Rip rap calculator for Arkansas

A ton of riprap covers around 17 square feet. It may sound low, but armor stone is placed in layers 12 inches thick or more. That means each ton covers only a small share of the area covered by landscape rock. This calculator uses the selected layer thickness, 3,100 lb per cubic yard, and riprap generally ranging from 4 to 12 inches or larger.

How to measure an armored bank

1. Find the bank area along its angled surface, rather than using the horizontal footprint. Measure from top to toe, then multiply by the shoreline or channel run.
2. For erosion protection, make the rock course 1.5 to 2 times the size of its biggest pieces. A 12 inches course is the minimum used for working control.
3. Calculate cubic yards as cubic feet / 27, then estimate tons by multiplying by 1.55.

Tons of coverage

Stone placement guidelines

• Place non-woven geotextile beneath the rock, or start with 3-4 inches of #4 stone. Without fabric or bedding, riprap set on bare soil may settle as erosion removes support beneath the bank.
• Set the toe below the waterline or the expected scour depth. The lowest row needs to be buried because bank failure can begin at the bottom.
• Before placing stone, shape the bank at 2:1 or flatter. A face that is steeper needs engineering, not extra rock.
• Fit the exposed finish layer by hand or machine, nesting each stone into the next instead of leaving it at its landing spot.
• Extend the outlet apron 4-6 pipe diameters below culverts. The downstream area is where flowing water scours.

Example calculation: 50 feet of pond bank

For 50 feet of shore with a 6-foot slope face, the coverage area is 300 square feet. An 18-inch course needs 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or roughly 26 tons of riprap and 27 after a small allowance. Plan on geotextile, or use about 4 tons of #4 stone as rock bedding. Before you order, walk the truck access. The final 20 feet at a pond bank can bog down a loaded truck, and relocating stone twice costs twice.

Following the first major storm

After the water recedes, make a pass along the creek bank. Check the toe for displaced rock, exposed soil in the layer, visible fabric, and scouring around the outfall apron. Put shifted stone back in place and use reserve rock to fill sparse areas. A brief check twice yearly keeps the protection in working order.

A quick guide to size classes

Choose the stone size based on water force. Riprap in the four to eight inch range is used for pond banks, slow streams, and downspout outlets. Eight to twelve works for lake edges exposed to boat wake and faster channels. Highway and spillway projects use their own engineered specifications and layer depths, so follow the project drawing. When deciding between sizes, choose the larger stone and increase the layer thickness; added weight provides more resistance.

Riprap estimates, sizing, and installation

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