Rip rap calculator for Iowa

Expect one ton of riprap to cover about 17 square feet. Its 12 inches and thicker armor layer uses far more material than landscape rock, which is why the delivered pile can look small beside the bank it will cover. This calculator accounts for installed thickness using 3,100 lb per cubic yard, with rip rap generally ranging from 4 to 12 inches and larger.

How to measure a bank for armor stone

1. Start with the bank face: measure from top to bottom along the slope, then multiply that distance by the shoreline or channel length.
2. For erosion protection, use a stone layer 1.5 to 2 times the largest rock dimension. A 12-inch layer is the practical minimum.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Tons of coverage

Installation details for stable riprap

• Put fabric or bedding beneath the rock: use non-woven geotextile, or 3-4 inches of #4 stone. On bare soil, riprap may look sound for one season before the bank erodes underneath and the stone settles.
• Install the toe below the waterline or anticipated scour depth. The lower row needs to be buried because failure usually starts at the bottom.
• Start by shaping the bank at 2:1 or flatter, then install the stone. A steeper slope calls for engineering, not simply additional rock.
• Make the exposed surface interlock by positioning the top stones with a machine or by hand, rather than leaving them where they land.
• Water scours below culverts, so carry the outlet apron 4-6 pipe diameters downstream.

Example calculation: 50 feet of pond bank

For a 6-foot bank over 50 feet of shore, plan on 300 square feet. With an 18-inch thickness, the calculation is 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or roughly 26 tons of riprap and 27 including a small extra amount. Use a geotextile roll beneath it, or choose about 4 tons of #4 stone for a rock bedding layer. Check the delivery path before you place the order, especially the final 20 feet approaching a pond, where loaded trucks can bog down. Moving stone a second time doubles that placement cost.

Following the first major storm

After the water recedes, inspect the bank for movement at the toe, exposed soil between the rock, or fabric showing through. Put shifted stone back in place, use reserve material to cover sparse areas, and look over the outfall apron for erosion. A ten-minute check a couple times yearly is all the upkeep required. Riprap is intended to handle the protection work itself.

A quick guide to size classes

Choose riprap size based on the force of the water. Four to eight inch material fits pond banks, downspout outlets, and slow-moving streams. Eight to twelve works for lake edges exposed to boat wake and faster channels. Highway and spillway projects have their own engineered stone classes and layer requirements, so use the project drawing instead of this guidance. When deciding between sizes, select the larger stone and increase the layer thickness; added weight provides more resistance.

Riprap quantity, sizing, and placement

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