The following points highlight the three main types of structures used for controlling the flow of irrigation water in canals. They are: 1. Erosion Control Structures 2. Distribution Control Structures 3. Crossing Structures.
Type # 1. Erosion Control Structures (Drop Structures and Chutes):
Drop structures or chutes (Figs 10.9 and 10.10) are required to reduce the bottom slope of canals lying on steeply sloping land in order to avoid high velocity of the flow and risk of erosion. These structures permit the canal to be constructed as a series of relatively flat sections, each at a different elevation.
Drop structures take the water abruptly from a higher section of the canal to a lower one. In a chute, water does not drop freely but is carried through a steep, lined canal section. Chutes are used where there are big differences in the elevation of canal.
Type # 2. Distribution Control (Water Control) Structures:
Distribution control structures are required for easy and accurate water distribution within the irrigation system and on the farm.
i. Diversion boxes:
Division boxes are used to divide or direct the flow of water between two or more canals or ditches. Water enters the box through an opening on one side and flows out through openings on the other sides. These openings are equipped with gates (Fig 10.11).
ii. Turnouts:
Turnouts are constructed in the bank of a canal. They divert part of the water from the canal to a smaller one.
Turnouts can be concrete structures (Fig 10.12) or pipe structures.
iii. Checks:
To divert water from the field ditch to the field, it is often necessary to raise the water level in the ditch. Checks are structures placed across the ditch to block it temporarily and to raise the upstream water level. Checks can be permanent structures or portable (Fig 10.13).
iv. Siphon tubes and spiles:
Siphon tubes convey water over the channel bank into the field (Fig 10.14). They are made up of plastic, rubber or sheet metal of different sizes (2.5 to 7.5 cm dia). Each tube is around 1.0 m or more in length, depending on the height of the channel bank above the field. Siphon tubes are dipped in water and filled completely. One end of the tube is then closed tightly with the palm. Keeping the other hand dipped in water; the closed end is taken out and released at the ground surface.
v. Spiles:
These are, usually, made up of bamboo, concrete or baked pipes. Size of pipes varies from 2.5 to 10 cm dia or even more, depending on the desired flow and depth of water at the inlet and outlet end (Fig 10.15). They should be placed below the bank to reduce erosion at the outlet end.
Type # 3. Crossing Structures:
Inverted siphons and culverts are the common crossing structures. These are buried pipes used to carry irrigation water underneath roadways, drainage canals, natural streams or depressions (Fig 10.16).
