Riprap

Riprap is a layer of loose, graded quarry stone placed to armor a slope, bank, or structure toe against wave and current attack. On a coastal slope the median stone mass comes from the Hudson formula or the Van der Meer formula, sized by the stability number Ns = Hs / (delta x Dn50), where delta is the buoyant relative density of the rock, about 1.6 for granite in seawater. It is laid over a filter or geotextile so the underlayer is not washed out, with a defined gradation so small stone wedges between large. The Hudson KD coefficient is roughly 2 for rough angular rock in two layers on a breakwater trunk.