A STUDY OF UNCONFINED SEEPAGE FROM PARALLEL CANALS

Abstract

Irrigation canals are generally constructed in a deep pervious alluvium and are largely unlined. These canals alongwith their conveyance system are a major source of seepage from them which recharges the aquifer. The continuous recharging of aquifer leads to the rise of ground water table in the in fluence area of the canals. In order to plan and design an irrigation canal network or to manage effi ciently an existing system of canals, a reasonably accurate assessment of seepage losses from canals is essential. When canals run continuously with a constant discharge, a steady state of seepage may be attained in course of time. However, generally the canals run in roster or with varying discharges, and the water table positions may vary with time due to which a steady state condition of seepage may not be reached at all. In a number of major canal systems in the state of Uttar Pradesh (India), parallel canals by the side of the existing main canals have been constructed to augment supplies in the command area. Since the parallel canals largely run intermittently the seepage from such canal systems would remain in unsteady state. The canal systems may be constructed in regions where the existing water table is too deep, or the canal may be located at high elevation, such that the canal is not hydraulically connected . with the aquifer. In general, however, the irrigation canals are located in pervious alluvium with shallow water table positions and the canals are hydraulically connected with the aquifer. At present analytical solutions of the unsteady seepage from parallel canals are not yet available. In the persent thesis a study has been made to analyse the unsteady seepage from a canal for deep as well as for shallow water table positions and it has then been extended to study the unsteady seepage from parallel canals and their interference. The analysis is based on linear theory of hydrologic system. Many complex ground water flow problems have been solved based on this theory. Ground water hydrology- is a quantitative science and mathematics is its important dialect. Discrete kernel approach is comparatively new in its ambit. The discrete kernels are the proper ties of a linear system. Using the discrete kernel response functions, unsteady seepage from canals has been studied in this thesis. From the study of a single canal, which is not hydraulically connected with the aquifer, and (11) which runs intermittently, it is found that there is no reversal of flow in consequence of intermittent running of the canal. For two parallel canals, which are not hydrauli cally connected with the aquifer, their interferrence relates to evolution of water table only. In case of two identical parallel canals, it has been found that in the beginning of recharge,. two distinct water mounds are formed below the centre of each canal. With lapse of time, the points of maximum rise move towards each other; but they do not move beyond the respective recharging strip. With further lapse of time, a stagnant zone gets created between the canals, and the region between the two parallel canals takes the shape of a plateau. It has been found for the case of unequal parallel canals that, sometime after the onset of recharge, only one point of maximum rise under the larger canal is established. When a canal is hydraulically connected with the aquifer, the seepage losses decrease with time. In the case of parallel canals, the seepage from each canal will be further reduced because of inter ference of one canal on the other one. The interfer ence is the decrease in seepage loss of .a canal due to the presence of the parallel canal. It has (iii) been taken as the difference between the seepage losses from a canal when it runs alone and when it runs alongwith the parallel canal. The seepage loss from canal may be linearly or non-linearly dependent on the potential difference between the canal and the aquifer. It has been found that for very shallow water table position below the canal bed the linear relationship can be used. However, as the potential difference between the canal and the aquifer increases or the width of canal increases, the non-linearity gets pronounced. The study of interference of parallel canals has been done assuming a linear relationship between the seepage and the potential difference. It is found that Herbert's formula for reach transmissivity is appropriate for use in canal-aquifer interaction studies. The study of unsteady seepage from two parallel canals, when the water table is located at shallow depth below the bed of the canals, has been carried out for equal and unequal canals which run conti nuously. The study has been extended for the case in which one of the canals runs intermittently. It has been found that in case of two continuously running parallel canals, the reduction in seepage from one canal due to interference of the other is zero in the beginning of seepage. The interference increases with time, attains a maximum value, and (iv) then decreases. The decrease in inteference is monotonic at large time. The interference of parallel canals is found to decrease with increase in the spacing between the canals. For unequal parallel canals, the inteference of bigger canal on smaller canal is more than that of the smaller canal on the bigger one. If one of the parallel canals runs intermittently, it is found that the reduction in seepage from the continuously running canal, due to interference of the intermittently running canal, starts from zero, increases from cycle to cycle, reaches a maximum value, and then decreases. Also, the intermittently running canal in the parallel canal system acts as a drain during its closure period after a few cycles of running. It has been seen that in case of two continuously running equal parallel canals a stagnant zone is formed between the canals with lapse of time. From the study of interference of two parallel canals, when one canal is situated on a high ridge and the other in the valley having hydraulic connec tion with the aquifer, it is found that interference of ridge canal increases with time and that reversal of flow to the valley canal is mainly controlled by the dimension of the ridge canal and its distance from the valley canal. (v) When there is a natural drainage in the vicinity of the canals it will influence the water table evolution and carry away part of the recharge after it gets activated. In the present thesis a solution has been given to find the time of activation of the drain located in the vicinity of two parallel ridge canals running continuously. Temporal variation of the return flow to the drainage channel has also been quantified. It is hoped that this study will be helpful in understanding the interference of seepage of parallel canals.