Rip rap calculator for New Mexico

A ton of riprap covers around 17 square feet. That figure is accurate. Armor stone is placed in layers of 12 inches or more, so it covers much less area per ton than landscape rock. Use this calculator to account for layer depth, with 3,100 lb per cubic yard for material commonly ranging from 4 to 12 inches and larger.

How to measure an armored bank

1. Run your tape along the sloped face instead of measuring the level ground, then multiply by the shoreline or channel length.
2. Use a layer 1.5 to 2 times the largest stone dimension. For effective erosion control, 12 inches is the practical minimum.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Tons of coverage

Installation details for stable stone

• Put fabric or bedding beneath the rock first. Use non-woven geotextile, or install 3-4 inches of #4 stone. Riprap set directly on soil can slump after the bank erodes below it.
• Build in the toe before placing the rest. Set the lowest row beneath the waterline or expected scour depth because failure can start at the bottom.
• Shape the bank to 2:1 or flatter before installing stone. A steeper slope calls for engineering rather than extra rock.
• Use equipment or hand placement for the exposed top layer, arranging stones so they interlock rather than stopping where they land.
• At a culvert outlet, carry the apron 4-6 pipe diameters downstream. Water tends to scour in that area.

Example: 50 feet of pond bank

For a 50-foot shoreline section with a 6-foot slope face, plan on 300 square feet. An 18-inch layer calculates as 300 x 1.5 = 450 cubic feet = 16.7 cubic yards. You will need about 26 tons of riprap, or 27 with a small allowance. Put down a roll of geotextile, or use about 4 tons of #4 stone for a rock bed. Check the truck access before you order. A loaded truck can bog down in the final 20 feet near a pond, and relocating stone twice costs twice as much.

Following the first major storm

After runoff subsides, walk along the protected slope. Watch for rock that has moved near the bottom edge, bare soil visible through the stone, or exposed fabric. Put displaced material back, use reserve rock to fill sparse areas, and inspect the outfall apron for erosion. A brief check after monsoon runoff and periodically through the year helps keep the coverage in place.

Stone sizes at a glance

Size riprap according to the force of the water. Four to eight inch stone works for slow streams, pond banks, and downspout outfalls. Use eight to twelve for faster channels and lake shorelines exposed to boat wake. Highway and spillway projects use engineered classes and layer specifications, so follow their drawings instead of this page. If you are between sizes, select the larger material and increase the layer thickness. Heavier stone allows more margin.

Riprap amounts, sizing, and placement

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