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, 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.
Formula and source
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).
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