Rip rap calculator for Alaska

Expect one ton of riprap to cover about 17 square feet. That is accurate. Armor stone is installed in layers 12 inches or thicker, so each ton covers about a tenth of the area landscaping rock can cover. The pile often seems small beside the bank it must hold. This calculator bases quantities on layer depth, using 3,100 lb per cubic yard for stone commonly 4 to 12 inches and larger.

How to measure an armor bank

1. Take measurements on the sloped face rather than level ground: tape down the slope, then multiply that distance by the shoreline or channel length.
2. Set the rock course at 1.5 to 2 times the diameter of the biggest stones. For erosion work that holds up, do not go below 12 inches.
3. To find cubic yards, take cubic feet / 27; then use x 1.55 to calculate tons.

Tons of coverage

Stone placement rules

• Put the base in before the riprap: lay non-woven geotextile beneath it, or use 3-4 inches of #4 stone. Rock placed straight on soil may look sound for one season, then settle when erosion removes support below.
• Start by setting a buried toe. The first course needs to sit below the waterline or anticipated scour depth, since undermining begins at the bottom.
• Prepare a 2:1 slope or flatter ahead of rock placement. A bank steeper than that calls for engineering rather than extra stone.
• Use equipment or hand setting on the exposed top course, arranging stones to lock together instead of leaving them where they land.
• Extend the outlet apron 4-6 pipe diameters beyond a culvert. The downstream end is where flowing water cuts into the ground.

Example: 50 feet of pond bank

For 50 feet of shoreline with a 6-foot slope face, plan on 300 square feet of coverage. An 18-inch-deep layer works out to 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or roughly 26 tons of riprap. Make it 27 to cover the small allowance. Put down a roll of geotextile, or use about 4 tons of #4 stone for a rock bedding layer. Check the access route before placing the order. A loaded truck can sink along the final 20 feet to the pond edge, and handling the stone twice doubles the cost.

Following the first major storm

After the water goes down, make a pass along the bank. Check the toe for displaced rock, exposed soil in the gaps, visible fabric, and scouring at the outfall apron. Put shifted pieces back in place and use reserve stone to fill light areas. Ten minutes a couple of times each year covers the maintenance, including after freeze-thaw movement: riprap is meant to carry the protection load itself.

Stone size classes

Select stone for the force of the water. Four to eight inch riprap fits pond banks, slow streams and downspout outfalls. Use eight to twelve for lake edges exposed to boat wake and for faster channels. Engineered jobs such as highways and spillways use their own stone classes and layer thicknesses, so follow the drawing instead of this page. If the choice is between sizes, use the larger material and a thicker layer. Added weight gives more forgiveness.

Riprap quantity, size, and placement

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