Hydraulics & drainage · Open channels & culverts
Riprap sizing (Isbash) calculator
Stone diameter for a design velocity: d = V²/(2g C²(Gs − 1)), stone mass and layer thickness.
Standard: Isbash equation
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Inputs in the worked example
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| Input | Value | Unit |
|---|---|---|
| Design velocity | 3 | m/s |
| Isbash C | 0.86 high turbulence | |
| Rock specific gravity | 2.65 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| d50 = V²/(2g C²(Gs − 1)) | 0.376 | m |
| Stone mass (sphere) | 73.69 | kg |
| Layer thickness 2 d50 | 0.752 | m |
- ✓ d50 ≤ 1 m (practical)
Method, formulas and assumptions
- d50 = V²/(2g C²(Gs − 1)) = 3^2/(2*9.81*0.86^2*(2.65-1)) = 0.37589 m
- Stone mass (sphere) = 2.65*1000*PI()*0.37589^3/6 = 73.694 kg
- Layer thickness 2 d50 = 2*0.37589 = 0.75178 m
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.