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REVIEWED PLN2021-0034+Geotech Report+2.15.2022_5.19.49_AM+2680439Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 www.cobaltgeo.com (206) 331-1097 December 10, 2020 Brett and Cheri Herzer C/O Mr. Lee Michaelis Lee.michaelis@pugetsoundplanning.com RE: Limited Geotechnical Evaluation Proposed Two Lot Subdivision 16209 75th Place West Edmonds, Washington In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our limited geotechnical evaluation at the referenced site. The purpose of our evaluation was to determine the feasibility of utilizing infiltration devices for stormwater runoff management along with providing recommendations for foundation and retaining wall design. Site and Project Description The site is located at 16209 75th Place West in Edmonds, Washington. The site consists of one irregularly shaped parcel (No. 00513100006600) with a total area of about 72,352 square feet. The west-central portion of the property is developed with two homes, a driveway, local short rockery and modular block walls, along with numerous accessory buildings. The property slopes downward from east to west at variable magnitudes of 5 to about 100 percent and total relief of about 170 feet. The eastern half of the property consists of steeper slope areas with magnitudes of 30 to 100 percent and total relief of about 140 feet. In general, the slope magnitudes decrease from east to west across the property. Within the eastern half, the site slopes downward from east to west at magnitudes of 5 to 60 percent and relief of about 30 feet. There are local graded or altered slopes around the two structures and backyard areas as evidenced by benches, walls, and the structures. The property is vegetated with grasses, blackberry vines, ivy, ferns, bushes, and variable diameter trees. The steep slope areas are well vegetated with trees, ivy, blackberry vines and other herbs. The property is bordered on all sides by residential properties with access from the west and 75th Place West. The project includes subdivision of the property into two lots followed by minimal water line modifications and possibly driveway creation. Future development will likely include a new residence in the southwest portion of the new Lot 2 along with a short driveway. These features are shown on Figure 1. December 10, 2020 Page 2 of 6 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 Area Geology The Geologic Map of the Edmonds East and West Quadrangles indicates that the site is near the contacts of several geologic units. These include Advance Outwash, Transitional Beds, Olympic Beds, and Whidbey Formation. In general, all of these deposits become denser with depth. Groundwater is typically present between the Advance Outwash and Transitional Beds. The weathered zone in these materials varies from a few feet up to 20 feet or more. The site appears to be located within a pre-historic landslide feature. The large bowl-shaped feature encompasses the property and numerous adjacent properties. This feature is indicative of a deep-seated rotational slide with the scarp located in the eastern portion of the property where slope magnitudes are steepest. Soil & Groundwater Conditions We advance shallow hand borings to determine the general soil conditions. The soils encountered included loose to medium dense, silty-fine to fine grained sand that generally became denser with depth. Groundwater was observed in two excavations just east of the rockery (Figure 1). It appears that this area could be near the contact between advance outwash and Transitional Beds. Groundwater was nearly at the surface in some areas and about 2 feet below grade in one test pit. We observed likely areas of fill near the south house, extending west onto the moderately steep slope area (Figure 1). We reviewed numerous historic explorations conducted on properties directly north of this property. The soils encountered included variable density silty-sands and silt, generally consistent with Transitional Beds. Steep Slope/Landslide Hazards The site and adjacent areas contain steep slope/landslide hazard areas and associated buffer zones. The property slopes downward from east to west at variable magnitudes of 5 to about 100 percent and total relief of about 170 feet. The eastern half of the property consists of steeper slope areas with magnitudes of 30 to 100 percent and total relief of about 140 feet. In general, the slope magnitudes decrease from east to west across the property. Within the eastern half, the site slopes downward from east to west at magnitudes of 5 to 60 percent and relief of about 30 feet. There are local graded or altered slopes around the two structures and backyard areas as evidenced by benches, walls, and the structures. The property is within an older rotational landslide feature. Based on our review of available information, there is no evidence that this landslide has been reactivated. We did not observe evidence of shallow slide activity during our site visit. The project consists of very limited work near the west property line. This work will not decrease slope stability or pose an unreasonable threat to persons or property on the site or adjacent areas provided the earthwork and grading activities are periodically monitored by the geotechnical engineer and performed according to the plans and this report. We note that it will be necessary to drill at least one boring near the location of any proposed residence in order to analyze global and local slope stability and develop geotechnical design recommendations for site development. The current proposal is very limited in nature and drilled borings do not appear warranted at this time. December 10, 2020 Page 3 of 6 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the east half of the property is underlain by Alderwood-Everett gravelly sandy loam (15 to 25 and 25 to 70 percent slopes). These soils can have severe and very severe erosion potential when exposed on slope magnitudes greater than about 15 percent. These soils are included in the Edmonds Municipal Code section 23.80.020 under A1. It is our opinion that typical erosion control measures will be suitable to limit/reduce erosion potential during the water meter and any limited driveway grading. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically, erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before the onset of wet weather. Code Information 23.80.060 Development standards – General requirements. A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. The site is within an older landslide feature that contains steep slopes and erosion hazards. The currently proposed project includes minimal grading associated with a driveway and water meter near the west property line. It is our opinion that this work will not affect global or local stability. Any future residential development will require geotechnical analysis that includes one or more drilled borings and slope stability analyses. The proposed water meter relocation work will not increase the threat of geologic hazards on adjacent properties, will not impact other critical areas, and are safe as designed under anticipated conditions. 23.80.070 Development standards – Specific hazards. A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060, Development standards – General requirements, and the specific following requirements: 1. Minimum Building Setback. The minimum setback shall be the distance required to ensure the proposed structure will not be at risk from landslides for the life of the structure, considered to be 120 years, and will not cause an increased risk of landslides taking place on or off the site. A setback shall be established from all edges of landslide hazard areas. The size of the setback shall December 10, 2020 Page 4 of 6 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of and concurrence with a critical areas report prepared by a qualified professional; Not applicable as part of the current project scope. 2. Buffer Requirements. A buffer may be established with specific requirements and limitations, including but not limited to, drainage, grading, irrigation, and vegetation. Buffer requirements shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by activities within the buffer area, based upon review of and concurrence with a critical areas report prepared by a qualified professional; The site currently fully developed. No specific buffer is required for the current project. 3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The alteration will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas; Provided earthwork activities are performed in accordance with the approved plans, all runoff is fully controlled, and periodic geotechnical oversight is performed, the development will not decrease slope stability on adjacent properties, will not increase surface water discharge or sedimentation beyond current levels, and will not impact other critical areas. Temporary and permanent erosion and sediment control devices should be in at all times during construction. 4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this title. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design standards are: a. The proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the proposed development site is below these limits, the proposed development shall provide practicable approaches to reduce risk to human safety and improve the factor of safety for landsliding. In no case shall the existing factor of safety be reduced for the subject property or adjacent properties; b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas; c. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography; d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation; December 10, 2020 Page 5 of 6 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties; f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and g. Development shall be designed to minimize impervious lot coverage; The very limited project scope does not pose a risk to critical areas or the need to increase buffers on adjacent properties. 5. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited; 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; provided, that the director may extend or shorten the dry season on a case-by-case basis depending on actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practice permit issued by the city of Edmonds or the Washington State Department of Natural Resources; 7. Point Discharges. Point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except as follows: a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge; b. Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed storm water runoff in the predeveloped state; or c. Dispersed discharge upslope of the steep slope onto a low-gradient, undisturbed buffer demonstrated to be adequate to infiltrate all surface and storm water runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope; and We concur with the above code items and have no additional comments at this time. Conclusions and Recommendations The site contains erosion and landslide hazard areas based on topography, geologic conditions, and soil types. There is evidence of historic landslide activity in this area. The currently proposed subdivision and limited driveway/water meter construction appears suitable at this time. We recommend installing typical erosion control devices when this work takes place. Due to the very limited grading and excavation work anticipated, full geotechnical analysis with borings and slope stability analyses are not currently warranted. These analyses should be performed for any new habitable structures if they are proposed in the future. Erosion and Sediment Control Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment control measures should be implemented, and these measures should be in general accordance with local regulations. At a minimum, the following basic recommendations should be incorporated into the design of the erosion and sediment control features for the site: Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance of the site soils, to take place during the dry season (generally May through September). December 10, 2020 Page 6 of 6 Limited Geotechnical Evaluation www.cobaltgeo.com (206) 331-1097 However, provided precautions are taken using Best Management Practices (BMP’s), grading activities can be completed during the wet season (generally October through April). All site work should be completed and stabilized as quickly as possible. Additional perimeter erosion and sediment control features may be required to reduce the possibility of sediment entering the surface water. This may include additional silt fences, silt fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration systems. Any runoff generated by dewatering discharge should be treated through construction of a sediment trap if there is sufficient space. If space is limited other filtration methods will need to be incorporated. Closure The information presented herein is based upon professional interpretation utilizing standard practices and a degree of conservatism deemed proper for this project. We emphasize that this report is valid for this project as outlined above and for the current site conditions and should not be used for any other site. Sincerely, Cobalt Geosciences, LLC 12/10/2020 Phil Haberman, PE, LG, LEG Principal PH/sc Proposed Two Lot Subdivision 16209 75th Place West Edmonds, Washington SITE PLAN FIGURE 1 Cobalt Geosciences, LLCP.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com 8th Avenue NE N