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22-105973-Geotech Report-01-12-23 Kirkland Office | 911 Fifth Avenue | Kirkland, WA 98033 P | 425.827.7701 F| 425.827.5424 Everett Office | 2911 ½ Hewitt Avenue, Suite 2 | Everett, WA 98201 P | 425.259.0522 F | 425.827.5424 Tacoma Office | 1552 Commerce Street, Suite 102 | Tacoma, WA 98402 P | 253.722.2992 F | 253.722.2993 www.aesgeo.com May 12, 2017 Project No. 160482E001 Amalani, LLC 105 S. Main Street, Suite 230 Seattle, Washington 98104 Attention: Mr. Barry Margolese Subject: Geotechnical Evaluation Creekwood Plat Federal Way, Washington References: “Updated Geotechnical Engineering Report, Creekwood Residential Plat, Federal Way, Washington,” prepared by GeoResources, LLC for Amalani, LLC, dated April 28, 2017. “Wetland and Fish and Wildlife Habitat Assessment, Creekwood Plat,” prepared by Soundview Consultants for Amalani, LLC, dated May 8, 2017. Dear Mr. Margolese: This letter presents the findings of the geotechnical evaluation recently completed for the subject project. Our work has been completed for the exclusive use of Amalani, LLC, and their authorized agents, for specific application to this project. No other warranty express or implied is made. INTRODUCTION The subject site, known as the Creekwood Plat property, consists of a rectangular-shaped parcel with a reported area of 19.86 acres located at the southern terminus of 22nd Avenue SW approximately 200 feet south of SW 309th Court in Federal Way, Washington (King County Tax Parcel No. 1221039037). The location of the site is shown on the “Vicinity Map,” Figure 1. The site is currently undeveloped and naturally forested. It is our understanding that current plans include subdividing the site into 20 residential lots. These lots will be located in the Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 2 flatter-lying northeastern and northwestern portions of the parcel. Access to these lots will be provided by a new road that will enter the northeast corner of the property off of 21st Avenue SW. The new road will also connect to 22nd Avenue SW, which currently approaches the property from the north, terminating at the north property boundary. A stormwater detention tract and several large, open space and conservation tracts will make up the remainder of the Creekwood Plat property. A copy of the proposed plat map is included in Appendix A. The east branch of Lakota Creek flows from east to west through the southern portion of the parcel. The location of this creek is shown on Figure 2. A northern tributary of the east branch of Lakota Creek originates from a spring located at the bottom of a ravine in the eastern half of the Creekwood Plat property. This tributary is referred to in the remainder of this report as the “North Tributary.” As shown on Figure 2, the head of the North Tributary ravine extends up to the northern property boundary near the southern terminus of 22nd Avenue SW. The proposed road to be constructed for the Creekwood Plat will cross the head of this ravine a short distance above the spring. Based on review of the April 2017 “Updated Geotechnical Engineering Report” prepared for the project by GeoResources, LLC (GeoResources) and the May 2017 “Wetland and Fish and Wildlife Habitat Assessment” prepared by Soundview Consultants, it is our understanding that the head of the North Tributary ravine will be filled in to accommodate the proposed road. A vegetated SierraScape reinforced soil slope (RSS) will be constructed along the downslope (south) side of the road to minimize the footprint of the fill. The toe of the RSS will be located upstream of the spring that is the source of the North Tributary. The maximum height of the RSS is anticipated to be approximately 30 feet. The SierraScape RSS will use geogrid reinforcement in the fill that will be connected to welded wire baskets faced with a plantable medium inside of the baskets. It is our understanding that permeable rock fill will be placed at the base of the road fill to accommodate the passage of water below the road. It is our understanding that approval of construction of the road across the head of the ravine was previously denied by the City because it lies within the stream buffer. In their April 2017 report, GeoResources indicated that the upper portion of the North Tributary ravine has been altered by erosion from stormwater discharge from a culvert located at the head of the ravine. The purpose of our study was to further evaluate this issue and how it relates to approval of site development as planned. SITE RECONNAISSANCE We conducted a reconnaissance of the upper portion of the North Tributary on September 30, 2016. During our reconnaissance we observed that the sides of the upper portion of the ravine, in the area of the proposed road crossing, are largely devoid of vegetation with plant growth limited to a few young trees and some sparse, low-growing vegetation (Photo 1). Many Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 3 of the plants growing on and adjacent to the top of the ravine slope exhibit exposed roots (Photo 2). These observations are indicative of accelerated erosion. Areas of bare soil indicative of accelerated erosion were observed on the sides of the ravine up to approximately 150 to 200 feet below the head of the ravine. Slope inclinations near the head of the ravine range from approximately 60 percent to near-vertical over a maximum height of approximately 30 feet. Photo 1. Eroded area at the head of the North Tributary ravine.∗ Photo 2. Eroded slope at the head of the North Tributary ravine with exposed roots.* ∗Black and white reproduction of this color original may reduce its effectiveness and lead to incorrect interpretation. Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 4 Some of the young trees growing in the ravine exhibit bowed trunks. Such bowing may be indicative of slope movement. Tension cracks, which were observed in the soil adjacent to the top of the slope near the head of the ravine, are indicative of localized slope movement. A large pile of debris was observed in the upper portion of the ravine. The debris consists largely of yard waste with numerous tires, and scattered pieces of lumber (Photos 3 and 4). As shown in Photo 4, a small rockery is present on the west side of the ravine near the lower limit of the debris pile. Trees growing in the upper portion of the ravine appear to be fairly young with trunk diameters significantly smaller than the mature trees growing in the upland areas adjacent to the top of the ravine (Photos 3 and 5). The lack of mature trees inside the head of the ravine suggests that the ground surface in this area has not been stable for a sufficient length of time to allow such trees to become established. However, the presence of the young trees also indicates that the area has remained sufficiently stable over the past 1 to 3 decades to allow growth of at least some trees. Photo 3. Eroded slopes and debris at the head of the North Tributary ravine.∗ ∗Black and white reproduction of this color original may reduce its effectiveness and lead to incorrect interpretation. Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 5 Photo 4. Debris and rockery near head of North Tributary ravine.∗ Photo 5. Large, mature trees growing in the upland area above the ravine.* Soil exposed on the eroded sidewalls of the ravine generally consist of medium dense, fine sand with variable silt content (Photo 6). We interpret these sediments to be representative of Vashon recessional outwash. The Vashon recessional outwash was deposited by meltwater streams that emanated from the retreating glacial ice during the Vashon Stade of the Fraser Glaciation, approximately 12,500 to 15,000 years ago. Review of the regional geologic map titled Geologic Map of the Poverty Bay 7.5-Minute Quadrangle, King and Pierce Counties, Washington by Booth, Waldron, and Troost (2004) indicates that the sediments underlying this ∗Black and white reproduction of this color original may reduce its effectiveness and lead to incorrect interpretation. Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 6 portion of the site consist of Vashon recessional outwash. Our interpretation of the sediments exposed in the ravine are consistent with the regional geologic map. Photo 6. Sandy outwash sediments exposed in an eroded slope in the upper ravine.∗ The sandy outwash sediments are highly sensitive to erosion, particularly when exposed to concentrated flows. LITERATURE REVIEW Municipal Stormwater System The subject parcel is bounded to the north by the Lakota Ridge neighborhood. Review of King County Tax Assessor records indicates that the homes in this neighborhood were constructed in the mid-1970s. It is our understanding that at the time of its construction, stormwater collected from Lakota Ridge discharged to a detention pond located in the backyard of a residence located at the south end of 22nd Avenue SW, adjacent to the subject site (30920 22nd Avenue SW). From the detention pond, the collected stormwater then discharged through a culvert into the head of the North Tributary ravine. Subsequent to development of Lakota Ridge, it is our understanding that the volume of runoff discharged to this stormwater system increased as residential development in the area expanded. In response to accelerated erosion problems in the North Tributary ravine, a stormwater bypass pipe was installed by the City of Federal Way to convey stormwater away from the ∗Black and white reproduction of this color original may reduce its effectiveness and lead to incorrect interpretation. Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 7 North Tributary. The bypass pipe extends south from the southern terminus of 22nd Avenue SW, across the Creekwood Plat property along the west side of the North Tributary ravine, and discharges to the east branch of Lakota Creek. Given the existing condition of the ravine, it does not appear that restoration of the portion of the ravine impacted by the stormwater related erosion was conducted as part of the bypass line installation project. Based on a telephone conversation with Mr. Tony Doucette, surface water engineer for the City of Federal Way, it is our understanding that installation of the bypass line was completed in or about 2006. The original detention pond and the discharge culvert from the detention pond to the North Tributary ravine remain in place. According to Mr. Doucette discharge to the culvert does not normally occur, but the culvert remains in place to serve as an emergency overflow if the capacity of the bypass line is exceeded. A copy of the municipal stormwater plan for the Lakota Ridge neighborhood is included in Appendix B. Historical Erosion Complaints Review of a November 20, 1981 article from a local newspaper (The News) titled “Erosion Problem Wears on Residents’ Patience” describes the erosion problem at the head of the ravine in the area of the proposed road crossing. In the article, a neighbor (Francis Cooley, 2205 SW 309th Street) is quoted as saying that “It (referring to the ravine) used to be a small, gradual gully. But now we have a huge canyon at the end of our street.” The article describes the problem as a culvert that discharges stormwater into the head of the ravine. Another neighbor mentioned in the article (Steve Kangas, 30925 22nd Avenue SW) states that “the size of the canyon nearly doubled during the record October 6 rainfall.” Mr. Kangas is reported to have “dumped about 1,000 old tires into the canyon” in an attempt to reduce further erosion. The article also states that David Aggerholm, manager of King County Surface Water Management (KCSWM) “agrees that the erosion problem is one of the worst in South King County.” A copy of this article is included in Appendix C. We also reviewed a February 20, 1982 letter written by Steve Kangas provided to us by Amalani, LLC. The letter, which has a KCSWM “Received” stamp dated February 23, 1982, is addressed to “Jerry.” The letter is a request by Mr. Kangas for King County to fix the erosion problem at the head of the ravine at the end of 22nd Avenue SW. Mr. Kangas indicates in the letter that the erosion problem in the ravine became severe after “a developer tapped into the pipe” that discharged into the head of the ravine at that time. A copy of this letter is included in Appendix C. We also reviewed what appears to be an interoffice memorandum from Mr. John Loutsis to Mr. Jerry Creek, manager for KCSWM. It is possible that this is the same “Jerry” to which the February 1982 letter from Steve Kangas is addressed. The memorandum was also provided to us by Amalani, LLC. The memorandum refers to a complaint received from a resident at 30920 22nd Avenue SW regarding erosion “around the south end of the pond and at the end of the discharge pipe.” The parcel at this address, which is located on the east side of 22nd Avenue Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 8 SW adjacent to the head of the North Tributary ravine, has a detention pond in its backyard that discharged into the North Tributary prior to construction of the bypass pipe in 2006. It appears that the discharge pipe mentioned in the complaint refers to the culvert discharging stormwater into the head of the North Tributary ravine. A copy of this memorandum is included in Appendix C. Pre-Development Topography A pre-development topographic map of the Lakota Ridge neighborhood was provided to Associated Earth Sciences, Inc. (AESI) by Amalani, LLC. The map indicates that the North Tributary ravine existed prior to the Lakota Ridge development, but lacks the near-vertical slopes at the head of the ravine present today. This is consistent with Francis Cooley’s description in the 1981 article from The News, which described the original topography at the head of the ravine south of 22nd Avenue SW as “a small, gradual gully.” A copy of the pre-development topographic map is included in Appendix D. CONCLUSIONS Although the North Tributary ravine was originally formed by natural processes, stormwater discharge into the North Tributary ravine associated with area development in the mid-1970s and early 1980s has resulted in accelerated erosion at the head of the ravine. The erosion and subsequent efforts by area residents to control the erosion by dumping tires, yard waste, and other debris, has resulted in steep, unstable slopes, unsightly debris, and a stream buffer area highly altered from its natural state. As previously mentioned, base flow in the North Tributary originates from a spring. This spring is currently located approximately 90 feet below the head of the ravine within the area of accelerated erosion/scouring. Although spring flow in the ravine is a natural process, the accelerated erosion associated with area development is not a natural process. Soil loss associated with the development-related erosion has resulted in scouring of the floor of the ravine. As a result of this scouring, the location of the spring has likely migrated up the ravine some distance from its original location. As the head water of the stream has migrated above its natural location, the associated stream buffer has also migrated north beyond its natural location. Consequently, the existing location of the stream buffer has been altered by stormwater discharge from area development. Section 19.145.330 of the Federal Way Municipal Code (FWMC) allows intrusion into stream buffers provided that: (a) It will not adversely affect water quality; Creekwood Plat Federal Way, Washington Geotechnical Engineering Study May 12, 2017 ASSOCIATED EARTH SCIENCES, INC. TJP/pc – 160482E001-3 - Projects\20160482\KE\WP Page 9 (b) It will not adversely affect the existing quality of wildlife habitat within the stream or buffer area; (c) It will not adversely affect drainage or stormwater retention capabilities; (d) It will not lead to unstable earth conditions nor create erosion hazards; (e) It will not be materially detrimental to any other property in the area of the subject property nor to the city as a whole; and, (f) It is necessary for reasonable development of the subject property. Each of these items is discussed below: Water Quality Under the existing conditions, accelerated soil erosion in the area of the proposed road crossing results in elevated sediment loading and turbidity in Lakota Creek. Construction of the proposed road crossing will improve water quality by eliminating over-steepened slopes at the head of the ravine that are at severe risk of erosion in their current state. Quality of Wildlife Habitat Wildlife habitat in the area of the proposed road crossing is severely degraded from its natural state, consisting mostly of steep, eroded slopes, dumped tires, yard waste, and other debris. Construction of the road crossing as planned will include removal of the dumped tires, yard waste, and other debris. It will also mitigate landslide and erosion hazards in the area of the crossing, thereby reducing the risk of sediment loading and elevated turbidity in Lakota Creek. Use of the proposed SierraScape reinforced soil slope will minimize the footprint of the proposed road crossing and will result in a plantable slope face. It is our understanding that a planting plan for the slope face and nearby heavily eroded areas in the upper ravine has been prepared for the project to enhance conditions in the stream buffer. Drainage/Stormwater Retention The project will be subject to the requirements of the 2009 King County Surface Water Design Manual. Unstable Earth Conditions and Erosion Hazards Based on analyses conducted by GeoResources, it is our understanding that the road crossing will provide an engineered solution for mitigation of existing unstable slopes in the area. Not Materially Detrimental to Other Properties in the Area Construction of the proposed road crossing will improve the stability of the existing unstable slopes at the head of the ravine reducing landslide hazard risks to the adjoining properties. Copyright:© 2013 National Geographic Society, i-cubed 0 20001000 FEE T ± NOTE : BLACK AND WHITEREPRODUCTION OF THIS COLORORIGINAL M AY RE DUCE ITSEFFECTIVENESS AND LEAD TOINCORRECT INTERPRETAT ION VICINITY MAP PROJ NO. DATE: FIGURE:KE 1604 82A 11/1 6 1 Document Path: G:\GIS_Projects\aTemplatesNTRP\aVM_Template\160XXX Fig1 ProjectVicinity_King.mxdDATA SOURCES / REFERENCES:USGS: 24K SERIES TOPOGRAPHIC MAPSKING CO: STREETS, CITY LIMITS, PARCELS 2016 LOCATIONS AND DISTANCES SHOWN ARE APPROXIM ATE Ki ts apCounty Sno hom i sh Co unty Pierc e C ount y King Cou nty22 ndAveSWSW309thSt 21stA v e S W S I T E CREE KW OO D PL ATFEDERAL WAY, WASHINGTON NOTES:1. BASE MAP REFERENCE: CORE DESIGN, CREEKWOOD,BOUNDARY / TOPOGRAPHIC SURVEY, SHEET P02, 2/5/14.160482 Creekwood \ 160482 F2 Site.cdrCREEKWOOD PLATFEDERAL WAY, WASHINGTONTOPOGRAPHIC SITE PLANPROJ NO. DATE: FIGURE:KE160482A 11/16 2BLACKAND WHITE REPRODUCTION OF THIS COLOR ORIGINALMAY REDUCE ITSEFFECTIVENESSAND LEAD TO INCORRECT INTERPRETATION.a s s o c i a t e de a r t h s c i e n c e si n c o r p o r a t e dFEET60 1200NCONTOUR INTERVAL = 2’LEGEND:TEST PIT - BY OTHERSSPRINGCULVERT OUTLET*NOTE: LOCATIONAND DISTANCES SHOWNAREAPPROXIMATE.NORTH TRIBUTARYSTORMWATERBYPASS PIPEEASEMENTSPRING - FLOW INRAVINE BELOWTHIS POINTNORTH TRIBUTARYLAKOTA CREEKEAST BRANCHLAKOTA CREEK3092022NDAVE SW* APPENDIX A Proposed Plat Map J:\2013\13150\Design\Engineering\Preliminary\13150 P01.dwg, 4/14/2017 12:55:13 PM APPENDIX B Municipal Stormwater Plan NOTES:1. BASE MAP REFERENCE: UNTITLED, UNDATED, PLAN SHEET,RECEIVED FROM CORE DESIGN GROUP, 11/16.160482 Creekwood \ 160482 Exhibit.cdrCREEKWOOD PLATFEDERAL WAY, WASHINGTONMUNICIPAL STORM DRAIN PLANPROJ NO. DATE: .KE160482A 11/16 .BLACKAND WHITE REPRODUCTION OF THIS COLOR ORIGINALMAY REDUCE ITSEFFECTIVENESSAND LEAD TO INCORRECT INTERPRETATION.a s s o c i a t e de a r t h s c i e n c e si n c o r p o r a t e dNO SCALENNOTE: LOCATIONAND DISTANCES SHOWNAREAPPROXIMATE.3092022NDAVE SWDETENTION PONDCULVERT TO NORTH TRIBUTARYNORTH TRIBUTARY RAVINENORTH TRIBUTARYSTORMWATERBYPASS PIPE APPENDIX C Historical Erosion Complaints APPENDIX D Lakota Ridge Pre-Development Topographic Map