IS 11522:1985 is the Indian Standard (BIS) for criteria for hydraulic design of drop structures. 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.
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Stilling Basin Length (General Approach) | Calculated using specific empirical formulas, e.g., L = 5 * (yc * HL)^0.5 for a vertical drop. | Determined from dimensionless charts based on Froude number and basin type (e.g., L/d2 vs Fr for USBR basins). | USBR Design of Small Canal Structures |
| Baffle Block Height | For baffled apron drops, height is equal to the downstream subcritical water depth (y2). | For SAF Stilling Basin, height is equal to the upstream supercritical flow depth (d1). | FHWA HEC-14 |
| End Sill Height (for a simple forced jump) | For a vertical drop with a cistern, a height of y2/10 is recommended. | For a USBR Type II basin, the dentated end sill height is 0.2 * d2. | USBR Design of Small Canal Structures |
| Required Tailwater Depth | Design requires actual tailwater depth to be equal to or slightly above the theoretical sequent depth (yj). | Varies by basin type. USBR Type III basins can perform with a tailwater depth as low as 85% of the sequent depth (d2). | USBR Design of Small Canal Structures |
| Minimum Freeboard for Structure Walls | Side walls should be at the same level as the upstream canal bank, implying a freeboard of 0.5 m to 0.75 m. | Side walls should extend above max water surface by the upstream canal freeboard, typically 1 to 2 ft (0.3 to 0.6 m). | USBR Design of Small Canal Structures |
| Applicability of Baffle Blocks/Piers | Used in baffled apron drops for low head structures. | Generally avoided for incoming velocities exceeding 60 ft/s (18 m/s) due to potential for cavitation damage. | FHWA HEC-14 |