1500 INDUSTRY ST 2018-01-01 MF Import

Jane Zimmerman City of Everett Engineering&Public Works -2- August 29,2002 The proposed 1 L4 acres "equivalent area", will consists of roof generated or clean s[orm runoff from Building A through Building G and will be collected and routed to an infiltration facility to be located on- site for each building. A separate gravity conveyance pipe system will be provided to connect thc overflow of each infiltration fncility and route the overflow to discharge to the Glemvood Creek Basin. Please refer to the Developed Basin Map. Hydrolody: The methodology udlized to size the detention system for this project was such that one-half of the 2-year pre-developed storm event was the release rate during the 2-year developed condition storm. In addition, the 10-year pre-developed pwk runoff rate was the release ra[e during the 10-year pcst-developed condition s[orm. Pinally, the 100-year pre-developed peak rate o[runoff was the release rate during a 100-year,24-hour developed condition storm event. In addition, after the initial pond sizing, the pond was upsized by 39 percent based on the amount af impervious surface on-site, which generated a larger pond and all of the hydrographs were routed through the final upsized pond ro determine their actual performance. We have included the drainage calculations in Attachment"B"for your reference. Alternative Drs�inage Design for"Equivalent Area": Although this project assumes infiltration for the 11.4 acre equivalent area draining to the, Glemvood Creek Bz;in, an alternative conceptual design has been prepared in the event that fuwre soils investigation determines that infiltration is not feasible at. isolated locations. The alternative replaces the infiltration facility with an individualized deten[ion vault, while utilizing a separate conveyance system that routes the discSarge to the Glemvood Crcek Basin. Refer to the Altemative Drainage Sys[em Exhibi[ attached. We have also included detention drainage calculations per acre of impervious roof in Attachment"B" for your reference. We trust that this letter satisfies the City's concerns regarding the issue of maintaining Qie natural discharge location of the site by providing an "equivalenP'area. Please feel free to wntact myself at[his office to further discuss this matter or with questions or comments that we may be able to address. Respectfully, ��CS """- Patrick A. Taitano,P.E. Project Engineer � PAT/dm 10470c.0�l.doc enc: A[tachment"A"—Exhibit Drawings Attachmen["B"—Drainage Calculatians cc: Patrick Gemma, Panattoni Development Company, LLC Daniel K. Balmelli, Darghuusen Consulting Engineers, Inc. Ali Sadr, IIarghausen Consulting Engineers, Inc. -/L