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HomeMy WebLinkAbout701311003 Geotech AssessmentStratum Group File: 6.19.19 1 PO Box 2546, Bellingham, Washington 98227 Phone: (360) 714-9409 October 10, 2022 Dan Gilmore 17622 12th Avenue SW Normandy Park, WA 98166-3600 glaciermutt@gmail.com Re: Geologic Hazard Assessment Jefferson County Parcel 701311003 East Quilcene Road Quilcene, WA It is my understanding you are planning to install a septic system on the site. This geologic hazard evaluation was conducted to assess the landslide, erosion and seismic hazards on the subject property and to inform you as to where development can take place on the site. The Jefferson County Public Land Records landslide map indicates a part of the western portion of the property is ‘high’ landslide hazard area and part is a ‘moderate’ landslide hazard area. The central portion of the property is mapped as ‘moderate’ and the eastern portion is mapped as ‘slight’. The shoreline slope stability map indicates that the shoreline bluff on the property is in part an ‘unstable old slide’ and the rest of the site is mapped as ‘unstable’. The seismic hazard map shows the bluff and area near the bluff as being in a seismic hazard area. The County map indicates the western portion of the property is 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 October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 2 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. GENERAL GEOLOGY The Geologic Map of Quilcene 7.5-Minute Quadrangle, Jefferson County, Washington (Contreras and others, 2014) indicates that the upland area of the property is underlain by ice contact deposits and the bluff slopes are underlain by Double Bluff Glacial Drift. Townsend Shale on the west side of the property west of East Quilcene Road and ice contact deposits on the east side of the property east of East Quilcene Road. Townsend Shale is also mapped at or very near the subject property. Contreras and others (2014) indicate a possible fault contact between the Townsend Shale and the Double Bluff Glacial Drift. Figure 1. Clipped and annotated geologic map (Contreras and others, 2014) showing the subject property location. Em2t= Townsend Shale, Qgdd = Double Bluff drift, Qcw = Whidbey Formation, Qgic = Vashon ice contact deposits, Qaf = alluvial fan deposits, Qls = landslide deposits. Black dotted lines are inferred faults. Black triangles are radiocarbon date sites. Orange diamond is site discussed in the map pamphlet. Gray circles with numbers are wells. The ice contact deposits consist of poorly sorted sediment that was deposited by glacial ice during the late stages of the last glacial period approximately 14,000 years ago. The Double Bluff Glacial Drift was deposited directly by glacial ice approximately 150,000 years ago during an earlier glacial period. Two later glacial periods have taken place since the Double Bluff glacial period. As such the Double Bluff Drift is very hard and compact. The Townsend Shale is a massive to well bedded marine mudstone, siltstone, and sandstone with locally abundant concretions. October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 3 My observations are consistent with the above-described mapping; however, I observed hard glacial drift underlying the ice contact deposits on the upper bluff slope and ravine slope of the property that I interpret to be Vashon glacial drift from the last glacial period. Like Contreras and others (2014) I noted the abrupt change in bluff composition essentially at the property with very hard, dense glacial sediments consistent with the interpretation that the unit is likely Double Bluff Glacial Drift and to the north (Figure 2) and bedrock consistent Townsend Shale to the south (Figure 3). The interpretation of the drift being Double Bluff is in part due to the overall stratigraphy of the area, the very hard condition of the sediment suggesting multiple periods of compaction and the tilted nature of the unit. The interpretation of a fault between the Double Bluff and the Townsend Shale is supported by the presence of highly deformed areas and possible brecciated areas within the Townsend Shale. Figure 2. Tilted sediments glacial related sediments to north of property interpreted as Double Bluff Drift October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 4 Figure 3. Bedrock consistent with Townsend Shale south of property. SITE SPECIFIC OBSERVATIONS The subject property is bounded on the east by East Quilcene Road. The eastern approximately 80 percent of the property has very gentle slopes towards the west towards the steep shoreline bluff on the west side of the property. This gentle slope upland area is underlain by moderately loose silty sand and gravel of variable composition. The area is forested with a mixed stand of trees with Douglas fir being the predominant tree with a mixed understory of vegetation as well. A steep side ravine is located on the southwest portion of the property and abuts the adjoining property to the south. The slopes of this ravine are forested. A trickle of water was flowing in the lower part of the ravine at the time of my site visit in May 2022. Most of the ravine stream channel is lined with boulders such that the stream is no longer capable of down cutting. Closer to the bluff the stream has down cut into the bedrock of the Townsend Shale and then the ravine intersects the shoreline bluff with a low waterfall drop over the underlying hard substratum. A trail from the neighboring property utilizes the stream valley as a path way to the shoreline. The shoreline bluff is approximately 65 feet high and with a slope of approximately 45 degrees. The slope is mostly forested throughout down to the shoreline. A few steeper areas with exposed thin soils are present and some minor erosion is evident along the base of the bluff where the forested slope is slightly undercut. The base of the slope at in the vicinity is resistant to erosion as it is underlain by hard glacial drift to the north and bedrock to the south. The contact between the two hard resistant units is obscured by a wedge of colluvium at the base of the slope that has trees growing on it (Figure 4). October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 5 Figure 4. View of lower slope with forested colluvium The shoreline reach at the site is characterized by a gravel/cobble beach and scattered driftwood logs. The beach is at least in part a platform type beach with thin beach sediment overlying either bedrock to the south or the hard glacial drift to the north. The shoreline at the subject property has been mapped as a feeder bluff on the Coastal Atlas and on the Jefferson County integrated shoretypes map (Coastal Geologic Services, 2015). Feeder bluffs are bluffs that eroded and “feed” the beach with sediment. A transport zone is mapped immediately to the north. My own observations are that there is really no discernable difference between what is mapped as transport or feeder bluff at this location. Based on site observations along the bluff and shoreline, the bluff slopes along this reach could be described as very slow feeder bluffs. Historic aerial photographs dating back to the 1940s including oblique aerials dating back to the 1970s reveal some shallow slides along this shore reach and evidence of past shallow surficial slides of various ages were present on the bluff in the vicinity both within areas underlain by bedrock and hard glacial drift. However, most of the bluff slopes are tree covered suggesting that the recurrence interval between shallow slide at any giver location and many decades. A slide deposit a short distance to the south that is of similar scale and appearance to the accumulated slide debris at the subject property and elsewhere along the bluff has been dated at an estimated age of 340 years old (Contreras and others, 2014). Maximum fetch is approximately 1.2 miles to the northwest towards the head of Quilcene Bay as Fisherman Point blocks waves from the south from Hood Canal. October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 6 GEOLOGIC HAZARD ASSESSMENT Shoreline Bluff Stability and Bluff Retreat Due to the bedrock and very hard glacial drift underlying the lower bluff and low wave energy at the site, bluff retreat at the subject property is very slow. Slow incremental bluff retreat at the site is driven by shallow thin upper weathered soil slides, minor raveling of mechanically weak mudstone, shale, and siltstone caused by oversteepening and undercutting by periodic wave erosion. Many bedrock bluffs in the Salish Sea region have bluff retreat rates that are essentially negligible (Shipman and others, 2014). However, the bedrock at the subject property is mechanically weak due to an abundance of discontinuities including bedding planes, joints, weathering fractures, and fault shear zones. The discontinuities create planes and zones of weakness in the rock mass that can act as failure surfaces for shallow rock slides and rock fall. The abundant discontinuities also make the rock highly fissile and friable and therefore subject to raveling. The very glacial drift is subject to slow weathering as well and appears to be forming bluff slopes essentially the same as the portions underlain by bedrock except that the glacial drift areas may be somewhat steeper as the result of the lack of fracturing in the drift relative to the bedrock. Based on my site observations, review or historic aerial photographs, the underlying geology as well as a radiocarbon date on a nearby slide, I estimate that the shoreline bluff retreat has been less than 1 inch 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 1 inch 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 16-21%. The chance of a 1-foot rise by 2120 is 90-98%. The chance of a 2.5-foot rise by 2120 is 37-59%. 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 until the bluff and shoreline adjust to the new level. October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 7 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 1.0 inch per year. The top of the bluff will not retreat at an incremental rate of 1.0 inch per year, but will retreat in an episodic manner where 1 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 1 inch per year, the top edge of the bluff can be expected to retreat about 9 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 14 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 will have 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. 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 to D. Based on our observation of very dense glacial drift and bedrock underlying the upland of the site with thin younger glacial drift, the ground shaking amplification is likely to be minimal and the site should be considered site class C and possibly B. There may be very local seismic energy focused at the October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 8 very top edge of the bluff and the steep bluff slopes 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. However, the site is no worse than any other locations with similar soils in the area of eastern Jefferson County. Seismic hazard maps have not been developed for the Dabob Fault Zone as there us a lack of record to determine its activity or recurrence interval, but its close proximity to the site may result in similar shaking as whet should be expected from Cascadia Fault. 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 drift and bedrock. Seismically Induced Landslides The very gentle sloping ground on the upland area 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. Stream Slope The stream valley on the southwest boundary area of the property has moderately steep slopes with deep incision only towards the top edge of the shoreline bluff. Some soil creep should be expected on the stream valley slopes and possible shallow slides near where the stream meets the top of the shoreline bluff. CONCLUSIONS AND RECOMMENDATIONS Based on my geologic hazard assessment, it is my opinion that a home and septic drainfield and access driveway can be located on the site such that they will be outside of any landslide or erosion hazard for well over 100 years. Furthermore, the development can be accomplished in a October 10, 2022 Jefferson County Parcel 701311003 Geology Hazard Assessment Stratum Group File: 5.20.22 9 manner that will not increase the risk of landslides or erosion on or off the site as long as the recommendations of this report are followed. Any building or development should be located at least 30 feet back from the top edge of the shoreline bluff and 25 feet back from the top edge of the stream valley slope on the southwest part of the property. Stormwater Management: Stormwater from the home and driveway access can be dispersed in the consistent with the Department of Ecology’s Stormwater Management Manual for Western Washington. The upper soil layers on the site consist of silty sand and gravel soils that will readily infiltrate stormwater if natural soils and vegetative cover are maintained consistent with the Manual and stormwater is dispersed and not concentrated. Yard waste and debris: No debris or yard waste should be placed on or near the shoreline bluff slope or on the steep slope down to the stream valley on the southwest part of the property. 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. Vegetation management: Currently the property is very well vegetated with native plants and tree cover. Thinning and limbing for view purposes is acceptable, but care should be taken to ensure vegetative cover of steep slopes is maintained. 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. Licensed Engineering Geologist