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 San Antonio
 Unified Development Code
 Appendix H. STORM WATER DESIGN CRITERIA MANUAL
 Chapter 7. STORM DRAIN SYSTEMS
 Appendix 7.2. Hydraulics Of Storm Drainage Systems
 Appendix 7.2.6. Energy Losses

§ 7.2.6.1. Pipe Friction Losses  

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  • The frictional loss from the conduit is one of the losses. The head loss due to friction is determined by the following formulas:

    (Equation 7.2.6.1a, Friction Loss Formula)

    H f = S f L

    H f = Friction loss (ft.)
    S f = Friction slope (ft./ft.)
    L = Length of pipe (ft.)

    Should the conduit have partial flow, then the frictional slope will match the pipe slope. For pressure flow or full flow of the conduit, the formula below can be used to determine the frictional slope of the conduit:

    (Equation 7.2.6.1b, Pressure Flow Formula)

    35-H7-2-6-1b.png

    S f = Friction slope (ft./ft.)
    Q = flow (cfs.)
    n = roughness coefficient of conduit
    A = cross section area (sq. ft.)
    R = hydraulic radius = A/WP (ft.)
    WP = wetted perimeter (ft.)