Loading...
HomeMy WebLinkAboutRS_Sierra Heights ES Pavement - Drainage Memo_20250516_v3 1932 First Avenue, Suite 500 Seattle, WA 98101 p. 206.725.1211 f. 206.973.5344 lpdengineering.com Memorandum Date: May 15, 2025 To: City of Renton Community & Economic Development 1055 South Grady Way, 6th Floor Renton, WA 98055 From: Lily Siu, PE Project: Sierra Heights Elementary School Parking Lot Upgrades z 2501 Union Avenue NE Subject: Drainage Memorandum This drainage memorandum addresses the improvements for the Sierra Heights Elementary School paving improvements project. The site is located on two parcels (#0423059069 and #0423059176), at 2501 Union Avenue NE in Renton, Washington. According to Section 1.1.1 of the 2022 City of Renton Surface Water Design Manual (SWDM), full drainage review is required for this project since it proposes more than 2,000 square feet (SF) of new plus replaced impervious surface. Project Overview The project includes approximately 17,913 SF [0.411 acres] of pavement replacement and 1,428 SF [0.033 acres] of concrete walkway replacement onsite, as well as 157 SF [0.004 acres] of new pavement and 914 SF [0.021 acres] of new concrete walkway are proposed. The total replaced impervious surfaces are 19,341 SF [0.444 acres] and the total new impervious surface is 1,071 SF [0.025 acres]. Within the right-of-way on Union Avenue NE, 943 SF [0.022 acres] of pavement replacement, 672 SF [0.015 acres] of concrete walkway replacement, and 23 SF [0.001 acres] of new concrete walkway is proposed. The total replaced impervious surfaces in the right-of-way are 1,616 SF [0.037 acres] and the total new impervious surface is 23 SF [0.001 acres]. Existing Conditions Sierra Heights Elementary School is located on two parcels (#0423059069 and #0423059176), totaling approximately 14.49 acres. The site is located in Section 4, Township 23 North, Range 5 East, Willamette Meridian. It is zoned School (Public). The school campus is bounded by a wooded area and single-family residential property to the north, Union Avenue NE to the east, single-family residential properties to the south, and a power line corridor to the west. The site is located within the May Creek subbasin of the Cedar River/Lake Washington drainage basin. Stormwater runoff from the existing parking lots is collected by catch basins and conveyed to the public storm system in Union Avenue NE. Topographically, the property slopes down from south to north. The south parking lot elevation ranges from about 451 to 446 while the north parking lot goes from 445 to 440. The USDA Natural Resources Conservation Service lists the underlying soil for the site as Alderwood gravelly sandy loam, 8 to 15 percent slopes. According to King County and City of Renton GIS mapping, there is a wetland on the far west side of the site. The proposed project is limited to the DEVELOPMENT ENGINEERING njanders 06/05/2025 May 15, 2025 Sierra Heights Elementary School Drainage Memorandum Page 2 parking lots on the east side of the site and will not impact the wetland. There are no other critical areas shown within or adjacent to the site. Core Requirements According to Table 1.1.2.A in the SWDM, projects triggering full drainage review are required to address Core Requirements #1-9 and Special Requirements #1-5. Core Requirement #1 – Discharge at Natural Location (1.2.1): The project proposes no changes to the flow regime. Core Requirement #2 – Off-site Analysis (1.2.2): Stormwater from the parking lots is collected by catch basins and conveyed to a structure near the northeast corner of the property. Flow crosses the east property line in a 12-inch pipe which connects to the 24-inch public storm system in Union Avenue NE. The piped system discharges into a roadside ditch just north of the site. The ditch continues north, eventually discharging to May Creek at a point approximately 0.28 miles north of the site. The project will not modify the drainage characteristics of the site and is not expected to have any adverse impact on the downstream system. Core Requirement #3 – Flow Control (1.2.3): Per the City of Renton Flow Control Application Map, Reference 15- A in the 2017 SWDM, the project site is located within Flow Control Duration Standard area. The project is a redevelopment, includes less than 2,000 SF of new impervious, does not include any new pervious surface, and the value of the proposed improvements does not exceed 50% of the assessed value of the existing site improvements. Therefore, according to Section 1.2.3.1 B in the SWDM, there are no target surfaces requiring flow control mitigation. Core Requirement #4 – Conveyance System (1.2.4): The previously proposed and existing onsite conveyance systems will not experience a change in flow characteristics as a result of the proposed project and will not need to be analyzed for conveyance capacity per SWDM 1.2.4.2. Core Requirement #5 – Erosion and Sedimentation Control (1.2.5): A Temporary Erosion and Sediment Control (TESC) plan has been included with the plan set. The TESC plan will be considered the minimum for anticipated site conditions. The Contractor will be responsible for implementing all TESC measures and upgrading as necessary. The TESC facilities will be in place prior to any demolition, clearing, grubbing or construction. Core Requirement #6 – Maintenance and Operations (1.2.6): The project will not add any new drainage facilities, all existing facilities are being regularly maintained by the School District. Core Requirement #7 – Financial Guarantees and Liability (1.2.7): This project is owned by a public entity, the Renton School District, and therefore the project will not need to comply with Section 1.2.7 of the SWDM. Core Requirement #8 – Water Quality (1.2.8): Per section 1.2.8 of the SWDM, this project meets exemption 3 for Core Requirement #8 since it is a redevelopment project with the following conditions: the total valuation of the project’s proposed improvements is less than 50% of the assessed value of the existing site; the project proposes less than 5,000 SF of new pollution generating impervious surfaces (157 SF proposed); the project proposes no new pervious surfaces. Therefore, water quality facilities are not required. Core Requirement #9 – On-Site BMPs (1.2.9): The project site proposed more than 2,000 SF of new plus replaced impervious surfaces which will require an evaluation of BMP feasibility for all new impervious surfaces, new pervious surfaces, and replaced impervious surfaces that are not already mitigated with an approved on-site BMP or flow control facility. Per section 1.2.9.2 of the SWDM, this project will be subject to Large Lot BMP Requirements since the project site is larger than 22,000 SF. The drainage system layout onsite will remain unchanged. The majority of the new and replaced impervious surfaces onsite is pavement replaced in-kind, resulting in no change to drainage May 15, 2025 Sierra Heights Elementary School Drainage Memorandum Page 3 characteristics. Due to the locations of the replaced surfaces within other existing improvements, on-site BMPs are infeasible. The new impervious surfaces are access and ADA improvements that widen existing walkways, or straighten the existing driveway. The widened areas of the proposed walkways are immediately adjacent to and slope towards the existing walkways. The realigned driveway entrance with 157 SF of new impervious surface and 401 SF of replaced impervious surface is also immediately adjacent to and slopes towards the existing pavement surfaces. All BMPs listed in section 1.2.9.2 of the SWDM are infeasible for these new and replaced target surfaces. Special Requirements Special Requirement #1 – Other Adopted Area-Specific Requirements (1.3.1): The City of Renton Surface Water Design Manual is one of several adopted regulations in the City of Renton that apply requirements for controlling drainage on an area-specific basis. Other adopted area-specific regulations include requirements that have a more direct bearing on the drainage design of a proposed project. These regulations include the following: · Master Drainage Plans (MDPs) – The project is not within an area covered by an approved Master Drainage Plan. The project is not a Master Planned Development, a Planned Unit Development, a subdivision that will have more than 100 lots, a commercial development that will construct more than 50 acres of impervious surface, and will not clear more than 500 acres within a drainage sub-basin. Therefore, a Master Drainage Plan is not required. · Basin Plans (BPs) – The May Creek Basin Action Plan was prepared in April, 2001. The recommendations listed in this basin plan do not impose any additional requirements applicable to the project. · Salmon Conservation Plans (SCPs) – A Chinook Salmon Conservation Plan was prepared for the Lake Washington / Cedar / Sammamish Watershed (WRIA 8), dated July, 2005. The recommendations listed in the Conservation Plan do not impose any additional requirements applicable to the project. · Lake Management Plans (LMPs) – The project is not within an area governed by an LMP. · Hazard Mitigation Plan – According to the Renton Annex Hazard Mitigation Plan, the site is not within any mapped hazard areas. The Hazard Plan does not list any additional requirements that would apply to this project. · Shared Facility Drainage Plans (SFDPs) – The proposed project is not within an area with an SFDP. Special Requirement #2 – Flood Hazard Area Delineation (1.3.2): According to the FEMA flood insurance rate map, the project is within Zone X – “areas determined to be outside the 0.2% annual chance floodplain.” Special Requirement #3 – Flood Protection Facilities (1.3.3): There are no existing flood protection facilities on- site, nor does the project propose new flood protection facilities. Special Requirement #4 – Source Control (1.3.4): This project will not involve any activities listed in the SWDM that require the use of structural source control measures. During construction, BMPs for activities A-20, Concrete and Asphalt Application, and A-31, Vehicle and Equipment parking and Storage, must be implemented as described in the King County Stormwater Pollution Prevention Manual. Special Requirement #5 – Oil Control (1.3.5): The project will not have high-use site characteristics nor is it an existing high-use site; therefore, oil control is not required.