, sand, and gravel consistent with Vashon till was observed on the central and eastern portions of the property. RECEIVED BY DCD 2/22/2022 February 21, 2022 Jefferson County Parcel 021202009 Geologic Hazard Assessment Stratum Group File: 12.6.21 3 Figure 2. Vashon advance outwash excavated from a hand dug test pit near the proposed building site. SITE SPECIFIC OBSERVATIONS The 6.02-acre subject property is located on the northwestern portion of Marrowstone Island. Access to the site is via Fort Gate Road. The property is currently undeveloped. The subject property is characterized by gentle planar slopes. Measured slope angles in the proposed building area were between 5 and 10 degrees (Figure 3 and 4). Slopes between 12 and 17 degrees are present approximately 150 feet to the east of the building site. No steep slopes are present on the subject property or vicinity. Vegetation on the site consists of mixed forest stands that include Douglas fir, western red cedar, red alder, and madrone with an understory of salal, ocean spray, trailing blackberry, other brush, and grass. RECEIVED BY DCD 2/22/2022 February 21, 2022 Jefferson County Parcel 021202009 Geologic Hazard Assessment Stratum Group File: 12.6.21 4 Figure 3. The proposed building site. Figure 4. The proposed building site. RECEIVED BY DCD 2/22/2022 February 21, 2022 Jefferson County Parcel 021202009 Geologic Hazard Assessment Stratum Group File: 12.6.21 5 GEOLOGIC HAZARD ASSESSMENT The subject property is gently sloping and is not at risk of landslides or erosion. The site is underlain by compact and well drained Vashon advance outwash and Vashon till. No evidence of previous, ongoing, or incipient landslides were observed during the field assessment. No conditions that indicate the potential for landslides were observed during the field assessment or using lidar hillshade imagery of the area. There is no surface water on the site and therefore no risk of rapid erosion. CONCLUSIONS Based on our geologic hazard assessment, the subject property is not a landslide or erosion hazard area and will not be at risk of landslides or erosion. Furthermore, the proposed development will not increase the risk of landslides or erosion on or off the site. 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 Geoff Malick, L.G., M.Sc. Licensed Geologist RECEIVED BY DCD 2/22/2022