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HomeMy WebLinkAbout947400023 Geotech Assessment 1 Project: 5.31.22 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 June 21, 2022 Mark Casteel Proman1951@gmail.com Re: Geologic Hazard Assessment 41 Manzanita Road Jefferson County Parcel 947400023 Port Townsend, WA 98368 This geologic hazard evaluation was conducted to assess the risk of landslide and erosion hazards at 41 Manzanita Road to determine the feasibility for the construction of a new home on the property. The Jefferson County Public Land Records landslide hazard map indicates that the subject property is located in a moderate landslide hazard area. The shoreline stability map indicates that the lower (southern) portion of the property has unstable slopes, while the upper (northern) half of the property is mapped as intermediate slopes. Based on our geologic hazard assessment, it is our opinion that construction of a home can be located on the site outside of any landslide or erosion hazard area. Furthermore, such construction will not increase the risk of landslides or erosion on or off the subject property as long as the recommendations in this report are followed. This geologic hazard assessment included a field inspection of the subject property and vicinity including the shoreline area and slopes on and in the vicinity of the site. The evaluation also included review of available geologic mapping, historical aerial photographs, historical land surveys, lidar (light detecting and ranging) imagery, and our own notes, photographs, and observations 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 & Slaughter, 2005) indicates that subject property is underlain primarily by Vashon recessional outwash delta deposits (Figure 1). The northwest corner of the property is mapped as being underlain by Vashon till. June 21, 2022 Jefferson County Parcel 947400023 Geologic Hazard Assessment 2 Project: 5.31.22 Figure 1. Clipped and annotated geologic map showing the subject property. Qml = modified land, Qb = beach deposits, Qgod = Recessional outwash delta deposits, Qgt = Vashon lodgment till, Qga = Vashon advance outwash. Vashon recessional outwash delta deposits (Qgod) are described as pebble to cobble gravel and sand with well-rounded clasts with foreset bedding deposited by glacial meltwater channels as they flowed into Discovery Bay. This unit was not overridden by glacial ice after deposition and is therefore generally loose to medium dense and relatively well drained. Vashon till (Qgt) 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 in places. Subject property Qgt June 21, 2022 Jefferson County Parcel 947400023 Geologic Hazard Assessment 3 Project: 5.31.22 The highly compact nature of the deposit is the result of overriding glacial ice that attained a maximum thickness in the area greater than 4,000 feet. Our observations from hand-dug test pits on the property are generally consistent with the above- described mapping. Soils observed on the property consist of medium dense silty sand with rounded pebble gravel (Figure 2), consistent with the mapped outwash delta deposits. Figure 2. Medium dense, silty sand with gravel deposits on the subject property, consistent with recessional outwash delta deposits. SITE SPECIFIC OBSERVATIONS The subject property consists of a single, 0.31-acre parcel located on an upland slope on the northwest side of Discovery Bay, approximately 10 miles southwest of the city of Port Townsend, Washington. The property is accessed via a gravel driveway that traverses the property between Manzanita Road and Holland Drive from northwest to southeast. The property June 21, 2022 Jefferson County Parcel 947400023 Geologic Hazard Assessment 4 Project: 5.31.22 is undeveloped at present except for a septic system previously installed in the northeastern portion of the property upslope of the driveway and a water well located along the north-central property boundary by Manzanita Road. The subject property surface consists of gentle to moderate slopes that descend from Manzanita Road to the north down to the south to southeast (Figures 3 and 4). Measured slope angles are generally 10 to 15 degrees with the steepest slopes being located immediately below Manzanita Road. These steeper slopes reach up to 20 degrees but are no greater than approximately 4 to 6 feet in height. Figure 3. View across the subject property from the Manzanita Road entrance, looking southeast. June 21, 2022 Jefferson County Parcel 947400023 Geologic Hazard Assessment 5 Project: 5.31.22 Figure 4. View across the subject property from near the Holland Drive entrance, looking northwest . Manzanita Road, located upslope of the subject property, is graded to drain water toward a drainage ditch along the upslope edge of the road, on the opposite side from the subject property (Figure 5). Vegetation on the property consists of stands of Douglas fir and western red cedar trees along the north and southwest portions of the property. The middle portion of the property, which has been cleared of trees, is vegetated with a mix of sword fern, grasses, salal, Oregon grape, and other shrubs. Trees on the property are straight. There is no evidence of surface water features or ephemeral surface water flow on the property and no evidence of groundwater perching or seeps. June 21, 2022 Jefferson County Parcel 947400023 Geologic Hazard Assessment 6 Project: 5.31.22 Figure 5. View of the north edge of the subject property along Manzanita Road, looking west . The subject property is on the left side of the road in this photo. GEOLOGIC HAZARD ASSESSMENT The area of proposed home construction is located on gentle natural slopes underlain by well- drained soils. Although the property is mapped as a moderate landslide hazard area possessing unstable slopes, we observed no evidence of previous, ongoing, or incipient slope failure or significant instability. We did not observe any topographic, geologic, or hydrologic conditions that indicate the potential for future slope failure on the site Soils observed in test pits on the property were well drained and capable of accepting additional water input contributed by stormwater runoff from a home. We did not observe drainage conditions upgradient of the subject property that are likely to pose an erosion hazard to the site. June 21, 2022 Jefferson County Parcel 947400023 Geologic Hazard Assessment 7 Project: 5.31.22 CONCLUSIONS AND RECOMMENDATIONS Based on this geologic hazard assessment, it is our opinion that construction of a new house on the subject property will not be at risk from landslides or erosion. Furthermore, the proposed development on the property will not increase the risk of landslides or erosion on or off the site as long as the recommendations of this report are followed. Stormwater management: Stormwater collected from structures built on the property should be dispersed consistent with the Stormwater Management Manual for Western Washington. Soils on the property upland appear capable of readily accepting stormwater infiltration. Concentrated stormwater should not be discharged within 10 feet from the top edge of fill slopes, if such slopes are created during construction. Yard waste and debris: Yard waste and/or cut vegetation should not be piled on or near slopes. Piles of vegetation waste can form an unstable mass that could slide down the slope damaging vegetation and soils on the slope and cause raveling. 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, Stratum Group Ben Carlson, L.G., M.Sc. Geoff Malick, L.G., M.Sc Licensed Geologist Licensed Geologist