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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 ft^{2}, 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, ft^{2}.

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References:

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