DESIGN OF CANALS FOR NONUNIFORM SEDIMENT

Abstract

Canals in alluvium are constructed for conveyance of water from storage and diversion structures for irrigation and hydropower generation over a period of time, various methods of their design have been evolved in India and abroad. To name some of them are those due to Kennedy, Lacey, Lindley, Inglis, Blench, Chitale, Engelund and Hansen, and Kondap. While some of them considered sediment transport rate implicitly, others included it in their design equations explicitly with other parameters. The alluvium and the sediment entering into the canal are invariably graded and nonuniform. Somehow none of the investigators in the past considered the nonuniformity of the sediment. Most of them adopted the median size d to represent sediment in their equations. In the case of nonuniform material, it is not only the transport rate of sediment, but also its size distribution for which the canal needs to be designed. In other words, the quality and quantity of sediment being transported by the canal should be the same as the quality and quantity of the sediment entering into it through canal head regulator. These being same,. the bed material constitution of the canal will reach an equilibrium, different from that of the former. In this thesis the methods of design of canal developed by Engelund and Hansen,and Kondap for uniform sediment are adopted for nonuniform sediment also. Different sizes are tried to represent the sediment and the size d r/d50for which the •designed canal has the capacity to transport the sediment entering into it is taken as the true representative size d /d r 50 of nonuniform sediment. The transport capacity is estimated by the fraction wise computations using the method developed by Samaga et.al. If the nonuniformity of sediment entering into the canal is described by the Kramer's uniformity coefficient P.d. d3 M = 1 1 and modified particle Reynolds number '. =~s501 T lo"D 'IPfv2j 2 8 P.d. 5a 1 1 it was seen that the bed material constitution is given by MD(=MT t AMT) and RD T ± L: T). Thus if the canal is designed for• MD and I'D along with other design parameters, the constitution of transported material will be nearly equal to MT and T. Dimensionless equations have been developed for the determination of dr/d50 in terms of the design parameters viz Q* [_ qT ~ 7 /2 5/2 t' #T 1 Y r Y 3 MD and =:D. g d50 s s f gd50 Predictors have been developed for determination of MD and °;D.Design procedure with a computer algorithm has presented. In an attempt to minimise the difference between the constitutions of bed material and transported material various designs of canal with different bed shears have been tried.. The difference was seen to continuously reduce with increased bed shear without giving an optimum. The transport capacity increased to impractical ranges. This attempt was discontinued as unsuccessful.