Input Parameters
Example Data Table
| Scenario | b0 (m) | z (H:V) | Hu (m) | Ht (m) | Method | Typical Peak Q (m³/s) |
|---|---|---|---|---|---|---|
| Small irrigation canal | 3.0 | 1.0 | 2.0 | 0.3 | Broad-crested | ~ 15 to 30 |
| Medium embankment breach | 6.0 | 1.5 | 3.0 | 0.5 | Combined | ~ 50 to 120 |
| Large breach, low tailwater | 10.0 | 2.0 | 4.0 | 0.2 | Orifice | ~ 150 to 300 |
These are illustrative ranges only. Use site-specific data and engineering judgment for design and emergency response planning.
Formula Used
Effective head: He = max(0, Hu − Ht)
Trapezoidal breach geometry at head He:
- Top width: b = b0 + 2 z He
- Area: A = He × (b0 + b) / 2
Broad-crested weir approximation: Q = Cw × b × He^(3/2)
Orifice/underflow approximation: Q = Cd × A × √(2 g He)
Submergence correction: A simple reduction factor is applied when Ht/Hu ≥ 0.7, clamped between 0.30 and 1.00.
How to Use This Calculator
- Pick a method that matches expected breach flow behavior.
- Enter breach width, side slope, and upstream head.
- Provide tailwater depth to represent downstream backwater.
- Keep coefficients within realistic engineering ranges.
- Set an average factor and duration for volume estimation.
- Press Calculate Outflow to view results above the form.
- Use Download CSV or Download PDF for reports.
Professional Notes: Canal Breach Outflow
1) Why breach outflow estimates matter
A canal breach can release large volumes within minutes, threatening workers, nearby roads, and downstream property. For response planning, field teams typically need peak discharge (m³/s) for hazard zoning and an average discharge for volume and pumping plans. Even a small irrigation canal may produce tens of cubic meters per second if head is 1–3 m and the opening widens quickly.
2) Head and tailwater control the driving energy
This calculator uses an effective head, He = Hu − Ht, to represent the portion of upstream depth not cancelled by downstream backwater. When tailwater rises, the discharge reduces. A practical trigger is a tailwater ratio Ht/Hu ≥ 0.7, where submergence becomes significant. In emergency situations, monitoring tailwater helps validate whether the breach is free-flowing or partially drowned.
3) Breach geometry assumptions used here
Many breaches form a trapezoid: a bottom width b0 and side slopes z (horizontal per vertical). At head He, the top width becomes b = b0 + 2 z He, and area is A = He × (b0 + b) / 2. Wider slopes and higher heads increase both width and area, raising flow rapidly.
4) Choosing a discharge model
The broad-crested approach, Q = Cw b He^(3/2), is useful when the breach crest behaves like a control section. Typical Cw values are about 1.6–1.9. The orifice approach, Q = Cd A √(2 g He), is useful for underflow-like openings; Cd is often 0.60–0.70. The combined option reports both and uses the larger as a conservative peak.
5) From peak flow to total released volume
Because breach conditions evolve, a constant peak can overstate total outflow. This tool applies an average factor (commonly 0.5–0.8) so Qavg = factor × Qpeak. Total volume is then V = Qavg × duration. Use results for screening and emergency planning, and confirm with site hydraulics, survey data, and professional review.
FAQs
1) Which method should I choose?
Use broad-crested when flow is controlled at a crest-like section. Use orifice when flow passes through an opening acting like underflow. If unsure, choose Combined and review both estimates.
2) What does “effective head” mean?
Effective head is the upstream depth above the breach bottom minus tailwater depth. It represents the net driving energy available to push flow through the breach.
3) Why does tailwater reduce discharge?
High tailwater submerges the opening and reduces the pressure difference across the breach. The calculator applies a simple submergence factor when Ht/Hu becomes large.
4) Are Cw and Cd fixed constants?
No. They depend on breach shape, roughness, approach conditions, and contraction. Use realistic ranges (Cw about 1.6–1.9, Cd about 0.60–0.70) and calibrate with site observations when possible.
5) How should I set the average factor?
For quick screening, 0.6 is a common starting point. Use lower values if the breach is expected to stabilize or head drops quickly, and higher values if head remains nearly constant.
6) What duration should I use?
Use the expected time until isolation, repair, or significant drawdown occurs. For emergency response, evaluate several durations (for example 0.5, 1, and 2 hours) to bracket volume.
7) Is this suitable for final design decisions?
This tool is best for planning and rapid assessment. For final design or public safety decisions, use detailed hydraulic modeling, field measurements, and qualified engineering review.