This equation or formula calculates the velocity and flowrate of the flow that will occur in a long channel of uniform cross-section (a 'friction' equation). It has the form:

Where the depth of flow is known, the equation allows the velocity and flowrate to be estimated. However, it is most often used to determine the depth of flow corresponding to a flowrate specified by a hydrological model.  This must be done using iterative calculations.  An example is the calculation of flow characteristics along an overflow routes in DRAINS when the Lite hydraulic model is used. See the results at the end of the help topic on overflow routes from pits.

While developed for steady flow, this equation can be applied to unsteady flow situations.

In English-speaking countries the Manning’s equation is the most commonly-used friction equation applied to open channels.  It is applied to all open channels in DRAINS.  Typical values of the Manning's coefficient n for channels are:

Manning's equation can also be used for pipes. QUDM presents the following roughness coefficients for pipes in average condition:

A form of Manning's roughness is used as a retardance coefficient in the kinematic wave equation for calculating flow travel times.