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The Manning Equation is the most commonly used equation to analyze open channel
flows. It is a semi-empirical equation for simulating water flows in channels and
culverts where the water is open to the atmosphere, i.e. not flowing under pressure, and
was first presented in 1889 by Robert Manning. The channel can be any shape -
circular, rectangular, triangular, etc. The units in the Manning equation appear to
be inconsistent; however, the value k has hidden units in it to make the equation
consistent. The Manning Equation was developed for uniform steady state flow (see Discussion and References for Open Channel Flow). S is the
slope of the energy grade line and S=h_{f}/L where h_{f} is energy (head)
loss and L is the length of the channel or reach. For uniform steady flows, the
energy grade line = the slope of the water surface = the slope of the bottom of the
channel.

The product A/P is also known as the hydraulic radius, R_{h}.

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