Rip rap calculator for Vermont

A ton of riprap covers roughly 17 square feet. That figure is correct. Because armor rock is installed in layers of 12 inches or more, each ton covers about a tenth as much area as landscape stone, and the needed pile can seem surprisingly small before it is spread across the bank. This calculator uses layer thickness and 3,100 lb per cubic yard for stone generally sized 4 to 12 inches and larger.

How to measure an armor bank

1. Use the distance along the slope face rather than the level ground, then multiply it by the bank or channel length.
2. Plan on a layer 1.5 to 2 times the size of the largest stone. For meaningful erosion protection, use 12 inches as the minimum working depth.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Ton coverage

Steps for keeping riprap in place

• Start with a base layer: place non-woven geotextile beneath the rock, or use 3-4 inches of #4 stone. On bare soil, riprap can appear stable for a season before erosion beneath the bank causes it to settle.
• Build the toe into the bank. Place the lowest row below the waterline or expected scour depth, as undermining usually starts from below.
• Shape the bank to 2:1 or flatter before setting the stone. Slopes steeper than that call for engineering, not just extra material.
• Use equipment or hand placement on the exposed upper course, fitting stones together so they lock in place instead of stopping where they fall.
• Extend an outlet apron 4-6 pipe diameters downstream from culverts. This is the section where flowing water cuts into the ground.

Example: 50 feet of pond bank

Along 50 feet of shoreline, a 6-foot bank face equals 300 square feet. For an 18-inch-thick layer, calculate 300 x 1.5 = 450 cubic feet, or 16.7 cubic yards. That works out to about 26 tons of riprap, or 27 including a small extra amount. Use a roll of geotextile beneath it, or bed the stone on about 4 tons of #4 stone instead. Check the truck access before you place an order, especially the final 20 feet near the pond, where a loaded truck can sink. Relocating stone costs twice as much when it has to be moved twice.

Following the first major storm

After water levels recede, make a pass along the bank. Note any rock displaced near the toe, exposed earth in the stone, visible fabric, or erosion around the outfall apron. Put shifted pieces back in place and use reserve stone to fill sparse areas. A brief check twice yearly, especially after freeze-thaw weather, keeps the armored slope in order.

A brief guide to stone sizes

Choose riprap according to water force. Four-to-eight-inch material works for pond banks, slow streams, and downspout outfalls. Eight-to-twelve-inch rock fits lake edges affected by boat wake and quicker channels. Highway and spillway work has specified stone classes and depths, so use the project drawing rather than this page. If two sizes are under consideration, select the larger material and increase layer thickness. More weight provides greater tolerance.

Riprap amounts, sizing and installation

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