1815 13TH ST SEATTLE CHILDRENS NORTH CLINIC 2019-07-08

MAGNUSSON KLEMENCIC ASSOCIATES ■ 5.5 feet above the dead storage level). With this detention and flow control design, all flow duration criteria are met. A precast below-grade detention vault has been selected to provide detention storage due to space constraints. The detention vault design has been performed in accordance with Volume III, Chapter 3, of the COESWMM, including accommodating six inches of sediment storage (dead storage) below the discharge invert elevation. The proposed vault is made up of precast concrete panels arranged together to provide the volume of detention required. A product overview for Oldcastle Precast's Panel Vault System is included in Appendix G. The panels can span a maximum of 20 feet, which requires two internal walls within the vault. A number of wall openings through these internal walls will hydraulically connect the entire vault while providing full access for maintenance operations. To aid in sediment removal,the detention vault floor will slope 5 percent from each wall towards the center, creating a "V" channel; the six inches of sediment storage is measured up from the highest point along the vault floor so there will be additional sediment storage within the floor's "V" channel. The vault will provide a minimum of 5 percent additional volume to allow for a small contingency for the future clinic facility's design, which is currently in the schematic design phase;this contingency will allow adjustments to the future clinic design as it progresses without recalculation of detention volumes. The flow control structure will be housed in a catch basin immediately outside of the vault outlet. The vault access will comply with the requirements of Volume III, Chapter 2, of the COESWMM. Access hatches with a clear opening of 5 feet by 10 feet are proposed above the inlet and outlet, which will provide easy maintenance access to these two critical locations as required. Four additional manhole access points will complete the required access coverage (any location in the vault needs to be accessible within 50 feet from one of the provided access points). Ventilation ports at each corner of the vault will allow for artificial ventilation prior to maintenance activities. Meeting the overflow requirements outlined in Volume III, Chapter 2, of the COESWMM, the minimum of one foot freeboard is provided. With the six inches of dead storage plus approximately six inches of sloped floor, 5.5 feet of required storage volume, and this one foot of freeboard,the total inside dimension required is 7.5 feet. In addition, the two-foot diameter outfall riser has been confirmed to handle the 100-year recurrence interval developed peak flow rate at the crest elevation. The conveyance pipes between this outfall riser and the combined sewer system connection have also been confirmed to meet the 100-year flow rate; this capacity check has been included in Appendix F. CONVEYANCE ANALYSIS The proposed storm drainage design includes two general routes of conveyance to the detention vault: the northern route and the southern route. These routes consist of piping, catch basins, and manholes. See Figure 6, Conveyance Catchment Areas,for the basin delineations to each structure overlaid on the full conveyance system layout. A breakdown of the pervious and impervious areas tributary to these structures is also included for each basin. In accordance with the COESWMM requirements, the project's drainage system has been analyzed for the 25-year recurrence interval peak flow and the Rational Method was used to calculate this peak flow rate. Runoff coefficients of 0.30 for pervious areas and 0.90 for impervious areas, along with coefficients "m" and "n" provided in Table 5.2 of Volume I, Chapter 5, of the COESWMM, were used to create an Intensity-Duration-Frequency (IDF) curve for the 25-year storm event. The IDF curve was then input into Bentley StormCAD V8i (SELECTseries 4) to complete the conveyance analysis. Calculation results are included in Appendix H. Storm Drainage Report 10 Providence Regional Medical Center Everett North Site, Everett, Washington