Distribution and Measurement Structures for Canal Flows (Part - 1) - Civil Engineering (CE) PDF Download

Introduction 

The flow of a main canal bifurcating into a branch canal with the rest flowing downstream is controlled with the help of a cross regulator across the parent canal and a head regulator across the branch canal.  At times, the flow of a canal divides into two or three smaller branch canals without any regulating structure by designing the entrance of the canals IN such a way that the flow enters each branch canal proportionate to its size.  Again, from a canal, outlet structures may take out water for delivery to the field channel or water courses belonging to cultivators. These outlet works, of course, are generally not provided on the main canal and branches, but are installed in the smaller distributaries.  Apart from these, there could be a need to measure the flow in a canal section and different structures have been tried, mostly based on the formation of a hydraulic jump and calibrating the discharge with the depths of flow. Typical structures of these kinds are graphically represented in Figure 1 and this lesson deals with each type in detail. 

Figure 1. Canal structures for flow distribution and measurement

Flow distributing structures 

The flow of a canal can be distributed in to smaller branches using a variety of structures which have been developed to suit a wide variety of conditions. The flow being diverted in to each branch is usually defined as a proportion of the total flow. Thus, these flow distributing structures differ from the flow regulating structures discussed in Lesson 3.9 since the latter are designed to draw off any amount of discharge irrespective of the flow in the parent channel.  The flow distributing structures require a control section in both the off-take channel and in the parent channel. Flow distributors of fixed proportion type are generally used in India, whereas in some countries a flow splitter with a mechanical arrangement is used to change the flow distribution proportions.  

The Punjab type proportional distributor has each opening or offtake constructed as a flume or free overfall weir and is dimensioned so as to pass a given fraction of the total flow. The controlling section consisting of the flume, elevated floor or weir crest is located in the individual offtakes, and not in the supply channel. A typical plan of a proportional distributor with two offtakes is shown in Figure 2.

FIGURE 2. Proportional distributor structure with two offtakes

Proportional distributors may have only one offtake as shown in the typical plans shown in Figure 3.  Generally, all offtakes should be designed to bifurcate at 60⁰ or 45⁰. The crest of all the offtakes and the flume in the parent channel should be at the same level and at least 0.15m above the downstream bed level of the highest channel.  The parent channel flume may have provisions for a stop log insertion for emergency closures. 

Figure 3. Proportional Offtakes With (A) One Offtake On Left Side (B) One Offtake On Right Side (C) One Offtake On Either Side

Canal outlet 

Canal outlets, also called farm turnouts in some countries, are structures at the head of a water course or field channel. The supply canal is usually under the control of an irrigation authority under the State government.  Since an outlet is a link connecting the government owned supply channel and the cultivator owned field channel, the requirements should satisfy the needs of both the groups.

Since equitable distribution of the canal supplies is dependent on the outlets, it must not only pass a known and constant quantity of water, but must also be able to measure the released water satisfactorily. Also, since the outlets release water to each and every farm watercourse, such structures are more numerous than any other irrigation structure.  Hence it is essential to design an outlet in such a way that it is reliable and be also robust enough such that it is not easily tampered with. Further the cost of an outlet structure should be low and should work efficiently with a small working head, since a larger working head would require higher water level in the parent channel resulting in high cost of the distribution system. Discharge through an outlet is usually less than 0.085 cumecs. 

Various types of canal outlets have been evolved from time to time but none has been accepted as universally suitable.  It is very difficult to achieve a perfect design fulfilling both the properties of ‘flexibility’ as well as ‘sensitivity’ because of various indeterminate conditions both in the supply channel and the watercourse of the following factors:  

• Discharge a nd s ilt
• Capacity fa ctor
• Rotation of channels
• Regime condition of distribution channels, etc. 

These modules are classified in three types, which are as follows: 

(a) Non-modular outlets  These outlets operate in such a way that the flow passing through them is a function of the difference in water levels of the distributing channel and the watercourse. Hence, a variation in either affects the discharge. These outlets consist of regulator or circular openings and pavement. The effect of downstream water level is more with short pavement.

(b) Semi-modular outlets The discharge through these outlets depend on the water level of the distributing channel but is independent of the water level in the watercourse so long as the minimum working head required for their working is available.

(c) Module outlets  The discharge through modular outlets is independent of the water levels in the distributing channel and the watercourse, within reasonable working limits. This type of outlets may or may not be equipped with moving parts. Though modular outlets, like the Gibb’s module, have been designed and implemented earlier, they are not very common in the present Indian irrigation engineering scenario. 

The common types of outlets used in India are discussed in the next sections. 

Pipe outlets

This is a pipe with the exit end submerged in the watercourse (Figure 4). The pipes are placed horizontally and at right angles to the centre line of the distributing channel and acts as a non-modular outlet.  

Discharge through the pipe outlet is given by the formula:

Q = CA (2gH) ^1/2            (1) 

In the above equation, Q is the discharge; A is the cross sectional area; g is the acceleration due to gravity; H is difference in water levels of supply channel and watercourse and C is the coefficient of discharge which depends upon friction factor (f), length (L) and diameter of the outlet pipe (d) related by the formula: 

         (2) 

The coefficient f is the fluid friction factor and its value may be taken as 0.005 and 0.01 for clear and encrusted iron pipes respectively.  For earthenware pipes, f may be taken as 0.0075.  All other variables are in SI units, that is, meters and seconds. 

It is a common practice to place the pipe at the bed of the distributing channel to enable the outlets to draw proportional amount of silt from the supply channel. The entry and exit ends of the pipe should preferably be fixed in masonry to prevent tampering. Since the discharge through this type of outlet can be increased by lowering the water surface level of the watercourse (thus increasing the value of H in the discharge equation), it is possible for the irrigator to draw more than fair share of water.  A pipe outlet may also be designed as a semimodular outlet, that is, one which does not depend upon the water level in the watercourse by allowing it to fall freely in to the watercourse (Figure 5). 

Figure 5, Pipe outlet with exit above water- course FSL

Pipe outlets require minimum working head and have higher efficiency. It is also simple and economical to construct and is suitable for small discharges. However, these outlets suffer from disadvantages like the coefficient discharge which varies from outlet to outlet and at the same outlet at different times apart from the possibility of tampering in the non-modular type.