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REVIEWED RESUB 1_ENG2024-0184_Geotechnical Memo_12.27.2024_3.49.19_PM_468886717311-135t" Ave. N.E. Suite A-500 Woodinville, WA 98072 NELSON GEOTECHNICAL ;---Reviewed by (425) 486-1669 ASSOCIATES. INC. City of Edmonds ; Planning Division ; www.nelsongeotech.com -- -- - -- - -- - - - --- MEMORANDUM RESUB DATE: December 18, 2024 TO: Chris Olson; seaview123@hotmail.com Greg Guillen; grego@cgengineering.com Jared Underbrink; jaredu@cgengineering.com FROM: Khaled M. Shawish, PE Alex B. Rinaldi, PE, LG RE: Geotechnical Comment Response Olson Drainage Improvements 9219 Olympic View Drive Edmonds, Washington NGA File No. 13594B22 INTRODUCTION DEC 27 2024 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT 2024 This memo presents our response to the City of Edmonds review of the proposed drainage improvements located at 9219 Olympic View Drive in Edmonds, Washington. We previously prepared a geotechnical site evaluation dated August 16, 2022. For use in preparing this memorandum, we have been provided with a civil plan set dated December 17, 2024, prepared by CG Engineering. The project includes improving an existing outdated and insufficient drainage system which conveys stormwater from the site and other upstream sources outside the property towards the upper portion of the steep bluff slope, near the western corner of the subject parcel. The following sections address the applicable Edmonds Community Development Code (ECDC) referenced in the City of Edmonds project review letter followed by our response. Geotechnical Comment Response Olson Drainage Improvements Edmonds, Washington NGA File No. 135941322 December 18, 2024 Page 2 Mitigation Sequencing (ECDC 23.40.120): A. Applicants shall demonstrate that all reasonable efforts have been examined with the intent to avoid and minimize impacts to critical areas. B. When an alteration to a critical area is proposed, such alteration shall be avoided, minimized, or compensated for in the following sequential order of preference: 1. Avoiding the impact altogether by not taking a certain action or parts of an action; 2. Minimizing impacts by limiting the degree or magnitude of the action and its implementation by using appropriate technology or by taking affirmative steps, such as project redesign, relocation, or timing, to avoid or reduce impacts, 3. Rectifying the impact to wetlands, frequently flooded areas, and fish and wildlife habitat conservation areas by repairing, rehabilitating, or restoring the affected environment to the historical conditions or the conditions existing at the time of the initiation of the project; 4. Minimizing or eliminating the hazard by restoring or stabilizing the hazard area through engineering or other methods; 5. Reducing or eliminating the impact or hazard over time by preservation and maintenance operations during the life of the action; 6. Compensating for the impact to wetlands, frequently flooded areas, and fish and wildlife habitat conservation areas by replacing, enhancing, or providing substitute resources or environments, and/or 7. Monitoring the hazard or other required mitigation and taking remedial action when necessary. The existing drainage system within the property consists of an open concrete -lined channel along the western portion of the property which receives stormwater from several upstream and offsite sources. It is conveyed to a vertical drainage pipe near the northwest corner of the parcel which discharges into a catch basin, approximately 16-feet vertically lower. The outlet pipe from the catch basin is damaged such that overflow from the catch basin is eroding and incising into the slope near and below the 130-foot contour. The project proposes to improve this system by implementing a new 12-inch diameter PVC drain line in place of the concrete trough system, which will extend onto a Type 1 catch basin along the crest of the slope. From the main catch basin, stormwater will be conveyed along the surface of the slope via an fl- inch diameter HDPE pipe to a diffuser tee below and to the north of the property along the lower, gently sloping portion of the bluff. Based on the topographic map, the gradient at the outfall is roughly 14- to 18-degrees and discharged water will flow along vegetated slope down towards the northwest. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Olson Drainage Improvements Edmonds, Washington NGA File No. 135941322 December 18, 2024 Page 3 The main catch basin along the crest of the slope will be setback approximately 8- to 10-feet from the crest of the bluff and will be anchored via a dead man setback approximately 15- to 20-feet south of the catch basin. The system consists of a buried ultra block connected to the catch basin with a galvanized all thread rod. The catch basin is also supported vertically by driven pipe piles on three sides. The HDPE pipe and catch basin are structurally connected and laterally restrained by the dead man anchor. Based on our review of the project plans, it is our opinion that the proposal has been designed in a manner that satisfies the mitigation sequencing by: • Limiting impacts by utilizing overland, durable HDPE pipe, along minimal disturbance associated with the placement of the new conveyance system along the western portion of the property (23.40.120 (B) (2). • Minimizing or eliminating the hazard through improving the drainage picture and eliminating the ongoing erosion that is occurring along the current outfall (23.40.120 (B) (5). Critical Areas Report (ECDC 23.40.090(D)(7)): D. Minimum Report Contents. At a minimum, the report shall contain the following: .7. A statement specifying the accuracy of the report and all assumptions made and relied upon. We prepared a report for the property dated August 16, 2022. The scope of work at this time was to evaluate the site conditions and provide and assessment of the geologic hazards. As such, the study was based on shallow hand tool explorations, reconnaissance of the bluff and overall site. Although the soil explorations performed at the site were relatively shallow, much of the subsurface conditions were observable in exposures along the slope below the site. Based on observation and extensive experience in this area and on similar sites, it is our opinion that the accuracy of the report and assumptions made are adequate and commensurate to the level of complexity of the project. Special Study and Report Requirements — Geologically Hazardous Areas (ECDC 23.80.050(C)(3)): ...3. Clearly state that the proposed project will not decrease slope stability or pose an unreasonable threat to persons or property either on or off site and provide a rationale as to those conclusions based on geologic conditions and interpretations specific to the project; Based on our review of the plans and site conditions, it is our opinion that the proposed drainage improvements will not decrease slope stability or pose an unreasonable threat to persons or property either on or off site. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Comment Response Olson Drainage Improvements Edmonds, Washington NGA File No. 135941322 December 18, 2024 Page 4 Slope stability conditions are measured by the ratio of resisting forces to driving forces. The proposed repairs are not significantly altering the driving forces, as the weight of the planned drain system is negligible and very little earthwork and overall disturbance is expected for its installation. For long-term consideration, erosion at the outfall may present a concern of localized, shallow slope failure, however, such concerns are addressed with the outfall diffuser design. Additionally, the surface conditions along the outfall areas are heavily vegetated with underbrush, ground cover, and mature tree canopy. Potential hazards levels to adjoining properties or the public are considered low, given the outfall location. Special Study and Report Requirements — Geologically Hazardous Areas (ECDC 23.80.050(C)(6)): ...6. If a landslide or erosion hazard is identified, provide minimum setback recommendations for avoiding the landslide or erosion hazard, other recommendations for site development so that the frequency or magnitude of landsliding or erosion on or off the site is not altered, and recommendations consistent with ECDC 23.80.060 and 23.80.070. The bluff slope below the existing home generally meets the definitions of erosion and landslide hazard areas, as defined in Edmonds code. The proposed repairs pass through and are within these geologically hazardous areas. As such, in our opinion a typical buffer or setback to the geologic hazard areas is not applicable forthe proposed drainage improvements and an exception for this improvement to an existing, documented drainage system should be considered. The main Type 1 catch basin nearest to the crest of the slope will be setback approximately 14-feet but will also be restrained laterally an additional approximately 20-feet by the proposed dead man anchoring system, as well as restrained vertically by drive pipe piles. In our opinion, the proposed catch basin structure setback is adequate. The outfall location is generally situated on sloping terrain in the range of approximately 25 to 32 percent. The ground surface in the vicinity of the outfall is heavily vegetated. The outfall diffuser design indicates two options both including the HDPE pipe terminating at an approximately 11-feet-long HDPE diffuser T. The T is either to be placed on an aggregate splash pad or an HDPE splash pad. Based on our review of this design, we expect this system will adequately dissipate the energy of the stormwater conveyed through the system and will disperse evenly over the vegetated ground surface. We do not expect an increase in slope stability or erosion hazard as a result of the planned outfall. We trust this memorandum satisfies your needs at this time. Please contact us if you have any questions or require additional services. 4 NELSON GEOTECHNICAL ASSOCIATES, INC.