The Manning Equation is an empirical equation that was developed by the French Engineer Philippe Gauckler in 1867. It was redeveloped by the Irish engineer Robert Manning in 1890. This formula gives the relationship between several parameters of interest for uniform flow of water in an open channel.
The Manning roughness coefficient, $n$, is a dimensionless empirical constant. Its value depends on the nature of the channel and its surfaces.
The flow must be in the turbulent regime in order to use the Manning equation for uniform open channel flow. The Manning equation was developed specifically for the flow of water.