Rip rap calculator for Washington

A ton of riprap covers only about 17 square feet, and that is expected. Armor rock is placed in layers 12 inches deep or more, so each ton goes about a tenth as far as landscape rock. The pile can seem small until you see the bank area it must cover. This calculator factors in layer depth, using 3,100 lb per cubic yard for material generally sized 4 to 12 inches and larger.

How to measure an armored bank

1. Measure along the slope, from top to bottom, rather than across the level ground. Multiply that distance by the shoreline or channel length.
2. For the rock layer, allow 1.5 to 2 times the diameter of the largest stone. A 12-inch depth is the practical floor for erosion control.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Per-ton coverage

How to place stone so it holds

• Prepare the base before placing rock: lay non-woven geotextile beneath it, or use 3-4 inches of #4 stone. Riprap set straight on soil can appear stable for a season, but undermining below the bank can cause it to settle.
• Build a buried toe by setting the lowest row beneath the waterline or expected scour depth. A riprap slope usually comes apart starting at its base.
• Prepare a 2:1 slope or flatter before installing stone. Banks steeper than that call for engineering, not just extra rock.
• Place the exposed upper course by hand or machine, fitting each stone against the next rather than leaving it where it lands.
• Extend the apron 4-6 pipe diameters beyond a culvert outlet. The flow can scour the ground in that downstream area.

Example: 50 feet of pond bank

For 50 feet of 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 the small allowance. Plan for a roll of geotextile, or about 4 tons of #4 stone if rock bedding is preferred. Before ordering, check the truck path. Along wet Puget Sound ground, the last 20 feet to a pond edge can trap a loaded truck, and moving stone twice costs twice.

Following the first major storm

After the water recedes, make a pass along the bank. Check the toe for displaced rock, exposed soil in the gaps, visible fabric, and any erosion around the outfall apron. Put shifted stones back in place, use reserve rock to cover sparse areas, and address any digging. Ten minutes of checking a couple of times each year keeps the riprap doing its job without much ongoing work.

Stone sizes at a glance

Choose stone for the water force involved. Riprap from four to eight inch works for pond banks, slow streams and downspout outfalls. Eight to twelve is suited to lake edges with boat wake and quicker channels. Highways and spillways use engineered specifications for class and depth, so follow the plans rather than this page. If choosing between sizes, use the larger stone with a thicker layer; added weight gives more margin.

Riprap sizing, tonnage and installation

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