A rectangular sharp-crested weir converts upstream head into a free-flow discharge estimate. It is useful for an early check because the controlling variables are visible: crest width, head, gravity, and a selected discharge coefficient.
The three-halves head relationship.
The CivilKit screen uses:
Because head appears to the power of 1.5, a small change in measured head can change the predicted flow noticeably. The result is reported in both cubic metres per second and litres per second.
What the discharge coefficient represents.
Cd collects the effects of the crest and approach conditions that are not written explicitly in the ideal equation. It should be selected from a relation or standard that matches the physical weir. It is not a universal constant for every rectangular opening.
Keep the head datum consistent.
H is the upstream energy head above the crest for the simplified relation. If approach velocity is material, a measured water-surface elevation alone may not represent the head used by the calibration. Submergence and tailwater can also invalidate a free-flow shortcut.
Limits before using the result.
- End contractions, side walls, crest thickness, nappe ventilation, approach flow, sediment, and debris are not modelled.
- The screen does not solve submerged or drowned-weir flow, upstream storage, transient routing, downstream energy, or freeboard.
- Use the appropriate local relation and verify the coefficient, head measurement, and discharge range against the actual structure.
next check
Compare the control with channel capacity.
Open the rectangular weir-flow calculator, then compare the result with the Manning open-channel screen.