What This Solves
Calculates the discharge velocity at pipe and culvert outlets to assess erosion potential, scour depth, and the need for outlet protection.
Best Used When
- You need to determine if outlet velocity exceeds permissible limits for the downstream channel
- You want to estimate scour hole dimensions at a pipe outlet
- You are evaluating whether outlet protection or energy dissipation is needed
Do NOT Use When
- You need to design a riprap apron or outlet protection pad — Use Outlet Protection Calculator
- You need to design a stilling basin or energy dissipation structure — Use Energy Dissipator Calculator
Key Assumptions
- Outlet velocity is calculated from pipe flow area and discharge using continuity
- Scour depth estimates use FHWA HEC-14 empirical relationships
- Tailwater conditions affect the effective outlet velocity
- The pipe outlet is unsubmerged or the degree of submergence is known
- No significant entrance or exit losses beyond standard coefficients
Input Quality Notes
Accurate pipe flow depth at the outlet is needed. For partial flow, use normal depth calculations. Tailwater depth affects both velocity and scour — use field data or downstream channel analysis.
Bed Material Critical Velocities
| Material | Vc (ft/s) |
|---|---|
| Fine sand (0.06-0.25mm) | 0.5-1.0 |
| Medium sand (0.25-0.5mm) | 1.0-1.5 |
| Coarse sand (0.5-2mm) | 1.5-2.5 |
| Fine gravel (2-6mm) | 2.5-4.0 |
| Coarse gravel (6-60mm) | 4.0-7.0 |
| Cobbles (60-250mm) | 7.0-12.0 |
| Clay/silt (cohesive) | 2.0-5.0 |
| Cohesive soil (mixed) | 3.0-6.0 |
Scour Potential Classification
Low (V/Vc < 0.8) No protection needed
Moderate (0.8-1.2) Minimal protection
High (1.2-2.0) Moderate protection
Severe (V/Vc > 2.0) Extensive protection
Ready to Calculate
Enter outlet geometry and flow parameters to calculate velocity and scour potential.
For educational purposes only. Not a substitute for professional engineering judgment.
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Last verified: February 2026