This standard provides criteria and guidelines for the hydraulic design of drop structures (also known as falls) in irrigation canals. It covers the classification of drops, the design of control structures like weirs and flumes, and the design of energy dissipation arrangements such as stilling basins to ensure the stability of the canal bed and banks.
Provides criteria for the hydraulic design of drop structures used in canals to dissipate energy due to change in elevation.
Quick Reference Values
Typical upstream glacis slope2:1 (H:V)
Typical downstream glacis slope3:1 to 5:1 (H:V)
Recommended minimum freeboard0.5 m to 0.75 m
Coefficient of discharge (Cd) for rectangular weir1.8
Minimum length of impervious floor (downstream)5 x (D2 - D1)
Length of stilling basin for hydraulic jump5 to 7 times the post-jump depth (y2)
Key Formulas
Q = Cd * L * H^(3/2) — Discharge formula for a rectangular weir
y2/y1 = 0.5 * [-1 + sqrt(1 + 8*Fr1^2)] — Hydraulic jump relationship (Belanger's momentum equation)
Lc = 2.28 * Hc / (q^2 / g)^(1/3) — Length of crest for Sarda type fall
Practical Notes
Proper design of the stilling basin and end sill is critical to prevent downstream scour, a common failure mode for these structures.
The choice between a vertical drop and a glacis (sloping) drop depends primarily on the drop height and discharge; glacis falls are preferred for larger drops.
Ensure adequate upstream and downstream protection works (like block pitching or concrete lining) to prevent erosion and outflanking.