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
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