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2017 D. R. STRONG Consulting Engineers Inc. Cedars at the Highlands Technical Information Report Renton, Washington TECHNICAL INFORMATION REPORT for CEDARS AT THE HIGHLANDS PRELIMINARY PLAT 14120 160th Avenue SE, Renton, Washington ___ ____________________________________________________________________________ DRS Project No. 16041 Renton File No. PRE16-000524 Applicant Jamie Walter Harbour Homes, LLC 400 North 34th Street, Suite 300 Seattle, WA 98103 Report Prepared by D. R. STRONG Consulting Engineers, Inc. 620 7th Avenue Kirkland, WA 98033 (425) 827-3063 Report Issue Date March 15, 2017 2017 D. R. STRONG Consulting Engineers Inc. Cedars at the Highlands Technical Information Report Renton, Washington TECHNICAL INFORMATION REPORT CEDARS AT THE HIGHLANDS PLAT TABLE OF CONTENTS SECTION I ...................................................................................................................... 4 Project Overview ......................................................................................................... 4 Predeveloped Site Conditions ..................................................................................... 4 Developed Site Conditions .......................................................................................... 4 SECTION II ................................................................................................................... 16 Conditions and Requirements Summary ................................................................... 16 Conditions of Approval............................................................................................... 18 SECTION III .................................................................................................................. 19 Off-Site Analysis ........................................................................................................ 19 Task 1: Define and Map Study Area ...................................................................... 19 Task 2: Resource Review ...................................................................................... 19 Task 3: Field Inspection ......................................................................................... 28 Task 4: Drainage System Description and Problem Descriptions .......................... 29 Task 5: Mitigation of Existing or Potential Problems .............................................. 30 SECTION IV .................................................................................................................. 36 Flow Control, Low Impact Development (LID) and Water Quality Facility Analysis and Design .................................................................................................. 36 Existing Site Hydrology .......................................................................................... 36 Developed Site Hydrology ...................................................................................... 38 Performance Standards ............................................................................................. 42 Flow Control System ................................................................................................. 43 Water Quality Treatment System ............................................................................... 53 SECTION V ................................................................................................................... 55 Conveyance System Analysis and Design ................................................................ 55 SECTION VI .................................................................................................................. 57 Special Reports and Studies ..................................................................................... 57 SECTION VII ................................................................................................................. 58 Other Permits ............................................................................................................ 58 SECTION VIII ................................................................................................................ 59 CSWPPP Analysis and Design (Part A) .................................................................... 59 SWPPP Plan Design (Part B) .................................................................................... 59 SECTION IX .................................................................................................................. 61 Bond Quantities, Facility Summaries, and Declaration of Covenant .......................... 61 2017 D. 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Cedars at the Highlands Technical Information Report Renton, Washington SECTION X ................................................................................................................... 62 Operations and Maintenance Manual ........................................................................ 62 APPENDICES ............................................................................................................... 63 Appendix “A” Legal Description ................................................................................. 64 List of Figures Figure 1 TIR Worksheet .................................................................................................. 6 Figure 2 Site Location ................................................................................................... 11 Figure 3 Drainage Basins, Subbasins, and Site Characteristics ................................... 12 Figure 4 Soils ................................................................................................................ 13 Figure 5 City of Renton Topography Map ..................................................................... 20 Figure 6 City of Renton Coal Mine Hazard Areas Map ................................................. 21 Figure 7 City of Renton Flood Hazards Map ................................................................. 22 Figure 8 City of Renton Streams and Wetlands Map .................................................... 23 Figure 9 City of Renton Landslide Hazards Map ........................................................... 24 Figure 10 City of Renton Seismic Hazard Areas Map ................................................... 25 Figure 11 FEMA Map .................................................................................................... 26 Figure 12 King County iMap Drainage Complaints Map................................................ 27 Figure 13 Offsite Analysis Downstream Map ................................................................ 32 Figure 14 Offsite Analysis Downstream Table .............................................................. 33 Figure 15 Predeveloped Area Map ............................................................................... 37 Figure 16 Developed Area Map .................................................................................... 41 Figure 17 Detention & Water Quality Facility Details ..................................................... 54 2017 D. R. STRONG Consulting Engineers Inc. Page 4 Cedars at the Highlands Level One Downstream Analysis Renton, Washington SECTION I PROJECT OVERVIEW The Project is the proposed subdivision of one existing parcel zoned R-4 into 14 single- family residential lots, per the City of Renton’s (City) subdivision process. The Project is located at 14120 160th Avenue SE, Renton, Washington (Site) also known as Tax Parcel Number 145750-0110. The Project will meet the drainage requirements of the 2017 City of Renton Surface Water Design Manual (CORSWDM). PREDEVELOPED SITE CONDITIONS The total existing Site area is approximately 195,531 s.f. (4.49 acres). The Site is currently developed with a workshop, one shed and gravel driveways. The remainder of the Site is undeveloped forest covered with dense brush. An upstream tributary flows onto the Site on the western property line. This upstream flow is conveyed through a series of privately maintained channels and pipes to the southeasterly section of the Site where it is conveyed in a southerly direction by a swale. Project frontage on 160th Avenue SE drains onto the site; contributing to the upstream tributary area. The predeveloped site consists of a shallow grade to the southeast and is contained within one Threshold Discharge Area (TDA). There exists an unnamed Type Ns stream flowing through the northeast corner of the Site. A natural ridge divides the Site into two distinct Natural Drainage Areas, NDA 1 and NDA 2. Runoff from NDA 1 is collected by the unnamed Type Ns stream and is conveyed in an easterly direction. NDA 2 results from runoff sheet flowing in a southeasterly direction and being collected by a swale located offsite. The two downstream paths convey stormwater in a southerly direction and converge in an existing detention pond within 1000’ downstream of the Site. This detention pond discharges into an existing conve yance system in SE 144th Street which then crosses underneath 154th Place SE where it outlets to Stewart Creek, a Class 3 stream. Stewart Creek outlets to Cedar River approximately 6,000 feet from the Site. DEVELOPED SITE CONDITIONS The applicant is seeking approval to subdivide 4.49 acres into 14 single–family residential lots (Project), with lot sizes ranging from approximately 9,002 s.f. to 16,821 s.f. The existing workshop and shed on site will remain and will occupy two of the fourteen proposed lots under a 5-year temporary use permit. The proposed retained structures account for approximately 8,400 s.f. of impervious area. After 5 years, these structures will be demolished and new single-family residences will be constructed. Therefore, the lots containing the existing structures will be modeled assuming build-out conditions per Renton Municipal Code zoning designations. The project is required to meet the City’s Flow Control Duration Standard – Matching Forested Conditions and Basic Water Quality treatment. This standard matches the developed Site flow durations to the flow durations of pre -developed rates for forested (historical) site conditions. These standards will be met through the use of a detention vault preceding a stormfilter. The proposed impervious surface areas are as follows: half street frontage improvements consisting of a 5’ sidewalk along 160th AVE SE, Road 2017 D. R. STRONG Consulting Engineers Inc. Page 5 Cedars at the Highlands Level One Downstream Analysis Renton, Washington A, the 14 new single-family residences and associated driveways, the existing structures to remain, and Tract A, the storm water detention facility, will generate approximately 86,552 s.f. of impervious area (1.99 acres). A area of non-target impervious surface along the north side of 160th AVE SE will be collected by the proposed detention facility. The facility has been designed with this area being proposed as a mitigation trade area. This area will be traded for a portion of the frontage that will not be collected by the proposed stormwater vault. Proposed Site landcover and surfaces are shown in Figure 16, Developed Site Conditions. (See Section IV). The Project proposes to combine runoff from NDA 2 and the target frontage into one detention system and discharge to the existing swale that is the natural discharge location for NDA 2. The Project is proposing to build a bypass conveyance system that will direct any pre-existing upstream runoff to its natural discharge location. Per Section 1.2.9.2.1 Small Lot BMP Requirements of the CORSWDM (Manual), projects are required to mitigate for impervious surfaces by use of Flow Control Best Management Practices (BMP’s). The Project’s lots fall within the requirements for small lots, as all lots are under 22,000 square feet. This requires all lots under 11,000 square feet to apply flow control BMPs to mitigate impervious areas equal to 10% of the total lot area. Lots 9 and 10 are required to mitigate impervious area equal to 20% of their total lot area due to lot areas greater than 11,000 square feet. The soils investigation summarized by the project geotechnical report are confined to the upper layers (up to 12 feet deep) of Site soils. The report indicates the presence of glacial till and glaciomarine deposits not conducive to infiltration. It is the recommendation of the geotechnical engineer to investigate alternat ive means of stormwater management. With limited native growth areas and negligible infiltration rates, the Project is proposing to utilize a combination of reduced impervious surface credits and basic dispersion to meet the small lot BMP requirements. (See Section IV). 2017 D. R. STRONG Consulting Engineers Inc. Page 6 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 1 TIR WORKSHEET TECHNICAL INFORMATION REPORT (TIR) WORKSHEET Part 1 PROJECT OWNER AND PROJECT ENGINEER Part 2 PROJECT LOCATION AND DESCRIPTION Project Owner: Gerald Smith Phone: Address: 14120 160th Ave SE, Renton WA 98059 Project Engineer: Yoshio L. Piediscalzi, P.E. Company: D. R. STRONG Consulting Engineers Inc. Phone: (425) 827-3063 Project Name: Cedars at the Highlands City Permit#: PRE16-00524 Location: Township: 23 North Range: 05 East Section: 29 Site Address: 14120 160th Ave SE, Renton, WA 98059 Part 3 TYPE OF PERMIT APPLICATION Part 4 OTHER REVIEWS AND PERMITS Landuse Services Subdivision / Short Subdivision / UPD Building Services: M/F / Commercial / SFR Clearing and Grading Right-of-Way Other: DFW HPA Shoreline Mngmt COE 404 Structural DOE Dam Safety Rockery/Vault FEMA Floodplain ESA Section 7 COE Wetlands Other: Part 5 PLAN AND REPORT INFORMATION Technical Information Report Type of Drainage Review (circle): Date (include revision dates): April 3, 2017 Date of Final: Site Improvement Plan (Engr. Plans) Type (circle one): Full / Modified / : Small Site Date (include revision dates): Date of Final: Part 6 SWDM ADJUSTMENT APPROVALS Type (circle one): Standard / Blanket Description: (include conditions in TIR Section 2) _____________________________________________________________________________________ _____________________________________________________________________________________ ____________________________________________________________________________________ Date of Approval: ☒ Full ☐ Targeted ☐ Simplified ☐ Large Project ☐ Directed 2017 D. R. STRONG Consulting Engineers Inc. Page 7 Cedars at the Highlands Level One Downstream Analysis Renton, Washington Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: TBD Completion Date Describe: Monitor discharge location during construction. Re: SWDM Adjustment No. _____________ Part 8 SITE COMMUNITY AND DRAINAGE BASIN Community Plan: Renton Comprehensive Plan Special District Overlays: N/A Drainage Basin: Lower Cedar River Drainage Basin Stormwater Requirements: Flow Control Duration Standard – Matching Forested and Basic WQ treatment Part 9 ONSITE AND ADJACENT SENSITIVE AREAS River/ Stream: Type Ns Stream Lake Wetlands Adjacent wetland to the east Closed Depression Floodplain Other Steep Slope Erosion Hazard Landslide Hazard Coal Mine Hazard Seismic Hazard Habitat Protection Part 10 SOILS Soil Type AgC Slopes 8-15% Erosion Potential Moderate to Severe High Groundwater Table Sole Source Aquifer other Seeps/Springs Additional Sheets Attached 2017 D. R. STRONG Consulting Engineers Inc. Page 8 Cedars at the Highlands Level One Downstream Analysis Renton, Washington Part 11 DRAINAGE DESIGN LIMITATIONS REFERENCE Core Requirement #2 – Offsite Analysis SEPA Sensitive/Critical Areas _______ LID Infeasibility Additional Sheet Attached LIMITATION / SITE CONSTRAINT Part 12 TIR SUMMARY SHEET Threshold Discharge Area: Site comprised of one TDA (name or description) Core Requirements (all 8 apply) Discharge of Natural Location Yes Number of Natural Discharge Locations: 2 Offsite Analysis Level: 1 / 2 / 3 dated: 01/25/2017 Flow Control Standard: Matching Forested (incl. facility summary sheet) or Exemption Number: __________________ On-site BMPS: Reduced Impervious Surface Credit and Basic Dispersion Conveyance System Spill containment located at: TBD Erosion and Sediment Control / CSWPP/CESCL/ESC Site Supervisor: TBD Construction Stormwater Pollution Contact Phone: TBD Prevention After Hours Phone: TBD Maintenance and Operation Responsibility: Private / Public If Private, Maintenance Log Required: Yes / No Financial Guarantees and Provided: Yes / No Liability Water Quality Type: Basic / Sens Lake / Enhanced Basic / Bog (include facility summary sheet) or exemption No. Landscape Management Plan: Yes / No Special Requirements (as applicable) Area Specific Drainage Type: CDA / SDO / MDP / BP / LMP / Shared / None Requirements Name: Floodplain/Floodway Delineation Type: Major / Minor / Exemption / None 100-year Base Flood Elevation (or range): Datum: Flood Protection Facilities Describe: N/A Source Control Describe Landuse: (comm. / industrial landuse) Describe any structural controls: Oil Control High-use Site: Yes / No 2017 D. R. STRONG Consulting Engineers Inc. Page 9 Cedars at the Highlands Level One Downstream Analysis Renton, Washington Treatment BMP: Maintenance Agreement: Yes / No with whom? Other Drainage Structures Describe: Runoff will be collected and conveyed to the detention vault located in Tract A. The proposed vault will be immediately followed by a Stormfilter to achieve water quality standards. Part 13 EROSION AND SEDIMENT CONTROL REQUIREMENTS MINIMUM ESC REQUIREMENTS DURING CONSTRUCTION Clearing Limits Cover Measures Perimeter Protection Traffic Area Stabilization Sediment Retention Surface Water Collection Dewatering Control Dust control Flow Control Control Pollutants Protect Existing and Proposed BMPs/Facilities Maintain Protective BMPs / Manage Project MINIMUM ESC REQUIREMENTS AFTER CONSTRUCTION Stabilize Exposed Surfaces Remove and Restore Temporary ESC Facilities Clean and Remove All Silt and Debris, Ensure Operations of Permanent BMPs/Facilities, restore operation of BMPs/Facilities as necessary Flag Limits of sensitive areas and open space Preservation areas Other 2017 D. R. STRONG Consulting Engineers Inc. Page 10 Cedars at the Highlands Level One Downstream Analysis Renton, Washington Part 14 STORMWATER FACILITY DESCRIPTIONS (Note: Include Facility Summary and Sketch Flow Control Type/Description Water Quality Type/Description Detention Infiltration Regional Facility Shared Facility On-site BMPs Other Detention Vault Reduced Impervious Surface Credit and Basic Dispersion __________________ Vegetated Flowpath Wetpool Filtration Oil Control Spill Control On-site BMPs Other _________________ Contech StormFilter See flow control Part 15 EASEMENTS/TRACTS Part 16 STRUCTURAL ANALYSIS Drainage Easement Covenant Native Growth Protection Covenant Tract Other: Sanitary Sewer Easement Cast in Place Vault Retaining Wall Rockery > 4’ High Structural on Steep Slope Other: Part 17 SIGNATURE OF PROFESSIONAL ENGINEER I, or a civil engineer under my supervision, have visited the site. Actual site conditions as observed were incorporated into this worksheet and the attached Technical Information Report. To the best of my knowledge the information provided here is accurate. Signed/Date 2017 D. R. STRONG Consulting Engineers Inc. Page 11 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 2 SITE LOCATION Site 2017 D. R. STRONG Consulting Engineers Inc. Page 12 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 3 DRAINAGE BASINS, SUBBASINS, AND SITE CHARACTERISTICS 0GRAPHIC SCALE9060301 INCH = 60 FT. 2017 D. R. STRONG Consulting Engineers Inc. Page 13 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 4 SOILS 2017 D. R. STRONG Consulting Engineers Inc. Page 14 Cedars at the Highlands Level One Downstream Analysis Renton, Washington King County Area, Washington AgC—Alderwood gravelly sandy loam, 8 to 15 percent slopes a) Map Unit Setting National map unit symbol: 2t626 Elevation: 50 to 800 feet Mean annual precipitation: 20 to 60 inches Mean annual air temperature: 46 to 52 degrees F Frost-free period: 160 to 240 days Farmland classification: Prime farmland if irrigated b) Map Unit Composition Alderwood and similar soils: 85 percent Minor components: 15 percent Estimates are based on observations, descriptions, and transects of the mapunit. c) Description of Alderwood (1) Setting Landform: Ridges, hills Landform position (two-dimensional): Shoulder Landform position (three-dimensional): Nose slope, talf Down-slope shape: Linear, convex Across-slope shape: Convex Parent material: Glacial drift and/or glacial outwash over dense glaciomarine deposits (2) Typical profile A - 0 to 7 inches: gravelly sandy loam Bw1 - 7 to 21 inches: very gravelly sandy loam Bw2 - 21 to 30 inches: very gravelly sandy loam Bg - 30 to 35 inches: very gravelly sandy loam 2Cd1 - 35 to 43 inches: very gravelly sandy loam 2Cd2 - 43 to 59 inches: very gravelly sandy loam (3) Properties and qualities Slope: 8 to 15 percent Depth to restrictive feature: 20 to 39 inches to densic material Natural drainage class: Moderately well drained Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) Depth to water table: About 18 to 37 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Very low (about 2.7 inches) (4) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 4s Hydrologic Soil Group: B Other vegetative classification: Limited Depth Soils (G002XN302WA), Limited Depth Soils (G002XS301WA), Limited Depth Soils (G002XF303WA) Hydric soil rating: No 2017 D. R. STRONG Consulting Engineers Inc. Page 15 Cedars at the Highlands Level One Downstream Analysis Renton, Washington d) Minor Components (1) Everett Percent of map unit: 5 percent Landform: Eskers, kames, moraines Landform position (two-dimensional): Shoulder, footslope Landform position (three-dimensional): Crest, base slope Down-slope shape: Convex Across-slope shape: Convex Hydric soil rating: No (2) Indianola Percent of map unit: 5 percent Landform: Eskers, kames, terraces Landform position (three-dimensional): Tread Down-slope shape: Linear Across-slope shape: Linear Hydric soil rating: No (3) Shalcar Percent of map unit: 3 percent Landform: Depressions Landform position (three-dimensional): Dip Down-slope shape: Concave Across-slope shape: Concave Hydric soil rating: Yes (4) Norma Percent of map unit: 2 percent Landform: Depressions, drainageways Landform position (three-dimensional): Dip Down-slope shape: Concave, linear Across-slope shape: Concave Hydric soil rating: Yes 2017 D. R. STRONG Consulting Engineers Inc. Page 16 Cedars at the Highlands Level One Downstream Analysis Renton, Washington SECTION II CONDITIONS AND REQUIREMENTS SUMMARY The Project must comply with the following Core and Special Requirements: C.R. #1 – Discharge at the Natural Location: Existing drainage patterns discharge from the Site at two locations. The two downstream paths converge within ¼ mile, maintaining one TDA. Natural discharge point 1 is an unnamed Type Ns Stream located in the northeast section of the Site. This stream will be enclosed in a Sensitive Area Tract. Natural discharge point 2 is along the southern property line, where stormwater is collected by a swale and conveyed to the south and east. Discharge from the detention facility will be directed to the existing swale located to the south and east of the Site, thus maintaining discharge at the natural locations. C.R. #2 – Offsite Analysis: Analysis is included in Section III. The Analysis describes the Site’s runoff patterns in detail. C.R. #3 – Flow Control: The Project is required to adhere to Flow Control Duration Standard – Matching Forested site conditions. One detention vault will provide flow control as required for the new and replaced impervious and pervious surfaces. The Site is required to “match the flow duration of pre-developed rates for forested (historic) site conditions over the range of flows extending from 50% of 2 - year up to the full 50-year flow and matches peaks for the 2- and 10-year returm periods,” (2017 CORSWDM, Sec. 1.2.3.1). A detention vault will accommodate this requirement. C.R. #4 – Conveyance System: New pipe systems are required to be designed with sufficient capacity to convey and contain (with minimum 6-inches of freeboard) the 25-year peak flow, assuming developed conditions for onsite tributary areas and existing conditions for any offsite tributary areas. Pipe system structures may overtop for runoff events that exceed the 25 -year design capacity, provided the overflow from a 100-year runoff event does not create or aggravate a “severe flooding problem” or “severe erosion problem” as defined in C.R. #2. Any overflow occurring onsite for runoff events up to and including the 100 -year event must discharge at the natural location for the project site. In residential subdivisions, such overflow must be contained within an onsite drainage easement, tract, covenant or public right-of-way. The proposed conveyance system will be analyzed using the KCBW program to determine if the proposed conveyance system is capable of conveying the 100-year peak storm storm without overtopping any structures or channels. This analysis will be done at the time of engineering submittal. C.R. #5 – Erosion and Sediment Control: The Project will provide the seven minimum ESC measures. A temporary erosion and sedimentation control plan will be presented as part of the engineering construction plan set. C.R. #6 – Maintenance and Operations: Maintenance of the proposed storm drainage facilities will be the responsibility of the City. An Operation and Maintenance Manual will be included in Section X at the time of construction plan preparation. 2017 D. R. STRONG Consulting Engineers Inc. Page 17 Cedars at the Highlands Level One Downstream Analysis Renton, Washington C.R. #7 – Financial Guarantees: Prior to commencing construction, the Applicant must post a drainage facilities restoration and site stabilization financial guarantee. For any constructed or modified drainage facilities to be maintained and operated by the City, the Applicant must: 1) Post a drainage defect and maintenance financial guarantee for a period of two years, and 2) Maintain the drainage facilities during the two-year period following posting of the drainage defect and maintenance financial guarantee. C.R. #8 – Water Quality: The Project is required to provide basic water quality treatment. A Stormfilter immediately following the detention vault will accommodate this requirement. C.R. #9 – On-Site BMPs: The Project is required to provide on-site BMPs to mitigate the impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, existing impervious surfaces, and replaced impervious surface targeted for mitigation. It has been determined, using the order of preference outlined in Section 1.2.9.2.1 of the CORSWDM that basic dispersion and reduced impervious surfaces are the most feasible options to meet the on-site BMP requirements. Full dispersion is not feasible due to the amount of native growth that is being retained on the Site. Required native growth areas and flowpath lengths could not be met. Per the geotechnical report dated 02.23.2017, it has been recommended that due to the existing soil conditions, any level of stormwater infiltration feasibility is negligible. The requirements for full infiltration, limited infiltration, rain gardens, bioretention , and permeable pavement all necessitate the recommendation of infiltration from a geotechnical engineer. S.R. #1 – Other Adopted Area-Specific Requirements: Not applicable for this Project. S.R. #2 – Flood Hazard Area Delineation: There is no flood hazard areas on the Site, therefore S.R. #2 is not applicable for this project. S.R. #3 – Flood Protection Facilities: Not applicable for this Project. S.R. #4 – Source Control: Not applicable for this Project. S.R. #5 – Oil Control: Not applicable for this Project. S.R. #6 – Aquifer Protection Area: Site not located within zones 1 and 2, therefore not applicable for this Project. 2017 D. R. STRONG Consulting Engineers Inc. Page 18 Cedars at the Highlands Level One Downstream Analysis Renton, Washington CONDITIONS OF APPROVAL Cedars at the Highlands PRE16-000524 TBD 2017 D. R. STRONG Consulting Engineers Inc. Page 19 Cedars at the Highlands Level One Downstream Analysis Renton, Washington SECTION III OFF-SITE ANALYSIS LEVEL ONE DOWNSTREAM ANALYSIS TASK 1: DEFINE AND MAP STUDY AREA This Offsite Analysis was prepared in accordance with Core Requirement #2, Section 1.2.2 of the 2017 City of Renton Surface Water Design Manual (CORSWDM). The Site is located at 14120 160th Avenue SE, Renton, Washington. The Project is the subdivision of one parcel into fourteen single-family lots. See Figures 5 through 12 for maps of the study area. TASK 2: RESOURCE REVIEW Adopted Basin Plans: King County Department of Permitting and Environmental Review (DPER) and Department of Natural Resources and Parks (DNRP) Lower Cedar River Basin Plan Summary Finalized Drainage Studies: Alpine Nursery TIR. Basin Reconnaissance Summary Reports: Cedar River Current and Future Conditions Report (April 1993). Comprehensive Plans: Renton’s Comprehensive Plan, adopted on June 22, 2015, effective July 1, 2015. Floodplain/Floodway (FEMA) Map: No floodplains exist on site, See Figure 11. Other Offsite Analysis Reports: Mindy’s Place Critical Area Mitigation Plan Sensitive Areas Map Folios, City of Renton GIS: See Figures 6-10. The City’s GIS system shows a wetland on Site. Sewell Wetland Consulting, Inc. prepared a Critical Area Study dated July 6, 2016 . This assessment found that there were no areas on the Site that met the definitions of wetlands. DNRP Drainage Complaints and Studies: Per King County Water and Land Resources Division, there are two complaints within the downstream paths, within approximately one mile from the Site within the last 10 years. See Figure 12. USDA King County Soils Survey: See Figure 4 King County Wetlands Inventory: No wetlands were identified via King County Wetland Inventory. See Figure 8. Migrating River Studies: None are applicable to this Site. King County Designated Water Quality Problems: Per the Washington State Water Quality Assessment 303(d)/305(b) Integrated Report current as of 2012, there are no water quality problems within 1 mile downstream of the Site. 2017 D. R. STRONG Consulting Engineers Inc. Page 20 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 5 CITY OF RENTON TOPOGRAPHY MAP Site 2017 D. R. STRONG Consulting Engineers Inc. Page 21 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 6 CITY OF RENTON COAL MINE HAZARD AREAS MAP Site 2017 D. R. STRONG Consulting Engineers Inc. Page 22 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 7 CITY OF RENTON FLOOD HAZARDS MAP Site 2017 D. R. STRONG Consulting Engineers Inc. Page 23 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 8 CITY OF RENTON STREAMS AND WETLANDS MAP Site 2017 D. R. STRONG Consulting Engineers Inc. Page 24 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 9 CITY OF RENTON LANDSLIDE HAZARDS MAP Site 2017 D. R. STRONG Consulting Engineers Inc. Page 25 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 10 CITY OF RENTON SEISMIC HAZARD AREAS MAP Site 2017 D. R. STRONG Consulting Engineers Inc. Page 26 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 11 FEMA MAP Site (Approximate) 2017 D. R. STRONG Consulting Engineers Inc. Page 27 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 12 KING COUNTY IMAP DRAINAGE COMPLAINTS MAP Location Complaint Year Description A X 2007 Letter regarding private channel. A C 2008 Water flows even w/ no rain lately. Inv found contractor working in Evendell pond. Still under DDES. A FCR 2007 No site visit needed. Discussed concerns about existing & proposed storm systems. Site A A 2017 D. R. STRONG Consulting Engineers Inc. Page 28 Cedars at the Highlands Level One Downstream Analysis Renton, Washington TASK 3: FIELD INSPECTION UPSTREAM TRIBUTARY AREA Upon evaluation of the upstream area through examining COR topographic map (see Figure 5) and by conducting field reconnaissance on January 25th, 2017, there is a moderate upstream tributary area draining onto the Site in two locations. The first is the existing Type Ns stream flowing through the northeast area of the Site. This area is located in a critical areas tract to remain undisturbed and does not impact nor is it impacted by the developable area of the Project. The second upstream runoff location is the frontage swale on 160th Ave SE. This swale collects runoff from northern parcels and drains onto the Site. This stormwater sheet and channel flows through the Site and discharges near the southeast of the Site. The proposal includes a bypass line that will convey the stormwater draining onto the site from the frontage swale to its natural discharge point located in the southeast of the Site. The parcels to the south and east sheet flow to the south, away from the Site. GENERAL ONSITE AND OFFSITE DRAINAGE DESCRIPTIONS The Site is sloped to the east at slope s ranging from 0-20 percent. A ridge exists in the northeastern portion of the Site, resulting in two distinct Natural Drainage Area’s (NDA’s), which converge within ¼ mile. Therefore, the Site is encompassed within a single Threshold Discharge Area (TDA). NDA 1 is a tributary area to a Type Ns Stream which flows through the northeast corner of the Site. This unnamed stream flows through adjacent parcels to the east before discharging into a retention pond south of the Site where runoff converges with NDA 2. This stream and tributary area will remain undisturbed NDA 2 contains the majority of the Site’s stormwater runoff. The Site’s stormwater sheet flows in a southeasterly direction before being collected by a private stormwater CPP that conveys runoff to a swale located off property at the southeast corner of the Site. This swale conveys runoff to a detention pond which discharges to the existing conveyance system located in SE 144th Street. Runoff continues west through the conveyance system in SE 144th Street and then crosses underneath 154th Place SE where it outlets to Stewart Creek, a Class 3 stream. Stewart Creek outlets to Cedar River approximately 6,000 feet from the Site. 2017 D. R. STRONG Consulting Engineers Inc. Page 29 Cedars at the Highlands Level One Downstream Analysis Renton, Washington TASK 4: DRAINAGE SYSTEM DESCRIPTION AND PROBLEM DESCRIPTIONS DRAINAGE SYSTEM DESCRIPTION The downstream analysis is further illustrated and detailed in the Dow nstream Map Figure 13 and Downstream Table Figure 14. The drainage area is located within the Lower Cedar River Drainage Basin. The drainage area was evaluated by reviewing available resources described in Task 2, and by conducting a field reconnaissance on January 25th, 2017 under overcast conditions. DOWNSTREAM PATH NDA 1 “A1” is a Natural Discharge Area (NDA) for the Site. It is located along the eastern property line (±0). From Point “A1” to Point “B1”, runoff continues to flow east as channel flow via an unnamed Type Ns Stream. Moderate flow was observed (±0’-927’). Point “B1” is the inlet of an 18” diameter CMP culvert. (±927’). From Point “B1” to Point “C1”, runoff is conveyed in a southwesterly direction as pipe flow via an 18” CMP culvert. Moderate flow was observed (±927’-960’). Point “C1” is the outlet of a 18” CMP culvert. Moderate flow was observed (±960’). Point “C1” is the convergence point of NDA 1 and NDA 2. DOWNSTREAM PATH NDA 2 Point “A2” is a NDA for the Site. It is channel flow located on the southern property line. (±0). From Point “A2” to Point “B2”, runoff is conveyed easterly as channel flow via a 3’ tall and 2’ wide swale. Moderate flow observed (±0’-60’). Point “B2” is the inlet of a 24” diameter CPP. Moderate flow observed. (±60’). From Point “B2” to Point “C2”, runoff is conveyed in an easterly direction as pipe flow via 24” CPP. Moderate flow was observed (±60’-142’). Point “C2” is the outlet of a 24” diameter CPP. Moderate flow observed. (±142’). From Point “C2” to Point “D2”, runoff is conveyed in a southerly direction as channel flow via a 6’ tall and 4’ wide swale. Moderate flow was observed. (±142’-269’). Point “D2” is the inlet of a 48” diameter CMP. Moderate flow observed. (±269’). From Point “D2” to Point “E2”, runoff is conveyed in a southerly direction as pipe flow via a 48” diameter CMP. Moderate flow was observed (±269’-277’). Point “E2” is the outlet of a 48” diameter CMP. Moderate flow observed. (±277’). From Point “E2” to Point “F2”, runoff is conveyed in a southerly direction as channel flow via a 9’ tall and 6’ wide swale. Moderate flow was observed (±277’-507’). Point “F2” is the inlet of a 18” diameter CPP. Moderate flow was observed. (±507’). 2017 D. R. STRONG Consulting Engineers Inc. Page 30 Cedars at the Highlands Level One Downstream Analysis Renton, Washington From Point “F2” to Point “G2”, runoff is conveyed in a southerly direction as pipe flow via a 18” diameter CPP culvert. Moderate flow was observed (±507’-515’). Point “G2” is the outlet of a 18” CPP. Moderate flow was observed. (±515’). From Point “G2” to Point “H2”, runoff is conveyed in a southerly direction as channel flow via a 1’ tall and 2’ wide channel. Moderate flow was observed (±515’-663’). Point “H2” is an existing detention pond. (±663’). From Point “H2” to Point “I2”, runoff is conveyed in a southerly direction as surface flow via an existing detention pond. Standing water was observed (±663’-779’). Point “I2”, is the inlet of a 18” CMP. Moderate flow was observed. (±779’). From Point “I2” to Point “J2”, runoff is conveyed in a southerly direction as pipe flow via a 18” diameter CMP. Moderate flow was observed. (±779’-839’). Point “J2” is a Type 1 catch basin. COR Facility ID No. 144177. (±839’). From Point “J2” to Point “K2”, runoff is conveyed in a westerly direction as pipe flow via an 18” CMP. Moderate flow was observed. (±839’-949’). Point “K2” is a Type 1 catch basin. COR Facility ID No. 501597. CB contained inlet protection from neighboring construction activities. Flow direction was confirmed via construction projects TIR and COR Maps. (±949’). From Point “K2” to Point “L2”, runoff is conveyed in a westerly direction via an 18” CMP. (±949’-1,232’). Point “L2” is a Type 1 catch basin. COR Facility ID No. 501598. CB contained inlet protection from neighboring construction activities. Flow direction was confirmed via construction projects TIR and COR Maps. (±1,232’). From Point “L2” to “M2”, runoff is conveyed in a westerly direction via an 18” CMP. (±1,232’-1,486’) Point “M2” is a Type 1 catch basin. COR Facility ID No. 501599. CB contained inlet protection from neighboring construction activities. Flow direction was confirmed via construction projects TIR and COR Maps. (±1,486’). This is the end of field investigation. TASK 5: MITIGATION OF EXISTING OR POTENTIAL PROBLEMS A review of the King County Water and Land Resources Division – Drainage Services Section Documented Drainage Complaints within one mile of the downstream flow paths revealed three complaints within the last ten years that have since been closed. These complaints can be seen in Figure 12. Several drainage complaints exist within the downstream path but are not within the last ten years and are not applicable for Level One Downstream Analysis. 2017 D. R. STRONG Consulting Engineers Inc. Page 31 Cedars at the Highlands Level One Downstream Analysis Renton, Washington Complaint Parcel Number Summary Recurring Type Required Mitigation 2007-0244 7253700130 Letter regarding a private channel. Yes 1 None 2008-0339 7253700130 Water flows even w/ no rain lately. Inv found contractor working in Evendell pond. Still under DDES. No 1 Complaint has been closed. None 2007-0726 7253700130 No site visit needed. Discussed concerns about existing & proposed storm systems. Yes Related to complaint 2007- 0244 1 None Evaluation of these complaints concludes that all drainage complaints from this parcel are regarding the privately maintained channel that is the NDL of the proposed Site. Per Table 1.2.3.A, Summary of Flow Control Performance Criteria Acceptable for Impact Mitigation, No additional flow control or other mitigation is needed for Type 1 Drainage Problems for Flow Control Duration Standard Matching Condition Areas. Despite no required mitigation, the improvements p roposed by the Project will mitigate the conveyance system nuisance problem by replacing all existing conveyance systems will a system of catch basins and pipes conforming to Core Requirement 4, Conveyance Analysis. The project should not create any problems as specified in Section 1.2.2.1 of the Manual and therefore is not required to provide Drainage Problem Impact Mitigation subject to the requirements of Section 1.2.2.2. A detention vault will provide flow control and basic water quality requirements will be met through the use of a Contech Stormfilter. During construction, standard sediment and erosion control methods will be utilized. This will include the use of a stabilized construction entrance, perimeter silt fencing, and other necessary measures to minimize soil erosion during construction. 2017 D. R. STRONG Consulting Engineers Inc. Page 32 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 13 OFFSITE ANALYSIS DOWNSTREAM MAP GRAPHIC SCALE01002004001 INCH = 200 FT. 2017 D. R. STRONG Consulting Engineers Inc. Page 33 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 14 OFFSITE ANALYSIS DOWNSTREAM TABLE DOWNSTREAM PATH NDA 1 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 mile = 1,320 feet 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. A1 Natural discharge area Runoff exits at the NDA along the eastern property line of the Site. 0’ None Observed None Anticipated Moderate flow observed A1-B1 East and south Stream Unnamed Type Ns Stream None Observed None Anticipated Moderate flow observed. Majority of stream not investigated due to being located on private property B1 Pipe inlet 18” CMP Culvert 927’ None Observed None Anticipated Moderate flow observed B1-C1 Westerly pipe flow 18” CMP Culvert None Observed None Anticipated Moderate flow observed C1 Pipe outlet 18” CMP Culvert ±960’ None Observed None Anticipated Moderate flow observed. This is the convergence point of NDA 1 and NDA 2. 2017 D. R. STRONG Consulting Engineers Inc. Page 34 Cedars at the Highlands Level One Downstream Analysis Renton, Washington DOWNSTREAM PATH NDA 2 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 mile = 1,320 feet 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. A2 Natural discharge area – Channel Runoff exits Site across the southern property line as easterly channel flow 0’ None Observed None Anticipated Moderate flow observed A2-B2 Easterly channel flow 3’ tall 2’ wide channel. None Observed None Anticipated Moderate flow observed B2 Pipe inlet 24” diameter CPP 60’ None Observed None Anticipated Moderate flow observed B2-C2 Easterly pipe flow 24” diameter CPP None Observed None Anticipated Moderate flow observed C2 Pipe outlet 24” diameter CPP ±142’ None Observed None Anticipated Moderate flow observed C2-D2 Southerly channel flow 6’ tall, 4’ wide swale None Observed None Anticipated Moderate flow observed D2 Pipe inlet 48” diameter CMP ±269’ None Observed None Anticipated Moderate flow observed D2-E2 Southerly pipe flow 48” diameter CMP None Observed None Anticipated Moderate flow observed E2 Pipe outlet 48” diameter CMP ±277’ None Observed None Anticipated Moderate flow observed E2-F2 Southerly channel flow 9’ tall, 6’ wide swale None Observed None Anticipated Moderate flow observed F2 Pipe inlet 18” diameter CPP ±507’ None Observed None Anticipated Moderate flow observed F2-G2 Southerly pipe flow 18” diameter CPP None Observed None Anticipated Moderate flow observed G2 Pipe outlet 18” diameter CPP ±515’ None Observed None Anticipated Moderate flow observed G2-H2 Southerly channel flow 1’ tall, 2’ wide channel None Observed None Anticipated Moderate flow observed 2017 D. R. STRONG Consulting Engineers Inc. Page 35 Cedars at the Highlands Level One Downstream Analysis Renton, Washington H2 Detention pond Existing detention pond ±663’ None Observed None Anticipated Moderate flow observed H2-I2 Southerly surface flow Existing detention pond None Observed None Anticipated Standing Water I2 Pipe inlet 18” CMP ±779 None Observed None Anticipated Moderate flow observed I2-J2 Southerly pipe flow 18” CMP None Observed None Anticipated Moderate flow observed J2 Type 1 CB. COR Facility No. 144177 18” CMP inlet from North, 18” CMP Inlet from East, 18” CMP outlet to West. ±839’ None Observed None Anticipated Moderate flow observed J2-K2 Westerly pipe flow 18” CMP None Observed None Anticipated Moderate flow observed K2 Type 1 CB. COR Facility No. 501597 Not field observable. ±949’ None Observed None Anticipated CB contains inlet protection from nearby construction activity. Flow direction confirmed via construction project’s TIR and COR Maps. K2-L2 Westerly pipe flow None Observed None Anticipated Moderate flow observed L2 Type 1 CB. COR Facility No. 501598 Not field observable. ±1,232’ None Observed None Anticipated CB contains inlet protection from nearby construction activity. Flow direction confirmed via construction project’s TIR and COR Maps. L2-M2 Westerly pipe flow None Observed None Anticipated M2 Type 1 CB. COR Facility No. 501599 Not field observable. ±1,486’ None Observed None Anticipated CB contains inlet protection from nearby construction activity. Flow direction confirmed via construction project’s TIR and COR Maps. End of field investigation. 2017 D. R. STRONG Consulting Engineers Inc. Page 36 Cedars at the Highlands Technical Information Report Renton, Washington SECTION IV FLOW CONTROL, LOW IMPACT DEVELOPMENT (LID) AND WATER QUALITY FACILITY ANALYSIS AND DESIGN EXISTING SITE HYDROLOGY Western Washington Hydrology Model (WWHM) 2012 was used to model runoff from the Site. The Site falls within the City’s Flow Control Duration Standard – Matching Forested area. The Site was modeled as predeveloped forested condtions for target surfaces (see Figure 15). Per Table 3.2.2.A of the 2017 CORSWDM the Soil group for Alderwood is modeled as “Till.” The Project currently proposes to retain two existing structures on site under a 5 -year temporary use permit. The temporary use permit is with the assumption that in 5 years, these structures will be removed and replaced with single family residences. Therefor, the existing site hydrology will be modeled as “Till Forest” in order to account for the future removal of existing structures. Results of the WWHM analysis are included in this section. Modeling Input for the Pre-developed Site PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Flat 4.305 Pervious Total 4.305 Impervious Land Use acre Impervious Total 0 Basin Total 4.305 Modeling Results Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.168177 5 year 0.277267 10 year 0.360079 25 year 0.47581 50 year 0.569667 100 year 0.669808 2017 D. R. STRONG Consulting Engineers Inc. Page 37 Cedars at the Highlands Technical Information Report Renton, Washington FIGURE 15 PREDEVELOPED AREA MAP GRAPHIC SCALE 0 30 60 120 1 INCH = 60 FT. 2017 D. R. STRONG Consulting Engineers Inc. Page 38 Cedars at the Highlands Technical Information Report Renton, Washington DEVELOPED SITE HYDROLOGY Soil Type The soil types are unchanged from predeveloped conditions. Land Cover WWHM was used to model the developed peak runoff from the Site. The portions of the Site within the developable area tributary to the proposed detention facility were modeled as “Till Grass”, and “Impervious” as appropriate. In order to accurately model the developed site hydrology, considerations were made for individual lots. Due to the existing structure occupying Lot 9, it has been modeled with the maximum impervious allowed per zoning code (R-4, 50%). All other lots have been modeled utilizing the proposed small lot BMP requirements; reduced impervious surface credit and basic dispersion. Lot impervious area restrictions vary from 600 to 800 square feet less than the minimum assumed impervious coverage of 4,000 square feet as defined in section 3.2.2.1 of the CORSWDM (Manual). Results of the WWHM analysis are included in this section. 2017 D. R. STRONG Consulting Engineers Inc. Page 39 Cedars at the Highlands Technical Information Report Renton, Washington Area Breakdown Lot Area Disturbed Area Undist Imp by Zoning BMP Reqd Minimum 4,000 s.f. Imp Reduced Imp Basic Dispersion BMP Credit total impervious pervious impervious pervious Lot 1 9,830 0 4,915 983 4,000 3,200 1,400 2,200 9,830 0 0 3,200 6,630 Lot 2 9,049 0 4,525 905 4,000 3,600 700 1,100 9,049 0 0 3,600 5,449 Lot 3 9,021 0 4,511 902 4,000 3,600 700 1,100 9,021 0 0 3,600 5,421 Lot 4 9,022 0 4,511 902 4,000 3,600 700 1,100 9,022 0 0 3,600 5,422 Lot 5 9,006 0 4,503 901 4,000 3,600 700 1,100 9,006 0 0 3,600 5,406 Lot 6 9,000 0 4,500 900 4,000 3,200 700 1,500 9,000 0 0 3,200 5,800 Lot 7 9,260 0 4,630 926 4,000 3,200 700 1,500 9,260 0 0 3,200 6,060 Lot 8 9,027 0 4,514 903 4,000 3,200 700 1,500 9,027 0 0 3,200 5,827 Lot 9 16,140 0 8,070 3,228 4,000 3,600 700 1,100 16,140 0 0 3,600 12,540 Lot 10 16,781 0 8,391 3,356 4,000 0 16,781 0 0 8,391 8,391 Lot 11 9,398 0 4,699 940 4,000 3,600 700 1,100 9,398 0 0 3,600 5,798 Lot 12 9,101 0 4,551 910 4,000 3,600 700 1,100 9,101 0 0 3,600 5,501 Lot 13 9,474 0 4,737 947 4,000 3,200 1,400 2,200 9,474 0 0 3,200 6,274 Lot 14 9,047 0 4,524 905 4,000 3,600 700 1,100 9,047 0 0 3,600 5,447 Total Lots 143,156 0 71,578 17,608 56,000 44,800 10,500 17,700 143,156 0 0 53,191 89,966 TRACT A 8,361 0 8,361 0 0 4,181 4,181 TRACT B 11,751 11,751 0 0 0 0 0 TRACT C 3,500 3500 0 0 0 0 0 Total Tracts 23,612 15,251 8,361 0 0 4,181 4,181 Total Lots & Tracts 166,768 151,517 0 0 57,371 94,146 ROW On Site ROW 28,777 0 28,777 0 0 23,022 5,755 Frontage ROW 5,253 0 5,253 0 0 4,202 1,051 Target Area - Bypass 1,957 0 1,957 0 0 1,957 0 Non-Target - Trade 1,957 0 1,957 0 0 1,957 0 Total ROW 35,987 35,987 0 0 29,181 6,806 BYPASS LOT TO DETENTIONBMP 2017 D. R. STRONG Consulting Engineers Inc. Page 40 Cedars at the Highlands Technical Information Report Renton, Washington Modeling Input MITIGATED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat 2.318 Pervious Total 2.318 Impervious Land Use acre ROADS FLAT 0.67 ROOF TOPS FLAT 1.317 Impervious Total 1.987 Basin Total 4.305 Modeling Results Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 1.173071 5 year 1.585458 10 year 1.880598 25 year 2.27942 50 year 2.595815 100 year 2.929138 ___________________________________________________________________ 2017 D. R. STRONG Consulting Engineers Inc. Page 41 Cedars at the Highlands Technical Information Report Renton, Washington FIGURE 16 DEVELOPED AREA MAP GRAPHIC SCALE 0 30 60 120 1 INCH = 60 FT. 2017 D. R. STRONG Consulting Engineers Inc. Page 42 Cedars at the Highlands Technical Information Report Renton, Washington PERFORMANCE STANDARDS The Site is required to adhere to Flow Control Duration Standard – Matching Forested site conditions of the existing site. A detention vault preceding a Stormfilter will provide flow control and basic water quality treatment. The Project is required to “match the flow duration of pre-developed rates for forested (historic) site conditions over th e range of flows extending from 50% of 2-year up to the full 50-year flow and matches peaks for the 2- and 10-year return periods.” (CORSWDM, Sec. 1.2.3.1). Detention vault and Stormfilter details will be provided at the time of final engineering. Per Section 1.2.9.1 of the CORSWDM, the project is required to supplement the flow mitigation provided by required flow control facilities by use of Best Management Practices (BMPs). Section 1.2.9.2 outlines individual lot BMP requirements and the preferred BMPs to be utilized to achieve flow mitigation. Lots that are 11,000 s.f. or less are required to mitigate 10% of the total lot size of impervious area, and lots that are greater than 11,000 s.f. are required to mitigate impervious area equal to 20% of the total lot size. Small lot BMPs were determined by the BMP requirements outlin ed in Section 1.2.9.2.1 of the CORSWDM. As required by the CORSWDM, small lot BMP requirements were analyzed in the order of preference listed in section 1.2.9.2.1. 1. Full Dispersion: There is an insufficient amount of native growth in order to utilize full dispersion. The required undisturbed area and flowpath lengths result in full dispersion for any of the lots to be infeasible. 2. Full Infiltration: Per geotechnical evaluation, the existing soil on the Site makes full infiltration infeasible. The full geotechnical report has been submitted in conjunction with the preliminary application. 3. Limited Infiltration: Per geotechnical evaluation, the existing soil on the Site makes any form of infiltration infeasible. The full geotechnical report has been submitted in conjunction with the preliminary application. 4. Rain Gardens: Per geotechnical evaluation, the existing soil on the Site makes any form of infiltration infeasible. The full geotechnical report has been submitted in conjunction with the preliminary application. 5. Bioretention: Per geotechnical evaluation, the existing soil on the Site makes any form of infiltration infeasible. 6. Permeable Pavement: Per geotechnical evaluation, the existing soil on the Site makes any form of infiltration infeasible. 7. Basic Dispersion: A combination of restricted impervious area and basic dispersion via splash blocks is being proposed in order to meet the BMP requirements of small lots. Per the CORSWDM, any lots utilizing Basic Dispersion require a supplemen tal BMP to be implemented to achieve compliance. In addition to splash blocks, the Project is proposing to implement Reduced Impervious Surface Credits to meet flow control BMP requirements. The amount of reduced impervious surface per lot varies; the Area Breakdown table included in this section shows individual requirements for each lot. 2017 D. R. STRONG Consulting Engineers Inc. Page 43 Cedars at the Highlands Technical Information Report Renton, Washington FLOW CONTROL SYSTEM The Project will utilize an detention facility designed to control site runoff. The Western Washington Hydrology Modeling (WWHM) software was used to size the facility. The detention vault design information is shown here. WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: PRELIM vault Site Name: Cedars at the Highlands Site Address: 14120 160th Ave SE City : Renton Report Date: 3/15/2017 Gage : Seatac Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.17 Version Date: 2016/11/18 Version : 4.2.13 ___________________________________________________________________ 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 4.305 Pervious Total 4.305 Impervious Land Use acre Impervious Total 0 Basin Total 4.305 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ 2017 D. R. STRONG Consulting Engineers Inc. Page 44 Cedars at the Highlands Technical Information Report Renton, Washington MITIGATED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat 2.318 Pervious Total 2.318 Impervious Land Use acre ROADS FLAT 0.67 ROOF TOPS FLAT 1.317 Impervious Total 1.987 Basin Total 4.305 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Vault 1 Vault 1 ___________________________________________________________________ Name : Vault 1 Width : 71.3903997634634 ft. Length : 71.3903997634634 ft. Depth: 12 ft. Discharge Structure Riser Height: 11 ft. Riser Diameter: 18 in. Orifice 1 Diameter: 1.15 in. Elevation: 0 ft. Orifice 2 Diameter: 2.01 in. Elevation: 7.857 ft. Orifice 3 Diameter: 1.22 in. Elevation: 8.77 ft. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Vault Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 0.0000 0.117 0.000 0.000 0.000 0.1333 0.117 0.015 0.013 0.000 0.2667 0.117 0.031 0.018 0.000 0.4000 0.117 0.046 0.022 0.000 0.5333 0.117 0.062 0.026 0.000 0.6667 0.117 0.078 0.029 0.000 0.8000 0.117 0.093 0.032 0.000 0.9333 0.117 0.109 0.034 0.000 2017 D. R. STRONG Consulting Engineers Inc. Page 45 Cedars at the Highlands Technical Information Report Renton, Washington 1.0667 0.117 0.124 0.037 0.000 1.2000 0.117 0.140 0.039 0.000 1.3333 0.117 0.156 0.041 0.000 1.4667 0.117 0.171 0.043 0.000 1.6000 0.117 0.187 0.045 0.000 1.7333 0.117 0.202 0.047 0.000 1.8667 0.117 0.218 0.049 0.000 2.0000 0.117 0.234 0.050 0.000 2.1333 0.117 0.249 0.052 0.000 2.2667 0.117 0.265 0.054 0.000 2.4000 0.117 0.280 0.055 0.000 2.5333 0.117 0.296 0.057 0.000 2.6667 0.117 0.312 0.058 0.000 2.8000 0.117 0.327 0.060 0.000 2.9333 0.117 0.343 0.061 0.000 3.0667 0.117 0.358 0.062 0.000 3.2000 0.117 0.374 0.064 0.000 3.3333 0.117 0.390 0.065 0.000 3.4667 0.117 0.405 0.066 0.000 3.6000 0.117 0.421 0.068 0.000 3.7333 0.117 0.436 0.069 0.000 3.8667 0.117 0.452 0.070 0.000 4.0000 0.117 0.468 0.071 0.000 4.1333 0.117 0.483 0.073 0.000 4.2667 0.117 0.499 0.074 0.000 4.4000 0.117 0.514 0.075 0.000 4.5333 0.117 0.530 0.076 0.000 4.6667 0.117 0.546 0.077 0.000 4.8000 0.117 0.561 0.078 0.000 4.9333 0.117 0.577 0.079 0.000 5.0667 0.117 0.592 0.080 0.000 5.2000 0.117 0.608 0.081 0.000 5.3333 0.117 0.624 0.082 0.000 5.4667 0.117 0.639 0.083 0.000 5.6000 0.117 0.655 0.084 0.000 5.7333 0.117 0.670 0.085 0.000 5.8667 0.117 0.686 0.086 0.000 6.0000 0.117 0.702 0.087 0.000 6.1333 0.117 0.717 0.088 0.000 6.2667 0.117 0.733 0.089 0.000 6.4000 0.117 0.748 0.090 0.000 6.5333 0.117 0.764 0.091 0.000 6.6667 0.117 0.780 0.092 0.000 6.8000 0.117 0.795 0.093 0.000 6.9333 0.117 0.811 0.094 0.000 7.0667 0.117 0.826 0.095 0.000 7.2000 0.117 0.842 0.096 0.000 7.3333 0.117 0.858 0.097 0.000 7.4667 0.117 0.873 0.098 0.000 7.6000 0.117 0.889 0.098 0.000 7.7333 0.117 0.904 0.099 0.000 7.8667 0.117 0.920 0.111 0.000 8.0000 0.117 0.936 0.143 0.000 8.1333 0.117 0.951 0.160 0.000 8.2667 0.117 0.967 0.173 0.000 8.4000 0.117 0.982 0.184 0.000 8.5333 0.117 0.998 0.195 0.000 2017 D. R. STRONG Consulting Engineers Inc. Page 46 Cedars at the Highlands Technical Information Report Renton, Washington 8.6667 0.117 1.014 0.204 0.000 8.8000 0.117 1.029 0.219 0.000 8.9333 0.117 1.045 0.237 0.000 9.0667 0.117 1.060 0.250 0.000 9.2000 0.117 1.076 0.262 0.000 9.3333 0.117 1.092 0.273 0.000 9.4667 0.117 1.107 0.283 0.000 9.6000 0.117 1.123 0.292 0.000 9.7333 0.117 1.138 0.301 0.000 9.8667 0.117 1.154 0.310 0.000 10.000 0.117 1.170 0.318 0.000 10.133 0.117 1.185 0.326 0.000 10.267 0.117 1.201 0.334 0.000 10.400 0.117 1.216 0.342 0.000 10.533 0.117 1.232 0.349 0.000 10.667 0.117 1.248 0.356 0.000 10.800 0.117 1.263 0.363 0.000 10.933 0.117 1.279 0.370 0.000 11.067 0.117 1.294 0.650 0.000 11.200 0.117 1.310 1.788 0.000 11.333 0.117 1.326 3.272 0.000 11.467 0.117 1.341 4.722 0.000 11.600 0.117 1.357 5.803 0.000 11.733 0.117 1.372 6.422 0.000 11.867 0.117 1.388 7.011 0.000 12.000 0.117 1.404 7.506 0.000 12.133 0.117 1.419 7.970 0.000 12.267 0.000 0.000 8.407 0.000 ___________________________________________________________________ ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:4.305 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:2.318 Total Impervious Area:1.987 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.168177 5 year 0.277267 10 year 0.360079 25 year 0.47581 50 year 0.569667 100 year 0.669808 2017 D. R. STRONG Consulting Engineers Inc. Page 47 Cedars at the Highlands Technical Information Report Renton, Washington Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.098487 5 year 0.158016 10 year 0.209782 25 year 0.292013 50 year 0.367418 100 year 0.456729 ___________________________________________________________________ Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.206 0.071 1950 0.240 0.092 1951 0.346 0.300 1952 0.116 0.064 1953 0.098 0.080 1954 0.141 0.084 1955 0.218 0.083 1956 0.178 0.096 1957 0.161 0.084 1958 0.163 0.088 1959 0.138 0.073 1960 0.268 0.243 1961 0.134 0.091 1962 0.091 0.062 1963 0.131 0.085 1964 0.165 0.084 1965 0.123 0.096 1966 0.109 0.079 1967 0.256 0.087 1968 0.149 0.079 1969 0.144 0.075 1970 0.121 0.079 1971 0.153 0.088 1972 0.264 0.204 1973 0.125 0.096 1974 0.141 0.088 1975 0.200 0.079 1976 0.143 0.085 1977 0.049 0.064 1978 0.120 0.089 1979 0.074 0.061 1980 0.376 0.263 1981 0.110 0.081 1982 0.261 0.148 1983 0.180 0.087 1984 0.113 0.069 1985 0.066 0.073 1986 0.281 0.097 1987 0.258 0.176 1988 0.111 0.069 1989 0.070 0.071 1990 0.720 0.271 1991 0.324 0.218 1992 0.135 0.091 2017 D. R. STRONG Consulting Engineers Inc. Page 48 Cedars at the Highlands Technical Information Report Renton, Washington 1993 0.131 0.069 1994 0.053 0.059 1995 0.175 0.090 1996 0.411 0.309 1997 0.325 0.288 1998 0.116 0.070 1999 0.449 0.198 2000 0.125 0.090 2001 0.033 0.054 2002 0.175 0.116 2003 0.239 0.078 2004 0.277 0.288 2005 0.182 0.086 2006 0.189 0.095 2007 0.533 0.464 2008 0.566 0.312 2009 0.255 0.098 ___________________________________________________________________ Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.7203 0.4636 2 0.5661 0.3124 3 0.5327 0.3091 4 0.4486 0.2999 5 0.4107 0.2877 6 0.3763 0.2876 7 0.3458 0.2708 8 0.3247 0.2626 9 0.3243 0.2429 10 0.2812 0.2177 11 0.2775 0.2040 12 0.2684 0.1981 13 0.2641 0.1763 14 0.2612 0.1478 15 0.2584 0.1159 16 0.2555 0.0982 17 0.2549 0.0970 18 0.2404 0.0965 19 0.2394 0.0963 20 0.2182 0.0960 21 0.2058 0.0953 22 0.2003 0.0918 23 0.1889 0.0911 24 0.1823 0.0911 25 0.1795 0.0899 26 0.1778 0.0897 27 0.1748 0.0892 28 0.1746 0.0877 29 0.1647 0.0877 30 0.1629 0.0875 31 0.1606 0.0873 32 0.1533 0.0871 33 0.1494 0.0860 34 0.1442 0.0847 35 0.1425 0.0845 2017 D. R. STRONG Consulting Engineers Inc. Page 49 Cedars at the Highlands Technical Information Report Renton, Washington 36 0.1414 0.0839 37 0.1405 0.0837 38 0.1384 0.0836 39 0.1355 0.0831 40 0.1342 0.0806 41 0.1314 0.0800 42 0.1309 0.0792 43 0.1253 0.0792 44 0.1247 0.0791 45 0.1227 0.0785 46 0.1210 0.0781 47 0.1197 0.0751 48 0.1158 0.0733 49 0.1156 0.0729 50 0.1134 0.0711 51 0.1111 0.0705 52 0.1097 0.0698 53 0.1089 0.0692 54 0.0977 0.0692 55 0.0910 0.0691 56 0.0745 0.0639 57 0.0695 0.0638 58 0.0664 0.0620 59 0.0535 0.0609 60 0.0491 0.0593 61 0.0326 0.0544 ___________________________________________________________________ Stream Protection Duration POC #1 The Facility PASSED The Facility PASSED. Flow(cfs) Predev Mit Percentage Pass/Fail 0.0841 18482 17992 97 Pass 0.0890 16525 12666 76 Pass 0.0939 14611 8425 57 Pass 0.0988 12949 4571 35 Pass 0.1037 11375 3371 29 Pass 0.1086 10031 3063 30 Pass 0.1135 8949 2843 31 Pass 0.1184 7914 2733 34 Pass 0.1233 7045 2642 37 Pass 0.1282 6320 2541 40 Pass 0.1331 5694 2449 43 Pass 0.1380 5174 2376 45 Pass 0.1429 4770 2321 48 Pass 0.1479 4348 2203 50 Pass 0.1528 3963 2064 52 Pass 0.1577 3570 1952 54 Pass 0.1626 3225 1870 57 Pass 0.1675 2898 1775 61 Pass 0.1724 2620 1683 64 Pass 0.1773 2363 1575 66 Pass 0.1822 2147 1490 69 Pass 0.1871 1954 1382 70 Pass 2017 D. R. STRONG Consulting Engineers Inc. Page 50 Cedars at the Highlands Technical Information Report Renton, Washington 0.1920 1819 1283 70 Pass 0.1969 1691 1161 68 Pass 0.2018 1538 1068 69 Pass 0.2067 1365 979 71 Pass 0.2116 1237 921 74 Pass 0.2165 1136 849 74 Pass 0.2214 1047 789 75 Pass 0.2263 974 753 77 Pass 0.2312 910 715 78 Pass 0.2361 839 683 81 Pass 0.2410 766 634 82 Pass 0.2459 717 587 81 Pass 0.2509 645 550 85 Pass 0.2558 579 500 86 Pass 0.2607 496 458 92 Pass 0.2656 432 415 96 Pass 0.2705 379 369 97 Pass 0.2754 343 326 95 Pass 0.2803 310 282 90 Pass 0.2852 271 234 86 Pass 0.2901 235 187 79 Pass 0.2950 205 170 82 Pass 0.2999 177 143 80 Pass 0.3048 149 121 81 Pass 0.3097 125 97 77 Pass 0.3146 110 78 70 Pass 0.3195 97 72 74 Pass 0.3244 85 66 77 Pass 0.3293 71 62 87 Pass 0.3342 62 56 90 Pass 0.3391 55 52 94 Pass 0.3440 45 47 104 Pass 0.3489 41 44 107 Pass 0.3539 37 39 105 Pass 0.3588 35 34 97 Pass 0.3637 29 22 75 Pass 0.3686 25 16 64 Pass 0.3735 22 11 50 Pass 0.3784 18 10 55 Pass 0.3833 15 10 66 Pass 0.3882 11 9 81 Pass 0.3931 9 9 100 Pass 0.3980 9 9 100 Pass 0.4029 8 6 75 Pass 0.4078 8 5 62 Pass 0.4127 7 4 57 Pass 0.4176 7 4 57 Pass 0.4225 7 4 57 Pass 0.4274 7 4 57 Pass 0.4323 7 4 57 Pass 0.4372 7 3 42 Pass 0.4421 7 3 42 Pass 0.4470 6 3 50 Pass 0.4520 5 2 40 Pass 0.4569 5 2 40 Pass 0.4618 5 1 20 Pass 0.4667 4 0 0 Pass 2017 D. R. STRONG Consulting Engineers Inc. Page 51 Cedars at the Highlands Technical Information Report Renton, Washington 0.4716 4 0 0 Pass 0.4765 4 0 0 Pass 0.4814 4 0 0 Pass 0.4863 4 0 0 Pass 0.4912 4 0 0 Pass 0.4961 4 0 0 Pass 0.5010 4 0 0 Pass 0.5059 4 0 0 Pass 0.5108 4 0 0 Pass 0.5157 3 0 0 Pass 0.5206 3 0 0 Pass 0.5255 3 0 0 Pass 0.5304 3 0 0 Pass 0.5353 2 0 0 Pass 0.5402 2 0 0 Pass 0.5451 2 0 0 Pass 0.5500 2 0 0 Pass 0.5550 2 0 0 Pass 0.5599 2 0 0 Pass 0.5648 2 0 0 Pass 0.5697 1 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 Vault 1 POC N 563.71 N 0.00 Total Volume Infiltrated 563.71 0.00 0.00 0.00 0.00 0% No Treat. Credit Compliance with LID Standard 8 Duration Analysis Result = Passed ___________________________________________________________________ POC #2 was not reported because POC must exist in both scenarios and both scenarios must have been run. POC #3 was not reported because POC must exist in both scenarios and both scenarios must have been run. POC #4 was not reported because POC must exist in both scenarios and both scenarios must have been run.Perlnd and Implnd Changes No changes have been made. ___________________________________________________________________ 2017 D. R. STRONG Consulting Engineers Inc. Page 52 Cedars at the Highlands Technical Information Report Renton, Washington 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 2017 D. R. STRONG Consulting Engineers Inc. Page 53 Cedars at the Highlands Technical Information Report Renton, Washington WATER QUALITY TREATMENT SYSTEM The Project is required to adhere to the City of Renton’s Basic Water Quality treatment criteria. A Stormfilter immediately following the proposed detention vault in Tract A is proposed to accommodate this requirement. This information will be completed at the time of engineering. 2017 D. R. STRONG Consulting Engineers Inc. Page 54 Cedars at the Highlands Technical Information Report Renton, Washington FIGURE 17 DETENTION & WATER QUALITY FACILITY DETAILS (To be completed at time of final engineering) 2017 D. R. STRONG Consulting Engineers Inc. Page 55 Cedars at the Highlands Technical Information Report Renton, Washington SECTION V CONVEYANCE SYSTEM AN ALYSIS AND DESIGN Per Core Requirement #4 of the KCSWDM, the conveyance system must be analyzed and designed for the existing tributary and developed onsite runoff. Pipe systems shall be designed to convey the 25-year storm with a minimum of 6-inches of freeboard between the design water surface and structure grate. Any overflow from the 100-year design storm must not create or aggravate a severe flooding problem . The Rational Method will be used to calculate the Q-Ratio for each pipe node. A conveyance system consisting primarily of pipes and catch basins will be designed for the Project. Onsite runoff will be collected by the multiple catch basins. Pipes are typically six-inch to twelve-inch diameter LCPE material. 2017 D. R. STRONG Consulting Engineers Inc. Page 56 Cedars at the Highlands Technical Information Report Renton, Washington BACKWATER ANALYSIS (To be completed at time of final engineering) 2017 D. R. STRONG Consulting Engineers Inc. Page 57 Cedars at the Highlands Technical Information Report Renton, Washington SECTION VI SPECIAL REPORTS AND STUDIES The following report and studies have been provided with this submittal. Critical Areas Study: Sewell Wetland Consulting, Inc. – July 6, 2016 Subsurface Exploration and Geotechnical Engineering Report: Earth Solutions NW LLC – February 23, 2017 Arborist Report: Greenforest, Inc – March 20, 2016 Trip Generation Letter Report: Traffex – January 19, 2017 2017 D. R. STRONG Consulting Engineers Inc. Page 58 Cedars at the Highlands Technical Information Report Renton, Washington SECTION VII OTHER PERMITS Request modification from RMC 4-6-060. Specifically, the half-street frontage improvements required for 162nd AVE SE. 2017 D. R. STRONG Consulting Engineers Inc. Page 59 Cedars at the Highlands Technical Information Report Renton, Washington SECTION VIII CSWPPP ANALYSIS AND DESIGN (PART A) The Erosion and Sedimentation Control Design will meet the seven minimum King County requirements: 1. Areas to remain undisturbed shall be delineated with a high visibility plastic fence prior to any site clearing or grading. 2. Site disturbed areas shall be covered with mulch and seeded, as appropriate, for temporary or permanent measures. 3. Perimeter protection shall consist of a silt fence down slope of any disturbed areas or stockpiles. 4. A stabilized construction entrance will be located at the point of ingress/egress (i.e. onsite access road). 5. The detention vault will act as a sediment pond for sediment retention. Perimeter silt fences will provide sediment retention within the bypass areas. 6. Surface water from disturbed areas will sheet flow to the sediment pond for treatment. 7. Dust control shall be provided by spraying exposed soils with water until wet. This is required when exposed soils are dry to the point that wind transport is possible which would impact roadways, drainage ways, surface waters, or neighboring residences. SWPPP PLAN DESIGN (PART B) Construction activities that could contribute pollutants to surface and storm wate r include the following, with applicable BMP’s listed for each item: 1. Storage and use of chemicals: Utilize source control, and soil erosion and sedimentation control practices, such as using only recommended amounts of chemical materials applied in the proper manner; neutralizing concrete wash water, and disposing of excess concrete material only in areas prepared for concrete placement, or return to batch plant; disposing of wash -up waters from water-based paints in sanitary sewer; disposing of wastes f rom oil-based paints, solvents, thinners, and mineral spirits only through a licensed waste management firm, or treatment, storage, and disposal (TSD) facility. 2. Material delivery and storage: Locate temporary storage areas away from vehicular traffic, near the construction entrance, and away from storm drains. Material Safety Data Sheets (MSDS) should be supplied for all materials stored, and chemicals kept in their original labeled containers. Maintenance, fueling, and repair of heavy equipment and vehicles shall be conducted using spill prevention and control measures. Contaminated surfaces shall be cleaned immediately following any spill incident. Provide cover, containment, and protection from vandalism for all chemicals, liquid products, petroleum products, and other potentially hazardous materials. 2017 D. R. STRONG Consulting Engineers Inc. Page 60 Cedars at the Highlands Technical Information Report Renton, Washington 3. Building demolition: Protect stormwater drainage system from sediment -laden runoff and loose particles. To the extent possible, use dikes, berms, or other methods to protect overland discharge paths from runoff. Street gutter, sidewalks, driveways, and other paved surfaces in the immediate area of demolition must be swept daily to collect and properly dispose of loose debris and garbage. Spray the minimum amount of water to help control windblown fine particles such as concrete, dust, and paint chips. Avoid excessive spraying so that runoff from the site does not occur, yet dust control is achieved. Oils must never be used for dust control. 4. Sawcutting: Slurry and cuttings shall be vacuumed during the activity to prevent migration offsite and must not remain on permanent concrete or asphalt paving overnight. Collected slurry and cuttings shall be disposed of in a manner that does not violate ground water or surface water quality standards. The complete CSWPPP to be completed at final engineering. 2017 D. R. STRONG Consulting Engineers Inc. Page 61 Cedars at the Highlands Technical Information Report Renton, Washington SECTION IX BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT 1. Bond Quantity Worksheet – will be submitted at final engineering 2. The Stormwater Facility Summary Sheet is included in this section 2016 KING COUNTY SURFACE WATER DESIGN MANUAL, REFERENCE D 4/24/2016 Page 1 STORMWATER FACILITY SUMMARY SHEET DPER Permit No.___________________ (provide one Stormwater Facility Summary Sheet per Natural Discharge Location)Date ___________________ OVERVIEW:NPDES Permit No.___________________ Project Name Parcel No.____________________________ Project Location Retired Parcel No.____________________________ Downstream Drainage Basins:Project includes Landscape Management Plan?yes Major Basin Name ______________________________________________(include copy with TIR as Appendix)no Immediate Basin Name ______________________________________________ GENERAL FACILITY INFORMATION: Leachable Metals Infiltration Impervious Surface Limit Type # of Type # of Type # of facilities Flow Control BMPs Ponds ______ Ponds ______ Ponds ______ Basic Clearing Limit Vaults ______ Tanks ______ Vaults ______ Conservation Drainage Facility Tanks ______ Trenches _____ Tanks ______ Flood Problem Landscape Management Plan If no flow control facility, check one: Project qualifies for KCSWDM Exemption (KCSWDM 1.2.3): Basic Exemption (Applies to Commercial parcels only)Area % of Total Redevelopment projects Cost Exemption for Parcel Redevelopment projects Direct Discharge Exemption Other _____________________ Total impervious surface served by Project qualifies for 0.1 cfs Exception per KCSWDM 1.2.3 flow control facility(ies) (sq ft) Impervious surface served by flow KCSWDM Adjustment No. ___________________control facility(ies) designed 1990 or later (sq ft) approved KCSWDM Adjustment No. __________________ Impervious surface served by Shared Facility Name/Location: _________________________ pervious surface absorption (sq ft) No flow control required (other, provide justification): Impervious surface served by approved ____________________________________________________ water quality facility(ies) (sq ft) Flow Control Performance Std Declarations of Covenant Recording No. Water QualityDetention TREATMENT SUMMARY FOR TOTAL IMPERVIOUS SURFACES ----- Total Impervious Acreage (ac) No flow control required per approved Flow control provided in regional/shared facility per approved PROVIDE FACILITY DETAILS AND FACILITY SKETCH FOR EACH FACILITY ON REVERSE. USE ADDITIONAL SHEETS AS NEEDED FOR ADDITIONAL FACILITIES Impervious Surface Exemption for Transportation Total Acreage (ac) Cedars at the Highlands 14120 160th Avenue SE, Renton, Washington 145750-0110 Lower Cedar River Lower Cedar River 1 04-31-2017 2016 KING COUNTY SURFACE WATER DESIGN MANUAL, REFERENCE D 4/24/2016 Page 2 STORMWATER FACILITY SUMMARY SHEET DPER Permit No.___________________ (provide one Stormwater Facility Summary Sheet per Natural Discharge Location) Project Name Downstream Drainage Basins: Major Basin Name _______________________________ Project Location Immediate Basin Name ___________________________ FLOW CONTROL FACILITY:Basin: Facility Name/Number _______________________________________ New Facility Project Impervious Facility Location ____________________________________________ Existing Facility Acres Served ________ UIC? □ yes □ no UIC Site ID:% of Total Project Impervious cu.ft.Volume Factor Acres Served ________ _____________ ac.ft.____________of Safety _______No. of Lots Served ________ Control Structure location: _______________________________________________ Type of Control Structure:No. of Orifices/Restrictions __________ Riser in vault Size of Orifice/Restriction (in.) No.1 ______ cu.ft. Riser in Type II CB (numbered starting with lowest No.2 ______ ac.ft. Weir in Type II CB orifice): No.3 ______ (inches in decimal format)No.4 ______ WATER QUALITY FACILITIES Design Information Indicate no. of water quality facilities/BMPs for each type:Water Quality design flow (cfs) _______Flow dispersion Water Quality treated volume (sandfilter) (cu.ft.) _______Filter strip Water Quality storage volume (wetpool) (cu.ft.) _______Biofiltration swale regular, wet or Landscape management plan Farm management plan continuous inflow _______Wetvault combined w/detention ______High flow bypass structure (e.g., flow-splitter catch basin) _______Wetpond basic large combined w/detention ______Oil/water separator baffle coalescing plate _______Pre-settling pond ______Storm filter _______Stormwater wetland ______Pre-settling structure (Manufacturer:______________________) _______Sand filter basic large Sand bed depth ______Catch basin inserts (Manufacturer:________________________) regular linear vault (inches)______________Source controls _________________________________________ ● Is facility lined? yes no If so, what marker is used above liner?_____________________________________________________ Facility Summary Sheet Sketch: All detention, infiltration and water quality facilities must include a detailed sketch (11"x17" reduced size plan sheets preferred). Dam Safety Regulations (WA State Dept of Ecology): Reservoir Volume above natural grade Depth of Reservoir above natural grade (ft) Live Storage Volume Live Storage Depth (ft) Cedars at the Highlands 14120 160th Avenue SE, Renton, Washington Lower Cedar River Lower Cedar River TBD TRACT A 55,716 11 Vault 3 1.15 2.01 1.22 1.986 100% .0985 141.0 2017 D. R. STRONG Consulting Engineers Inc. Page 62 Cedars at the Highlands Technical Information Report Renton, Washington SECTION X OPERATIONS AND MAINTENANCE MANUAL The Home Owners Association will be responsible for maintenance of several private catch basins on various lots as depicted on the plans. Maintenance guidelines set forth in the 2017 CORSWDM are to be included at time of engineering. All other drainage elements are to be publicly maintained. 2017 D. R. STRONG Consulting Engineers Inc. Page 63 Cedars at the Highlands Technical Information Report Renton, Washington APPENDICES 2017 D. R. STRONG Consulting Engineers Inc. Page 64 Cedars at the Highlands Technical Information Report Renton, Washington APPENDIX “A” LEGAL DESCRIPTION LOT 6, BLOCK 3, CEDAR PARK FIVE ACRE TRACTS, ACCORDING TO PLAT THEREOF RECORDED IN VOLUME 15 OF PLATS, PAGE 91, RECORDS OF KING COUNTY, WASHINGTON.