|Design and Operation of Smallholder Irrigation in South Asia (WB, 1995, 134 p.)|
|Chapter 8 - Hydraulics of canal regulation and types of control structures|
The term "upstream control. is used in the popular sense, meaning that water releases to the primary canal are decided by the operating- agency, with consideration being given to the supply/demand situation, rather than being directly (hydraulically) responsive to downstream demand. The term will later also be used in the technical sense, implying operation of a gate (usually automatically) in response to the water level immediately upstream of it.
The alternative approaches to the upstream control type of system have been noted in the previous chapter. They are as follows:
(a) Continuous flow throughout, including tertiaries, with division of flow in proportion to area served, through use of flow dividers This system is widely used in small village schemes. In larger public systems the flow divider may be used at branches in primary and possibly secondary canals, but not at offtakes to tertiaries, as the resulting flow during periods of limited supply could be too small for efficient conveyance in unlined tertiaries and field channels, and for field application for non-paddy crops. Continuous flow throughout is not further considered herein.
(b) Continuous regulated flow in primary canals, with full flow supply in fixed rotation to secondaries and their tertiaries. This system requires no operation of controls on the secondaries or the tertiary offtakes. However the proportionate reduction in supply/demand which can be accommodated depends on the divisibility of the secondaries into groups (discussed later). The system is particularly applicable to situations in which the supply in high and low flow seasons is assured (a special situation), and the ratio between the two can be accommodated numerically in designing the grouping of secondaries.
(c) Continuous regulated flow in both primary and secondary canals, with full flow supply in fixed rotation to tertiaries. This removes the above constraint of divisibility of the number of secondaries and also permits tailoring of supply to the needs of individual tertiary commands. Any proportionate reduction in supply can be accommodated, including adjustment to conserve storage during rainfall. It is a very flexible system, but requires operation of hydraulic structures down the length of the secondaries.
(d) Continuous regulated flow in primary canals, with full-flow supply in rotation to secondaries and their tertiaries, subject to a system of rotating priorities contingent upon the amount of water available. This system, widely used in N.W. India, can accommodate any degree of reduction in supply. Its principal disadvantage is the relatively long time between irrigations which may occur in low priority rotations. However, in the area in which the system is currently in use, canal irrigation is widely supplemented by farmer-owned tubewells. The system would not be appropriate in an area with limited availability of groundwater and planned diversified cropping, such as the case under consideration. It is not further considered herein.
(e) Supply to portions of the service area is deleted for the entire season in dry years. This type of operation is appropriate only where a large proportion of the seasonal supply comes from storage and the deficiency is predictable. It is not further considered.
Of the systems listed above the two which could be appropriate to the project situation under discussion are (b) and (c). The difference between the two is that in alternative (b) supply to the secondaries is full-flow and rotational in periods of reduced supply or demand. While in alternative (c) it is variable (regulated) and continuous. As a corollary, in (b) rotational supply begins at the secondary; in (c) it begins at the tertiary. (In another usage, system (b) is "structured" down to the head of the secondary, system (c) down to the head of the tertiary). There are advantages and disadvantages with both alternatives and choice should be project specific.
Such group rotations, operated in conjunction with the project reservoir, may be sufficient to meet all seasonal variations in supply and demand. However, the period between irrigations may be a problem. The minimum practical duration of running time of a secondary operating on/off depends upon its length. If filling and emptying are not to take up too great a proportion of the "on" cycle the minimum practical running time for a secondary may be ten days. With a one on/two off rotation of secondaries (supply at one-third of maximum) the corresponding period between irrigations of a particular holding supplied once from its tertiary during each rotation would be thirty days. For a basic crop such as wheat this irrigation interval would not be a problem, provided that the dates of supply were know in advance. For specialty crops it would be excessive unless cultivators had farm storage, such as a large-diameter dug well. (It is presumed that only limited groundwater is available in the case under discussion).
Apart from the practical length of the "on" period of the secondary, as previously noted there are soil conditions in which rotational operation of unlined secondaries would be unfeasible due to excessive sloughing of banks. There are also ground conditions (high watertable) in which rotational operation would be destructive to a lined secondary, due to back-pressure on the lining.
Summarizing, full-flow rotation of secondaries permits operation of the system at two or three levels of delivery, in addition to delivery at full capacity. As the rotational cycle is likely to be as much as three or four weeks this is not a rapid response type of operation, capable of short-term adjustment. It is more appropriate to making seasonal changes in rate of delivery which can be planned in advance, a situation which generally requires substantial reservoir capacity. The irrigation interval is relatively long during periods of reduced delivery, being determined by the length and filling time of the secondaries. The use of such a system is contingent upon ground conditions which permit on/off cycling of secondaries without deterioration of the canal section. The system is most appropriate where sufficient groundwater is available to provide supplemental irrigation of stress sensitive crops. Its principal advantage is simplicity of operation, gate operation being required only down to the head of the secondary.
In alternative (c), the secondaries and the primary canal operate continuously at variable flow. The tertiaries served by a particular secondary operate rotationally at full-flow, the tertiaries being grouped for this purpose in the same manner as the secondaries in alternative (b). However, there are many more tertiaries than secondaries, with more possible groupings, and the rotational period can be much shorter due to the shorter filling time of tertiaries. Furthermore stability of the channel section under on/off conditions is not generally a problem with the much smaller tertiaries.
For example, with alternative (c), operation the situation is taken in which the supply to the area is reduced to one-quarter of system capacity, either due to reduced availability or low demand. The flow in the primary canal is reduced to one-quarter of capacity by operation of control structures, and also the flow in the secondaries. (The division of flow between primary and secondaries may be by flow dividers on the primary canal, discussed later). To reduce flow in the tertiaries to one-quarter of capacity would be ineffective due to high proportionate seepage losses in these small channels with such low flow and low field efficiency. Consequently, the tertiaries on each secondary are divided into four groups, one group at a time taking the whole flow in the parent secondary and operating at full design capacity. Each group operates for three days on and nine days off, for a rotational cycle of twelve days.
While such an arrangement may appear straight-forward, and it is indeed operable, it poses a number of hydraulic and other operational problems. First, to permit full-flow diversion to tertiaries the water-level in the parent secondary must be maintained at or near full supply level, even while flow in secondary is reduced to one-quarter of capacity. This requires a considerable number of hydraulic structures on the secondary and their operation. Second, and probably more importantly, the tertiary intakes have to be gated and the gates must remain closed other than during the appropriate rotational turn. The question is what agency operates the tertiary intake gates, and ensures that they are not opened out of turn, particularly in periods of severely reduced supply when crop survival is at stake.