Rip rap calculator for Montana

Plan on about 17 square feet from one ton of riprap. It sounds low, but it is right. Armor rock is placed in courses 12 inches thick or more, which gives it only a fraction of the coverage of landscape stone. This rip rap calculator uses installed depth and a density of 3,100 lb per cubic yard, based on material commonly sized 4 to 12 inches and above.

How to measure an armored bank

1. Take your measurement along the bank face, not across the level ground. Multiply that sloped distance by the shoreline or channel length.
2. Make the rock layer 1.5 to 2 times the biggest stone dimension. For erosion control, do not go below 12 inches.
3. Find cubic yards by taking cubic feet / 27, then use x 1.55 to estimate tons.

Tons and coverage

Installation details for stable riprap

• Put fabric or bedding beneath the rock: use non-woven geotextile, or 3-4 inches of #4 stone. Riprap laid straight on soil can hold its appearance for a season, then settle when the underlying bank starts washing away.
• Bury the toe row below the waterline or anticipated scour depth. Bank protection often comes apart from the bottom first, so key that lower edge in.
• Bring the bank to a 2:1 slope or flatter before placing rock. More stone does not solve a face that is too steep; that calls for engineering.
• Use hand placement or a machine for the exposed upper course, fitting the stones together so they lock in place instead of stopping where they happen to roll.
• For culvert outfalls, extend the apron 4-6 pipe diameters beyond the outlet. That downstream area takes the digging force.

Example calculation: 50 feet of pond bank

For 50 feet of shoreline with a 6-foot slope face, the area is 300 square feet. With an 18-inch course: 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, which is about 26 tons of riprap, or 27 allowing a little extra. Include a geotextile roll, or plan on about 4 tons of #4 stone for a rock bedding layer. Check the truck path before you order. That final 20 feet near a pond can trap a loaded truck, and moving stone a second time costs twice.

Following the first major storm

After water recedes, take a pass along the bank. Check the toe for displaced rock, exposed soil between stones, visible fabric, and any scour at the outfall apron. Put shifted pieces back in place and use reserve stone to fill sparse areas. A couple of short checks each year, about Ten minutes apiece, helps keep the rock cover doing its job.

A quick guide to size classes

Size riprap for the water force at the site. Four to eight inch material works for pond banks, downspout outfalls, and slow streams. Use eight to twelve for lake edges receiving boat wake and for faster channels. Highway and spillway projects have their own engineered classes and depths, so use the drawing rather than this guide. If the choice is between sizes, go larger and increase the layer thickness; added stone weight gives more tolerance.

Riprap sizing, tonnage, and installation

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