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HomeMy WebLinkAbout001061005 Geotech Report PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 October 3, 2022 Suzanne Carlson 1431 Washington Street Port Townsend, WA 98368 suzannelizette@gmail.com Re: Geology Hazard Assessment Jefferson County Parcel 001061005 McCurdy Point Road Port Townsend, Washington It is my understanding that you are planning on building a home on the property off of McCurdy Point Road. The Jefferson County Public Land Records landslide map indicates the shoreline bluff area is ‘high’ landslide hazard area. The shoreline slope stability map indicates that the shoreline bluff on the property is within an area the map indicates as ‘unstable old slides’. The seismic hazard map shows the bluff and area near the bluff as being in a seismic hazard area. The County maps do not include the area as an erosion hazard area. Based on my assessment of the subject property and vicinity, I conclude that a home can be located on the site outside of any landslide or shoreline bluff retreat areas and will not be at risk from landslides or shoreline bluff retreat for at least 100 years. Furthermore, development can be completed on the site such that it will not increase the risk of landslides or erosion on or off the property. Based on my geologic hazard assessment, it is my opinion that the risk of seismic hazards at the site will be no greater than the regional seismic hazard from area faults and there is no risk of potential liquefaction or seismic force enhancement at the site. This evaluation included a visual inspection of the property and vicinity, a visual inspection of the steep slopes and other slopes in the area, an inspection of the shoreline conditions at the site and vicinity including multiple inspections of this shoreline reach over the past 25 years, review of available geologic mapping in the area, review of lidar (light distance and ranging) imagery of the vicinity, and my own notes and observations I have made in the vicinity of the site and at locations with similar geologic conditions. DCD RECEIVED 12/02/2022 2 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 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 upland portion of the subject property above the steep shoreline bluff is underlain by glacial ablation till, glacial till and glacial advance outwash (Figure 1). The bluff is mapped as being underlain by undifferentiated glacial and non glacial deposits. A stratigraphic column a short distance to the suggests the bluff is underlain from top to bottom by glacial till, glacial advance outwash, Olympia glacial and non glacial deposits and Possession Drift. Figure 1. Portion of geologic map (Schasse and Slaughter, 2005). Qb = beach, Qguc = glacial and non glacial deposits undivided, Qgdme = glacial marine drift of Everson Stade, Qga = glacial advance outwash, Qgt = glacial till, Qgta = glacial ablation till, Qco = non glacial deposits of Olympia age, Qgpp = Possession Drift Observations on the uplands, bluff and shoreline on and in the vicinity of the subject property are generally consistent with the mapping described above. The uppermost part of the bluff all along the shoreline of the subject property and at least portions of the upland portions of the property are underlain by glacial till consisting of silty sand and gravel in a moderately compact condition. The till is on the order of a few feet to 15 feet thick. Most of the upland on the property appears to be underlain by slightly reworked glacial till and the till appears to be mostly ice wasting ablation till. The reworking is from wave Qgdme DCD RECEIVED 12/02/2022 3 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 action that has left very subtle wave cut terrace strands on the upland area that are only apparent in lidar bare earth imagery (Figure 2) than visually on the ground. Figure 2. Lidar bare earth via Jefferson County GIS. Note the very subtle wave strands across the uplands indicated by arrows. The strand lines were formed during the late stages of the last glacial period. During the last glacial period the mass of glacial ice depressed the local land surface hundreds of feet lower than current elevation. As the glacial ice thinned the area was briefly inundated with sea water. The melting ice on the shallow sea surface then ‘rained’ sediment onto the sea floor. The sediment deposited in this manner is called glacial marine drift. Glacial marine drift is present along the top of the shoreline bluff to the northeast (see Figure 1). The steep bluff is underlain by dense sand, silt and gravel beneath the till all along the bluff face. This dense sand and gravel varies in composition along the bluff face and is consistent with glacial advance outwash as wells as under ice flow as blocks of till and very high energy flows are embedded within the unit. No glacial drift is present at the base of the bluff at the subject property, but is present towards the northeast at McCurdy Point and to the southwest. The stratigraphy is consistent with this glacial drift at the base of the bluff being Possession Drift, a glacial drift unit deposited during an older ice age that predates that last glacial period. Historic aerial photographs of the shoreline dating back to the 1940s are consistent with this reach being an area of relatively slow bluff retreat over the historic record. However, the Site DCD RECEIVED 12/02/2022 4 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 shortness of the historic record should be considered as bluff retreat can be episodic as beach conditions change or the bluff becomes very undermined. Oblique aerial images dating back to 1990 do indicate that shoreline erosion on the lower portions of the bluff has taken place at the site and along much of the shoreline in this area. I have also observed that the lower portions of the bluff all along this shore reach have undergone erosion. For example a trail route that I utilized to access the shore in the past is no longer a functional route as a portion of the slope has eroded away leaving an impassable oversteep area. However, the top edge of the bluff at the property and along most of this shore line reach has not retreated during the past 50 years and the upper bluff slopes on the property remain tree covered. SPECIFIC SITE OBSERVATIONS The property consists of an upland area bounded on the northwest by a steep northwest facing shoreline bluff. Except for the shoreline bluff on the outer edge of the property, the upland portion slopes gently towards the northwest. The upland area is forested throughout with a mixed stand of predominantly evergreen trees. Soils on the upland consist of silty sand and gravel consistent with glacial till, but the relatively medium dense condition suggests that the upper soil has been reworked (see geology section above). The shoreline bluff on the property is approximately 95 feet in height. There is a lower are of the bluff where the shoreline bluff intersects a dry valley at the southwest corner area of the site. At that location the bluff is about 65 feet in height. The bluff is very steep throughout, with essentially vertical areas on the lower slopes and slopes on the order of 65 degrees on portions of the upper slope. Most of the upper slope is tree covered. Past shallow slope failures are apparent all along the northwest facing bluff. This entire section of the bluff has very little bluff vegetation on the lower bluff slopes and the bluff is generally very steep, but can hold steep slopes for long periods of time. Wedges of loose soil with vegetation have developed along the lower slope and wave erosion of this lower slope appears to be infrequent based on driftwood and vegetation at the base of the slope and areas of the uppermost beach; however, periodic storm surge events have and will erode the accumulated soil at the base of the bluff. The bluff slopes on the subject property are eroded primarily by wave action that undermines the base of the bluff. The shoreline at the site is exposed to open waters of the Straight of Juan de Fuca. Large waves are common during storms and hence periodically the shore and the base of the bluff are exposed to very large waves. DCD RECEIVED 12/02/2022 5 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 Because of the very compact nature of the bluff soils, the slope failures appear to consist of relatively narrow slab type failures. Ravelling of material off the exposed soils on the unvegetated portions of the bluff and periodic shallow topsoil failures also play a minor role. The material eroded from the bluff face is removed from the base of the bluff and transported along the shore towards the northeast by wave action. I did not observe any evidence (tension cracks or trees rotated inward away from the top of the bluff or indications of past uplift of the beach area) indicating an incipient global-type or deep- seated failure on the subject property. The lack of large mature trees along the bluff on the subject property and adjoining property bluff slopes is indicative of erosion rates that do not allow enough time to for trees to be become established for long periods. The beach at the base of the bluff is a sand, gravel and cobble beach. Overall the beach on this reach of shoreline is built up such that there is no large undermining of slope as the upper beach abuts the base of the slope. This may periodically change for short periods of time during large wave events with high tides and storm surge. Long shore drift moves sediment from the southwest to the northeast along this section of coast. With very little shoreline armoring and active erosion of the high bluffs to the southwest, the sediment supply to the beach remains very near natural conditions. Overall the beach appears to be in very good condition with no lowering of the beach that could lead to an increased rate of shoreline erosion. Large volumes of sediment are provided by the very high eroding bluffs to the southwest of the subject property. This sediment supply maintains the beach such that erosion at the toe of the bluff slope is limited to very high tides and storm wave events. 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 dense glacial advance outwash and with hard glacial till on the upper slopes. The beach on this shore reach is in good condition such that erosion of the base of the slope is not frequent, but it doe take place. 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. DCD RECEIVED 12/02/2022 6 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 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 3 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 3 inches per year, relative sea level rise projections will likely cause an increase of erosion and shoreline bluff retreat at this shoreline. Lavin and others (2019) using data from Miller and others (2018) projected 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 13-19%. The chance of a 1-foot rise by 2120 is 93-99%. The chance of a 2.5-foot rise by 2120 is 36-60%. For the longer term out to 2120 there is greater uncertainty primarily due to uncertainty regarding the 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, 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 4.0 inches per year. The top of the bluff will not retreat at an incremental rate of 4.0 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 4.0 inches per year, the top edge of the bluff can be expected to retreat 33 feet over the next 100 years. However, due to the potential for DCD RECEIVED 12/02/2022 7 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 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 38 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 120 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 8. 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. 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 map at the subject property has areas mapped as within site class C and areas mapped as in site class C to D. 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. DCD RECEIVED 12/02/2022 8 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 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. 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. CONCLUSION AND RECOMMENDATIONS 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) as long as the home and is located at least 38 feet back from the top edge of the slope. 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. Based on my geologic hazard assessment, it is my opinion that the risk of seismic hazards at the DCD RECEIVED 12/02/2022 9 October 3, 2022 Jefferson County Parcel 001061005 Geology Hazard Assessment Stratum Group File: 7.1.22 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. 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. Stratum Group appreciates the opportunity to be of service to you. Should you have any questions regarding our assessment please contact our office at (360) 714-9409. Sincerely yours, Stratum Group Dan McShane, L.E.G., M.Sc. Licensed Engineering Geologist DCD RECEIVED 12/02/2022