Input Data
Example Data Table
| Method | Cd | Geometry | Hup | Hdown | Outlets | Approx. Total Discharge (SI) |
|---|---|---|---|---|---|---|
| Circular orifice outlet | 0.62 | D = 0.15 m | 0.30 m | 0.00 m | 1 | ~0.036 m³/s (≈36 L/s) |
| Rectangular weir | 0.61 | b = 0.50 m | 0.20 m | 0.00 m | 1 | ~0.026 m³/s (≈26 L/s) |
| V-notch weir (90°) | 0.62 | θ = 90° | 0.18 m | 0.00 m | 1 | ~0.010 m³/s (≈10 L/s) |
Example values are indicative. Use measured coefficients for final design decisions.
Formula Used
- Orifice outlet: Q = Cd × A × √(2 g H)
- Rectangular sharp-crested weir: Q = (2/3) × Cd × b × √(2 g) × H^(3/2)
- V-notch weir: Q = (8/15) × Cd × √(2 g) × tan(θ/2) × H^(5/2)
- Broad-crested weir (critical): Q = Cd × (8/27) × b × √(2 g) × H^(3/2)
When submergence is enabled, the calculator uses differential head for orifices. For weirs, it applies an empirical correction suitable for quick checks.
How to Use This Calculator
- Select the unit system that matches your field measurements.
- Choose the turnout method installed at the canal offtake.
- Enter geometry, coefficient, and the upstream head above datum.
- If tailwater affects the flow, enter downstream head and enable correction.
- Enter operating time to estimate delivered volume, then calculate.
For commissioning, collect paired head and flow measurements to calibrate Cd for the site.
Canal turnout guidance
Discharge pathways at canal turnouts
Turnouts deliver water through gates, pipes, or measuring structures. Orifice outlets respond quickly to head changes because flow depends on area and the square root of head. Weirs transform head over a crest into a repeatable estimate, supporting fair distribution between users. V‑notch weirs are sensitive at low flows, while broad‑crested crests suit sturdier concrete works.
Selecting realistic discharge coefficients
The discharge coefficient, Cd, represents contraction and energy losses at the entrance and along the outlet. Smooth approaches and clean edges often justify Cd around 0.60–0.65 for orifices and about 0.60–0.62 for sharp‑crested weirs. Sediment, algae, rough formwork, or damaged crests lower effective Cd, so field checks should update assumptions. For multi‑outlet turnouts, keep Cd consistent across identical units and verify symmetry with simple bucket timing tests.
Head measurement and datum control
Accurate head is the backbone of turnout estimation. Measure Hup from the weir crest elevation or from the orifice centerline, and keep the datum consistent across visits. Use a staff gauge, hook gauge, or pressure sensor in calmer water away from local drawdown. Record gate opening, debris, and approach condition, because these influence Cd and repeatability. In SI, use meters; in imperial, heads are feet and orifice sizes inches typically.
Submergence and tailwater effects
Tailwater can partially drown a turnout and reduce the driving head. For orifices, differential head Hup minus Hdown is a practical approach that matches observed behavior when the outlet is submerged. Weirs lose free overflow under submergence, so corrected discharge or local calibration becomes important. When submergence is frequent, consider improving downstream conveyance or raising control levels.
Reporting checks and construction decisions
Construction teams often need both discharge and delivered volume. Multiply total flow by operating time to estimate volume and compare it against allocation targets. High outlet velocity can trigger erosion, so results help justify aprons, riprap, or energy dissipators. Exporting CSV supports logs and audits, while PDFs provide a dated record for supervisors, clients, and commissioning files.
FAQs
1) Which turnout method should I select?
Use the method that matches your installed control: orifice for gated outlets, rectangular or broad‑crested weir for crest structures, and V‑notch for low‑flow measurement. If unsure, select the structure you can measure most reliably.
2) What coefficient value should I start with?
Start with Cd 0.60–0.65 for orifice outlets and about 0.60–0.62 for sharp‑crested weirs under clean conditions. Treat it as a baseline, then calibrate with field flow checks to reflect roughness, debris, and entrance losses.
3) How should I measure upstream head?
Measure head from the correct datum: weir crest elevation or orifice centerline. Take readings in calmer water upstream of the control, away from drawdown and turbulence. Use a fixed gauge or sensor so repeated measurements reference the same level.
4) When should I enable submergence correction?
Enable it when downstream water level influences flow—such as a backed‑up drain or submerged outlet. For orifices, differential head is appropriate. For weirs, correction is an approximation, so confirm with field measurements when submergence is high or persistent.
5) Can the calculator estimate delivered volume?
Yes. Enter operating time to convert discharge to delivered volume, then compare against the required volume field to estimate the time needed. This supports shift planning, rotational supply schedules, and checking whether a turnout can meet an allocation window.
6) Why did my computed discharge change after maintenance?
Cleaning debris, sharpening a crest edge, smoothing approach flow, or adjusting gate alignment can change effective Cd and the measured head. Recheck the datum and take new calibration points. Updated coefficients often explain improved consistency and reduced losses.