Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 13

Later on, different scientists and engineers worked on this formula. After con-
ducting a series of experiments, Kutter, Basin, and Manning proposed a method for
determining “ C ” in Chezy's formula. But due to simplicity, Manning's formulation
is widely used.

1.2.4.2 Manning's Equation

1
N

R 1 / 6

= ×

Manning suggested C in Chezy's formula. Manning's equation is

commonly expressed as

1
N R 2 / 3 S 1 / 2

V = (1.8)

where

V = average flow velocity, m/s

N = Manning's roughness coefficient

S = Channel slope, in m per m

R = Hydraulic radius, m, calculated as R = A / P

Flow cross-sectional area, in square meter (m 2 )

A =

P = Wetted perimeter, m

The Manning's equation is best used for uniform steady-state flows. Though
these assumptions are rarely achieved in reality, Manning's equation is still used
to model most open channel flows. Manning's equation is a semiempirical equation.
Thus, the units are inconsistent and handled through the conversion factor.

1.2.4.3 Manning's “N” Values

Manning's “ N ” value is an important variable describing material roughness in open
channel flow computations. Manning's “ N ” values depend on many physical char-
acteristics of channel surface. Changes in this variable can significantly affect flow
discharge, depth, and velocity estimates. So care and good engineering judgment
must be exercised in the selection process of “ N ”. The composite “ N ” value should
be calculated where the lining material, and subsequently Manning's “ N ” value,
changes within a channel section (Table 1.2 )

Table 1.2 Manning's
roughness coefficient for
different artificial channels

Channel type/lining type N value

Concrete 0.011-0.013

Stone masonry 0.03-0.042

Soil cement 0.02-0.025

Bare soil 0.02-0.023

Vegetative waterway a

0.15-0.35

a For medium, dense, and very dense grass, N should
be 0.15, 0.25, and 0.35, respectively.