This means the greater the hydraulic radius, the larger volume of water the channel can carry.
In wide rectangular channels, the hydraulic radius is approximated by the flow depth.
The friction coefficients across weirs and orifices are less subjective than n along a natural (earthen, stone or vegetated) channel reach.
Accordingly, more error is expected in estimating the average velocity by assuming a Manning's n, than by direct sampling (i.e., with a current flowmeter), or measuring it across weirs, flumes or orifices.
Overbank n values for a given reach will vary greatly depending on the time of year and the velocity of flow.
It is the only best and sound method to estimate the energy loss in human made open channels.
It has long been accepted that the value of n varies with the flow depth in partially filled circular pipes.
[10] These equations account for the variation of n with the depth of flow in accordance with the curves presented by Camp.