Dr Lai Sai Hin <email@example.com>
Nor Azazi bin Zakaria <firstname.lastname@example.org>
Aminuddin bin Ab Ghani <email@example.com>
Zorkeflee Abu Hasan <firstname.lastname@example.org>
Chang Chun Kiat <email@example.com>
Mohd Fazly Bin Yusof <firstname.lastname@example.org>
Leow Cheng Siang <email@example.com>
Dr Onni Suhaiza bt Selaman <firstname.lastname@example.org>
In analyzing flow through river channels, one of the most common tasks of a
river engineer is to make estimates of discharge based on an estimated,
recorded or simulated water level. This is very important not only to
ensure sufficient water supply and waste disposal etc. during low flow, but
also for practical purposes such as flood forecasting and flood mitigation
during overbank flow or extreme water level.
For inbank flow the theoretical determination of the stage-discharge
relationship at a given cross-section of a river is a straightforward
issue. It is sufficient, in general, to use the overall hydraulic radius as
the parameter, which characterizes the properties of the cross section. It
is then possible to calculate the discharge through the channel from one of
a range of well-known uniform flow formulas (such as manning or Darcy-Weisbach)
in term of the channel roughness, slope and depth.
However, once the river is in flood, and flowing
out-of-bank, it becomes much more difficult due to the complex 3D turbulent
structure and interactions existed at the interface region between main
channel and flood plain. These interactions (Figures 1 and 2), i.e.
momentum transfer and apparent shear can significantly reduce the discharge
capacity of a river. Due to this reason, traditional flow equations and
methods in overbank flow discharge estimation were found to be not very
accurate, and may lead to very serious over-or underestimation of discharge
capacity. This has become the subject of considerable researches in the
past 30 years focusing on various aspect of compound channel such as flow
distribution, stage discharge relationship, surface roughness,
apparent shear, discharge estimation, and etc.
Figure 1: Mechanisms of overbank flow in a
straight compound channel
(after Knight and Shiono, 1990).
Vortices form at the interface of main channel and flood plain
"compound" covers channel cross-sections having berms or flood
plains that come into action at high flows but which are
normally dry. Due to the two-stage geometry similar to that of
overbank flow in natural rivers, this type of channel has been
study extensively to understand the interaction mechanisms at
the interface region of overbank flow such as Myers (1978);
Baird & Ervine (1984); Myers (1987); Brennon & Myers (1990);
Myers (1991); Lambert & Myers (1998); Abril & Knight (1999);
Myers et al. (1999); Myers, Lyness & Cassells (2001). These
studies have shed light on how the flow behaves when it goes
overbank, and provides invaluable insights into the interaction
between the main channel and flood plain.
studies carried out, various methods as well as empirical
formulas have also been proposed for overbank flow discharge
calculation. Based on the calculation involved, these methods
can be divided into several groups, such as: 1. Traditional
methods such single channel method, channel division method,
etc.; 2. The apparent shear force methods (Wormleaton, 1982;
Knight & Demetriou, 1983; Prinos &Townsend, 1984; Christodolou &
Myers, 1999; 3. The correction factor methods (Radojkovic, 1985;
Wormleaton & Merrett, 1990; Ackers, 1992, 1993); 4. The
numerical methods (Keller & Rodi, 1988; Tominaga & Nezu, 1991;
Abida & Townsend, 1994; Knight & Shiono, 1990); etc.
Unfortunately, none yet commands wide spread acceptance. The
main reason for this is that most of the previous researches are
based on small, and some in large-scale laboratory
investigations, with certain idealized conditions, i.e. uniform
channel cross-section, flood plain topography, surface roughness
and bed slope etc. Under such conditions, the equations derived
will be specific for a certain channel type, and it is not
general applicable to channels / rivers with different
geometrical shapes and boundary conditions.
recently, much research effort on compound channel has been
directed towards a better understanding of the complex turbulent
structure and secondary current, and moved towards the
development of multiple dimensional (2 or 3-D) models, such as
and algebraic stress models. While some success have been
achieved, these methods are too difficult to be used. The
present stage of development does not yet encourage its use in
normal engineering design, partly because of the complexity, but
mainly because of uncertainty about the turbulence coefficient
and any variation in it. Therefore, despite the significance
progress so far, the problem with discharge estimation still
In the case of flooding rivers, the
flood plains are often heavily wooded or vegetated with very
complex configurations, the shapes are irregular, and the flow
is usually turbulent with a considerable mixing. These leading
to a completely different roughness conditions than that modeled
in laboratory flumes, not surprisingly, there is as yet no
commonly accepted method for discharge estimation in compound
natural rivers. In this case, full-scale field experiments
would be the best way to further understanding of flow in
compound river channels, as well as evolving accurate methods of
Unfortunately, fieldwork is rare,
partly because compound channel flow conditions occur typically
under flood conditions when acquisition of data is difficult and
sometimes dangerous. Clearly, further studies need to be
carried out for overbank flow in natural rivers.
of the Study
In view of the shortage of
investigation of flow in natural rivers during flood time, it
was decided to:
Study the geometrical relationships for
flooded natural rivers
Study the velocity profiles and
stage-discharge relationship for flooded natural rivers;
Determine the boundary resistance factors for
natural rivers with floodplain;
Determine the influence of main channel and
flood plain interaction (momentum transfer and apparent
friction) to discharge capacity;
Developed a reliable method for discharge
estimation in natural compound channels.
Significance of the Study
stage-discharge relationship (H v Q) for overbank
flow is of great practical importance, as it not only links
discharge with water level in flood routing models, but also it
is frequently used to obtain estimates of flood discharges at
extreme water level for practical purposes. Therefore, this
study has been proposed. It enables river engineers to estimate
more accurately the discharge in flooded natural rivers, and
hence provides the engineering community with a reliable method
for flood forecasting and flood mitigation projects
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Townsend. (1994). A model for routing unsteady flows in
compound channels, Journal of Hydraulic Research, Vol. 32,
Abril J.C. and D.W.
Knight. (1999). Accurate stage-discharge prediction in
compound channels using a finite element method for
depth-averaged turbulent flow. Proceedings of the
Institution of Civil Engineers, Water, Maritime and Energy,
(submitted for publication).
Ackers P. (1992). Hydraulic design of
two-stage channels. Proceedings of the Institution of Civil
Engineering, Water, Maritime and Energy, 96. 247-257.
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Conference on Hydraulic Design in Water Resources
Engineering: Channels and Channel Control Structures,
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W.R.C. Myers. (1999). Apparent Friction Factor on the
Flood Plain-Main Channel Interface of Compound Channel
Sections, Proceedings of 28th Congress of the
International Association for Hydraulic Research, Craz,
Austria, SE3, A379.
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of flow characteristics in main channel/flood plain flows,
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channel, Journal of Hydraulic Research, Vol. 128, No. 2,
Lambert M.F. and W.R. Myers. (1998).
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channels. Proceedings of the Institution of Civil
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compound channel. Journal of Hydraulic Research, 16, No.2,
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in compound channels, Journal of Hydraulic Engineering, Vol.
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Engineering, ASCE, Vol. 117, 676-680
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Cassells. (1999). Resistance coefficients for inbank and
overbank flows. Proceedings of the Institution of Civil
Engineers, Water, Maritime and Energy, 136, 105-115.
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(2001). Influence of boundary on velocity and discharge in
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Djordjevic. (1985). Computation of discharge distribution
in compound channels, Proceedings of the 21st
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(1991). Turbulent structure in compound open-channel flows,
Journal of Hydraulic Engineering, ASCE, Vol. 117, No. 1,
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Merritt. (1990). An improved method of calculation for
steady uniform flow in prismatic main channel flood plain
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Allen and P. Hadjipanos. (1982). Discharge assessment in
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