What This Solves
Calculates the required riprap stone size (D50) for channel bank and bed protection using Isbash, USACE, HEC-11, or Maynord methods.
Best Used When
- You are designing channel bank protection with riprap for a stream or drainage channel
- You need to determine the median stone size (D50) and gradation for erosion protection
- You want to compare riprap sizing results from multiple standard methods
Do NOT Use When
- You are designing outlet protection at a pipe or culvert outlet specifically — Use Outlet Protection Calculator
- You need to evaluate grass or other flexible lining adequacy instead of riprap — Use Channel Lining Calculator
Key Assumptions
- Stone sizing equations assume angular, durable rock with standard specific gravity (2.65)
- Flow velocity or shear stress is the primary design parameter
- The riprap blanket thickness is typically 1.5 * D50 or the D100 size, whichever is greater
- Filter fabric or granular bedding is provided beneath the riprap
- The channel geometry is stable and not actively migrating
Input Quality Notes
Design velocity or shear stress should be computed from a hydraulic analysis (not estimated). Use the velocity at the toe of slope or bank, which may differ from the average channel velocity.
Standard Riprap Classes
| Class | D50 (in) | Application |
|---|---|---|
| Class I | 6-9 | Light duty, low velocity |
| Class II | 9-15 | Medium duty |
| Class III | 15-21 | Standard protection |
| Class IV | 21-27 | Heavy duty |
| Class V | 27-36 | Very heavy duty |
| Class VI | 36-48 | Severe conditions |
Isbash Coefficients
| Condition | C |
|---|---|
| Loose, minimum stability | 0.86 |
| Loose, average | 1 |
| Loose, stable | 1.2 |
| Placed, stable | 1.4 |
| Keyed (embedded) | 1.6 |
Ready to Calculate
Select a sizing method and enter flow parameters to determine riprap requirements.
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Last verified: February 2026