Conveyance Structures for Canal Flows (Part - 3) - Civil Engineering (CE) PDF Download

Structures to carry canal water below a natural stream 

A canal can be conveyed below a natural stream with the help of structures like a super-passage or a siphon. These are exactly opposite in function to that of the aqueducts and siphon aqueducts, which are used to carry the canal water above the natural stream. The natural stream is flumed and made to pass in a trough above the canal. If the canal water flows with a free surface, that is, without touching the bottom of the trough, it is called a super-passage (Figure 16). Else, when the canal passes below the trough as a pressure flow, then it is termed as a syphon or a canal syphon. 

FIGURE 16a. Typical layout of a Super-passage

FIGURE 16b. Section through the Super-passage shown in Figure 16a.

Instead of a trough, the canal flow may be conveyed below the natural stream using small pre-cast RCC pipes (for small discharges) and rectangular or circular barrels, either in single or multiple cells, may be used (for large discharges),  as  shown  in  Figure 

FIGURE 17. Plan and section of canal siphon

Structures to carry canal water at the same level as a natural stream 

A  structure in which the water of the stream is allowed to flow into the canal from one side and allowed to leave from the other, known as a level crossing , falls into this category  (Figure 18). 

FIGURE 18. General view level crossing (gate hoisting arrangements not shown)

This type of structure is provided when a canal approaches a large sized drainage with high flood discharges at almost the same level. The flow control is usually provided on either side of the canal and on the outlet side of the drain. As such, this type of arrangement is very similar to canal head-works with a barrage. Advantage may be taken of the flow of the natural drainage to augment the flow of the outgoing canal. The barrage type regulator is kept closed during low flows to head up the water and allows the lean season drainage flow to enter the outgoing canal. During flood seasons, the barrage gates may be opened to allow much of the silt-laden drainage discharge to flow down. 

Another structure, called an inlet, is sometimes provided which allows the entry of the stream water into the canal through an opening in the canal bank, suitably protected by pitching the bed and sides for a certain distance upstream and downstream of the inlet. If the natural stream water is not utilized in the canal then an outlet, which is an opening on the opposite bank of the canal is provided. The canal bed and sides suitably pitched for protection. 

Transitions at changes in canal cross-sections 

A canal cross section may change gradually, in which case suitable flaring of the walls may be made to match the two sections (Figure 19). 

FIGURE 19. Transition canal banks warped to vertical and then flurried

For more abrupt changes, like a normal canal section being changed to a vertical walled aqueduct, suitable transitions have been designed which would avoid formation of any hydraulic with consequent loss of energy. A typical view of transition of a normal canal bank to a vertical walled flume section is shown in (Figure 20).  

FIGURE 20. Transition with canal banks warped to vertical along with fluming

As may be observed, the banks of the normal canal section are first changed to vertical walls keeping the same canal bed width (B_c). Beyond this, the vertical section is reduced gradually to form a reduced sized flume of width (B_f). Various formulae have been proposed for deciding the intermediate curve, that is, an equation deciding the width (B_x) at any distance x from the start of the fluming, assuming a length L for the transition. One formula that is commonly used for this kind of transition is the UPIRI method, commonly known as Mitra’s transition and is given as follows: 

Bx = (B_c  * B_f *L ) / (L * B_c – X (B_c – B_f))                      (1)

The length L of the transition is assumed to be equal to 2 *(B_c – B_f).  In another type of transition, the vertical curved walls of a normal canal section is both transformed in to vertical walls of a flume as well as its section is reduced gradually, as shown in Figure 20. This results in reduction of the canal bed width from B_c to B_f and the side slopes from M₀ to O. The values for the bed width Bx at any length X from the start of the transition and the corresponding side slope mx are given by the following expressions

B = Bc    + X/L [1- (1- X/L)ⁿ ] (Bc -Bf )             (2)

m _x = m₀ [1 – (1- X/L)^1/2]                  (3)

Where n = 0.8 - 0.26(m₀) ½ and the length of transition L, is expressed as  

L = 2.35 (B_c-B_f) + 1.65 m0 h c                (4)