Rip rap calculator for Rhode Island

Plan on about 17 square feet from one ton of riprap. The figure is right. Armor stone is installed in layers of 12 inches or more, so each ton covers roughly a tenth as much as landscape rock. The pile can seem small until it is spread across the bank it protects. This calculator uses layer depth and 3,100 lb per cubic yard for stone commonly sized 4 to 12 inches and above.

How to measure an armor bank

1. Run your tape over the slope face instead of across level ground, then multiply by the shoreline or channel length.
2. Make the rock layer 1.5 to 2 times the biggest stone dimension. For erosion control, use no less than 12 inches.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Ton coverage

Stone placement basics

• Put fabric or bedding below the rock: use non-woven geotextile, or 3-4 inches of #4 stone. Rock set on bare soil can look stable for a season before the bank erodes underneath and it settles.
• Bury the toe below the waterline or expected scour depth. Riprap breaks down from its base upward.
• Before placing stone, cut the slope to 2:1 or flatter. A steeper bank calls for engineering rather than added 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 beyond culvert outlets. Downstream is where scour forms.

Example: 50 feet of pond bank

For a 50 feet shoreline with a 6-foot slope face, the area is 300 square feet. Using an 18-inch layer: 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or about 26 tons of riprap and 27 with a small allowance. Include a geotextile roll, or use about 4 tons of #4 stone for rock bedding. Check the truck path before placing an order. The final 20 feet near a pond edge can trap loaded trucks, and moving stone a second time costs twice.

Following the first major storm

When the water recedes, inspect the bank. At the toe, check for displaced rock, bare soil between stones, or fabric showing through. Reset loose pieces, cover sparse sections with reserve stone, and look over the outfall apron for washout. Spend Ten minutes on this a year a couple of times. Riprap needs little upkeep because the stone handles the protection.

Stone sizes at a glance

Size the stone for the water force. Four to eight inch riprap suits pond banks, downspout outfalls and gentle streams. Use eight to twelve for lake shorelines exposed to boat wake and for faster channels. Engineered projects, including highways and spillways, specify their own stone classes and layer depths. Follow the drawing rather than this guide. If two sizes seem possible, choose the larger stone and increase layer thickness. Added weight gives more margin.

Riprap sizing, tonnage and installation

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