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HomeMy WebLinkAboutGeotechnical ReportStratum Group Project: 1.31.22 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 February 15, 2022 Richard Rathvon 20 Liberty Knoll Colts Neck, NJ 07722 rrathvon@gmail.com Re: Geologic Hazard Assessment for Proposed Septic System 660 Twana Way Jefferson County Parcel 701164005 Quilcene, WA 98376 This geologic hazard evaluation was conducted to assess the risk of landslide and erosion hazards at 660 Twana Way to determine the feasibility and appropriate locations for the construction of a new home and septic system on the property. The Jefferson County Public Land Records landslide hazard map indicates that part of the subject property is located in a moderate landslide hazard area and part of the property is located in a high landslide hazard area. The shoreline stability map indicates that part of the subject property has unstable slopes and an unstable recent slide is mapped as extending onto the north portion of the shoreline bluff on the property. The shoreline area of the property is mapped as an erosion hazard area and the lower portions of the property are mapped as a potential seismic hazard area. Based on our geologic hazard assessment, construction of a home and septic system can be located on the site outside of any landslide hazards or shoreline bluff retreat from erosion for the next 100 years as long as the home and septic system are located at least 30 feet from the top edge of the shoreline bluff. 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. 06/27/2024 February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 2 Project: 1.31.22 GENERAL GEOLOGY The Geologic Map of the Quilcene 7.5 Minute Quadrangle, Jefferson County, Washington (Contreras and others, 2014) indicates that subject property is underlain primarily by Whidbey Formation sediments with the lower western third of the property mapped as being underlain by Vashon ice contact deposits, which mantle older glacial and non-glacial formations such as the Whidbey Formation in the area. The map also indicates Double Bluff drift glacial deposits may be present at exposures at the southwest corner of the property. Landslides and landslide scarps are indicated just to the south of the subject property. Figure 1. Clipped and annotated geologic map showing the subject property. Qb = beach deposits, Qaf = alluvial fan, Qgic = ice contact deposits, Qcw = Whidbey Formation, Qgdp = Possession glacial rift, Qco = Olympia non- glacial deposits, Qgdd = Double Bluff glacial drift, Qc = Pre-Fraser non-glacial deposits, Qls = landslide. Hachures are landslide scarps. Heavy dotted line with query is a surmised potential fault. The Whidbey Formation is a non-glacial deposit that is between 125,000 and 80,000 years old. The Whidbey Formation in the vicinity of the property consists predominantly of sand but also includes units of silt and clay. The formation is dense and hard having been overridden by glacial ice over two separate glacial periods. The Vashon ice contact deposits (Qgic) are a mix of poorly sorted sediments deposited directly by glacial ice during the late stages of the last glacial period between about 14,000 years ago. Subject property Qb Qco Qgic Qcw Qls February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 3 Project: 1.31.22 Contreras and others (2014) mapped the unit where deposits appeared to be associated with ice wasting and the surface area of the deposits lacked streamlines associated with ice movement. The Double Bluff drift (Qgdd) consists of sediment deposited during a glacial period that is estimated to have taken place between 190,000 and 125,000 years ago. This formation represents a glacial period that occurred three ice ages ago. The basis of the interpretation is from age estimates and map relationships in the larger vicinity than shown on the map; however, this area is geologically complex due to the presence of a fault zone and local offsets and folding. Our observations are generally consistent with the above-described mapping. Soils observed on the property upland and exposed along the shoreline bluff primarily consist of very dense, stratified, sand and gravel with lesser silt lenses (Figure 2). These dense deposits are commonly overlain by a surficial cap of medium dense, poorly sorted, and unstratified, sand, silt, and gravel interpreted to represent Vashon ice contact deposits (Figure 3). A very dense, poorly sorted, and unstratified sand, silt, and gravel unit exposed at the base of the shoreline bluff on the subject property and to the south is consistent with Contreras and others (2014) interpretation of the unit being the Double Bluff drift mapped in the vicinity (Figure 2). February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 4 Project: 1.31.22 Figure 2. Whidbey Formation sand and gravel with Double Bluff drift at the base of the bluff a few hundred feet south of the subject property. Double Bluff Whidbey Fm. February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 5 Project: 1.31.22 Figure 3. Medium dense, poorly sorted sediment consistent with Vashon ice contact deposits on the site upland. SITE SPECIFIC OBSERVATIONS An annotated lidar hillshade image of the subject property and vicinity is provided in Figure 4. The property is accessed by Twana Way, a gravel/dirt road, which crosses through the subject property from the southeast corner to access residential properties to the north. February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 6 Project: 1.31.22 Figure 4. Annotated lidar hillshade map of the subject property showing key site features. Potential building area outside of any landslide or erosion hazards is outlined with green dashed line. The west edge of the property consists of a shoreline bluff (Figure 5). The bluff is nonexistent to very low at the northwest corner of the property where the bluff descends down to an accretion shore area and steadily rises to the south before dipping down somewhat near the southwest corner where the bluff intersects a valley. The bluff at the property is approximately 40 feet high at its maximum height. The bluff face slopes at approximately 10 degrees on the low, north end and slopes up to approximately 30 degrees toward the south end of the bluff. The bluff is underlain by compact sand and gravel consistent with pre glacial alluvial sediment and very dense silt/clay with gravel consistent with glacial till. The bluff face is vegetated with Douglas fir, western red cedar, and madrone trees with an understory of mostly grass with some other shrubs. We observed no concentration of hydrophilic vegetation or evidence of springs on the bluff face. Dabob Bay Accretionary shore Bluff erosion Gentle lowland slopes Twana Way Suitable building area Valley Valley Ridge February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 7 Project: 1.31.22 Figure 5. View of the beach and shoreline bluff on the subject property, looking northwest. The beach at the base of the bluff is a gravel and sand beach. Minor past erosion of the toe of the bluff is evident at or very near the southwest corner of the property where the very hard likely Double Bluff glacial drift deposits are exposed at the base of the bluff. The upper part of the beach consists of sand and gravel with driftwood. It appears that water did reach the base of the bluff recently, likely during the recent high tide/storm surge event on January 11, 2022. However, no significant erosion was evident and it appears that erosion occurs on an infrequent basis. A grass-covered accretion beach fronts the northwest end of the property and extends as a long shoreline spit for over a mile to the north of the subject property. No erosion was evident on the accretion shore area ad water did not reach the slope area fronted by the accretion shore reach. The shoreline at the property is located in a transition zone between areas of bluff erosion south of the subject property and shoreline accretion to the north. These observations are consistent with shoreline type designations on the Washington Coastal Atlas and Jefferson County Public Lands Records (Coastal Geologic Services, 2015). Slope face February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 8 Project: 1.31.22 The upland area above the shoreline bluff slopes up at approximately 10 degrees or less (Figure 6). This gentle slope area is located between two valleys. No water was present in the bottom of the northern valley on the property and the valley area is underlain by well drained sand. The valley on the south is on and/or just south of the property line. There is indication of periodic water flow in this valley, but no water was present at the time of our site visit on February 1, 2022. These valleys appear to be primarily remnant erosional features that formed at the end of the last glacial period when erosion rates were very high on the bare landscape right after ice retreated from the area. Figure 6. Gentle slopes on the lower (western) upland of the subject property. The eastern portion of the property consists of a plunging ridge between two valleys. The valley side slopes range between 15 and 35 degrees (Figure 7) up to the ridge line and ridge line plunges around 15 degrees to the west-southwest. The south sides of the ridgelines are steeper than the north sides. The Twana Way roadway and a driveway that descend from the road toward the shoreline are cut into the flanks of the slopes of the ridges, which on the steeper south sides of the ridges results in 2- to 4-foot high cut slopes above the road that reach 35 to 50 degrees in slope and 2- to 4-foot tall, 30-40 degree fill slopes below. February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 9 Project: 1.31.22 Figure 7. Moderate slopes descending from the south ridgeline on the eastern portion of the property upland. The upland area is forested with a mix of trees including Douglas fir, western red cedar, western hemlock and big leaf maple. The understory consists predominantly of sword fern and includes bracken fern, salal, evergreen huckleberry and other brush. GEOLOGIC HAZARD ASSESSMENT The upland slope area between Twana Road and the shoreline bluff slope is gently sloping and is outside of any landslide or erosion hazard area. Soils observed in test pits on this lower upland were well drained and capable of accepting additional water input contributed by a septic system and stormwater runoff from the home. The soils are well drained and generally coarse grained and will not be subject to seismic induced liquefaction. The portion of the property outside of any landslide or erosion hazards areas is indicated on Figure 4 above. Shoreline Bluff Slope Stability The shoreline bluff at the site is underlain by dense alluvial sediments consisting primarily of dense sand and gravel consistent with Whidbey Formation and very hard silty glacial till consistent with Double Bluff Drift. These bluff face sediments are capable of maintaining steep slope angles for long periods of time. The glacial drift is very hard and resistant to erosion. The steeper areas will be subject to ravel and shallow surface failures as the upper soil weathers or if February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 10 Project: 1.31.22 the slope is undercut by erosion. Currently there is only minimal erosion at the base of the shoreline bluff at the very southwest corner of the site (Figure 8). We observed no evidence of recent sliding other than minor ravel on the shoreline bluff slope. The ‘unstable recent slide’ indicated on the shoreline stability map does not appear in any aerial images dating back to 1951 or within the lidar bare earth imagery. There was an area of bare ground on a slope area to the north visible in the 1976 aerial photograph, but that appears to be associated with land grading and/or mining associated with road construction. We suspect the designation was mismapped. Figure 8. View of shoreline bluff from the south. Note minor erosion at hard glacial till located at southwest corner of property. Slopes on Eastern Portion of Site The slopes above Twana Road consist of two valleys with a ridge between. The ridge slopes and valley sides are moderate slopes and no indication of potential landslides is present. Some minor ravel of soil is present on the cut slopes associated with Twana Road. The steep headwall areas of these valleys appear to have had some shallow sliding in the past that sent slides part way down the valleys, but we interpret these landforms to be relict. The upper headwall areas of the north valley appear to be steep enough that shallow slides could take place, but the gradient of the valley, width of the valley and lack of water flow is such that we do not anticipate that shallow slides from the steep headwall of the valley would reach the subject property. February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 11 Project: 1.31.22 Beach Processes and Bluff Retreat The beach at the property is located towards the northern end of an approximately 10 mile long drift sector with sediment transported from south to north along the shore. Most of this drift sector is lined with fairly high feeder bluffs (Figure 9) and there is very little shoreline armoring that would reduce the sediment supply. Numerous very large landslides are located along the drift sector including a slide 0.4 miles to the south that covered the entire beach area 10s of feet deep with sediment over a distance of 550 feet in 1998, and there are multiple large areas of high bluffs of exposed glacial sediment that are eroding and raveling onto the beach feeding the beach. Figure 9. View of eroding bluffs south of property and large 1998 slide area 0.4 miles to the south. There is a slight change in shore orientation and openness to larger fetch along that shore area just to the south of the property. This is likely due to the presence of the very hard glacial till that is resistant to erosion at the base of the bluff at the subject property. The combination of the hard glacial till and the slight change in shore orientation is such that shoreline erosion at the subject property is very low relative to the shore areas to the south. The shore form at the property transitions from a feeder bluff to the south to a transport zone (bluff backed beach on Jefferson County map) to an accretion zone on the north part of the shore at the site. There was no indication of recent erosion having taken place during the very high tide storm surge of January 11, 2022 along the base of the slope on the property. No erosion of the accretion area below the north portions of the site was observed and water did not reach the base of the slope on the northern portion of the site where the trail is located during the January 11, 2022 event. There has been no appreciable change of the shoreline conditions or shoreline bluff 1998 large landslide February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 12 Project: 1.31.22 over the historic record based on our review or aerial photographs dating back to 1951 and historic maps dating back to 1883. Based on the beach conditions, shoreline bluff materials of dense sand and gravel, the very low erosion rate observed over the past 50 years in the oblique aerial imagery, the long term shoreline bluff retreat at the site has been less than 1 inch per year. The top edge of the bluff retreat will be episodic with many years where no retreat will take place at the top edge of the bluff; however, over time the upper bluff will become undermined as erosion and ravel take place on the slopes below and shallow small scale slides should be expected to reach the top edge of the bluff. Consideration of Sea Level Rise While past shoreline bluff retreat has been very slow and the long-term rate has likely been less than 1 inch per year, relative sea level rise projections for the year 2100 are estimated to be between 1.0 and 2.8 feet for the likely range of sea level rise under low to high green house gas levels (Miller et al., 2018). The increase in sea level will result in a greater frequency of high water reaching the base of the slope and hence an increase in erosion rate. However, the increase in erosion will take place along the entire shoreline drift sector and will be even more pronounced at the current feeder bluff areas to the south. The increased erosion to the south will add sediment to the beach fronting the subject property and thus will partially off set the sea level rise increase of erosion at the subject property as the entire shore reach and beach adjust to the change in sea level. Due to the change of seal level, we assume that there will be some increase in erosion rate and a corresponding change in bluff retreat. We estimate that the long term bluff rate accounting for sea level rise over the next 100 years will be 2 inches per year. This corresponds to top of bluff retreat of 8.3 feet over the next 100 years. An additional 20 feet should be added as an addition safety buffer for larger scale ravel or higher erosion rates than what we are estimating or other potential change in conditions along this shore reach. Seismic event considerations Test pits on the site found that the upland area where the home would be located is underlain by coarse well drained sediment. This sediment will not be susceptible to soil liquefaction from seismic events. Given the steepness of the shoreline bluff, small scale slides should be expected on the bluff slope in the event of a large seismic event. We observed no evidence of past seismic induced slope movement such as lateral spread on the uplands of the property on the ground or within lidar bare earth imagery The shoreline at the site could be impacted by earthquake-induced tsunamis or from large waves generated by landslides into the bay. Waves and currents produced by tsunamis or landslide waves could result in erosion of the toe of the shoreline bluff on the subject property that would February 15, 2022 Jefferson County Parcel 701164005 Geologic Hazard Assessment 13 Project: 1.31.22 not occur under more typical conditions. However, we anticipate this impact to be minor. The proposed building area as indicated on Figure 4 is well above the reach of any earthquake or landslide generated wave heights anticipated for this area. CONCLUSIONS AND RECOMMENDATIONS Based on our geologic hazard assessment, it is our opinion that construction of a single-family house and associated septic system on the lower upland on the subject property will not be at risk from landslides or erosion within the expected life of the structure as long as the building and septic systems are located at least 30 feet from the top of the steep shoreline bluff slope. Furthermore development within the area indicated on Figure 4 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. Stormwater should not be discharged onto or within approximately 30 feet of the shoreline bluff slope. Yard waste and debris: Ground cover vegetation within 20 feet of the top edge of the slope and on the bluff slope should not be disturbed. Trees within the buffer may be thinned or limbed for view purposes. However, if the trees are thinned, trees should be left on at least 30-foot centers. In addition, no debris or yard waste should be placed on or near the steep coastal bluff or other steep slopes on the property. Yard waste can eventually builds up and can form a wet unstable mass that will slide down the bluff damaging the slope and increasing the likelihood of a future slope failure. In addition to our construction recommendations, new construction on the subject property may also be subject to buffer requirements established in the Jefferson County Shoreline Master Program. Final construction setbacks should reflect whichever setback is largest. 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 Ben Carlson, L.G., M.Sc. Dan McShane, L.E.G., M.Sc. Licensed Geologist Licensed Engineering Geologist