Researchers: 

Dr. Lai Sai Hin

Prof. Dr. Nor Azazi Zakaria <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Prof. Dr. Aminuddin Ab. Ghani <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Zorkeflee Abu Hasan

Chang Chun Kiat <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Mohd Fazly Yusof <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Leow Cheng Siang <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Dr. Onni Suhaiza Selaman <This email address is being protected from spambots. You need JavaScript enabled to view it.>

Introduction:

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)

Figure 2:  Vortices form at the interface of main channel and flood plain

Compound Channel:

The term "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.

From the 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.

More 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 the k-e, 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 remains.

Flooding Rivers:

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 discharge prediction.

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.

Study Objectives:

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 Study:

The 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.

References:

• Abida H. and R.D. Townsend.  (1994).  A model for routing unsteady flows in compound channels, Journal of Hydraulic Research, Vol. 32, 145-153.
• 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.  (1991).  Hydraulic design of straight compound channels.  Report SR281, HR Wellingford, UK.
• Ackers P.  (1992).  Hydraulic design of two-stage channels.  Proceedings of the Institution of Civil Engineering, Water, Maritime and Energy, 96.  247-257.
• Baird J.I. and D.A. Ervine.  (1984). A resistance to flow in channels with overbank floodplain flow.  Proceedings of the 1^st International Conference on Hydraulic Design in Water Resources Engineering: Channels and Channel Control Structures, University of Southampthon,137-4.150.
• Brennon E.K. and W.R.C. Myers.  (1990).  Flow resistance in a compound channels.  Journal of Hydraulic Research, 28 No.2.  141-146.
• Christodolou G.C. and W.R.C. Myers.  (1999).  Apparent Friction Factor on the Flood Plain-Main Channel Interface of Compound Channel Sections, Proceedings of 28^th Congress of the International Association for Hydraulic Research, Craz, Austria, SE3, A379.
• http://www.iahr.org/membersonly/grazproceedings99/doz/000/000/367.htm
• Keller R.J. and W. Rodi.  (1988).  Prediction of flow characteristics in main channel/flood plain flows, Journal of Hydraulic Research, Vol. 24, No. 4, 425-441.
• Knight D.D. and J.D. Demetriou.  (1983).  Flood plain and main channel flow interaction.  Journal of Hydraulic Engineering, ASCE, 109, No.8, August, 1073-1092.
• Knight D.W. and K. Shiono.  (1990).  Turbulence measurements in a shear region of a compound channel, Journal of Hydraulic Research, Vol. 128, No. 2, 175-196.
• Lambert M.F. and W.R. Myers.  (1998).  Estimating the discharge capacity in straight compound channels.  Proceedings of the Institution of Civil Engineering, Water, Maritime and Energy, 130.  84-94.
• Myers W.R.C.  (1978).  Momentum transfer in a compound channel.  Journal of Hydraulic Research, 16, No.2, 139-150.
• Myers W.R.C.  (1987).  Velocity and discharge in compound channels, Journal of Hydraulic Engineering, Vol. 113, 753-766.
• Myers WRC. (1991).  Influence of geometry on discharge capacity of open channels, Journal of Hydraulic Engineering, ASCE, Vol. 117, 676-680
• Myers W.R.C., D.W. Knight, J.F. Lyness, J.B. Cassells.  (1999).  Resistance coefficients for inbank and overbank flows.  Proceedings of the Institution of Civil Engineers, Water, Maritime and Energy, 136, 105-115.
• Myers W.R.C., J.F. Lyness and J. Cassells.  (2001).  Influence of boundary on velocity and discharge in compound channels, Journal of Hydraulic research, Vol. 39, No. 3, 311-319.
• Prinos P. and R. Townsend.  (1984).  Comparison of methods for predicting discharge in compound open channels.  Adv. in water resour., 7(Dec), 180-187.
• Radojkovic M. and S. Djordjevic.  (1985).  Computation of discharge distribution in compound channels, Proceedings of the 21^st Congress of International Association for Hydraulic Research, Melbourne, Australia, Vol. 3, 367-371.
• Tominaga A. and I. Nezu.  (1991).  Turbulent structure in compound open-channel flows, Journal of Hydraulic Engineering, ASCE, Vol. 117, No. 1, 21-41.
• Wormleaton P.R. and D.J. Merritt.  (1990).  An improved method of calculation for steady uniform flow in prismatic main channel flood plain sections.  Journal of hydraulic research, V28, No2, 157-174.
• Wormleaton P.R., J. Allen and P. Hadjipanos.  (1982).  Discharge assessment in compound channel flow.  Journal of Hydraulic Division, ASCE, 108, No. HY9, 975-994.