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HomeMy WebLinkAbout821333014 Geotech AssessmentApril 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 1 Geotechnical Evaluation of Parcel 821333014 and 821333002 Jefferson County, Washington. David S. Parks M.S. Principal Geologist L.G., L.E.G., L.H.G # 533 Crescent Environmental PLLC 424 East First Street, Box 429 Port Angeles WA 98362 crescentenvironmental@gmail.com Cell: 360-640-3187 Client: Carl Peterson Email: sp32091@hotmail.com April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 2 1.0 INTRODUCTION Crescent Environmental PLLC is pleased to present the results of a Geotechnical Evaluation of parcels 821333014 and 821333002, Jefferson County, Washington (Figure 1). (Note: all figures are at the end of this report). I have qualitatively evaluated the relative slope stability of the property (parcel 821333014 and 821333002), Jefferson County, Washington, based on soil and geologic characteristics, slope morphology, hillslope gradient, and plant community types and ages. I have performed an on-site review of the subject parcel and have evaluated the relative slope stability of the parcel with respect to the presence/absence of landslide features and coastal erosion hazards. This report is an instrument of service subject to the limitations and conditions set out in Appendix B and was prepared by an Engineering Geologist and Hydrogeologist licensed in the State of Washington. In preparing this report Crescent Environmental PLLC visited parcel 821333014 and 821333002, Jefferson County, Washington on March 15, 2021. This work was conducted in accordance with the signed scope-of-work between Crescent Environmental PLLC and Carl Peterson dated March 3, 2021. 2.0 Purpose and Scope The purpose of this geotechnical evaluation is to qualitatively characterize the landslide and erosion hazards of parcels 821333014 and 821333002, Jefferson County, Washington, and to evaluate the feasibility of locating two residential structures on the low gradient central portion of the two parcels adjacent to a Landslide Hazard Area as defined by Jefferson County Code (e.g., slopes steeper than 40% and greater than 10 vertical feet). In order to qualitatively characterize landslide hazards, I performed an on-site review of the parcels to make observations of the land surface characteristics with respect to any deep-seated or shallow landslides or other indicators of active mass wasting processes such as exposed scarps, tension cracks, hummocky topography, tilted or swept trees, seeps, springs or concentrated water, sag ponds, or closed depressions. 3.0 Site Location The subject parcels (821333014 and 821333002) are located about 3.4 miles West of the Hood Canal Bridge near Teal Lake Road, Washington in Jefferson County, April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 3 Washington (Figure 1). The two parcels are approximately 3.67 acres (northern) and 0.52 acres (southern) in area according to the assessors’ survey record (Figure 2). 4.0 Site Characteristics The subject parcels are situated on a mostly flat to moderately sloping terraced outwash and till plain bordering the Olympic Mountains to the West and Squamish Harbor to the south. The parcels encompass flat to moderately sloping (>35%) hillslopes above Squamish Harbor that have been used historically as a gravel mine but were reclaimed in 2009. (Figures 3 & 4). The elevations of the subject parcels range from approximately 80 to 120 feet above mean sea level (m.s.l.) based on LiDAR bare earth elevation data using the National Vertical Datum of 1988 (NAVD88) (Figure 3 & 4). Precipitation is seasonally variable, characterized by wet mild winters and relatively dry, cool, summers. Annual average precipitation (1981-2010) is 22 inches primarily occurring as rainfall. Native vegetation on the parcel is a mixed second-growth conifer-dominated forest composed of Douglas fir, Western red cedar, Big Leaf Maple, Red alder and Madrone. The understory vegetation is dominated by upland plant species such as Salal, Snowberry, Huckleberry, and Alder. A dense community of non-native Scotch Broom (Cytisus scoparius) occupies the area previously used as a gravel mine. Jefferson County critical areas mapping derived from the Coastal Zone Atlas of Washington has identified the area of the two parcels as having a “Slight” landslide hazard (Figure 5). 5.0 Geologic Conditions Regional The project area is located on the northeast side of the Olympic Mountains, a range of sedimentary and volcanic rocks uplifted as a result of tectonic convergence and subduction along the Cascadia margin of North America. The highlands to the south are in Crescent Formation, mostly high-relief terrain formed on volcanic rocks. The lower and less steep lands to the north are sedimentary bedrock units overlain by recessional outwash and till plains, where ice sheets that occupied the Strait of Juan de Fuca and April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 4 Hood Canal during the Pleistocene lapped on the edge of the Olympic Mountains. Surficial Geology On the rolling uplands the sedimentary bedrock is overlain by weathered till, outwash and alluvial fan deposits (Pessl, Jr. et al. 1989). During major ice ages, continental glaciers pushed south into the lower terrain around the Olympic Mountains and the continental ice sheet pushed southward from the west end of the Puget Lowlands and south from the Juan de Fuca Strait. The glacial drift once formed an irregular blanket over the bedrock throughout the area, but post-glacial incision by the streams and the Strait of Juan de Fuca and Hood Canal cut through the glacial deposits into the sedimentary bedrock units, leaving drift mantling the rolling uplands and irregular strath terraces within the major canyons. Weathering and mass- wasting (creep and landslide) processes along the new canyons, ravines and coastal bluffs have continued to erode into the canyon walls. Locally the project area is mapped with surficial deposits of older alluvium, recessional outwash and Juan de Fuca lobe glacial till (Figure 6). Recessional outwash and glaciomarine drift are Pleistocene in age and include gravel, sand, silt, clay, and locally peat; and are characterized by northern rock types. Juan de Fuca lobe glacial till is Pleistocene in age and includes unsorted and highly compacted mixture of clay, silt, sand, gravel, and boulders deposited directly by glacial ice; gray where fresh and light yellowish brown where oxidized; permeability is very low where lodgment till is well developed; clasts are of northern source but with abundant Olympic rock types where Olympic sediments are present in the substrate. Till is most commonly matrix-supported but locally clast-supported; matrix more angular than water worked sediments; cobbles and boulders commonly faceted and (or) striated; and forms a patchy cover ranging from 0.5 ft-20 ft. thick. Glacial till is frequently overlain by recessional outwash and underlain by advance outwash deposits or other older units (Othberg and Palmer, 1979; Pessl, Jr., et al., 1989). Geologic mapping by Pessl, Jr. et al. (1989) of the subject parcel (Figure 4) shows two geologic units on the parcel; undifferentiated pre-late Quaternary Glacial till (Qgt) and Quaternary glacial outwash. No landslide features are mapped on or near the subject parcel by Othberg and Palmer, 1979 or Pessl., Jr. et al., 1989. Soil Textures There are two soil textural series mapped on parcels 821333014 and 821333002, Jefferson County, Washington (McCreary, 1975) as shown in Figure 7. The slopes April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 5 composing the eastern portions of the parcels are underlain by Carlsborg gravelly loamy sand slopes of 0-15 percent and the western areas of the parcels are composed of Everett gravelly sandy loam soils. The Carlsborg gravelly loamy sand is described as a well-drained, gravelly loamy sand soil that developed in glacial outwash. The soil is typically 20-60 inches deep, with a gravelly sandy loam extending to a depth of 20 inches. This soil is typically excessively drained, runoff is slow and the hazard of erosion is slight. McCreary (1975) provides an (USCS) classification for the Clallam soil series as gravelly sandy loam, very gravelly sandy loam (SM-GM) (McCreary, 1975). The Everett soil series consists of somewhat excessively drained, gravelly soils from glacial outwash. McCreary (1975) provides an (USCS) classification for the Clallam soil series as gravelly sandy loam, very gravelly sandy loam (GP-GM) (McCreary, 1975). Runoff is slow and the hazard of water erosion is slight to moderate. (McCreary, 1975). The soils and underlying glacial till on the subject property have relatively high values for soil cohesion, ranging in value from 1500-5000 pounds per square foot (psf) (Koloski, Schwarz, and Tubbs, 1989). Figure 8 shows the locations of adjacent water wells and figures 9-11 show well driller’s logs for each of these three wells documenting the subsurface conditions surrounding the two parcels of interest. 6.0 Seismic Hazards The site location of the project parcels is mapped as having a low liquefaction hazard rating by the Department of Natural Resources (Figure 12). The Seismic Design Criteria rating for the project parcels are listed as D1 by the Department of Natural Resources. (Figure 13). Criteria D1 relates to the International Building Code requirements for short period ground accelerations between 0.67 g and 0.83 g (Note: g is the acceleration of gravity, 9.8 m/s2). 7.0 Review of General Slope Stability Processes When considering the relative stability of a natural hillslope, two primary factors exert a dominant control on shallow landslide processes, slope gradient and slope shape. Slope gradient is a measure of slope steepness and as the slope gradient approaches April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 6 70% (rise over run) the incidence of shallow landslides increases dramatically (Carson and Petley, 1970). Slope shape is important because convergent hillslopes tend to concentrate water in the center or axis of the hillslope and this concentration of water increases the pore water pressure in the slope, reducing the strength of the material within the hillslope (Sidle, 1984). Divergent slopes disperse water and reduce pore water pressure in the slope. Planar slopes similarly also do not tend to concentrate water in any one location and are more stable than convergent hillslopes. In the Pacific Northwest, where hillslopes are typically vegetated with over story trees and dense understory vegetation, plant roots help to reinforce the soil mass in the rooting zone of vegetation. When trees are cut for harvest or to maintain views, the rooting strength diminishes over a time period of 3-7 years (Sidle, 1992). This reduction in root reinforcement in the upper part of the soil can lead to the initiation of shallow landsliding if the slope is steep enough. Therefore, the most hazardous combination of factors that can lead to landslide occurrence is vegetation cutting on steep, convergent hillslopes, without providing for immediate revegetation of the hillslope to offset the root strength reduction as the plant roots decay and lose strength. For the soils steeper than 40% found on parcels 821333014 and 821333002, Jefferson County, Washington, the relative reinforcement effects of plant roots are an important factor in maintaining slope stability due to the root network function in providing cohesion to the well-rounded clasts found in the glacial outwash and till on the site. Additionally, the role of woody tree species in forming soil arches on non-cohesive hillslope soils where tree trunks are firmly anchored in the soil at appropriate intervals has been recognized (Gray and Megahan, 1981) as an important factor that maintains slope stability. The role of lateral root cohesion has been identified by many authors (Sidle et al., 1985; Tsukamoto and Minematsu, 1986; Tsukamoto, 1987) and the soil reinforcement strength of lateral plant roots of non-tree species relative to shallow landsliding has been quantified in the Oregon Coast Range (Roering, 2001). A list of plant species that provide effect rooting strength that are appropriate to the Pacific Northwest is included in Appendix A. Deep-seated landslides are mass wasting features that exhibit a failure plane below the depth of rooting vegetation to a depth of hundreds of feet. Deep-seated landslides occur in all lithologies (rock types) and can be relatively slow-moving on the order of centimeters/year to extremely fast (meters/second). Deep-seated landslides typically have a characteristic morphology in the horizontal April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 7 plane with a concentric or “half-circle” shape and are typically composed of a series of moving blocks of rock and soil in the vertical plane that give the slope a bench-like appearance in cohesive materials. Deep-seated landslides can also occur in fine- grained (silt or clay) materials in the form of an earth flow. Finally, some deep-seated landslides exhibit a block-glide type failure mode where a large mass of rock or soil material moves vertically down a slope. Recent examples of reactivated glacial deep-seated landslides in Washington State include the Oso landslide in Snohomish County and the Ledgewood landslide in Island County. 8.0 Aerial Photography Review Aerial photographs reviewed of the subject parcel include 1951, 1968,1990, 2006, 2009, 2011, 2013, 2015, 2017, and 2019 (Figures 14-23) and oblique photographs of the site were reviewed for years 1977, 1996, 2006 and 2016 (Figures 24-27) (WDOE, 2021). The 1951 and 1968 photography shows the site fully forested prior to active gravel mining that is first observed in the 1990 photography (Figure 16). Subsequent photographs from 2006-2009 show the site undergoing active gravel mining and reclamation in 2009 (Figure18). Following 2009, the site begins to revegetate and becomes increasingly covered with plant and tree cover until the present time (Figures 19-23). Oblique shoreline aerial photographs show the subject parcel from 1977 and 1996 through the years 2006 and 2016 (Figures 24-27). There is no evidence of landsliding observed from the aerial photography record that would potentially threaten proposed residential structures located on the gently sloping areas of the former gravel pit floor. 9.0 On-Site Observations Observations of the land surface on parcel 821333014 and 821333002, Jefferson County, Washington were made on March 25, 2021. I walked the lot boundaries and reviewed ground conditions including the area to the east of the edge of parcel. The upper northern portion of the northern parcel is a level to gently sloping (0-20%) forested hillslope which transitions to a moderately sloping (35%) hillslope in the upper portion of the old gravel mine (Photograph 1). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 8 Photograph1: View looking east from the flat central portion of parcel 821333014. Areas on the east and west end of the moderate hillslope are steeper (>70%) in limited areas. The southern sides of the parcels round over into a moderately (>20%-40%) sloping forested hillslope above Teal Lake Road (Photograph 2). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 9 Photograph 2: View looking west on moderately sloping hillslope directly north of Teal Lake Road. The slope directly above Teal Lake Road is vegetated with mature conifer trees and dense understory vegetation consisting of madrone, salal, ocean spray, and alder. On-site observations made on 3/25/2021 revealed a complete absence of observable landslide hazards on the two parcels. Review of historical aerial photographs from 1951-2019 and oblique aerial photographs from the Department of Ecology from 1977-2016 indicate no observable evidence of landsliding on the site. 10.0 Qualitative Slope Stability Assessment Based on a review of published geologic mapping, soil series descriptions, review of historic aerial photographs and on-site observations, the subject parcel has been stable over the assessment period with respect to landsliding that would threaten proposed residential structures located on the gently sloping central portion of the former gravel mine. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 10 The moderate (35-40%) slopes on the north side of the parcel once formed the working face of the former gravel mine and have been adequately reclaimed and left in a stable condition. 11.0 Jefferson County Critical Areas Designation A Geologically Hazardous Area designation defined by the Jefferson County Critical Areas Code is listed for the subject parcel as shown on Figure 5, and the description of geologically hazardous areas contained in the Jefferson County Code is given below: 18.22.510 Classification/designation. Geologically hazardous areas are areas that because of their susceptibility to erosion, sliding, earthquake, or other geological events, are not suited to siting commercial, residential, or industrial development consistent with public health or safety concerns. Unless specifically noted below, principal sources of geologically hazardous areas mapped information are the Washington Department of Natural Resources (DNR) Geologic Hazard Maps (https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/geologic-hazard- maps). (1) The following are geologically hazardous areas and subject to the standards of this article when mapped as high or moderate geologically hazardous areas31: (a) Erosion Hazard Areas (as defined in JCC 18.10.50). (b) Landslide Hazard Areas (as defined in JCC 18.10.120). Landslide hazard areas include any areas susceptible to landslide because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors, as follows: (i) Areas of historic failures, such as: (A) Areas delineated by United States Department of Agriculture, Natural Resources Conservation Service as having a significant limitation for building site development; (B) Coastal areas mapped by the Department of Ecology Coastal Atlas32 as unstable, unstable old slides, and unstable recent slides in; or, (C) Areas designated and mapped as quaternary slumps, earthflows, mudflows, lahars, or landslide hazards by the Washington State Department of Natural Resources or the United States Geological Survey. (ii) Areas where all three of the following conditions occur: (A) Slopes are steeper than 15 percent; (B) Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and, (C) Spring or groundwater seepage. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 11 (iii) Areas that have shown movement during the Holocene epoch (from 10,000 years ago to present) or have been underlain or covered by mass wastage debris of this epoch. (iv) Areas with slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials. 31 Inserted by staff after public hearing; was unintentionally omitted and now appears throughout this article. 32 Reinserted by staff; was removed from PC draft as too outdated but no known other source is available. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 12 Jefferson County Code Chapter 18.22 CRITICAL AREAS DRAFT WORK PRODUCT SUBJECT TO REVIEW EXHIBIT A Page 41/109 (v) Areas with slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking. (vi) Areas that are potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action, including stream channel migration zones. (vii) Areas that show evidence of, or are at risk from, snow avalanches. (viii) Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding. (ix) Areas with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet, except areas composed of bedrock. (c) Seismic Hazard Areas (as defined in JCC 18.10.190). (d) Channel Migration Zones (CMZs) (as defined in JCC.18.10.030). (d) Seiche and Landslide Generated Wave Hazard Areas (as defined in JCC 18.10.190). (e) Tsunami Hazard Areas (as defined in JCC 18.10.200). (f) Other geologic events, such as coal mine hazards and volcanic hazards are not a known risk in Jefferson County, and are therefore, not subject to review as part of the permitting process. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 13 12.0 Recommendations for Erosion and Landslide Hazard Areas 1. One geologically hazardous area as defined in the Jefferson County Code is located on the southern side of the two parcels above Teal Lake Road. Figure 28 shows the delineation of the slopes 40% and steeper with a vertical elevation of 10 feet or greater above Teal Lake Road. It is my recommendation that a 20 horizontal foot buffer be established from the edge of slopes 40% and steeper adjacent to the top of the slope above Teal Lake Road. 2. Based on a complete absence of landsliding over the 70-year period 1951-2021, I have no other recommendations regarding mitigation of landslide hazards. The former working face of the gravel mine has been adequately reclaimed and has maintained a stable configuration since 2009. 3. All existing and future drainage from the proposed building sites should be contained on-site. I recommend that you retain a qualified civil engineer to design a suitable storm water system for your parcel that will minimize discharges to the northern and eastern side of the parcels. 4. Standard erosion control measures (sediment fences, erosion blankets on disturbed areas) will be adequate during site preparation and construction due to the well- drained soils on site, the lack of surface water drainage, the heavily vegetated conditions of slopes down-gradient from the parcel, and the relatively large distance to any receiving waters near the site 5. It should be anticipated that utility trenches may need shoring due to the coarse- grained nature of the soils mapped on the site. 6. Clearing of non-native Scotch Broom plants on the two parcels is unlikely to affect hillslope stability. I do recommend consideration of native plant species shown in Appendix A when removing non-native plants. 13.0 Conclusions On-site inspection and review of historical aerial photographs over a 70-year period (1951 and 2021) show no observable landsliding on the two parcels over the evaluation period. Implementation of actions listed above are likely to reduce the risk to the environment from residential structures proposed for the subject parcels over typical engineering design lifespans from the potential earth hazards that exist in this area. This report is subject to the conditions and limitations set forth in Appendix B. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 14 Sincerely, David S. Parks M.S. Licensed Engineering Geologist, Hydrogeologist L.G., L.E.G., L.H.G. #533 Crescent Environmental PLLC 424 East First Street, Box 429 Port Angeles, WA 98362 Cellular: 360-640-3187 Email: crescentenvironmental@gmail.com April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 15 14.0 References Carson, M.A., and D. J. Petley., 1970. The existence of threshold hillslopes in the denudation of the landscape. Trans. Inst. British Geogr. 49, 71-95. Contreras, T.A. 2013. Geologic Map of the Lofall 7.5-Minute Quadrangle, Jefferson and Kitsap Counties, Washington. Geologic Map 2013-03, Washington Department of Natural Resources. Gray, D.H., and W.F. Megahan., 1981. Forest vegetation removal and slope stability in the Idaho Batholith. Res. Paper. INT-271, 23 pp. U.S. Forest Service, U.S. Dept. of Agriculture, Ogden, Utah. Koloski, J.W., S.D. Schwarz, and D. W. Tubbs., 1989. Geotechnical Properties of Geologic Materials. Engineering Geology in Washington, Volume 1. Washington Division of Geology and Earth Resources Bulletin 78. McCreary, F.R., 1975. Soil Survey of Jefferson County Area, Washington. United States Department of Agriculture, Soil Conservation Service. Othberg, K.L. and P. Palmer. 1979. Preliminary Surficial Geologic Map of Part of the Gardiner Quadrangle, Jefferson County, Washington. Open File Report (OFR) 79-19. Washington Department of Natural Resources, Olympia, Washington. Pessl, Jr., F., D.P. Dethier, D.B. Booth, J.P Minard, 1989, Surficial Geologic Map of the Port Townsend 30-60 Minute Quadrangle, Puget Sound Region, Washington. IMAP Series 1198. United States Geologic Survey. Roering, J., 2001. Root cohesion variability and shallow landslide susceptibility in the Oregon Coast Range. Canadian Geotechnical Journal, DOI: 10.1139/cgj-38-5-995. Sidle, R.C., 1984. Relative importance of factors influencing landsliding in coastal Alaska. Proc. 21st Ann. Eng. Geol. and Soils Eng. Symp., pp. 311- 325. Univ. of Idaho, Moscow. Sidle, R.C., A.J. Pearce, and C.L. O’Loughlin.,1985. Hillslope Stability and Land Use, Water Resources Monograph, vol. 11, American Geophysical Union, Washington D.C. Sidle, R.C., 1992. A theoretical model of changes in root cohesion in response to vegetation management. Journal of Environmental Quality, 20(1), 43-52. Tsukamoto, Y. and H. Minematsu., 1986. Evaluation of the effect of lateral roots on April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 16 slope stability, Proceedings of the 18th IUFRO World Congress, Ljubliana, Yugoslavia. Tsukamoto, Y., 1987. Evaluation of the effect of tree roots on slope stability. Bulletin of the Experimental Forest, 23, 65-124. Washington Department of Ecology, Shoreline Aerial Photography Viewer, 2021. Washington Department of Ecology, Well Log Viewer, 2021. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 17 Figure 1: Location map of parcels 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 18 Figure 2: Assessor’s survey map of parcels 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 19 Figure 3: Project Area LiDAR Hillslope Gradient (% slope). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 20 Figure 4: Map showing parcel boundaries and LiDAR topography on parcels 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 21 Figure 5: Jefferson County Critical Areas Geologically Hazardous Areas designation for the subject parcels. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 22 Figure 6: Geologic Mapping of parcel 821333014 and 821333002 Jefferson County, Washington (WDNR, 2013). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 23 Figure 7: Soils Mapping of parcel 821333014 and 821333002 Jefferson County, Washington (McCreary, 1975). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 24 Figure 8: Locations of water wells adjacent to parcels 821333014 and 821333002 Jefferson County, Washington (WDOE, 2021). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 25 Figure 9: Driller’s well log of water well adjacent to the west of parcels 821333014 and 821333002 Jefferson County, Washington (WDOE, 2021). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 26 Figure 10: Driller’s well log of water well adjacent to the east of parcels 821333014 and 821333002 Jefferson County, Washington (WDOE, 2021). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 27 Figure 11: Driller’s well log of water well adjacent to the south of parcels 821333014 and 821333002 Jefferson County, Washington (WDOE, 2021). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 28 Figure 12: Liquefaction Susceptibility Mapping (Washington Department of Natural Resources) for the Project Area (2021). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 29 Figure 13: Seismic Design Criteria for the Project Area (Washington Department of Natural Resources, 2021). April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 30 Figure 14: 1951 Aerial Photograph of parcels 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 31 Figure 15: 1968 Aerial Photograph of parcels 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 32 Figure 16: 1990 Aerial Photograph of parcels 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 33 Figure 17: 2006 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 34 Figure 18: 2009 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 35 Figure 19: 2011 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 36 Figure 20: 2013 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 37 Figure 21: 2015 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 38 Figure 22: 2017 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 39 Figure 23: 2019 Aerial Photograph of parcel 821333014 and 821333002, Jefferson County, Washington. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 40 Figure 24: 1977 Shoreline Aerial Photograph of the Subject Parcel, WDOE, 2021. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 41 Figure 25: 1996 Shoreline Aerial Photograph of the Subject Parcel, WDOE, 2021. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 42 Figure 26: 2006 Shoreline Aerial Photograph of the Subject Parcel, WDOE, 2021. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 43 Figure 27: 2016 Shoreline Aerial Photograph of the Subject Parcel, WDOE, 2021. April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 44 Figure 28: Subject Parcels Map showing building envelope and 20-foot setback from hillslopes steeper than 40% April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 45 Native Plant Species for Maintaining Slope Stability Appendix A: Native Plant Species for Maintaining Slope Stability Reference: Plant Indicator Species of Coastal Forested Slopes. Elliott Manashe. Botanical Name Common Name Hydrologic Regime Erosion Control Value Relative Slope Stability See Common Name Index for Cross Reference W=Wet M=Moist D=Dry D=Detrimental V=Variable EC=Surface Erosion Control SS=Slope Stabilization U=Unstable V=Variable D=Disturbed S=Stable Hydrologic Regime refers to a plants water tolerance. A Wet Hydrologic Regime indicates that a plant would be best suited to a wet site such as a seep, spring or wetland. Erosion control value is an indicator of how well a plant species inhibits soil erosion, either Surface Erosion (EC) or plants used to stabilize slopes (SS). Relative Slope Stability is an indicator of where the plant is typically found, (e.g. plants rated U, for Unstable, are often found in areas disturbed by landsliding. Middle Strata: Shrubs and Small Trees Botanical Name Common Name Hydrologic Regime Erosion Control Value Relative Slope Stability Acer circinatum Vine Maple M, D EC, SS D, S Acer glabrum (4) Douglas Maple D SS D, S Amelanchier alnifolia Serviceberry D EC, SS S Arctostaphylos columbiana Hairy Manzanita D EC, SS S Ceanothus sanguineus (5) Red-stem Ceanothus D EC, SS D, S Ceanothus velutinus (5) Snowbrush D EC, SS D, S Cornus stolonifera Redtwig Dogwood W, M, D EC, SS D, V Corylus cornuta Beaked Hazel M, D EC, SS D, S April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 46 Crataegus douglasii Black Hawthorn W, M, D EC, SS D, S Gaultheria shallon Salal D EC S, V Holodiscus discolor Oceanspray D EC, SS S, V Lonicera hispidula Purple Honeysuckle D EC S Lonicera involucrata Twinberry W, M, D EC, SS S Mahonia aquifolium Tall Oregon Grape D EC, SS S Oemleria cerasiformis Indian Plum D EC S Philadelphus lewisii Mock-orange D EC, SS S Physocarpus capitatus Pacific Ninebark W, M EC, SS S Rhododendron macrophyllum Bigleaf Rhododendron D EC S Ribes lacustre Swamp Gooseberry W, M EC U, V Ribes sanguineum Redflowering Currant M, D EC, SS S Rosa gymnocarpa Baldhip Rose D EC, SS S Rosa nutkana Nootka Rose M, D EC, SS D, S Rosa pisocarpa Swamp Rose W, M, D EC, SS S R. laciniatus Cutleaf Blackberry M, D D U, D Rubus parviflorus Thimbleberry M, D EC D, V Rubus spectabilis Salmonberry W, M, D EC, D D, V Sambucus racemosa Red Elderberry M, D EC D, V Shepardia canadensis Soapberry D EC, SS D, S Symphoricarpos albus Common Snowberry D, M EC, SS D, S Vaccineum ovatum Evergreen Huckleberry D EC S Vaccineum parvifolium Red Huckleberry D EC S April 7,2021 GEOTECHNICAL EVALUATION OF PARCELS 821333014 and 821333002, JEFFERSON COUNTY WASHINGTON 47 Appendix B REPORT LIMITATIONS AND GUIDELINES FOR USE This attachment provides information to help you manage your risks with respect to the use of this report. Geotechnical Services Are Performed for Specific Purposes, Persons and Projects This report has been prepared for use by client Carl Peterson. This report is not intended for use by others, and the information contained herein is not applicable to other sites. Crescent Environmental PLLC structures our services to meet the specific needs of our clients. For example, a geotechnical or geologic study conducted for a civil engineer or architect may not fulfill the needs of a construction contractor or even another civil engineer or architect that are involved in the same project. Because each geotechnical or geologic study is unique, each geotechnical engineering or geologic report is unique, prepared solely for the specific client and project site. No one except Carl Peterson should rely on this report without first conferring with Crescent Environmental PLLC. This report should not be applied for any purpose except the one originally contemplated. A Geotechnical Engineering or Geologic Report is Based on a Unique Set of Project-Specific Factors This report has been prepared for parcels 821333014 and 821333002, located in Jefferson County, Washington. Crescent Environmental PLLC considered a number of unique, project-specific factors when establishing the scope of services for this project and report. Unless Crescent Environmental PLLC specifically indicates otherwise, do not rely on this report if it was: 1. Not prepared for you. 2. Not prepared for your project. 3. Not prepared for the specific site explored. 4. Completed before important changes were made. For example, changes that can affect the applicability of this report include those that affect: a. Elevation, configuration, location, or orientation of activities and structures. b. Project ownership. Within the limitations of scope, schedule, and budget, our services have been executed in accordance with the standard of care in this area at the time this report was prepared. No subsurface explorations by the author were included in the scope-of-work nor conducted as part of this evaluation. It is recommended that the user of this report conduct their own verification of subsurface conditions. All dimensions and locations on included maps and figures are approximate and do not represent nor are intended to represent a legal survey. Users of this report should verify all locations and distances of shown features independently prior to relying on stated locations and/or displayed distances. It is not currently possible to predict the timing or extent of natural earth hazards such as landslides or marine bluff erosion with existing scientific principles except for generalized hazard mapping. No warranty or other conditions, express or implied, should be understood.