11215 19TH AVE SE 2025-07-29 - Laserfiche WebLink

5.0 CONVEYANCE ANALYSIS AND DESIGN Conveyance analysis and design methods were selected per the City of Everett Stormwater Management Manual, Volume 1, Chapter 6 - Conveyance Systems and Hydraulic Structures. The conveyance system was designed for the 25-year, 24-hour storm event (precipitation rate = 2.60 in.), using the rational method with Everett IDF Tables. Per 6.4.1, the Uniform Flow Method (Mannings Equation) was performed to ensure that during the 25-year, 24-hour storm event, there would be a minimum of a half foot of freeboard between water surface elevation and the structure rim elevation. The conveyance analysis for the drainage system was completed using StormShed 3G. The tailwater elevation for Vaults A and B, as necessary for the HGL analysis, was set at the 25-year design water surface elevations of 443.32 and 438.43 feet respectively. During the 25-year storm, all headwater elevations within the system as designed remain more than a half foot below the catch basin grate elevations. The conveyance system is therefore adequately designed. Conveyance calculations and reports are included below. The vault discharge system is comprised of the primary flow intake(s), the vault detention volume, the vault outfall, the control structure and a control structure with overflow riser. The conveyance analysis for the vault discharge system was done as a Manning's equation evaluation of the most restrictive downstream pipe length, contrasted with having the flattest slope and the greatest unmitigated flow rates. Vault A Discharge Pipe: Diameter: 12" Minimum Slope: 0.6% Pipe Capacity: 3.99 cfs Maximum Flow: 2.83 cfs Evaluation: System Adequate Vault B Discharge Pipe: Diameter: 18" Minimum Slope: 0.55% Pipe Capacity: 11.28 cfs Maximum Flow: 6.98 cfs Evaluation: System Adequate Combined Discharge Pipe (Vault A and Vault B): Diameter: 18" Minimum Slope: 0.6% Pipe Capacity: 11.69 cfs July 2014 5-1 LDC Job No. 13-144