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
Checked against published worked examples from: Catchments & Creeks Pty Ltd.
Details, formula, and sources
ScopeD50 = SF x V^2 / (2 g C^2 (Ss - 1)). The published Isbash form has no safety factor (SF = 1); the default here is SF 1.2. An 8 ft/s flow in turbulent water (C 0.86) wants a 12 in stone at SF 1.2 (9.8 in by plain Isbash); 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.
Formula or tabled50_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.
Source and editionIsbash riprap-sizing equation by name (public-domain field estimate); first-principles unit arithmetic (g = 32.2 ft/s^2).
Free accessThe Isbash equation is public-domain; every input is user-entered (no gradation table or design chart reproduced).
What governsEstimate. AHJ and licensed professional govern.
API field namesvelocity_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
- Safety factor default 1.2 applied to D50; the published Isbash equation (NRCS TS14C) carries none, so enter 1 to reproduce itdesign practice