5901 23RD DR W SEAWAY CORP 3C 2025-07-29

I Stormwater Management Report Everett, WA Off-site conveyance calculation: 11 For the offsite portion of the drainage system, a backwater analysis was performed. This confirms that the drainage system has capacity if the Lot 3D development connects to any catch basin in the system. The conveyance system for off-site conveyance system was analyzed using the 1998 King County Backwater (KCBW) hydraulic design and StormSHED SCS Method for hydrologic design. Adequate capacity to convey peak rate runoff and adherence to all applicable system composition requirements per City of Everett Standards are incorporated within the design. See the following KCBW analysis program printout sheets for results of the backwater analysis. This section demonstrates compliance with an analysis of conveyance capacity for the storm drainage network. Storm drainage capacity analysis is based on the following assumptions: • The undetained 100-year discharge from both Lot 3C and 3D of 5.18 cfs was used for the backwater analysis. • The design 100-year water surface of the proposed detention (elev. = 420.6) was used as the tailwater elevation for this analysis. REVIEW OF THE PIPE-DATA PI OUTLET INLET IN OVERFLO END STRU Q # LENGTH DI TY ELEV ELEV TY ICE K M C Y ELEV ANG WIDE RAT I/ 1 30.00 18 1 414.50 417.25 6 .20 .0078 2.0 .0292 .74 422.25 87 2.0 0.00 2 263.00 18 1 417.25 420.00 6 .20 .0078 2.0 .0292 .74 423.70 87 2.0 0 .00 3 74.00 18 1 420.00 421.00 6 .20 .0078 2.0 .0292 .74 425.00 45 2.0 0.00 4 135.00 12 1 421.50 426.00 6 .20 .0078 2.0 .0292 .74 430.00 5 2.0 0.00 5 130.00 12 1 426.00 431.00 6 .20 .0078 2.0 .0292 .74 436.00 45 2.0 0.00 6 142.00 12 1 431.00 433.00 6 .20 .0078 2.0 .0292 .74 438.00 15 2.0 0.00 7 107.00 12 1 433.00 435.00 6 . 20 .0078 2.0 .0292 .74 BACKWATER COMPUTER PROGRAM FOR PIPES Pipe data from file:runl.bwp Surcharge condition at intermediate junctions Tailwater Elevation:420. 6 feet Discharge Range: l. to 6. Step of 0.5 [cfs] Overflow Elevation: 444 . feet Weir:NONE Upstream Velocity:3. feet/sec PIPE NO. 1: 30 LF - 18"CP @ 9. 17% OUTLET: 414 .50 INLET: 417.25 INTYP: 6 JUNC NO. 1: OVERFLOW-EL: 422.25 BEND: 87 DEG DIA/WIDTH: 2 .0 Q-RATIO: 0.00 Q(CFS) HW(FT) HW ELEV. * N-FAC DC DN TW DO DE HWO HWI 1.00 3.37 420. 62 * 0.012 0.38 0. 18 6. 10 6. 10 3.36 3. 37 0.44 1.50 3.37 420. 62 * 0.012 0. 46 0.22 6. 10 6. 10 3.36 3. 37 0.57 2.00 3.39 420. 64 * 0.012 0.54 0.25 6. 10 6. 10 3. 36 3. 39 0. 68 PACLAND Job# 50108001 Page 20