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Home My WebLink AboutBLD2020-00572 - 09 GEOTECHStratum Group File: 10.23.19 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 December 10, 2019 Jane Uhlir 1138 Broadway Avenue E 2A Seattle, WA 98102 Re: Geologic Hazard Assessment Jefferson County Parcel 999200601 183 Aldrich Road, Port Townsend, WA Dear Ms. Uhlir: This geologic hazard evaluation was conducted to assess the risk of erosion and landslide hazards on the subject property. It is our understanding that you are planning to construct a new home and septic drainfield on the site. The property is located on a gentle upland slope above a shoreline bluff along Discovery Bay. Based on our geologic hazard assessment, it is our opinion that the subject property will not be at risk of landslides or shoreline bluff retreat for well over 100 years. Furthermore, the proposed development will not increase the risk of landslides or erosion on or off the site. This geologic hazard assessment included an inspection of the property and vicinity, slopes on the property, the shoreline bluff, and review of available geologic mapping, historic aerial photographs and lidar (light detecting and ranging) imagery of the site and vicinity. Past observations along the shoreline and at sites with similar geologic condition including areas in the vicinity of the subject property also aided in our interpretations. 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 and Slaughter, 2005) indicates that the subject property is underlain by Vashon advance outwash. December 10, 2019 Jefferson County Parcel 999200601 Geologic Hazard Assessment Stratum Group File: 10.23.19 2 Figure 1. Clipped and annotated geologic map (Schasse and Slaughter, 2005) showing the subject property location. Qga = Vashon advance outwash, Qgo = Vashon recessional outwash, Qgpc = undivided glacial and no glacial pre- Vashon deposits, Qp = peat, Qm = marsh deposits, Qb = beach deposits, Qls = landslide deposits. Our observations are not consistent with the above described mapping. Hand dug test pits on the subject property revealed medium stiff massive silt and clay with highly scattered pebbles consistent with Everson glaciomarine drift. The discrepancy between the above described mapping and our observations is likely due to mapping scale issues that do not allow for all areas to be precisely mapped. 1:24,000 scale mapping cannot account for all small-scale variability. Everson glaciomarine drift is unsorted clay-rich diamicton with lenses and discontinuous beds of moderately to well-sorted silt, clay, sand, and gravel. The marine drift was deposited during the late stages of the last glacial period when glacial ice had thinned and the area was submerged below sea level with glacial ice floating on the water above. The melting floating ice released sediments that were deposited on the sea floor. Dropstones of various sizes are commonly rounded, polished and striated. These dropstones are typically sourced from local areas as well as the Coast Plutonic Complex of British Columbia. We also observed bluff exposures along the shoreline downslope of the property as part of our assessment. The upper bluff is underlain by compact diamicton composed of clay, silt, sand, and gravel with scattered cobbles consistent with Vashon till (Figure 2). Observed pebble gravel and cobbles had a strong Coast Plutonic Complex signature indicative of glacial transport and deposition. December 10, 2019 Jefferson County Parcel 999200601 Geologic Hazard Assessment Stratum Group File: 10.23.19 3 The lower bluff is underlain by stratified dense pebble gravel with minor sand (Figure 2). Bedding gently tilted, likely due to subglacial deformation or isostatic loading. Unlike the overlying till, the pebble gravel had a strong Olympic Mountain signature indicative of nonglacial deposition. Based on this composition and the unit’s stratigraphic position underlying Vashon till, it is our opinion that the lower bluff is underlain by Olympia nonglacial deposits. Figure 2. Bluff exposure on an adjacent property to the southwest. SITE SPECIFIC OBSERVATIONS The subject property is located in a gently sloping upland area (Figure 3). Measured slopes on the site range from 0 to 12 degrees. Vegetation on the site consists of mixed forest stands that include western red cedar, Douglas fir, and madrone with an understory of salal, Oregon grape, snowberry, ocean spray, and other brush. The property waterward edge is setback located over 75 feet from the top edge of the shoreline bluff on the neighboring properties to the west and southwest. December 10, 2019 Jefferson County Parcel 999200601 Geologic Hazard Assessment Stratum Group File: 10.23.19 4 Figure 3. The gently sloping subject property. The shoreline bluff to the west-southwest ranges in height from 13 to 22 feet (Figure 4). On the properties directly downslope of the subject property, a wooden bulkhead protects the base of the bluff from wave action. Here, the lower bluff consists of a roughly 40-degree apron of material that raveled and failed from the upper bluff. The moderately sloping apron is covered in grass and weeds. The upper 3 to 5 feet of the bluff is near vertical and exposes the underlying soil (Figure 4). We observed evidence of previous and ongoing raveling on the steep upper portion of the bluff. Where the bluff is not protected by wooden bulkhead to the southeast, failed material from above has been eroded away and there is no apron on the lower bluff (Figure 4). Here, the bluff is near vertical with very little vegetation cover other than brush and weeds hanging from above. On unarmored portions of the bluff, we observed evidence of previous and ongoing raveling as well as periodic 1-to 2-foot-thick slab failures. December 10, 2019 Jefferson County Parcel 999200601 Geologic Hazard Assessment Stratum Group File: 10.23.19 5 Figure 4. The shoreline bluff on an adjacent property to the southwest. The shoreline is characterized by a sand and gravel beach with scattered cobbles (Figure 5). Driftwood logs on the upper beach mark the ordinary high-water mark (OHWM). Waves appear to reach the bulkhead (and nearby unarmored sections of the lower bluff) on a periodic basis. Maximum fetch is approximately 3.75 miles northwest across Discovery Bay. Shoreline landform and drift mapping completed for the Washington State Department of Ecology mapped the shoreline near the subject property as a feeder bluff and/or transport zone. A reach just to the northwest was mapped as accretion. This is mostly consistent with our observations; however, a portion of the bluff has been armored and that section no longer “feeds” sediment to the beach. Net shore drift is from the south-southeast to the north-northwest. December 10, 2019 Jefferson County Parcel 999200601 Geologic Hazard Assessment Stratum Group File: 10.23.19 6 Figure 5. The shoreline to the west of the subject property. GEOLOGIC HAZARD ASSESSMENT Bluff retreat on adjacent properties to the west and southwest of the site is very slow and is caused by minor raveling of the oversteepened upper portion of the bluff. Raveling is caused by weakening of the soils by weathering (rain and wind) and fracture propagation. Undercutting from wave erosion does not currently drive bluff retreat where the wooden bulkhead armors the lower bluff. In these areas, bluff retreat is approximately 1 inch per year. Using this bluff retreat rate, the subject property will not be at risk of shoreline erosion for well over 100 years. If the bulkhead were to be removed, the bluff retreat rate would increase due to erosion and undercutting of the lower bluff. Shallow 1- to 2-foot thick slab failures would also be possible on oversteepened portions of the bluff. Without the bulkhead, the long-term bluff retreat rate may be as high as 3 inches per year. However, even with this increased bluff retreat rate, the subject property would not be at risk of shoreline bluff retreat for over 100 years. During out site investigation, we did not observe any evidence of previous, ongoing, or incipient landslides on the subject property or vicinity. We did not observe any groundwater perching, seepage, or springs. There are no geologic units underlying the site that could act as perching layers for groundwater flow or failure surfaces for deep-seated landslides. There are no other conditions on the site that portend future landslides on the site. December 10, 2019 Jefferson County Parcel 999200601 Geologic Hazard Assessment Stratum Group File: 10.23.19 7 CONCLUSIONS Based on our geologic hazard assessment, it is our opinion that the subject property will not be at risk of landslides or shoreline bluff retreat for well over 100 years. Furthermore, the proposed development will not increase the risk of landslides or erosion on or off the site. For foundation bearing, a soil bearing value of 2,500 psf may be used for the glacial drift soils. The glacial marine drift at this location has been desiccated and hardened and the underlying glacial till or preglacial deposits are very compact or hard. Stormwater should be dispersed or infiltrated in a dispersed manner consistent with the Western Washington Stormwater Manual. 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