In India, canal irrigation schemes form an important component for agricultural development. In conditions of prevailing water shortage situations, it is necessary that the present day irrigation practices with respect to canal operations have to be improved to manage the available water resources in an equitable manner. Various projects like National Water Management Project and Water Resources Consolidation Project, supported by the World Bank are focusing on ways to improve the canal irrigation schemes and one of the strategies adopted towards this is by a system called structured irrigation network. The structured irrigation network involves hydraulic control structures functioning as flow rate control devices for equitable supply of water to the offtakes. These flow rate control structures are called offtake control structures. The available guidelines in literature for the design of offtake control structures are scanty and the design procedure adopted in these guidelines are approximate. As these offtake control structures form an important and integral part of the structured irrigation network, it is necessary, to review the analysis and design algorithms of these structures and to study their hydraulic characteristics.

The available guidelines do not focus on the hydraulic flow conditions at minimum discharge in the parent canal. As some amount of variation in the parent canal discharge becomes inevitable depending on the season and the cropping pattern, the flow at both maximum and minimum discharge conditions in the parent canal is to be studied. At maximum discharge condition, the offtake structures are designed for a particular share of parent canal discharge. But at minimum discharge condition, the offtake may not draw the same proportion that is originally designed at maximum flow condition. The offtake control structures should be designed such that at minimum discharge condition also, the deviation from true proportionality is minimum.

The offtake control structures considered in the scope of the present study are : (a) open flume (b) pipe semimodule and (c) pipe outlet. Modification of existing pipe outlets are also considered. A review of the hydraulics of these offtake control structures in done and an attempt is made to develop an algorithm, for more accurate analysis and design of offtake control structures taking into account both maximum and minimum discharge conditions. The performance characteristics of offtake control structures are studied with respect to various parameters. The overdesigned pipe outlets are very common in existing canal irrigation schemes. These structures draw more discharge than the design discharge at both maximum and minimum flow conditions. A general procedure is suggested for correcting such overdesigned pipe outlets using a constriction sleeve in the pipe and a flume in the offtake.

The offtake control structures are designed such that uniform flow conditions prevails in the parent canal at full supply discharge. For this purpose, it is necessary that sufficient energy loss is available at the offtake. This is verified through data from several field canals.

The dimensions of the offtake control structure are decided from analysis at maximum flow condition and the performance of the structure is verified from analysis for minimum flow condition. It is found that the available equations are insufficient for analysis of minimum flow conditions and hence an additional principle based on energy is developed to analyze the flow at minimum discharge conditions. The analysis involves a set of non linear algebraic equations and an iterative procedure is developed for the solution which is valid for different types of offtake structures. For all the offtake structures, there is one degree of freedom in the design, which gives a number of alternative designs. A general procedure is evolved to select the most optimal design from the set of alternative designs based on a set of desirable characteristics. The parameters used for this purpose are: (a) deviation from proportionality (b) throat width (c) submergence ratio and (d) crest setting. Two methods are suggested for this purpose namely : (a)screening technique and (b) user defined weightage method. The relative merits of the selection techniques are discussed. An algorithm is developed for the design, analysis and selection of the most optimal design of the offtake control structures and to study the hydraulic characteristics of the structure for a given range of discharge in the parent canal.

The performance of the offtake control structures are studied in a range of operating discharge, with respect to various parameters. Seven field problems are considered for this purpose. For the open flume, the crest height is found to be an important factor which has a significant effect on the proportionality characteristics of the structure. It is found that, the practice of fixing the crest at 10% of full supply depth, provides good proportionality characteristics for broader channels. However, for narrow channels, a larger crest height is found to give good proportionality characteristics. Such a crest setting also has the advantage of a larger throat width. The final design of open flume is studied through the test problems, using both the selection methods namely (a) screening technique and (b) user defined weightage method. It is found that the Ordinary Least Square solution in the user defined weightage technique matches with the choice of design obtained form the screening technique. The Weighted Least Square solution with increase in weightage for throat width and submergence gives a higher crest setting with no specific advantage.

The effect of crest setting on the proportionality characteristics of pipe semimodule is studied and it is seen that a higher crest setting with an associated better throat width is possible than that of open flume. The velocity of flow in the pipe part of the pipe semimodule is found to be an important factor on the proportionality characteristics. If the velocity of flow in the pipe part is such that the effect of head loss through the pipe is dominant, then the pipe semimodule will behave like a pipe outlet. On the other hand if the velocity of flow in the pipe part is such that the effect of head loss through the pipe is not significant, then the structure is found to behave like an open flume.

The flow conditions and the setting of the pipe outlet have a significant effect on the proportionality characteristics of the pipe outlet. The flow conditions at maximum and minimum discharge may be submerged- submerged, free-free or submerged-free flow. The deep set pipe outlets, irrespective of the flow conditions, have poor proportionality characteristics, with offtake drawing a large share of parent canal discharge at the reduced discharge conditions in the parent canal. This favors the upstream farmers and as a result, the tail end command areas suffer from water shortage. The shallow setting of fixing the pipe at 70$\%$ of the full supply depth from the canal bed as presently recommended, to achieve flexibility of 1.0 (based on approximate theory) is not superior to slightly deeper setting ( 50 to 60$\%$ of full supply depth) from proportionality considerations (Flexibility of 1.0 means that any deficit or surplus is shared proportionally by the offtake and the parent canal). The deeper setting will be more acceptable to the farmers than the very shallow setting and these also have good proportionality characteristics.

A general procedure is developed for the correction of the overdesigned pipe outlets, to supply the desired proportion of discharge at both design and reduced discharge conditions. Three types of correction are suggested for this purpose, namely: (a) correction by a constriction sleeve (b) correction by construction of flume in the offtake canal (c) correction by both sleeve and flume. It is found that, for shallow pipe setting, if the proportionality characteristics of the original pipe outlet is good, it can be corrected by a constriction sleeve at the entrance of the pipe. However, for deep set pipe outlets, the correction by a sleeve does not improve the proportionality characteristics. The correction by a flume is suggested for deep set over designed pipe outlets, and in this case, a better design with a good throat width for the flume is obtained, if the effect of the head loss in the pipe is significant. If the effect of the head loss in the pipe is insignificant, it is not possible to attain adequate throat width even at very high crest setting, and to achieve good proportionality. For this case, a correction technique with both flume and sleeve is presented. A general algorithm is developed for the correction of overdesigned pipe outlets by both sleeve and flume and its application is demonstrated.