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HomeMy WebLinkAbout16834-R-TIR-2014-04-21-PRLM Preliminary Technical Information Report Plat of Copperwood (Stuth Property) Renton, Washington Our Job No. 16834 16834.003.doc TABLE OF CONTENTS 1.0 PROJECT OVERVIEW Figure 1 – TIR Worksheet Figure 2 – Site Location Figure 3 – Drainage Basins, Subbasins, and Site Characteristics Figure 4 – Soils Figure 5 – FEMA Map Figure 6 – Sensitive Areas Map 2.0 CONDITIONS AND REQUIREMENTS SUMMARY 2.1 Analysis of the Core Requirements 2.2 Analysis of the Special Requirements 3.0 OFF-SITE ANALYSIS 4.0 FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN A. Existing Site Hydrology B. Developed Site Hydrology C. Performance Standards D. Flow Control System E. Water Quality System 5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN 6.0 SPECIAL REPORTS AND STUDIES 7.0 OTHER PERMITS 8.0 CONSTRUCTION STORMWATER POLLUTION PREVENTION PLAN (CSWPPP) ANALYSIS AND DESIGN 9.0 BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT 10.0 OPERATIONS AND MAINTENANCE MANUAL APPENDIX A - Lower Cedar River Basin and Nonpoint Pollution Action Plan Tab 1.0 16834.003.doc 1.0 PROJECT OVERVIEW The proposed Plat of Copperwood is a single-family residential project consisting of 47 lots zoned R4, and will be utilizing the small lot clustering provisions. The project is 10.33 acres in size, containing 8 tax parcels (152305-9066, 152305-9067, 152305-9201, 152305-9100, 152305-9093, 152305-9170, 152305-9043, and 152305-9221). The site is located at the intersection of 143rd Avenue S.E. and S.E. 2nd Place, in a portion of Section 15, Township 23 North, Range 5 East, W.M., in the City of Renton. The site is rectangular in shape and fronts S.E. 2nd Place. The proposed on-site roads are to be tied into S.E. 2nd Place, and will be extended into the site to provide public access and circulation. This site is currently developed, with 7 single-family residences, and is currently zoned R4. The existing on-site improvements consist of storm and water utilities, and gravel driveways that access S.E. 2nd Place. There is a creek on-site (Maplewood Creek Tributary 0303), near the east property line. This creek has an associated 75-foot buffer that will be maintained through development. The existing site drains to Maplewood Creek Tributary 0303, where it then leaves the site near the southeast corner. The existing discharge location will be maintained after development as well. The on-site creek is identified by the City of Renton as a Class 3 waterbody. The site is bound along the east property line by 143rd Avenue S.E. The site is bound by single- family residences to the south and west. The southeast corner of the site contains steep slopes, ranging from 15% to 70%. These steep slopes will be contained within a Natural Growth Protection Area (NGPA) Tract. S.E. 2nd Place bounds the site to the north. Access to the site from S.E. 2nd Place will require frontage improvements, including new asphalt, new curb and gutter, sidewalk, and planter strip. On-site soils are mapped as Alderwood. Please refer to the Soils Map in this section. All drainage calculations were modeled as till soils. The project will be constructing roadways consisting of curb and gutter, sidewalk, and street trees. The site will tie into the existing road at S.E. 2nd Place. All roads for this project have been designed to be 26 feet wide, with an 8-foot planter strip at the back of curb and a 5-foot sidewalk (both sides). Overall, the proposed public right-of-way is to be 53 feet in width. The topography on-site is generally flat, with a slight rise in the westernmost portion of the site. The site generally slopes from west to east, toward Maplewood Creek Tributary 0303, from elevation 398 to 386. The project will be mass graded with cuts and fills balanced on site. The Maplewood Creek tributary and the area within its associated 75-foot buffer will remain undisturbed; no buffer averaging is proposed. The proposed development also includes further restoration of the Maplewood Creek Tributary 0303 buffer area. The drainage facilities are required to meet the requirements of the 2009 King County Surface Water Design Manual (KCSWDM), and the 2010 City of Renton Amendments to the KCSWDM. The drainage design shall meet, at a minimum, the Conservation Flow Control (Level 2) and Basic Water Quality Treatment. The drainage facility located in Tract D is a combination detention/wetpond, sized for Level 2 Flow Control. The wetpond will be sized pursuant to the 2009 KCSWDM and its amendments to ensure that the Basic Water Quality requirement is met. The project will be discharging the drainage from the pond to Maplewood Creek Tributary 0303. Please refer to Section 4.0 for detailed drainage calculations. KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2009 Surface Water Design Manual 1/1/09 1 16834.005.doc Part 1 PROJECT OWNER AND PROJECT ENGINEER Part 2 PROJECT LOCATION AND DESCRIPTION Project Owner The Quadrant Corporation Phone Address 14725 SE 36TH Street, Suite 100 Bellevue, WA 98006 Project Engineer Chris Jensen, P.E. Company Barghausen Consulting Engineers , Inc. Phone (425) 251-6222 Project Name Plat of Copperwood DDES Permit # Location Township 23N Range 5E Section 15 Site Address Renton, WA Part 3 TYPE OF PERMIT APPLICATION Part 4 OTHER REVIEWS AND PERMITS Landuse Services Subdivision / Short Subd. / UPD Building Services M/F / Commercial / SFR Clearing and Grading Right-of-Way Use Other DFW HPA COE 404 DOE Dam Safety FEMA Floodplain COE Wetlands Other Shoreline Management Structural Rockery/Vault/ ESA Section 7 Part 5 PLAN AND REPORT INFORMATION Technical Information Report Type of Drainage Review Full / Targeted / (circle): Large Site Date (include revision April 18, 2014 dates): Date of Final: Site Improvement Plan (Engr. Plans) Type (circle one): Full / Modified / Small Site Date (include revision April 18, 2014 dates): Date of Final: Part 6 ADJUSTMENT APPROVALS Type (circle one): Standard / Complex / Preapplication / Experimental / Blanket Description: (include conditions in TIR Section 2) Date of Approval: KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2009 Surface Water Design Manual 1/1/09 2 16834.005.doc Part 7 MONITORING REQUIREMENTS Monitoring Required: Yes / No Start Date: Completion Date: Describe: Part 8 SITE COMMUNITY AND DRAINAGE BASIN Community Plan : Special District Overlays: Drainage Basin: Lower Cedar River Basin Stormwater Requirements: Part 9 ONSITE AND ADJACENT SENSITIVE AREAS River/Stream Lake Wetlands Closed Depression Floodplain Other Steep Slope Erosion Hazard Landslide Hazard Coal Mine Hazard Seismic Hazard Habitat Protection Part 10 SOILS Soil Type Slopes Erosion Potential AgC 6-15 Percent High High Groundwater Table (within 5 feet) Sole Source Aquifer Other Seeps/Springs Additional Sheets Attached KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2009 Surface Water Design Manual 1/1/09 3 16834.005.doc Part 11 DRAINAGE DESIGN LIMITATIONS REFERENCE LIMITATION / SITE CONSTRAINT Core 2 – Offsite Analysis Sensitive/Critical Areas SEPA Other Additional Sheets Attached Part 12 TIR SUMMARY SHEET (provide one TIR Summary Sheet per Threshold Discharge Area) Threshold Discharge Area: (name or description) Discharge to Maplewood Creek Tributary 0303 near southeast corner of site. Core Requirements (all 8 apply) Discharge at Natural Location Number of Natural Discharge Locations: 1 Offsite Analysis Level: 1 / 2 / 3 dated: Flow Control Level: 1 / 2 / 3 or Exemption Number (incl. facility summary sheet) Small Site BMPs Conveyance System Spill containment located at: N/A Erosion and Sediment Control ESC Site Supervisor: TBD Contact Phone: TBD 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 Basicm / 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 Fac. / None Requirements Name: Floodplain/Floodway Delineation Type: Major / Minor / Exemption / None 100-year Base Blood Elevation (or range): Datum: Flood Protection Facilities Describe: N/A Source Control Describe landuse: N/A (comm./industrial landuse) Describe any structural controls: N/A KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2009 Surface Water Design Manual 1/1/09 4 16834.005.doc Oil Control High-use Site: Yes / No Treatment BMP: Maintenance Agreement: Yes / No with whom? Other Drainage Structures Describe: 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 Control Dewatering Control Dust Control Flow Control MINIMUM ESC REQUIREMENTS AFTER CONSTRUCTION Stabilize Exposed Surfaces Remove and Restore Temporary ESC Facilities Clean and Remove All Silt and Debris Ensure Operation of Permanent Facilities Flag Limits of SAO and open space preservation areas Other 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 Flow Control BMPs Other Pond Biofiltration Wetpool Media Filtration Oil Control Spill Control Flow Control BMPs Other Wetpond KING COUNTY, WASHINGTON, SURFACE WATER DESIGN MANUAL TECHNICAL INFORMATION REPORT (TIR) WORKSHEET 2009 Surface Water Design Manual 1/1/09 5 16834.005.doc Part 15 EASEMENTS/TRACTS Part 16 STRUCTURAL ANALYSIS Drainage Easement Covenant Native Growth Protection Covenant Tract Other 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 barg h au senCONSUL TINGENGINE E R S ,INC.Horizontal: Scale: Vertical: For: Title: VICINITY MAP Job Number 18215 72ND AVENUE SOUTH KENT, WA 98032 (425) 251-6222 (425) 251-8782 CIVIL ENGINEERING, LAND PLANNING, SURVEYING, ENVIRONMENTAL SERVICES N.T.S.N/A 16834 DATE: 04/16/14 PLAT OF COPPERWOOD RENTON, WASHINGTON P:\16000s\16834\exhibit\graphics\16834 vmap.cdr REFERENCE: Rand McNally (2014) SITE barg h au senCONSUL TINGENGINE E R S ,INC.Horizontal: Scale: Vertical: For: Title: ASSESSOR MAP Job Number 18215 72ND AVENUE SOUTH KENT, WA 98032 (425) 251-6222 (425) 251-8782 CIVIL ENGINEERING, LAND PLANNING, SURVEYING, ENVIRONMENTAL SERVICES N.T.S.N/A 16834 DATE: 04/16/14 PLAT OF COPPERWOOD RENTON, WASHINGTON P:\16000s\16834\exhibit\graphics\16834 amap.cdr SITE REFERENCE: King County Department of Assessments (Dec. 2011) barg h au senCONSUL TINGENGINE E R S ,INC.Horizontal: Scale: Vertical: For: Title: SOIL SURVEY MAP Job Number 18215 72ND AVENUE SOUTH KENT, WA 98032 (425) 251-6222 (425) 251-8782 CIVIL ENGINEERING, LAND PLANNING, SURVEYING, ENVIRONMENTAL SERVICES N.T.S.N/A 16834 DATE: 04/16/14 PLAT OF COPPERWOOD RENTON, WASHINGTON P:\16000s\16834\exhibit\graphics\16834 soil.cdr REFERENCE: USDA, Natural Resources Conservation Service LEGEND: AgC = Alderwood gravelly sandy loam, 6-15% slopes SITE barg h au senCONSUL TINGENGINE E R S ,INC.Horizontal: Scale: Vertical: For: Title: FEMA MAP Job Number 18215 72ND AVENUE SOUTH KENT, WA 98032 (425) 251-6222 (425) 251-8782 CIVIL ENGINEERING, LAND PLANNING, SURVEYING, ENVIRONMENTAL SERVICES N.T.S.N/A 16834 DATE: 04/16/14 PLAT OF COPPERWOOD RENTON, WASHINGTON P:\16000s\16834\exhibit\graphics\16834 fema.cdr REFERENCE: Federal Emergency Management Agency (Portion of Map 53033C0982 F, May 1995) SITE barg h au senCONSUL TINGENGINE E R S ,INC.Horizontal: Scale: Vertical: For: Title: SENSITIVE AREAS MAP Job Number 18215 72ND AVENUE SOUTH KENT, WA 98032 (425) 251-6222 (425) 251-8782 CIVIL ENGINEERING, LAND PLANNING, SURVEYING, ENVIRONMENTAL SERVICES N.T.S.N/A 16834 DATE: 04/16/14 PLAT OF COPPERWOOD RENTON, WASHINGTON P:\16000s\16834\exhibit\graphics\16834 amap.cdr SITE REFERENCE: (2014)www.rentonweb.org Tab 2.0 16834.003.doc 2.0 CONDITIONS AND REQUIREMENTS SUMMARY 2.1 Analysis of the Core Requirements Core Requirement No. 1: Discharge at the Natural Location. Response: The runoff from the existing site leaves near the southeast corner via an on-site creek (Maplewood Creek Tributary 0303). The proposed detention pond will discharge into the creek, maintaining the natural discharge location for the site. Core Requirement No. 2: Off-Site Analysis. Response: An Off-Site Analysis has been performed pursuant to the 2009 KCSWDM. See Section 3.0 for the Level 1 Off-Site Analysis. Core Requirement No. 3: Flow Control. Response: Conservation Flow Control (Level 2) is required. Conservation flow control requirements will be met by a detention pond, designed pursuant to the 2009 KCSWDM and the 2010 City of Renton Amendments to the KCSWDM. See Section 4.0 for more information. Core Requirement No. 4: Conveyance System. Response: The conveyance system has been design pursuant to the 2009 KCSWDM and the 2010 City of Renton Amendments to the KCSWDM. Conveyance and backwater analysis will be provided in the final engineering. Core Requirement No. 5: Erosion and Sediment Control. Response: Temporary erosion control measures will be provided in the final engineering. Core Requirement No. 6: Maintenance and Operations. Response: All proposed storm structures will be publically maintained by the City of Renton; therefore, no operation and maintenance manuals are required. Core Requirement No. 7: Financial Guarantees and Liability. Response: The project will provide a Site Improvement Bond Quantity Worksheet to establish a bond amount for drainage facility restoration and site stabilization financial guarantee prior to construction. Core Requirement No. 8: Water Quality. Response: Basic water quality is required for this project. This requirement is met by a wetpond. The wetpond is in combination with the detention pond, and sizing will be completed during final engineering. 2.2 Analysis of the Special Requirements Special Requirement No. 1: Other Adopted Area-Specific Requirements. Response: The proposed project is not located in a designated Critical Drainage Area. 16834.003.doc Special Requirement No. 2: Flood Hazard Area Delineation. Response: As indicated by the FEMA Map included in this report, the proposed site does not lie within a floodplain or floodway of a stream, so this special requirement does not apply. Special Requirement No. 3: Flood Protection Facilities. Response: This project does not rely on an existing flood protection facility or propose to modify or construct a new flood protection facility, so this special requirement does not apply. Special Requirement No. 4: Source Control. Response: The project does not require a commercial building or commercial site development permit, so this special requirement does not apply. Special Requirement No. 5: Oil Control. Response: This site is not classified as a High Use Site given the criteria in the 2009 KCSWDM, so this special requirement does not apply and no special control treatment is necessary. Special Requirement No. 6: Aquifer Protection Area Response: The project site is not located within an Aquifer Protection Area and, therefore, this special requirement does not apply. Tab 3.0 16834.003.doc 3.0 OFF-SITE ANALYSIS Task 1 STUDY AREA DEFINITION AND MAPS The proposed Plat of Copperwood is a single-family residential project consisting of 47 lots zoned R4, and will be utilizing the small lot clustering provisions. The project is 10.33 acres in size containing 8 tax parcels (152305-9066, 152305-9067, 152305-9201, 152305-9100, 152305-9093, 152305-9170, 152305- 9043, and 152305-9221). The site is located at the intersection of 143rd Avenue S.E. and S.E. 2nd Place, in a portion of Section 15, Township 23 North, Range 5 East, W.M., in the City of Renton. The site is rectangular in shape, and fronts S.E. 2nd Place. The proposed on-site roads are to be tied into S.E. 2nd Place, and will be extended into the site to provide public access and circulation. This site is currently developed, with 7 single-family residences, and is currently zoned R4. The existing on-site improvements consist of storm and water utilities, and gravel driveways that access S.E. 2nd Place. There is a creek on-site (Maplewood Creek Tributary 0303), near the east property line. This creek has an associated 75-foot buffer that will be maintained through development. The existing site drains to Maplewood Creek Tributary 0303, where it then leaves the site near the southeast corner. The existing discharge location will be maintained after development as well. The on-site creek is identified by the City of Renton as a Class 3 waterbody. The site is bound along the east property line by 143rd Avenue S.E. The site is bound by single- family residences to the south and west. The southeast corner of the site contains steep slopes, ranging from 15% to 70%. These steep slopes will be contained within a Natural Growth Protection Area (NGPA) Tract. S.E. 2nd Place bounds the site to the north. Access to the site from S.E. 2nd Place will require frontage improvements, including new asphalt, new curb and gutter, sidewalk, and planter strip. On-site soils are mapped as Alderwood. Please refer to the Soils Map in this section. All drainage calculations were modeled as till soils. The project will be constructing roadways consisting of curb and gutter, sidewalk, and street trees. The site will tie into the existing road at S.E. 2nd Place. All roads for this project have been designed to be 26 feet wide, with an 8-foot planter strip at the back of curb and a 5-foot sidewalk (both sides). Overall, the proposed public right-of-way is to be 53 feet in width. The topography on-site is generally flat, with a slight rise in the westernmost portion of the site. The site generally slopes from west to east, toward Maplewood Creek Tributary 0303, from elevation 398 to 386. The project will be mass graded with cuts and fills balanced on site. The Maplewood Creek tributary and the area within its associated 75-foot buffer will remain undisturbed; no buffer averaging is proposed. The proposed development also includes further restoration of the Maplewood Creek Tributary 0303 buffer area. The drainage facilities are required to meet the requirements of the 2009 King County Surface Water Design Manual (KCSWDM), and the 2010 City of Renton Amendments to the KCSWDM. The drainage design shall meet, at a minimum, the Conservation Flow Control (Level 2) and Basic Water Quality Treatment. The drainage facility located in Tract D is a combination detention/wetpond, sized for Level 2 Flow Control. The wetpond will be sized pursuant to the 2009 KCSWDM and its amendments to ensure that the Basic Water Quality requirement is met. The project will be discharging the drainage from the pond to Maplewood Creek Tributary 0303. Please refer to Section 4.0 for detailed drainage calculations. UPSTREAM DRAINAGE ANALYSIS: Upstream of the site, to the west, are existing single-family residences. This area is approximately 3.80 acres. The west property line serves as a high point between the upstream basin and the project site. Due to the high point between the upstream basin and the project site, it is safe to assume that the amount of runoff flowing onto the site from the upstream basin in negligible. 16834.003.doc TASK 2 RESOURCE REVIEW · Adopted Basin Plans: The site is located within the Lower Cedar River Basin. Refer to Appendix A for the portions of the basin that applies to this project. · Finalized Drainage Studies: This is not applicable. · Basin Reconnaissance Summary Report: This site is located in the Lower Cedar River Basin, which is covered by the Lower Cedar River Basin and Nonpoint Pollution Action Plan dated July 1997 (included in Appendix A). · Critical Drainage Area Maps: This project will not discharge to any critical areas or wetlands as it will be discharging runoff from the detention pond to Maplewood Creek Tributary 0303, matching pre-developed flows. Therefore, no critical areas are to be affected. · Floodplain and Floodway FEMA Maps: Please see the attached FEMA Map (Section 1.0) utilized for this analysis. As indicated on the map, the site is located in Zone X and is outside of the 500-year floodplain. · Other Off-Site Analysis Reports: A site investigation was conducted in preparation of this Level 1 Off-Site Drainage Analysis. The United States Department of Agriculture Soils Conservation Service Map is also provided. See Figure 4 – Soils Map in Section 1.0. · Sensitive Areas Folios: Based on review of the King County Sensitive Areas Map Folios, the site does not contain any On-Site Wetlands, Erosion Areas, Sensitive Areas, or Land Slide Areas. · Road Drainage Problems: This is not applicable. · United States Department of Agriculture King County Soils Survey: Based on the Soils Map (Figure 4 – Soils Map, Section 1.0) for this area, the site is underlain with Alderwood gravelly sandy loam. The soils were modeled as till soils. · Wetland Inventory Map: From the Wetland and Fish and Wildlife Habitat Assessment Report by Soundview Consultants, there is no known documentation or inventory of wetlands for the project site. · Migrating River Studies: This is not applicable. · City of Renton Aquifer Protection Zones: Per the City of Renton’s GIS Map, the project site is not located within an Aquifer Recharge Area. 16834.003.doc TASK 3 FIELD INSPECTION 3.1 Conveyance System Nuisance Problems (Type 1) Conveyance system nuisance problems are minor but not chronic flooding or erosion problems that result from the overflow of a constructed conveyance system that is substandard or has become too small as a result of upstream development. Such problems warrant additional attention because of their chronic nature and because they result from the failure of a conveyance system to provide a minimum acceptable level of protection. There were no conveyance system nuisance problems observed during the site visit. Furthermore, based on a review of the drainage complaints received from the City of Renton, there is no evidence of past conveyance system nuisance problems occurring in the direct downstream drainage course, as there is a record of none having been submitted. This site will have a Level 2 Flow Control facility, which will restrict the flow of the 2-year release rate to 50% of the pre-developed site and will provide adequate mitigation to prevent any future drainage complaints as a result of this proposed site development. 3.2 Severe Erosion Problems (Type 2) Severe erosion problems can be caused by conveyance system overflows or the concentration of runoff into erosion-sensitive open drainage features. Severe erosion problems warrant additional attention because they pose a significant threat either to health and safety or to public or private property. Based on our site visit, there was no evidence of, or potential for, erosion/incision sufficient to pose a sedimentation hazard downstream within the limits of the study. All runoff sheet flows to Maplewood Creek Tributary 0303, where it is then conveyed off site. Stormwater runoff from the proposed roads will be collected and conveyed to a detention and water quality facility where it will then be discharged to the on-site creek tributary. As a result no future erosion problems should occur downstream because of this development. 3.3 Severe Flooding Problems (Type 3) Severe flooding problems can be caused by conveyance system overflows or the elevated water surfaces of ponds, lakes, wetlands, or closed depressions. Severe flooding problems are defined as follows: · Flooding of the finished area of a habitable building for runoff events less than or equal to the 100-year event. Examples include flooding of finished floors of homes and commercial or industrial buildings. Flooding in electrical/heating systems and components in the crawlspace or garage of a home. Such problems are referred to as "severe building flooding problems." · Flooding over all lanes of a roadway or severely impacting a sole access driveway for runoff events less than or equal to the 100-year event. Such problems are referred to as "severe roadway flooding problems." Based on a review of the FEMA Map (Section 1.0) the proposed site is outside of the 500-year floodplain, and there is no evidence of severe flooding problems encountered during our visit. 16834.003.doc TASK 4 DRAINAGE SYSTEM DESCRIPTION AND PROBLEM DESCRIPTIONS DOWNSTREAM DRAINAGE ANALYSIS: In the pre-developed condition, all runoff drains to the Maplewood Creek Tributary 0303. Once it is collected by the creek tributary, it leaves the site at the southeast corner. In the developed condition, stormwater that is discharged from the site will be conveyed from the detention pond to Maplewood Creek Tributary 0303. Steep slopes exist near the southeast corner of the NGPA, ranging from 15% to 70%. Stormwater from the detention pond is discharged via a bubble-up manhole that is armored with crushed rock. The crushed rock will help dissipate the outflows of the pond. In addition to the bubble-up manhole and crushed rock armoring, the outlet will be placed at the toe of the stream channel to ensure no runoff is being introduced over the slopes of the channel. The proposed detention pond was designed to match developed flows to pre-developed flows, from half of the 2-year storm event up to the full 50-year storm event. With the above proposed measures for stormwater management, we believe that the proposed development will not increase the amount of runoff entering Maplewood Creek Tributary 0303. Since the amount of runoff entering the creek tributary will remain the same, there will be no erosion of the existing steep slopes located near the southeast corner of the site. Tab 4.0 16834.003.doc 4.0 FLOW CONTROL AND WATER QUALITY FACILITY ANALYSIS AND DESIGN A. Existing Site Hydrology This site is currently developed, with 7 single-family residences containing 8 tax parcels (152305-9066, 152305-9067, 152305-9201, 152305-9100, 152305-9093, 152305-9170, 152305-9043, and 152305-9221) and is currently zoned R4. The existing on-site improvements consist of storm and water utilities, and gravel driveways that access S.E. 2nd Place. There is a creek on site (Maplewood Creek Tributary 0303), near the east property line. This creek has an associated 75-foot buffer that will be maintained through development. The existing site drains to Maplewood Creek Tributary 0303, where it then leaves the site near the southeast corner. The existing discharge location will be maintained after development as well. The on-site creek is identified by the City of Renton as a Class 3 waterbody. B. Developed Site Hydrology The completed project will create 47 lots. The total developed area will be 10.33 acres in size. New impervious surfaces will include roadways, driveways, sidewalks, and roof areas. The project will be providing landscaped pervious areas, open space areas, and a drainage facility. A conveyance system consisting of catch basins and storm pipes will be constructed in the roadways to collect drainage from impervious surfaces and lots, and conveyed to the new drainage facility. A combination detention/wetpond will be constructed in Tract D to provide water quality and flow control for the project. The pond will contain a control structure fitted with a riser overflow. Stormwater from the pond will be conveyed to Maplewood Creek Tributary 0303 via a proposed 12-inch pipe. The runoff will enter the creek and them be conveyed off site via Maplewood Creek. C. Performance Standards The KCRTS Runoff Time Series Program was used to size the detention facility. The detention pond was sized for Conservation Flow Control (Level 2) based on the requirements of the 2009 KCSWDM and the 2010 City of Renton Amendments to the KCSWDM. Individual lot BMPs will be applied to an impervious area equal to 10% of the lot size for lots less than 11,000 square feet and 20% of the lot size for lots greater than 11,000 square feet. The BMP credit has been applied to the impervious land cover data (impervious areas with applied BMPs modeled as 50% grass/50% impervious), which was used to size the detention pond. Please refer to the KCRTS computations included in this section. Based on Level 2 detention pond sizing calculations, the pond volume required is 121,975 cubic feet. The project is also subject to Basic Water Quality. To meet this requirement, a wetpond is proposed, and will be in combination with the detention pond. The wetpond was sized pursuant to the 2009 KCSWDM and the 2010 City of Renton Amendments to the KCSWDM. Based on the Wetpool Sizing Worksheet, the required wetpond volume is 34,409 cubic feet. 16834.003.doc D. Flow Control System Flow Control System details will be submitted during final engineering. E. Water Quality System A Wetpond will be used for water quality. Sizing of the wetpond will be prepared during final engineering. Tab 5.0 16834.003.doc 5.0 CONVEYANCE SYSTEM ANALYSIS AND DESIGN Conveyance system analysis and backwater calculations will be provided in final engineering. Tab 6.0 16834.003.doc 6.0 SPECIAL REPORTS AND STUDIES The following special reports have been prepared for this project and are included in this section: 6.1 Geotechnical Report by Terra Associates, Inc., dated February 10, 2014 Tab 7.0 16834.003.doc 7.0 OTHER PERMITS This section will be completed in final engineering. Tab 8.0 16834.003.doc 8.0 CONSTRUCTION STORMWATER POLLUTION PREVENTION PLAN (CSWPP) ANALYSIS AND DESIGN This section will be completed in final engineering. Tab 9.0 16834.003.doc 9.0 BOND QUANTITIES, FACILITY SUMMARIES, AND DECLARATION OF COVENANT This section will be provided in final engineering. Tab 10.0 16834.003.doc 10.0 OPERATIONS AND MAINTENANCE MANUAL This section will be provided in final engineering.