Design stilling basins using hydraulic jump relationships. Check velocity, sequent depth, and stability with inputs. Get an aligned basin length estimate for construction teams.
| Case | Q | b | y₁ | g | C | Lₛ | SF |
|---|---|---|---|---|---|---|---|
| A | 12.5 m³/s | 3.0 m | 0.45 m | 9.81 | 6.0 | 0.50 m | 1.10 |
| B | 25.0 m³/s | 4.5 m | 0.60 m | 9.81 | 6.5 | 0.75 m | 1.15 |
| C | 420 ft³/s | 12.0 ft | 1.6 ft | 32.2 | 6.0 | 2.0 ft | 1.10 |
Values are illustrative; match your site geometry and hydraulics.
This is a general estimator. Confirm with your governing standard and basin type.
Stilling basins reduce erosive velocity downstream of spillways, chutes, and outlets by forcing a hydraulic jump. The calculator estimates approach velocity, Froude number, and sequent depth. When Fr₁ exceeds one, the jump can convert kinetic energy into turbulence, raising the water surface and lowering unit discharge intensity at the outlet. Under common operating ranges.
Discharge Q, width b, and approach depth y₁ define the approach area A₁ and velocity v₁. Small changes in y₁ strongly affect Fr₁ because depth appears inside the square‑root term. Use section dimensions at the jump location, not at a distant upstream control. Keep units consistent and match gravity to the selected system to avoid hidden scaling errors.
The sequent depth y₂ is the downstream depth required to satisfy momentum across the jump. If tailwater is much lower than y₂, the jump may sweep out of the basin. If tailwater is much higher, the jump can drown, reducing mixing and increasing uplift risk. The optional tailwater input provides quick notes, but final confirmation should use the governing design standard and profiles.
The jump length is approximated as Lⱼ ≈ C·(y₂−y₁). Coefficient C commonly ranges from 5 to 7 for preliminary sizing, while special basin types may require different correlations. Add Lₛ for end sills, transitions, or a slab extension, then apply a safety factor to cover uncertainty in roughness, aeration, and operating ranges.
Field success depends on geometry, concrete quality, and drainage details. Verify invert elevations, joint sealing, and embedded items so the basin depth matches the computed section. Check that downstream protection, such as riprap or a cutoff wall, aligns with expected residual velocity. During commissioning, observe jump location across flows and adjust appurtenances if needed. Document assumptions, store calculations, and coordinate with structural detailing so basin walls, anchors, and drains remain fully compatible throughout.
It is an estimated slab length that contains the hydraulic jump and provides room for rollers, turbulence, and appurtenances such as an end sill.
Use it for preliminary sizing of rectangular channels where a hydraulic jump is expected. Final design must follow the selected basin type and standard details.
Approach depth affects velocity and Froude number. Because Fr₁ depends on √(g·y₁), small depth errors can shift the computed sequent depth and length.
For quick estimates, many projects use C between 5 and 7. If your standard or basin type specifies a different relationship, use that value instead.
If tailwater is below the sequent depth, the jump can sweep downstream. If it is well above, the jump can drown, affecting energy dissipation and pressures.
Confirm widths, inverts, joint layout, drainage, and downstream protection. Align the slab, walls, and end treatment with the expected jump location across flows.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.