Regulating Structures for Canal Flows (Part - 1) - Irrigation Engineering - Civil Engineering (CE) PDF Download

Introduction

A canal obtains its share of water from the pool behind a barrage through a structure called the canal head regulator.  Though this is also a regulation structure for controlling the amount of water passing into the canal (with the help of adjustable gates), it shall be discussed under diversion works (Module 4). In this lesson, attention is focussed on structures that regulate the discharge and maintain the water levels within a canal network (Figure 1).

Figure 1. Canal structures for flow regulation and control

These structures may be described as follows:

1. Drops and falls to lower the water level of the canal
2. Cross regulators to head up water in the parent channel to divert some of it through an off take channel, like a distributary.
3. Distributary head regulator to control the amount of water flowing in to off take channel.
4. Escapes, to allow release of excess water from the canal system. 

Canal drops and falls

A canal has a designed longitudinal slope but has to pass through an undulating terrain. When a canal crosses an area that has a larger natural surface slope, a canal drop, also called fall in India, has to be provided suitably at certain intervals (Figure 2)  

Figure 2 . Typical location for providing canal drop or fall

The location of a fall has to be judiciously worked out such that there should be a balance between the quantities of excavation and filling. Further the height of the fall has to be decided, since it is possible to provide larger falls at longer intervals or smaller falls at shorter intervals. It may be observed that the portion of the canal which is running in filling (Figure2) may be able to serve the surrounding area by releasing water by gravity. For the portion of the canal that is running in excavation, if surrounding areas have to be irrigated, it has to be done through pumping. 

There are various types of fall structures, some of which are no more provided these days.  However, there are many irrigation projects in India which have these structures in the canal network, as they were designed many years ago. Many of these structures used boulder masonry as their construction material, whereas now brick masonry or, more commonly, mass concrete is being used commonly in modern irrigation projects. 

Falls of antiquity 

The Ogee type of fall has been one of the first to be tried in the Indian canal irrigation system, probably since more than a century back (Figure 3a).  However, according to the earliest structures provided, the crest of the fall was in the same elevation as that of the upstream section of the canal.  This caused a sharp draw-down of the water surface on the upstream side. On the downstream, the drop in elevation added energy to the falling water which exited the falls as a shooting flow, causing erosion of the canal bed immediately downstream.  These difficulties were later removed by raising the crest level of the fall above the upstream canal bed level and providing suitable stilling basin with end sill at the downstream end of the fall which kills most of the excess energy of the leaving water by helping to form a hydraulic jump (Figure 3b). 

FIGURE 3. (a) Ogee-type fall made of rubble masonry
(b) Same type of fall, but made of concrete and equipped with a stilling basin for energy dissipation

The rapid-fall was tried in some of the north-Indian canals which were constructed with boulders cemented together by lime concrete (Figure 4).  These were quite effective but, the cost being prohibitive, was gradually phased out. 

The trapezoidal-notch fall consists of one or more notches in a high crested wall across the channel with a smooth entrance and a flat circular lip projecting downstream from each notch to disperse water (Figure 5).  This type of fall was started around the late nineteenth century and continued to be constructed due to its property of being able to maintain a constant depth-discharge relationship, until simpler and economical alternatives were designed. 

FIGURE 5. The notch-fall

Some falls have been commonly used in the recent times in the canal systems of India. These are described in the following sections. Detailed references may be had from the following two publications of the Food and Agriculture Organisation (FAO):

1. FAO Irrigation and Drainage paper 26/1: Small Hydraulic Structures, Volume 1 (1982) 
2. FAO Irrigation and Drainage paper 26/2: Small Hydraulic Structures, Volume 2 (1982) 

Falls with vertical drop

These are falls with impact type energy dissipators. The vertical-drop fall (Figure 6) uses a raised crest to head up water on the upstream of the canal section and allows it to fall with an impact in a pool of water on a depressed floor which acts like a cushion to dissipate the excess energy of the fall.  This type of fall was tried in the Sarda canal of Uttar Pradesh, which came to be commonly called as the Sarda-type fall.  

FIGURE 6 . Vertical drop fall

Typical plan and section of a Sarda-type fall is shown in Figure 7. Usually, two different crests for the fall are adopted, as shown in Figure 8. For canals conveying discharges less that 14m³/s, crest with rectangular cross section is adopted, and for discharges more than that, trapezoidal crest with sloping upstream and downstream faces is chosen.

Longitudinal section 

FIGURE 7. SARDA TYPE FALL

FIGURE 8 Types of cross-section for Sarada Fall (a)    Rectangular Crest (Drowned Flow)(b)    Trapezoidal Crest {Free flow)

For smaller discharges, the following a may be provided. 

• Well drop fall (Figure 9)  
• Pipe drop fall (Figure 10)
• Baffled apron drop (Figure 11) 

Longitudinal section

Half plan of top and half plan of bottom

FIGURE 9 . WELL DROP

FIGURE 10. Pipe drop spillway

Figure.11 Plan and Section of Baffled Apron Drop