Rip rap calculator for Kansas

A ton of riprap covers roughly 17 square feet. That figure reflects the deep layers required for armor stone, commonly 12 inches or more, rather than the shallow spread used for landscape rock. This calculator uses layer depth and a density of 3,100 lb per cubic yard for material generally sized 4 to 12 inches and larger.

Measuring an armored bank

1. Follow the tape along the bank face instead of measuring level ground, then multiply that distance by the channel or shoreline length.
2. Set the rock layer at 1.5 to 2 times the dimension of the largest stone. For erosion control, use no less than 12 inches.
3. To find cubic yards, divide cubic feet by 27; then multiply by 1.55 for tons.

Tons and coverage

Placement details for stable stone

• Install a base before the rock: use non-woven geotextile or 3-4 inches of #4 stone. Riprap placed directly on soil can appear stable for a season, then settle when the underlying bank erodes away.
• Set the toe below the waterline or expected scour depth by burying the lowest stones. Failures commonly begin at the bottom of a riprap bank.
• Before placing stone, shape the slope at 2:1 or flatter. A steeper bank needs engineering review rather than added rock.
• Use hand placement or equipment on the exposed top course, fitting stones together so they interlock rather than stopping where they roll.
• For a culvert outlet, carry the apron 4-6 pipe diameters beyond the discharge point downstream, where scour develops.

Example: 50 feet of pond bank

A shoreline with a 6-foot bank face and 50 feet of length measures 300 square feet. For an 18-inch layer: 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or about 26 tons of riprap and 27 with the small allowance. Include a roll of geotextile, or use about 4 tons of #4 stone as rock bedding. Review the truck access before ordering. The final 20 feet to a pond bank can catch a loaded truck, and relocating stone twice costs twice.

Following the first heavy storm

After water recedes, make a pass along the bank. Check the toe for displaced rock, exposed soil in the cover, visible fabric, and erosion at the outfall apron. Put shifted stone back in place and use reserve material to fill shallow areas. A short check a few times each year helps keep the riprap layer in order.

Stone size classes

Select riprap size for the water's force. Four to eight inch stone works on pond banks, downspout outfalls and slow streams. Use eight to twelve for lake shorelines subject to boat wake and quicker channels. Highway and spillway work has project-specific stone classes and depths, so follow the drawing rather than this page. If two sizes seem possible, use the larger stone with a thicker layer; added weight offers greater resistance.

Riprap quantity, sizing and placement

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