Rip Rap Calculator

Rip rap is a blanket of loose angular stone sized so that individual pieces resist being dislodged by flowing water or wave attack. Stability is a balance between the drag and lift forces of the current and the submerged weight of each stone, which is why design charts key stone size to design velocity and bank slope rather than to appearance. The quantity side is straightforward volume work: plan area times blanket thickness gives cubic feet, dividing by 27 gives cubic yards, and multiplying by a placed density of about 1.30 to 1.50 tons per cubic yard gives tonnage. That placed density sits well below the 2.6 to 2.7 specific gravity of the parent rock because a dumped stone blanket carries roughly 35 to 45 percent voids. Gradation classes run from about 2 to 15 lb stone at 4 to 8 in (Class I) up to 250 to 2000 lb stone at 24 to 36 in (Class IV), with class names varying by state highway agency. Blanket thickness is normally specified as at least 1.5 times the maximum stone diameter and never less than 12 in, so Class II is commonly placed 18 to 24 in thick. A geotextile or graded granular filter beneath the stone is essential: without it, fines migrate upward, the blanket settles into the bank, and the section fails by piping rather than by stone movement. A plan-area estimate also misses toe trenches and keyed-in terminations, and quarry run rarely matches the specified gradation exactly, so ordering 10 to 15 percent over the computed tonnage is normal.

Armoring banks with stone long predates engineering literature; the term rip rap appears in English from the seventeenth century, and levee, canal and harbor builders in Europe and along the Mississippi used hand-placed stone revetments throughout the 1800s. Systematic sizing rather than rule of thumb came in the twentieth century. The Soviet engineer S. V. Isbash published work in the 1930s on the stability of stone dropped into flowing water, producing the Isbash relation between critical velocity and stone size, and the incipient-motion framework published by Albert Shields in 1936 gave the sediment-transport basis that later rip rap criteria drew on. In the United States the Army Corps of Engineers consolidated rip rap design in its engineer manuals for channel stabilization and bank protection, and the Federal Highway Administration issued HEC-11, Design of Riprap Revetment, whose 1989 revision remains a standard reference for highway embankments and bridge scour protection. State highway agencies developed the lettered and numbered gradation classes used in bidding, and ASTM published D6825 as a standard guide for placing rip rap revetment. Practice shifted over the same period from purely velocity-based sizing toward methods using boundary shear stress and accounting for bank angle, filter performance and toe scour, which is why modern specifications pair a stone class with both a thickness and a filter requirement instead of quoting a stone size alone.