CURVES IN CANALS
Because of economic and other considerations, the canal alignment does not remain straight all through the length of the canal, and curves or bends have to be provided. The curves cause disturbed flow conditions resulting in eddies or cross currents which increase the losses. In curved channel portion, the water surface is not level in the transverse direction. There is a slight drop in the water surface at the inner edge of the curve and a slight rise at the outer edge of the curve. This result in slight increase in the velocity at the inner edge and slight near the bed move to the inner bank and the high-velocity fluid particles near the surface gradually cross to the outer bank. The cross currents tend to cause erosion along the outer bank. The changes in the velocity on account of cross currents depend on the approach flow condition and the characteristics of the curve. When separate curves follow in close succession either in the same direction or in the reversed direction, the velocity changes become still more complicated.
Therefore, wherever possible, curves in channels excavated through loose soil should be avoided. If it is unavoidable, the curves should have a long radius of curvature. The permissible minimum radius of curvature for a channel curve depends on the type of channel, dimensions of cross-section, and velocities during full-capacity operations, earth formation along channel alignment and dangers of erosion along the paths of curved channel. In general, the permissible minimum radius of curvature is shorter for flumes or lined canals then earth canals, shorter for small cross-sections than for large cross-sections, shorter for low velocities than for high velocities, and shorter for tight soils than for loose soils. Table indicates the values of minimum radii of channel curves for different channel capacities.
Table: Radius of curvature for channel curves
Channel capacity (m3/s) Minimum radius of curvature (metres)
Less than 0.3 100
0.3 to 3.0 150
0.3 to 15.0 300
15.0 to 30.0 600
30.0 to 85.0 900
More than 85 1500