Input Parameters
Units: SI (m, m/s, m³/s)Result
Enter inputs and run calculation to see discharge and derived values.
Run Log
| # | Time | Units | Profile | g | L | H | C | Va | kv | k | N | Contract? | Ht | Leff | Q | q |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| No runs yet. Calculate to populate the log. | ||||||||||||||||
Example Data
Click a row to load values into the form, then adjust and calculate.
| Units | Profile | L (m/ft) | H (m/ft) | C | Va | kv | k | N |
|---|---|---|---|---|---|---|---|---|
| SI | Sharp‑Crested | 6 | 0.9 | 1.84 | 0 | 1.0 | 0.1 | 2 |
| US | Broad‑Crested | 15 | 1.8 | 3.087 | 1.5 | 1.0 | 0.15 | 0 |
| SI | Ogee | 10 | 1.2 | 2.137 | 0.8 | 1.0 | 0.1 | 2 |
Formulas Used
This tool uses a unified overflow relation for simple crest profiles:
Discharge:
Q = C · Leff · Ht3/2
- Total head above crest:
Ht = H + kv · Va2/(2g) - Effective crest length (optional end contractions):
Leff = L − 2k · Ht · N - Unit discharge:
q = Q / Leff
Typical default C values (user‑adjustable):
SI (m³/s; L,H in m)
Sharp‑crested:
Broad‑crested:
Ogee:
Sharp‑crested:
1.84Broad‑crested:
1.705Ogee:
2.137
US (cfs; L,H in ft)
Sharp‑crested:
Broad‑crested:
Ogee:
Sharp‑crested:
3.33Broad‑crested:
3.087Ogee:
3.44
The coefficients above are widely used practical values; project‑specific calibration or guidance may prescribe different values depending on geometry, approach conditions, and crest shape quality.
How to Use
- Select the unit system and spillway profile.
- Enter crest length, head over crest, and optional approach velocity.
- Choose whether to apply end contractions and set k and N.
- Accept default coefficient C or enter a calibrated value.
- Click Calculate Discharge to compute Q, Ht, and Leff.
- Review the run log and export CSV or PDF for records.
FAQs
Sharp‑crested weirs have a thin crest where nappe springs clear. Broad‑crested weirs maintain near‑critical flow over a longer crest. Ogee spillways use a curved crest matched to the design head.
Upstream approach velocity adds kinetic energy that effectively increases the head over the crest. Including Va2/(2g) yields a more realistic total head Ht.
A simple reduction uses
Leff = L − 2k·Ht·N, where k is an empirical coefficient and N is the number of end contractions (0–2).
Yes. Defaults are provided for convenience, but you can enter any project‑specific or calibrated coefficient prescribed by local guidance.
SI uses cubic meters per second (m³/s). US customary uses cubic feet per second (cfs). Unit discharge is reported per unit crest length.
This simple model assumes free overflow without downstream submergence and adequate aeration. For submerged or ventilated conditions, consult advanced manuals and apply appropriate corrections.