analysis of flow resistance in open channel for proper flow prediction

International Journal of Civil, Mechanical and Energy Science (IJCMES) [Vol-2, Issue-1,Jan-Feb, 2016]
Infogain Publication (Infogainpublication.com) ISSN : 2455-5304
www.ijcmes.com Page | 1
Analysis of Flow Resistance in Open Channel for
Proper Flow Prediction
Lakshmi Mitra1
, Prof. (Dr.) Mimi Das Saikia2
1
M.Tech. Student, Department of Civil Engineering, Assam downtown University, Panikhaiti, Guwahati, India
2
Professor, Department of Civil Engineering, Assam downtown University, Panikhaiti, Guwahati, India
Abstract— In an open channel flow, the Manning’s n
depends on the bed material of the channel, the slope of
the channel and the rate of discharge in the channel.
Experiments are done with two bed materials, the
original bed surface of the channel and with grass carper.
The roughness coefficient is determined in various cases.
The variations in roughness coefficient is tested with
respect to flow parameters such as discharge, depth and
bed slope. The effect of variations the flow parameters on
different bed roughness is also analyzed by taking into
account the original flume surface and grass carpet
surface.
Keywords— Channel bed, Manning’s “n”, Open
channel flow, Rectangular flume, Roughness
coefficient.
I. INTRODUCTION
Roughness coefficient is very important to determine the
flow rate and water level during flood, design of
hydraulic structure; open channel drainage system etc.
Various types of vegetation and gravel bed give resistance
to the flood flows which results turbulence in the flow.
Vegetation prevents the erosion on the bank of the
channel and controls the movement of soil particles along
the channel bottom. Two roughness coefficients are
generally used for open channel flow – one is Chezy’s
coefficient and the other is Manning’s coefficient. Out of
these two coefficients Manning’s n is widely used and it
depends on the geometry of the channel, slope of the
channel, discharge and roughness of the channel, silting
and scouring and obstruction in flow etc. The value of
Manning’s n extremely varies with the bed and side
material. The effect of roughness also affects the state of
flow. The Manning’s n also depends on the depth of flow,
if the depth increases the Manning’s n also increases. The
bed with varying material it is very difficult to calculate
the average e Manning’s n. The purpose of this study is
to determine the influence of bed material, slope and
discharge on roughness coefficient in open channel.
Manning’s Equation
Where,
V = magnitude of velocity, A = Area, R = A/P where,
P= wetted perimeter
S = slope, n = Manning’s coefficient
II. LITERATURE REVIEW
In 1891 Robert Manning first introduced the Manning’s
formula in his paper published in Ireland. Limerinos J. T.
(1970) found the Manning Coefficient from measured bed
roughness in natural channels. Yen B. C. (2002) pointed
out the effects of cross-sectional shape, boundary no
uniformity and flow unsteadiness in addition to viscosity
and wall roughness that are commonly considered. In
2006 Ismail, Z and Shiono, K. Francisco J.M. Simões,(
2010.) derived some equations which was carried out by
using only laboratory data with plane beds made of sand
and gravel, with both uniform and graded sediments.
Omid M. H. (2010) et al conducted an experiment on
effects of bed-load movement on flow resistance over bed
forms. In 2010, Ji-Sung KIM et.al estimated Manning’s
roughness coefficient for a gravel-bed river reach using
field measurements of water level and discharge, and the
applicability of various methods used for estimation of
the roughness coefficient was evaluate. Ali1 Md Z. and
Saib N. A. (2011) did the laboratory experiment to
determine the affects of gravel bed to roughness
characteristics in channel.
III. LABORATORY EXPERIMENT
A tilting flume 6m long, 0.30 m wide and 0.46 m deep is
taken here for laboratory work as shown fig 1and fig 2.
Nine cases are considered; here with three discharges Q1,
Q2 and Q3and each discharge each with three slopes
1:100, 1:150 and 1:250 (table 1). Variations of Manning’s
n with respect to change in Depth of water, Froude’s
number has been observed and plotted for the above
mentioned nine cases from Fig 4 to Fig 12. Another test
case number 10 is also studied that is for grass carpet
roughness with discharge Q2= 0.00367 m3/sec with bed
slope 1:100. The laboratory observations for the tenth
case it has been plotted from Fig 13 to Fig 15. Fig-16

shows variation in water depth for different discharge
with different slope for the original bed material.
Table 1
No. Slope Discharge(m3/sec)
Case-1 s=1:100 Q1=0.00384
Case-2 s=1:150 Q1=0.00384
Case-3 s=1:250 Q1=0.00384
Case-4 s=1:100 Q2=0.00367
Case-5 s=1:150 Q2=0.00367
Case-6 s=1:250 Q2=0.00367
Case-7 s=1:100 Q3=0.0012
Case-8 s=1:150 Q3=0.0012
Case-9 s=1:250 Q3=0.0012
Fig 1: Rectangular flume of 6meter length
Fig 2:Flow in the original bed material of the channel
Fig 3:Flow through grass carpet
Fig 4: Manning’s “n” for Discharge Q1
Fig 5:Depth of water for Discharge Q1
Fig 6:Froude’s no. for Discharge Q1
Fig 7:Manning’s “n” for Discharge Q2

Fig 8:Depth of water for Discharge Q2
Fig 10:Manning’s “n” for Discharge Q3
Fig 11: Depth of water for Discharge Q3
Fig 13: Manning’s “n” for Discharge Q2 & slope 1:100
Fig 14:Depth of water for Discharge Q2 & slope 1:100
Fig 15:Froude’s no. for Discharge Q2 & slope 1:100

Fig 16: Depth of water for different discharges at
different slopes
IV. CONCLUSION
In this experimental study the Manning roughness coefficient
“n” is investigated for water flows, within rectangular open
channels. The Manning’s n is not constant throughout, i.e. it
depends on various parameters and not only on roughness
factors, especially in channels of combined roughness. “n”
appears not to be constant at any present roughness condition
and this is due to the nature of flow, where even at the same
run its behavior is not constant. The variation in n is more in
the original channel bed. “n” increases with the increase in
discharge as well as increase in slope. The maximum
percentage variation in n is at a distance of 2m to 3.5m from
the upstream and it is about 3% to 83% of the average “n”
value. The Manning’s n for carpet grass is very high and the
average value is 0.12. In the carpet grass almost the state of
the flow is coming to be uniform i.e. subcritical flow with less
variation. In grass carpet the maximum variation is 12%
which is at a distance 2m to 2.5m from the upstream. The
original bed material of the channel has combined roughness
which has a huge effect on the Froude No. also. The variation
in Froude’s is from 13 to 0.3 i.e. the state of flow changes
from highly supercritical flow to subcritical flow.
Irregularities of the bed surface of the channel causes an
increase in roughness coefficient of the channel. The
percentage variation in roughness coefficient in an uniform
bed materials is very less as compared to an irregular bed
material. The study was mainly concentrated on steady
slightly non uniform as a part of our detail study on channel
resistance. Further more study is required and accordingly
planned for unsteadiness effect on open channel flow
resistance. It is also proposed for the extension of the present
work for the determination of other open channel resistance
like Darcy’s “f”.
REFERENCES
[1] Yen B C, F.ASCE2
(1989) Uniform flow in a smooth
open channel. Journal of Hydraulic Research, 27(5),
603-616
[2] Dabrowski W. et.al Department of Environmental
Engineering, Cracow University of Technology,
(2010) “FLOW RATE MEASUREMENTS BY
FLUMES”
[3] Francisco J.M. , (2010) USGS Geomorphology and
Sediment Transport Laboratory, Flow resistance in
open channels with fixed and movable
[4] Ismail, Z and Shiono, K.Department of Civil and
Building Engineering Loughborough University
Loughborough, Leicestershire, LE11 3TU UK “ The
effect of vegetation along cross-over floodplain edges
on stagedischarge and sediment transport rates in
compound meandering”
[5] KIM J S, LEE C J, Won KIM, Yong-Jeon KIM,
River and Coastal Research Division, Korea Institute
of Construction Technology, Goyang 411-712,
(Korea Water Science and Engineering, 2010),
“Roughness coefficient and its uncertainty in gravel-
bed river”
[6] Hossein O M et.al, Department of Irrigation and
Reclamation Engineering, University of Tehran,
(2010) “Effects of bed-load movement on flow
resistance over bed forms”
[7] Yen, B.C. (1992). “Dimensionally homogeneous
Manning’s formula” J.Hydraulic Engineering.
[8] Yen, C.L. And Overton. D.E.(1973).Shapes effects
on resistance in flood plain channels. J. Hydraulic
Div., Am Society of Civil Engineering.
[9] Ali Z M and Saib A N (2011) Department of Water
and Environmental Engineering, Universiti Tun
Hussein Onn Malaysia 86400 Parit Raja Batu Pahat,
Johor MALAYSIA Influence of bed roughness in
open channel.

analysis of flow resistance in open channel for proper flow prediction

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