Rip rap calculator for Texas

One ton of riprap covers about 17 square feet. Not a typo. Armor stone goes down in layers 12 inches and thicker, so coverage per ton is a tenth of what landscape rock manages, and first-time buyers are reliably startled by how small the pile looks against how much bank it has to protect. This rip rap calculator works in layer thickness, at 3,100 lb per cubic yard, for stone that typically runs 4 to 12 inches and up.

Measuring a bank for armor

1. Measure the slope face, not the flat ground: run the tape down the slope itself, times the length of shoreline or channel.
2. Layer thickness is 1.5 to 2 times the largest stone size, and 12 inches is the working minimum for real erosion control.
3. Cubic feet / 27 = cubic yards, x 1.55 = tons.

Coverage per ton

Placement rules that keep the stone working

• Fabric or bedding first: non-woven geotextile under the rock, or 3-4 inches of #4 stone. Riprap on bare soil looks fine for a season, then slumps as the bank washes out from underneath.
• Key in the toe. Bury the bottom row below the waterline or expected scour depth, because riprap fails from the bottom up.
• Grade the slope to 2:1 or flatter before stone goes down. Steeper faces are an engineering problem, not a more-rock problem.
• Set the visible top course by machine or by hand so stones interlock instead of resting where they rolled.
• At culvert outlets, run the apron 4-6 pipe diameters downstream. That's where the water digs.

Worked example: 50 feet of pond bank

A 6-foot slope face along 50 feet of shoreline is 300 square feet. At an 18-inch layer: 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, about 26 tons of riprap, 27 with the small allowance. Add a roll of geotextile, or about 4 tons of #4 stone if you'd rather bed with rock. Walk the truck route before ordering; the last 20 feet to a pond edge is where loaded trucks get stuck, and stone placed twice costs twice.

After the first big storm

Walk the bank once the water drops. Look for stones that shifted at the toe, fresh soil showing between rocks, and any spot where the fabric peeks through. Re-seat what moved, top the thin spots from your reserve stone, and check the outfall apron for digging. Ten minutes of inspection a couple of times a year is the entire maintenance program, which is the point of riprap: the material does the work so you don't.

Size classes, briefly

Match stone to water energy. Four to eight inch riprap handles pond banks, downspout outfalls and slow streams. Eight to twelve suits lake shorelines with boat wake and faster channels. Engineered work (highways, spillways) specs its own classes and thicknesses; follow the drawing, not this page. Between two sizes, take the larger and thicken the layer; heavier stone forgives more.

Riprap tonnage, size and placement

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