Rip rap calculator for Delaware

Expect about 17 square feet from a ton of riprap. That limited coverage is normal: armor rock is installed 12 inches deep or deeper, unlike landscape material. A pile can look undersized until it is spread across the bank it must cover. This calculator bases the result on installed depth and 3,100 lb per cubic yard for rip rap generally ranging from 4 to 12 inches and larger.

How to measure a bank for armor stone

1. Measure along the bank face rather than across level ground, then multiply that distance by the shoreline or channel length.
2. Build the layer at 1.5 to 2 times the largest rock dimension. For active erosion control, use no less than 12 inches.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Tons of coverage

Stone placement details that matter

• Start with fabric or bedding: install non-woven geotextile below the rock, or use 3-4 inches of #4 stone. Rock set directly on soil may hold briefly, but the bank can wash away below it and leave the layer slumped.
• Start with the toe. Place the lowest stones below the waterline or the expected scour level, since a riprap bank can unravel from its base.
• Before placing rock, shape the face to 2:1 or flatter. If the bank is steeper, it calls for engineering rather than extra stone.
• Machine-place or hand-set the exposed upper layer so the stones lock together instead of stopping wherever they fall.
• At culvert outlets, extend the apron 4-6 pipe diameters downstream, where discharge flow starts cutting into the ground.

Example calculation: 50 feet of pond bank

For 50 feet of shoreline with a 6-foot slope face, the area comes to 300 square feet. Using an 18-inch depth, the calculation is 300 x 1.5 = 450 cubic feet = 16.7 cubic yards. Plan on about 26 tons of riprap, or 27 with a small allowance. Use a roll of geotextile beneath it, or bed the rock on about 4 tons of #4 stone. Check the truck access before you order. The last 20 feet at a pond edge can bog down a loaded truck, and relocating stone costs twice as much.

Following the first heavy storm

When the water level falls, make a pass along the bank. Check the toe for displaced rock, exposed soil in the cover, visible fabric, and scour around the outfall apron. Put loose stones back, fill sparse areas with extra rock, and correct any digging at the outlet. A quick check twice a year keeps a Delaware Bay or pond edge protected with little ongoing work.

A quick guide to size classes

Size the rock for the force of the water. Four to eight inch riprap is suited to pond edges, downspout outlets, and slow streams. Use eight to twelve for lake banks exposed to boat wake or for faster channels. Highways, spillways, and other engineered installations have specified rock classes and layer depths, so use the plans rather than this guidance. When the choice is between sizes, select the larger material and increase the depth. Added rock weight allows more margin.

Riprap sizing, tonnage and installation

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