Chapter 7. Discussion
- Table of Contents
- 7.1. Flow over weirs
- 7.2. Procedure for modeling flows in fishways
- 7.3. Flow below fishways
- 7.4. Velocity distribution at a weir with a V-shaped crest
In terms of hydraulics, fishways are energy dissipating structures. According to an energy dissipation scheme, three basic types are usually recognized: the vertical slot, the Denil, and the pool-and-weir fishway (Katopodis & Rajaratnam 1983).
It is often asked, whether there is one special fishway type that is superior over the others. Unfortunately, the fishway type is defined by the fish species in concern and the site specific characteristics, e.g. available space, discharge, and material, economical resources, scenery, and the values placed on nature, etc. There are no such fishway types that could fulfil all these demands and all the fishway types should be considered for use.
Of the three basic types, pool-and-weir fishways are suitable for sites where the upper and lower water level are almost constant, and where there is enough space. Pool-and-weir fishways can be modified to tolerate water level fluctuations to some extent by forming the weir crest or adding notches or orifices. Denil fishways are suitable for sites where the space is a limiting factor and the slope of the fishway would become steep. Denil fishways tolerate water level fluctuations better than pool-and-weir fishways, and may carry a large amount of water. According to this study, the attraction of Denil fishways to fish may be weak, although with improvements and a preceding entrance pool the situation could be improved. Vertical slot fishways are suitable for sites where water level fluctuations are extensive, and where a large number of fish species with varying swimming ability is in concern.
The entrance conditions are actually more important for the proper operation of a fishway than the actual fishway type, especially when it comes to fish passage. One important point is that flow pattern both inside the fishway and at the fishway entrance should be clear and concise. All this does not, however, mean that the exit is of minor importance. On the contrary, it is important to locate the fishway exit far enough from possible water intakes in order to avoid unnecessary migration back downstream.
In this paper, flows in fishways have been considered partly as flows over weirs and partly using the procedures developed for uniform flow. Fishways with sharp crested weirs, baffles and vanes are dealt with, in addition to flows over sharp crested weirs. In addition, flow conditions below different fishway structures have been evaluated.
7.1. Flow over weirs
Practically all hydraulic investigations of weir flows aim at studying the relation between the discharge coefficient and the parameters influencing the flow. Lakshamana Rao (1975) stated that the factors influencing the flow over weirs may be categorized into three groups, which are channel conditions, weir characteristics, and liquid properties. The influence of approach velocity was realized long ago, already in the late 1880’s. In the 1950’s, it was shown that the approach velocity head term can easily be eliminated while dealing with standing wave weirs and thus discharge can be expressed in terms of the water depth at the weir crest. The distribution of velocities in the approach channel definitely influences the discharge over the weir. It is well known that the cross-sectional shape of the channel influences the velocity distribution. Boundary irregularities of sufficiently large magnitude, and obstructions in flow create considerable loss of energy and make the approach flow nonuniform, resulting in increased α values (Lakshamana Rao 1975).
When rapidly varied flow occurs in a sudden-transition structure, the physical characteristics of the flow are basically fixed by the boundary geometry of the structure as well as by the state of flow (Chow 1959). The separation zones, eddies, and rollers that may occur in rapidly varied flow tend to complicate the flow pattern and to distort the actual velocity distribution in the stream. In such cases, the flow is actually confined by one or more separation zones, rather than solid boundaries (Chow 1959).
Experimental formulas developed for the discharge over sharp-crested weir can be expressed in a general form
where C is the discharge coefficient, B is the effective width of the weir crest, and H is the measured head above the crest, excluding the velocity head. Experiments have shown that the coefficient C in Eq. (28) remains approximately constant for sharp crested weirs under varying heads if the nappe is aerated. Insufficient aeration below the nappe means a reduction of pressure beneath the nappe due to the removal of air by the overfalling jet. This reduction of pressure will cause, among other things, an increase in discharge, sometimes accompanied by fluctuations or pulsation of the nappe, and unstable performance of the hydraulic model (Chow 1959).
For sharp-crested weirs, it has been noticed that the discharge coefficient is highly sensitive and varies with modified nappe conditions. At low heads, the nappe is found to be depressed; and as the head increases, the nappe varies from the clinging type to the drowned or wetted underneath type (nappe types are shown in Fig. 32). When compared with the free nappe conditions, there was about an 8% increase and about a 28% increase in the discharge coefficient for depressed nappes and clinging nappes, respectively. When the nappe became drowned (plunging type), the increase dropped to 19%, and with further increase in head the increase was only 12%. In all these cases, the discharge coefficient was not influenced by the tailwater elevation below the weir crest, but only by the head-weir height ratio. With further increase in tailwater elevation, the nappe became the riding type and the discharge was influenced by both the upstream and downstream water levels. For submerged flows, the estimation is more difficult to give, depending greatly on the submergence factor h1/h2, where h1 and h2 are upstream and downstream heads, respectively. However, modification of the nappe conditions results in small variations of 1 to2 % in the discharge, and therefore needs to be considered only for accurate gagings (Lakshamana Rao 1975).
Submergence of a weir will reduce the coefficient of discharge of the corresponding unsubmerged flow (Chow 1959). Weirs are said to be submerged when the tailwater is higher than the crest. In submerged weirs, the nappe is always unaerated. Submerged flow is usually unstable, having considerable surface disturbance immediately downstream. For submerged flow, the flow resistance is higher compared with unsubmerged flow.
For a given discharge, as submergence increases, the submerged-flow over the rectangular sharp-crested weir passes through several regimes, as indicated in Fig. 33 (a-d). From an exploratory experimental study, Wu and Rajaratnam (1996) found that these regimes can be classified as (1) the impinging jet, (2) breaking wave (or surface jump), (3) surface wave, and (4) surface jet. For the impinging jet regime, the flow over the weir plunges into the tailwater, diffuses as a plane submerged jet, and eventually hits the bed of the downstream channel. For the other three regimes, which might be collectively called the surface flow regime, the flow remains as a jet at the surface in the downstream channel, with its thickness increasing downstream because of turbulent mixing.
Figure 33. Regimes of flow for rectangular sharp-crested weirs a) impinging jet, b) breaking waves, c) surface wave, d) surface jet (Wu & Rajaratnam 1996).
The surface jet regime (Fig 33 d) was studied by Rajaratnam and Muralidhar (1969), who found that the flow forms into a surface jet over a longitudinal distance of 2t, where t is the depth of flow below the weir, measured from the weir crest. After that, the flow behaves like a turbulent surface jet. Furthermore, Rajaratnam and Muralidhar (1969) found that a large recirculation region was formed below the expanding surface jet, having a length of about seven to eight times the height of the weir, P. They also found that in deeply submerged flow, an eddying region, or standing eddy, is formed below rectangular weirs with sharp crest (Fig. 34).
Lakshamana Rao (1975) stated that ratio h/P, where h is the head over the weir and P is the height of the weir, can be considered as a depth or area contraction and consequently a measure of approach velocity. In studies, this has been observed to be a very important factor that affects the discharge coefficient. Also, other geometric parameters become important in influencing the coefficient. For smaller weir heights or for a given weir height at larger heads, the reduction in discharge coefficient becomes more gradual. These trends may be attributed to the fact that with smaller weir heights, the downstream eddy becomes smaller in size and consumes less energy. In the studies of Wu and Rajaratnam (1996), relative weir heights h/P varied from 0.076 to 1.5. In this range, the height of the weir did not affect the discharge to a great extent.
Figure 34. Definition sketch for flow below a deeply submerged rectangular weir; a general view of the flow (from Rajaratnam & Muralidhar 1969).
In fishways, weirs and vanes that produce the energy dissipation may be considered to be constrictions in an open channel. A constriction in an open channel constitutes a reach of a sudden reduction in a channel cross section. The effect of a constriction on the flow depends mainly on the boundary geometry, the discharge, and the state of flow. The phenomenon is usually so complicated that the resulting flow pattern is not readily subject to any analytical solution. The flow through constriction may be subcritical or supercritical. When the flow is subcritical, the constriction will induce a pronounced backwater effect that extends a long distance upstream. When the flow is supercritical, the constriction will disturb only the water surface that is adjacent to the upstream side of the constriction and will not extend farther upstream (Chow 1959). Usually, the depth of approaching flow in fishways is higher than the critical flow depth, and the flow is in subcritical state.