The Chezy Formula

The Chezy formula is the most common method of calculating flow in an open channel. The Chezy coefficient (C) can be calculated multiple different ways shown below.

The most common method of calculating flow in and open channel:

V = C\sqrt{RS}

where

  • V = velocity (ft/sec)
  • C = Chezy's frictional coefficient (aka coefficient of rugosity). C has been empirically calculated by Manning and Darcy-Weisbach (shown below)
  • R = hydraulic radius (ft)
  • S = slope of channel bed

C By Darcy-Weisbach

For a small, very smooth channel, the following can be used:

C = \sqrt{\frac{8g}{f}}

where

C By Manning

Large channels with fully turbulent flow (friction no longer plays a significant role but rather the channel roughness is important):

C = \frac{1.49}{n}R^{1/6} \text{ [US]}
C = \frac{1.00}{n}R^{1/6} \text{ [SI]}

where

The Chezy-Manning Equation

For simplicity, combining the Chezy Formula with the Manning roughness coefficient you'll get:

V = \frac{1.49}{n}R^{2/3}\sqrt{S} \text{ [US]}
V = \frac{1.00}{n}R^{2/3}\sqrt{S} \text{ [SI]}

where

References:

  1. http://en.wikipedia.org/wiki/Ch%C3%A9zy_formula

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