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2018 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 June 29, 2018 2018 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 ............................................................................... 52 SECTION V ................................................................................................................... 54 Conveyance System Analysis and Design ................................................................ 54 SECTION VI .................................................................................................................. 56 Special Reports and Studies ..................................................................................... 56 SECTION VII ................................................................................................................. 57 Other Permits ............................................................................................................ 57 SECTION VIII ................................................................................................................ 58 CSWPPP Analysis and Design (Part A) .................................................................... 58 SWPPP Plan Design (Part B) .................................................................................... 58 SECTION IX .................................................................................................................. 60 Bond Quantities, Facility Summaries, and Declaration of Covenant .......................... 60 2018 D. R. STRONG Consulting Engineers Inc. Cedars at the Highlands Technical Information Report Renton, Washington SECTION X ................................................................................................................... 61 Operations and Maintenance Manual ........................................................................ 61 APPENDICES ............................................................................................................... 62 Appendix “A” Legal Description ................................................................................. 63 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 ..................................................... 53 2018 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 13 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 and category III wetland in 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 wetland 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 conveyance 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 13 single–family residential lots (Project), with lot sizes ranging from approximately 9,000 s.f. to 16,911 s.f. The existing workshop on site will remain and will occupy one of the thirteen proposed lots under a 5-year temporary use permit. The proposed retained structure will account for approximately 8,400 s.f. of impervious area. After 5 years, this structure 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. However, the project will meet the City’s Flood Problem Flow Control Standard due to the existing downstream flooding problem within the drainage basin tributary to the SE 144 th Street storm system. This standard matches the developed Site flow durations to the flow durations of pre - 2018 D. R. STRONG Consulting Engineers Inc. Page 5 Cedars at the Highlands Level One Downstream Analysis Renton, Washington developed rates for forested (historical) site conditions from 50% of the 2-year peak flow up to the full 50-year peak flow. Also, developed peak discharge rates shall not exceed predeveloped peak discharge rates for the 2-,10-, and 100-year return periods. The Flood Problem Flow Control Standard and Basic Water Quality requirement 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 A, the 13 new single-family residences and associated driveways, the existing structure to remain, and Tract A, the storm water detention facility, will generate approximately 87,641 s.f. of impervious area (2.012 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 8 and 9 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 alternative 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). 2018 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): July 2, 2018 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 2018 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 Category III 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 2018 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 2018 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 2018 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 2018 D. R. STRONG Consulting Engineers Inc. Page 11 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 2 SITE LOCATION Site 2018 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 0 GRAPHIC SCALE 906030 1 INCH = 60 FT. 2018 D. R. STRONG Consulting Engineers Inc. Page 13 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 4 SOILS 2018 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 2018 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 2018 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 and category III wetland located in the northeast section of the Site. This stream and wetland 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. However, the project will meet the City’s Flood Problem Flow Control Standard due to the existing downstream flooding problem within the drainage basin tributary to the SE 144th Street storm system. 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 match peaks for the 2-, 10-, and 100-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. 2018 D. R. STRONG Consulting Engineers Inc. Page 17 Cedars at the Highlands Level One Downstream Analysis Renton, Washington 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. 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 infiltration evaluation letter dated July 13th, 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. 2018 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 2018 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 thirteen 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. There is a category III wetland located in the northeast corner of the Site. 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. 2018 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 2018 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 2018 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 2018 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 2018 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 2018 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 2018 D. R. STRONG Consulting Engineers Inc. Page 26 Cedars at the Highlands Level One Downstream Analysis Renton, Washington FIGURE 11 FEMA MAP Site (Approximate) 2018 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 2018 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 and category III wetland 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 stormwate r 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. 2018 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’). 2018 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 no t applicable for Level One Downstream Analysis. 2018 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, the project will meet the City’s Flood Problem Flow Control Standard due to the existing downstream flooding problem within the drainage basin tributary to the SE 144th Street storm system. In addition to the increase flow control, the improvements proposed by the Project will mitigate the conveyance system nuisance problem by replacing all existing conveyance systems with 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. 2018 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 SCALE 0 100 200 400 1 INCH = 200 FT. 2018 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. 2018 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 2018 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. 2018 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. However, the project will meet the City’s Flood Problem Flow Control Standard due to the existing downstream flooding problem within the drainage basin tributary to the SE 144th Street storm system. 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 one existing structure on site under a 5-year temporary use permit. The temporary use permit is with the assumption that in 5 years, this structure will be removed and replaced with single family residences. Therefo re, the existing site hydrology will be modeled as “Till Forest” in order to account for the future removal of the existing structure. 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 4.109 Pervious Total 4.109 Impervious Land Use acre Impervious Total 0 Basin Total 4.109 Modeling Results Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.1605 5 year 0.2646 10 year 0.3437 25 year 0.4541 50 year 0.5437 100 year 0.6393 2018 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. 2018 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 400 to 700 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 39 Cedars at the Highlands Technical Information Report Renton, Washington Area Breakdown Lot Area Disturbed LOT Area Undist Imp by Zoning BMP Reqd Minimum 4,000 s.f. Imp Reduced Imp Basic Dispersion BMP Credit total impervious pervious impervious pervious 1 9,830 0 4,915 983 4,000 3,300 700 1,400 9,830 0 0 3,300 6,530 2 9,001 0 4,501 900 4,000 3,400 700 1,300 9,001 0 0 3,400 5,601 3 9,000 0 4,500 900 4,000 3,400 700 1,300 9,000 0 0 3,400 5,600 4 9,001 0 4,501 900 4,000 3,400 700 1,300 9,001 0 0 3,400 5,601 5 9,005 0 4,503 901 4,000 3,400 700 1,300 9,005 0 0 3,400 5,605 6 9,002 0 4,501 900 4,000 3,400 700 1,300 9,002 0 0 3,400 5,602 7 9,279 0 4,640 928 4,000 3,600 700 1,100 9,279 0 0 3,600 5,679 8 12,832 0 6,416 2,566 4,000 3,600 700 1,100 12,832 0 0 3,600 9,232 9 16,911 0 8,456 3,382 4,000 0 16,911 0 0 8,456 8,456 10 9,398 0 4,699 940 4,000 3,400 700 1,300 9,398 0 0 3,400 5,998 11 9,101 0 4,551 910 4,000 3,400 700 1,300 9,101 0 0 3,400 5,701 12 9,474 0 4,737 947 4,000 3,300 1,400 2,100 9,474 0 0 3,300 6,174 13 9,047 0 4,524 905 4,000 3,400 700 1,300 9,047 0 0 3,400 5,647 Total Lots 130,881 0 65,441 16,062 52,000 41,000 9,100 16,100 130,881 0 0 49,456 81,426 TRACT A 10,037 0 10,037 0 0 7,026 3,011 TRACT B 20,248 20,248 0 0 0 0 0 TRACT C 3,500 3500 0 0 0 0 0 Total Tracts 33,785 23,748 10,037 0 0 7,026 3,011 Total Lots & Tracts 164,666 140,918 0 0 56,481 84,437 ROW On Site ROW 30,866 0 30,866 0 0 25,000 5,866 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 38,076 38,076 0 0 31,159 6,917 BYPASS LOT TO DETENTIONBMP 2018 D. R. STRONG Consulting Engineers Inc. Page 40 Cedars at the Highlands Technical Information Report Renton, Washington Modeling Input MITIGATED LAND USE Name : RDIN Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat 2.097 Pervious Total 2.097 Impervious Land Use acre ROADS FLAT 0.715 ROOF TOPS FLAT 1.09 DRIVEWAYS FLAT 0.207 Impervious Total 2.012 Basin Total 4.109 Modeling Results Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 1.1574 5 year 1.5532 10 year 1.8349 25 year 2.2140 50 year 2.5135 100 year 2.8282 ___________________________________________________________________ 2018 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. 2018 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. However, the project will meet the City’s Flood Problem Flow Control Standard due to the existing downstream flooding problem within the drainage basin tributary to the SE 144th Street storm system. 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 the range of flows extending from 50% of 2-year up to the full 50-year flow and matches peaks for the 2 -, 10-, and 100-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 infiltration 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 infiltration 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 supplemental 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. 2018 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: Preliminary Vault Site Name: Cedars Site Address: 14120 160th Ave SE City : Renton Report Date: 7/2/2018 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 : PREDEV Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Flat 4.109 Pervious Total 4.109 Impervious Land Use acre Impervious Total 0 Basin Total 4.109 __________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : RDIN Bypass: No 2018 D. R. STRONG Consulting Engineers Inc. Page 44 Cedars at the Highlands Technical Information Report Renton, Washington GroundWater: No Pervious Land Use acre C, Lawn, Flat 2.097 Pervious Total 2.097 Impervious Land Use acre ROADS FLAT 0.715 ROOF TOPS FLAT 1.09 DRIVEWAYS FLAT 0.207 Impervious Total 2.012 Basin Total 4.109 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater Vault 1 Vault 1 ___________________________________________________________________ Name : Vault 1 Width : 71.2373251505034 ft. Length : 71.2373251505034 ft. Depth: 12 ft. Discharge Structure Riser Height: 11 ft. Riser Diameter: 18 in. Orifice 1 Diameter: 1.11 in. Elevation: 0 ft. Orifice 2 Diameter: 1.96 in. Elevation: 7.787 ft. Orifice 3 Diameter: 1.19 in. Elevation: 8.7 ft. Element Flows To: Outlet 1 Outlet 2 ___________________________________________________________________ Vault Hydraulic Table Stage(feet) Area(ac.) Volume(ac-ft.) Discharge(cfs) Infilt(cfs) 0.0000 0.116 0.000 0.000 0.000 0.1333 0.116 0.015 0.012 0.000 0.2667 0.116 0.031 0.017 0.000 0.4000 0.116 0.046 0.021 0.000 0.5333 0.116 0.062 0.024 0.000 0.6667 0.116 0.077 0.027 0.000 0.8000 0.116 0.093 0.029 0.000 0.9333 0.116 0.108 0.032 0.000 1.0667 0.116 0.124 0.034 0.000 1.2000 0.116 0.139 0.036 0.000 1.3333 0.116 0.155 0.038 0.000 1.4667 0.116 0.170 0.040 0.000 1.6000 0.116 0.186 0.042 0.000 1.7333 0.116 0.201 0.044 0.000 1.8667 0.116 0.217 0.045 0.000 2.0000 0.116 0.233 0.047 0.000 2.1333 0.116 0.248 0.048 0.000 2.2667 0.116 0.264 0.050 0.000 2.4000 0.116 0.279 0.051 0.000 2018 D. R. STRONG Consulting Engineers Inc. Page 45 Cedars at the Highlands Technical Information Report Renton, Washington 2.5333 0.116 0.295 0.053 0.000 2.6667 0.116 0.310 0.054 0.000 2.8000 0.116 0.326 0.055 0.000 2.9333 0.116 0.341 0.057 0.000 3.0667 0.116 0.357 0.058 0.000 3.2000 0.116 0.372 0.059 0.000 3.3333 0.116 0.388 0.061 0.000 3.4667 0.116 0.403 0.062 0.000 3.6000 0.116 0.419 0.063 0.000 3.7333 0.116 0.434 0.064 0.000 3.8667 0.116 0.450 0.065 0.000 4.0000 0.116 0.466 0.066 0.000 4.1333 0.116 0.481 0.068 0.000 4.2667 0.116 0.497 0.069 0.000 4.4000 0.116 0.512 0.070 0.000 4.5333 0.116 0.528 0.071 0.000 4.6667 0.116 0.543 0.072 0.000 4.8000 0.116 0.559 0.073 0.000 4.9333 0.116 0.574 0.074 0.000 5.0667 0.116 0.590 0.075 0.000 5.2000 0.116 0.605 0.076 0.000 5.3333 0.116 0.621 0.077 0.000 5.4667 0.116 0.636 0.078 0.000 5.6000 0.116 0.652 0.079 0.000 5.7333 0.116 0.667 0.080 0.000 5.8667 0.116 0.683 0.081 0.000 6.0000 0.116 0.699 0.081 0.000 6.1333 0.116 0.714 0.082 0.000 6.2667 0.116 0.730 0.083 0.000 6.4000 0.116 0.745 0.084 0.000 6.5333 0.116 0.761 0.085 0.000 6.6667 0.116 0.776 0.086 0.000 6.8000 0.116 0.792 0.087 0.000 6.9333 0.116 0.807 0.088 0.000 7.0667 0.116 0.823 0.088 0.000 7.2000 0.116 0.838 0.089 0.000 7.3333 0.116 0.854 0.090 0.000 7.4667 0.116 0.869 0.091 0.000 7.6000 0.116 0.885 0.092 0.000 7.7333 0.116 0.900 0.093 0.000 7.8667 0.116 0.916 0.123 0.000 8.0000 0.116 0.932 0.142 0.000 8.1333 0.116 0.947 0.156 0.000 8.2667 0.116 0.963 0.168 0.000 8.4000 0.116 0.978 0.178 0.000 8.5333 0.116 0.994 0.187 0.000 8.6667 0.116 1.009 0.196 0.000 8.8000 0.116 1.025 0.216 0.000 8.9333 0.116 1.040 0.230 0.000 9.0667 0.116 1.056 0.241 0.000 9.2000 0.116 1.071 0.252 0.000 9.3333 0.116 1.087 0.262 0.000 9.4667 0.116 1.102 0.271 0.000 9.6000 0.116 1.118 0.280 0.000 9.7333 0.116 1.133 0.288 0.000 9.8667 0.116 1.149 0.296 0.000 10.000 0.116 1.165 0.304 0.000 10.133 0.116 1.180 0.312 0.000 2018 D. R. STRONG Consulting Engineers Inc. Page 46 Cedars at the Highlands Technical Information Report Renton, Washington 10.267 0.116 1.196 0.319 0.000 10.400 0.116 1.211 0.326 0.000 10.533 0.116 1.227 0.333 0.000 10.667 0.116 1.242 0.340 0.000 10.800 0.116 1.258 0.346 0.000 10.933 0.116 1.273 0.352 0.000 11.067 0.116 1.289 0.632 0.000 11.200 0.116 1.304 1.769 0.000 11.333 0.116 1.320 3.253 0.000 11.467 0.116 1.335 4.703 0.000 11.600 0.116 1.351 5.784 0.000 11.733 0.116 1.366 6.402 0.000 11.867 0.116 1.382 6.991 0.000 12.000 0.116 1.398 7.486 0.000 12.133 0.116 1.413 7.949 0.000 12.267 0.000 0.000 8.386 0.000 ___________________________________________________________________ ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:4.109 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:2.097 Total Impervious Area:2.012 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.16052 5 year 0.264644 10 year 0.343685 25 year 0.454147 50 year 0.543731 100 year 0.639312 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.093008 5 year 0.150518 10 year 0.200863 25 year 0.28129 50 year 0.355414 100 year 0.443565 ___________________________________________________________________ Stream Protection Duration Annual Peaks for Predeveloped and Mitigated. POC #1 Year Predeveloped Mitigated 1949 0.196 0.067 2018 D. R. STRONG Consulting Engineers Inc. Page 47 Cedars at the Highlands Technical Information Report Renton, Washington 1950 0.229 0.086 1951 0.330 0.286 1952 0.111 0.060 1953 0.093 0.075 1954 0.135 0.078 1955 0.208 0.077 1956 0.170 0.090 1957 0.153 0.078 1958 0.156 0.082 1959 0.132 0.069 1960 0.256 0.233 1961 0.128 0.086 1962 0.087 0.059 1963 0.125 0.079 1964 0.157 0.080 1965 0.117 0.091 1966 0.104 0.074 1967 0.244 0.082 1968 0.143 0.075 1969 0.138 0.071 1970 0.116 0.075 1971 0.146 0.082 1972 0.252 0.209 1973 0.119 0.090 1974 0.134 0.082 1975 0.191 0.073 1976 0.136 0.079 1977 0.047 0.061 1978 0.114 0.084 1979 0.071 0.057 1980 0.359 0.252 1981 0.105 0.076 1982 0.249 0.145 1983 0.171 0.081 1984 0.108 0.065 1985 0.063 0.068 1986 0.268 0.090 1987 0.247 0.173 1988 0.106 0.066 1989 0.066 0.066 1990 0.688 0.258 1991 0.310 0.206 1992 0.129 0.085 1993 0.125 0.064 1994 0.051 0.056 1995 0.167 0.085 1996 0.392 0.293 1997 0.310 0.274 1998 0.110 0.065 1999 0.428 0.190 2000 0.120 0.084 2001 0.031 0.051 2002 0.167 0.114 2003 0.229 0.074 2004 0.265 0.276 2005 0.174 0.080 2006 0.180 0.090 2007 0.508 0.457 2018 D. R. STRONG Consulting Engineers Inc. Page 48 Cedars at the Highlands Technical Information Report Renton, Washington 2008 0.540 0.295 2009 0.243 0.092 ___________________________________________________________________ Stream Protection Duration Ranked Annual Peaks for Predeveloped and Mitigated. POC #1 Rank Predeveloped Mitigated 1 0.6875 0.4568 2 0.5404 0.2948 3 0.5084 0.2925 4 0.4282 0.2861 5 0.3920 0.2764 6 0.3592 0.2743 7 0.3300 0.2575 8 0.3099 0.2515 9 0.3095 0.2330 10 0.2684 0.2092 11 0.2649 0.2061 12 0.2562 0.1898 13 0.2521 0.1726 14 0.2494 0.1450 15 0.2466 0.1144 16 0.2439 0.0915 17 0.2433 0.0906 18 0.2294 0.0904 19 0.2285 0.0904 20 0.2083 0.0902 21 0.1965 0.0900 22 0.1912 0.0857 23 0.1803 0.0857 24 0.1740 0.0854 25 0.1713 0.0846 26 0.1697 0.0841 27 0.1668 0.0836 28 0.1667 0.0821 29 0.1572 0.0819 30 0.1555 0.0818 31 0.1533 0.0816 32 0.1463 0.0813 33 0.1426 0.0799 34 0.1376 0.0796 35 0.1361 0.0791 36 0.1350 0.0790 37 0.1341 0.0781 38 0.1321 0.0780 39 0.1293 0.0771 40 0.1281 0.0760 41 0.1254 0.0754 42 0.1250 0.0752 43 0.1196 0.0746 44 0.1191 0.0736 45 0.1171 0.0736 46 0.1155 0.0735 47 0.1142 0.0711 48 0.1106 0.0685 49 0.1104 0.0684 50 0.1082 0.0672 51 0.1060 0.0663 2018 D. R. STRONG Consulting Engineers Inc. Page 49 Cedars at the Highlands Technical Information Report Renton, Washington 52 0.1047 0.0658 53 0.1040 0.0652 54 0.0932 0.0647 55 0.0869 0.0645 56 0.0711 0.0608 57 0.0663 0.0604 58 0.0634 0.0589 59 0.0510 0.0574 60 0.0469 0.0559 61 0.0311 0.0514 ___________________________________________________________________ Stream Protection Duration POC #1 The Facility PASSED The Facility PASSED. Flow(cfs) Predev Mit Percentage Pass/Fail 0.0803 18576 16074 86 Pass 0.0849 16523 11182 67 Pass 0.0896 14540 6564 45 Pass 0.0943 12776 3287 25 Pass 0.0990 11462 3161 27 Pass 0.1037 10059 3048 30 Pass 0.1083 8915 2941 32 Pass 0.1130 7843 2832 36 Pass 0.1177 7142 2742 38 Pass 0.1224 6340 2659 41 Pass 0.1271 5683 2541 44 Pass 0.1318 5251 2453 46 Pass 0.1364 4738 2355 49 Pass 0.1411 4291 2246 52 Pass 0.1458 3970 2117 53 Pass 0.1505 3559 1995 56 Pass 0.1552 3195 1889 59 Pass 0.1598 2920 1794 61 Pass 0.1645 2627 1699 64 Pass 0.1692 2363 1605 67 Pass 0.1739 2175 1512 69 Pass 0.1786 1975 1406 71 Pass 0.1833 1804 1270 70 Pass 0.1879 1694 1148 67 Pass 0.1926 1532 1052 68 Pass 0.1973 1352 950 70 Pass 0.2020 1245 906 72 Pass 0.2067 1141 845 74 Pass 0.2113 1047 795 75 Pass 0.2160 970 760 78 Pass 0.2207 916 728 79 Pass 0.2254 843 687 81 Pass 0.2301 766 644 84 Pass 0.2348 715 587 82 Pass 0.2394 639 543 84 Pass 0.2441 572 494 86 Pass 0.2488 496 456 91 Pass 0.2535 432 410 94 Pass 0.2582 378 355 93 Pass 2018 D. R. STRONG Consulting Engineers Inc. Page 50 Cedars at the Highlands Technical Information Report Renton, Washington 0.2628 344 327 95 Pass 0.2675 312 277 88 Pass 0.2722 271 226 83 Pass 0.2769 245 186 75 Pass 0.2816 202 164 81 Pass 0.2862 172 132 76 Pass 0.2909 151 116 76 Pass 0.2956 125 86 68 Pass 0.3003 110 79 71 Pass 0.3050 97 73 75 Pass 0.3097 85 67 78 Pass 0.3143 71 62 87 Pass 0.3190 61 56 91 Pass 0.3237 55 53 96 Pass 0.3284 45 47 104 Pass 0.3331 40 43 107 Pass 0.3377 37 39 105 Pass 0.3424 35 33 94 Pass 0.3471 29 21 72 Pass 0.3518 25 16 64 Pass 0.3565 22 12 54 Pass 0.3612 17 11 64 Pass 0.3658 15 11 73 Pass 0.3705 12 10 83 Pass 0.3752 9 9 100 Pass 0.3799 9 9 100 Pass 0.3846 8 7 87 Pass 0.3892 8 6 75 Pass 0.3939 7 6 85 Pass 0.3986 7 5 71 Pass 0.4033 7 5 71 Pass 0.4080 7 5 71 Pass 0.4126 7 4 57 Pass 0.4173 7 4 57 Pass 0.4220 7 4 57 Pass 0.4267 6 3 50 Pass 0.4314 5 3 60 Pass 0.4361 5 3 60 Pass 0.4407 5 3 60 Pass 0.4454 4 3 75 Pass 0.4501 4 1 25 Pass 0.4548 4 1 25 Pass 0.4595 4 0 0 Pass 0.4641 4 0 0 Pass 0.4688 4 0 0 Pass 0.4735 4 0 0 Pass 0.4782 4 0 0 Pass 0.4829 4 0 0 Pass 0.4876 4 0 0 Pass 0.4922 3 0 0 Pass 0.4969 3 0 0 Pass 0.5016 3 0 0 Pass 0.5063 3 0 0 Pass 0.5110 2 0 0 Pass 0.5156 2 0 0 Pass 0.5203 2 0 0 Pass 0.5250 2 0 0 Pass 0.5297 2 0 0 Pass 2018 D. R. STRONG Consulting Engineers Inc. Page 51 Cedars at the Highlands Technical Information Report Renton, Washington 0.5344 2 0 0 Pass 0.5390 2 0 0 Pass 0.5437 1 0 0 Pass _____________________________________________________ ___________________________________________________________________ Water Quality BMP Flow and Volume for POC #1 On-line facility volume: 0.3704 acre-feet On-line facility target flow: 0.384 cfs. Adjusted for 15 min: 0.384 cfs. Off-line facility target flow: 0.2131 cfs. Adjusted for 15 min: 0.2131 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 546.68 N 0.00 Total Volume Infiltrated 546.68 0.00 0.00 0.00 0.00 0% No Treat. Credit Compliance with LID Standard 8 Duration Analysis Result = Failed ___________________________________________________________________ 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. ___________________________________________________________________ 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. 2005-2018; All Rights Reserved. 2018 D. R. STRONG Consulting Engineers Inc. Page 52 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 53 Cedars at the Highlands Technical Information Report Renton, Washington FIGURE 17 DETENTION & WATER QUALITY FACILITY DETAILS (To be completed at time of final engineering) 2018 D. R. STRONG Consulting Engineers Inc. Page 54 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 55 Cedars at the Highlands Technical Information Report Renton, Washington BACKWATER ANALYSIS (To be completed at time of final engineering) 2018 D. R. STRONG Consulting Engineers Inc. Page 56 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 Geotechnical Infiltration Evaluation: Earth Solutions NW LLC – July 13, 2017 Arborist Report: Greenforest, Inc – March 20, 2016 Trip Generation Letter Report: Traffex – January 19, 2017 Conceptual Wetland Buffer Reduction and Enhancement Plan: Soundview Consultants – June 28, 2018 2018 D. R. STRONG Consulting Engineers Inc. Page 57 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 58 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 water 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 from 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 59 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 60 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. Lower Cedar RiverLower Cedar River14.1092.01287,64187,641087,641 Cedars at the Highlands14120 160th Avenue SE, Renton, WashingtonLower Cedar RiversLower Cedar River2.012100%TBDVault1.111.961.191155.716TBDTract A3 2018 D. R. STRONG Consulting Engineers Inc. Page 61 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. 2018 D. R. STRONG Consulting Engineers Inc. Page 62 Cedars at the Highlands Technical Information Report Renton, Washington APPENDICES 2018 D. R. STRONG Consulting Engineers Inc. Page 63 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.