18-03-2014, 10:56 AM
Regulating Structures for Canal Flows
Instructional objectives
On completion of this lesson, the student shall be able to learn:
The necessity of providing regulating structures in canals.
The basics of canal drops and falls.
The importance of canal regulators.
The need for Groyne Walls, Curved Wings and Skimming Platforms.
The functions of escapes in a canal.
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).
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).
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.
Groyne walls or curved wings
(Please refer to IS: 7871-1975 “Criteria for hydraulic design of groyne wall (curved wing)
for sediment distribution at off-take points in a canal” for more details)
These are curved vertical walls, also called Gibb’s groyne walls, which project out in to
the parent canal from the downstream abutment of the off-taking canal. The groyne wall
is provided in such a way that it divides the discharge of the parent canal in proportion
of the discharge requirement of the off-taking canal with respect to the flow in the
downstream parent canal. The groyne wall extends upstream in to the parent canal to
cover 3⁄4 to full width of the off-take. The proportional distribution of flow in to the off-
taking canal is expected to divert proportional amount of sediment, too. A general view
of a groyne wall is shown in Figure 23.