Input Form
Example Data Table
| Case | Q (m³/s) | B (m) | z (H:V) | y₁ (m) | yᵗʷ (m) | ΔZ (m) | Key output (Lᵦ, m) |
|---|---|---|---|---|---|---|---|
| Typical small canal | 2.50 | 3.00 | 1.50 | 0.80 | 1.20 | 0.60 | ≈ 6.5 |
| Higher velocity reach | 4.00 | 3.00 | 1.00 | 0.70 | 1.10 | 0.80 | ≈ 8.0 |
| Wide canal, mild jump | 3.50 | 5.00 | 2.00 | 0.90 | 1.30 | 0.50 | ≈ 5.5 |
Formula Used
- Area: A = y (B + z y)
- Wetted perimeter: P = B + 2 y √(1+z²)
- Hydraulic radius: R = A / P
- Velocity: V = Q / A
- Froude: Fr₁ = V₁ / √(g y₁)
- Sequent depth: y₂ = 0.5 y₁ ( √(1+8Fr₁²) − 1 )
- Energy loss: ΔE = (y₂−y₁)³ / (4 y₁ y₂)
- Basin length: Lᵦ = max(6(y₂−y₁), 5y₂) × SF
- Broad-crested head: Q = C b H^(3/2) ⇒ H = (Q/(Cb))^(2/3)
- Total head above crest: Hₜ = H + V²/(2g)
- Manning check: Q = (1/n) A R^(2/3) S₀^(1/2)
How to Use This Calculator
- Enter canal geometry (B, z) and upstream depth y₁.
- Provide discharge Q and tailwater depth yᵗʷ from your profile.
- Set advanced values (n, S₀, crest b, C) if known.
- Press Calculate to display results above the form.
- Review the tailwater message to judge jump stability.
- Download CSV/PDF for records, then iterate with scenarios.
Flow Conditions at the Drop
Approach depth and velocity govern how aggressive the drop becomes. The calculator converts discharge and section geometry into area, hydraulic radius, and velocity. It then evaluates the approach Froude number to judge whether a hydraulic jump can form. Supercritical approach flow typically needs energy dissipation and downstream safety for reliability. Subcritical flow may require a different control concept, such as a check structure or gated drop.
Stilling Basin Length Selection
Stilling basin sizing is treated as preliminary and transparent. The sequent depth relationship estimates the post‑jump depth using the approach Froude number. A rule based on multiples of sequent depth and depth difference provides an initial basin length, then a user safety factor scales it. This helps compare alternatives quickly while you confirm lining type, blocks, joints, construction tolerances, plus reinforcement detailing, and local standards.
Tailwater Compatibility and Jump Control
Tailwater depth is compared with the predicted sequent depth to indicate stable, swept, or submerged jump behavior. When tailwater is low, the jump may move downstream and erode the apron. When tailwater is high, the jump can drown, raising uplift pressures and vibration, plus toe protection needs. Use the message as a screening check and adjust basin floor elevation, downstream control, or appurtenances to stabilize dissipation.
Crest Head and Freeboard Planning
A broad‑crested control estimate links discharge to head over crest through a coefficient and crest length. The calculator also adds the velocity head term to show total head above the crest. Use this to check upstream afflux, gate settings, and available freeboard. Freeboard input is carried into recommended basin depth, supporting wall height selection, safer inspection access, maintenance clearance, and flood routing for extreme events.
Capacity Check and Iteration Workflow
The Manning check uses the entered upstream depth to estimate conveyance for the approach reach. Reporting the percentage of demanded flow against this capacity highlights when geometry or roughness assumptions are unrealistic. Iterate by adjusting depth, width, slope, and roughness to match expected operating conditions. Export CSV or PDF for design notes, stakeholder review, in early planning, then also validate with detailed hydraulic and structural review.
FAQs
1) What does this calculator size for a canal drop?
It estimates approach hydraulics, sequent depth for a preliminary hydraulic jump, and a recommended stilling basin length and depth. It also provides a broad‑crested head check and a Manning capacity comparison for the entered approach depth.
2) Which inputs most affect the basin length result?
Discharge, upstream depth, and channel width dominate because they control velocity and Froude number. Tailwater depth influences the stability message, while the safety factor directly scales the suggested basin length.
3) How should I choose the weir coefficient and crest length?
Use values consistent with your crest shape, upstream contraction, and field calibration where available. Start with typical broad‑crested assumptions for screening, then replace with project-specific coefficients from standards or model tests.
4) Why does the tool say the jump may sweep out or be submerged?
The message compares tailwater depth to the predicted sequent depth. Low tailwater can push the jump downstream, increasing scour risk. High tailwater can drown the jump, increasing uplift and requiring added dissipation features.
5) Is the trapezoidal section handled exactly?
Section properties are computed for a trapezoid, but the jump relationship uses a rectangular approximation based on the entered upstream depth. This is suitable for early sizing; confirm final geometry with detailed hydraulic analysis.
6) Can I use the CSV and PDF exports for submissions?
Exports are useful for traceable notes, option comparisons, and internal reviews. For approvals, attach the governing design standard checks, drawings, and any model or CFD results required by your client or authority.