Rip rap calculator for Louisiana

Riprap installed at 12 inches or more covers far less area than landscape stone: one ton covers about 17 square feet. This calculator estimates material by layer depth, using 3,100 lb per cubic yard for rip rap commonly sized 4 to 12 inches and above.

How to measure an armored bank

1. Take the measurement along the sloped bank face, then multiply it by the shoreline or channel length. Do not measure across level ground.
2. For effective erosion protection, use a 12 inches minimum layer. Set thickness at 1.5 to 2 times the largest rock dimension.
3. Find cubic yards by dividing cubic feet by 27, then use x 1.55 to calculate tons.

Tons of coverage

Steps for stable stone placement

• Install non-woven geotextile beneath the rock, or start with 3-4 inches of #4 stone. Bare soil can wash away below riprap, particularly where wet clay loses its shape, leaving the rock to settle after a season.
• Set the toe before placing the rest of the stone. Place the lowest row under the waterline or anticipated scour depth, since erosion can undermine riprap from below.
• Before placing stone, cut the bank to a 2:1 slope or gentler. A face that is steeper needs engineering, not added rock.
• Use equipment or hand placement for the exposed top course, arranging stones so they lock together rather than sit where they land.
• For culvert outlets, carry the apron 4-6 pipe diameters beyond the outlet downstream. This is the area where flow begins to scour the ground.

Example: 50 feet of pond bank

For a 6-foot bank face running 50 feet along the water, the area is 300 square feet. Using an 18-inch layer, calculate 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or about 26 tons of riprap. Plan for 27 with a small extra amount. Include geotextile, or use about 4 tons of #4 stone as a rock bed. Check the truck path before placing an order, since the final 20 feet near a pond can be soft enough to trap a loaded truck. Moving stone twice doubles the handling.

Following the first major storm

After the water recedes, walk along the bank and inspect the toe for rock that has moved, newly exposed soil between stones, or visible fabric. Put shifted stone back in place, use reserve material to cover thin areas, and look over the outfall apron for erosion. Ten minutes spent checking it a few times each year is generally all the upkeep needed. Riprap is intended to protect the bank with the stone doing the work.

Stone sizes at a glance

Select riprap size based on water force. 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. Highways, spillways and other engineered work have specified stone classes and depths, so follow the project drawing instead of this guide. When between sizes, choose the larger rock and use a thicker layer; added weight provides more tolerance.

Riprap quantities, sizing and placement

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