Riprap Median Stone Size (Isbash)
Median stone size D50 to armor a channel or outlet against a scouring velocity by the public-domain Isbash equation.
Example
You enter
- Flow velocity against the stone (ft/s) 8
- Stone specific gravity Ss 2.65
- Isbash turbulence coefficient C 0.86
- Safety factor SF 1.2
You get
- D50 (ft) 0.97722
- D50 (in) 11.727
Details, formula, and sources
D50 = SF x V^2 / (2 g C^2 (Ss - 1)). An 8 ft/s flow in turbulent water (C 0.86) wants a 12 in stone; the same flow in a calm reach (C 1.20) needs only 6 in - turbulence, not just velocity, sets the rock. The hydraulic engineer governs.
d50_ft = safety_factor x velocity_fps^2 / (2 x 32.2 x turbulence_coeff^2 x (specific_gravity - 1)); d50_in = d50_ft x 12.
Isbash riprap-sizing equation by name (public-domain field estimate); first-principles unit arithmetic (g = 32.2 ft/s^2).
The Isbash equation is public-domain; every input is user-entered (no gradation table or design chart reproduced).
Estimate. AHJ and licensed professional govern.
Field names used by the API: velocity_fps, specific_gravity, turbulence_coeff, safety_factor, d50_ft, d50_in
- Turbulence coefficient C ~ 0.86 high-turbulence (below outlets, bends), ~1.20 low-turbulence (straight channels)Isbash / USACE riprap practice
- Stone specific gravity 2.65 default (typical quarried stone); the stone source governsmaterials practice