Regulating Structures for Canal Flows (Part - 2) - Civil Engineering (CE) PDF Download

Falls with drop along inclined glacis 

These are falls with and inclined glacis along which the water glides down and the energy is dissipated by the action of a hydraulic jump at the toe of the structure. Inclined drops are often designed to function as flume measuring devices. These may be with and without baffles as shown in Figures 12 and 13 respectively and supplemented by friction blocks and other energy dissipating devices (Figure 14).  

Figure 12. Sectional View through Glacis Falls without Baffles

Figure 13. Plan and Section Oh Standing Wave Flume- Fall

FIGURE 14 Plumed glacis wrth metered fall

Similar type of fall was also developed in Punjab which was called the CDO type fall, as shown in Figure 15 (for hydraulic drop up to 1m) and Figure 16 (for hydraulic drop above 1m). 

Figure.15. Cdo Punjab Type Fall Upto 1m Drop

Figure.16. Cdo Punjab Type Fall For Greater Than 1m Drop

The glacis type falls may be modified in the following ways: 

(a) Flumed or un-flumed, depending upon the crest width being smaller or equal to the bed width of the canal (Figure17).

(b) Meter or non-meter fall depending upon whether the canal fall may be used to measure the discharge as well. Details of a meter-fall is described in Lesson 3.10 

Figure 17 (A) A flumed glacis fall with fall width(BF) being less than canal width(Bc). (B) An un - flumed glacis fall, where (Bf) is the same as (Be).

The following appurtenant structures should be considered while providing a verticaldrop or a glacis-type fall:

• The floor of the falls should be able to resist the uplift pressure under the condition of dry canal and a high ground water table.
• Cut-off walls or curtain walls either of masonry or concrete should be provided at the upstream and downstream ends of the floors of the falls.
• Bed protection with dry brick pitching should be provided in the canal just upstream and downstream of the fall.
• Side protection should be provided at the upstream and downstream splays with brick pitching.

Since falls are structures across a canal, it is usual for providing a bridge along with the fall structure for crossing the canal. 

Canal regulators 

These include the cross regulator and the distributary head regulator structures for controlling the flow through a parent canal and its off-taking distributary as shown in Figure 1. They also help to maintain the water level in the canal on the upstream of the regulator. Canal regulators, which are gated structures, may be combined with bridges and falls for economic and other considerations, like topography, etc.

A typical view of a distributary head regulator and a cross regulator (shown partly in section) is illustrated in Figure 18.  

Figure 18 . Distributary head regulator and parent canal cross regulator showing combination with glacis fall and bridgeGates and gate hoisting arrangements have not been shown for clarity

In the figure, the gates and gate hoisting arrangements have not been shown, for clarity. Further, the floor of the regulators would be protected on the upstream and downstream with concrete blocks and boulder apron. A typical sectional drawing through a regulator is shown in Figure 19. 

FIGURE 19. Section through a typical regulator

The angle at which a distributary canal off-takes from the parent canal has to be decided carefully. The best angle is when the distributary takes off smoothly, as shown in Figure 20(a).  Another alternative is to provide both channels (off-taking and parent) at an angle to the original direction of the parent canal (Figure 20b). When it becomes necessary for the parent canal to follow a straight alignment, the edge of the canal rather than the centre line should be considered in deciding the angle of off-take (Figure 20c). 

FIGURE 20. Alignment types for off taking canal from a parent canal
(a)    Smooth off take
(b)    Both inclined to original flow;
(c)    Parent canal flows straight with reduced width.

To prevent excessive entry of silt deposition at the mouth of the off-take, the entry angle should be kept to between 600 and 800. For the hydraulic designs of cross regulators, one may refer to the Bureau of Indian Standard code IS: 7114-1973 “Criteria for hydraulic design of cross regulators for canals”. The water entering in to the off-taking distributary canal from the parent canal may also draw suspended sediment load. 

The distributary should preferably be designed to draw sediment proportional to its flow, for maintaining non-siltation of either the parent canal or itself. For achieving this, three types of structures have been suggested as discussed below along with the relevant Bureau of Indian standard codes.