Main irrigation canal

Main irrigation canal is the canal that delivers water to the lands to be irrigated and is composed of the off-load part (from the headworks to the head of distribution canals) and working part where distributing conducting canals branch off from the main canal.

Main canal is the major part of the conductive network, because water is delivered through it from the source to the entire irrigated territory. The main canal is composed of the head section, and off-load and working sections.

Head section is the inlet water-intake section where water is cleared from sediments if sediment tanks are available. The canalís off-load part delivers water from an irrigation source to the first distribution canal. The canalís working section distributes water between distribution canals.

The main canal is laid orienting on the highest points of a given irrigated area.

An irrigated land can be supplied with water from a source by gravity or by pump irrigation. At gravity water supply, water intake can be of dam intake and river-canal (damless) intake types. At that, water comes from a river (reservoir) through a check structure to the off-load part of the main canal by which it is transported to a particular irrigated land. At pumping irrigation, water comes through pipes.

The location and length of the main canalís off-load section depend on the slope of the ground and main canal, water withdrawal source water level rise above the earthís surface where the water needs to be taken to.

In some cases, minimum slope does not provide cost-effective solution, since the cross section becomes too large and considerably increases the volume of earthwork operations. And so the most advantageous slope is established, at which no silting will happen and the main canal construction cost will be lowest.

When laying canal, the command point to which water is required to be delivered from a river by gravity or a water withdrawal point. In the first case, it would be necessary to determine the length of the main canalís off-load section and a water withdrawal point. Canal laying is carried out from the command point towards the river. In the second case, it would be necessary to determine the length of the off-load section of the main canal and determine the upper boundary of the command area. The canal is laid from a water withdrawal point towards the irrigated area under design.

The main canal must command the entire irrigated area, from the end of the off-load section to the end of the last cluster of structures from which only a distribution network is constructed.

Depending on the relief, one can outline three key layouts of the main canal and inter-farm network:

The main canal is routed towards the horizontals (across the slope) at a design gradient along the upper boundary of the respective irrigated area. First-order distributors are designed as double-sided command type along the maximum slope. If there are hollows, uplands and subdivides, the main canal is designed based on the same principle; however, along the upper boundary it loses its straightness. Distribution canals are routed at an allowable gradient along the dividing crests and to ensure commanding both slopes.

The main canal is laid along the maximum surface slope, approximately in the middle of the designed irrigated land. The distributors are laid at minimum allowable gradient along the land contours. Such a layout of canals is reasonable to apply for the irrigated lands with long length along the slope (gradient) and narrow width. In this case, drop structures are designed only on the main canal, while in the first layout version those have to be designed on all first-order distributors. At considerable flow rate in the main canal, a hydropower plant is constructed in the places of concentrated water fall.

Under the conditions of complex relief, the main canal, inter-farm, and farm distributors are arranged on command points and watershed divides so that the flow velocities through canals should be lower than the scouring (erosive) velocity and higher than the silting velocity and the requirements to the management of such an irrigated area as well as other conditions of canal designing should be met.