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Home My WebLink AboutRS_Drainage_Technical_Information_Report_221227_v1Patrick E. Carroll, P.E. PEC Design and Consulting LLC 2621 NE 109th Street Vancouver, WA 98686 (503) 849-1646 pat@pecdesign.net Date: August 23, 2022 Rev: Subject: Technical Information Report (TIR) – Billings 2 Lot Short Plat, 14103 160th Avenue SE, Renton, WA 98059 Table of Contents: Project Overview Conditions and Requirements Summary / Core Requirements Off Site Analysis Flow Control and Water Quality Facility Analysis and Design Conveyance System Analysis and Design Special Reports and Studies Other Permits ESC Analysis and Design Bond Quantities, Facility Summaries and Declarations of Covenant Operations Manual Special Requirements Project Description: The proposed project will divide the existing 36,469 sf parcel into 2 Lots. An existing single family residence is on the proposed parcel B and a future residence will be constructed on parcel A. A 16-foot- wide access driveway will be constructed to parcel A. A driveway with a fire department required hammer head will be constructed in the future and included in the building permit submittal for the residence. The total new impervious area for the driveway, road improvements on 160th Avenue SE (road asphalt paving, driveway drop and sidewalk improvements) is a total of 3,992 sf. The general grade of the residence site slopes gently to the west. The existing soils consist of fine to medium sand (based upon a geotechnical report from Nelson Geotechnical Associates, Inc. (NGA)), therefore runoff from the new driveway impervious area will flow into a proposed inlet and into the existing storm system inlet in 160th Ave. SE. The street improvements, and sidewalk will flow into the existing roadside ditch on 160th Ave. SE. The runoff from the future fire turnaround and the proposed residence will flow from to a future infiltration trench area on Parcel A. The individual system design will be provided at time of the building permit application. Under this permit, it is assumed that the only area to be disturbed, will be for the installation of the proposed driveway to parcel A, and for the installation of the street, sidewalk, and driveway drop on 160th Avenue SE. The approximate area to be disturbed during the installation of the improvements is approximately 7,000 sf. No existing native growth areas or significant trees will be disturbed. The proposed site earthwork is estimated as follows. DocuSign Envelope ID: 65DBE03F-B538-48F3-A86D-7E3E13222F87 Earthwork Calculations: Access Driveway Exc. = 2,879 sf = 160 cy Residence Exc. = 1,013 sf = 50 cy Total excavation to be spread onsite. = 210 cy. Conditions and Requirements Summary: The following specifics about the site apply: The site will use the City of Renton/King County Stormwater Manual allow for direct discharge to the existing storm system since the total additional flow is less than 0.10cfs . Due to the size of the site and area disturbed, a Full Dra inage Review format will be used. The following sections address the project as relates to the nine Core Requirements and the five Special Requirements. Core Requirement #1 - Discharge at the Natural Location The impervious areas will discharge to the existing storm system since the added flow is less than 0.10cfs. Core Requirement #2 – Offsite Analysis There are no identified drainage problems “downstream” of the site. Max. mitigated flow to the existing storm system per WWHM calculations is less than 0.10cfs. Core Requirement #3 - Flow Control No flow control is required since the new flow to the storm system per WWHM calculations is less than 0.10cfs, and the driveway runoff will be infiltrated onsite. Core Requirement #4 - Conveyance System The use of PVC pipe for runoff collection and routing to the existing storm system provides accepted and common conveyance practice. Three feet of cover is provided under pavement. Any storm piping under 3 feet of cover will be Ductile Iron Pipe. Core Requirement #5 - Construction Stormwater Pollution Prevention A TESCP Plan is provided integral with the drainage design drawing. The site is small and warrants only the normal placement of a downslope silt fence at its perimeter. A standard stabilized construction entrance BMP is provided for site entrance silt and debris protection of adjacent pavement. Core Requirement #6 - Maintenance and Operations An Operations and Maintenance Manual will be provided as part of the building permit process. A new manual will be provided in Exhibit B of a new Declaration of Covenant which is required by the City of Renton. Core Requirement #7 - Financial Guarantees and Liability The STE and Performance and Maintenance Bonds are required, and a right-of-way driveway installation. DocuSign Envelope ID: 65DBE03F-B538-48F3-A86D-7E3E13222F87 Core Requirement #8 - Water Quality The use of a catch basin with downturned elbows protects the infiltration trenches (and groundwater) from serious oil and other contaminant spills or seeps from vehicular areas. Also, the roof is considered a non-polluting impervious surface. This site meets the criteria for a Surface Area Exemption. Core Requirement #9 - Flow Control BMP's No flow control is required since the added storm flow is less than 0.10cfs. Core Requirement #10 - Special Reports and Studies: No previous reports or studies have been id entified for this site, except for the older and more recent short plat. Copies of these documents were not available for review. Other Permits: Permits for normal residential construction such as plumbing, electrical, etc. will be required. A right- of-way permit is required for driveway installation. Special Requirements for Full Drainage Review: Special Requirement #1 - Other Adopted Requirements - None Special Requirement #2 - Flood Hazard Area - None Special Requirement #3 - Flood Protection - None Special Requirement #4 - Source Control - None Special Requirement #5 - Oil Control - None Flow Control and Water Quality Facility Analysis and Design: Per the Core Requirements above, the stormwater from the impervious roof areas will flow to the existing storm system. Conveyance System Analysis and Design: 6” and 4” PVC pipe will be used in all stormwater locations. All slopes will be in excess of 2 percent due to the site topography, and are adequate by inspection. Minimum cover of 3 feet will be provided in any paved or concrete driveway areas. Special Reports and Studies: No special reports of studies were performed for this site. Previous short plat documents were not available for review. Other Permits: No special permits are required for site. The building permits are the only required permits for the construction. ESC Analysis and Design: DocuSign Envelope ID: 65DBE03F-B538-48F3-A86D-7E3E13222F87 As noted in the Core Requirements, the site is small and only requires the normal placement of downslope filter fence. A standard stabilized construction entrance will be used at the residence construction site entrance. Bond Quantities, Facility Summaries and Declaration of Covenant: A Declaration of Covenant with attached Operations and Maintenance Manual will be provided if previous documents are not considered applicable. Operations and Maintenance Manual: Provided with the Declaration of Covenant (see above note). Any questions, or comments may be sent via email or mail to the above address. Patrick E. Carroll, P.E. PEC Design and Consulting LLC 2621 NE 109th Street Vancouver, WA 98686 pat@pecdesign.net DocuSign Envelope ID: 65DBE03F-B538-48F3-A86D-7E3E13222F87 8/23/2022 WWHM2012 PROJECT REPORT ___________________________________________________________________ Project Name: Billing Short Plat Site Name: Billing SP Site Address: 14103 160th Ave SE City : Renton, WA Report Date: 8/23/2022 Gage : Seatac Data Start : 1948/10/01 Data End : 2009/09/30 Precip Scale: 1.00 Version Date: 2019/09/13 Version : 4.2.17 ___________________________________________________________________ Low Flow Threshold for POC 1 : 50 Percent of the 2 Year ___________________________________________________________________ High Flow Threshold for POC 1: 50 year ___________________________________________________________________ PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Lawn, Flat .17 Pervious Total 0.17 Impervious Land Use acre Impervious Total 0 Basin Total 0.17 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ MITIGATED LAND USE Name : Basin 1 Bypass: No GroundWater: No DocuSign Envelope ID: 65DBE03F-B538-48F3-A86D-7E3E13222F87 Pervious Land Use acre C, Lawn, Flat .08 Pervious Total 0.08 Impervious Land Use acre DRIVEWAYS MOD 0.09 Impervious Total 0.09 Basin Total 0.17 ___________________________________________________________________ Element Flows To: Surface Interflow Groundwater ___________________________________________________________________ ___________________________________________________________________ ANALYSIS RESULTS Stream Protection Duration ___________________________________________________________________ Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.17 Total Impervious Area:0 ___________________________________________________________________ Mitigated Landuse Totals for POC #1 Total Pervious Area:0.08 Total Impervious Area:0.09 ___________________________________________________________________ Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.01339 5 year 0.024287 10 year 0.033154 25 year 0.046203 50 year 0.057252 100 year 0.06943 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.04489 5 year 0.058748 10 year 0.068625 25 year 0.081931 50 year 0.092463 100 year 0.103541 ___________________________________________________________________ DocuSign Envelope ID: 65DBE03F-B538-48F3-A86D-7E3E13222F87