Concept:
Manning’s formula
Manning’s formula gives an empirical formula according to which the mean velocity is expressed in terms of a coefficient of roughness (n), called Manning’s roughness coefficient.
V=n1×R32×S21;
Where,
R = hydraulic radius of the channel section,S = bed slope of the channel
Geometrics of channel
- Hydraulic Radius or hydraulic mean depth (R):
R=wetted;perimeterwetted;area=PA
- Hydraulic depth (D)
D=Top;width;of;channelwetted;area=TA
Discharge in the channel
The flow of water through the channel is given as
Q = A × V
Where,
A = Area of channel section, V = Mean velocity of water flowing through the channel
Calculation:
Given,
S = 1 in 1000, n = 0.01
1) when the pipe is flowing just full
Qfull = Afull × V
Afull = (πr2), P = 2πr
R=PA=2πrπr2=2r=4d;
Qfull = A × V
Qfull=;(πr2)×;n1×R32×S21
Qfull=;(πr2)×;0.011×(4d)32×(10001)21
2) when the pipe is flowing half
Qhalf = Ahalf × V
Ahalf = (πr2)/2, Phalf = πr
R=PA=πr2πr2=2r=4d;
Qhalf = A × V
Qhalf=(2πr2)×;n1×R32×S21
Qhalf=;(2πr2)×;0.011×(4d)32×(10001)21
3) The ratio of Qfull to Qhalf is
\(\frac{{{{\rm{Q}}{{\rm{full}}}}}}{{{{\rm{Q}}{{\rm{half}}}}}} = \frac{{{\rm{;}}\left( {{\rm{\pi }}{{\rm{r}}^2}} \right){\rm{;;}} \times {\rm{;}}\frac{1}{{0.01}} \times {{\left( {\frac{{\rm{d}}}{2}} \right)}^{\frac{2}{3}}} \times {{\left( {\frac{1}{{1000}}} \right)}^{\frac{1}{2}}}}}{{\left( {\frac{{{\rm{\pi }}{{\rm{r}}^2}}}{2}} \right){\rm{;;}} \times {\rm{;}}\frac{1}{{0.01}} \times {{\left( {\frac{{\rm{d}}}{2}} \right)}^{\frac{2}{3}}} \times {{\left( {\frac{1}{{1000}}} \right)}^{\frac{1}{2}}}}} = \frac{2}{1};\)
\(\frac{{{{\rm{Q}}{{\rm{full}}}}}}{{{{\rm{Q}}{{\rm{half}}}}}} = \frac{2}{1};\)
Important points:
The circular channel section running half-full on one day and running full on another day, the ratio of the velocity of flow of section running full to running half is 1. As the hydraulic mean depth (R) is same in both the condition.
\({\rm{i}}.{\rm{e}}\frac{{{{\rm{V}}{{\rm{full}}}}}}{{{{\rm{V}}{{\rm{half}}}}}} = 1\)
The circular channel section running half-full on one day and running full on another day, the ratio of the headloss due to friction of section running full to running half is 1. As the mean velocity is same in both the condition.
\({\rm{i}}.{\rm{e}}\frac{{{{\rm{(h_f)}}{{\rm{full}}}}}}{{{{\rm{(h_f)}}{{\rm{half}}}}}} = 1\)