2207 GIBSON PL 2018-05-29 - Laserfiche WebLink

i , OMEGA. 2707 Wetmore Avenue Everett, WA 98201 ENGINEERING, INC. t 425.387.3820 f 425.259.1958 October 24, 2016 APPROVED PUBLIp WORKSe_-�ADEPART ENT City , InJh� of Everett BY 3000 Cedar Ave DATE I/—lb- Everett, WA 98201 CITY OF EVERETT REVIEWED FOR PERMIT pw16/l-oa7 RE: Robinett Short Plat— 8" HDPE As-Built Conveyance Calculations Dear Staff, • This letter is being prepared to define the change in the conveyance capacity for the 8" HPDE pipe that was installed as part of the Robinett Short Plat. The 8" HDPE system collects runoff from the two short plat lots along with two other existing lots, one of which has recently applied for a building permit with the City. Project As-Builts. The proposed 8" HDPE pipe from CB #2 to the bluff was designed with a slope of 1.3%. The as-built slope of that same pipe was found to be 2.9% by ASPI the project surveyor. As a result the conveyance capacity for the as-built pipe will be greater than the original design capacity. The following is an excerpt from the approved drainage report Section 9: The peak Q for the 100-year event was calculated to be 2.16 cfs. Using Manning's equation an 8"pipe with a minimum slope of 1.32% has the capacity to convey that runoff without any backwater-conditions. Therefore, the pipe system north of CB#2 will be 8"HDPE pipe to the • bottom of the slope with diffuser tee for energy dissipation. Using the same approach, the peak capacity of the as-built 8" HDPE can be calculated to determine the addition flow capacity in the steeper pipe (s=2.9%). The new peak capacity for this system is 2.98 cfs. Refer to the attached Manning's Equation spreadsheet. The rational method calculation for the maximum impervious areas allowed to be tributary to this system can be increased. The attached rational method calculation sheet shows that the maximum impervious area can be increased to 0.97 ac. As noted above the existing SFR parcel directly to the west of the short plat has already had a building permit with drainage plan/report submitted to the City. This project proposed to construct 0.20 acres of impervious surfaces to be tributary to the 8" HDPE pipe. As a result that leaves 0.77 impervious acres ofcapacity for this system.- Lot 2 of this short plat has submitted building permit with site plan showing that 0.19 acres of impervious surfaces will be directed to the 8" pipe. Therefore, at this time there remains 0.57 impervious acres (25,000 sf) of capacity in the as-built 8" HDPE pipe system constructed for this project to serve the SP Lot 1 and the existing adjacent parcel west of Lot 1. This available capacity is expected to be more than enough since the longer driveways