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HomeMy WebLinkAbout938400812 Geotech AssessmentStratum Group File: 8.10.19 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 August 28, 2019 Charles Boulay crboulay@gmail.com Re: Geology Hazard Assessment Jefferson County Parcel 938400812 Port Townsend, Washington Dear Mr. Boulay: This geologic hazard evaluation was conducted to assess the risk of erosion and landslide hazards on the subject property. It is our understanding that you are planning to construct a new home, driveway, and septic system on the site. The property is located on gentle inland slope approximately 1,500 feet east of Cape George. Based on our geologic hazard assessment, it is our opinion that the subject property is not located in a geologically hazardous area. Furthermore, the proposed development will not increase the risk of landslides or erosion on or off the site. This geologic hazard assessment included a field inspection of the subject properties and vicinity including all slopes. Our 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 Townsend South and Part of the Port Townsend North 7.5-minute Quadrangles, Jefferson County, Washington (Schasse and Slaughter, 2005) maps areas just to the east of the subject property. That map indicates that gently sloping areas approximately 500 feet to the east are underlain by Vashon recessional and advance outwash deposits as well as Vashon till. Vashon recessional outwash is composed of pebble to cobble gravel and sand with both planar and cross bedding. The unit was deposited by meltwater streams emanating from a retreating ice front during the late stages of the last glaciation approximately 14,000 years ago. In the Port Townsend area, Vashon recessional outwash typically forms a thin veneer over thicker deposits of till. August 28, 2019 Jefferson County Parcel 938400812 Geologic Hazard Assessment Stratum Group File: 8.10.19 2 Vashon advance outwash consists of compact sand and pebble to cobble gravel with minor silts and clays. The unit is typically parallel bedded but 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. Following deposition, the outwash was compacted by the mass of the overriding ice sheet. 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 14,000 years ago. The unit is highly compacted to the point of resembling concrete. The highly compact nature of the deposit is from the glacial ice that had a maximum ice thickness in the area of 4,000 feet (Schasse and Slaughter, 2005; Thorsen, 1981). Hand dug test pits on the site revealed compact diamicton composed of clay, silt, sand, gravel with scattered cobbles and boulders consistent with Vashon till (Figure 1). Our observations are consistent with lidar imagery showing that the subject property and vicinity are located on the northern side of a remnant meltwater river bank (Figure 2). During the late stages of the last glacial period, a meltwater river emanating and flowing along the edge of the glacial ice front down cut into previously deposited glacial sediments including Vashon till deposited earlier in the glacial period. This down cutting created the slope on the property, The slope angle is well below the angle of repose for hard compact glacial. Figure 1. Vashon till on the subject property. August 28, 2019 Jefferson County Parcel 938400812 Geologic Hazard Assessment Stratum Group File: 8.10.19 3 Figure 2. Lidar image (Stratum Group, 2017) showing late-Pleistocene geomorphic features in the Cape George region. SITE SPECIFIC OBSERVATIONS The 0.28-acre subject property is located on a gentle inland slope approximately 1,500 feet east of Cape George in Jefferson County. Slopes on the property are planar and range from 0 to 12 degrees on the lower half to the site to 10 to 17 degrees on the upper half of the site (Figure 3). The subject property is currently undeveloped with the exception of an existing septic drainfield. Access to the property is via San Juan Drive and Vancouver Drive. There is a shallow roadside ditch just south of the property boundary within the Vancouver Drive right of way. Vegetation on the site consists of grass, thistle, horsetail, holly, salal, weeds, and brush. August 28, 2019 Jefferson County Parcel 938400812 Geologic Hazard Assessment Stratum Group File: 8.10.19 4 Figure 3. Gentle slopes on the subject property. GEOLOGIC HAZARD ASSESSMENT The topography of the subject property is characterized by gentle modest slopes. During our site visit we did not observe any evidence of previous, ongoing, or incipient slope failure on the site. We also did not observe any evidence of groundwater perching, seepage, or springs. There are no conditions on the site that portend future landslides. Surface water on the site is limited to a drainage trench within the Vancouver Drive right of way. Therefore, there is no risk of rapid erosion on the site. August 28, 2019 Jefferson County Parcel 938400812 Geologic Hazard Assessment Stratum Group File: 8.10.19 5 CONCLUSIONS Based on our geologic hazard assessment, the subject property is not located in a geologic hazard area. Furthermore, development of the site will not increase the risk of landslides or erosion on or off the site. Stormwater should be dispersed and infiltrated on the southern, lower area of the site consistent with the Western Washington Stormwater Manual. 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. Geoff Malick, G.I.T. M.Sc. Licensed Engineering Geologist Geologist-in-Training