Today, the Manning equation is probably the most popular for practical open-channel flow computations, including hydraulic computer models. It is easy to use, gives results that range from reasonable to accurate in many situations, and is accepted by the industry. It has served us well for many years and is to be commended. However, the results from the Manning equation are essentially at the mercy of n -values. And the selection of appropriate n values is as much an art as a science.
Many sources offering guidance are available on n selection.
But, due to the variability found in nature, it is difficult, if not impossible, to accurately estimate n in complex hydraulic situations. Manning s equation is commonly used in natural channels for conditions that are not consistent with that from which it was developed. These conditions include non-uniform reaches, unsteady flow, irregular shaped channels, turbulence, steep channels, sediment and debris transport, moveable beds, etc. As a result, much research as been performed on n -values. Most improvements pertaining to the Manning equation have come about by variation on a theme the original design of the Manning equation remains an industry standard.