Inputs
Saved calculations
No calculations saved yet. Run the calculator to populate this table.
Example data
These are sample entries to illustrate common weir setups and typical field magnitudes.
| Scenario | Weir Type | H | L / b | θ | Cd | Expected Use |
|---|---|---|---|---|---|---|
| Site dewatering check | Rectangular (Suppressed) | 0.25 m | 1.00 m | — | 0.62 | Quick discharge estimate for temporary channels |
| Small flow measurement | V-Notch | 0.18 m | — | 90° | 0.62 | Higher sensitivity at low heads |
| Plant overflow structure | Broad-crested | 0.40 m | 1.50 m | — | 0.99 | Robust crest suitable for higher flows |
| Contracted opening | Rectangular (Contracted) | 0.30 m | 0.90 m | — | 0.62 | Temporary box opening with end contractions |
Formula used
The calculator uses standard sharp-crested and broad-crested discharge relations with an optional approach-velocity correction.
- Effective head: He = H + (Va² / 2g)
- Sharp-crested rectangular: Q = (2/3) · Cd · Le · √(2g) · He^(3/2)
- Contracted rectangular (effective length): Le = L − 0.1 · n · H
- V-notch (triangular): Q = (8/15) · Cd · tan(θ/2) · √(2g) · He^(5/2)
- Broad-crested (critical-flow form): Q = (8/27) · Cd · b · √(2g) · He^(3/2)
How to use this calculator
- Select a unit system and the weir type used on site.
- Enter head over crest (H) measured upstream of the weir.
- Provide crest width (L/b) for rectangular or broad-crested types, or notch angle (θ) for V-notches.
- Keep the typical Cd for a quick estimate, or enter a tested coefficient.
- Optionally enter approach velocity (Va) to include velocity head in the effective head.
- Press Calculate; the result appears above the form and is saved in the table.
- Use Download CSV or Download PDF to attach results to reports.
Overflow measurement in construction drainage
Temporary channels, sumps, and plant outfalls often need a fast overflow estimate. A sharp‑crested or broad‑crested weir converts a measured head above crest into discharge, supporting pump sizing, bypass planning, and environmental reporting. It helps validate diversion flows during storm events. This calculator saves each run so crews can compare scenarios during shifts.
Choosing the correct weir geometry
Rectangular weirs suit wider flows where crest width is known and stable. Use suppressed for full‑width crests, and contracted when there are side gaps that reduce effective length. A V‑notch is preferred for small flows because discharge varies strongly with head, giving better sensitivity at low depths. Common notch angles are 60° and 90°. Broad‑crested structures are common where a robust crest is needed.
Interpreting coefficients and corrections
Discharge depends on gravity, effective head, and a coefficient that reflects crest details and approach flow. If approach velocity is known, velocity head is added to form an effective head and reduce under‑prediction. For contracted rectangular openings, an end‑contraction factor estimates effective crest length. Typical Cd is about 0.58–0.65 for sharp‑crested and 0.90–1.05 for broad‑crested.
Quality checks and typical ranges
Field readings are most reliable when head is taken upstream in a calm pool, away from drawdown and turbulence. Keep consistent units, confirm the crest is level, and ensure the nappe is ventilated for sharp‑crested types. Verify free overflow conditions; submergence or tailwater influence can reduce accuracy and requires different methods. If results appear extreme, recheck head measurement, crest width, and whether the selected weir type matches the structure.
Documentation for audits and handover
Use the saved table to capture time, geometry, coefficient, and discharge in one place. Up to 25 recent entries are kept per session, making shift‑to‑shift comparison simple. Export CSV for spreadsheets or daily logs, and export PDF for attaching to method statements, inspection packs, or commissioning records. Consistent documentation supports decisions on pumping rates, diversion controls, and compliance limits.
FAQs
What is the minimum information needed to estimate discharge?
You need the head over the crest and the correct geometry. Enter crest width for rectangular or broad‑crested types, or notch angle for V‑notches. Use a typical coefficient for screening, or a tested value for reporting.
When should I use a V‑notch instead of a rectangular weir?
Use a V‑notch for small or variable flows where sensitivity at low heads matters. Because discharge changes strongly with head, small measurement differences become more visible than with a rectangular crest.
How does approach velocity affect the result?
Approach velocity adds velocity head to the measured head, increasing the effective head used in the formula. This can reduce underestimation when upstream flow is fast, especially in narrow channels or short approach pools.
What does the end‑contraction setting change?
For contracted rectangular openings, end contractions reduce the effective crest length. The calculator applies a simple correction based on the number of contractions and measured head, helping approximate real discharge when side clearances exist.
Why can my calculated flow look unrealistically high?
Common causes include measuring head in a turbulent zone, using the wrong weir type, entering crest width incorrectly, or ignoring submergence. Re‑measure in a calm pool upstream, confirm units, and check the structure matches the selection.
Are the CSV and PDF exports suitable for project records?
Yes for documentation and traceability. Exports capture date/time, inputs, coefficients, and discharge so reviewers can reproduce calculations. For contractual measurement, align the coefficient and measurement procedure with your project specifications.