Riprap Sizing Methods — Reference
Sizing stone for channel, bank, and outlet protection. The design quantity is the median stone size D50, set by velocity or shear; gradation, layer thickness, and an underlying filter complete the design.
Velocity-Based D50 (Isbash)
| Term | Meaning / typical value |
|---|---|
| V | Design (local) velocity at the stone |
| Ss | Stone specific gravity, ~2.65 |
| C | Isbash coefficient: ~0.86 high-turbulence, ~1.20 low-turbulence |
| g | 32.2 ft/s² (9.81 m/s²) |
FHWA HEC-11 (revetment) and HEC-23 (bridge/abutment countermeasures) give velocity- and depth-based D50 relations with bank-angle and specific-gravity corrections; use the method your reviewer requires.
Gradation Limits (Well-Graded Riprap)
| Ratio | Target |
|---|---|
| D85 / D15 (uniformity) | 1.5–2.5 (up to ~4.6 allowed) |
| D100 (max) / D50 | ≤ 2.0 |
| D50 / D15 | ~1.5–2.0 |
Well-graded stone interlocks and resists displacement; uniform (gap-graded) stone is unstable and prone to winnowing.
Layer Thickness & Filter
| Item | Rule |
|---|---|
| Blanket thickness | ≥ larger of 1.5·D50 or D100; increase for steep / submerged placement |
| Granular filter | Terzaghi: D15f/D85b < 4–5 < D15f/D15b |
| Geotextile filter | Alternative to granular; AOS sized to retain base soil |
Sources: FHWA HEC-11 Design of Riprap Revetment; FHWA HEC-23 Bridge Scour and Stream Instability Countermeasures; USACE EM 1110-2-1601; Isbash (1936). State DOT riprap classes (e.g., NCDOT Class A/B/1/2) define standard gradations — check the governing spec.
Related cheat sheets and tools
Get the design velocity from Manning's and the channel geometry card, then size stone in the riprap tool. Downstream of an energy dissipator, pair with the hydraulic-jump card. For erosion and sediment within a full watershed model, see HydroComplete.