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HomeMy WebLinkAbout09A- GEOTECH REPORTStratum Group File: 8.24.20 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 September 28, 2020 Sharon Redmon Shredmon@gmail.com Re: Geologic Hazard Assessment 2590 East Quilcene Road, Quilcene, WA Jefferson County Parcel 701301006 Dear Ms. Redmon: This geologic hazard assessment was conducted to assess the risk of landslide and erosion hazards on the subject property. It is our understanding that you are planning site improvements including a remodel to the existing home and a new workshop. Preliminary site plans show a proposed additions to the existing home with the footprint expanded to the east and northeast sides of the home. The Jefferson County Public Land Records landslide hazard map indicates that the subject property is a moderate landslide hazard area. The shoreline stability map indicates that the subject property is potentially unstable. Based on our geologic hazard assessment, it is our opinion that the remodeled home and new workshop will not be at risk of landslides or erosion for at least 100 years. Furthermore, the proposed home remodel and workshop will not increase the risk of landslides on or off the site as long as the recommendations of this report are followed. 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. GENERAL GEOLOGY The Geologic Map of the Quilcene 7.5-Minute Quadrangle, Jefferson County, Washington (Contreras and others, 2014) indicates that western third of the property is underlain by pre- Fraser glacial drift (Figure 1). The eastern two-thirds of the property are mapped as being underlain by Vashon ice-contact deposits (Figure 1). September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 2 Figure 1. Clipped and annotated geologic map showing the subject property location. Qpd = pre-Fraser glacial drift, Qcw = Whidbey Formation, Qls = landside deposits, Qgic = Vashon ice-contact deposits, Qaf = alluvial fan deposits. Geologic observations on the site and vicinity are generally consistent with the above described mapping; however, we interpret a glacial till on the lower portion of the site to be Vashon till. Based on our observations, it is our opinion that the upland portion of the subject property between the top of the bluff and East Quilcene road is underlain by Vashon ice-contact deposits of variable thickness. The shoreline bluff on the subject property is underlain by Vashon till covered by a thin veneer of Vashon ice-contact deposits. Pre-Fraser deposits including Olympic Mountain sourced alpine till and a silt clay unit were observed underlying Vashon till on bluff exposures to the north of the subject property. Vashon ice contact deposits Medium stiff, poorly sorted clay, silt, sand, and gravel with scattered cobbles consistent with Vashon ice-contact deposits were observed in hand dug test pits on the upper 1 to 2 feet of the bluff as well as the upland portion of the property (Figure 2). Vashon ice-contact deposits consist of unconsolidated sediments that were deposited directly by thin wasting glacial ice as well as sediments deposited by meltwater that was in close proximity to glacial ice as the Puget Lobe was retreating during the late stages of the last glacial period. This unit can be highly variable in composition, even within a relatively small area. Contreras September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 3 and others (2014) describe the unit as loose to compact, unsorted, and unstratified sediment, silty pebble to cobble gravel, sand, lacustrine mud, and isolated boulders. Figure 2. Vashon ice contact deposits underlying the upland portion subject property. Vashon till Compact, unsorted clay, silt, sand, and gravel with scattered cobbles and boulders consistent with glacial till were observed in natural exposures on the shoreline bluff (Figure 3). Many of the observed pebbles, cobbles, and boulders were sourced from the Coast Plutonic Complex in British Columbia, indicative of transport and deposition by glacial ice sourced from Canada. Based on the deposit’s Coast Plutonic Complex signature, lack of observed stratigraphic evidence of pre-Fraser deposition and the hardness of the till, it is our opinion that the shoreline bluff is underlain by Vashon till deposited directly by glacial ice during the Fraser Glaciation approximately 14,000-18,000 years before present. Vashon till is typically well-compacted due to the significant overburden pressure induced by the mass of the overlying ice. However, it is not possible to eliminate the possibility of the till being pre-Fraser in age due to the lack of numerical age constraints or stratigraphic context. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 4 Figure 3. Vashon till underlying the shoreline bluff just south of the subject property. Pre-Fraser deposits Very hard till composed of unsorted clay, silt, sand, and gravel with scattered cobbles was observed on the lower bluff approximately 90 feet north of the subject property (Figure 4). This till was discernably harder than the Vashon till on the subject property. Observed pebbles and cobbles were predominantly composed of Olympic Mountain lithologies (Figure 4). Based on the unit’s Olympic Mountain signature and very compact nature, it is our opinion that pre-Fraser Olympic Mountain alpine till underlies the lower bluff to the north of the subject property. Hard, thinly to thickly bedded silt and clay with abundant desiccation fractures was observed on the lower bluff approximately 200 feet to the north of the subject property (Figure 5). Based on the unit’s very compact nature and stratigraphic position underlying pre-Fraser Olympic Mountain alpine till, it is our opinion that this unit is also pre-Fraser in age. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 5 Figure 4. Pre-Fraser Olympic Mountain alpine till on the lower bluff approxiamtely 90 feet north of the subject property. Figure 5. Pre-Fraser silt clay unit on the lower bluff approximately 200 feet north of the subject property. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 6 SITE SPECIFIC OBSERVATIONS The 2.07-acre subject property is located on the east shore of Quilcene Bay. The site is accessed via a gravel driveway from East Quilcene Road. The site is currently developed with an existing home. Relevant features are shown below in Figure 6. Figure 6. Lidar hillshade image of the subject property and vicinity. The upland portion of the property is characterized by gentle to moderate slopes. The eastern half of the property including the workshop location (Figure 7) is gently sloping to the west with angles between 0 and 15 degrees. A level bench area is located in the central portion of the property (Figure 6). A moderate slope between 10 and 20 degrees descends from the west side of the bench towards the existing home site. The existing driveway curves down a gentle slope on the north side of the bench (Figure 6). A driveway loop is located in a gently sloping area directly upslope to the northeast and east of the existing home (Figure 8). September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 7 Figure 7. Gentle slopes in the vicinity of the workshop. Figure 8. Part of the driveway loop and the eastern side of the existing home. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 8 The home has been constructed into the moderate slope with a crawl space between the ground surface and main floor of the home. A level area between 10 and 18 feet wide is located on the west side of the home (Figure 9). Figure 9. The west side of the home and backyard platform. A shoreline access trail descends from the backyard platform to the beach on a moderate ramp that cuts diagonally down the bluff from northeast to southwest (Figure 10). The trial effectively cuts the bluff into an upper slope and lower slope. The upper slope is a 2- to 8-foot high cut slope that is partially supported by wood cribbing (Figure 11). Minor soil raveling and minor settlement (Figures 11 and 12) was observed on this cut slope. The lower slope is between 2 and 12 feet high with slope angles predominantly between 30 and 40 degrees with limited steeper areas (Figure 10). North of the bottom of the shoreline access trail, the lowermost 2 feet of the bluff is undercut from previous wave erosion by up to 16 inches (Figure 13). Minor raveling, soil creep, and tilting of trees was observed on oversteepened slopes directly above undercut areas. The bluff north and south of the shoreline access trail area is between 20 and 35 feet high with measured slope angles predominantly between 30 and 55 degrees with limited steeper areas (Figures 10 and 13). Vegetation on the site consists of mixed forest stands that include Douglas fir, madrone, western red cedar, red alder, and big leaf maple with an understory of sword fern, bracken fern, ocean spray, Oregon grape, salal, vine maple, and other brush. Evidence of recent brush removal was apparent on portions of the shoreline bluff. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 9 Figure 10. The shoreline bluff. View looking southeast. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 10 Figure 11. The shoreline access trail. Note wood cribbing and minor raveling on cut slope above trail. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 11 Figure 12. Gap created by minor soil settlement beneath wood cribbing above the cut slope above the trail. Figure 13. Undercut areas and minor raveling on the lower bluff. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 12 SHORELINE CONDITIONS The shoreline reach fronting the subject property consists of a gravel and sand beach with scattered cobbles, boulders, and woody debris (Figure 14). Waves appear to reach the base of the bluff during large storm events combined with high tides. However, wave heights are limited by a maximum fetch of approximately 2.25 miles southwest down Quilcene Bay. Shoreline landform maps (Washington Coastal Atlas) indicate that the shoreline at the subject property is a transport zone (Figure 15). Transport zones are areas where sediment is transported by wave and current movement along the beach and with little to no sediment being added to the beach by erosion of the bluff area above the beach. Overall shoreline drift (sediment transport) is indicated as south to north on this shoreline reach (Figure 16). This mapping is consistent with our shoreline observations. Figure 14. The shoreline reach fronting the subject property. View is to the south. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 13 Figure 15. Annotated Washington Coastal Atlas coastal landform mapping. Figure 16. Annotated Washington Coastal Atlas shoreline drift mapping. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 14 GEOLOGIC HAZARD ASSESSMENT Shoreline bluff retreat The shoreline bluff on the subject property is subject to bluff retreat that is predominantly driven by soil raveling. On the lower bluff slope, raveling is currently driven by a combination of weakening of the soils through weathering processes and undercutting and oversteepening by infrequent wave erosion. Raveling is most likely to occur following infrequent storm events that undercut and/or over steepen the lower bluff. Over time, raveling will slowly work its way up the bluff towards the shoreline access trail. Root throw from infrequent tree toppling also contributes to long term bluff retreat by removing soil from the bluff face and also exposing soil to ravel. Very infrequent small shallow landslides between 1 and 2 feet thick may also occur at undercut areas on the oversteepened lower bluff. However, the areas possibly subject to small-scale shallow sliding are located over 35 feet from the footprint of the proposed home remodel. Historical oblique aerial photographs from 1977 to 2016 show previous bluff conditions over an approximately 40-year period (Figures 17 to 21). No evidence of shoreline bluff retreat is visible in the sequence of photographs. Figure 17. Historical oblique aerial photograph from 1977. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 15 Figure 18. Historical oblique aerial photograph from 1994. Figure 19. Historical oblique aerial photograph from 2001. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 16 Figure 20. Historical oblique aerial photograph from 2006. Figure 21. Historical oblique aerial photograph from 2016. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 17 Fully quantifying the long-term top of bluff retreat at the site is not possible due to the relatively short historic record (Shipman, MacLennan and Johannessen, 2014 and Johannessen and MacLennan, 2007). However, based on the site geology, current bluff and shoreline conditions, as well as the record of previous conditions from available historic aerial photographs, it is our opinion that a conservatively cautious estimate for long-term top of the bluff retreat for the site is 1.5 inches per year. Using this conservative estimate long-term average retreat rate of 1.5 inches per year, the shoreline bluff can be expected to retreat 12.5 feet over the next 100 years. The existing home and proposed additions are located at least 27 feet from the lower bluff slope currently subject to shoreline erosion and bluff retreat. Therefore, it is our opinion that the proposed home remodel will not be at risk of shoreline bluff retreat for at least 100 years. Furthermore the home is founded on very hard glacial till that will be resistant to erosion The proposed workshop is located over 200 feet from the shoreline bluff and is not at risk of shoreline bluff retreat. The minor raveling on the cut slope above the shoreline access trail is distinct from the shoreline erosion and raveling processes occurring on the lower bluff slope. The upper slope raveling is the result of the slope being oversteepened because it was cut for the shoreline access trail. Although this raveling and associated minor settlement should be monitored and possibly remediated for aesthetic purposes, it does not pose a threat to the proposed home remodel. Deep-seated landsliding During the site investigation, evidence of previous deep-seated landslides including slide scarps, back-rotated benches with deformed trees, and slide deposits were observed on the shoreline bluff to the north of the subject property (Figure 22). The closest area with evidence of previous deep-seating landsliding is approximately 145 feet north of the subject property (Figure 22) and is visible in historical oblique aerial photographs from 2001, 2006, and 2016 (Figures 20 to 22). September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 18 Figure 22. Lidar hillshade image of the subject property and vicinity. The observed deep-seated landslides were caused by a combination of undercutting and oversteepening of the bluff by wave erosion and either water perching above very hard, impermeable pre-Fraser Olympic Mountain alpine till or fracture propagation within mechanically weak pre-Vashon silt clay unit (Figure 5). These two geologic units are not present at the subject property. The bluff at the subject property is underlain by Vashon till covered by a thin veneer of Vashon ice-contact deposits, deposits that do not create the deep-seated instability present in the areas to the north. Furthermore, the bluff at the subject property is substantially lower and gentler than the bluff areas to the north subject to deep-seated failure. No evidence of previous, ongoing, or incipient deep-seated landsliding was observed on the subject property. This includes a lack of evidence of landslide deposits sourced from the slopes above (to the east of) East Quilcene Road. No evidence of groundwater perching, seepage, or springs were observed on the subject property. There are no other conditions on the site that portend future deep-seated landslides on the site. Based on our observations, it is our opinion that the potential for deep-seated landslides on the subject property is very low. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 19 Erosion hazards The creek on the adjacent property to the south (Figure 22) has a well established channel within hard till and no evidence of any recent bank erosion towards the subject property. Therefore, the creek does not pose an erosion hazard risk to the subject property. CONCLUSIONS AND RECOMMENDATIONS Based on our geologic hazard assessment, it is our opinion that the proposed remodel and addition to the home and the new workshop will not be at risk of landslides or erosion for well over 100 years. Infrequent shallow failures are possible on the lower bluff; however, these potential slide areas are over 35 feet from the proposed home remodel. The proposed site improvements including the home remodel and workshop will not increase the risk of landslides on or off the site as long as the recommendations of this report are followed. Stormwater management: Due to the potential to increase raveling and shallow failure on the lower bluff, stormwater from the remodeled home should be collected using gutters and downspouts and piped to the base of the bluff. The stormwater pipe should be inspected on a regular basis. If damage or leaks are found, the pipe should be replaced as soon as possible. Vegetation management: Due to the potential to increase raveling and shallow failure on the lower bluff, vegetation clearing should not occur on the bluff face. Limbing or topping of trees and brush trimming for view corridors is acceptable. Yard waste/debris: No debris or yard waste should be placed on or near the shoreline bluff. 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 shallow slope failure. September 28, 2020 Jefferson County Parcel 701301006 Geologic Hazard Assessment Stratum Group File: 8.24.20 20 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 References: Johannessen and MacLennan, 2007, Beaches and Bluffs of Puget Sound, Puget Sound Nearshore Partnership Report 2007-04, Washington Sea Grant Program, University of Washington, Seattle, Washington. Contreras, Trevor A., Patton, Annette I., Legorreta, Paulín, Gabriel Ian J., Hubert, Cakir, Recep and Carson, Robert J., 2014, Geologic Map of The Quilcene 7.5-Minute Quadrangle, Jefferson County, Washington. Washington Division of Geology and Earth Resources. Shipman, H., MacLennan, A., and Johannessen, J. 2014. Puget Sound Feeder Bluffs: Coastal Erosion as a Sediment Source and its Implications for Shoreline Management. Shorelands and Environmental Assistance Program, Washington Department of Ecology, Olympia, WA. Publication #14-06-016.