Permissible velocities and variability of hydraulic resistances in concrete canal channels

Abstract

1. Processes of siltation by sediments and overgrowth by algae are, as a rule, observed in concrete channels of irrigation canals at low flow velocities, which leads to a considerable increase of the roughness coefficient and hydraulic resistance of the channel. Algae develop more intensely in concrete channels than in earth channels as a consequence of their self-purifying capacity. 2. Relations are proposed for determining the minimum and maximum design velocities in concrete canal channels providing their high hydraulic efficiency and operating reliability. On the basis of calculations and an analysis of on-site data, it is recommended to assign the design velocity in concrete channels from 0.55 to 4.0 m/sec. 3. The validity of the logarithmic law of resistance in concrete canal channels both for tranquil and rapid flow was confirmed. However, the regularities of the change in the hydraulic resistances of these flows are different: for tranquil flows the resistance coefficients decrease with increase of the Reynolds number, and for rapid flows they increase. 4. On the basis of a statistical analysis of on-site data, a normal distribution law of the values of the roughness coefficient of concrete canal channels and a reliable interval of variation of their values within n=0.0165–0.0174 were established. In practical calculations the roughness coefficient of concrete linings of irrigation canals should be taken equal to n=0.0170, which corresponds to the on-site data to a maximum degree with a probability greater than 98%.