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RS_Drainage_Technical_Information_ Report_251102_v1
Y & C 2 Short Plat 3607 NE 12th Street Renton, WA 98056 TECHNICAL INFORMATION REPORT October 22, 2025 Prepared By: Prepared For: C2MY Engineers, LLC. Y & C 2 LLC PO Box 52883 10065 Wallingford Ave N Bellevue, WA 98015 Seattle, WA 98133 PH: (206) 451-7856 Contact: Thomas Chung Contact: Choomeng Chin, PE Ph: (206) 708-5538 cmchin.c2my@gmail.com thomaschung_1999@yahoo.com 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 2 This page intentionally left blank 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 3 TABLE OF CONTENTS I. PROJECT OVERVIEW ...................................................................................................................... 7 II. CONDITIONS & REQUIREMENTS SUMMARY ........................................................................ 15 III. OFFSITE ANALYSIS .................................................................................................................... 19 IV. FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN ............. 33 Water Quality Treatment Requirements: ........................................................................................ 36 V. CONVEYANCE SYSTEM ANALYSIS AND DESIGN ............................................................... 41 VI. SPECIAL REPORT AND STUDIES ............................................................................................. 45 VII. OTHER PERMITS ........................................................................................................................ 49 VIII. EROSION & SEDIMENT CONTROL (ESC) ANALYSIS AND DESIGN .............................. 53 IX. BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION OF COVENANT ... 61 X. MAINTENANCE AND OPERATION MANUAL ....................................................................... 65 APPENDIX ........................................................................................................................................... 71 WWHM REPORT AND SUMMARY FOR FLOW CONTROL EXCEPTION: ................................ 73 WWHM REPORT AND SUMMARY FOR WATER QUALITY TREATMENT: ............................ 78 GEOTECHNICAL REPORT: .............................................................................................................. 88 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 4 This page intentionally left blank 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 5 SECTION I PROJECT OVERVIEW 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 6 This page intentionally left blank 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 7 I. Project Overview This Technical Information Report (TIR)/Drainage Assessment provides the technical information and design analysis required for preparing Drainage Plan for the proposed development of 3-Lot Short Plat. This project will follow the 2022 City of Renton Surface Water Design Manual (SWDM). The proposed site is located on the south side of NE 12th Street, Renton, WA (see Figure 1 – Vicinity Map). The address is 3607 NE 12th Street, Renton, WA 98056. The total site within the property as surveyed is approximately 28,162 s.f. (0.65 +/- acres). Existing Drainage System Presently, the project site consists of a single-family homes, a detached garage and sheds with paved driveway, concrete walkways and open space. The surrounding area consists of single-family residential lots. The general slope ground slopes southwesterly approximately 0% -5% (see Figure 2, Existing Site Map). Proposed Drainage System The development of the site will implement on-site storm water management to the maximum extent feasible and keep the existing drainage patterns. All pervious area runoff from the area will continue to sheet flow southwesterly direction to the existing street drainage system. The driveways will be constructed of concrete pavement and drain to a water quality treatment system before discharge to future street storm system. The runoff from the proposed impervious roofs area will be connect to a perforated sub-out and drain to future roadway storm system. All pervious disturbed area with have amended soil. All existing improvements will be demolished. Using the maximum impervious area allowed for the R-8 Zoning of 65%, the maximum total proposed main residential buildings roof area for the proposed 3-lot is approximately 11,013 s.f. and the total driveway area of 996 s.f. The off-site frontage improvements area for widening and sidewalk is 8,522. The drainage design will follow the City of Renton SWDM design guidelines for drainage requirements. See page 33 for Areas Summary. According to the soil map of USGS soil survey map, the on-site soil consists of Arents, Alderwood material (AmC), 6-15% slopes. This type of soil consists of gravelly sandy loam. It is considered as hydrologic soil group C, till soil classification. Please also see geotechnical report prepared by Gary A. Flower, PLLC dated December 2, 2024 in appendix for additional information. 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 8 FIGURE 1: VICINITY MAP – (SCALE: NTS) North 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 9 FIGURE 2: SOIL SURVEY MAP - (SCALE: NTS) North 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 10 FIGURE 3: EXISTING SITE CONDITIONS MAP (NTS) 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 11 FIGURE 4: PROPOSED SITE DEVELOPMENT MAP (NTS) 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 12 This page intentionally left blank 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 13 SECTION II CONDITIONS AND REQUIREMENTS 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 14 This page intentionally left blank 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 15 II. Conditions & Requirements Summary This site development is required to comply with current 2022 Surface Water Design Manual. The proposed project is a 3-lot Short Plat development with frontage improvements and roadway widening with sidewalk and will add new and/or replaced impervious area of greater than 2,000 s.f., it will require drainage review. The project site is proposed to add a total new plus replaced impervious surface of 14,333 s.f. on-site and 8,533 sf off-site and does not subject to Simplified Drainage Review or Large Project Drainage review. It falls into the Directed Drainage Review and is required to comply per Section 1.1.2.3 of SWDM and all Core Requirements per Table 1.1.2.A. Core Requirements: 1. Discharge at a natural location (1.2.1): Runoff from the site will be discharged at the natural location. 2. Offsite Analysis (1.2.2): The development site drains to street drainage system that eventually discharges to Lake Washington, approximately 1.6 mile away. 3. Flow Control (1.2.3): The site meets the exception for Peak Rate Flow Control Standard Areas with threshold discharge area generate less than 0.15 cfs increase in 100-yr peak flow compared to existing site conditions. See Appendix for WWHM calculation. 4. Conveyance System (1.2.4): All runoff from the impervious building roof areas, driveway turn-around, and sidewalk will be tightlined to discharge to infiltration trenches with 6” PVC pipes at a minimum slope of 2%. 5. Temporary Erosion and Sediment Control (1.2.5): See Temporary Erosion and Sediment Control Plan per SWDM requirements and Section VIII. 6. Maintenance and Operation (1.2.6): On-site storm drainage system is owned and maintained by the owner. See Section X for maintenance guidelines information. 7. Financial Guarantees and Liability (1.2.7): The owner and contractor will obtain all necessary permits and provide financial guarantees required by the City prior to the beginning of construction. 8. Water Quality (1.2.8): The new and replaced PGIS pavement area is greater than 5,000 s.f., water quality treatment is required. All driveways pavement and roadway widening area will drain to a basic linear sand filter to remove pollutants from stormwater before discharge to the public storm system in the roadway. 9. On-Site BMPs (1.2.9): Implement on-site BMPs to the maximum extend feasible. Special Requirements: 1. Other Adopted Area-Specific Requirements (1.3.1): N/A. 2. Flood Hazard Area Delineation (1.3.2): The project site is not located within a 100-year floodplain. 3. Flood Protection Facilities (1.3.3): The project site does not contain a class 1 or 2 stream and will not be constructing or modifying an existing flood protection facility. 4. Source Control (1.3.4): N/A. 2422 TIR Y & C 2 Short Plat G:\2422\2422tir.doc\Page 16 5. Oil Control (1.3.5): N/A. 6. Aquifer Protection Area (1.3.6): N/A 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 17 SECTION III OFF-SITE ANALYSIS 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 18 This page intentionally left blank 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 19 III. Offsite Analysis Upstream Analysis Upstream area consists of established single-family lots that drain from the northeast corner through the subject property. The offsite drainage path will bypass the proposed development area to continue to the roadway drainage system as currently existed. The upstream drainage will not affect the drainage system on site. Downstream drainage Analysis: The outfall from the project site is at the southwestern portion of the subject property to the existing roadway drainage system on Pierce Ave NE. Task 1. Study Area Definition and Maps The upstream tributary area to this project consists of established single-family lots and roadway with established drainage system, away from the area to be developed. There is no likelihood that the proposed project activities could impact the upstream area with backwater conditions as the runoff flow path will be maintained. The current discharge point of the project is at the southwestern portion of the subject property and to the existing roadway drainage system on Pierce Ave NE. The proposed storm discharge will connect to the drainage system on Queen Ave NE and since both the system will merge less than ¼ mile away, the downstream analysis will be conducted on the Queen Ave NE storm system. A reduced copy of the existing conditions map is included as Figure 3, an existing site map showing the property lines. Task 2. Resource Review In our effort to determine, if there were any existing or potential problems with this downstream portion of the city drainage system, the following resources recommended by the SWDM were reviewed: a) Adopted Basin Plans: Not Applicable (N/A) b) Finalized Drainage Studies: N/A c) Basin Reconnaissance Summary Reports: N/A d) Critical Drainage Area Maps: N/A e) Floodplain/floodway (FEMA) Maps: Not present f) Other Offsite Analysis Reports: N/A g) Sensitive Area Folio: None h) DNR Drainage Problem Maps: N/A i) Road Drainage Problems: None known. j) U.S. Department of Agriculture Soil Survey: Arents, Alderwood material (AmC), hydrologic Soil Group C, till soil classification. k) Wetland Inventory Maps: None. 1) Migrating River Studies: N/A m) WA Ecology Water Quality Assessment 303(d)/305(b) Integrated Report – None. 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 20 FIGURE 5: DRAINAGE SYSTEM MAP (NTS) 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 21 FIGURE 6: FEMA FLOOD MAP (NTS) 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 22 FIGURE 7: KING COUNTY TOPOGRAPHIC MAP (NTS) 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 23 Task 3. Field Inspection A field observation was conducted to gather information including a level 1 Downstream Analysis and off-site conveyance system. Level 1 Field Study 1. No reported or observed problems. 2. No existing or potential constrictions or lack of capacity in the existing drainage system was apparent. 3. No reported flooding areas or sign of flooding were discovered. 4. No existing/potential overtopping, scouring, bank sloughing, or sedimentation is apparent. 5. No known aquatic habitats in the conveyance route. 6. The downstream area consists mainly of established roadway drainage system with pipes, structures and open ditches. The topography is moderate for the drainage route. 7. The pipe sizes encountered are from 18 inches up to 24 inches in diameter. 8. Off-site areas tributary to the project site were consistent with the site map included. 9. No known complaints of flooding. 10. The site visit was conducted at 2:30 pm on July 2, 2025. The weather was sunny and 80 ºF. Task 4. Drainage System Description and Problem Descriptions Upstream The upstream tributary area to this project consists of established single-family subdivisions and roadway. Runoff from these lots have an established drainage system that discharges to street drainage system. The offsite drainage path will bypass the proposed development area to continue to the natural location as currently existed. The upstream drainage will not affect the drainage system on site. No anticipation of any impact to the upstream drainage system due to this development Down Stream Drainage System Description: The runoff from the proposed site is proposed to drain to Queen Avenue NE drainage system, portion of the storm extension has not been constructed and currently no storm system exists between the subject property and the existing CB at the end of Queen Ave NE that is approximately 352 feet away area (A to D). From this point it continues southerly through a 12” CMP to CB (E )located at the end of NE 11th Ct cul-de-sac approximately 120’. Then it continues easterly in a shared driveway to another CB (F) approximately 46’ away and continues in the same direction to the end of the driveway CB (G) approximately 119’ away. At this point, the runoff turns southerly direction through the single-family lot approximately 341’ to a CB (H) located on the east side of St Andrew Church property. It then continues southerly for another 93’ to a CB (I) located in the lawn at the north side of NE10th Ct. From here it travels southeasterly another 47’ to a CB (J) located at the south travel lane with solid lid. Then, it continues southerly direction between the single-family lot lines to another CB (K) approximately 137’ at the back yard of house number 3643 NE 10th Lane, it continues in the same direction along the side of the building approximately 108’ to yet another CB (L) with solid grate located on NE 10th Lane. At this point, it continues easterly approximately 30’ on the street to another 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 24 CB (M) with solid grate. It then connects to a south curb line CB (N) approximately 10’. Then it continues southerly between the houses to a CB (O) approximately 208’ located on NE 10th St with solid grate where the analysis terminated for a total distance of an approximately 1,584 l.f., that is greater than ¼ miles from the project site. During the downstream drainage field visit no areas of an existing or potential drainage problems were apparent. Problem description: No apparent problem was noticed or observed along the runoff route. All pipes opening in the catch basins are not districted by any sediment in the catch basins. The downstream drainage system as described above is a not prone to stream bank erosion, siltation, and slides and does not threaten destruction of aquatic habitats. Task 5. Mitigation of Existing or Potential Problems No off-site mitigation is necessary as there are no observed problems during the analysis. 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 25 OFF-SITE ANALYSIS DRAINAGE SYSTEM TABLE Surface Water Design Manual, Core Requirement #2 Downstream Path to South SEOLA CREEK BASIN Symbol Drainage Component Type, Name, and Size Drainage Component Description Slope Distance from site discharge Existing Problems Potential Problems Observations of field inspector resource reviewer, or resident see map Type: sheet flow, swale stream, channel, pipe, pond; Size: diameter surface area drainage basin, vegetation, cover, depth, type of sensitive area, volume % 1/4 mi=1,320 ft constrictions, under capacity, ponding, overtopping, flooding, habitat or organism destruction, scouring, bank sloughing, sedimentation, incision, other erosion tributary area, likelihood of problem, overflow pathways, potential impacts. A-D FUTURE PIPE 12” PIPE FLOW 0.5% 0-325’ NONE OBSERVED NONE ANTICIPATED D-E PIPE 12” CMP PIPE FLOW 0.5% 325-445’ NONE OBSERVED NONE ANTICIPATED NO VISIBLE ISSUE E-F PIPE 12” CMP PIPE FLOW 0.5% 445-491’ NONE OBSERVED NONE ANTICIPATED NO VISIBLE ISSUE F-G PIPE 12”CMP PIPE FLOW 0.4% 491-610’ NONE OBSERVED NONE ANTICIPATED NO VISIBLE ISSUE G-H PIPE 12” CMP PIPE FLOW 0.5% 610-951’ NONE OBSERVED NONE ANTICIPATED NO VISIBLE ISSUE H-I PIPE 12” CMP PIPE FLOW 0.5% 951-1044’ NONE OBSERVED NONE ANTICIPATED NO VISIBLE ISSUE I-J PIPE 12” CMP PIPE FLOW 2% 1044-1091’ NONE OBSERVED NONE OBSERVED NO VISIBLE ISSUE J-K PIPE 12” CONC PIPE FLOW 1.2% 1091-1228’ NONE OBSERVED NONE OBSERVED NO VISIBLE ISSUE K-L PIPE 10” CONC PIPE FLOW 1.2% 1228-1336’ NONE OBSERVED NONE OBSERVED NO VISIBLE ISSUE L-M PIPE 10” CONC PIPE FLOW 1.4% 1336-1366’ NONE OBSERVED NONE OBSERVED NO VISIBLE ISSUE M-N PIPE 12” PVCC PIPE FLOW 1.5% 1366-1376’ NONE OBSERVED NONE OBSERVED NO VISIBLE ISSUE N-O PIPE 12” CONC PIPE FLOW 0.5% 1376-1584’ NONE OBSERVED NONE OBSERVED NO VISIBLE ISSUE 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 26 2422 TIR Chung Short Plat G:\2422\2422tir.doc\Page 27 Looking down stream at end of street Point (D) CB at Point (E) Pipe entrance in CB at point (E) CB at point (F) in driveway 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 28 CB at point (H) in lawn area looking north CB at Point (I) Pipe entrance in Catch Basin at Point (I) Solid grate CB (J) at NE 10th Ct. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 29 CB at Point (L) NE 10th Lane CB at point (M) on NE 10th Lane CB at point (N) on NE 10th Lane Pipes in CB at point (N) on NE 10th Lane 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 30 CB at point (O) on NE 10th Street with a sewer lid looking west-downstream 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 31 SECTION IV FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 32 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 33 IV. Flow Control and Water Quality Facility Analysis and Design This project falls into the Directed Drainage Review. All new proposed runoff from the site will be required to meet all core requirements. Flow control duration standards is Peak Rate Flow Control matching existing site conditions. Drainage Cover Area Summary PRESENT CONDITION Area (SF) AREA (ac) Pervious: Open space Total Pervious 22,847 0.52449 Impervious: House/Buildings 2,897 0.06650 Asphalt driveway 1,933 0.04438 Walkway 485 0.01113 Total Impervious 5,315 0.12202 Total Area: 28,162 0.64651 PROPOSED CONDITION Pervious Landscaping/grass/wetland 7,952 0.18255 Total Pervious 7,952 0.18255 Impervious Main Building Roofs(max. allowed) 13,133 0.30149 Driveways 966 0.02218 Total Impervious 14,099 0.32367 Total Area 22,051 0.50622 ROW Dedication - 6,111 - 0.14029 Off-site: Driveway 1,119 0.02569 Sidewalk/ramps 3,084 0.07080 Pavement widening 4,279 0.09823 Curb and Gutter 806 0.01850 Open space 5,345 0.12270 Total offsite Area: 14,633 0.33592 Basin area summary for WWHM input: Existing: Basin area = 36,684 sf = 0.84215 acs Total impervious = 5,315 sf =0.12202 acs Total pervious = 31,369 sf = 0.72013 acs Proposed: Basin area = 36,684 sf = 0.84215 acs On-site: Total maximum impervious roofs = 13,133 sf = 0.30149 acs (before restricted footprint) Total driveways = 966 sf = 0.02218 acs Total Pervious = 7,952 sf = 0.18255 acs Offsite: 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 34 Total impervious = 1119+3084+4279+806=9288 sf = 0.21322 acs Total pervious = 5,345 sf = 0.12270 acs WWHM Runoff Summary: The peak runoff as calculated based on the proposed land cover without any BMP credit with maximum impervious area allowed per Zoning and input into the WWHM modeling program in one basin. As calculated, the pre-developed and developed peak runoffs are as follow: The flow rates result in the Appendix are as follow: Storm Event: 2 –yr 10-yr 50-yr 100-yr Pre-developed 0.0996 cfs 0.1965 cfs 0.3041 cfs 0.3567 cfs Developed 0.2181 cfs 0.3315 cfs 0.4449 cfs 0.4974 cfs Per Core Requirement #3, 2022 SWDM, Section 1.2.3, this site is located in Peak Rate Flow Control Standard matching existing site conditions area and meets the Exceptions 1 per the peak flow calculation using the existing site conditions as follow, i.e. The facility requirement in peak Rate Flow Control Area is waived for any threshold discharge area in which the target surfaces subject to this requirements will generate no more than a 0.15-cfs difference (when modeled using 15-minute time steps) in the existing site conditions 100-year peak flow (modeled using same time step unit (e.g., 15 minute) used to calculate the developed flow. The existing 100-yr peak flow rate as calculated is 0.3567 cfs compared to the developed rate of 0.4974 cfs, an increases of 0.1407 cfs, that is less than 0.15 cfs and it meets the exception and no flow control facility is proposed. See Appendix page 71 for WWHM calculations. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 35 On-Site BMP Selection for Core Requirement #9: Roof Area: BMP On-Site List infeasibility Additional information Full Dispersion • The minimum native vegetation flow path length is less than 100 feet. Full Dispersion is not feasible Full Infiltration • Not feasible due to hydrological restricted layer and 0”/hr infiltration rate per Geotechnical Evaluation Report See Geotechnical Evaluation Report Limited Infiltration • Not feasible as above Bioretention Facilities • Not feasible as above Downspout Basic Dispersion • The flowpath does not meet the minimum horizontal setback requirements to property lines, structures etc. Restricted Footprint Lot 1 : Maximum allowable Impervious coverage per zoning is 65%. = 8,063 x 0.65 = 5,241 s.f. 10% minimum BMP implementation reduction = 806 sf Restricted max impervious area = 5,241-806 = 4,435 sf Allowable maximum building coverage (50%) = 4032 sf New driveway = 322 sf Allowable maximum new proposed building coverage and other impervious area =3,678+322=4000 sf < 4,435 sf, ok Lot 2 : Maximum allowable Impervious coverage per zoning is 65% = 6,993 x 0.65 = 4,545 s.f. 10% minimum BMP implementation reduction = 699 sf Restricted max impervious area = 4,545-699 = 3,846 sf Allowable maximum building coverage (50%) = 3,497 sf New driveway = 322 sf Allowable maximum new proposed building coverage and other impervious area =3,524+322=3,846 sf <= 3,846 sf ok Lot 3 : Maximum allowable Impervious coverage per zoning is 65% = 6,995 x 0.65 = 4,547 s.f. 10% minimum BMP implementation reduction = 700 sf Total Reduce Impervious area = 4,547-700 = 3,847 sf Maximum building coverage (50%) = 3,498 sf New driveway = 322 sf Allowable maximum new proposed building coverage and other impervious area =3,525+322=3,847 sf <= 3,847 sf ok Provided (C2.9) meet section 1.2.9.2.1 requirement 5. (Restricted footprint summary: (Maximum total impervious area including all existing impervious area to remain) Lot 1 = 4,000 sf Lot 2 = 3,846 sf Lot 3 = 3,847 sf Soil Amendment Provide for all disturbed pervious area Provided (C2.9) meet section 1.2.9.2.1 requirement 6. Perforated Stub-out Connection • Use BMP Provided (C2.9) meet section 1.2.9.2.1 requirement 7. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 36 Other Hard Surface: Pavement/Driveway BMP On-Site List infeasibility Additional information Full Dispersion • The site has less than a 65 to 10 ratio of the native vegetation area to the impervious area. • The minimum native vegetation flow path length is less than 100 feet. Full Dispersion is not feasible Sheet Flow Dispersion • The minimum vegetated flow path for sheet flow cannot be met. Not feasible Permeable Pavement Surfaces • Not feasible due to hydrological restricted layer and 0”/hr infiltration rate per Geotechnical Evaluation Report See Geotechnical Evaluation Report Full Infiltration • Not feasible due to hydrological restricted layer and 0”/hr infiltration rate per Geotechnical Evaluation Report Limited Infiltration • Not feasible due to hydrological restricted layer and 0”/hr infiltration rate per Geotechnical Evaluation Report Water Quality Treatment Requirements: The site is located within the basic water quality treatment for all new pollution generated pavement area with the treatment goal of 80% removal of total suspended solids from leaving the site. The total PGIS is 7,170 (966+1119+4279+806) s.f. (0.16460 acs) and hence water quality treatment is required per Core Requirement #8. Basic sand filter is proposed for the water quality treatment before being discharged to the storm system. A linear sand filter with 16’ long by 1’ wide sediment chamber with 3’ wide sand filter chamber is proposed and meet the treatment of 80% of the runoff volume, see Appendix for calculation. The treatment is 81.75% minimum of the runoff volume. Therefore, the proposed sand filter meets the requirement for 80% TSS removal. See appendix for WWHM sand filter sizing. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 37 FIGURE 7A: EXISTING IMPERVIOUS AREA SUMMARY MAP (NTS) 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 38 FIGURE 7B: PROPOSED IMPERVIOUS AREA SUMMARY MAP (NTS) 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 39 SECTION V CONVEYANCE SYSTEM ANALYSIS AND DESIGN 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 40 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 41 V. Conveyance System Analysis and Design CONVEYANCE REQUIREMENTS FOR NEW SYSTEMS (1.2.4.1): The new conveyance system mainly consists of roof drain tightlines. The 6” PVC tightline with minimum slope of 2% has a capacity of 0.79 cfs. It has sufficient capacity to convey the proposed impervious roof area for conveyance design of 25-yr storm of 0.083 cfs and also 100-yr storm. Areas of Contribution Summary Downspout System (4”): Assumed maximum contribution to each section of area = 2,000 s.f. = 0.0459 ac. C = 0.9 Conveyance Calculations: Roof Runoff From SWDM Figure 3.2.1.A-D aR bR Tc Pr iR IR 2-yr 1.58 0.58 6.30 2.00 0.54 1.09 10-yr 2.44 0.64 6.30 2.90 0.75 2.18 25-yr 2.66 0.65 6.30 3.40 0.80 2.73 100-yr 2.61 0.63 6.30 3.90 0.82 3.19 IR=(Pr) (iR) Peak Rainfall Intensity iR = (aR)*(Tc)^-(bR) Unit peak rainfall intensity factor Downspout System : Peak Runoff C Area (A) IR Q2 = 0.045 cfs 0.9 0.0459 1.09 Q25 = 0.083 cfs 0.9 0.0459 2.01 Q100 = 0.132 cfs 0.9 0.0459 3.19 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 42 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 43 SECTION VI SPECIAL REPORT AND STUDIES 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 44 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 45 VI. Special Report and Studies Geotechnical Report. See appendix. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 46 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 47 SECTION VII OTHER PERMITS 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 48 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 49 VII. Other Permits Water main extension and Wetland Variance 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 50 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 51 SECTION VIII EROSION AND SEDIMENT CONTROL ANALYSIS AND DESIGN 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 52 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 53 VIII. Erosion & Sediment Control (ESC) Analysis and Design Project Description: The project will consist of a single-family residence and ADU with a total lot area of approximately 0.65 acres and a total disturbed area of 0.84 acres will be involved in the development activity. Existing Site Conditions: The existing project site consists of a single-family and a detached garage with paved driveway, walkway, sheds open grass and a few trees. Existing drainage for the project site is from the northeast to southwest of the site to the street drainage system that discharges to Lake Washington. During the proposed construction this drainage pattern will be maintained. The discharge from the project will exit the site to street drainage system. Erosion and Sediment Control Measures: Delineate Clearing and easement limits The development drawings clearly indicate the limits of development activities for this developed site. Prior to any clearing or construction on site, the clearing limits for the project stage shall be clearly marked. The stakes shall be vividly marked with surveyors flagging and located with surveying methods. Cover measures According to the standard ESC notes included all disturbed areas to remain unworked for more than seven days during the dry season (May 1 to September 30) or two days during the wet season (October 1 to April 30), unless otherwise determined by the City of Renton. Any disturbed areas to remain unworked for more than 30 days will be seeded or sodded, unless the City of Renton determines that winter weather makes the vegetation establishment unfeasible. During the wet season any stockpiles to remain unworked for more than 12 hours will remain covered. Perimeter Protection A filter fabric silt fence shall be installed at the edge of the disturbed areas of the site. The silt fence/wattles shall be so located that any sediment laden runoff that does not directly enter the protected inlets will be filtered prior to offsite discharge. The perimeter protection will be installed prior to upslope grading. Traffic Area Stabilization Prior to the start of general construction, the construction vehicle entrances to the site shall be stabilized with a rock pad as detailed on the drawings, unless the proposed location of the construction entrance at the existing grave driveway access is acceptable to the County. Sediment retention Due to the current vegetated area that will be preserved at the perimeter of the lot for this development, the silt fence/wattle will be sufficient to trap sediment on site. However, contractor will be required to prevent any sediment laden runoff discharges offsite and will need to implement 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 54 sediment control measure such as additional silt fence/wattles, diversion dike, straw/mulch application, erosion control matting, or temporary sediment trap if necessary. Surface Water Collection Surface runoff on to the side is not anticipated as the upstream is stabilized single-family with roadway drainage system. However, at the construction site, if the sediment retention measures are found to be insufficient for management of surface flow, other surface water collection measures (e.g., ditches, berms, etc.) shall be installed to intercept all surface water from disturbed areas, convey it to a temporary sediment pond or trap, and discharge it downstream of any disturbed areas. Dewatering Control Dewatering of the site is not anticipated as the ground water is not encountered during soil exploration per Geotechnical Evaluation Report. Dust Control Dust control will be conducted by watering bare soil areas using a water truck or hoses fed by the water truck, as needed. Mulching of bare soil areas may also be used to prevent dust liberation from the site. Flow Control Surface runoff for flow control is not anticipated as the project will be constructed during the dry season. However, if condition change during construction, surface water from the disturbed area will be routed to a temporary sediment trap and install flow control if necessary. 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. If required, BMPs to be implemented to control specific sources of pollutants are discussed below. Vehicles, construction equipment, and/or petroleum product storage/dispensing: All vehicles, equipment, and petroleum product storage/dispensing areas will be inspected regularly to detect any leaks or spills, and to identify maintenance needs to prevent leaks or spills. On-site fueling tanks and petroleum product storage containers shall include secondary containment. Spill prevention measures, such as drip pans, will be used when conducting maintenance and repair of vehicles or equipment. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 55 In order to perform emergency repairs on site, temporary plastic will be placed beneath and, if raining, over the vehicle. Contaminated surfaces shall be cleaned immediately following any discharge or spill incident. Protect Flow Control BMPs Protect all infiltration trench BMPs from sedimentation through installation and maintenance of erosion and sediment control BMPs on portions of the site that drain into the BMPs. Restore the BMPs to their fully functioning condition if they accumulate sediment during construction. Restoring the BMP must include removal of sediment and any sediment-laden soils. Prevent compacting infiltration trench BMPs by excluding construction equipment and foot traffic. Protect completed lawn and landscaped areas from compaction due to construction equipment. Control erosion and avoid introducing sediment from surrounding land uses onto permeable pavements. Do not allow muddy construction equipment on the base material or pavement. Do not allow sediment-laden runoff onto permeable pavements. Keep all heavy equipment off existing soils under LID facilities that have been excavated to final grade to retain the infiltration rate of the soils. 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. Manage the Project 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. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 56 Retain sediment on site. Thoroughly monitor site and maintain all ESC measures. Schedule major earthwork during the dry season. Phasing of Construction The construction project is being phased to the extent practicable in order to prevent soil erosion, and, to the maximum extent possible, the transport of sediment from the site during construction. Revegetation of exposed areas and maintenance of that vegetation shall be an integral part of the clearing activities during each phase of construction. Seasonal Work Limitations From October 1 through April 30, clearing, grading, and other soil disturbing activities shall only be permitted if shown to the satisfaction of the local permitting authority that silt-laden runoff will be prevented from leaving the site through a combination of the following: Site conditions including existing vegetative coverage, slope, soil type, and proximity to receiving waters; and Limitations on activities and the extent of disturbed areas; and Proposed erosion and sediment control measures. Based on the information provided and/or local weather conditions, the local permitting authority may expand or restrict the seasonal limitation on site disturbance. The following activities are exempt from the seasonal clearing and grading limitations: Routine maintenance and necessary repair of erosion and sediment control BMPs; Routine maintenance of public facilities or existing utility structures that do not expose the soil or result in the removal of the vegetative cover to soil; and Activities where there is 100 percent infiltration of surface water runoff within the site in approved and installed erosion and sediment control facilities. Coordination with Utilities and Other Jurisdictions Care has been taken to coordinate with utilities, other construction projects, and the local jurisdiction in preparing this Construction SWPPS and scheduling the construction work. Inspection and Monitoring All BMPs shall be inspected, maintained, and repaired as needed to assure continued performance of their intended function. Site inspections shall be conducted by a person who is knowledgeable in the principles and practices of erosion and sediment control. This person has the necessary skills to: 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 57 Assess the site conditions and construction activities that could impact the quality of stormwater, and Assess the effectiveness of erosion and sediment control measures used to control the quality of stormwater discharges. A Certified Erosion and Sediment Control Lead shall be on-site or on-call at all times. Whenever inspection and/or monitoring reveals that the BMPs identified in this TESC Plan are inadequate, due to the actual discharge of or potential to discharge a significant amount of any pollutant, appropriate BMPs or design changes shall be implemented as soon as possible. Maintaining an Updated Construction SWPPS This TESC shall be retained on-site or within reasonable access to the site. The TESC plan shall be modified whenever there is a change in the design, construction, operation, or maintenance at the construction site that has, or could have, a significant effect on the discharge of pollutants to waters of the state. The TESC plan shall be modified if, during inspections or investigations conducted by the owner/operator, or the applicable local or state regulatory authority, it is determined that the TESC plan is ineffective in eliminating or significantly minimizing pollutants in stormwater discharges from the site. The TESC plan shall be modified as necessary to include additional or modified BMPs designed to correct problems identified. Revisions to the TESC plan shall be completed within seven (7) days following the inspection. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 58 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 59 SECTION IX BOND QUANTITIES, FACILITY SUMMARIES AND DECLARATION OF COVENANT 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 60 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 61 IX. Bond Quantities, Facility Summaries and Declaration of Covenant Declaration of Covenant Attached in Appendix 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 62 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 63 SECTION X MAINTENANCE AND OPERATION MANUAL 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 64 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 65 X. MAINTENANCE AND OPERATION MANUAL The project site development proposes on-site flow control BMP systems. The following is brief description of the facilities, their purpose, and their operation. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 66 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 67 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 68 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 69 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 70 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 71 Appendix 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 72 This page intentionally left blank 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 73 WWHM Report and Summary for Flow Control Exception: Predeveloped Input 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 74 Developed Input 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 75 Flow Frequencies 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 76 WWHM Report Summary: WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: 2422-3607 NE 12th St Renton-Exempt-R0.1 Site Name: Chung Short Plat Site Address: 3607 NE 12th St City : Renton, WA Report Date: 10/22/2025 Gage : Seatac Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.00 Version Date: 2025/05/13 Version : 4.3.2 ___________________________________________________________________ Low Flow Threshold for POC 1 : 50 Percent of the 2 Year ___________________________________________________________________ High Flow Threshold for POC 1: 50 year ___________________________________________________________________ PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat .72013 Pervious Total 0.72013 Impervious Land Use acre ROOF TOPS FLAT 0.06651 DRIVEWAYS FLAT 0.04438 SIDEWALKS FLAT 0.01113 Impervious Total 0.12202 Basin Total 0.84215 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : 42312119 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat .30525 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 77 Pervious Total 0.30525 Impervious Land Use acre ROADS FLAT 0.11674 ROOF TOPS FLAT 0.30149 DRIVEWAYS FLAT 0.04787 SIDEWALKS FLAT 0.0708 Impervious Total 0.5369 Basin Total 0.84215 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.72013 Total Impervious Area:0.12202 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0.30525 Total Impervious Area:0.5369 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.099605 5 year 0.154612 10 year 0.196469 25 year 0.255561 50 year 0.304133 100 year 0.356655 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.218114 5 year 0.284411 10 year 0.331528 25 year 0.394864 50 year 0.444893 100 year 0.497433 ___________________________________________________________________ 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. 2005-2024; All Rights Reserved. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 78 WWHM Report and Summary for Water Quality Treatment: 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 79 Predeveloped Input Developed Input 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 80 Linear Sandfilter Design 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 81 Stage Frequency 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 82 Sandfilter WQ Flow Frequency WQ Sandfilter LID Report 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 83 WWHM Report Summary: WQ-Linear Sandfilter WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: 2422-3607 NE 12th St Renton-Sandfilter Site Name: Y&C2 SP Site Address: 3607 NE 12th St City : Renton Report Date: 10/22/2025 Gage : Seatac Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.00 Version Date: 2025/05/13 Version : 4.3.2 ___________________________________________________________________ Low Flow Threshold for POC 1 : 50 Percent of the 2 Year ___________________________________________________________________ High Flow Threshold for POC 1: 50 year ___________________________________________________________________ PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat .1646 Pervious Total 0.1646 Impervious Land Use acre Impervious Total 0 Basin Total 0.1646 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : Basin 1 Bypass: No 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 84 GroundWater: No Pervious Land Use acre Pervious Total 0 Impervious Land Use acre ROADS FLAT 0.11674 DRIVEWAYS FLAT 0.04786 Impervious Total 0.1646 Basin Total 0.1646 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Sand Filter 1 ___________________________________________________________________ Name : Sand Filter 1 Bottom Length: 16.00 ft. Bottom Width: 3.00 ft. Depth: 3.5 ft. Side slope 1: 0 To 1 Side slope 2: 0 To 1 Side slope 3: 0 To 1 Side slope 4: 0 To 1 Filtration On Hydraulic conductivity: 1.5 Depth of filter medium: 1.5 Total Volume Infiltrated (ac-ft.): 21.118 Total Volume Through Riser (ac-ft.): 4.714 Total Volume Through Facility (ac-ft.): 25.832 Percent Infiltrated: 81.75 Total Precip Applied to Facility: 0 Total Evap From Facility: 0 Discharge Structure Riser Height: 3 ft. Riser Diameter: 24 in. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Sand Filter Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 0.0000 0.001102 0.000000 0.000 0.000 0.0389 0.001102 0.000043 0.000 0.001 0.0778 0.001102 0.000086 0.000 0.001 0.1167 0.001102 0.000129 0.000 0.001 0.1556 0.001102 0.000171 0.000 0.001 0.1944 0.001102 0.000214 0.000 0.001 0.2333 0.001102 0.000257 0.000 0.001 0.2722 0.001102 0.000300 0.000 0.002 0.3111 0.001102 0.000343 0.000 0.002 0.3500 0.001102 0.000386 0.000 0.002 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 85 0.3889 0.001102 0.000429 0.000 0.002 0.4278 0.001102 0.000471 0.000 0.002 0.4667 0.001102 0.000514 0.000 0.002 0.5056 0.001102 0.000557 0.000 0.002 0.5444 0.001102 0.000600 0.000 0.002 0.5833 0.001102 0.000643 0.000 0.002 0.6222 0.001102 0.000686 0.000 0.002 0.6611 0.001102 0.000728 0.000 0.002 0.7000 0.001102 0.000771 0.000 0.002 0.7389 0.001102 0.000814 0.000 0.002 0.7778 0.001102 0.000857 0.000 0.002 0.8167 0.001102 0.000900 0.000 0.002 0.8556 0.001102 0.000943 0.000 0.002 0.8944 0.001102 0.000986 0.000 0.002 0.9333 0.001102 0.001028 0.000 0.002 0.9722 0.001102 0.001071 0.000 0.002 1.0111 0.001102 0.001114 0.000 0.002 1.0500 0.001102 0.001157 0.000 0.002 1.0889 0.001102 0.001200 0.000 0.002 1.1278 0.001102 0.001243 0.000 0.002 1.1667 0.001102 0.001286 0.000 0.003 1.2056 0.001102 0.001328 0.000 0.003 1.2444 0.001102 0.001371 0.000 0.003 1.2833 0.001102 0.001414 0.000 0.003 1.3222 0.001102 0.001457 0.000 0.003 1.3611 0.001102 0.001500 0.000 0.003 1.4000 0.001102 0.001543 0.000 0.003 1.4389 0.001102 0.001586 0.000 0.003 1.4778 0.001102 0.001628 0.000 0.003 1.5167 0.001102 0.001671 0.000 0.003 1.5556 0.001102 0.001714 0.000 0.003 1.5944 0.001102 0.001757 0.000 0.003 1.6333 0.001102 0.001800 0.000 0.003 1.6722 0.001102 0.001843 0.000 0.003 1.7111 0.001102 0.001886 0.000 0.003 1.7500 0.001102 0.001928 0.000 0.003 1.7889 0.001102 0.001971 0.000 0.003 1.8278 0.001102 0.002014 0.000 0.003 1.8667 0.001102 0.002057 0.000 0.003 1.9056 0.001102 0.002100 0.000 0.003 1.9444 0.001102 0.002143 0.000 0.003 1.9833 0.001102 0.002185 0.000 0.003 2.0222 0.001102 0.002228 0.000 0.003 2.0611 0.001102 0.002271 0.000 0.004 2.1000 0.001102 0.002314 0.000 0.004 2.1389 0.001102 0.002357 0.000 0.004 2.1778 0.001102 0.002400 0.000 0.004 2.2167 0.001102 0.002443 0.000 0.004 2.2556 0.001102 0.002485 0.000 0.004 2.2944 0.001102 0.002528 0.000 0.004 2.3333 0.001102 0.002571 0.000 0.004 2.3722 0.001102 0.002614 0.000 0.004 2.4111 0.001102 0.002657 0.000 0.004 2.4500 0.001102 0.002700 0.000 0.004 2.4889 0.001102 0.002743 0.000 0.004 2.5278 0.001102 0.002785 0.000 0.004 2.5667 0.001102 0.002828 0.000 0.004 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 86 2.6056 0.001102 0.002871 0.000 0.004 2.6444 0.001102 0.002914 0.000 0.004 2.6833 0.001102 0.002957 0.000 0.004 2.7222 0.001102 0.003000 0.000 0.004 2.7611 0.001102 0.003043 0.000 0.004 2.8000 0.001102 0.003085 0.000 0.004 2.8389 0.001102 0.003128 0.000 0.004 2.8778 0.001102 0.003171 0.000 0.004 2.9167 0.001102 0.003214 0.000 0.004 2.9556 0.001102 0.003257 0.000 0.005 2.9944 0.001102 0.003300 0.000 0.005 3.0333 0.001102 0.003343 0.129 0.005 3.0722 0.001102 0.003385 0.411 0.005 3.1111 0.001102 0.003428 0.784 0.005 3.1500 0.001102 0.003471 1.229 0.005 3.1889 0.001102 0.003514 1.733 0.005 3.2278 0.001102 0.003557 2.287 0.005 3.2667 0.001102 0.003600 2.883 0.005 3.3056 0.001102 0.003642 3.512 0.005 3.3444 0.001102 0.003685 4.168 0.005 3.3833 0.001102 0.003728 4.842 0.005 3.4222 0.001102 0.003771 5.525 0.005 3.4611 0.001102 0.003814 6.210 0.005 3.5000 0.001102 0.003857 6.887 0.005 3.5389 0.001102 0.003900 7.549 0.005 ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.1646 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0 Total Impervious Area:0.1646 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.012965 5 year 0.023515 10 year 0.032101 25 year 0.044736 50 year 0.055433 100 year 0.067224 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.055612 5 year 0.07326 10 year 0.085131 25 year 0.100385 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 87 50 year 0.111953 100 year 0.123709 ___________________________________________________________________ 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 Sand Filter 1 POC Y 23.51 25.83 0.00 N 0.00 21.12 81.75 Total Volume Infiltrated 23.51 25.83 0.00 0.00 21.12 21 / 26 = 82% Treat. Credit = 82% Compliance with LID Standard 8 Duration Analysis Result = Failed ___________________________________________________________________ CHANGES TO PERLND AND IMPLND PARAMETER VALUES. A Total of 0 changes have been made. PERLND NAME Parameter Name Original Value New Value No PERLND changes have been made.IMPLND NAME Parameter Name Original Value New Value IMPLND 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. 2005-2025; All Rights Reserved. 2422 TIR C & Y 2 Short Plat G:\2422\2422tir.doc\Page 88 Geotechnical Report: Attached Gary A. Flowers, PLLC Geological & Geotechnical Consulting 5205 23rd Avenue W Everett, WA 98203 Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 1 December 2, 2024 Project No. 24015 Mr. Thomas Chung 1625 180th Pl SW Lynnwood, WA 98037 Subject: Geotechnical Services Report 3607 NE 12th Street Renton, Washington, 98056 This report summarizes the results of our geological and infiltration evaluation of the property located at the above noted address in Renton, WA. The property was a portion of a larger parcel that was divided into 2 separate lots (Parcel A and Parcel B) via a boundary line adjustment. Parcel A has an address of 1157 Queen Ave NE and Parcel B has an address of 3607 NE 12th Street. The purpose of our site evaluation was to document existing shallow soil and ground water conditions on the property, provide geotechnical recommendations for site development, determine the feasibility of on-site storm water infiltration and provide a design infiltration rate for the encountered soils, if feasible. It is our understanding that the property will be subdivided into 3 separate single family residential building lots with access off of Queen Ave NE. Each lot will be 6,000 to 7,000 sf in area. EXISTING CONDITIONS The subject property (Parcel B, 3607 NE 12th Street) is rectangular shaped and measures about 210 feet in the north-south direction by 134 feet in the east-west direction. The property is bounded on the north by NE 12th Street, on the east by Queen Avenue NE, on the west by Pierce Avenue NE and on the south by undeveloped Parcel A and single family residences. The property is flat lying with a barely discernible downward slope from SE to NW. Overall relief is about 2 feet. The site is currently developed with an old, single story house, detached garage, several outbuildings and an asphalt driveway. The site is primarily vegetated with tall field grass, several large and small deciduous and evergreen trees and some shrubs around the house. At the time of our study there was no evidence of standing or flowing water on the parcel. Thomas Chung Property Geotechnical Services Report 3607 NE 12th Street, Renton, Washington Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 2 Subsurface Exploration In order to confirm subsurface conditions on the property a series of 3 exploration pits were excavated on October 13, 2021. Logs of the exploration pits are attached to this report. Generally, medium dense to very dense silty sand with gravel was observed on the site. This sediment is interpreted to be Vashon age lodgement till. Lodgement till is a nonhomogeneous mixture of sand and gravel in a silt/clay matrix. This material was deposited in front of the advancing ice sheet on the order of 13,500 years ago and subsequently overrun by the ice which is thought to have been in excess of 3,000 feet thick in this area. The result is a densely consolidated soil with high strength and low permeability characteristics. Ground water seepage was not observed in any of the exploration pits and no caving conditions were observed within the sidewalls of the exploration pits. Although we had no access to Parcel B at the time of our field study, exploration pit EP-3 was specifically placed adjacent to the boundary adjustment line in order to confirm that the lodgement till sediments continued to the north and onto the subject property (Parcel B). Mappings and Nearby Studies According to the Geologic Map of King County, Washington, by Booth, Troost and Wisher, March 2007, the subject site is mapped as Vashon age glacial till (Qvt). Our explorations are in agreement with the published geology map of the area. A study performed for a small housing development at 3716 NE 12 Street, about 200 feet to the east and across NE 12th Street from the subject site, excavated 6 exploration pits and confirmed the presence of glacial till soils and potential shallow perched water during the wetter season. Hydrology No indication of standing or flowing water was present on the subject parcel or adjacent properties at the time of our field work. There was no evidence of erosion anywhere on the site. No hydrophilic or water loving plants were observed on the property or adjacent properties. CONCLUSIONS AND RECOMMENDATIONS On the basis of our geologic research and field explorations, the site is underlain by dense, silty sand with gravel soils which have been glacially consolidated. Overlying the glacially consolidated sediments is a weathered zone that is on the order of 18 inches thick and about 12 inches of grass sod and topsoil. Although not directly encountered in our explorations it is likely that some amount of fill soil may be present around the existing structures on the site. Thomas Chung Property Geotechnical Services Report 3607 NE 12th Street, Renton, Washington Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 3 The weathered glacial till is suitable for foundation support unless disturbed during clearing operations. If disturbed it must be sufficiently re-compacted to a medium dense or better condition. Compaction may be difficult depending upon time of year and inclement weather conditions. If suitable compaction cannot be attained the contractor should plan on removing all disturbed soil and placing building footings on the dense, un-weathered portion of the soil or on compacted crushed rock placed atop the undisturbed dense soil. . The on-site sediments should be considered extremely moisture sensitive and if worked during inclement weather or when ground water is present, will require special handling and care. If these soils become over optimum in their moisture content, they will be easily disturbed and additional, unplanned excavation may be necessary to establish suitable bearing for foundations and driveways. There are no geologic hazards present on this property. Foundation Recommendations All footings should be extended to bear onto undisturbed, dense, silty sand with gravel sediments. The on-site sediments are generally in a loose condition down to an approximate depth of 12 inches and then medium dense to a depth of 2.5 feet and may be locally shallower or deeper. Depending upon planned site grading suitable foundation bearing soils may not be reached with a typical spread footing foundation excavation of 18 inches below grade. If loose or disturbed soil is exposed in the foundation excavation the excavation should either be extended to medium dense or better soils or suitably re-compacted to a medium dense or better condition. For footings that bear on undisturbed or recompacted, medium dense or better native sand/silt sediments, or on structural fill placed over undisturbed native sediments, we recommend an allowable soil bearing value of 2500 psf be used in the design of these footings, including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. Total settlement of footings placed as detailed herein should be less than ½ inch in a 20 foot span with differential settlement less than ¼ inch between comparably loaded footings. However, foundations placed on soil that is not at least medium dense may result in increased settlement. All foundation excavations should be inspected by a representative of this firm, prior to concrete placement, to verify that the design bearing capacity of the soils has been attained and that construction conforms to the recommendations contained in this report. Lateral loads can be resisted by friction between the foundations, slabs and the supporting soils, and/or by passive earth pressure acting on the buried portions of the foundations. We recommend a passive equivalent fluid pressure of 200 pcf for buried foundations, provided the backfill around the footings and foundation walls is compacted to a medium dense or better condition. A coefficient of friction of 0.40 between concrete foundations elements may also be used if needed. These values are allowable and include a factor of safety of at least 1.5. Thomas Chung Property Geotechnical Services Report 3607 NE 12th Street, Renton, Washington Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 4 Retaining Walls Building plans were not available at the time this report was prepared. If any walls are planned in the final site grading plans, the following general recommendations are provided. Any retaining or foundation walls taller than 3 feet must be lined with a minimum of 12 inches of washed rock to within 1 foot of finish grade or utilize an approved drainage mat such as Mira Drain 6000 or equivalent. The washed rock or drain mat must tie into the footing drain for the wall footing. Horizontally backfilled walls that are free to yield laterally at least 0.1 percent of their height may be designed using an active equivalent fluid pressure equal to 35 pcf (triangular distribution). Fully restrained, horizontally backfilled, rigid walls that cannot yield should be designed for an at rest equivalent fluid pressure of 55 pcf. Additional surcharges such as traffic, other structures, or heavy equipment must be added to these design values. In addition, a seismic surcharge of 8H (rectangular distribution) should be added to the design. The lateral pressures presented above are based on the conditions of a uniform horizontal backfill consisting of imported sand and gravel with less than 5 percent fines content, compacted to a firm and unyielding condition. Over compaction should be avoided as this will increase the pressure acting on the wall. Where driveways, stairways, sidewalks, etc. will be constructed above a retaining wall the compaction of the soils should be closely monitored to avoid either over or under compaction of the soil and only hand operated equipment should be used within 3 feet of the wall. Floor Support Recommendations Slab-on-grade floors should be supported by undisturbed or re-compacted native soils or structural fill soils that are fully compacted to a medium dense or better condition and be firm and unyielding. A capillary break layer consisting of 4 inches of washed pea gravel, clean crushed rock or coarse sand and a heavy duty (minimum 10 mil), polyethylene plastic vapor barrier should be provided under any floor slabs where moisture intrusion is a concern. If the vapor barrier becomes compromised in any way during construction, it should be replaced or an additional layer added. Penetrations through the vapor barrier should be wrapped and taped. Erosion Protection The site is relatively level and as such, standard erosion control best management practices will be suitable during construction. This includes a rocked construction entrance at least 20 feet long comprised of clean 2-4 inch crushed rock underlain by geotextile filter fabric, covering stockpiles with plastic sheeting during inclement weather, placing and maintaining silt fences along the lower reaches of the cleared areas. Thomas Chung Property Geotechnical Services Report 3607 NE 12th Street, Renton, Washington Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 5 Site Drainage All storm water runoff from impervious surfaces should be collected and piped into an approved storm water drainage system. Due to the presence of shallow glacial till soils it is our opinion that this site is unsuitable for any infiltration of storm water. In order to verify whether or not there was any infiltration potential within the observed sediments an infiltration test was performed on the site (on Parcel A). Testing was completed in general accordance with City of Renton 2017 Storm Water Design Manual and Reference 6-A for a small scale pilot infiltration test (PIT). This test procedure was not changed in the more current 2022 Storm Water Design Manual. The pit was excavated to a depth of 3.1 feet and the bottom measured 4.7 feet long and 4.1 feet wide for a total area of 19.27 square feet. Presoaking and testing was completed using a garden hose running into a 4-foot long, 4-inch diameter, perforated, pvc pipe with a cap for a splash plate on the bottom end. The pvc pipe was marked in half-inch increments as required. Water was then added to the pit for the pre-soak period and maintained at a depth of 12 to 14 inches for 6 hours. It should be noted that little to no water flow was required in order to maintain the water depth. In fact, at the lowest water flow setting attainable with the available equipment the water level continued to rise throughout the soak period. Following the pre-soak, the water flow was shut off to allow the water level to adjust to the required 6-inch head. After 2 hours the water level had only reduced by less than 1 inch. Due to the conditions detailed above, the constant head test was not performed. The falling head test was then performed for an hour. During the falling head test the water level reduced by approximately .5 inches in one hour even with a 12-inch head instead of the recommended 6-inch head. Based on the results of our explorations, and the other study explorations across NE 12th Street that encountered glacial till sediments at shallow depth it is apparent that the subject site is underlain by very dense, glacially consolidated sediments (glacial till) that forms an impermeable layer at shallow depth. As such there is no adequate separation prior to encountering the impermeable sediments and there is a high potential for shallow groundwater atop the glacial till sediments during the wet season. In addition, applying the measured infiltration rate for our previous infiltration test on Parcel A and the required correction factors for testing, geometry and plugging, it is our opinion that the design infiltration rate is nil, and no consideration should be given to infiltration of storm water on this property. This is typical for sites that are underlain by glacial till sediments. It is our understanding that the site is subject to Core Requirement #9 of the 2022 City of Renton Storm Water Design Manual. As such, multiple BMPs must be considered prior to allowing on site storm water drainage to be connected to the local street storm water discharge system via a perforated pipe system. Thomas Chung Property Geotechnical Services Report 3607 NE 12th Street, Renton, Washington Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 6 Neither full nor basic dispersion are considered feasible as the required design requirements likely cannot be met regarding flow path lengths. Neither full nor partial infiltration should be considered given the underlying impermeable sediments at shallow depth. Bioretention facilities likely cannot meet the required vertical separation distance from the underlying impermeable glacial till sediments. Permeable pavements should only be considered if the sites are raised with permeable fill soils. Currently the underlying impermeable layer would create saturated conditions within one foot of the bottom of the lowest gravel base course or sand layer. Additionally, although not tested for this study, glacial till soils typically do not meet the requirements for minimum organic matter content of 1.0% or the minimum cation exchange capacity of 5 milliequivalents per 100 grams. Based on the above recommendations, a perforated pipe connection to the City storm water conveyance system appears to be the most applicable for these sites. SUMMARY Our findings and recommendations provided in this report were prepared in accordance with generally accepted principles of engineering geology and geotechnical engineering as practiced in the Puget Sound area at the time this report was submitted. We make no other warranty, either express or implied. Respectfully submitted, Gary A. Flowers, P.G., P.E.G. Robert M. Pride, P.E., G.E. Principal Engineering Geologist Geotechnical Engineer Attachments: Exploration Pit Logs Site & Exploration Plan Thomas Chung Property Geotechnical Services Report 3607 NE 12th Street, Renton, Washington Gary A. Flowers, PLLC. 5205 23rd Avenue W Everett, Washington 98203 206-819-4304 7 EXPLORATION PIT LOGS 1157 Queen Avenue NE & 3607 NE 12th Street Renton, WA EP-1 Approximate elevation 432 feet. 0.0’ – 1.0’ Loose, moist, grayish-black, silty SAND (topsoil) and grass sod 1.0’ – 2.5’ Medium dense, light brown, silty sand with gravel (glacial till) [SM] 2.5’ – 4.0’ Very dense, light brown, silty sand with gravel (glacial till) [SM] Bottom of pit at 4.0 feet on 10/13/2021. No ground water. No caving. EP-2 Approximate elevation 432 feet. 0.0’ – 1.0’ Loose, moist, grayish-black, silty SAND (topsoil) and grass sod 1.0’ – 2.5’ Medium dense, light brown, silty sand with gravel (glacial till) [SM] 2.5’ – 4.0’ Very dense, light brown, silty sand with gravel (glacial till) [SM] Bottom of pit at 4.0 feet on 10/13/2021. No ground water. No caving. EP-3 Adjacent to Parcel B boundary adjustment line. Approximate elevation 431 feet. 0.0’ – 1.0’ Loose, moist, grayish-black, silty SAND (topsoil) and grass sod 1.0’ – 2.5’ Medium dense, light brown, silty sand with gravel (glacial till) [SM] 2.5’ – 4.0’ Very dense, light brown, silty sand with gravel (glacial till) [SM] Bottom of pit at 4.0 feet on 10/13/2021. No ground water. No caving. IP-1 Approximate elevation 430 feet 0.0’ – 1.0’ Loose, moist, grayish-black, silty SAND (topsoil) and grass sod 1.0’ – 2.5’ Medium dense, light brown, silty sand with gravel (glacial till) [SM] 2.5’ – 3.1’ Very dense, light brown, silty sand with gravel (glacial till) [SM] Bottom of pit at 4.0 feet on 10/13/2021. No ground water. No caving. SHEET OF C2MY ENGINEERS, LLC 1 CHUNG SHORT PLAT 3607 NE 12TH STREET RENTON, WA 98056 C0.0 1