An overflow weir in an intake or canal system is not only a spillway formula. It is a level-control element, so the calculation should start from the water level to be maintained, the safe overflow level, and the downstream rating condition.
Recommended workflow
- Define the normal operating water level in the canal or basin.
- Select the overflow crest level based on the maximum water level that can be allowed before spilling.
- Use the available crest length and estimate discharge using a broad-crested or sharp-crested relationship as applicable.
- Check tailwater: if downstream water submerges the crest, apply a submerged flow correction.
- Check freeboard, approach velocity, side wall height, and safe energy dissipation downstream.
Quick design table
| Item | Check |
|---|---|
| Crest level | Set from allowable canal/basin water level, not randomly from ground level. |
| Discharge coefficient | Use a conservative coefficient first, then calibrate if site data is available. |
| Downstream control | Use a rating curve or hydraulic control level when available. |
For a professional spreadsheet, keep separate inputs for maintained water level, spill crest level, downstream water level, and coefficient assumptions.