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HomeMy WebLinkAbout021212010 Geotech AssessmentPO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 August 19, 2022 Good Homes Construction 1401 Cleveland St. Port Townsend WA, 98368 goodhomes01@gmail.com Re: Geology Hazard Assessment, Shoreline Bluff Parcel 021212010 1280Schwartz Road Norland, Washington This geologic hazard assessment was conducted to assess the risk of landslide and erosion hazards on the above-mentioned property. It is my understanding that you are planning on building a home on the property. The proposed home will be located 160 feet back from the ordinary high water mark which is approximately 125 feet back from the top edge of the bluff. The Jefferson County Public Records landslide map maps the shoreline bluff as a ‘high’ landslide area. The Jefferson County Public Records shoreline stability map maps shoreline bluff as ‘recent slide’. The Jefferson County Public Records seismic hazard map maps the bluff area as a seismic hazard area. The County maps do not include the area as an erosion hazard area. Based on my assessment of the geologic conditions at the site, shallow landslides on the shoreline bluff should be expected; however, it is my opinion that the proposed home on the site will not be at risk from slope failures for the life of the structure (at least 100 years). Based on my geologic hazard assessment, it is my opinion that the risk of seismic hazards at the proposed home site is no greater than the regional seismic hazard and there is no risk of potential liquefaction or seismic forces enhancement at the site. This evaluation included a review of available geologic maps of the area, a visual inspection of the property vicinity and shoreline bluff including multiple inspections of this reach of shoreline associated with other projects on the east shore of Marrowstone Island over a period of the past 25 years, review of available geologic mapping in the area, review of historic aerial photographs and lidar (light detecting and ranging) imagery of the site and vicinity and review of proposed development plans for the site. 08/23/2022 2 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 GENERAL GEOLOGY The Surficial Geologic Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region, Washington (Pessl, Dethier, Booth and Minard, 1989) indicates the property is underlain by Vashon glacial till. The glacial till was deposited directly by glacial ice during the last glacial period between approximately 18,000 and 14,000 years ago when the area was over ridden by the Puget ice lobe. The map shows advance outwash deposits are located to the north and south of the property near the shoreline. Figure 1. Portion of geology map (Pessl and others, 1989). Qvt = Vashon glacial till, Qva (light blue) = Vashon glacial advance outwash and Qb = beach. Observations on the bluff face at the subject property and in the vicinity of the property are generally consistent with the mapping described above. The upper approximately half of the bluff face and the entire upland area on the property are underlain by very compact glacial till. The rest of the bluff is underlain by compact advance glacial outwash sand and gravel. Portions of the bluff are underlain by a wedge of colluvium soils associated with past slide deposits and raveling of soils off the upper slopes. 3 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 SPECIFIC SITE OBSERVATIONS The property consists of a gentle sloping upland that slopes very gently towards the east towards the shoreline bluff. There is an existing access driveway that enters the property from Schwartz Road and extends to the eastern portion of the property. Most of the upland is forested with Douglas fir being the predominant species. The upland area of the property is underlain by very dense sandy silt with scattered cobbles consistent with glacial till. The shoreline bluff is approximately 65 feet high. The upper 2/3rds of the shoreline bluff slope is very steep with vertical areas. The upper 15 feet is tree covered and very steep with a vertical area below. Below the upper portion the slope is steep to near the shoreline area where some evidence of waves reaching the toe of the slope is present. The bluff slope is consistent with the bluff being a feeder bluff, that is, erosion of the bluff ‘feeds’ the beach along this shore reach. The upper part of the shoreline bluff consists of very dense glacial till that forms the very steep upper slopes. The lower approximately 1/3rd of the bluff is underlain sand units consistent with glacial advance outwash. A wedge of loose soil is accumulated on the lower half of the bluff such that the underlying outwash sand is partially obscured but can be seen on better exposed portions of the bluff just to the north of the subject property. The bluff is eroded primarily by wave action that undermines the base of the bluff. Recent shallow soil failures are evident on the property and adjoining shore areas. Portions of the base of the bluff at the subject property as well as areas in the near vicinity show evidence of recent erosion. Deep-seated large scale landslides have not been observed on this shoreline reach and the underlying geologic units are not susceptible to large scale landslides. No seeps or areas of seasonal wet areas were observed on the bluff face. Based on site observations over the past 20 years and review of historic aerial photographs of the shoreline reach, there have been slope failures at the site, but these shallow slope failures have been limited to just a few feet thick. Over time, additional shallow slope failures on the lower slope and mid slope will steepen the slope and shallow slides will reach the upper slope. Based on the shape of the upper bluff slope on the property and in the near vicinity, the scale of upper slope failures when they do take place will be on the order of 5 feet or less into the upland area. 4 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 GEOLOGIC HAZARD EVALUATION Shoreline Bluff Stability and Bluff Retreat The shoreline bluff is an eroding bluff and over time the base of the bluff is eroded such that the slopes above are undermined and shallow slope failures will extend to the top edge of the bluff. The bluff consists primarily of hard and dense glacial advance outwash and hard glacial till and hence is fairly resistant to erosion by the moderate waves that periodically reach the base of the slope. Fully quantifying the long-term top of bluff retreat at the site is not possible due to the relatively short historic record and somewhat episodic nature top of bluff retreat. A review of historic aerial photographs and maps does not reveal any discernable top of bluff retreat at the property. Shallow surficial slides that have resulted in top of bluff retreat are evident in the near vicinity with the top of bluff retreat appearing to be a few feet as the result of the tope edge failures. Based on the geology of the site bluff, shoreline conditions, and available historic aerial photographs, it is my opinion that a reasonable conservative estimate for long-term overall top edge of bluff retreat for the site has been less than 2 inches per year. Consideration of Relative Sea-Level Rise While past long-tern top of shoreline bluff retreat has been very slow and the long-term rate has likely been less than 2 inches per year, relative sea level rise projections will likely cause an increase of erosion and shoreline bluff retreat at this shoreline. Utilizing Lavin and others (2019) and their use of data from Miller and others (2018) project chances of relative sea-level rise at various shore reaches of Washington State under a range of greenhouse gas emission scenarios. At the site Lavin and others (2019) project the chance of relative sea increase of 1-foot by the year 2050 is from 12-18%. The chance of a 1-foot rise by 2120 is 95-99%. The chance of a 2.5- foot rise by 2120 is 37-62%. For the longer term out to 2120 there is greater uncertainty primarily due to stability of ice sheets and variable emission scenarios, but in the shorter term out to 2050 the range of uncertainty has narrowed regardless of future emission scenarios and estimates of global sea level rise over the next 30 years are projected to be equal to the past 100 years of global sea-level rise (Sweet and others, 2022). The increase in sea level will result in a greater frequency of high water reaching the base of the slope and higher, and hence, there will be an increase in erosion rate. The increase in erosion will take place along the entire shoreline drift sector. The increased erosion will add sediment to the beach; however, the relatively short updrift length to the south will limit the sediment supply 5 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 to the beach and thus erosion rates and top of bluff retreat should be expected to increase over time due to the relative sea level change. Estimated Long-Term Top of Bluff Retreat Based on the geology of the site bluff, shoreline conditions, available historic aerial photographs, and consideration of sea level rise, it is our opinion that a reasonable conservative estimate for long-term top edge of bluff retreat for the site is 3.0 inches per year. The top of the bluff will not retreat at an incremental rate of 3.0 inches per year, but will retreat in an episodic manner where 2 to 5 feet of the upper bluff will fail and then many years to decades will pass by with only minimal retreat at the top edge of the bluff. Using a conservative long-term average retreat rate of 3.0 inches per year, the top edge of the bluff can be expected to retreat 25 feet over the next 100 years. However, due to the potential for additional failures along the top edge of the bluff beyond the simple average retreat rate, the retreat of the top edge of the bluff could be an additional 5 feet. Therefore, it is our opinion bluff retreat over the next 100 years may be a maximum of 30 feet. There is no evidence of ongoing or incipient large scale slope failures on the site and given the geology of the bluff, none are anticipated to develop on this bluff reach. Seismic Hazard Assessment Seismic Shaking Western Washington is seismically active and seismic shaking may occur on the site due to earthquakes along the Cascadia Subduction Zone (CSZ) or other faults present in the area. The subject property is located approximately 150 miles from the CSZ. The Washington Department of Natural Resources Cascadia Seismic Scenario Shaking intensity map indicates that the subject property along with all of eastern Jefferson County will have very strong shaking intensity with a modified Mercalli intensity level of 7. Expected ground conditions for this scale include negligible damage to well-built structures of good design and construction and slight to moderate damage in well-built ordinary structures. Damage may be considerable to poorly built structures. The relative near proximity of the South Whidbey Fault zone suggests the potential for a somewhat higher probability of an intensity level 7 earthquake at this location relative to some 6 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 other areas in Western Washington. However, the potential hazard associated with the South Whidbey Fault is not well understood as there are very few constraints on the potential hazard associated with this fault as well as other faults in the area. Future earthquakes along these local faults zone could produce moderate to strong ground shaking. National Earthquake Hazard Reduction Program (NEHRP) seismic site class mapping has been completed by the Washington State Department of Natural Resources (Palmer and others, 2004a). This mapping provides guidance as to the potential ground-motion amplifying effects of soft soils during an earthquake. The map indicates that “Site classes C, D, and E represent increasingly softer soil conditions which result in a progressively increasing amplification of ground shaking.” The NEHRP mapping indicates that the subject property is located within site class B to C. Based on our observation of very dense glacial till underlying the upland of the site, the ground shaking amplification is likely to be minimal and the site should be considered site class B. There may be very local seismic energy focus at the very top edge of the bluff that may result of the outer few feet of the top edge of the bluff fracturing and collapsing to the shoreline below. Based on my assessment, it is my opinion that the seismic shaking hazard at the site is moderate due to the potential for damaging shaking to occur during a Cascadia Subduction Zone, the South Whidbey Fault Zone or the Devils Mountain Fault Zone. However, the site is no worse than any other locations with similar soils in the area of eastern Jefferson County. Liquefaction Liquefaction is a process whereby saturated, unconsolidated sandy soils temporarily become liquefied and loose strength due to strong ground shaking. Liquefaction can result in differential settlement and/or lateral displacement. The soils found to be most susceptible to liquefaction are loose, fine sands containing very little silt or clay. Potential liquefaction decreases with coarser grained sand or higher silt content or if the fine sand is denser. Higher ground shaking accelerations or long periods of shaking can increase the potential for soil to liquefy. Hence, a close proximity quake may cause high accelerations and liquefaction and a distant quake with lower site acceleration but long time periods of shaking can also cause soils to liquefy. The Liquefaction Susceptibility Map of Jefferson County, Washington (Palmer and others, 2004b) indicates that the subject property has very low liquefaction susceptibility. This is consistent with my observations that the site is underlain by very hard and dense glacial till. 7 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 Seismically Induced Landslides The subject property is located on very gentle sloping ground and is not susceptible to slope failures. Shallow failures should be expected on the steep shoreline bluff and along top outer 5 feet of the top edge of the bluff. A seismic event may result in already marginally stable steep shoreline bluff areas to slide. CONCLUSIONS AND RECOMMENDATIONS Based on my geologic hazard assessment, it is my opinion that the risk of seismic hazards at the subject property is no different than other sites in the area. The site is not subject to enhanced seismic movement and is not at risk from soil liquefaction. Furthermore, the proposed development will not result in an increase in seismic hazards at the site. Based on my geology assessment of the property and vicinity including observations made over the past 20 years during multiple inspections of this shoreline bluff reach, I conclude that the proposed home and septic system will not be at risk from shoreline bluff retreat or slope failures within the expected life of the structure (at least 100 years). Furthermore, it is my opinion that the proposed development of the property will not increase the risk of landslides or erosion on or off the site. It is my understanding that proposed home and septic system will be located 160 feet back from the ordinary high water mark which will place the development well outside of the any expected shoreline bluff retreat area, landslide hazard or seismic hazard area. Stormwater should be dispersed on site consistent with the Washington State Department of Ecology Western Washington Stormwater Manual. Soil bearing for foundations can utilize 3,000 psf for soil bearing. Because the bluff is an eroding bluff, slope failures should be expected to occur on a periodic basis. The only way to prevent continued erosion of the base of the bluff is to construct hard armoring at the shoreline at the base of the bluff. However, the eroding bluff acts as a feeder bluff (erosion of the bluff provides sediment) for the beaches north of the subject property, and any shoreline armoring will have a negative impact on properties down drift from the bluff. The construction of shoreline armoring on other properties may cause an increase of erosion of the subject property. The owner of the subject property should contact Jefferson County officials regarding any proposed shoreline protection projects along the bluff or any shoreline protection construction. There are currently no shoreline-armored areas along this stretch of coastline. 8 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 If conditions appear different than those described in this report, or other concerns arise, we request that we be notified so we can review those areas and modify our recommendations as required. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, M.Sc., L.E.G. Licensed Engineering Geologist 9 August 19, 2022 1280 Schwartz Road, Norland, WA Geology Hazard Assessment Stratum Group File: 5.1.22 References: Lavin, P., Roop, H.A., Neff, P.D., Morgan, H., Cory, D., Correll, M., Kosara, R., and Norheim, R., 2019. Interactive Washington State Sea Level Rise Data Visualizations. Prepared by the Climate Impacts Group, University of Washington, Seattle. Updated 7/20. Miller, I.M., Morgan, H., Mauger, G., Newton, T., Weldon, R., Schmidt, D., Welch, M., Grossman, E. 2018. Projected Sea Level Rise for Washington State – A 2018 Assessment. A collaboration of Washington Sea Grant, University of Washington Climate Impacts Group, University of Oregon, University of Washington, and US Geological Survey. Prepared for the Washington Coastal Resilience Project. updated 07/2019 Palmer,S.P., Magsino, S.L., Bilderback, E.L., Poelstra, P.L, Folger, D.S, and Niggemann, R.A., 2004a. Site Class Map of Jefferson County, Washington. Washton Division of Geology and Earth Resources open file report 2004-20 Liquefaction Susceptibility and Site Class Maps of Washington State, By County Map 16B—Jefferson County NEHRP Site Class Sheet 32 of 78. Palmer,S.P., Magsino, S.L., Bilderback, E.L., Poelstra, P.L, Folger, D.S, and Niggemann, R.A., 2004b. Site Class Map of Jefferson County, Washington. Washington Division of Geology and Earth Resources open file report 2004-20 Liquefaction Susceptibility and Site Class Maps of Washington State, By County Map 16B—Jefferson County NEHRP Site Class Sheet 31 of 78. Pessl, F.Jr., Dethier, D.P., Booth, D.B., and Minard, J.P., 1989. Surficial Geologic Map of the Port Townsend 30 by 60 Quadrangle, Puget Sound Region, Washington. Folio of the Port Townsend Quadrangle, Washington, Miscellaneous Investigations Series Map I1198-F. Sweet, W.V., Hamlington, B.D., Kopp, R.E., Weaver, C.P, Barnard, P.L., Bekaert, D, Brooks, W, Craghan, M., Dusek, G., Frederikse, T., Garner, G., Genz, A.S., Krasting, J.P., Larour, E., Marcy, D., Marra, J.J., Obeysekera, J., Osler, M., Pendleton, M., Roman, D., Schmied, L., Veatch, W., White, K.D. and Zuzak, C, 2022. Global and Regional Sea Level Rise Scenarios for the United States: Up dated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines. NOAA Technical Report NOS 01. National Oceanic and Atmospheric Administration, National Ocean Service, Silver Spring, MD, 111 pp.