| Design and operation of smallholder irrigation in South Asia |
|Chapter 8 - Hydraulics of canal regulation and types of control structures|
Supply to smallholders requires a large number of hydraulic structures at the tertiary level. For instance, a 20,000 ha service area with holdings averaging one hectare in size would need 500 to 600 tertiary intakes and some 20,000 farm turnouts, together with a large number of "junction structures", checks, and drops.
As previously noted, installation of tertiary structures has been responsible for much of the delay (often several years) between completing the main canal system and bringing the whole command into effective operation. Traditional brick construction of small hydraulic structures is satisfactory technically, but installation is very slow and limited to the dry season. The alternative material is pre-cast concrete. Manufacture of the units can continue throughout the year, and installation, although also limited to the dry season, is relatively rapid and requires largely unskilled labor.
While pre-cast concrete is undoubtedly the indicated solution to the problem, experience with small pre-cast structures underlines several design considerations unique to the smallholder situation. The tertiary outlet, in particular, is generally regarded by cultivators as the main constraint on the rate of diversion to their holdings, which is certainly true. The structure is consequently subject to attack and much ingenuity is devoted to increasing its discharge. Small structures in general are also particularly susceptible to destructive soil pressures and movement in expansive clays, a factor which operates against the use of light-weight sectionalized construction in pre-cast units. Robust, relatively heavy construction is preferable, economy being served by low labor requirements in installation rather than in cost of materials.
The problem of theft of gates on tertiary outlets and farm outlets, also on junction structures, has been discussed earlier. No material has proved immune and chains and padlocks are equally susceptible. The only material not subject to theft is soil, and this is eventually the fall-back material for closures. Repeated taking of mud from the channel-bed adjacent to structures, for closure purposes, can result in depressions which remain water-filled after each rotation, eventually seeping away and representing water-loss. The problem cannot be entirely avoided, but it can be reduced by designing the structures so as to minimize the amount of soil, or mud, required for closure. Steel bars embedded for this purpose in the throat of the opening have been incorporated in some designs.
Means of providing a tamper-proof adjustable gate for tertiary outlets has received much attention in the past, but without notable success. None of the operational systems discussed above in fact require adjustment of the intake in normal service; they are all full-flow rotational systems. Capability of one-time adjustment of capacity at the time of initial installation is, however, a desirable feature, for two reasons. First, because the size of tertiary command unavoidably varies, the appropriate capacity of outlet also varies from one to another. It is convenient to deliver standardized pre-cast units to the field and to make the appropriate final adjustment to capacity at each location. Second, the head in the intake and its capacity are functions of water-level in the parent secondary and in some cases water-level in the tertiary (if the intake is operating "submerged"). Both levels can be estimated in advance approximately only. It is convenient to make final adjustment of the intake in the field, in accordance with actual measured levels. One method of doing so is to supply the intake in the form of a standard body with oversized opening in which an insert sleeve is permanently grouted at the site. The sleeves are supplied in a range of sizes of opening, from which selection is made appropriate to the size of service area and the actual head on the particular intake.