Rip rap calculator for Minnesota

A ton of riprap covers roughly 17 square feet. That figure is correct. Because armor stone is installed in layers 12 inches thick or more, each ton covers only a small share of the area covered by landscape rock. This calculator bases the estimate on layer thickness and 3,100 lb per cubic yard for riprap generally ranging from 4 to 12 inches and larger.

How to measure an armored bank

1. Take the slope measurement along the face of the bank, rather than across level ground, then multiply it by the shoreline or channel length.
2. Use a layer depth of 1.5 to 2 times the largest stone dimension. For erosion control, 12 inches is the minimum working depth.
3. To find cubic yards, divide cubic feet by 27; then multiply by 1.55 for tons.

Tons and coverage

Stone placement basics

• Place fabric or bedding beneath the rock: use non-woven geotextile, or 3-4 inches of #4 stone. Riprap laid directly on soil can appear stable for a season before erosion below the bank causes it to settle.
• Build the toe into the bank. Set the lowest stones below the waterline or anticipated scour depth, since failure works upward from the bottom.
• Before placing stone, shape the slope to 2:1 or flatter. A steeper bank calls for engineering rather than more rock.
• Use a machine or hand placement for the exposed top course, fitting stones together so they interlock instead of stopping where they land.
• Extend the apron 4-6 pipe diameters downstream from culvert outlets. That is the area where flowing water cuts into the ground.

Example: 50 feet of pond bank

Start with the shoreline area: 50 feet of bank at a 6-foot slope face equals 300 square feet. For an 18-inch-thick layer, 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or roughly 26 tons of riprap. Plan for 27 to cover the small allowance. Use a roll of geotextile beneath it, or choose about 4 tons of #4 stone for a rock bed. Before you order, check the truck path. The final 20 feet near a pond can be soft, and relocating placed stone twice means paying for placement twice.

Following the first major storm

After the water level recedes, walk the bank and inspect the toe for moved rock, exposed soil between the stones, or visible fabric. Put shifted stones back in place, use reserve stone to fill sparse areas, and inspect the outfall apron for erosion. A quick inspection a couple of times each year, especially after spring melt, keeps the riprap in order with little upkeep.

Stone sizes at a glance

Select riprap for the water conditions. Four to eight inch stone works at pond banks, downspout outfalls and slow streams. Use eight to twelve for lake shorelines facing boat wake and for faster channels. Highway and spillway work has its own engineered stone classes and layer depths, so follow the project drawing instead of this page. If two sizes seem possible, use the larger stone with a thicker layer; added weight allows more margin.

Riprap amounts, sizes and installation

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