What This Solves
Designs stilling basins and impact basins to dissipate kinetic energy at culvert or pipe outlets, preventing downstream erosion.
Best Used When
- Outlet velocities exceed permissible limits for the downstream channel material
- You need to design a USBR-type stilling basin or FHWA impact basin
- You want to calculate the required basin dimensions and tailwater depth for a hydraulic jump
Do NOT Use When
- A simple riprap apron is sufficient for energy dissipation — Use Outlet Protection Calculator
- You only need to calculate the outlet velocity, not design a dissipation structure — Use Outlet Velocity Calculator
Key Assumptions
- Hydraulic jump forms within the basin at the calculated sequent depth
- Tailwater depth is sufficient to confine the jump within the basin
- Basin dimensions follow USBR or FHWA HEC-14 standard proportions
- Approach flow is fully developed and approximately uniform
- No significant debris or sediment loading that would reduce basin effectiveness
Input Quality Notes
Tailwater depth is critical for proper jump formation. Use downstream channel analysis or field data to estimate tailwater. If tailwater is too low, the jump will sweep out of the basin.
Design an energy dissipator for a culvert outlet, spillway, or drop structure. Enter the discharge and inlet flow conditions to size a USBR stilling basin or impact basin, locate the hydraulic jump, and check tailwater adequacy using FHWA HEC-14 and USBR methodology.
USBR Stilling Basin Selection
| Type | Froude | L/y2 |
|---|---|---|
| Type I | < 2.5 | 4 |
| Type II | > 4.5 (high V) | 4.3 |
| Type III | 4.5 - 17 | 2.8 |
| Type IV | 2.5 - 4.5 | 6 |
Type VI Impact Basin Limits
Impact basins are effective for small discharges and high velocities where space is limited.
Ready to Design
Select a dissipator type and enter flow parameters to design the basin.
For educational purposes only. Not a substitute for professional engineering judgment.
How energy dissipator design works
Energy dissipators force a hydraulic jump — the abrupt transition from fast, shallow (supercritical) flow to slow, deep (subcritical) flow — inside a controlled basin so that the destructive energy is spent there rather than scouring the downstream channel. The design follows four core relationships from FHWA HEC-14 and USBR Engineering Monograph No. 25.
1. Inlet Froude number classifies the incoming flow and selects the basin type:
Fr₁ = V₁ / √(g · y₁)
2. Sequent (conjugate) depth y₂ — the depth after the jump — comes from the momentum (Bélanger) equation:
y₂ / y₁ = ½ · ( √(1 + 8·Fr₁²) − 1 )
3. Tailwater check confirms enough downstream depth exists to hold the jump in the basin:
TW ≥ 0.85 · y₂
4. Energy dissipation efficiency compares the specific energy entering and leaving the basin (E = y + V²/2g, plus any drop height at inlet):
η = (E₁ − E₂) / E₁ × 100%
- V₁ — inlet (approach) velocity, ft/s or m/s
- y₁ — inlet flow depth (or pipe diameter), ft or m
- g — gravitational acceleration, 32.2 ft/s² (9.81 m/s²)
- Fr₁ — inlet Froude number (dimensionless)
- y₂ — sequent / conjugate depth after the hydraulic jump
- TW — tailwater depth in the receiving channel
- E₁, E₂ — specific energy at inlet and exit; η is the percent dissipated
The basin length is then sized as a multiple of y₂ that depends on the basin type (L = coefficient × y₂), and appurtenances (chute blocks, baffle blocks, end sills) are proportioned from y₁ and y₂ as shown in the reference table below.
USBR basin length & appurtenance proportions
Basin length is expressed as a multiple of the sequent depth (L / y₂), and appurtenance heights are proportioned from the inlet depth y₁ or sequent depth y₂. Values follow FHWA HEC-14 / USBR Engineering Monograph No. 25 and match the figures used by this calculator.
| Basin type | Froude range | Length L / y₂ | Chute blocks | Baffle blocks | End / dentated sill |
|---|---|---|---|---|---|
| USBR Type I | Fr < 2.5 | 4.0 | — | — | ~0.1 · y₂ |
| USBR Type IV | 2.5 – 4.5 | 6.0 | — | — | ~0.1 · y₂ |
| USBR Type III | 4.5 – 17 | 2.8 | 1.0 · y₁ | 0.8 · y₁ | 0.8 · y₁ |
| USBR Type II | > 4.5 (high V) | 4.3 | 1.0 · y₁ | — | 0.2 · y₂ (dentated) |
y₁ = inlet depth, y₂ = sequent depth. Type II is used for high-velocity flow (above 60 ft/s) and uses a dentated end sill; Type IV applies to the oscillating-jump range and should be used with caution. Confirm final dimensions against the current HEC-14 figures for your conditions.
Type VI impact basin sizing
For smaller culvert outfalls where tailwater is unreliable, the USBR Type VI impact basin dissipates energy with a hanging baffle instead of a hydraulic jump. Its width is set from the design discharge, and the remaining dimensions scale off that width:
W = 0.23·Q0.5 + 1.65 L = 1.33·W d = 0.5·W
Applicability limits (HEC-14, Chapter 9): discharge up to about 400 cfs and inlet velocity up to roughly 50 ft/s. Beyond these, the calculator flags a warning and a stilling basin (or multiple basins) is preferred.
Frequently asked questions
What is an energy dissipator and when do I need one?
An energy dissipator is a hydraulic structure—such as a USBR stilling basin, impact basin, or riprap basin—placed at a culvert outlet, spillway toe, or drop structure to convert high-velocity, high-energy flow into slower, calmer flow. You typically need one where the outlet velocity exceeds what the downstream channel can withstand without scour. Per FHWA HEC-14, a forced hydraulic jump inside a designed basin is one of the most effective ways to dissipate that excess energy.
How is the sequent (conjugate) depth y2 calculated?
The depth after the hydraulic jump (sequent depth y2) is found from the momentum equation for a rectangular channel: y2 = (y1 / 2) × (√(1 + 8·Fr₁²) − 1), where y1 is the incoming (supercritical) depth and Fr₁ is the inlet Froude number Fr₁ = V₁ / √(g·y₁). The basin length is then sized as a multiple of y2 that depends on the basin type.
Which USBR stilling basin type should I use?
Basin selection is driven mainly by the inlet Froude number. Type I suits Fr below 2.5; Type IV covers the 2.5–4.5 oscillating-jump range; Type III is used for 4.5–17 at velocities below 60 ft/s; and Type II is used for Fr above 4.5 at high velocity (above 60 ft/s). The calculator computes Fr₁ and recommends a type, but you can override it to compare designs.
Why does tailwater depth matter so much?
The hydraulic jump only forms and stays inside the basin if there is enough downstream depth to hold it. A common HEC-14 check is that tailwater should be at least about 85% of the sequent depth (TW ≥ 0.85·y2). If tailwater is short, the jump sweeps downstream and the basin fails to dissipate energy, so the design depresses the basin floor or adds an end sill to create the needed depth.
When is an impact basin (USBR Type VI) a better choice than a stilling basin?
A Type VI impact basin uses a hanging baffle to kill energy and does not rely on tailwater, so it is well suited to culvert outfalls where space is limited and downstream depth is unreliable. It is intended for smaller flows—typically up to about 400 cfs and inlet velocities up to roughly 50 ft/s. Above those limits a stilling basin or multiple basins is generally more appropriate.
Standards & related tools
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Last verified: February 2026