Rip rap calculator for North Dakota

Expect about 17 square feet of coverage from one ton of riprap. Armor stone is installed in layers 12 inches or thicker, not at landscape-rock depth, so a ton covers much less area. This calculator bases quantities on layer thickness and 3,100 lb per cubic yard for rock generally sized 4 to 12 inches and larger.

How to measure an armored bank

1. Take the slope-face measurement down the bank, then multiply it by the shoreline or channel length, not the level-ground distance.
2. Use a layer 1.5 to 2 times the largest rock dimension. Effective erosion protection needs a working minimum of 12 inches.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 to get tons.

Tons of coverage

Placement details for stable stone

• Put the support layer in place before the rock: non-woven geotextile beneath it, or 3-4 inches of #4 stone. On bare soil, riprap can look settled for a season before erosion below the bank lets it sink.
• Bury the toe below the waterline or anticipated scour depth. Riprap problems can begin at the bottom row.
• Before setting stone, cut the slope to 2:1 or flatter. A steeper bank needs engineering, not simply more rock.
• Machine-set or hand-place the exposed finish course so each stone locks against the next rather than staying where it lands.
• For culvert outlets, carry the apron 4-6 pipe diameters downstream. Water starts cutting there.

Example calculation: 50 feet of pond bank

A shoreline stretch of 50 feet with a 6-foot slope face covers 300 square feet. For an 18-inch layer, 300 x 1.5 = 450 cubic feet = 16.7 cubic yards. Plan on about 26 tons of riprap, or 27 with a small allowance. Add a roll of geotextile, or use about 4 tons of #4 stone for a rock bed. Check the truck route before you order. A loaded truck can sink in the last 20 feet by a pond edge, and placing stone twice doubles the cost.

Following the first major storm

After the water recedes, inspect the bank. Watch for rock displaced along the toe, bare soil visible through the stone, or exposed fabric. Put shifted pieces back, use reserve rock to cover sparse areas, and inspect the outfall apron for erosion. A ten-minute check twice yearly keeps riprap on the Missouri doing its job with little upkeep.

A quick guide to size classes

Size the rock for the force of the water. Four to eight inch riprap fits pond banks, downspout outfalls, and slow-moving streams. Eight to twelve works along lake edges exposed to boat wake and in faster channels. Highways and spillways use engineered size classes and layer depths, so use the project drawing instead of this guide. When size is uncertain, select the larger stone and increase layer thickness. More weight handles greater force.

Riprap sizing, tonnage, and placement

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