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HomeMy WebLinkAbout09A- Geotech ReportStratum Group File: 5.14.20 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 June 26, 2020 Theresa Crowell PO Box 1169 Kingston, WA 98346 Re: Geologic Hazard Assessment 240 Robinson Road, Brinnon, WA Jefferson County Parcel 502151018 Dear Ms. Crowell: This geologic hazard assessment was conducted to assess the risk of landslide, erosion, and liquefaction hazards on the subject property. It is our understanding that you are planning on building a new home on the site to replace the existing structure. A new septic system will also be constructed including a septic drainfield on the eastern portion of the property. The Jefferson County Public Land Records landslide hazard map indicates that the property includes high and slight landslide hazard areas. The shoreline stability map indicates that the property includes unstable old landslides, unstable recent landslides, areas of intermediate slope stability, and modified land. The liquefaction susceptibility map indicates that the lowland portion of the property is a moderate to high liquefaction hazard area. Based on our geologic hazard assessment, it is our opinion that the proposed new home and septic system will not be at risk of landslides or erosion as long as they are located at least 30 feet from the existing bulkhead along the shoreline. However, Jefferson County shoreline regulations may require a larger distance from the ordinary high-water mark (OHWM). The proposed new home and septic system will not increase the risk of landslides or erosion as long as the recommendations of this report are followed. Based on our assessment of underlying soils, the risk of liquefaction during an earthquake at the site is negligible. This geologic hazard assessment included a visual inspection of the property and vicinity including the upland area, shoreline bluff, and shoreline reach. It also included a review of available geologic mapping, historical aerial photographs, lidar (light detecting and ranging) imagery of the site and vicinity. Past observations at sites with similar geologic conditions including areas in the vicinity of the subject property also aided in our interpretations. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 2 GENERAL GEOLOGY The Geologic map of the Brinnon 7.5-minute Quadrangle, Jefferson and Kitsap Counties, Washington (Polenz and others, 2012) indicates that southern portion of the property is underlain by beach deposits (Figure 1). The central and northern portion of the property including the bluff slope and upland areas are mapped as Vashon ice-contact deposits (Figure 1). Figure 1. Clipped and annotated geologic map (Polenz and others, 2012) showing the subject property location. Qb = beach deposits, Qgic = Vashon ice-contact deposits, Qpu = pre-Vashon deposits. Our observations are somewhat consistent with the above described mapping. We observed medium dense sand and pebble gravel consistent with beach deposits in multiple hand dug test pits on the lowland (southern) portion in the vicinity of the existing home (Figure 2). We observed fine to medium sand with highly scattered pebbles and cobbles in shallow hand dug test pits on the bluff and upland portion of the property (Figure 3). The observed sand on the upland was medium dense. The observed sand on the bluff was loose and appeared to be raveled material from above. We also observed compact clay, silt, sand and gravel with scattered cobbles and boulders in cut face exposures in the vicinity of the proposed septic drainfield location just above the bluff (Figure 4). The observed deposit was poorly sorted and unstratified to weakly stratified. Observed pebbles, cobbles, and boulders were Olympic sourced. Based on our observations, it is our opinion that the upland portion of the property is underlain by a relatively thin layer of Vashon recessional outwash or ice-contact deposits overlying pre- Vashon Olympic sourced glacial drift. The shoreline bluff is underlain by thin sandy colluvium June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 3 deposit mantling pre-Vashon Olympic sourced glacial drift and possibly other pre-Vashon deposits. Figure 2. Beach deposits underlying the southern portion of the property. Figure 3. Vashon recessional outwash or ice-contact deposits underlying the upland portion of the property. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 4 Figure 4. Pre-Vashon Olympic sourced glacial drift below shallow recessional outwash or ice-contact deposits on the upland portion of the property. SITE SPECIFIC OBSERVATIONS The 1.1-acre subject property is located adjacent to the north side of the Duckabush River delta of Hood Canal. Access to the property is via Robinson Road. The property is currently developed with a manufactured home, shed, and shared driveway. The following sections describe four terrain areas on the site: the upland area north of Daphne Road, the bluff area north of Robinson Road, the lowland area south of Robinson Road, and the shoreline reach (Figure 5). June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 5 Figure 5. Annotated aerial photograph showing different portions of the property discussed below. Upland The upland portion of the property includes all areas north of Daphne Road. The area is characterized by graded bench areas and cut slopes. The proposed septic drainfield location is a 50-foot x 120-foot level graded area approximately 50 feet north of Daphne Road (Figure 6). Cut slopes range in height from 4 to 12 feet in height. We observed evidence of minor raveling on cut slopes adjacent to the proposed septic drainfield area and Daphne Road. Vegetation on the upland consists of scattered Douglas fir, madrone, red alder, and western red cedar trees as well grass and brush. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 6 Figure 6. The proposed septic drainfield location. Bluff The bluff on the property is located between Robinson Road and Daphne Road. The slope is an old shoreline bluff that now stands at the angle of repose due to no longer being subject to oversteepening from wave erosion. The slope is approximately 45 feet high with an average slope angle of 36 degrees. Measured slopes angles ranged from approximately 29 to 40 degrees. During our site visit we observed very minor raveling of loose sand on portions of the slope steeper than 35 degrees. Vegetation on the slope consists of mixed stands that include red alder, Douglas fir, and western red cedar with an understory of sword fern, salal, trailing blackberry, and other brush. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 7 Figure 7. The bluff slope on the subject property. Lowland The lowland portion of the property includes the developed area south of Robinson Road. This area is very gently sloping with measured angles between 0 and 5 degrees (Figure 8). Existing development includes a manufactured home, shed, and shared driveway (Figure 8). A 3-foot high concrete bulkhead separates the developed area from the beach. The home is located approximately 32 feet from the bulkhead (Figure 9). Vegetation includes mostly grass and planted bushes. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 8 Figure 8. The developed lowland portion of the property. Figure 9. The backyard area and bulkhead. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 9 Shoreline Reach The shoreline reach consists of a sandy gravel beach with a gradient (Figures 10 and 11) that transitions into a tidal zone composed of gravelly mudflats (Figure 11). The mudflats form the outer delta for the Duckabush River. Maximum fetch is approximately 4.5 miles to the south down Hood Canal. Waves appear to reach the bulkhead on an infrequent basis during large storms combined with high tides. Due to the extensive tidal mudflats fronting the beach area, wave energy at the site is further limited. Shoreline landform maps (Washington Coastal Atlas) indicate that the shoreline at the subject property is an accretion shoreform (Figure 12). Accretion shoreforms are areas where beach sediment accumulates and the beach overall is expanding. Overall shoreline drift (sediment transport) is indicated as west to east (Figure 13). Our observations are consistent with the above described mapping. Figure 10. The sandy gravel beach fronting the subject property. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 10 Figure 11. Steep beach face and extensive mudflats fronting the beach area. Figure 12. Annotated Washington Coastal Atlas coastal landform mapping. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 11 Figure 13. Annotated Washington Coastal Atlas shoreline drift mapping. GEOLOGIC HAZARD ASSESSMENT Landslide Hazards The bluff slope on the property is an angle of repose slope underlain by well drained outwash or ice-contact deposits and Pre-Vashon Olympic sourced till. During our site investigation, we did not observe any evidence of groundwater perching, seepage, or springs. We did not observe any evidence of previous, ongoing, or incipient slope failure on the slope. We did not observe any other conditions that portend future landslides on the site. Observed slope activity was limited to minor raveling of sandy soils on slopes steeper than 35 degrees. Shoreline erosion hazards The lowland portion of the property is currently protected from wave erosion and tidewater inundation by a 3-foot high concrete bulkhead. The combination of a steep beach gradient and extensive mudflats fronting the beach also limit the frequency and energy of waves reaching the bulkhead on the site. Historical aerial photographs dating from 1977 to 2016 do not show any evidence of waves or tidewater overtopping the bulkhead or reaching the developed portion of the property. There is no evidence of previous or ongoing beach erosion undercutting or otherwise compromising the bulkhead structure. Based on our assessment, it is our opinion that the home on the site will not be at risk of shoreline erosion. June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 12 Liquefaction hazards The developed lowland portion of the property is underlain by medium dense sandy gravel with sufficient coarse-grained sediment (coarse sand and pebble gravel) so as not be prone to seismic shaking induced liquefaction. Furthermore, the sediments underlying the site have achieved efficient grain packing structure from previous seismic shaking. Since the grains are already densely packed, the soil will not compress during shaking which causes a rapid decrease in pore space between grains leading to high porewater pressures and liquefaction. Tsunami hazards Recent tsunami inundation modeling competed by the Washington Department of Natural Resources (WA DNR) indicates that tsunami amplitudes (wave heights) from a large CSZ earthquake will not exceed 8 feet in the Duckabush/Brinnon area. The model utilizes a large fault rupture and offset that would generate a large wave. Smaller ruptures would generate smaller waves. The developed portion of the property is approximately 12 feet above sea level. The bulkhead on the southern edge of the property rises above the ground surface level of the developed portion of the property and provides additional protection equivalent to an extra 1.5 feet of elevation. Based on available modeling, tsunami inundation is unlikely at the subject property. However, due to the lack of detailed tsunami modeling in the area and relatively low elevation at the site, it is not possible to rule out shallow inundation due to higher than modelled wave heights caused by local bathymetric or topographic amplification or other seismic events on other local fault zones. The exact level of threat to the home and human life is dependent on inundation depths. Depths from 0 to 2.5 feet are unlikely to destroy a residential home and are likely survivable. Depths between 2.5 to 6 feet would likely cause severe damage to a residential home but may be survivable if the roof of the home or second story could be reached. Depths in excess of 6 feet are not survivable in a residential home (Eungard and others, 2017). According to recent regional modeling (WA DNR), peak wave heights in the Duckabush/Brinnon area will be approximately 2.5 hours after the earthquake although smaller tsunami waves will be possible for several hours after the initial tsunami. This lag time would allow for evacuation of the home and vicinity between the time of the quake and the arrival of the tsunami. Tsunami hazards are not included in the geologic hazard regulations of Jefferson County. CONCLUSIONS AND RECOMMENDATIONS Based on our geologic hazard assessment, it is our opinion that the proposed new home and septic system will not be at risk of landslides or erosion as long as they are located at least 30 feet from the existing bulkhead along the shoreline. However, Jefferson County shoreline regulations may require a larger distance from the ordinary high-water mark (OHWM). The June 18, 2020 Jefferson County Parcel 502151018 Geologic Hazard Assessment Stratum Group File: 5.14.20 13 proposed new home and septic system will not increase the risk of landslides or erosion as long as the recommendations of this report are followed. The subject property is not at risk of liquefaction during an earthquake. It is our opinion that tsunami inundation is unlikely at the site based on current tsunami modeling. However, it is not possible to rule out shallow inundation based on a lack of detailed modeling for the area and the potential for localized tsunamis from faults in closer proximity. Erosion control: Erosion control best management practices should be implemented during installation of the septic pressure line on the bluff slope. The project should be completed with minimal soil and vegetation disturbance. Vegetation management: Due to the potential for increased raveling and soil creep, vegetation clearing should not occur on the bluff slope on the property. Brush and trees may be trimmed for view and maintenance purposes. Yard waste and debris: No debris or yard waste should be placed on the bluff slope. Yard waste and cut vegetation eventually builds up and can kill the vegetation on the slope. It also can form a wet unstable mass that may slide down the slope damaging the slope and increasing the likelihood of a raveling and soil creep. Stormwater management: Stormwater from the new home can be dispersed or infiltrated on site 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. Geoff Malick, G.I.T., M.Sc. Licensed Engineering Geologist Geologist-in-Training