Semi-theoretical approach in canal design

The semi-theoretical approach designs a canal using a standard flow formula together with empirical limits obtained from experience.

• Flow equation: Manning's, $V = \frac{1}{n}R^{2/3}S^{1/2}$, with $Q = AV$ (or Chezy's formula).
• Non-silting and non-scouring conditions are met by keeping $V$ between permissible limits: not less than about 0.3-0.6 m/s (to avoid silting and weed growth) and not more than the maximum non-scouring velocity of the boundary material.
• The ratio $B/D$ and side slopes are taken from standard tables of design practice for the given discharge range.
• The slope is chosen to suit the land slope, so the design is checked by trial and adjusted until the section carries $Q$ at the desired velocity.

It is used for canals which are not fully alluvial (hilly regions, lined canals, canals in rigid boundaries) where regime theories are not applicable. It is simple but depends on experience (empirical limits), unlike Kennedy's and Lacey's theories, which are based on silt transport concepts.