 |  | Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.) | |||
 |  | Chapter 16 - Pumped lift irrigation distribution | |||
|  | Background | |||
| | The application of individually owned small pumping units | |||
| | Centralized pumped-lift systems | |||
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Major diversions from a river generally involve construction of a weir, raising the water level high enough to enter a canal system supplying the command area. Such a diversion has the merit of operating entirely by gravity. However, the cost of the weir can be considerable. If the amount of flow to be diverted is small in relation to the flow in the river, and particularly if the river channel is deeply incised, it may be more practical to dispense with the weir and to pump from the river up to bank level, from which point distribution is by open canal system or, in some cases, by pipe.
Pumped lift installations range in size from major pumping stations supplying thousands of hectares down to small portable pumps owned by individual cultivators and serving one or two hectares only. The limitation of the small privately-owned units is that they can reach only a narrow strip of land paralleling the river and about a kilometer in width. If it is desired to serve an area several kilometers in width there are two options. One is to excavate small supply canals, at river-level, into the area to be irrigated. Small pumps then draw from these canals. The other is to install government-owned centralized pumping units at intervals along the riverbank and to distribute by gravity canals or pipeline, into the service area. Cultivators do not use pumps, but take water from the canal or pipe system by gravity. Both types of system are discussed below.
The practicability of constructing river-level supply canals to serve small pump units depends largely on the depth of cut required. If the river is deeply incised, this course would be impractical. Where the river is tidal, however, the supply canals can be designed to fill during the high portion of the tidal range, being closed by tidal gates during the low portion of the range and functioning as storage until again filled on the next tide.
The small pumps, generally diesel or kerosine operated, may be owned by the cultivator or may be leased from a central agency. It is noted that one of the largest pumped-lift operations, using small group-owned units, is supplied not by a river but by major canals. This is the system used since historic times in the Nile delta, where the major canals have water level below ground level, and water is lifted 1 or 2 m by animal-driven water wheel, or manually, from small lowlevel supply canals. A current issue is the advantage of filling in the supply canals (a saving in cultivable land) and substituting central pumping units on the main and branch canal banks, with buried pipe distribution.
Unauthorized low-lift pumping from major canals is widely practiced by cultivators in some areas in South Asia, to the extent that it may well be questioned whether this should not be considered a legitimate component of distribution in canal systems. It would permit irrigation of a strip of land on one or both sides of a principal canal, an area which is usually difficult to serve directly by the gravity canal system. There would, of course, be a problem with control of the amount of water pumped, particularly as the primary canals from which such pumps would usually operate run continuously, not rotationally.
The same problem is encountered with direct gravity outlets on such canals, illustrating the fact that technically desirable, logical, features in a distribution system may be ruled out on management grounds if solution cannot be found to the problem of control
A centralized pumped-life intake on a major river can pose a number of technical problems including changes in the course of the river, siltation of the intake during high-flows, and the wide range in river-level between high and low-flow seasons. The magnitude of the monsoonal flood-flows of the major South Asian rivers, often in highly erosible channels, makes river control extremely difficult. Protection of the site of a pumping station against erosion may be feasible, but little can be done to prevent the low-season channel on which the pumping station depends from changing its position during a flood, to reappear half a kilometer away from the station when the flood recedes. Location of the station on a stable reach of the river is of course desirable, but such a site may not exist in the vicinity of the area which it is intended to irrigate. Work may be necessary in the river-bed at the end of each flood season, to re-establish a lowflow channel at the intake. However, it must be acknowledged that considerations of channel instability can rule out the installation of fixed pumping stations in some situations, making smaller moveable units the only feasible solution.
Some of the rivers in question carry extremely high silt loads during the flood season. Pumping is stopped at such times, partly to prevent carrying silt into the distribution system and partly because there is little demand for irrigation at that time. However, the intake can remain exposed to siltation. If the intake has a conventionally-shaped convergent approach structure, a low velocity eddy is likely to form in the intake area (the pumps being shut down), with heavy deposition of silt in the intake and the pump chamber. The solution to this major problem in one installation was to provide closure of the intake structure Bush with the river-channel, providing no opportunity for eddy formation.
The very wide range in river-level between low-flow and high-Dow seasons has caused much ingenuity to be exercised in the design of pumping stations for lift irrigation. One system widely used has the pumps and motors mounted on a moored floating pontoon. This requires the use of a telescopic or other adjustable-length pipe arrangement connecting the floating pump station to the fixed outlet at the top of the bank, a system which requires considerable attention during periods of varying river-flow. Another alternative utilizes a propeller pump at the lower end of a fixed inclined pipe extending up the bank. The propeller is driven by a shaft extending up the inside of the delivery pipe and connected to a motor at its upper end. At considerably greater structural cost, a pump chamber may be provided at low river-level, with vertical drive-shaft extending up to a motor on a platform above maximum flood level.
Water distribution from a major pumped-lift installation is generally by canal system, and the earlier discussion of such systems is relevant. However, pumped-lift diversions offer the possibility of sufficient head to permit the use of buried-pipe distribution, particularly in smaller and medium capacity installations. Distribution can then be basically similar to that previously described for medium and large capacity tubewells, the river-lift pump substituting for the tubewell pump. Differences are simply of scale, river-lift installations ranging up to a greater capacity than commonly encountered with tubewells. The river-lift pumping station is also located on one boundary of the service area, while the tubewell is more often located within that area.
One type of layout used for pumped-lift distribution systems has delivery lines arranged in radial fashion, extending from the pump station towards the perimeter of the service area. Outlet valves are located at intervals along each line. A deficiency in this arrangement is the fact that the area served by an outlet valve is much greater near the outer perimeter than near the hub of the radial system (the pump station). The distance to be run by earthen channel from the outlet to the individual plot is also greater and reliability of supply is consequently less. Furthermore, if several outlets are open together on one of the lines, the hydraulic head on the outlets near the pump and their discharge are substantially greater than further along the line. Consequently, the area near the pump is usually more intensively irrigated than the remainder, the net result being an undesirably small effective service area.
Several efforts have been made to install flow regulating devices on outlet valves, to ensure equality of discharge regardless of head. While this should be technically feasible, there has been little success in designing a tamper-proof system. In fact, a robust low-cost, low head flow control valve (20 to 30 liters/sec, head ranging from 1 to 5 m) is much needed, but has not yet been developed by irrigation equipment manufacturers.
As previously noted, an adaptation of the system used for medium and large tubewells may be employed as an alternative to the radial (or branching) layout. This divides the pump command into sub-areas each of about 30 ha, and each with a pipe loop system and outlet valves. Supply to each sub-area is controlled by an elevated float chamber, the demand within the sub-area being signalled back hydraulically, to a central chamber at the pump house which regulates the amount being pumped. Such regulation is facilitated in the case of pumped-lift installations, compared with tubewells, as the pumping capacity can be divided between several pumps. Output is adjusted by running the appropriate number of pumps. The system described offers the possibility of largely self-management by the cultivators within each sub-area.
A final factor in pumped-lift installation is largely riparian. Many such installations are pumping from the braided channel systems of deltaic areas. Further consumptive extraction can cause the saline/freshwater tidal interface to move upstream, adversely affecting existing pumping installations lower in the delta. Mathematical modelling of the often intricate tidal flow pattern within the deltaic channel system may be necessary in such a situation to determine the feasibility of further pumped-lift installations and their location.
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