Dr. Lai Sai Hin
Zorkeflee Abu Hasan
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 2: Vortices form at the interface of main channel and flood plain|
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.
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.
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.
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