Overbank Flow Condition in a River Section

ACEE Int. J. on Civil and Environmental Engineering, Vol. 01, No. 01, Feb2011
© 2011 ACEE 44
DOI:01.IJCEE.01.01.522
Overbank Flow Condition in a River Section
Dr. Kishanjit Kumar Khatua1
, Prof. Kanhu Charan Patra1
and Sabyasachi Behera
NIT, Rourkela, 1
Department ofCivil Engineering -769008, kkkhatua@nitrkl.ac.in
Abstract—When the flows in natural or man made channel
sections exceed the main channel depth, the adjoining
floodplains become inundated and carry part of the river
discharge. Due to different hydraulic conditions prevailing in
the river and floodplain of a compound channel, the mean
velocity in the main channel and in the floodplain are different.
This leads to the transfer of momentum between the main
channel water and that of the floodplain making the flow
structure more complex. Results of some experiments
concerning the overbank flow distribution in a compound
channel are presented. Flow sharing in river channels is
strongly dependant on the interaction between flow in the
main channel and that in the floodplain. The influence of the
geometry on velocity and flow distribution and different
functional relationships are obtained. Dimensionless
parameters are used to form equations representing the over
bank flow sharing in the subsections. The equations agree
well with experimental discharge data and other published
data. Using the proposed method, the error between the
measured and calculated discharge distribution for the a
compound sections is found to be the minimum when compared
with that using other investigators.
Index Terms—Over bank Flow, flow interaction, horizontal
interface, Flow distribution, Velocity distribution, Main
channel, Flood plain
I. INTRODUCTION
During floods, a part of the river discharge is carried by
the main channel and the rest is carried by the floodplains
located to its sides. Due to different hydraulic conditions
prevailing in the river and floodplain, mean velocity in the
main channel and in the floodplain are different. This leads to
the transfer of momentum between the channel section and
the floodplain. At the junction region between the main
channel and that of the floodplain [9] indicated the presence
ofartificial banks made ofvortices, which acted as a medium
for transfer of momentum. At low depths of flow over
floodplain, transfer of momentum takes place from the main
channel flow to the floodplain leading to the decrease in the
main channel velocity and discharge, while its floodplain
components are increased. And at higher depths over
floodplains the process of momentum transfer reverses, the
floodplain supplies momentum to the main channel.
Information regarding the nature of flow distribution in a
river channel is needed to solve a variety of river hydraulics
and engineering problems such as to give a basic
understanding of resistance relationship, to understand the
mechanism of sediment transport etc.. The flow and velocity
distribution in compound sections have been investigated
bymany investigators (e.g. [3], [4], [5], [1], [6], [7], [2], etc.).
The zonal or sub-area flow distributions in the main channel
and floodplain of compound channel mainly depend on the
channel geometry and flow parameters. An investigation is
made to obtain the flow distribution between lower main
channel, and floodplain for compound sections.
II. EXPERIMENTALSETUPANDPROCEDURE
Using the fund from the department of science and
technology, Government of India, the authors have built a
numbers of flumes in the water resources and hydraulic
engineering laboratory of the Civil Engineering Department
of the National Institute of Technology Rourkela, India.
Within the flumes, experimental channels with floodplains
are built using Perspex sheets. Details of the geometrical
parameter and hydraulics parameter of the experimental
channels are given at Table 1 and Table2 respectively.
Photo. 1. Experimental Set up for the compound
Figure 1. Division of a compound section into sub areas by an
assumed vertical interface.
A recirculating system of water supplyis established with
pumping of water from an underground sump to an overhead
tank from where water could flow under gravity to a stilling
tank. From the stilling tank water is led to the experimental
channel through a baffle wall. A transition zone helped to
reduce turbulence of the flow water. An adjustable tailgate at
the downstream end of the flume is used to achieve uniform
flow over the test reach in the channel for a given discharge.
Water from the channel is collected in a volumetric tank
(Photo. P 3)for measuring the flow discharge, from where
water runs back to the under ground sump, this establishing
a closed circuit of flow. The channel sections are made from
Perspex sheets for which the roughness of floodplain and
main channel are taken as smooth and identical. The
observations are made at the section of maximum curvatures
(bend apex) of the meandering channel geometries. The
measuring devices consist of a point gauge mounted on a
traversing mechanism to measure flow depths having a least
count of 0.1 mm. Point velocities are measured using a 16-
Mhz MicroADV (Acoustic Doppler Velocity-meter) having
least count of 0.001m/s. Guide rails are provided at the top of
the experimental flume on which a traveling bridge is moved
in the longitudinal direction of the entire experimental
channel. The point gauge and the micro-ADV attached to

© 2011 ACEE 45
DOI:01.IJCEE.01.01.522
the traveling bridge can move in both longitudinal and the
transverse direction of the experimental channel at the bridge
position.
TABLE I. GEOMETRY PARAMETERS OF THE EXPERIMEN-
TAL COMPOUND CHANNELS
TABLE II.
FLOW AND FLOW DISTRIBUTION FOR OVER BANK FLOW
AT BEND APEX OF EXPERIMENTAL CHANNELS.
The micro-ADV readings are recorded in a computer placed
besides the bridge.
III. OVERBANK FLOWDISCHARGE RESULTS
Duetotransfer ofmomentum between floodplain andmain
channel, the percentage of flow carried by the main channel
with depth does not followsimple area ratios.At lower depths
of flow over floodplain, the difference between percentage
offlowin main channel and percentageofarea ofmain channel
is positive indicating that the main channel carries a greater
percentage of flow than the simple area percentage. As the
depth of flow over floodplain increases, the percentage of
flow in main channel reduces. Plots of the isovels for the
longitudinal velocities are used to obtain the area-velocity
distributions that are subsequently integrated to get the
discharge of the main channel and floodplains sub-areas
separated by assumed vertical interface planes. The total
discharge of the compound channel is used as a divisor to
calculate the percentages of discharge carried by the main
channel and floodplain subareas or zones. When a horizontal
interface is used, the area of main channel is denoted by the
area aRSa (Fig.1). The flowpercentage carried bythis area is
represented as %Qlmc with depth ratio [b?= (H–h)/H)] for
the compound channels are given in Table 2.
IV. THEORITICALANALYSISANDMODEL
DEVELOPMENT
From investigations it is a well known fact that, in overbank
flow conditions of a river channel, there are two significant
dimensionless channel geometries i.e. width ratio ( and the
relative depth ( that effects the flow distributions in a river.
Therefore for an overbank river flow under uniform
conditions, the percentages of ratio of flow in main channel
to the total flow can be written as
),(% lmcQ (3)
Figre.3 Variation of percentage of flow in lower main channel
(%Q lmc
) against corresponding area of lower main channel for a
compound channels
where %Qlmc
= the percentage of flow in the main channel
subsection ofa compound channel obtained by the imaginary
horizonal interface plains of separation. Knight and
Demetrious [3] have presented an empirical equation for flow
carried bythe main channel (%Qlmc
) as
  
  




 9.152
25.1
3.5
1
300
11
)1(100
% 





 



 eQlmc (4)
where andhave their usual meanings defined before.
Patra and Kar [8] modified equation (4) for their meandering
compound channel and proposed %Qmc
as
  
    



 
/3613.5
1
300
11
)1(100
% 9.152
25.1
rlmc SLneQ 




 



 
(5)
where Sr
= the sinuosityof the meandering channels and =
the aspect ratio of main channel = b/h, b = width of main
channel and h = bank full depth of main channel. Adequacy
of equations (4 and 5) for flow distribution in straight and
meandering compound channel for the range of a up to 5.25
are discussed by the respective authors. For channels having
higher width ratio like, equation (5) gives higher percentages
of error between observed and calculated discharges.
Though the equation gives satisfactoryresults for low width
ratio channels (=2.00) and lesser satisfactory for higher
width ratio channels (=4.0). Using the present compound
channel data, further analysis is made here to improve
equation (4 and 5) for better generalization of equations. The
equations developed by [3] and [8]shows that the percentage
of flow carried by the lower main channel follow linearly to
the simple area ratios (%Almc
). To know the dependency, the
variation of (%Qlmc
) with the area ratio(%Almc
) for the present
compound channel Type-I and the straight channel of Knight

© 2011 ACEE 46
DOI:01.IJCEE.01.01.522
and Demetrious (1983) are plotted in Fig.3.
Figre.4 Variation of calculated verses observed value of flow
distribution in Lower main channel (%Q lmc
) for straight
compound section
From the plots the best fit power function ?is found instead
of a linear function. The equation is therefore modeled as
  0067.1
%0277.1% lmclmc AQ  ( 6)
Since for a rectangular main
channel
  11
1





A
Almc
substituting in (6a) we get
  
0067.1
11
)1(100
0277.1% 










lmcQ (7)
The variation ofcomputedpercentage offlow in main channel
with the observed value of Type-I along with channels of
Knight and Demetrious[3]isshown in Fig. 4.The figureproves
the adequacy ogf the present model for overbank flow
conditions of a compound river section.
CONCLUSIONS
The following conclusions are drawn:
1. The flow between the lower main channel and floodplain
sub-sections of a compound river section are examined
and a reasonable relationship to predict the sub-section
discharge for the types of geometry is proposed.
2. A set of compound channel data of different width ratio
varying from 2.00 to4.00 and depth ratiotested up to 0.4
are studied. For a compound channels the important
parameters effecting the flow distribution are relative
depth () and the width ratio (),
These dimensionless parameters are used to form general
equations representing the total overbank flow
percentage carried by lower main channel.
3. Theproposed analytical model issimple but morereliable
and found to gives reasonable results for the compound
channel ofall types of geometry. The proposed equations
give less error for the present channels. The models also
have been validated well to the data of Knight and
Demetrious [3].
ACKNOWLEDGMENT
The authors wish toexpress their thanks tothe department
of Science and Technology, India for providing the financial
support for building the flumes, channel set up and other
accessories for carrying out the research project in the
National Institute of Technology, Rourkela, India.
REFERENCES
[1] Bhattacharya, A. K.‘‘Mathematical model of flow in compound
channel.’’ PhD thesis, IIT, Kharagpur, India. 1995
[2] Khatua,K.K,”Interaction of flow and estimation of discharge
in two stage compound channels”, Thesis Presented to the
National Institute of Technology, Rourkela, in partial
fulfillments of the requirements for the Degree of Doctor of
Philosophy, 2007.
[3] Knight, D.W., and Demetriou, J.D., “Flood Plain and Main
Channel Flow Interaction”. Journal of Hyd. Engg., ASCE
Vo.109, No.8, pp-1073-1092, 1983.
[4] Myers, W.R.C., “Velocity and Discharge in Compound
Channels”, Jr. of Hydr. Engg., ASCE, Vol.113, No.6, pp.753-
766, 1987.
[5] Myer, W.R.C., and Lyness, J.F., “Discharge Ratios in Smooth
and Rough Compound Channels”, Jr. of Hydr. Eng., ASCE,
Vol., 123, No.3, pp.182-188, 1997.
[6] Patra,K.C,,” Flow interaction of Meandering River with Flood
plains “, Thesis Presented to the Indian Institute of Technology,
Kharagpur, at Kharagpur, in partial fulfillment of the
requirements for the Degree of Doctor of Philosophy, 1999.
Patra, K. C., & Kar, S.K., “Flow interaction of Meandering
River with Flood plains” Journal of Hydr. Engineering, ASCE,
Vol., 126, No.8, pp.593-603, 2000.

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