Manning’s Formula

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Definition - What does Manning’s Formula mean?

Manning's formula is an equation used to calculate water's rate of flow -- in cubic feet per second -- in an open channel, such as a ditch or flood control channel. The factors used in Manning's equation are as follows:

  1. The water's velocity in cubic feet per second
  2. The area of the flow in square feet
  3. Manning's Roughness Coefficient
  4. The hydraulic radius in feet and
  5. The channel slope expressed in feet of drop divided by feet of run.


Trenchlesspedia explains Manning’s Formula

While Manning's Equation is similar to other formulas for determining the rate of flow (the "Q") within a channel, Manning’s Roughness Coefficient is a wild card. It represents the "friction factor" that occurs when friction exists (as it always does) between the sides and bottom of a channel, and the water flowing through the channel. In other words, a channel lined with rocks applies more friction to water flow than a smooth channel lined with Teflon. The rocky channel reduces the Q within the channel.

There are tables that provide Manning's Roughness Coefficient for various materials, such as rocks, flat concrete, asphalt, grass, dirt, etc. Take care when choosing the coefficient as the choice will affect your flow calculations and any design based on those calculations. Selecting a coefficient that's too high could result in a design that promises a lower Q and may lead to channel overtopping or unmanageable downstream flows.

An alternative to picking the coefficient from a table is to use a stream flow gauge in the channel under observation. The roughness coefficient can then be calculated based on real-life measurements.

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