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Flow Velocity in Straight Sewers Formula and Calculator

Civil Engineering and Design
Fluid Flow Hydraulic and Pneumatic Engineering and Design Menu

Flow Velocity in Straight Sewers Formula and Calculator

Obstruction-free straight sewers will have a flow velocity of

Formula 1.0,
V = ( C / n ) · ( R^2/3 · S^1/2 )

Where:

V = Flow velocity ft/s
n = coefficient dependent on roughness of conduit surface
R = A / P - hydraulic radius, ft = area in ft², of fluid divided by wetted perimeter, ft
S = energy loss, ft/ft of conduit length; approximately the slope of the conduit invert for uniform flow
C = 1.486 (conversion factor to account for change from metric units used in development of the formula)

The value of n can range from 0.013 in smooth concrete sewers to 0.02 in plain pipe. Further, n usually increases as a sewer pipe ages. Specific n values are given in engineering handbooks such as those cited in the references section of this webpage.

The water flow, cu ft/s is:

Q = A · V

where:

A = cross-sectional area of flow, ft².

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• Rainfall Runoff Rate Formulas and Calculator
• Pipe Roughness Coefficients Table Charts | Hazen-Williams Coefficient | Manning Factor
• Darci's Equation Fluids Flow Equation
• Head Loss Darcy - Weisback Equation
• Head Loss Darcy Calculator - Weisback Equation
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• Function for open channel flow rate with input variable for inflow

References:

• Hicks - Handbook of Civil Engineering Calculations,
• McGraw-Hill.
• Kevin D. Wills, M.S.E., P.E., Consulting Engineer.