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HomeMy WebLinkAbout09- GEOTECH REPORTStratum Group File: 5.1.20 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 July 14, 2021 Ann Burkhart pamonawoods@gmail.com P.O. Box 145 Port Hadlock, WA 98339 Re: Geologic Hazard Assessment Jefferson County Parcels 921183008 and 921183002 Jefferson County, WA This geologic hazard evaluation was conducted to assess the risk of landslide and erosion at the property. It is my understanding that you are planning on building an access drive from Oak Bay Road to the upper western portion of the property and the western, upper portion of the site will be developed with a retreat center. Jefferson County Public Land Records landslide map indicates that the properties are located within an area mapped as a slight landslide hazard area and the Shoreline Stability map indicates the properties are in an intermediate slope stability area. Based on my geology hazard assessment, it is my opinion that the subject properties are not at risk from landslides or erosion. Furthermore, building an access drive onto the property to access the upper west portion of the property and development of the upper western part of the property can be accomplished without increasing the risk of landslides or erosion on or off the site as long as the recommendations in the report are followed and grading and stormwater for the development area managed in a manner consistent with best management practices and with the Western Washington Stormwater Manual. The moderate slopes on the property are underlain compact gravely sand with silt glacial drift and is not subject to failure or erosion given the moderate slopes. This geologic hazard assessment included a field inspection of the site and vicinity. The evaluation also included review of the 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 Hainsville 7.5-minute Quadrangles, Jefferson and Kitsap Counties, Washington (Polenz and others, 2015) indicates that the property is underlain by Vashon glacial till deposits (Figure 1). *Exhibit H* July 14, 2021 Jefferson County Parcels 921183008 and 921183002 Geologic Hazard Assessment Stratum Group File: 5.1.21 2 Figure 1. Clipped and annotated geologic mapped (Polenz and others, 2015) showing the subject property location. Qgt = Vashon Stade lodgment till, Qgic = Vashon Stade ice-contact deposits, Qgos = Vashon Stade recessional outwash sand, Qgdme = Everson glacial marine drift, Qmw = mass wasting, ΦEmq = Quimper Sandstone, blue line is late ice age shoreline. The Vashon till was deposited directly by glacial ice during the last glacial period when glacial ice from the north covered the area between 18,000 and 14,000 years ago. The till is dense and hard and consist of very poorly sorted clay through boulders. My observations of the property and area are consistent with the geology map described above. The overall aspect of the properties is they are located on the east slope of a glacial scoured drumlin (elongate hill formed by the flowing ice). SITE SPECIFIC OBSERVATIONS Oak Bay Road bounds the east of the properties. The properties slope down to the east with moderate slopes. The slope has valley-like features associated with past drainage across the slope from west to east (Figure 2). These valleys are dry features with no evidence of water flow or active or ongoing erosion (Figure 3). I interpret these valleys as relict drainages that formed shortly after glacial ice left the area. At the late stages of the last glacial period freshly exposed bare land under harsher conditions with remnant ice in the vicinity was highly erodable. Numerous valleys like these are present in the region and others (Polenz and others, 2014 and 2015 and Contreras and others, 2014) have made similar interpretations. Periodic water flow may take place during unusual weather events such as rapid heavy snow melt on frozen ground. However, no significant erosion was observed in any of the valleys on the properties and with the exception of the very lower end of two of the valleys, there was no evidence of water. July 14, 2021 Jefferson County Parcels 921183008 and 921183002 Geologic Hazard Assessment Stratum Group File: 5.1.21 3 Figure 2. Lidar bare earth imagery from Jefferson County GIS. Note valleys crossing the property. Figure 3. View of relict valley on central portion of property. Slopes throughout the properties are moderate slopes. Slopes along some of the relict valley slides are somewhat steeper. There is a moderately steep low cut slope along portions of the east side of the property associated with Oak Bay Road (Figure 4). No slope failures are evident along the cut slope. The slopes are forested with a mix stand of mature trees including western red cedar, Douglas fir, western hemlock, big leaf maple and red alder. The understory vegetation is predominantly sword fern throughout most of the site. The upper very gentle slopes have thicker brush areas and more red alder. The property is underlain by dense glacial till with silty sand being the dominate sediment (Figure 5). July 14, 2021 Jefferson County Parcels 921183008 and 921183002 Geologic Hazard Assessment Stratum Group File: 5.1.21 4 Figure 4. View of the east edge of property looking south showing the road cut for Oak bay Road Figure 5. Typical very dense silty sand glacial till found throughout the site, July 14, 2021 Jefferson County Parcels 921183008 and 921183002 Geologic Hazard Assessment Stratum Group File: 5.1.21 5 Geologic Hazard Assessment The property is underlain by well drained dense silty to sandy hard glacial till. No evidence of previous, ongoing, or incipient landsliding is present on the property. The relict valleys on the property have stable slopes and the road cut above Oak Bay Road appears to be stable with no indications of potential sliding on the cut slopes. Valleys on the property are relict landforms and have stable side slopes and there is no erosion taking place within the valley areas. Review of lidar bare earth imagery revealed no indications of any large scale or small scale sliding on the site of vicinity. Based on my understanding of the geology units and observations, it is my opinion that there are no landslide hazards on the site. July 14, 2021 Jefferson County Parcels 921183008 and 921183002 Geologic Hazard Assessment Stratum Group File: 5.1.21 6 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 long as the recommendations of this report are followed. The building of a road onto the site can be accomplished such that the road will not be at risk from landslides and will not increase the risk of landslides on or off the site as long. Grading for the road should be done in a manner consistent with standard grading practices and consistent with the Department of Ecology’s Stormwater Management Manual for Western Washington for pollution prevention. No specific access road plans had been developed at the time of my site visit, but I noted that there were multiple routes that could be followed to access the upper part of the property where buildings will be proposed. The only issue will be grading and angling the road up the slope to meet desired grade. This can be readily accomplished particularly from the area on the northeast part of the site. Stormwater from the access drive and buildings should be fully dispersed consistent with the Stormwater Manual and based on the size of the property, forested condition and soil types this can be readily accomplished. For building foundations, soil bearing of 3,000 psf may be used as long as foundation footings are placed on unweathered glacial till. 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