Rip rap calculator for Utah

Plan on about 17 square feet of coverage from one ton of riprap. Armor stone is installed in layers of 12 inches or more, unlike landscape rock spread in a shallow layer, so the pile can look modest before it is placed. This rip rap calculator accounts for layer depth, using 3,100 lb per cubic yard for rock commonly sized 4 to 12 inches and larger.

How to measure an armored bank

1. Run your tape along the bank face rather than across the level ground, then multiply that distance by the shoreline or channel length.
2. Set the rock layer at 1.5 to 2 times the largest stone dimension. For active erosion control, use no less than 12 inches.
3. To find tons, divide cubic feet by 27 and multiply the result by 1.55.

Tons of coverage

Stone placement basics

• Start with fabric or bedding: place non-woven geotextile beneath the riprap, or use 3-4 inches of #4 stone. Rock set directly on soil can look acceptable at first, but the bank may slump when water removes soil below it.
• Start by setting the toe. Place the lowest course below the waterline or the expected scour depth, since undermining begins at the base.
• Shape the bank to 2:1 or flatter before placing the stone. If the face is steeper, it calls for engineering rather than extra rock.
• Machine-place or hand-place the exposed upper stones so they lock together, instead of leaving them where they land.
• For culvert outlets, extend the apron 4-6 pipe diameters past the discharge point downstream, where water can scour the ground.

Example: 50 feet of pond bank

A shoreline length of 50 feet with a 6-foot bank face equals 300 square feet. For an 18-inch layer, calculate 300 x 1.5 = 450 cubic feet = 16.7 cubic yards. That is about 26 tons of riprap, or 27 including the small allowance. Install a roll of geotextile, or use about 4 tons of #4 stone as rock bedding. Before ordering, check the truck path, especially the last 20 feet to a pond edge where loaded vehicles may lose traction. Rehandling stone doubles the cost.

Check after the first major storm

After the water recedes, make one pass along the bank. Check the toe for displaced rock, exposed soil in the gaps, or fabric visible beneath the stone. Put shifted pieces back, use reserve stone to fill sparse areas, and inspect the outfall apron for erosion. Ten minutes of review a couple of times a year covers the maintenance: riprap is intended to protect the bank with little ongoing work.

A quick guide to size classes

Base the rock size on the water's force. Four to eight inch riprap works for pond banks, downspout outlets, and slow-moving streams. Eight to twelve is suited to lake edges with boat wake and quicker channels. Highway and spillway projects have engineered stone classes and layer depths, so follow the project drawing instead of this page. If deciding between two sizes, choose the larger material and increase layer depth. Heavier rock tolerates more force.

Riprap sizing, tonnage and placement

Other calculators