Loading...
HomeMy WebLinkAboutEX_11_M_Civil_Narrative_220812_v1Civil Engineering Narrative Project Name: Sounders FC Conditional Use Permit Project Address: Prepared By: Tim Brockway, PE Date: August 8th, 2022 INTRODUCTION The Seattle Sounders FC Headquarters & Training Center Project is located within the City of Renton, situated west of Oakesdale Ave SW. The site is in the NE ¼ of NW ¼ and NW ¼ of NE ¼ of Section 25, Township 23 North, Range 4 East, Willamette Meridian The site will occupy lots 088670-0110, -0120, -0130, -0210, and - 0220 for a total disturbed area of approximately 14.9 acres. Overall, the project will include 575,000 square-feet of sports fields, approximately 718,000 cubic-feet of floodplain mitigation, and the installation of storm conveyance infrastructure. Soils for the area were mapped using the King County Soil Survey maps (See Figure 3 – USDA Soil Survey Map), and a Geotechnical exploration has been performed to address slope and soil characteristics. This project proposes to construct a drainage system with a new network of underground pipes and catch basins to collect surface water runoff throughout the site and direct it to a water quality treatment facility that will be constructed adjacent to Pond B which exists on the property. Treated water from that facility will flow into the existing Pond B, along with higher storm events that are not subject to the requirements for water quality treatment. Pond B was designed for the Boeing Master Plan, and was constructed to accommodate the full detention requirements for the overall Longacres Campus, of which this Sounders Field Complex site is a part. From Pond B, the storm runoff flows into the North Pond (Pond A), which in turn flows out to Springbrook Creek and ultimately the Duwamish River. This system is consistent with the conditions of the original Longacres Development Agreement with the City which was the basis for the existing development on the property. Current drainage code matches the original design requirements for the existing property, making this proposal to avoid new detention feasible within City code. EXISTING CONDITIONS The existing site consists of open grassy areas, a large existing office building with associated plaza and walkways. See Table 1 for site surface cover information. The site was previously home to the Longacres Racetrack outbuilding stable and support services area, which was demolished during the development of the Longacres Office Park. Minimal conveyance systems exist within the project site area and runoff now leaves the site via surface flow which makes its way ultimately to the South Pond B. There is a total elevation change of approximately 7 feet across the site, from 14.7 feet along the pond edge in the western region to 21.5 feet in the eastern region of the project limits. The site is generally otherwise considered “flat” terrain. GRADING AND FLOOD PLAIN MITIGATION Overall, the project will include approximately 575,000 square-feet of sports fields, which will result in the placement of approximately 718,000 cubic-feet of fill within the currently delineated floodplain, as shown in City and FIRM documents. As required by code, floodplain fill mitigation will be required in the form of compensatory added volume below the record Base Flood Elevation (BFE). This BFE is listed as at 20.0 feet elevation at the southern area of the Sounders fields earthwork, and roughly 19.6 to 19.7 at the northern edge. In order to be conservative and to account for the immediate need to provide mitigation, our proposed compensatory volume is calculated using a BFE of 20.0 across the entire field project limits. RECEIVED 10/12/2022 JDing PLANNING DIVISION DocuSign Envelope ID: A56F3DC7-D564-4883-A9F5-21D84A1EE521 CUP CIVIL NARRATIVE 2 The required compensatory volume is proposed to be provided using a series of underground storage chambers (StormTech or Eq) to hold the required flood plain fill mitigation volume of floodwater, in order to comply with floodplain requirements as outlined in the City code. These systems are recognized as reliable and are widely used for detention storage throughout Western Washington. These chambers will connect to open, screened inlet/outlet pipes that extend below the Pond B water surface. The chambers will begin to store replacement flood volume when the pond elevation reaches its maximum detention water surface elevation of 14.7 feet. Greater volume than the 718,000 cubic feet that is required to compensate for the fill of the field project, is proposed with this project. The intent is to set aside or “bank” additional compensation volume for future fill placed elsewhere on the Longacres campus under potential future redevelopment projects. PROPOSED DRAINAGE SYSTEM This project proposes to construct a drainage system with a new network of underground pipes andcatch basins, to collect the drainage from the surface and under-drain system of the sports fields on the site, as well as the surface water runoff throughout the walkways and other areas adjacent to the fields within the project site. This system will direct runoff to a flow splitter that will direct larger flows that do not require water quality treatment, directly to Pond B. Smaller flows will be directed to the water quality treatment system. Flows from this facility will also be directed into the existing Pond B. See Table 2 for site surface cover information. According to Figure 1.1.2.A of the 2022 Renton Surface Water Design Manual (RSWDM), this project meets the criteria for a Full Drainage Review, including water quality requirements. All water quality on site will be provided in a system, designed according to Chapter 6 of the 2022 RSWDM and complying as required in that section with State Ecology GULD designation requirements. In order to provide the required enhanced basic (I did not think we were code-mandated to treat for Phosphorus?) water quality treatment for the soccer fields, the project plans to implement a Filterra Bioscape system, along the Pond B edge, downstream of the fields. This system has General Use Level Designation (GULD) approval for enhanced and phosphorous treatment, therefore allowing its use per the RSWDM. UTILITIES This project will construct a maintenance building in the SE corner of the project site, and will be extending water service to this building from an existing on-site main, and will extend a sewer main into the site from the main in Oakesdale. This will require a King County permit along with the City development review and permitting. There will also be one or more irrigation meters proposed for the fields, and the exact count and location are being determined. A section of existing water main lies beneath an area of improvements, and will require re- location and replacement, along with a revision or replacement of the existing easement. DocuSign Envelope ID: A56F3DC7-D564-4883-A9F5-21D84A1EE521