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APPROVED STM BLD2020-0866+Storm_Drainage_Report+2.18.2021_3.13.25_PM+2054086IEC INSIGHT ENGINEERING CO. Applicant: Home Development Company 502 92" d St SE Everett, WA 98208-1901 STORMWATER SITE PLAN For 1741h SFR Prepared for The City of Edmonds 121 5th Ave N Edmonds, WA 98020 425-771-0220 Project Site Location: 7115 1741h St SW Edmonds, WA 98026 Tax Id:00513100013804 IECO Project: 20-1069 RESUB Feb 19 2021 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT APPLIES WITH APPLICABLE CITY STROMWATER CODE Contact: IECO P.O. Box 1478 Everett, WA 98206 425-303-9363 Certified Erosion and Sedimentation Control Lead: To be named by contractor Stormwater Site Plan Prepared By: Nicole Maas, E.I.T Stormwater Site Plan Preparation Date: October 29, 2020 Revision Date: February 10, 2021 Approximate Construction Date: May 1, 2021 P.O Box 1478 •Everett, WA98206• P: 425.303.9363 info@insightengineering.net 02/17/2021 TABLE OF CONTENTS 1.0 Executive Summary................................................................................................................3 1.1 Minimum Requirements Summary..............................................................................................7 2.0 Existing Conditions.................................................................................................................9 3.0 Offsite Analysis.....................................................................................................................14 3.1 Upstream Analysis.....................................................................................................................14 3.2 Downstream Analysis................................................................................................................14 4.0 Developed Conditions..........................................................................................................18 5.0 Site Hydraulic Analysis.........................................................................................................19 5.1 Existing Basin Summary............................................................................................................19 5.2 Developed Basin Summary........................................................................................................19 6.0 Water Quality.......................................................................................................................21 7.0 Appendix...............................................................................................................................22 *Geotechnical approval of the proposed design was added as Appendix E Figures Figure1 - Vicinity Map.................................................................................................................6 Figure2 - Soil Map......................................................................................................................10 Figure 3 - Downstream Analysis Map 1....................................................................................16 Figure 4 - Downstream Analysis Map 2....................................................................................17 Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -1- Acronyms and Abbreviations BLA Boundary Line Adjustment BMP Best Management Practices DOE Department of Ecology ECDC Edmonds Community Development Code EDDS Engineering Design and Development Standards ESC Erosion and Sediment Control IECO Insight Engineering Company MR Minimum Requirement SWPPP Stormwater Pollution Prevention Plan SWMMWW Stormwater Management Manual for Western Washington TESC Temporary Erosion and Sediment Control WWHM Western Washington Hydrology Model Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -2- 1.0 Executive Summary The proposed project 1741 SFR is located at 7115 174th St SW Edmonds, Washington. More generally, the site is in Section 8, Township 27 North, and Range 4 East of the Willamette Meridian in Snohomish County, Washington. Please refer to the Vicinity Map attached later in the section. This project is subject to the City of Edmond's 2017 requirements and the requirements defined in DOE's 2019 SWMMWW. The site area is 0.28 Acres. The site is zoned as city zoning. The site is currently developed with an existing home, detached garage, and a gravel driveway from 174h St SW. The remainder of the site exists as residential landscaping and native vegetation. The site contains two drainage basins, one that slopes to the north, and another that slopes to the southeast. Please refer to the downstream analysis map for more details. There are no critical areas located on the site. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Please refer to the soils map and descriptions attached later in this report for more details. The proposal is to construct a new single-family home, with a total roof area of 2,283 SF, a walkway, and a driveway off 174th St SW. The existing detached garage, house, and driveway will be removed. Access to the home will be provided from a driveway off 1741h St SW. The total new and replaced hard surface area will be 3,955 SF. Per Figure 3.1, (Flow Chart for Determining Requirements for Development) from the 2017 Edmonds Stormwater Addendum, Minimum requirements #1 through 5 shall apply for this project. See section 1.1 for Minimum Requirements Summary included later in this report. Flow control requirements are exempt as the proposed development will increase the site's peak discharge by 0.04 CFS which is less than the 0.1 CFS threshold requiring flow control. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -3- Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMPs must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. A 50' x 2.95' rain garden will be used to infiltrate the runoff from the roof. It will be located on the south-eastern corner of the site, according to Geotech recommendation, 10' away from the proposed home and 5' away from the property lines. An infiltration rate of 0.31 in/hr per the Geotechnical Report was used for sizing the rain garden. The design water surface is at an elevation of 396.67' and will have an emergency overflow system in place to prevent back -watering in the roof lines. Refer to the WWHM report located in Appendix B for more details. Please refer to Appendix A for tables detailing BMP feasibility. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the BMPs for other hard surfaces must be considered in the following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. Permeable pavement will be used to infiltrate the runoff from the driveway. Please refer to Appendix A for tables detailing BMP feasibility. Post -Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -4- Flow Chart for Determining Requirements for Development. Ducs the project result in 2,000 square feet, or greater, of new plus replaced hard surface area? OR Dees the land disturbing activity total 7,000 square feet or greater? Yes No Minimum requirements No. I through 5 apply Minimum requirement No. 2 applies Next Question Dues the project add 5,000 square feet or more of new plus replaced hard surfaces? OR Convert 0.75 acres or more of vegetation to lawn or landscaped areas? OR Convert 2.5 acres or more of native vegetat ion to pasture? No Yes Is this a road related project` All Minimum Requirements apply to the new and replaced hard surfaces and converted vegetation areas_ All Minimum Requiren-iunts apply to the new hard surfaces and converted vegetation areas. Yes Insight Engineering Co. - Stormwater Site Plan Yee Does the project add 5,000 square feet or more of new hard surfaces' Yes E)o new hard surfaces add 50%car more to the existing hard surtaccs within the project limits? No No iti g7 No additional requirements. 2/10/2021 -5- INSIGHT ENGINEERING CO. P.O. Box 1478Everett, WA98206 425-303-9363, 425-303-9362 f. Info@insightengineering.net FIGURE]. VICINITYMAP Taken from PDS Map Portal Figure 1 -Vicinity May 174th SFR Edmonds, Washington SCALENT: I EAT 8/6/20 I JOB#: 20-1069 BY NAM I FILE NAME: 20-1069/doc/Stormwater Site Plan Insight Engineering Co. - Stormwater Site Plan 2/10/2021 1.1 Minimum Requirements Summary MR : Minimum Requirement SWPPP : Stormwater Pollution Prevention Plan MR #1 Stormwater Site Plan Narrative: The Stormwater Site Plan preparation follows the City of Edmonds 2017 requirements and in accordance with DOE's 2019 SWMMWW. Refer to the executive summary within Section 1.0. MR #2 SWPPP Narrative: A SWPPP has been included in the Appendix A under Section 6. MR #3 Water Pollution Source Control for New Development: No source control pollutants pertain to the proposed single-family project; therefore, no BMPs are required for the project. MR #4 Preservation of Natural Drainage Systems and Outfalls: The developed site's runoff is proposed to be infiltrated into the native soils. MR #5 Onsite Stormwater Management: Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMPs must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. A 50' x 2.95' rain garden will be used to infiltrate the runoff from the roof. It will be located on the south- eastern corner of the site, according to Geotech recommendation, 10' away from the proposed home and 5' away from the property lines. An infiltration rate of 0.31 in/hr per the Geotech Report was used in sizing the rain garden. The design water surface is at an elevation of 396.67' and will have an emergency overflow system in place to prevent back -watering in the roof lines. Refer to the WWHM report in Appendix B for more sizing details. Please refer to Appendix A for tables detailing BMP feasibility. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the BMPs for other hard surfaces must be considered in the following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. Permeable Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -7- Pavement will be used to infiltrate the runoff from the driveway. Please refer to Appendix A for tables detailing BMP feasibility. Post -Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 2.0 Existing Conditions The proposed project 1741h SFR is located at 7115 1741h St SW Edmonds, Washington. More generally, the site is in Section 8, Township 27 North, and Range 4 East of the Willamette Meridian in Snohomish County, Washington. The site area is 0.28 Acres. The site is zoned as city zoning. The site is currently developed with an existing home, detached garage, and a gravel driveway from 174h St SW. The remainder of the site exists as residential landscaping and native vegetation. The site contains two drainage basins, one that slopes to the north, and another that slopes to the south. Please refer to the downstream analysis map for more details. There are no critical areas located on the site. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Please refer to the soils map and descriptions attached later in this report for more details. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -9- FIGURE 2. SOIL MAP SOILS LEGEND 5— Alderwood-Urban land complex, 2 to 8 percent slopes 4- Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes Figure 2 - Soil May 174th SFR Edmonds, Washington SCALE: DATE: 8/6/20 JOB #: 20-1069 NONE P.O. Box 1478 Everett, WA 98206 FILE NAME: 425-303-9363, 425-303-9362 f. 20-1069/ doc/Stormwater Site Plan Info@insightengineering.net BY: NAM Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -10- Snohomish County Area, Washington 5—Alderwood-Urban land complex, 2 to 8 percent slopes Map Unit Setting • National map unit symbol: 2hz9 • Elevation: 50 to 800 feet • Mean annual precipitation: 25 to 60 inches • Mean annual air temperature: 48 to 52 degrees F • Frost -free period: 180 to 220 days • Farmland classification: Not prime farmland Map Unit Composition • Alderwood and similar soils: 60 percent • Urban land: 25 percent • Minor components: 15 percent • Estimates are based on observations, descriptions, and transects of the mapunit. Description of Alderwood Setting • Landform: Till plains • Parent material: Basal till Typical profile • HI - 0 to 7 inches: gravelly ashy sandy loam • H2 - 7 to 35 inches: very gravelly ashy sandy loam • H3 - 35 to 60 inches: gravelly sandy loam Properties and qualities • Slope: 2 to 8 percent • Depth to restrictive feature: 20 to 40 inches to densic material • Natural drainage class: Moderately well drained • Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) • Depth to water table: About 18 to 36 inches • Frequency of flooding: None • Frequency of ponding: None • Available water storage in profile: Low (about 3.0 inches) Interpretive groups • Land capability classification (irrigated): None specified • Land capability classification (nonirrigated): 4s • Hydrologic Soil Group: B • Forage suitability group: Limited Depth Soils (G002XN302WA) • Hydric soil rating: No Minor Components Terric medisaprists, undrained • Percent of map unit: 5 percent • Landform: Depressions • Hydric soil rating: Yes Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -11- Norma, undrained • Percent of map unit: 5 percent • Landform: Depressions • Hydric soil rating: Yes Mckenna • Percent of map unit: 5 percent • Landform: Depressions • Hydric soil rating: Yes Snohomish County Area, Washington 4—Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes Map Unit Setting • National map unit symbol: 2hyy • Elevation: 50 to 800 feet • Mean annual precipitation: 25 to 60 inches • Mean annual air temperature: 48 to 52 degrees F • Frost -free period: 180 to 220 days • Farmland classification: Not prime farmland Map Unit Composition • Alderwood and similar soils: 60 percent • Everett and similar soils: 25 percent • Minor components: 15 percent • Estimates are based on observations, descriptions, and transects of the mapunit. Description of Alderwood Setting • Landform: Till plains • Parent material: Basal till Typical profile • HI - 0 to 7 inches: gravelly ashy sandy loam • H2 - 7 to 35 inches: very gravelly ashy sandy loam • H3 - 35 to 60 inches: gravelly sandy loam Properties and qualities • Slope: 25 to 70 percent • Depth to restrictive feature: 20 to 40 inches to densic material • Natural drainage class: Moderately well drained • Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) • Depth to water table: About 18 to 36 inches • Frequency of flooding: None • Frequency of ponding: None • Available water storage in profile: Low (about 3.0 inches) Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -12- Interpretive aroups • Land capability classification (irrigated): None specified • Land capability classification (nonirrigated): 7e • Hydrologic Soil Group: B • Hydric soil rating: No Description of Everett Setting • Landform: Terraces, plains • Parent material: Glacial outwash Typical profile • H1 - 0 to 6 inches: gravelly ashy sandy loam • H2 - 6 to 18 inches: very gravelly ashy sandy loam • H3 - 18 to 60 inches: extremely gravelly sand Properties and qualities • Slope: 25 to 70 percent • Depth to restrictive feature: 14 to 20 inches to strongly contrasting textural stratification • Natural drainage class: Somewhat excessively drained • Capacity of the most limiting layer to transmit water (Ksat): High (1.98 to 5.95 in/hr) • Depth to water table: More than 80 inches • Frequency of flooding: None • Frequency of ponding: None • Available water storage in profile: Very low (about 2.1 inches) Interpretive arouos • Land capability classification (irrigated): None specified • Land capability classification (nonirrigated): 7e • Hydrologic Soil Group: A • Hydric soil rating: No Minor Components Mckenna • Percent of map unit: 5 percent • Landform: Depressions • Hydric soil rating: Yes Terric medisaprists, undrained • Percent of map unit: 5 percent • Landform: Depressions • Hydric soil rating: Yes Norma, undrained • Percent of map unit: 5 percent • Landform: Depressions • Hydric soil rating: Yes Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -13- 3.0 Offsite Analysis A site reconnaissance was performed by Brian Kalab of Insight Engineering on June 29, 2020 to verify the downstream flow paths and observe any drainage problems downstream of the site. The sky was overcast and rainy with a temperature of 59 degrees. The site area is 0.28 Acres. The site is currently developed with an existing house, detached garage, and driveway from 174th St SW. The remainder of the site exists as residential landscaping and native vegetation. The site contains two drainage basins, one that drains to the north and one that drains to the south. Please refer to the downstream analysis map for more details on basin designation. 3.1 Upstream Analysis Based on the site reconnaissance and the topographic survey of the site, the upstream flows appear to be minimal. 3.2 Downstream Analysis In the existing conditions, runoff from Sub -Basin Al drains to the northeast and flows through four neighboring properties (located at 7027 174th St SW, 7107 174th St SW, 17226 Meadowdale Beach Rd, and 7002 Meadowdale Beach Rd) for about 500 ft before draining into Stilthouse Creek. The creek flows to the northwest for about 3,500 ft before draining into the Puget Sound. There are no conveyance features on the neighboring properties. Runoff from Sub -Basin A2 flows onto the driveway to 7027 174th St SW and flows north for about 160 ft before joining with the flows from Sub -Basin Al. Basin B drains to the southeast portion of the property and flows into a catch basin on 174th St SW. The flow continues along the existing drainage system for about 300 ft, then flows south for about 150 ft, then flows west for about 1,400 ft before draining into Terrace Creek. The creek meanders to the northwest for about 2,000 ft before draining into the Puget Sound. This is where the downstream analysis was concluded. There did not appear to be any restrictions or erosional problems downstream of the site. The flows from Basin A and Basin B do not converge within a quarter of a mile. In developed conditions, all effective impervious areas of the site are located in Basin A and will drain into the rain garden, which will infiltrate the runoff so the subsurface flows continue their natural drainage path. The infiltrated stormwater will follow the same flow pattern as in the Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -14- existing conditions. The rain garden is located more than 50 ft from the top of the steep slope in the northeast corner of the site and will therefore not cause any downstream erosion hazards to Basin A. The rain garden will have a 2.5' wide dispersion trench overflow system located on the most northern edge. Overflow runoff will flow to the north and will follow the existing drainage pattern for Sub -Basin Al. Piped overflow for the rain garden is not feasible because the only accessible storm drainage system is located in a different drainage basin than the rain garden. The rain garden infiltrates 99.87% of runoff and the 100-yr flow rate in the mitigated state is 0 cfs therefore, overflow runoff is expected to be minimal. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -15- FIGURE 3. DOWNSTREAM ANALYSIS MAP 1 IEC INSIGHT ENGINEERING CO. P.O. Box 1478 Everett, WA 98206 425-303-9363, 425-303-9362 f. Info@insightengineering.net z 4 Map taken from Citv of Edmonds GIs Figure 3 - Downstream Analysis Map 174th SFR Edmonds, Washington ISCALE: I DATE: 8/6/20 I JOB #: 20-1069 I NONE I FILE NAME: BY: NAM 20-1069/doc/Stormwater Site Plan Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -16- FIGURE 4. DOWNSTREAM ANALYSIS MAP 2 IEC INSIGHT ENGINEERING CO P.O. Box 1478 Everett, WA 98206 425-303-9363, 425-303-9362 f. Info@insightengineering.net Page Break Map taken from City of Edmonds GIs Figure 4 - Downstream Analysis Map 174th SFR Edmonds, Washington I SCALE:DATE: 8/6/20 JOB #: 20-1069 ONE I I I I FILE NAME: BY: NAM 20-1069/doc/Stormwater Site Plan Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -17- 4.0 Developed Conditions The proposed project 174" SFR is located at 7115 1741h St SW, Edmonds, Washington. More generally, the site is in Section 8, Township 27 North, and Range 4 East of the Willamette Meridian in Snohomish County, Washington. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". The project site is zoned as city zoning. The proposal is to construct a new single-family home, with a total roof area of 2,283 SF, a walkway, and a driveway off 1741h St SW. The existing detached garage, house, and driveway will be removed. Access to the home will be provided from a driveway off 174th St SW. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMPs must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. A 50' x 2.95' rain garden will be used to infiltrate the runoff from the roof. It will be located on the south-eastern corner of the site, according to Geotech recommendation, 10' away from the proposed home and 5' away from the property lines. An infiltration rate of 0.31 in/hr per the Geotechnical Report was used for sizing the rain garden. The design water surface is at an elevation of 396.67' and will have an emergency overflow system in place to prevent back -watering in the roof lines. Refer to the WWHM report located in Appendix B for sizing details. Please refer to Appendix A for tables detailing BMP feasibility. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the BMPs for other hard surfaces must be considered in the following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. Permeable pavement will be used to infiltrate the runoff from the driveway. Please refer to Appendix A for tables detailing BMP feasibility. Post -Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -18- 5.0 Site Hydraulic Conditions Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Site Area = 0.28 Acres (12,015 SF) Clearing Limit = 0.19 Acres ( 8,386 SF) 5.1 Existing Basin Summary Site Area = 0.28 Acres Clearing Limit = 0.19 Acres Sub -basin A Sub -basin B = 0.16 Acres = 0.03 Acres Both sub -basins were modeled as forested area in the existing conditions. 5.2 Developed Basin Summary Site Area = 0.28 Acres Clearing Limit = 0.19 Acres Sub -basin A = 0.16 Acres Impervious: Roof Area = 0.05 Acres (2,283 SF) Walkway = 0.00 Acres ( 72 SF) Total Impervious = 0.05 Acres (2,355 SF) Pervious: Pervious Area = 0.16 - 0.05 0.11 Acres Sub -basin B = 0.03 Acres Impervious: Driveway = 0.02 Acres ( 777 SF) Total Impervious = 0.02 Acres ( 777 SF) Pervious: Pervious Ares = 0.03 — 0.02 = 0.01 Acres Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -19- Frontage Area = 0.06 Acres Impervious: Asphalt = 0.01 Acres ( 162 SF) Driveway = 0.02 Acres ( 661 SF) Total Impervious = 0.03 Acres ( 823 SF) Pervious: Pervious Area = 0.06 0.03 = 0.03 Acres The roof area will be served by a 50' x 2.95' rain garden located on the southeastern corner of the site. Please refer to the WWHM report in Appendix B for more sizing details. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -20- 6.0 Water Quality The site is exempt for water quality because the pollution generating surface is only 1,600 SF, which is less than the threshold of 5,000 SF. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -21- 7.0 Appendix A. LID Feasibility Table B. WWHM Report C. Stormwater Pollution Prevention Plan D. Geotech Report Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -22- A. LID FEASIBILITY TABLE Roof BMPs (List #1) Per ECDC 18.30.060.D.5.d Minimum Requirement #5 BMP Feasibility BMP Feasibility Conditions T5.30 Full Dispersion Infeasible Full Dispersion is infeasible because the required native vegetation preservation could not be achieved for the City zoned project per Vol. V Ch. 5 of the SCDM. T5.10A Downspout Infeasible Downspout Full Infiltration is infeasible because Full Infiltration the site contains medium dense to dense glacial till deposits (hardpan) at shallow depths. Refer to the geotechnical report for more information. T5.14 Rain Gardens Feasible Rain Gardens are feasible for this site and will be used to mitigate runoff from the roof area. An infiltration rate of 0.31 in/hr was used. Please refer to the Geotech report for more details. T5.I013 Downspout N/A Dispersion T5.IOC Perforated N/A Stub -out Connections Detention Pipe or Vault N/A Other Hard Surface BMPs (List #1) Per ECDC 18.30.060.D.5.d Minimum Requirement #5 BMP Feasibility BMP Feasibility Conditions T5.30 Full Dispersion Infeasible Full Dispersion is infeasible because the required native vegetation preservation could not be achieved for the City zoned project per Vol. V Ch. 5 of the SCDM. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -23- Permeable Pavement or Feasible Permeable Pavement has been considered feasible Rain Gardens and will be used for the driveway area. An infiltration rate of 0.31 in/hr was used. Please refer to the Geotech report for more details. T5.12 Sheet Flow N/A Dispersion Detention Pipe or Vault N/A Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -24- C ,SAMAA:1► ■INaEel 1114 Refer to the following pages for the report detailing rain garden sizing. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -25- WWHM2012 PROJECT REPORT Project Name: Rain Garden Updated Site Name: 174th SFR Site Address: 7115 174th St SW City : Edmonds Report Date: 2/11/2021 MGS Regoin Puget East Data Start 1901/10/1 Data End : 2058/09/30 DOT Data Number: 03 Version Date: 2019/09/13 Version : 4.2.17 Low Flow Threshold for POC 1 : 50 Percent of the 2 Year High Flow Threshold for POC 1: 50 year PREDEVELOPED LAND USE Name : Predeveloped Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Flat .05 Pervious Total 0.05 Impervious Land Use acre Impervious Total 0 Basin Total 0.05 Element Flows To: Surface Interflow Groundwater MITIGATED LAND USE Name : Developed Bypass: No Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -26- Groundwater: No Pervious Land Use acre Pervious Total 0 Impervious Land Use acre ROOF TOPS FLAT 0.05 Impervious Total 0.05 Basin Total 0.05 Element Flows To: Surface Interflow Groundwater Surface retention 1 Surface retention 1 Name : Bioretention 1 Bottom Length: 50.00 ft. Bottom Width: 2.95 ft. Material thickness of first layer: 1 Material type for first layer: SMMWW 12 in/hr Material thickness of second layer: 0 Material type for second layer: Sand Material thickness of third layer: 0 Material type for third layer: GRAVEL Infiltration On Infiltration rate: 0.31 Infiltration safety factor: 1 Total Volume Infiltrated (ac-ft.): 19.672 Total Volume Through Riser (ac-ft.): 0.026 Total Volume Through Facility (ac-ft.): 19.698 Percent Infiltrated: 99.87 Total Precip Applied to Facility: 2.044 Total Evap From Facility: 1.16 Underdrain not used Discharge Structure Riser Height: 1 ft. Riser Diameter: 12 in. Element Flows To: Outlet 1 Outlet 2 Bioretention 1 Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 0.0000 0.0100 0.0000 0.0000 0.0000 Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -27- 0.0275 0.0100 0.0000 0.0000 0.0000 0.0549 0.0098 0.0001 0.0000 0.0000 0.0824 0.0096 0.0001 0.0000 0.0000 0.1099 0.0094 0.0002 0.0000 0.0000 0.1374 0.0092 0.0002 0.0000 0.0001 0.1648 0.0090 0.0003 0.0000 0.0005 0.1923 0.0088 0.0003 0.0000 0.0007 0.2198 0.0086 0.0004 0.0000 0.0010 0.2473 0.0084 0.0004 0.0000 0.0011 0.2747 0.0082 0.0005 0.0000 0.0011 0.3022 0.0080 0.0006 0.0000 0.0011 0.3297 0.0078 0.0006 0.0000 0.0011 0.3571 0.0076 0.0007 0.0000 0.0011 0.3846 0.0075 0.0008 0.0000 0.0011 0.4121 0.0073 0.0009 0.0000 0.0011 0.4396 0.0071 0.0009 0.0000 0.0011 0.4670 0.0069 0.0010 0.0000 0.0011 0.4945 0.0067 0.0011 0.0000 0.0011 0.5220 0.0065 0.0012 0.0000 0.0011 0.5495 0.0063 0.0013 0.0000 0.0011 0.5769 0.0062 0.0013 0.0000 0.0011 0.6044 0.0060 0.0014 0.0000 0.0011 0.6319 0.0058 0.0015 0.0000 0.0011 0.6593 0.0056 0.0016 0.0000 0.0011 0.6868 0.0054 0.0017 0.0000 0.0011 0.7143 0.0053 0.0018 0.0000 0.0011 0.7418 0.0051 0.0019 0.0000 0.0011 0.7692 0.0049 0.0020 0.0000 0.0011 0.7967 0.0047 0.0021 0.0000 0.0011 0.8242 0.0046 0.0022 0.0000 0.0011 0.8516 0.0044 0.0024 0.0000 0.0011 0.8791 0.0042 0.0025 0.0000 0.0011 0.9066 0.0041 0.0026 0.0000 0.0011 0.9341 0.0039 0.0027 0.0000 0.0011 0.9615 0.0037 0.0028 0.0000 0.0011 0.9890 0.0035 0.0030 0.0000 0.0011 1.0000 0.0034 0.0030 0.0000 0.0011 Surface retention 1 Hydraulic Table Stage(feet) Area(ac.) Volume(ac-£t.) Discharge(cfs) To Amended(c£s) Wetted Surface 1.0000 0.0100 0.0030 0.0000 0.0409 0.0000 1.0275 0.0102 0.0033 0.0000 0.0409 0.0000 1.0549 0.0104 0.0036 0.0000 0.0432 0.0000 1.0824 0.0106 0.0039 0.0000 0.0443 0.0000 1.1099 0.0108 0.0041 0.0000 0.0454 0.0000 1.1374 0.0110 0.0044 0.0000 0.0465 0.0000 1.1648 0.0112 0.0048 0.0000 0.0477 0.0000 1.1923 0.0114 0.0051 0.0000 0.0488 0.0000 1.2198 0.0116 0.0054 0.0000 0.0499 0.0000 1.2473 0.0119 0.0057 0.0000 0.0510 0.0000 1.2747 0.0121 0.0060 0.0000 0.0522 0.0000 1.3022 0.0123 0.0064 0.0000 0.0533 0.0000 1.3297 0.0125 0.0067 0.0000 0.0544 0.0000 1.3571 0.0127 0.0071 0.0000 0.0555 0.0000 1.3846 0.0129 0.0074 0.0000 0.0567 0.0000 Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -28- 1.4121 0.0131 0.0078 0.0000 0.0578 0.0000 1.4396 0.0133 0.0081 0.0000 0.0589 0.0000 1.4670 0.0135 0.0085 0.0000 0.0600 0.0000 1.4945 0.0137 0.0089 0.0000 0.0612 0.0000 1.5220 0.0140 0.0093 0.0000 0.0623 0.0000 1.5495 0.0142 0.0096 0.0000 0.0634 0.0000 1.5769 0.0144 0.0100 0.0000 0.0645 0.0000 1.6044 0.0146 0.0104 0.0000 0.0656 0.0000 1.6319 0.0148 0.0108 0.0000 0.0668 0.0000 1.6593 0.0150 0.0112 0.0000 0.0679 0.0000 1.6868 0.0153 0.0117 0.0000 0.0690 0.0000 1.7143 0.0155 0.0121 0.0000 0.0701 0.0000 1.7418 0.0157 0.0125 0.0000 0.0713 0.0000 1.7692 0.0159 0.0129 0.0000 0.0724 0.0000 1.7967 0.0162 0.0134 0.0000 0.0735 0.0000 1.8242 0.0164 0.0138 0.0000 0.0746 0.0000 1.8516 0.0166 0.0143 0.0000 0.0758 0.0000 1.8791 0.0168 0.0147 0.0000 0.0769 0.0000 1.9066 0.0171 0.0152 0.0000 0.0780 0.0000 1.9341 0.0173 0.0157 0.0000 0.0791 0.0000 1.9615 0.0175 0.0162 0.0000 0.0803 0.0000 1.9890 0.0177 0.0166 0.0000 0.0814 0.0000 2.0165 0.0180 0.0171 0.0225 0.0818 0.0000 2.0440 0.0182 0.0176 0.0977 0.0818 0.0000 2.0714 0.0184 0.0181 0.2020 0.0818 0.0000 2.0989 0.0187 0.0186 0.3281 0.0818 0.0000 2.1264 0.0189 0.0192 0.4712 0.0818 0.0000 2.1538 0.0191 0.0197 0.6273 0.0818 0.0000 2.1813 0.0194 0.0202 0.7923 0.0818 0.0000 2.2088 0.0196 0.0207 0.9624 0.0818 0.0000 2.2363 0.0198 0.0213 1.1332 0.0818 0.0000 2.2637 0.0201 0.0218 1.3006 0.0818 0.0000 2.2912 0.0203 0.0224 1.4606 0.0818 0.0000 2.3187 0.0206 0.0230 1.6096 0.0818 0.0000 2.3462 0.0208 0.0235 1.7445 0.0818 0.0000 2.3736 0.0210 0.0241 1.8629 0.0818 0.0000 2.4011 0.0213 0.0247 1.9637 0.0818 0.0000 2.4286 0.0215 0.0253 2.0472 0.0818 0.0000 2.4560 0.0218 0.0259 2.1153 0.0818 0.0000 2.4835 0.0220 0.0265 2.1721 0.0818 0.0000 2.5000 0.0222 0.0268 2.2515 0.0818 0.0000 Name : Surface retention 1 Element Flows To: Outlet 1 Outlet 2 Bioretention 1 ANALYSIS RESULTS Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -29- Stream Protection Duration Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.05 Total Impervious Area:O Mitigated Landuse Totals for POC #1 Total Pervious Area:O Total Impervious Area:0.05 Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.000605 5 year 0.000939 10 year 0.001145 25 year 0.001386 50 year 0.001549 100 year 0.001699 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0 5 year 0 10 year 0 25 year 0 50 year 0 100 year 0 Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.000 0.000 1950 0.001 0.000 1951 0.001 0.000 1952 0.000 0.000 1953 0.000 0.000 1954 0.001 0.000 1955 0.001 0.000 1956 0.001 0.000 1957 0.001 0.000 1958 0.001 0.005 1959 0.001 0.000 1960 0.001 0.000 1961 0.001 0.000 1962 0.000 0.000 1963 0.001 0.000 1964 0.001 0.000 1965 0.001 0.000 1966 0.000 0.000 1967 0.001 0.000 Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -30- 1968 0.001 0.000 1969 0.000 0.000 1970 0.000 0.000 1971 0.001 0.000 1972 0.001 0.000 1973 0.000 0.000 1974 0.001 0.000 1975 0.001 0.000 1976 0.000 0.000 1977 0.000 0.000 1978 0.000 0.000 1979 0.001 0.000 1980 0.001 0.000 1981 0.000 0.000 1982 0.001 0.000 1983 0.001 0.000 1984 0.001 0.000 1985 0.001 0.000 1986 0.002 0.000 1987 0.001 0.000 1988 0.001 0.000 1989 0.000 0.000 1990 0.001 0.000 1991 0.001 0.000 1992 0.001 0.000 1993 0.000 0.000 1994 0.000 0.000 1995 0.001 0.000 1996 0.001 0.004 1997 0.003 0.000 1998 0.000 0.000 1999 0.001 0.000 2000 0.000 0.000 2001 0.000 0.000 2002 0.001 0.000 2003 0.000 0.000 2004 0.001 0.000 2005 0.000 0.000 2006 0.001 0.000 2007 0.001 0.000 2008 0.001 0.000 2009 0.001 0.000 Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.0025 0.0048 2 0.0023 0.0037 3 0.0014 0.0003 4 0.0014 0.0000 5 0.0013 0.0000 6 0.0011 0.0000 7 0.0010 0.0000 8 0.0009 0.0000 Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -31- 9 0.0009 0.0000 10 0.0009 0.0000 11 0.0009 0.0000 12 0.0009 0.0000 13 0.0008 0.0000 14 0.0008 0.0000 15 0.0008 0.0000 16 0.0008 0.0000 17 0.0007 0.0000 18 0.0007 0.0000 19 0.0007 0.0000 20 0.0007 0.0000 21 0.0007 0.0000 22 0.0007 0.0000 23 0.0007 0.0000 24 0.0007 0.0000 25 0.0006 0.0000 26 0.0006 0.0000 27 0.0006 0.0000 28 0.0006 0.0000 29 0.0006 0.0000 30 0.0006 0.0000 31 0.0006 0.0000 32 0.0006 0.0000 33 0.0006 0.0000 34 0.0006 0.0000 35 0.0005 0.0000 36 0.0005 0.0000 37 0.0005 0.0000 38 0.0005 0.0000 39 0.0005 0.0000 40 0.0005 0.0000 41 0.0005 0.0000 42 0.0005 0.0000 43 0.0005 0.0000 44 0.0005 0.0000 45 0.0005 0.0000 46 0.0005 0.0000 47 0.0004 0.0000 48 0.0004 0.0000 49 0.0004 0.0000 50 0.0004 0.0000 51 0.0004 0.0000 52 0.0004 0.0000 53 0.0004 0.0000 54 0.0004 0.0000 55 0.0004 0.0000 56 0.0003 0.0000 57 0.0003 0.0000 58 0.0002 0.0000 59 0.0002 0.0000 60 0.0001 0.0000 61 0.0001 0.0000 Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -32- Stream Protection Duration POC #1 The Facility PASSED The Facility PASSED. Flow(cfs) Predev Mit Percentage Pass/Fail 0.0003 21945 76 0 Pass 0.0003 20037 75 0 Pass 0.0003 18379 74 0 Pass 0.0003 16771 74 0 Pass 0.0004 15366 74 0 Pass 0.0004 14014 74 0 Pass 0.0004 12823 74 0 Pass 0.0004 11849 74 0 Pass 0.0004 10840 74 0 Pass 0.0004 9997 74 0 Pass 0.0004 9184 72 0 Pass 0.0004 8479 72 0 Pass 0.0005 7854 72 0 Pass 0.0005 7249 72 0 Pass 0.0005 6718 72 1 Pass 0.0005 6196 71 1 Pass 0.0005 5741 71 1 Pass 0.0005 5260 70 1 Pass 0.0005 4827 70 1 Pass 0.0005 4460 70 1 Pass 0.0006 4115 70 1 Pass 0.0006 3820 70 1 Pass 0.0006 3542 70 1 Pass 0.0006 3275 70 2 Pass 0.0006 3033 70 2 Pass 0.0006 2796 70 2 Pass 0.0006 2616 69 2 Pass 0.0006 2447 68 2 Pass 0.0007 2299 68 2 Pass 0.0007 2186 68 3 Pass 0.0007 2053 67 3 Pass 0.0007 1957 67 3 Pass 0.0007 1858 67 3 Pass 0.0007 1741 67 3 Pass 0.0007 1658 66 3 Pass 0.0007 1558 66 4 Pass 0.0008 1460 65 4 Pass 0.0008 1372 63 4 Pass 0.0008 1312 63 4 Pass 0.0008 1243 63 5 Pass 0.0008 1195 62 5 Pass 0.0008 1157 62 5 Pass 0.0008 1117 62 5 Pass 0.0008 1076 61 5 Pass 0.0009 1033 61 5 Pass 0.0009 996 60 6 Pass 0.0009 968 59 6 Pass 0.0009 936 59 6 Pass Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -33- 0.0009 904 59 6 Pass 0.0009 868 59 6 Pass 0.0009 833 59 7 Pass 0.0009 797 59 7 Pass 0.0010 768 58 7 Pass 0.0010 741 58 7 Pass 0.0010 720 58 8 Pass 0.0010 689 58 8 Pass 0.0010 669 56 8 Pass 0.0010 649 55 8 Pass 0.0010 638 54 8 Pass 0.0010 615 54 8 Pass 0.0011 592 54 9 Pass 0.0011 577 52 9 Pass 0.0011 565 52 9 Pass 0.0011 550 52 9 Pass 0.0011 538 51 9 Pass 0.0011 524 51 9 Pass 0.0011 513 51 9 Pass 0.0011 504 51 10 Pass 0.0012 495 51 10 Pass 0.0012 482 50 10 Pass 0.0012 472 50 10 Pass 0.0012 456 50 10 Pass 0.0012 439 50 11 Pass 0.0012 427 50 11 Pass 0.0012 417 50 11 Pass 0.0012 407 49 12 Pass 0.0013 390 49 12 Pass 0.0013 376 48 12 Pass 0.0013 360 48 13 Pass 0.0013 345 48 13 Pass 0.0013 328 48 14 Pass 0.0013 319 48 15 Pass 0.0013 313 48 15 Pass 0.0013 303 48 15 Pass 0.0014 294 48 16 Pass 0.0014 289 48 16 Pass 0.0014 283 46 16 Pass 0.0014 277 44 15 Pass 0.0014 272 44 16 Pass 0.0014 268 43 16 Pass 0.0014 265 42 15 Pass 0.0014 261 41 15 Pass 0.0015 257 40 15 Pass 0.0015 250 39 15 Pass 0.0015 245 39 15 Pass 0.0015 241 39 16 Pass 0.0015 237 39 16 Pass 0.0015 232 39 16 Pass 0.0015 227 39 17 Pass 0.0015 224 38 16 Pass Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -34- Water Quality BMP Flow and Volume for POC #1 On-line facility volume: 0 acre-feet On-line facility target flow: 0 cfs. Adjusted for 15 min: 0 cfs. Off-line facility target flow: 0 cfs. Adjusted for 15 min: 0 cfs. LID Report LID Technique Used for Total Volume Volume Infiltration Cumulative Percent Water Quality Percent Comment Treatment? Needs Through Volume Volume Volume Water Quality Treatment Facility (ac-ft.) Infiltration Infiltrated Treated (ac-ft) (ac-ft) Credit retention 1 POC N 17.93 N 99.87 Total Volume Infiltrated 17.93 0.00 0.00 99.87 0.00 0% No Treat. Credit Compliance with LID Standard 8 Duration Analysis Result = Passed Perind and Impind Changes No changes have been made. This program and accompanying documentation are provided 'as -is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by Clear Creek Solutions, Inc. 2005-2021; All Rights Reserved. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -35- C. STORMWATER POLLUTION PREVENTION PLAN This Stormwater Pollution Prevention Plan (SWPPP) has been prepared as part of the Construction stormwater permit requirements for the project 170 SFR in 7115 174th St SW, Edmonds, Washington. It is located in Section 8, Township 27 North, and Range 4 East of the Willamette Meridian in Snohomish County, Washington. The site area is 0.28 Acres. The site is currently developed with an existing house, detached garage, and driveway. The remainder of the site exists as residential landscaping and native vegetation. The site contains two drainage basins, one that slopes to the north, and another that slopes to the south. Please refer to the downstream analysis map for more details. There are no critical areas located on the site. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Please refer to the soils map and descriptions attached earlier in this report for more details. The project site is zoned as city zoning. The proposal is to construct a new single-family home, with a total roof area of 2,283 SF, a walkway, and a driveway off 174th St SW. The existing detached garage and home will be removed. Access to the home will be provided from a walkway and driveway off 174th St SW. The total new and replaced hard surface area will be 3,955 SF. Post -Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. Permeable Pavement per BMP T5.15 will be used to provide stormwater management for other hard surfaces and a Rain Garden per BMP T5.14 will be used for the roof area. The purpose of this SWPPP is to describe the proposed construction activities and all temporary and permanent erosion and sediment control (TESC) measures, pollution prevention measures, inspection/monitoring activities, and recordkeeping that will be implemented during the proposed construction project. The objectives of the SWPPP are to: 1. Implement Best Management Practices (BMPs) to prevent erosion and sedimentation, and to identify, reduce, eliminate or prevent stormwater contamination and water pollution from construction activity. 2. Prevent violations of surface water quality, ground water quality, or sediment management standards. Insight Engineering Co. - Stormwater Site Plan ?/10/2021 -36- 3. Prevent, during the construction phase, adverse water quality impacts including impacts on beneficial uses of the receiving water by controlling peak flow rates and volumes of stormwater runoff at the Permittee's outfalls and downstream of the outfalls. This SWPPP was prepared using the Ecology SWPPP Template. This SWPPP was prepared based on the requirements set forth in the Construction Stormwater General Permit and in the Stormwater Management Manual for Western Washington (SWMMWW 2019). The 13 BMP Elements Element #1— Mark Clearing Limits To protect adjacent properties and to reduce the area of soil exposed to construction, the limits of construction will be clearly marked before land -disturbing activities begin. Element #2 — Establish Construction Access Construction access or activities occurring on unpaved areas shall be minimized, yet where necessary, access points shall be stabilized to minimize the tracking of sediment onto public roads, and wheel washing, street sweeping, and street cleaning shall be employed to prevent sediment from entering state waters. Install the temporary construction entrance, according to the approved construction plans, prior to any clearing or grading activities. Maintain until the access road is paved. Element #3 — Control Flow Rates In order to protect the properties and waterways downstream of the project site, stormwater discharges from the site will be controlled. In general, discharge rates of stormwater from the site will be controlled where increases in impervious area or soil compaction during construction could lead to downstream erosion, or where necessary to meet local agency stormwater discharge requirements. Element #4 — Install Sediment Controls Install silt fencing, according to the approved plans, prior to any clearing or grading activities. Maintain until all construction activities are completed. Install catch basin filters, according to the approved construction plans, as catch basins become operable. Maintain until all construction activities are completed. Element #5 — Stabilize Soils Exposed and un-worked soils shall be stabilized with the application of effective BMPs to prevent erosion throughout the life of the project. Apply temporary hydro -seed to exposed and un-worked soils, according to the approved construction plans, as needed to prevent erosion during site grading. Apply permanent hydro -seed to areas at final grade as site grading is completed. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -37- Apply mulching to exposed and un-worked soils, according to the approved construction plans, as needed to prevent erosion during site grading. Maintain until site grading is completed and permanent hydro -seed is applied. Cover stockpiles with plastic sheeting, according to the approved construction plans, as needed to prevent erosion during site grading. Maintain until stockpiles are removed from site. Element #6 — Protect Slopes All cut and fill slopes will be designed, constructed, and protected in a manner than minimizes erosion. The following specific BMPs will be used to protect slopes for this project. Element #7 — Protect Drain Inlets All storm drain inlets and culverts made operable during construction shall be protected to prevent unfiltered or untreated water from entering the drainage conveyance system. However, the first priority is to keep all access roads clean of sediment and keep street wash water separate from entering storm drains until treatment can be provided. Element #8 — Stabilize Channels and Outlets Where site runoff is to be conveyed in channels, or discharged to a stream or some other natural drainage point, efforts will be taken to prevent downstream erosion. Element #9 — Control Pollutants All pollutants, including waste materials and demolition debris, that occur onsite shall be handled and disposed of in a manner that does not cause contamination of stormwater. Good housekeeping and preventative measures will be taken to ensure that the site will be kept clean, well organized, and free of debris. Element #10 — Control Dewatering There will be no dewatering expected as part of this proposal. If it occurs, Baker tanks will be used for dewatering. Element #11— Maintain BMPs All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to assure continued performance of their intended function. Maintenance and repair shall be conducted in accordance with each particular BMP's specifications. Visual monitoring of the BMPs will be conducted at least once every calendar week and within 24 hours of any rainfall event that causes a discharge from the site. If the site becomes inactive, and is temporarily stabilized, the inspection frequency will be reduced to once every month. All temporary erosion and sediment control BMPs shall be removed within 30 days after the final site stabilization is achieved or after the temporary BMPs are no longer needed. Trapped sediment shall be removed or stabilized on site. Disturbed soil resulting from removal of BMPs or vegetation shall be permanently stabilized. Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -38- Element #12 — Manage the Proiect Erosion and sediment control BMPs for this project have been designed based on the following principles: Design the project to fit the existing topography, soils, and drainage patterns; Emphasize erosion control rather than sediment control; Minimize the extent and duration of the area exposed; Keep runoff velocities low; Retain sediment on site; Thoroughly monitor site and maintain all ESC measures and Schedule major earthwork during the dry season. The SWPPP shall be modified as necessary to include additional or modified BMPs designed to correct problems identified. Revisions to the SWPPP shall be completed within seven (7) days following the inspection. Element #13 — Protect On -site Stormwater Management BMPs for Runoff from Roofs and Other Hard Surfaces On -site stormwater management BMPs used for runoff from roofs and other hard surfaces include: full dispersion, roof downspout full infiltration or dispersion systems, perforated stubout connections, rain gardens, bioretention systems, permeable pavement, sheetflow dispersion, and concentrated flow dispersion. The areas on the site to be used for these BMPs shall be protected from siltation and compaction during construction by sequencing the construction in a fashion to install these BMPs at the latter part of the construction grading operations, by excluding equipment from the BMPS and the associated areas, and by using the erosion and sedimentation control BMPs listed below. Additional requirements for protecting these BMPs during the construction process, testing functionality, and restoring functionality are needed at the final stage of the construction process. Relevant BMPs BMP C102: Buffer Zone BMP C103: High Visibility Fence BMP C231: Brush Barrier BMP C233: Silt Fence BMP C234: Vegetated Strip Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -39- D. GEOTECH REPORT Insight Engineering Co. - Stormwater Site Plan 2/10/2021 -40- COBALT 0 E 0 S C I E N C E S Geotechnical Investigation Proposed Residence 7115 1,74th Street SW Edmonds, Washington June i8, 2020 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON Table of Contents i.o INTRODUCTION............................................................................................................. 1 2.0 PROJECT DESCRIPTION.............................................................................................. 1 3.0 SITE DESCRIPTION....................................................................................................... 1 4.o FIELD INVESTIGATION............................................................................................... 2 4.1.1 Site Investigation Program................................................................................... 2 5.0 SOIL AND GROUNDWATER CONDITIONS .............................................................. 2 5.1.1 Area Geology........................................................................................................ 2 5.1.2 Groundwater........................................................................................................ 3 6.o GEOLOGIC HAZARDS................................................................................................... 3 6.1 Steep Slope Hazard.............................................................................................. 3 6.2 Erosion Hazard.................................................................................................... 3 6.3 Seismic Hazard.................................................................................................... 4 7.o DISCUSSION................................................................................................................... 4 7.1.1 General.................................................................................................................4 8.o RECOMMENDATIONS.................................................................................................. 4 8.1.1 Site Preparation................................................................................................... 4 8.1.2 Temporary Excavations........................................................................................ 5 8.1.3 Erosion and Sediment Control.............................................................................. 6 8.1.4 Foundation Design............................................................................................... 6 8.1.5 Stormwater Management..................................................................................... 7 8.1.6 Slab-on-Grade...................................................................................................... 7 8.1.E Groundwater Influence on Construction .............................................................. 8 8.1.8 Utilities................................................................................................................ 8 9.o CONSTRUCTION FIELD REVIEWS............................................................................ 9 io.o CLOSURE.................................................................................................................... 9 LIST OF APPENDICES Appendix A — Statement of General Conditions Appendix B — Figures Appendix C — Exploration Logs GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 i.o Introduction COBALT GEOSCIENCES In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical investigation for the proposed residential development located at 7115 174th Street SW in Edmonds, Washington (Figure 1). The purpose of the geotechnical investigation was to identify subsurface conditions and to provide geotechnical recommendations for foundation design, stormwater management, earthwork, soil compaction, and suitability of the on -site soils for use as fill. The scope of work for the geotechnical evaluation consisted of a site investigation followed by engineering analyses to prepare this report. Recommendations presented herein pertain to various geotechnical aspects of the proposed development, including foundation support of the new building and stormwater management. 2.0 Project Description The project includes construction of a new single-family residence in the central portion of the property. Associated construction will include a driveway, utilities, and stormwater management systems. Anticipated building loads are expected to be light to moderate and site grading will include cuts and fills on the order of 4 feet or less. Stormwater management will include infiltration devices, if feasible. We should be provided with the final plans when they become available. 3.0 Site Description The site is located at 7115 174th Street SW in Edmonds, Washington (Figure i). The property consists of one rectangular shaped parcel (No. 00513100013804) with a total area of about 0.28 acres. The central portion of the property is developed with a dilapidated residence. The remainder of the property is undeveloped and vegetated with grasses, bushes, ferns, ivy, blackberry vines, and sparse trees. A majority of the property is level to slightly sloping. There is a moderately steep slope area extending downward to the north and northeast from the northern portion of the property. The slope is about 10 to 15 feet tall and has magnitudes of 15 to 30 percent. Short areas may have magnitudes approaching 40 percent. The overall topography continues downward to the north and northeast. The site is bordered to the north, east, and west by residential properties, and to the south by 174th Street SW. PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 4.o Field Investigation 4.1.1 Site Investigation Program L 2 COBALT GEOSCIENCES The geotechnical field investigation program was completed on June 11, 202o and included excavating and sampling two test pits within the property for subsurface analysis. The soils encountered were logged in the field and are described in accordance with the Unified Soil Classification System (USCS). A Cobalt Geosciences field representative conducted the explorations, collected disturbed soil samples, classified the encountered soils, kept a detailed log of the explorations, and observed and recorded pertinent site features. The results of the sampling are presented on the exploration logs enclosed in Appendix C. 5.0 Soil and Groundwater Conditions 5aa Area Geology The site lies within the Puget Lowland. The lowland is part of a regional north -south trending trough that extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington, this lowland is glacially carved, with a depositional and erosional history including at least four separate glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to the east by the Cascade Range. The lowland is filled with glacial and non -glacial sediments consisting of interbedded gravel, sand, silt, till, and peat lenses. The Geologic Map of Washington — Northwest Quadrant, indicates that the site is underlain by Vashon Glacial Till. Vashon Glacial Till is typically characterized by an unsorted, non -stratified mixture of clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden and incorporated by the glacial ice. Explorations Both test pits encountered approximately 6 to 12 inches of topsoil and vegetation underlain by 2 to 3 feet of loose to medium dense, silty -fine to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by dense to very dense, silty -fine to medium grained sand with gravel (Glacial Till), which continued to the termination depths of the test pits. 2 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 5.1.2 Groundwater L 2 COBALT GEOSCIENCES Groundwater was not encountered in any of the explorations. There is a slight chance that perched groundwater may be encountered during late winter and early spring months. We anticipate that groundwater would be perched between fill or weathered till and underlying unweathered glacial till. Note: we did not observed mottled soils in the test pits. Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. 6.o Geologic Hazards 6a Steep Slope Hazard A majority of the property is level to slightly sloping. There is a moderately steep slope area extending downward to the north and northeast from the northern portion of the property. The slope is about 10 to 15 feet tall and has magnitudes of 15 to 30 percent. Short areas may have magnitudes approaching 40 percent. The overall topography continues downward to the north and northeast. These areas do not appear to consist of steep slope or landslide hazard areas. The site and adjacent slopes are stable at this time with no evidence of severe erosion, soil creep, or landslide activity. In general, these slopes will remain stable provided they are fully vegetated. Similarly, the erosion potential of the shallow soils will remain `slight' if the soils remain vegetated. The proposed development is located in a nearly level area, at least 30 feet from slopes with magnitudes of 15 percent or more. The proposed development should not adversely affect erosion or slope stability in sloped areas or areas underlain by soils with a relatively high erosion potential. No additional setback or buffer is required. 6.2 Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is underlain by Alderwood-Urban land complex (2 to 8 percent slopes) and Alderwood-Everett gravelly sandy loams (25 to 70 percent slopes). These soils would have a slight to severe erosion potential in a disturbed state, depending on the slope magnitude. 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 315t to April 1st. Erosion control measures should be in place before the onset of wet weather. 3 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 6.3 Seismic Hazard L 2 COBALT GEOSCIENCES The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015 International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of dense to very dense soils within the upper too feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss, S,, F,,. The USGS website includes the most updated published data on seismic conditions. The site specific seismic design parameters and adjusted maximum spectral response acceleration parameters are as follows: PGA (Peak Ground Acceleration, in percent of g) Ss 131.6o% of g S, 46.70% of g Fa 1.00 Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The relatively dense soil deposits that underlie the site have a low liquefaction potential. 7.o DISCUSSION 7-1.1 General The site is underlain by weathered and unweathered glacial till. The proposed residential structure may be supported on a shallow foundation system bearing on medium dense or firmer native soils and/or structural fill placed on suitable native soils. Local overexcavation of fill and/or loose soils may be necessary below proposed foundation elements. Infiltration of stormwater runoff is feasible in some portions of the property and within the upper weathered glacial till. Systems could include very shallow trenches or rain gardens. Overflow should extend into available storm or storm -sewer infrastructure. We should be provided with the civil plans once they have been prepared. 8.o Recommendations 8.1.1 Site Preparation Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich soil and fill. Based on observations from the site investigation program, it is anticipated that the stripping depth will be 6 to 12 inches. Deeper excavations will be necessary below large trees and in any areas underlain by undocumented fill materials. 4 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 L 2 COBALT GEOSCIENCES The native soils consist of silty -sand with gravel. These soils may be used as structural fill provided they achieve compaction requirements and are within 3 percent of the optimum moisture. Some of these soils may only be suitable for use as fill during the summer months, as they will be above the optimum moisture levels in their current state. These soils are variably moisture sensitive and may degrade during periods of wet weather and under equipment traffic. Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the ASTM D 1557 test method. 8.1.2 Temporary Excavations Based on our understanding of the project, we anticipate that the grading could include local cuts on the order of approximately 4 feet or less for foundation and utility placement. Any deeper excavations should be sloped no steeper than 1.5H:1V in loose soils, 1H:1V in medium dense native soils and 3/4H:1V in dense to very dense native soils. If an excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no steeper than 2H:1V, where room permits. Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a qualified person during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather, and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Materials should not be stored or equipment operated within 10 feet of the top of any temporary cut slope. Soil conditions may not be completely known from the geotechnical investigation. In the case of temporary cuts, the existing soil conditions may not be completely revealed until the excavation work exposes the soil. Typically, as excavation work progresses the maximum inclination of temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be made. Soil and groundwater conditions can be highly variable. Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that the project can proceed and required deadlines can be met. If any variations or undesirable conditions are encountered during construction, we should be notified so that supplemental recommendations can be made. If room constraints or groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed by the WAC, temporary shoring systems may be required. The contractor should be responsible for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences and the project structural engineer review temporary shoring designs prior to installation, to verify the suitability of the proposed systems. 5 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 8.1.3 Erosion and Sediment Control L 2 COBALT GEOSCIENCES 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). 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. 8.1.4 Foundation Design The proposed residential building may be supported on a shallow spread footing foundation system bearing on undisturbed medium dense or firmer native soils or on properly compacted structural fill placed on the suitable native soils. If structural fill is used to support foundations, then the zone of structural fill should extend beyond the faces of the footing a lateral distance at least equal to the thickness of the structural fill. For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed structure. Provided that the footings are supported as recommended above, a net allowable bearing pressure of 2,500 pounds per square foot (psf) may be used for design. A 1/3 increase in the above value may be used for short duration loads, such as those imposed by wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing excavations should be inspected to verify that the foundations will bear on suitable material. Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch. Differential settlement, along a 25-foot exterior wall footing, or between adjoining column footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most settlement is expected to occur during construction, as the loads are applied. However, additional post -construction 6 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 L 2 COBALT GEOSCIENCES settlement may occur if the foundation soils are flooded or saturated. All footing excavations should be observed by a qualified geotechnical consultant. Resistance to lateral footing displacement can be determined using an allowable friction factor of 0.40 acting between the base of foundations and the supporting subgrades. Lateral resistance for footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior areas). The allowable friction factor and allowable equivalent fluid passive pressure values include a factor of safety of 1.5. The frictional and passive resistance of the soil may be combined without reduction in determining the total lateral resistance. Care should be taken to prevent wetting or drying of the bearing materials during construction. Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the footing excavations, should be removed prior to placing concrete. The potential for wetting or drying of the bearing materials can be reduced by pouring concrete as soon as possible after completing the footing excavation and evaluating the bearing surface by the geotechnical engineer or his representative. 8.1.5 Stormwater Management The site is underlain by weathered and unweathered glacial till. Limited infiltration is generally feasible at shallow depths within the weathered glacial till. Any trenches should be located at least 20 feet from the top of the moderately steep slope as shown in Figure 2. One or more small rain gardens could be considered. We performed a small-scale pilot infiltration test in TP-2 at a depth of 2 feet below grade. Following testing and application of correction factors for site variability (o.9), testing (0.4), and influent control (o.9), the infiltration rate was determined to be 0.31 inches per hour. Dense to very dense glacial till, which is nearly impermeable, was observed in this test pit at about 3.5 feet below grade. Any trench or rain garden must have at least 12 inches of clearance above this restrictive layer. Systems may be sized using the Loamy Sand designation (if allowed) or the infiltration rate shown above. Any trench must be located at least 20 feet from the top of the moderately steep slope and setback from structures per stormwater requirements. The geotechnical engineer should be on site during trench construction to determine if the soil conditions are suitable or if local loosening or removal of glacial till is required. We can provide additional recommendations once a civil plan has been prepared. We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional modifications are needed. 8.1.6 Slab -on -Grade We recommend that the upper 12 inches of the existing fill and/or native soils within slab areas be re - compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). 7 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 L 2 COBALT GEOSCIENCES Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers. The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302AR-04 Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier selection and floor slab detailing. A minimum 4 inch thick capillary break should be placed over the prepared subgrade. This may consist of pea gravel or 5/8 inch clean angular rock. Slabs on grade may be designed using a coefficient of subgrade reaction of 18o pounds per cubic inch (pci) assuming the slab -on -grade base course is underlain by structural fill placed and compacted as outlined in Section 8.1. A 4 inch thick capillary break material should be placed over the prepared subgrade. This could include pea gravel or 5/8 inch clean angular rock. A perimeter drainage system is recommended unless interior slab areas are elevated a minimum of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should consist of a 4 inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain rock wrapped in a non -woven geosynthetic filter fabric to reduce migration of soil particles into the drainage system. The perimeter drainage system should discharge by gravity flow to a suitable stormwater system. Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate surface water flow away from the building and preferably with a relatively impermeable surface cover immediately adjacent to the building. 8.1.7 Groundwater Influence on Construction Groundwater was not encountered in any of the explorations. There is a chance that light groundwater could be encountered above the unweathered glacial till, where encountered. The depth to groundwater would likely be 3 to 8 feet below grade in those areas. It should be noted that we did not observe mottled soils; therefore, there may not be seasonal groundwater present below the site at shallow depths. If groundwater is encountered, we anticipate that sump excavations and small diameter pumps systems will adequately de -water short-term excavations, if required. Any system should be designed by the contractor. We can provide additional recommendations upon request. 8.1.8 Utilities Utility trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty and sandy soils were encountered at shallow depths in the explorations at this site. These soils have low cohesion and density and will have a tendency to cave or slough in excavations. Shoring or sloping back trench sidewalls is required within these soils in excavations greater than 4 feet deep. 8 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 L 2 COBALT GEOSCIENCES All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5 feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench backfill in pavement areas should be compacted to at least go percent of the maximum dry density based on ASTM Test Method D1557• Pipe bedding should be in accordance with the pipe manufacturer's recommendations. The contractor is responsible for removing all water -sensitive soils from the trenches regardless of the backfill location and compaction requirements. Depending on the depth and location of the proposed utilities, we anticipate the need to re -compact existing fill soils below the utility structures and pipes. The contractor should use appropriate equipment and methods to avoid damage to the utilities and/or structures during fill placement and compaction procedures. 9.o Construction Field Reviews Cobalt Geosciences should be retained to provide part time field review during construction in order to verify that the soil conditions encountered are consistent with our design assumptions and that the intent of our recommendations is being met. This will require field and engineering review to: ■ Monitor and test structural fill placement and soil compaction ■ Observe bearing capacity at foundation locations ■ Observe slab -on -grade preparation ■ Verify stormwater facility soil conditions ■ Observe excavation stability Geotechnical design services should also be anticipated during the subsequent final design phase to support the structural design and address specific issues arising during this phase. Field and engineering review services will also be required during the construction phase in order to provide a Final Letter for the project. lo.o Closure This report was prepared for the exclusive use of Dave Nolan and his appointed consultants. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved in writing by Cobalt Geosciences, LLC. The recommendations contained in this report are based on assumed continuity of soils with those of our test holes, and assumed structural loads. Cobalt Geosciences should be provided with final architectural and civil drawings when they become available in order that we may review our design recommendations and advise of any revisions, if necessary. Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the responsibility of Dave Nolan who is identified as "the Client" within the Statement of General Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of these not be satisfied. 9 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON June 18, 2020 Respectfully submitted, Cobalt Geosciences, LLC Original signed by: t��HONYy�t WAsyq k.. a, 54896 OVAL Exp. 6/26/2020 Phil Haberman, PE, LG, LEG Principal PH/sc PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 I 513T��f�j of C� _ 11. Habi rm-m E��lCOBALT GEOSCIENCES 10 APPENDIX A Statement of General Conditions Statement of General Conditions USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent and may not be used by any third party without the express written consent of Cobalt Geosciences and the Client. Any use which a third party makes of this report is the responsibility of such third party. BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report are in accordance with Cobalt Geosciences present understanding of the site specific project as described by the Client. The applicability of these is restricted to the site conditions encountered at the time of the investigation or study. If the proposed site specific project differs or is modified from what is described in this report or if the site conditions are altered, this report is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the report to reflect the differing or modified project specifics and/or the altered site conditions. STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in accordance with the normally accepted standard of care in the state of execution for the specific professional service provided to the Client. No other warranty is made. INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements regarding their condition, made in this report are based on site conditions encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or sampling locations. Classifications and statements of condition have been made in accordance with normally accepted practices which are judgmental in nature; no specific description should be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock and groundwater conditions as influenced by geological processes, construction activity, and site use. VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be encountered that are different from those described in this report or encountered at the test locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected conditions are substantial and if reassessments of the report conclusions or recommendations are required. Cobalt Geosciences will not be responsible to any party for damages incurred as a result of failing to notify Cobalt Geosciences that differing site or sub -surface conditions are present upon becoming aware of such conditions. PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next project stage (property acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated project specifics and that the contents of this report have been properly interpreted. Specialty quality assurance services (field observations and testing) during construction are a necessary part of the evaluation of sub -subsurface conditions and site preparation works. Site work relating to the recommendations included in this report should only be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be responsible for site work carried out without being present. APPENDIX B Figures: Vicinity Map, Site Plan 10.2 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 206-331-1097 � nnn' w nnn' %N nnn' w 177.14 nnn' LIV V1C CR4 1a7-1R nAl 0 0 0 tv 0 0 0 N 0 0 0 N 7 z 0 0 0 J 1747stSt5W syfi d WASHINGTON I �f 11 1L,. - a 743rd Si SW _ y. '� •pa. r ■Edmonds` - r Project Location f 1s or Rd r i ;.., •- �. �'' I _ _ 752nd St SW ' - 1153 ••=m r ) � + 1 � 156th f W lt' a .osa 00 1 152 Lh PI: SLV , 1) .n r..1 ^ sr 3 76 h 5 3 � f L $ Meada7lyd�le ' - R .d = r'� MIN �' 1 .!i�'i� . Ti+ �r. ,r I ' _2 ro �' xr.�>_w rl, L r �'•'}-�LI - �'!-' _ _ _ _ _ F. _ _ •. i — ,I 768i1ii5-_w +,• a "a .ter ••_ • .. iirpuP4� JJ r / D,we.rn •� _ __Y .�V f = s _ 7d2nd PI SW •. r ; e "Rock.. � pdRd` I r ] • 1 � - - Z3td_P! 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Box 82243 Proposed Single -Family Residence VICINITY MAP Kenmore, WA 98028 11 1 th Street SW (2o6) 331-1097 % J� %4 _ www.cobaltgeo.com Edmonds, Washington FIGURE i cobaltgeo(d-) gmail.com TP-i Approximate Test Pit N Location A Proposed Residence 7115174th Street SW Edmonds, Washington Not to Scale SITE PLAN FIGURE 2 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeoogmail.com APPENDIX C Exploration Logs Unified Soil Classification System (USCS) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION Clean Gravels GW Well -graded gravels, gravels, gravel -sand mixtures, little or no fines Gravels (more than 50% (less than 5% fines) GP Poorly graded gravels, gravel -sand mixtures, little or no fines COARSE GRAINED SOILS of coarse fraction retained on No. 4 sieve) Gravels with Fines (more than 12% fines) GM Silty gravels, gravel -sand -silt mixtures GC Clayey gravels, gravel -sand -clay mixtures (more than 50% retained on Clean Sands ;°e sw Well -graded sands, gravelly sands, little or no fines No. 200 sieve) Sands (50% or more of coarse fraction (less than 5% fines) SP Poorly graded sand, gravelly sands, little or no fines passes the No. 4 sieve) Sands with Fines sM Silty sands, sand -silt mixtures (more than 12% fines) sc Clayey sands, sand -clay mixtures ML Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, FINE GRAINED SOILS (50% or more Silts and Clays (liquid limit less than 50) Inorganic or clayey silts with slight plasticity cL Inorganic clays of low to medium plasticity, gravelly clays, sandy clays silty clays, lean clays Organic oL Organic silts and organic silty clays of low plasticity passes the MH Inorganic silts, micaceous or diatomaceous fine sands or silty soils, No. 200 sieve) Silts and Clays (liquid limit 50 or more) Inorganic elastic silt CH Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay Organic OH Organic clays of medium to high plasticity, organic silts HIGHLY ORGANIC SOILS primarily organic matter, dark in color, and organic odor PT Peat, humus, swamp soils with high organic content (ASTM D4427) 1 Classification of Soil Constituents 1 MAJOR constituents compose more than 50 percent, by weight, of the soil. Major constituents are capitalized (i.e., SAND). Minor constituents compose 12 to 50 percent of the soil and precede the major constituents (i.e., silty SAND). Minor constituents preceded by "slightly" compose 5 to 12 percent of the soil (i.e., slightly silty SAND). Trace constituents compose o to 5 percent of the soil (i.e., slightly silty SAND, trace gravel). Relative Density (Coarse Grained Soils) Consistency (Fine Grained Soils) N, SPT, Relative N, SPT, Relative Blows/FT Density Blows/FT Consistency 0-4 Very loose Under 2 Very soft 4 -10 Loose 2-4 Soft 10 - 30 Medium dense 4-8 Medium stiff 30 - 50 Dense 8 -15 Stiff Over 50 Very dense 15 - 30 Very stiff Over 3o Hard Grain Size Definitions Description Sieve Number and/or Size Fines <#200 (o.o8 mm) Sand -Fine #200 to #40 (o.o8 to 0.4 mm) -Medium #40 to #10 (0.4 to 2 mm) -Coarse #10 to #4 (2 to 5 mm) Gravel -Fine #4 to 3/4 inch (5 to 19 mm) -Coarse 3/4 to 3 inches (19 to 76 mm) Cobbles 3 to 12 inches (75 to 305 mm) Boulders >12 inches (305 mm) Moisture Content Definitions Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA98028 Soil Classification Chart Figure C1 (2o6) 331-1097 _ www.cobalt eg o.com cobaltgeo(d-) gmail.com Date: June 11, 2020 Depth: 8' Groundwater: None Contractor: Client provided Elevation: Logged By: PH Checked By: SC U) o Moisture Content (%) - Plastic Liquid U E Limit Limit } t 0 N Material Description o c ? o DCP Equivalent N-Value G 0 10 20 30 40 50 ---- 2 ---- - -- SM Topsoil/Grass --------------------------------------------- Loose to medium dense, silty -fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. (Weathered Glacial Till Dense to very dense, silty -fine to medium grained sand with gravel, 4 grayish brown, moist. (Glacial Till) 5 b 7 End of Test Pit 8' 9 10 Test Pit TP-2 Date: June 11, 2020 Depth: 6' Groundwater: None Contractor: Client provided Elevation: Logged By: PH Checked By: SC N a> JO o -0Plastic Moisture Content (%) 1 Liquid LL E 3 Limit Limit N Material Description DCP Equivalent N-Value ? o o G 0 10 20 30 40 50 Topsoil/Grass ------ 1 ---- SM --------------------------------------------- Loose to medium dense, silty -fine to medium grained sand with gravel yellowish brown, moist. (Weathered Glacial Till) 2 3 ------ 4 ---- --- SM --------------------------------------------- Dense to very dense, silty -fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) 5 b 7 End of Test Pit b' 9 10 Cobalt Geosciences, LLC Proposed Residence Test Pit P.O. Box 82243 Kenmore, WA 98028 COBALT11 1 th Street SW 7115 %4 (2o6) 331-1097 Edmonds, Washington Logs www.cobaltgeo.com cobaltgeoogmail.com E. Geotechnical Design Approval RESUB 3/5121 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT COBALT G E 0 S C I E N C E S March 4, 2021 Dave Nolan Dave.nolan214 (&gmail.com RE: Plan Review Proposed Residence 7115174th Street SW Edmonds, Washington Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss our plan review of the provided civil stormwater plan. We have reviewed the civil plan by IECO dated February 18, 2021 which shows the location of a bioretention system and other stormwater features. The proposed system location appears suitable with respect to the nearby steep slope hazard areas. We should be on site to verify soil conditions during excavation. 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 K-A 54896 NAL 3/4/2021 Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097