The flow of irrigation water in the canals must always be under control. Maintenance is reduced for lined canals, but skilled labour is required. The construction cost is much higher than for earthen canals. The materials mostly used for canal lining are concrete (in precast slabs or cast in place), brick or rock masonry and asphaltic concrete (a mixture of sand, gravel and asphalt). Lining canals is also an effective way to control canal bottom and bank erosion. Construction of a canal lined with bricks Maintenance of an earthen canalĮarthen canals can be lined with impermeable materials to prevent excessive seepage and growth of weeds (Fig. 77b) in order to control weed growth and to repair damage done by livestock and rodents.įig. They also require continuous maintenance (Fig. The disadvantages of earthen canals are the risk of the side slopes collapsing and the water loss due to seepage. 76):Įarthen canals are simply dug in the ground and the bank is made up from the removed earth, as illustrated in Figure 77a.įig. It is commonly expressed in percent or per mil.Īn example of the calculation of the bottom slope of a canal is given below (see also Fig. The bottom slope of the canal does not appear on the drawing of the cross-section but on the longitudinal section (see Fig. For example, if the side slope of the canal has a ratio of 1:2 (one to two), this means that the horizontal distance (w) is two times the vertical distance (h) (see Fig. The side slope of the canal is expressed as ratio, namely the vertical distance or height to the horizontal distance or width. It is required to guard against overtopping by waves or unexpected rises in the water level. The freeboard of the canal is the height of the bank above the highest water level anticipated.
The typical cross-section of a trapezoidal canal is shown in Figure 74.įig. For the purposes of this publication, only this type of canal will be considered. The most commonly used canal cross-section in irrigation and drainage, is the trapezoidal cross-section. Field ditches have smaller dimensions and convey water from the farm entrance to the irrigated fields.Īccording to the shape of their cross-section, canals are called rectangular (a), triangular (b), trapezoidal (c), circular (d), parabolic (e), and irregular or natural (f) (see Fig. Channels and canals refer to main waterways supplying water to one or more farms. 5.2.1 Open canalsĪn open canal, channel, or ditch, is an open waterway whose purpose is to carry water from one place to another. Canal structures are required for the control and measurement of the water flow. The conveyance and distribution systems consist of canals transporting the water through the whole irrigation system. The centrifugal pump will only operate when the case is completely filled with water. The water is immediately caught by the rapidly rotating impeller and expelled through the discharge pipe. Water enters the case at the center, through the suction pipe. 72a) consists of a case in which an element, called an impeller, rotates driven by a motor (see Fig. There are several types of pumps, but the most commonly used in irrigation is the centrifugal pump. Then a pump must be used to supply water to the irrigation system (see Fig. In some cases, the irrigation water source lies below the level of the irrigated fields. Its purpose is to direct water from the original source of supply (lake, river, reservoir etc.) into the irrigation system. The intake structure is built at the entry to the irrigation system (see Fig. The drainage system removes the excess water (caused by rainfall and/or irrigation) from the fields. The field application system assures the transport of water within the fields. The distribution system assures the transport of water through field ditches to the irrigated fields. The conveyance system assures the transport of water from the main intake structure or main pumping station up to the field ditches. The (main) intake structure, or (main) pumping station, directs water from the source of supply, such as a reservoir or a river, into the irrigation system. The irrigation system consists of a (main) intake structure or (main) pumping station, a conveyance system, a distribution system, a field application system, and a drainage system (see Fig.
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