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2380-CI-COMM-TIR Comments-2018-03-30.pdf
March 30, 2018 Marion Nielson Lanktree Land Surveying Inc. 421 B St NE Auburn, WA 98002 SUBJECT: "On Hold" Notice Cho Short Plat / LUA18-000177, SHPL-A Dear Ms. Nielson: The Planning Division of the City of Renton accepted the above master application for review on March 13, 2018. During our review, staff has determined that additional information is necessary in order to proceed further. The following four (4) Technical Information Report (TIR) items will need to be addressed before June 29, 2018 so that we may continue the review of the above subject application: 1. The sizing calculations for the detention facility only includes about twenty five percent (25%) of the total area that should be included, as the existing pervious surface area is incorrectly excluded from the calculations. 2. Raingardens are proposed as an onsite BMPs for the roof run-off. The sizing calculations for the detention facility takes ten percent (10%) credit that is specified for bioretention areas. Raingardens and bioretention areas are fundamentally different. Raingardens do not receive sizing credits for the sizing of detention facilities. Either remove the credits from the sizing calculations or bioretention will need to be utilized in place of the proposed raingardens and sized as outlined in Appendix C of the 2017 Renton Surface Water Design Manual. 3. The Engineer who prepared the TIR is claiming exemption from water quality treatment since the pollution generating impervious surface (PGIS) is less than 5,000 square feet (SF). However, this total does not include the roadway widening PGIS. The total PGIS for the project is approximately 7,749 square feet (SF), which exceeds the maximum criteria to qualify for the water quality treatment exemption. Either reduce the PGIS to less than 5,000 SF or provide water quality treatment as outlined in the 2017 Renton Surface Water Design Manual. 4. The TIR does not reference Special Requirement #6, which is applicable to the project. At this time, your project has been placed “on hold” pending receipt of the requested information. Please contact me at (425) 430-7289 or cclose@rentonwa.gov should you have any questions. Sincerely, Clark H. Close Senior Planner cc: Woong Hee Cho and Hee Yong Cho / Owners Grant / Part(ies) of Record T ECHNICAL I NFORMATION R EPORT C HO S HORT P LAT 11840 SE 192ND S TREET R ENTON, W ASHINGTON 98058 The Concept Group 4701 SW Admiral Way, Ste 353 ● Seattle, WA 98116 ● (206) 446-1291 Stormwater Technical Information Report Cho Short Plat 11840 SE 192nd Street RENTON, WASHINGTON 98058 REVISIONS January 18, 2018 Original Submission Developer/Owner Contact: Woong Hee Cho Address: 3037 67th Ave SE; Mercer Island, WA 98040 Phone: 206-697-1332 Professional Preparing TIR Name: Anna Nguyen, P.E. Company: The Concept Group Address: 4701 SW Admiral Way, Ste 353; Seattle, WA 98116 Phone: (206) 446-1291 Email: anna@conceptbusinessgroup.com I certify that this technical information report and all attachments were prepared either by me or my technical staff working directly under my supervision. Table of Contents SECTION 1: PROJECT OVERVIEW .......................................................................................................................................... 1 SITE INFORMATION .......................................................................................................................................................................... 1 SOIL DATA ..................................................................................................................................................................................... 1 EXISTING CONDITIONS...................................................................................................................................................................... 2 PROPOSED CONDITIONS ................................................................................................................................................................... 2 SECTION 2: CONDITIONS AND REQUIREMENTS .................................................................................................................... 3 DESIGN STANDARDS ........................................................................................................................................................................ 3 CORE REQUIREMENT #1: DISCHARGE LOCATION AT NATURAL LOCATION ................................................................................................... 3 CORE REQUIREMENT #2: OFFSITE ANALYSIS ......................................................................................................................................... 3 CORE REQUIREMENT #3: FLOW CONTROL ........................................................................................................................................... 5 CORE REQUIREMENT #4: CONVEYANCE SYSTEM ................................................................................................................................... 5 CORE REQUIREMENT #5: EROSION AND SEDIMENT CONTROL .................................................................................................................. 6 CORE REQUIREMENT #6: MAINTENANCE AND OPERATIONS .................................................................................................................... 7 CORE REQUIREMENT #7: FINANCIAL GUARANTEES AND LIABILITY ............................................................................................................. 7 CORE REQUIREMENT #8: WATER QUALITY .......................................................................................................................................... 7 CORE REQUIREMENT #9: FLOW CONTROL BMP ................................................................................................................................... 8 SPECIAL REQUIREMENT #1 – OTHER ADOPTED REQUIREMENTS ............................................................................................................... 8 SPECIAL REQUIREMENT #2 – FLOOD HAZARD AREA DELINEATION ............................................................................................................ 8 SPECIAL REQUIREMENT #3 – FLOOD PROTECTION FACILITIES ................................................................................................................... 9 SPECIAL REQUIREMENT #4 – SOURCE CONTROL.................................................................................................................................... 9 SPECIAL REQUIREMENT #5 – OIL CONTROL .......................................................................................................................................... 9 Appendices Appendix A: Soil Data Appendix B: Proposed Drainage Plan & Drainage Calculations Appendix C: Erosion Control Plan & Details Appendix D: Maintenance Procedures Cho Short Plat January 2018 The Concept Group Page 1 SECTION 1: PROJECT OVERVIEW The property is located at 11840 SE 192nd Street in the City of Renton, Washington. There are currently three structures on the subject property. The project proposes to develop the site with a mixed-use building, with the existing structures to be demolished. Site Information Location: 11840 SE 192nd Street Size: 44,296 sq. ft. City, County, State: Renton, King County, Washington Governing Agency: City of Renton Design Criteria: 2017 City of Renton Surface Water Design Manual 2016 King County Surface Water Manual Figure 1 – Vicinity Map / Site Location (Not-to-Scale) Soil Data The Soils Conservation Service (SCS) mapped the soils information in the project as predominately AmB, Alderwood gravelly sandy loam (0% - 8%). This type of soil is moderately well drained. An Infiltration Assessment was prepared by Cascade GeotechNW, dated October 2, 2017. The report states that site “soils encountered in the test pits appear to be consistent with this SCS [AgB] soil type.” Measured infiltration rates ranged between 0.45 – 0.50 inches/hour. Refer to Appendix A for the Preliminary Geotechnical Report. Cho Short Plat January 2018 The Concept Group Page 2 Figure 2 – Soils Map (Not-to-Scale) Existing Conditions The site is currently developed with one (1) single-family residential structure and two (2) detached sheds. Vegetation consists of lawn, landscaping and mature evergreen and deciduous trees. Existing impervious surface coverage is detailed in Table 1 below. Existing impervious to remain. Table 1: Existing Impervious Surfaces (SF) SFR House 1,460 Shed 252 Gravel Driveway 2,960 Total 4,672 Proposed Conditions The applicant is proposing to develop the site into a total of three (3) parcels. Projected project buildout of the lot area (impervious surface) is detailed in Table 2 below. Table 2: Proposed New Impervious Surfaces (SF) Roofs (2,500 SF per lot) 7,500 Driveways 1,300 Frontage Improvement (Sidewalk) 1,260 Frontage Improvement (Roadway Widening) 2,789 Private Road 3,663 Total New Impervious Surface 16,512 Project Location Cho Short Plat January 2018 The Concept Group Page 3 SECTION 2: CONDITIONS AND REQUIREMENTS Design Standards The 2017 City of Renton Surface Water Design Manual, as adopted from the 2016 King County Surface Water Design Manual sets forth the drainage requirements for this project. The project is subject to full drainage review. The nine (9) Core Requirements and five (5) Special Requirements of the SWM drainage requirements are detailed in this section. Table 3 provides a summary to the requirements. Core Requirement #1: Discharge Location at Natural Location Drainage Basin The property lies within the Soos Creek drainage basin. The general topography of the site slopes from north to south. Elevations on the site vary from a high point of 503 feet at the northern property line to 297 feet at the southern property line. Discharge at the Natural Location Existing drainage patterns and a topographic data available on King County’s GIS iMap website indicate that the runoff from the developed areas sheet flows south towards SE 192nd Street. In the developed condition, stormwater will be routed to an on-site detention BMP with overflows discharging to the public storm drain system along SE 192nd Street, maintaining the natural location of discharge. Figure 3: Drainage Study Area Map (Not-to-Scale) Core Requirement #2: Offsite Analysis Level 1 Downstream Analysis A Level 1 downstream analysis was performed on November 9, 2017. Stormwater currently sheet flows south towards SE 192nd Street. Stormwater enters the 12” public storm drain system located within SE 192nd Street continues to flow east, in the closed- Table 3 - Jurisdictional Requirements Peak Run-off Control: Flow Control Duration Standard Water Quality: N/A – based on surface area exemption Conveyance: 25-year storm event Downstream Analysis: Level 1, ¼ mile Project Location Discharge Location Cho Short Plat January 2018 The Concept Group Page 4 pipe system along SE 192nd Street for approximately 1,950 feet and discharging into Big Soos Creek, beyond ¼ mile from the site. Drainage System Description and Problem Descriptions A resource review was conducted to document existing and potential flooding and erosion problems. Based on this field inspection there appears to be no current significant erosion or capacity problems within a ¼ mile downstream of the property. Other Offsite Reports: Per the stormwater map on King County’s iMap, there are no significant existing drainage or flooding problems within a ¼ mile downstream within the past 10 years. Wetlands Inventory: The project site is not located in or near a mapped wetland per the King County sensitive areas inventory maps. Hazard Area and Landslide Area Maps: The project site is not located in or near a hazard or landside area per the King County sensitive areas inventory maps. Label Type Date Closed #1 DRNG 4/4/1996 #2 DRNG 12/18/1991 #3 DRNG 9/3/1997 #4 DRNG 1/28/1999 #5 DDM not directly downstream of property #6 DRNG 9/26/1996 #7 WQ NON-CAPACITY ISSUE contractor dumped waste material into wetland Mitigation of Existing or Potential Problems No existing or potential flooding, capacity, or erosion problems were observed during the site visit requiring mitigation. Based on this field inspection and research of King County records; there are no apparent erosion or capacity problems within ¼ mile downstream of this project. Project Location 1 2 3 4 5 6 7 Cho Short Plat January 2018 The Concept Group Page 5 Core Requirement #3: Flow Control Per the 2017 City of Renton Surface Water Design Manual flow maps, this project is located within the Flow Control Duration Standard. Western Washington Hydrology Model The Western Washington Hydrology Model (WWHM) was used to analyze the pre and post developed runoff rates. Table 4 outlines the assumed existing conditions as 100% forested condition. Table 4 – Existing Conditions (ac) Forest Grass Roof Driveway/Patio Road 0.286 0.00 0.00 0.00 0.00 There are no new pervious surfaces proposed within of the project. New pervious surface is defined as “…the conversion of native pervious surface to non-native pervious surface…” The disturbed area of the site within the clearing limits does not contain native pervious surface. Table 5 outlines the assumed developed conditions, taking into account flow control sizing credits for proposed LID BMP’s (bioretention and permeable pavement). Table 5 – Developed Conditions Target Surfaces (ac) Grass Roof Driveway/Parking Sidewalk Road 0.074 0.155 0.015 0.00 0.042 A 5 FT diameter x 275 LF CMP pipe detention pipe is proposed to mitigate the on-site stormwater from the development. Refer to Appendix B for the WWHM report showing the detention facility passing the duration control requirements. Core Requirement #4: Conveyance System The Rational Method was used to calculate the 25-year and 100-year storm events. Manning’s Equation was used to calculate capacity of the proposed conveyance system. As shown in the Table 4, the capacity of this conveyance system exceeds the 100-year event and 100-year discharge from the detention vault. Refer to the Appendix B for detailed the conveyance calculations. Table 5: Conveyance Analysis Pipe ID Area (sf) C value Q25 Q100 Pipe Pipe Capacity (cfs) Pipe #17 (Lot 3) 2,500 0.90 0.16 0.19 6” @ 1.0% 0.61 Pipe #10 (Lot 1, 3, access driveway) 7,789 0.90 0.51 0.59 8” @ 0.5% 0.92 Pipe #6 (Lot 1, 2, 3, access driveway) 10,289 0.90 0.68 0.79 8” @ 0.5% 0.92 Pipe #12 - Discharge from detention 0.019 6” @ 1.0% 0.61 Cho Short Plat January 2018 The Concept Group Page 6 Figure 4: Conveyance Diagram (Not-to-Scale) Core Requirement #5: Erosion and Sediment Control All erosion and sediment control measures shall be governed by the requirements of the City of Renton and the King County Surface Water Manual. An Erosion and Sedimentation Control plan has been prepared to assist the contractor in complying with these requirements and designed to prevent sediment-laden run-off from leaving the site during construction. Refer to Appendix C for Erosion Control plans and details. The erosion potential of the site is influenced by four major factors: soil characteristics, vegetative cover, topography, and climate. Erosion/ sedimentation control is achieved by a combination of structural measures, cover measures, and construction practices that are tailored to fit the specific site. Construction Sequence and Procedure Prior to the start of any grading activity upon the site, all erosion control measures, including installation of a stabilized construction entrance, shall be installed in accordance with the construction documents. The best construction practice will be employed to properly clear and grade the site and to schedule construction activities. The planned construction sequence for the construction of the site is as follows: 1. Flag or fence clearing limits. 2. Install catch basin protection if required. 3. Grade and install construction entrance(s). 4. Install perimeter protection (silt fence, brush barrier, etc.). 5. Construct sediment ponds and traps. Pipe #10: 8” @ 0.5% Pipe #17: 6” @ 1.0% Pipe #6: 8” @ 0.50% Cho Short Plat January 2018 The Concept Group Page 7 6. Grade and stabilize construction roads. 7. Construct surface water controls (interceptor dikes, pipe slope drains, etc.) simultaneously with clearing and grading for project development. 8. Maintain erosion control measures in accordance with City of Renton and King County standards and manufacturer’s recommendations. 9. Relocate erosion control measures or install new measures so that as site conditions change the erosion and sediment control is always in accordance with the King County Erosion and Sediment Control Standards. 10. Cover all areas that will be unworked for more than seven days during the dry season or two days during the wet season with straw, wood fiber mulch, compost, plastic sheeting or equivalent. 11. Stabilize all areas that reach final grade within seven days. 12. Seed or sod any areas to remain unworked for more than 30 days. 13. Upon completion of the project, all disturbed areas must be stabilized and BMPs removed if appropriate. Trapping Sediment Structural control measures will be used to reduce erosion and retain sediment on the construction site. The control measures will be selected to fit specific site and seasonal conditions. The following items will be used to control erosion and sedimentation processes: Temporary gravel construction entrance Filter fabric fences (Silt fences) Ground cover measures such as straw cover and/or hydroseeding Inlet protection Vehicle tracking of mud off-site shall be avoided. Installation of a gravel construction entrance will be installed at a location to enter the site. The entrances are a minimum requirement and may be supplemented if tracking of mud onto public streets becomes excessive. Core Requirement #6: Maintenance and Operations Maintenance and operations of all drainage facilities located within the private residential properties is the responsibility of the applicant or property owner in accordance with the requirements as attached in Appendix D. Proper maintenance is important for adequate functioning of the stormwater facilities. If it is unclear whether a problem exists, contact a Professional Engineer. Core Requirement #7: Financial Guarantees and Liability Proposed BMP’s are privately owned and maintained. The project does not require financial guarantees. Core Requirement #8: Water Quality This project is exempt from Water Quality requirements as defined by Core Requirement #8 based on the surface area exemption because only 4,963 square feet is pollution- generating impervious surface (PGIS). PGIS is defined as “an impervious surface considered to Cho Short Plat January 2018 The Concept Group Page 8 be a significant source of pollutants in surface and storm water runoff. Such surfaces include those subject to vehicular use or storage of erodible or leachable materials, wastes, or chemicals, and which receive direct rainfall or the run-on or blow-in of rainfall.” Non-metal rooftops are not considered PGIS; therefore, are not included in the PGIS area calculations. Water quality is not required for this project because the project does not create more than 5,000 sq ft of pollution generating impervious surfaces. Core Requirement #9: Flow Control BMP The project utilizes Better Site Design by generally matching existing grades where feasible. Existing trees and other vegetation around the site perimeter will be preserved where possible to help minimize disturbance to the hydrologic cycle. Native soils in all disturbed pervious areas will be amended with compost. Soil Management Plan Within the limits of site disturbance, duff and topsoil will be retained in an undisturbed state and stockpiled for later use to stabilize and amend soils throughout the Site. Postconstruction soil amendment will meet the requirements of KCSWDM Appendix C, Section C.2.13. BMP Feasibility and Applicability Discussion Per the 2017 City of Renton SWDM, Section 1.2.9.2, Small Lot BMP is required for this project. Full dispersion, per Appendix C, Section C.2.1, of runoff from impervious surfaces is not feasible due to insufficient area on the site for dispersion flow paths. The site does not contain native vegetation; therefore, a native vegetative flow path of 100 ft is not feasible within the project limit. Full infiltration, per Appendix C, Section C.2.2, is not feasible. Infiltration tests were conducted; yielding measured low infiltration rates of 0.45 inches/hour. Limited Infiltration. Infiltration is not feasible for this site. Infiltration tests were conducted; yielding measured low infiltration rates of 0.45 inches/hour. Rain Gardens, per Appendix C, Section C.2.12, is feasible because the field testing indicates native soil saturated hydraulic conductivity of at least than 0.30 inches per hour. No other BMP is required to be evaluated. Roof stormwater will be mitigated with bioretention rain gardens. Raingardens were sized per the 2016 KCSWDM C.2.6. Refer to Appendix B for the calculations. Special Requirement #1 – Other Adopted Requirements Based on available data at the time this report was prepared, there are no one Special Adopted Requirements for this project. Special Requirement #2 – Flood Hazard Area Delineation Based on FEMA Flood Map # FM53033C0991F and King County iMap Districts and Development Conditions report, the project area is located in Zone X, a 500-year flood plain. Cho Short Plat January 2018 The Concept Group Page 9 Special Requirement #3 – Flood Protection Facilities This project is exempt from Special Requirement #3 because it does not rely on an existing flood protection facility or propose to modify or construct a new flood protection facility. Special Requirement #4 – Source Control This project does not exceed the water quality requirement threshold; therefore, it is exempt from Special Requirement #4. Special Requirement #5 – Oil Control This project not defined as a high-use site per Chapter 1 of the 2017 City of Renton Surface Water Design Manual; therefore, it is exempt from Special Requirement #5 Cho Short Plat –Technical Information Report The Concept Group Appendices Appendix A Soils Information Page 1 Cascade GeotechNW 4957 Lakemont Blvd SE, C-4, #325 Bellevue, WA 98006 cascadegeonw@gmail.com 206-491-0081 October 2, 2017 File No. 2017-15 Mr. Willy Cho 3037 - 67th Avenue SE Mercer Island, WA 98040 Subject: Infiltration Assessment Proposed Development 11840 SE 192nd Street, Renton, WA King County Parcel # 6198400341 Dear Mr. Cho, As requested, Cascade GeotechNW co nducted field exploration and field infiltration testing to assist you and your project team with the proposed development at the above- referenced site. This study was performed in general accordance with our mutually agreed scope of work outlined in our proposal dated September 12, 2017, which was subsequently approved by you on September 13, 2017. The following sections present our understanding of the projects, the scope of services, document the subsurface conditions encountered at the test locations, the field infiltration testing procedure, the measured field infiltration rates, and the recommended long-term (design) infiltration rate. SITE DESCRIPTION AND PROJECT UNDERSTANDING The subject site is an approximately 1.02-acre property located at 11840 SE 192nd Street in Renton, Washington (see Vicinity Map, Figure 1). The site is bounded by SE 192nd Street to the south, a vacant lot to the east, and existing single-family residences to the west and north (see Figure 2). The site is currently occupied by a one-story house in the southern portion of the site. The site is practically flat. Based on information provided to us, we understand that you plan to remove the existing house, and to subdivide the subject property into three single-family residential parcels Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 2 PanGEO, Inc. and to construct one single-family residence on each subdivided parcel. We further understand that a geotechnical report is required by the City of Renton to evaluate the soil and groundwater conditions at the site and the feasibility of infiltration characteristics of the site soils, and to provide geotechnical recommendation related to the flow control and BMP design. The conclusions and recommendations outlined in this report are based on our understanding of the proposed development, which is in turn based on the project information provided. If the above project description is incorrect, or the project information changes, we should be consulted to review the recommendations contained in this study and make modifications, if needed. PROJECT SCOPE The purpose of our work is to characterize subsurface conditions at the project site, and to conduct field infiltration testing and provide infiltration rate to support the stormwater drainage design. The scope of our work for this project included the following tasks and work efforts: 1. Document Review – Review readily available geologic and geotechnical data for the project area. 2. Site Reconnaissance – Conduct a site reconnaissance to observe the existing site conditions, and to identify site conditions that may impact the proposed short plat development from a geotechnical standpoint. 3. Infiltration Testing – Excavate four (4) test pits and conduct four small scale pilot infiltration tests (PIT) at the site at about 5 feet deep. The infiltration testing was performed in general accordance with the procedures outlined in the 2017 Renton Surface Water Design Manual and 2014 WSDOE Srormwater Management Manual For western Washington . 4. Report – Preparation of a geotechnical report summarizing our work on the project and presenting our findings and opinions. Please note that our current geotechnical evaluation is limited to evaluation of infiltration feasibility of the site soils, and is not intended for providing geotechnical Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 3 PanGEO, Inc. recommendation for foundation design of the future houses. It should also be noted that our scope of work does not include an evaluation of chemical properties of soil and groundwater. SITE GEOLOGY According to the geology maps of the area (The Geologic Map of King County, Washington: scale 1:100,000, Booth, D. B., Troost, K. A., and Wisher, A. P., 2007), the project area is underlain by Vashon glacial till (Map Unit Qvt). Glacial till (Qvt) is a very dense heterogeneous mixture of silt, sand, and gravel laid down at the base of an advancing glacial ice sheet. SUBSURFACE EXPLORATIONS AND CONDITIONS Four test pits (TP-1 through TP-4) were excavated at the site on September 23 and 24, 2017, to explore the general subsurface conditions at the site and feasibility of infiltration for stormwarer design. The approximate test pit locations were taped from existing site features and are indicated on Figure 2. The test pits were excavated to depths of about 5 feet for infiltration testing and to about 7 feet to evaluate the conditions below the test depth. The tests were excavated using a backhoe owned and operated by G & R Excavating LLC of Hobart, Washington. An engineer from Cascade GeotechNW was present during the field exploration to observe the test pit excavations, obtain representative samples, and to describe and document the soils encountered in the explorations. Summary test pit logs are included in Appendix A of this report. The relative in-situ density of cohesionless soils, or the relative consistency of fine-grained soils was estimated from the excavating action of the excavator, and the stability of the test pit sidewalls. The stratigraphic contacts indicated on the test pit logs represent the approximate depth to boundaries between soil units. Where soil contacts were gradual or undulating, the average depth of the contact was recorded in the logs. Actual transitions between soil units may be more gradual or occur at different elevations. After PIT tests are completed and test pits were logged, the excavation was backfilled with the excavated soils and the surface was tamped and re- graded smooth. Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 4 PanGEO, Inc. In general, the test pits generally encountered 1 to 2 feet of topsoil consisting of brown to dark brown, loose, damp, silty sand. Below the topsoil, the test pits generally encountered light gray, loose to medium dense, moist, slightly silty to silty sand with trace to some gravel that extended to the bottom of the test pits at about 7 feet below the surface. No groundwater was observed in the test pits at the time of excavation. Detailed descriptions of the subsurface conditions encountered at the test pit location are described in the Test Pit Logs in Appendix A of this report. According to Soil Survey map, King County Area, Washington (USDA, 1973), the site is underlain by one soil type; AgB (Alderwood gravelly sandy loam). Our observations of the soils encountered in the test pits appear to be consistent with this SCS soil type. This soil type is described as to be moderately well drained for natural drainage class and relatively low permeability. IN-SITU INFILTRATION TESTING We conducted four in-situ infiltration tests at the site (PIT-1 through PIT-4). The approximate locations of the infiltration tests were specified by the project civil engineer and are depicted on Figure 2. The infiltration tests were conducted in general accordance with the procedure for small-scale pilot infiltration test (PIT) as outlined in the Department of Ecology’s Stormwater Management Manual for Western Washington (SMMWW) (DOE, 2012 as amended in 2014). The infiltration test procedure consisted of the following: • The test pits/small PIT locations were excavated using a 24- and 36-inch buckets to the approximate design bottom of the proposed infiltration facility. The bottom pit area was about 3 feet by 4 feet or 3.5 feet by 4 feet or base areas of 12 to 14 square feet. • The pit was then pre-soaked by maintaining at least 12 inches of water in the bottom of the small PIT for six hours. The water supply was sourced from a garden hose bib at the existing residence. • At the end of pre-soak period, a one hour steady state test was performed, using a flow meter to measure the volume of water needed to maintain a constant head of 12 inches. Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 5 PanGEO, Inc. • After completing the steady state test, the water supply was turned off and a falling head test was conducted by recording the drop the water level over regular time intervals. The infiltration rate was then calculated based on the results of the steady state testing and the bottom area of the small PITs. IN-SITU INFILTRATION TESTING RESULTS After the pre-soak period, a constant head of 12 inches was maintained at each test location. Based on the test surface area and the measured volume of water per hour needed to maintain the constant head, we calculated a field infiltration rate for each test location. The results are summarized in Table 1 below. Table 1 – Summary of Field Infiltration Test Data (No factor of safety included) Test Location Constant Head Field Infiltration Rate (inches/hour) PIT-1 0.50 PIT-2 0.45 PIT-3 0.45 PIT-4 0.50 Plate 1. Testing at PIT-1 Plate 1. Testing at PIT-2 Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 6 PanGEO, Inc. After completing the infiltration test, the pit was excavated to a depth of 7 feet below the test elevation to evaluate the presence of impermeable soils or groundwater mounding that may impact the long-term performance of the infiltration system. We observed no perched groundwater or seepage throughout the depth of excavation. For design purposes, the field infiltration rates must be reduced through correction factors, as discussed in the following section. DESIGN INFILTRATION RATE AND RECOMMENDATIONS The field test provides a short-term infiltration rate. To provide a long-term design infiltration rate, field rate is adjusted by applying a series of correction factors to account for site variability, the number of locations tested and the degree of influent control to prevent siltation and bio-buildup. Table 2 below, sourced from Table 3.3.1 of the DOE manual, outlines the correction factors to be applied to the field infiltration rate in order to estimate the long-term design infiltration rate for proposed infiltration facility. Table 2 - Correction Factors to be used with In-Situ Saturated Hydraulic Conductivity Measurements to Estimate Design Rates (source: Table 3.3.1, Vol. 3, DOE Stormwater Manual) Issue Partial Correction Factor Site variability and number of locations tested CFv = 0.33 to 1.0 Test Method: Large-Scale PIT Small-Scale PIT Grain Size Method CFt = 0.75 CFt = 0.50 CFt = 0.40 Degree of influent control to prevent siltation and bio- buildup CFm = 0.9 Total Correction Factor (CFT) = CFv * CFt * CFm The partial correction factor for site variability (CFv) is selected based on the number of locations tested and the consistency of the underlying soil conditions and ranges from 0.33 to 1.0. Based on our experience and engineering judgment, we recommend a correction factor of 0.9 for site variability. From the Table 2 above, the partial correction factor for CFm is 0.9 and the partial correction factor for CFt is 0.5 when using the small- Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 7 PanGEO, Inc. scale PIT method. Using the partial correction factors described above, in our opinion a Total Correction Factor (CFT) of 0.405 is appropriate. The total correction factor (CFT = CFv x CFt x CFm) is then applied to the field rate to estimate to obtain a long-term design infiltration rate. The estimated long-term infiltration rates for infiltration facility are provided in Table 3 below. Table 3: Estimated Long-Term Infiltration Rates for Infiltration Facility Test Location, Depth Correction Factor (CFv x CFt x CFm)* Long-Term Infiltration Rate (inches/hour) PIT-1, 5’ 0.405 0.202 PIT-2, 5’ 0.405 0.182 PIT-3, 5’ 0.405 0.182 PIT-4, 5’ 0.405 0.202 *CFv = 0.9, CFt = 0.5, CFm = 0.9 Conclusions – In summary, based on the results of our field exploration, infiltration tests, and observed soil texture, in our opinion, roof water disposal using the infiltration method is likely infeasible. However, it is our opinion that limited infiltration may be used for permeable pavement design and other on-site BMPs, as appropriate. LIMITATIONS This report has been prepared for the exclusive use of Mr. Willy Cho and the project team for specific application to the proposed development. This report is intended to provide infiltration feasibility and recommendations based on a site reconnaissance, field infiltration testing, and our understanding of the project. The study was performed using a mutually agreed-upon scope of work. Variations in soil conditions may exist between the locations of the explorations and the actual conditions underlying the site. The nature and extent of soil variations may not be evident until construction occurs. If any soil conditions are encountered at the site that are different from those described in this report, we should be notified immediately to Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 8 PanGEO, Inc. review the applicability of our recommendations. Additionally, we should also be notified to review the applicability of our recommendations if there are any changes in the project scope. The scope of our work does not include services related to construction safety precautions. Our recommendations are not intended to direct the contractors’ methods, techniques, sequences or procedures, except as specifically described in our report for consideration in design. Additionally, the scope of our work specifically excludes the assessment of environmental characteristics, particularly those involving hazardous substances. We are not mold consultants nor are our recommendations to be interpreted as being preventative of mold development. A mold specialist should be consulted for all mold-related issues. This report has been prepared for planning and design purposes for specific application to the proposed project in accordance with the generally accepted standards of local practice at the time this report was written. No warranty, express or implied, is made. This report may be used only by the client and for the purposes stated, within a reasonable time from its issuance. Land use, site conditions (both off and on-site), or other factors including advances in our understanding of applied science, may change over time and could materially affect our findings. Therefore, this report should not be relied upon after 24 months from its issuance. Cascade Group should be notified if the project is delayed by more than 24 months from the date of this report so that we may review the applicability of our conclusions considering the time lapse. It is the client’s responsibility to see that all parties to this project, including the designer, contractor, subcontractors, etc., are made aware of this report in its entirety. The use of information contained in this report for bidding purposes should be done at the contractor’s option and risk. Any party other than the client who wishes to use this report shall notify Cascade GeotechNW of such intended use and for permission to copy this report. Based on the intended use of the report, Cascade GeotechNW may require that additional work be performed and that an updated report be reissued. Noncompliance with any of these requirements will release Cascade Group from any liability resulting from the use this report. Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 9 PanGEO, Inc. CLOSURE We appreciate the opportunity to be of service and trust that the information outlined in this letter meets your need at this time. Please call if you have any questions. Sincerely, 10/2/2017 H. Michael Xue, P.E. Principal Geotechnical Engineer Enclosures: Figure 1 Vicinity Map Figure 2 Site and Exploration Plan Appendix A Summary Test Pit Logs A-1 thr. A-4 Logs of Test Pits TP-1 through TP-4 Infiltration Evaluation Proposed Development – 11840 SE 192nd Street, Renton, WA October 2, 2017 2017-15 11840 SE 192nd St Infiltration Rpt Page 10 PanGEO, Inc. REFERENCES Booth, D. B., Troost, K. A., and Wisher, A. P., 2007, The Geologic Map of King County, Washington: scale 1:100,000. City of Renton, 2017, Surface Water Design Manual. King County Department of Natural Resources and Parks, 2016, Surface Water Design Manual. WSDOE, 2014, Stormwater Management Manual for Western Washington. Figure No.Project No.2017-15 Infiltration Evaluation 11840 SE 192nd Street Renton, Washington 1file.grf w/ file.dat 9/18/17 (08:59) SDDBase Map: Google Maps VICINITY MAP SITE Cascade GeotechNW Not to Scale Infiltration Evaluation11840 SE 192nd StreetRenton, WashingtonSITE AND EXPLORATIONS PLAN2017-152Original Contours.grf 10/2/17 (09:19) AAEProject No. Figure No.Note:Basemap modified from Site Plan prepared by Lanktree Land Surveying, Inc.Approx. Test Pit/PIT LocationLegend:Appro. Scale:1" = 30'TP-3/PIT 3TP-4/PIT 4TP-1/PIT 1TP-1Cascade GeotechNWTP-2/PIT 2500500498498499497499497 APPENDIX A SUMMARY TEST PIT LOGS 2017-15 11840 SE 192nd St Infiltration Rpt Page A-1 Cascade GeotechNW Test Pit TP-1/PIT-1 Approximate ground surface elevation: N/A Ground Surface Conditions: Thick Blackberry Bushes Depth (ft) Material Description 0 – 1.5 Loose, brown-dark brown, silty SAND (SM), some roots, damp (Topsoil) 1.5 – 7 Loose to medium dense, light gray, silty SAND (SM), trace to some gravel, moist Test Pit terminated approximately 7 feet below ground surface. No caving and groundwater/seepage observed in the test pit. 2017-15 11840 SE 192nd St Infiltration Rpt Page A-2 Cascade GeotechNW Test Pit TP-2/PIT-2 Approximate ground surface elevation: N/A Ground Surface Conditions: Thick Blackberry Bushes Depth (ft) Material Description 0 – 2 Loose, brown to dark brown, silty SAND (SM), trace roots, damp to moist (Topsoil) 2 – 7 Loose to medium dense, light gray-brown, silty SAND (SM), trace gravel, moist Test Pit terminated approximately 7 feet below ground surface. No caving and groundwater/seepage observed in the test pit. 2017-15 11840 SE 192nd St Infiltration Rpt Page A-3 Cascade GeotechNW Test Pit TP-3/PIT-3 Approximate ground surface elevation: N/A Ground Surface Conditions: Short grass Depth (ft) Material Description 0 – 1.5 Loose, brown-dark brown, silty SAND (SM), trace roots and gravel, damp to moist (Topsoil) 1.5 – 7 Loose to medium dense, light gray-brown, silty SAND (SM), trace gravel, moist Test Pit terminated approximately 7 feet below ground surface. No caving and groundwater/seepage observed in the test pit. 2017-15 11840 SE 192nd St Infiltration Rpt Page A-4 Cascade GeotechNW Test Pit TP-4/PIT-4 Approximate ground surface elevation: N/A Ground Surface Conditions: Gravel driveway Depth (ft) Material Description 0 – 2 Loose, brown-dark brown, silty SAND (SM), trace roots and gravel, damp to moist (Topsoil) 2 – 7 Loose to medium dense, light gray-brown, silty SAND (SM), trace to some gravel, moist Test Pit terminated approximately 7 feet below ground surface. No caving and groundwater/seepage observed in the test pit. Date Test Pits Excavated: September 23, 2017 Test Pits Logged by: HX N ( blows/ft ) Approximate Consistency N (blows/ft ) Approximate Relative Density (%) Undrained Shear Strength (psf) 0 to 4 0 - 15 Very Soft 0 to 2 < 250 5 to 10 16 - 35 Soft 3 to 4 250 - 500 11 to 30 36 - 65 Medium Stiff 5 to 8 501 - 1000 31 to 50 66 - 85 Stiff 9 to 15 1001 - 2000 over 50 86 - 100 Very Stiff 16 to 30 2001 - 4000 Hard over 30 > 4000 CASCADE GEOTECHNW Project No. 2017-15 Figure A-5 KEY: Indicates 3-inch OD Dames & Moore Sample. Indicates 2-inch OD Split Spoon Sample (SPT). Indicates Disturbed Sample. Indicates No Recovery. Indicates Bag Sample. Indicates Shelby Tube Sample. COMPONENT DEFINITIONS COMPONENT SIZE RANGE Boulders Cobbles Gravel Coarse gravel Fine gravel Sand Coarse sand Medium sand Fine sand Silt and Clay Larger than 12 in 3 in to 12 in 3 in to No 4 (4.5mm ) 3 in to 3/4 in 3/4 in to No 4 ( 4.5mm ) No. 4 ( 4.5mm ) to No. 200 ( 0.074mm ) No. 4 ( 4.5 mm ) to No. 10 ( 2.0 mm ) No. 10 ( 2.0 mm ) to No. 40 ( 0.42 mm ) No. 40 ( 0.42 mm ) to No. 200 ( 0.074 mm ) Smaller than No. 200 ( 0.074 mm ) DESCRIPTIVE TERMS RANGE OF PROPORTION Trace or little Some Clayey, silty, sandy, gravelly And 1 - 5% 6 - 12% 13 - 30% 31 - 50% COMPONENT PROPORTIONS MOISTURE CONTENT DRY DAMP MOIST WET Absence of moisture, dusty, dry to the touch. Some perceptible moisture; below optimum No visible water; near optimum moisture content Visible free water, usually soil is below water table. RELATIVE DENSITY OR CONSISTENCY VERSUS SPT N -VALUE COHESIONLESS SOILS COHESIVE SOILS Density Very Loose Loose Medium Dense Dense Very Dense ATD : At Time of Drilling BGS: Below Ground Surface KEY: Indicates 3-inch OD Dames & Moore Sample. Indicates 2-inch OD Split Spoon Sample (SPT). Indicates No Recovery. Indicates Bag Sample. COMPONENT DEFINITIONS COMPONENT SIZE RANGE Boulders Cobbles Gravel Coarse gravel Fine gravel Sand Coarse sand Medium sand Fine sand Silt and Clay Larger than 12 in 3 in to 12 in 3 in to No 4 (4.5mm ) 3 in to 3/4 in 3/4 in to No 4 ( 4.5mm ) No. 4 ( 4.5mm ) to No. 200 ( 0.074mm ) No. 4 ( 4.5 mm ) to No. 10 ( 2.0 mm ) No. 10 ( 2.0 mm ) to No. 40 ( 0.42 mm ) No. 40 ( 0.42 mm ) to No. 200 ( 0.074 mm ) Smaller than No. 200 ( 0.074 mm ) DESCRIPTIVE TERMS RANGE OF PROPORTION Trace Few Little Some And 1 - 5% 6 - 10% 11 - 20% 21 - 35% 36 - 50% COMPONENT PROPORTIONS MOISTURE CONTENT DRY DAMP MOIST WET Absence of moisture, dusty, dry to the touch. Some perceptible moisture; below optimum No visible water; near optimum moisture content Visible free water, usually soil is below water table. RELATIVE DENSITY OR CONSISTENCY VERSUS SPT N -VALUE COHESIONLESS SOILS COHESIVE SOILS Density Very Loose Loose Medium Dense Dense Very Dense N ( blows/ft ) Approximate Consistency N (blows/ft ) Approximate Relative Density (%) Undrained Shear Strength (psf) 0 to 4 0 - 15 Very Soft 0 to 2 < 250 5 to 10 16 - 35 Soft 3 to 4 250 - 500 11 to 30 36 - 65 Medium Stiff 5 to 8 501 - 1000 31 to 50 66 - 85 Stiff 9 to 15 1001 - 2000 over 50 86 - 100 Very Stiff 16 to 30 2001 - 4000 Hard over 30 > 4000 Indicates Shelby Tube Sample. CASCADE GEOTECHNW Project No. 2017-15 Figure A-6 ATD : At Time of Drilling BGS: Below Ground Surface Cho Short Plat –Technical Information Report The Concept Group Appendices Appendix B Proposed Drainage Plan & Drainage Calculations WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: Cho Short Plat 5 FT Site Name: Cho Short Plat Site Address: 11840 SE 192nd Streer City : Renton Report Date: 11/11/2017 Gage : Seatac Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.00 Version Date: 2016/02/25 Version : 4.2.12 ___________________________________________________________________ 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, Forest, Flat .286 Pervious Total 0.286 Impervious Land Use acre Impervious Total 0 Basin Total 0.286 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat .074 Pervious Total 0.074 Impervious Land Use acre ROADS FLAT 0.042 ROOF TOPS FLAT 0.155 DRIVEWAYS FLAT 0.015 Impervious Total 0.212 Basin Total 0.286 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Tank 1 Tank 1 ___________________________________________________________________ Name : Tank 1 Tank Name: Tank 1 Dimensions Depth: 5 ft. Tank Type : Circular Diameter : 5 ft. Length : 275 ft. Discharge Structure Riser Height: 4 ft. Riser Diameter: 24 in. Orifice 1 Diameter: 0.32 in. Elevation: 0 ft. Orifice 2 Diameter: 0.45 in. Elevation: 2.348 ft. Orifice 3 Diameter: 0.27 in. Elevation: 2.68 ft. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Tank Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 0.0000 0.000 0.000 0.000 0.000 0.0556 0.006 0.000 0.000 0.000 0.1111 0.009 0.000 0.000 0.000 0.1667 0.011 0.001 0.001 0.000 0.2222 0.013 0.001 0.001 0.000 0.2778 0.014 0.002 0.001 0.000 0.3333 0.015 0.003 0.001 0.000 0.3889 0.016 0.004 0.001 0.000 0.4444 0.018 0.005 0.001 0.000 0.5000 0.018 0.006 0.002 0.000 0.5556 0.019 0.007 0.002 0.000 0.6111 0.020 0.008 0.002 0.000 0.6667 0.021 0.009 0.002 0.000 0.7222 0.022 0.011 0.002 0.000 0.7778 0.022 0.012 0.002 0.000 0.8333 0.023 0.013 0.002 0.000 0.8889 0.024 0.014 0.002 0.000 0.9444 0.024 0.016 0.002 0.000 1.0000 0.025 0.017 0.002 0.000 1.0556 0.025 0.019 0.002 0.000 1.1111 0.026 0.020 0.002 0.000 1.1667 0.026 0.022 0.003 0.000 1.2222 0.027 0.023 0.003 0.000 1.2778 0.027 0.025 0.003 0.000 1.3333 0.027 0.026 0.003 0.000 1.3889 0.028 0.028 0.003 0.000 1.4444 0.028 0.029 0.003 0.000 1.5000 0.028 0.031 0.003 0.000 1.5556 0.029 0.032 0.003 0.000 1.6111 0.029 0.034 0.003 0.000 1.6667 0.029 0.036 0.003 0.000 1.7222 0.030 0.037 0.003 0.000 1.7778 0.030 0.039 0.003 0.000 1.8333 0.030 0.041 0.003 0.000 1.8889 0.030 0.042 0.003 0.000 1.9444 0.030 0.044 0.003 0.000 2.0000 0.030 0.046 0.003 0.000 2.0556 0.031 0.048 0.004 0.000 2.1111 0.031 0.049 0.004 0.000 2.1667 0.031 0.051 0.004 0.000 2.2222 0.031 0.053 0.004 0.000 2.2778 0.031 0.055 0.004 0.000 2.3333 0.031 0.056 0.004 0.000 2.3889 0.031 0.058 0.005 0.000 2.4444 0.031 0.060 0.006 0.000 2.5000 0.031 0.062 0.006 0.000 2.5556 0.031 0.063 0.006 0.000 2.6111 0.031 0.065 0.007 0.000 2.6667 0.031 0.067 0.007 0.000 2.7222 0.031 0.069 0.008 0.000 2.7778 0.031 0.070 0.008 0.000 2.8333 0.031 0.072 0.009 0.000 2.8889 0.031 0.074 0.009 0.000 2.9444 0.031 0.075 0.010 0.000 3.0000 0.030 0.077 0.010 0.000 3.0556 0.030 0.079 0.010 0.000 3.1111 0.030 0.081 0.011 0.000 3.1667 0.030 0.082 0.011 0.000 3.2222 0.030 0.084 0.011 0.000 3.2778 0.030 0.086 0.011 0.000 3.3333 0.029 0.087 0.012 0.000 3.3889 0.029 0.089 0.012 0.000 3.4444 0.029 0.091 0.012 0.000 3.5000 0.028 0.092 0.012 0.000 3.5556 0.028 0.094 0.013 0.000 3.6111 0.028 0.095 0.013 0.000 3.6667 0.027 0.097 0.013 0.000 3.7222 0.027 0.099 0.013 0.000 3.7778 0.027 0.100 0.014 0.000 3.8333 0.026 0.102 0.014 0.000 3.8889 0.026 0.103 0.014 0.000 3.9444 0.025 0.104 0.014 0.000 4.0000 0.025 0.106 0.014 0.000 4.0556 0.024 0.107 0.292 0.000 4.1111 0.024 0.109 0.800 0.000 4.1667 0.023 0.110 1.454 0.000 4.2222 0.022 0.111 2.221 0.000 4.2778 0.022 0.112 3.075 0.000 4.3333 0.021 0.114 3.995 0.000 4.3889 0.020 0.115 4.955 0.000 4.4444 0.019 0.116 5.933 0.000 4.5000 0.018 0.117 6.904 0.000 4.5556 0.018 0.118 7.843 0.000 4.6111 0.016 0.119 8.728 0.000 4.6667 0.015 0.120 9.540 0.000 4.7222 0.014 0.121 10.26 0.000 4.7778 0.013 0.122 10.88 0.000 4.8333 0.011 0.122 11.40 0.000 4.8889 0.009 0.123 11.83 0.000 4.9444 0.006 0.123 12.17 0.000 5.0000 0.000 0.124 12.48 0.000 5.0556 0.000 0.000 12.96 0.000 ___________________________________________________________________ ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.286 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0.074 Total Impervious Area:0.212 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.008409 5 year 0.013206 10 year 0.015925 25 year 0.018805 50 year 0.020588 100 year 0.022104 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.005278 5 year 0.008003 10 year 0.010179 25 year 0.01339 50 year 0.016145 100 year 0.019234 ___________________________________________________________________ Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.008 0.004 1950 0.010 0.004 1951 0.019 0.012 1952 0.006 0.003 1953 0.005 0.004 1954 0.007 0.004 1955 0.012 0.004 1956 0.009 0.009 1957 0.007 0.004 1958 0.008 0.004 1959 0.007 0.004 1960 0.013 0.011 1961 0.007 0.006 1962 0.004 0.003 1963 0.006 0.004 1964 0.008 0.005 1965 0.006 0.008 1966 0.005 0.004 1967 0.011 0.004 1968 0.007 0.004 1969 0.007 0.004 1970 0.006 0.004 1971 0.006 0.004 1972 0.014 0.011 1973 0.006 0.007 1974 0.007 0.004 1975 0.009 0.004 1976 0.007 0.004 1977 0.001 0.004 1978 0.006 0.005 1979 0.004 0.003 1980 0.013 0.011 1981 0.005 0.004 1982 0.010 0.009 1983 0.009 0.004 1984 0.006 0.003 1985 0.003 0.004 1986 0.015 0.006 1987 0.013 0.010 1988 0.005 0.004 1989 0.003 0.004 1990 0.027 0.011 1991 0.016 0.010 1992 0.006 0.005 1993 0.007 0.003 1994 0.002 0.003 1995 0.009 0.006 1996 0.020 0.012 1997 0.016 0.012 1998 0.004 0.004 1999 0.015 0.010 2000 0.007 0.005 2001 0.001 0.003 2002 0.007 0.007 2003 0.009 0.004 2004 0.012 0.012 2005 0.008 0.004 2006 0.010 0.008 2007 0.020 0.014 2008 0.026 0.012 2009 0.013 0.007 ___________________________________________________________________ Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.0270 0.0145 2 0.0258 0.0124 3 0.0200 0.0122 4 0.0197 0.0122 5 0.0185 0.0120 6 0.0165 0.0119 7 0.0162 0.0111 8 0.0154 0.0110 9 0.0146 0.0109 10 0.0138 0.0107 11 0.0131 0.0105 12 0.0129 0.0103 13 0.0127 0.0101 14 0.0125 0.0090 15 0.0118 0.0087 16 0.0116 0.0083 17 0.0114 0.0076 18 0.0103 0.0074 19 0.0100 0.0073 20 0.0100 0.0072 21 0.0093 0.0065 22 0.0092 0.0065 23 0.0092 0.0064 24 0.0091 0.0055 25 0.0090 0.0054 26 0.0084 0.0054 27 0.0084 0.0050 28 0.0083 0.0045 29 0.0079 0.0042 30 0.0074 0.0042 31 0.0072 0.0041 32 0.0072 0.0041 33 0.0071 0.0041 34 0.0071 0.0041 35 0.0071 0.0041 36 0.0070 0.0041 37 0.0068 0.0040 38 0.0067 0.0040 39 0.0065 0.0040 40 0.0065 0.0040 41 0.0063 0.0040 42 0.0063 0.0039 43 0.0062 0.0039 44 0.0060 0.0039 45 0.0058 0.0039 46 0.0058 0.0039 47 0.0058 0.0038 48 0.0057 0.0038 49 0.0056 0.0037 50 0.0055 0.0037 51 0.0052 0.0037 52 0.0051 0.0036 53 0.0047 0.0036 54 0.0044 0.0036 55 0.0037 0.0035 56 0.0035 0.0035 57 0.0033 0.0034 58 0.0033 0.0034 59 0.0022 0.0032 60 0.0012 0.0032 61 0.0008 0.0031 ___________________________________________________________________ Stream Protection Duration POC #1 The Facility PASSED The Facility PASSED. Flow(cfs) Predev Mit Percentage Pass/Fail 0.0042 17552 11283 64 Pass 0.0044 16166 9364 57 Pass 0.0045 14964 9071 60 Pass 0.0047 13854 8814 63 Pass 0.0049 12812 8566 66 Pass 0.0050 11811 8325 70 Pass 0.0052 10898 8132 74 Pass 0.0054 10121 7922 78 Pass 0.0055 9383 7646 81 Pass 0.0057 8731 7317 83 Pass 0.0059 8143 7011 86 Pass 0.0060 7593 6712 88 Pass 0.0062 7058 6312 89 Pass 0.0064 6590 5867 89 Pass 0.0065 6145 5439 88 Pass 0.0067 5777 5020 86 Pass 0.0069 5431 4626 85 Pass 0.0070 5097 4301 84 Pass 0.0072 4808 3957 82 Pass 0.0073 4524 3700 81 Pass 0.0075 4252 3444 80 Pass 0.0077 4017 3253 80 Pass 0.0078 3782 3159 83 Pass 0.0080 3546 3067 86 Pass 0.0082 3337 2975 89 Pass 0.0083 3138 2875 91 Pass 0.0085 2950 2751 93 Pass 0.0087 2785 2594 93 Pass 0.0088 2597 2453 94 Pass 0.0090 2447 2304 94 Pass 0.0092 2304 2194 95 Pass 0.0093 2160 2067 95 Pass 0.0095 2024 1946 96 Pass 0.0097 1898 1815 95 Pass 0.0098 1790 1659 92 Pass 0.0100 1687 1522 90 Pass 0.0102 1586 1351 85 Pass 0.0103 1486 1218 81 Pass 0.0105 1381 1090 78 Pass 0.0107 1295 989 76 Pass 0.0108 1222 858 70 Pass 0.0110 1158 754 65 Pass 0.0112 1098 635 57 Pass 0.0113 1048 571 54 Pass 0.0115 997 499 50 Pass 0.0117 930 412 44 Pass 0.0118 884 347 39 Pass 0.0120 837 276 32 Pass 0.0121 790 208 26 Pass 0.0123 743 164 22 Pass 0.0125 716 152 21 Pass 0.0126 670 147 21 Pass 0.0128 634 140 22 Pass 0.0130 596 133 22 Pass 0.0131 565 128 22 Pass 0.0133 540 122 22 Pass 0.0135 497 115 23 Pass 0.0136 473 109 23 Pass 0.0138 434 102 23 Pass 0.0140 401 78 19 Pass 0.0141 368 53 14 Pass 0.0143 348 38 10 Pass 0.0145 323 16 4 Pass 0.0146 296 0 0 Pass 0.0148 273 0 0 Pass 0.0150 256 0 0 Pass 0.0151 235 0 0 Pass 0.0153 217 0 0 Pass 0.0155 198 0 0 Pass 0.0156 180 0 0 Pass 0.0158 158 0 0 Pass 0.0160 145 0 0 Pass 0.0161 130 0 0 Pass 0.0163 119 0 0 Pass 0.0165 109 0 0 Pass 0.0166 97 0 0 Pass 0.0168 91 0 0 Pass 0.0169 82 0 0 Pass 0.0171 76 0 0 Pass 0.0173 69 0 0 Pass 0.0174 61 0 0 Pass 0.0176 55 0 0 Pass 0.0178 48 0 0 Pass 0.0179 41 0 0 Pass 0.0181 38 0 0 Pass 0.0183 33 0 0 Pass 0.0184 27 0 0 Pass 0.0186 22 0 0 Pass 0.0188 21 0 0 Pass 0.0189 20 0 0 Pass 0.0191 19 0 0 Pass 0.0193 17 0 0 Pass 0.0194 14 0 0 Pass 0.0196 12 0 0 Pass 0.0198 8 0 0 Pass 0.0199 4 0 0 Pass 0.0201 3 0 0 Pass 0.0203 3 0 0 Pass 0.0204 3 0 0 Pass 0.0206 3 0 0 Pass _____________________________________________________ ___________________________________________________________________ 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 Volumn Volumn Infiltration Cumulative Percent Water Quality Percent Comment Treatment? Needs Through Volumn Volumn Volumn Water Quality Treatment Facility (ac-ft.) Infiltration Infiltrated Treated (ac-ft) (ac-ft) Credit Tank 1 POC N 35.66 N 0.00 Total Volume Infiltrated 35.66 0.00 0.00 0.00 0.00 0% No Treat. Credit Compliance with LID Standard 8 Duration Analysis Result = Passed ___________________________________________________________________ Perlnd and Implnd 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. 2005-2017; All Rights Reserved. Manning's Equation for Pipe Capacity Calculations 6-inch Pipe @ 1.0% Diameter of pipe (inches)6 Inches Hydraulic Radius: 0.143 % Full Flow (see chart)95 % 0.151 0.125 Slope of Pipe (percent)1 % 0.073 Manning's Coefficient 0.013 Area in Flow: 0.193 Hydraulic Radius 0.143 0.158 0.098 Area in Flow 0.193 SF 0.038 Pipe Capacity 0.6045 CFS Velocity 3.14 FPS 8-inch Pipe @ 0.5% Diameter of pipe (inches)8 Inches Hydraulic Radius: 0.191 % Full Flow (see chart)95 % 0.201 0.167 Slope of Pipe (percent)0.5 % 0.098 Manning's Coefficient 0.013 Area in Flow: 0.343 Hydraulic Radius 0.191 0.281 0.175 Area in Flow 0.343 SF 0.068 Pipe Capacity 0.9205 CFS Velocity 2.69 FPS 75% full-flow = 50% full-flow = 25% full-flow = 95% Full Flow = 75% Full Flow = 50% Full Flow = 25% Full Flow = 95% full-flow = 75% full-flow = 50% full-flow = 25% full-flow = 95% Full Flow = 75% Full Flow = 50% Full Flow = 25% Full Flow = 95% full-flow = King County Rational Method Legend Q = C x IR x A entry block Calculated values Q = peak flow rate in cubic feet per second C = runoff coefficients, for pavements & roofs C = 0.90 A = contributing area in acres IR = rainfall intensity in inches per hour IR = (PR) x (iR) PR = precipitation for the 24-hour duration storm event as interpolated from isopluvial maps iR = (aR) x (Tc) (-bR) Tc= time of concentration to initial inlet=5 min aR and bR = coefficients used to adjust the equation for the design storm return frequency R Rational Method Per 2016 King County Surface Water Manual Project: 3429 E Lake Sammamish Designed By: ATN Date: 17-Jan-18 Design Storm Event aR bR 2 years 1.58 0.58 5.0 0.62 5 years 2.33 0.63 5.0 0.85 10 years 2.44 0.64 5.0 0.87 25 years 2.66 0.65 5.0 0.93 50 years 2.75 0.65 5.0 0.97 100 years 2.61 0.63 5.0 0.95 Event PR iR IR 2 yr 0.62 0.00 10 yr 2.9 0.87 2.53 25 yr 3.4 0.93 3.18 100 yr 3.9 0.95 3.69 Basin C value Surface Area (sf) Surface Area (ac)Q10 Q25 Q100 Pipe #8 (Lot 3) 0.90 2500 0.06 0.13 0.16 0.19 Pipe #10 (Lot 1, 3, private road) 0.90 7789 0.18 0.41 0.51 0.59 Pipe #6 (Lot 1, 2, 3, private road) 0.90 10289 0.24 0.54 0.68 0.79 King County Stormwater Management Manual Table 3.2.1B Tc iR **Runoff coefficients "C" values from King County Stormwater Management Manual Table 3.2.1A, Impervious area assumed roof and pavement, Pervious pavement area assumed c value of gravel. Cho Short Plat –Technical Information Report The Concept Group Appendices Appendix C Erosion Control Plan & Details Cho Short Plat –Technical Information Report The Concept Group Appendices Appendix D Maintenance Procedures APPENDIX A MAINTENANCE REQUIREMENTS FOR FLOW CONTROL, CONVEYANCE, AND WQ FACILITIES NO. 3 – DETENTION TANKS AND VAULTS Maintenance Component Defect or Problem Conditions When Maintenance is Needed Results Expected When Maintenance is Performed Site Trash and debris Any trash and debris which exceed 1 cubic foot per 1,000 square feet (this is about equal to the amount of trash it would take to fill up one standard size office garbage can). In general, there should be no visual evidence of dumping. Trash and debris cleared from site. Noxious weeds Any noxious or nuisance vegetation which may constitute a hazard to County personnel or the public. Noxious and nuisance vegetation removed according to applicable regulations. No danger of noxious vegetation where County personnel or the public might normally be. Contaminants and pollution Any evidence of contaminants or pollution such as oil, gasoline, concrete slurries or paint. Materials removed and disposed of according to applicable regulations. Source control BMPs implemented if appropriate. No contaminants present other than a surface oil film. Grass/groundcover Grass or groundcover exceeds 18 inches in height. Grass or groundcover mowed to a height no greater than 6 inches. Tank or Vault Storage Area Trash and debris Any trash and debris accumulated in vault or tank (includes floatables and non-floatables). No trash or debris in vault. Sediment accumulation Accumulated sediment depth exceeds 10% of the diameter of the storage area for ½ length of storage vault or any point depth exceeds 15% of diameter. Example: 72-inch storage tank would require cleaning when sediment reaches depth of 7 inches for more than ½ length of tank. All sediment removed from storage area. Tank Structure Plugged air vent Any blockage of the vent. Tank or vault freely vents. Tank bent out of shape Any part of tank/pipe is bent out of shape more than 10% of its design shape. Tank repaired or replaced to design. Gaps between sections, damaged joints or cracks or tears in wall A gap wider than ½-inch at the joint of any tank sections or any evidence of soil particles entering the tank at a joint or through a wall. No water or soil entering tank through joints or walls. Vault Structure Damage to wall, frame, bottom, and/or top slab Cracks wider than ½-inch, any evidence of soil entering the structure through cracks or qualified inspection personnel determines that the vault is not structurally sound. Vault is sealed and structurally sound. Inlet/Outlet Pipes Sediment accumulation Sediment filling 20% or more of the pipe. Inlet/outlet pipes clear of sediment. Trash and debris Trash and debris accumulated in inlet/outlet pipes (includes floatables and non-floatables). No trash or debris in pipes. Damaged Cracks wider than ½-inch at the joint of the inlet/outlet pipes or any evidence of soil entering at the joints of the inlet/outlet pipes. No cracks more than ¼-inch wide at the joint of the inlet/outlet pipe. 2016 Surface Water Design Manual – Appendix A 4/24/2016 A-5 APPENDIX A MAINTENANCE REQUIREMENTS FLOW CONTROL, CONVEYANCE, AND WQ FACILITIES NO. 3 – DETENTION TANKS AND VAULTS Maintenance Component Defect or Problem Conditions When Maintenance is Needed Results Expected When Maintenance is Performed Access Manhole Cover/lid not in place Cover/lid is missing or only partially in place. Any open manhole requires immediate maintenance. Manhole access covered. Locking mechanism not working Mechanism cannot be opened by one maintenance person with proper tools. Bolts cannot be seated. Self-locking cover/lid does not work. Mechanism opens with proper tools. Cover/lid difficult to remove One maintenance person cannot remove cover/lid after applying 80 lbs of lift. Cover/lid can be removed and reinstalled by one maintenance person. Ladder rungs unsafe Missing rungs, misalignment, rust, or cracks. Ladder meets design standards. Allows maintenance person safe access. Large access doors/plate Damaged or difficult to open Large access doors or plates cannot be opened/removed using normal equipment. Replace or repair access door so it can opened as designed. Gaps, doesn't cover completely Large access doors not flat and/or access opening not completely covered. Doors close flat; covers access opening completely. Lifting Rings missing, rusted Lifting rings not capable of lifting weight of door or plate. Lifting rings sufficient to lift or remove door or plate. 4/24/2016 2016 Surface Water Design Manual – Appendix A A-6 MAINTENANCE INSTRUCTIONS FOR A RAIN GARDEN Your property contains a stormwater management flow control BMP (best management practice) called a "rain garden," which was installed to mitigate the stormwater quantity and quality impacts of some or all of the impervious or non-native pervious surfaces on your property. Rain gardens, often described as "bioretention," are vegetated closed depressions or ponds that retain and filter stormwater from an area of impervious surface or non-native pervious surface. The soil in the rain garden has been enhanced to encourage and support vigorous plant growth that serves to filter the water and sustain infiltration capacity. Depending on soil conditions, rain gardens may have water in them throughout the wet season and may overflow during major storm events. MAINTENANCE RESTRICTIONS The size, placement, and design of the rain garden as depicted by the flow control BMP site plan and design details must be maintained and may not be changed without written approval either from the King County Water and Land Resources Division or through a future development permit from King County. Plant materials may be changed to suit tastes, but chemical fertilizers and pesticides must not be used. INSPECTION FREQUENCY AND MAINTENANCE GUIDELINES • Rain gardens must be inspected annually for physical defects and sediment accumulation. • After major storm events, the system should be checked to see that the overflow system is working properly and sedimentation is not occurring at the inlet. If erosion channels or bare spots are evident, they should be stabilized with soil, plant material, mulch, or landscape rock. Sediment deposits should be carefully removed and the sediment source eliminated. • A supplemental watering program may be needed the first year to ensure the long-term survival of the rain garden's vegetation. • Chemical fertilizers and pesticides must not be used. • Mulch may be added and additional compost should be worked into the soil over time. • Plant materials may be changed to suit tastes. • Vegetation should be maintained as follows: 1) replace all dead vegetation as soon as possible; 2) remove fallen leaves and debris as needed; 3) remove all noxious vegetation when discovered; 4) manually weed without herbicides or pesticides; 5) to protect infiltration performance, do not compact soils in the bioretention cell with heavy maintenance equipment and/or excessive foot traffic; 6) during drought conditions, use mulch to prevent excess solar damage and water loss. RECORDING REQUIREMENT These rain garden flow control BMP maintenance and operation instructions must be recorded as an attachment to the required declaration of covenant and grant of easement per Requirement 3 of Section C.1.3.4 of the King County Surface Water Design Manual. The intent of these instructions is to explain to future property owners, the purpose of the BMP and how it must be maintained and operated. These instructions are intended to be a minimum; the King County Department of Permitting and Environmental Services (DPER) may require additional instructions based on site-specific conditions. See King County’s Surface Water Design Manual website for additional information and updates. TYPICAL RAIN GARDEN (SPILLWAY OR CATCHBASIN OUTLET) TREES, SHRUBS GROUND COVER BIORETENTION AREA PLAN VIEW NTS VEGETATED COMPACTED EARTH BERM ROCKED SPILLWAY, 2'Wx4'L (OR BEYOND BERM IF LONGER), OVERFLOW TO SUITABLE DISCHARGE AREA TREES, SHRUBS GROUND COVER BIORETENTION AREA PLAN VIEW NTS VEGETATED COMPACTED EARTH BERM, TOP WIDTH 2' MIN 4" RIGID PIPE OUTLET TO STORM SYSTEM CATCH BASIN w/GRATE SECTION A-A NTS BIORETENTION AREA GROUND COVER TREES TREES SHRUBS 6" MIN TO 12" MAX WATER DEPTH OVERFLOW TO SUITABLE SURFACE DISCHARGE AREA 2' MIN TOP WIDTH COMPACTED EARTH BERM (AS NEEDED) 6" MIN FREEBOARD ABOVE OVERFLOW WS TO TOP OF BERM OR ADJACENT ROADWAY BIORETENTION SOIL MIX PER REFERENCE 11-C, MIN.18" DEPTH MAX SLOPE 3H:1V BELOW OVERFLOW WATER SURFACE ELEVATION (TYP.)