Rip Rap Calculator for Banks, Ditches and Outfalls

Every number this rip rap calculator gives you follows from one decision: how hard the water hits. A tidal creek bank on the Northern Neck taking storm tide and boat wake is a different problem from a culvert outfall in a steep hollow that runs four times a year and runs hard. Pick the stone for the water, and the layer thickness, the area and the tonnage all fall out of that choice in order.

Which stone size is this water asking for?

Four to eight inch material suits pond banks, downspout outfalls and gentle streams. Eight to twelve inch handles lake and tidal edges exposed to wave action and boat wake, and faster channels. Where the choice is genuinely borderline, take the larger stone and thicken the layer with it; the extra weight absorbs the variation between one storm and the next.

Highway, spillway and other engineered work is designed rather than estimated. Those projects carry their own specified stone class and layer depth, so build to the drawing and use this page for the yard-scale jobs.

How do I measure a bank that is not flat?

Follow the face with the tape. Run it down the slope from the top of the bank to the toe and use that reading, not the flat ground you could pace at the top, then multiply by the length of shoreline or channel you are armoring. A bank that reads 5 ft across on a plan is nearer 5.6 ft down a 2:1 face, and steeper ground widens the gap further. Take the reading off the slope.

Enter it as a rectangle, the slope face as one dimension and the run as the other, both in feet. Multiple areas takes up to five entries, which is how a bank that steepens partway along gets handled without averaging away the worst section. A round outfall apron goes in as a circle with its diameter in feet.

How thick does the layer have to be?

One and a half to two times the largest stone in the load, and never less than 12 inches. Four to twelve inch residential material generally wants 12 to 18 inches; large stone or swift water can call for 24, which is the top of the depth (inches) field.

Shape the bank to 2:1 or flatter before any stone goes on. A steeper face needs engineering rather than more rock, and cutting a bank back usually means moving soil, which is sized as fill behind a repaired bank rather than as stone. Bury the toe: set the lowest course below the waterline or the expected scour depth, because bank failures start at the bottom and work up.

Check with your state and local authorities before stone goes in below the ordinary high water line of a stream, pond or tidal shoreline. Placing rock in and along water is commonly permitted work, what is required varies by waterbody, and this page is not a substitute for asking.

What does that come to in tons?

Take 80 ft of tidal creek bank with a 7 ft slope face, armored at 15 inches. That is 560 square feet. Fifteen inches is 15/12 of a foot, so 560 x 1.25 gives 700 cubic feet. Divided by 27 that is 25.93 cubic yards, and armor stone is published here at about 3,100 lb a cubic yard, or 1.55 tons, so 25.93 x 1.55 comes to 40.2 tons. The example is hypothetical, but the arithmetic is what the panel does.

The pile is smaller than the tonnage suggests and the coverage is much smaller than landscape stone: about 17 square feet to the ton at 12 inches, 12 at 18 inches and 9 at 24. Cubic yards on the result panel tell you what the drop spot has to hold, tons are what you order, and Email my quote keeps the set. Check access and the drop spot before ordering, since the last stretch of track to a pond can hold a loaded truck, and stone handled twice is a day lost.

Is the optional 5 per cent enough here?

It is the wrong instrument for this job. The tick box adds 5 per cent for compaction and spillage, which suits material that is spread; armor stone is placed piece by piece, and what you lose goes on shaping the face and filling the gaps between large stones. On 40.2 tons the box gives 42.2, which is a reasonable place to land, but the better habit is to round to the next whole load and keep a small reserve for the toe after the first big storm.

What goes underneath it?

Stone laid straight on soil holds for a season and then settles as water pulls the soil out from beneath it. Put down non-woven geotextile, which a landscape supplier carries and this site does not, or 3 to 4 inches of #4 crushed stone as a bedding course. On the same 560 square foot face, 4 inches of bedding is 186.7 cubic feet, or 6.9 cubic yards, and that material runs about 2,900 lb a yard, so roughly 10 tons.

Place the top layer so each stone locks against its neighbors rather than landing where the bucket drops it. On a culvert outfall, carry the apron four to six pipe diameters downstream, which is where the water does its scouring. Stone up to about 6 to 8 inches can be set by hand with gloves; above that a machine earns its hire.

Three neighboring materials are not armor and should not be ordered as though they were. Large river rock for a dry creek bed is rounded and decorative, drain rock behind a wall answers water sitting in the ground rather than water working at the face, and heavier stone for hard standing is for tracks and pads.

Where this work goes on in Virginia

Along the Chesapeake Bay, the ocean shore and the tidal rivers and creeks, waves, storm tides and rising water take the bank apart at the waterline, so the toe and the stone size carry the job. Inland, the pattern is scour: stream banks cut after heavy rain, and in the steep hollows of the far southwest, creeks rise fast and pour down onto anything unprotected. On Piedmont clay the problem is usually sediment washing off bare slopes and ditch banks, where a ditch check or a lined channel does more than a heavier stone.

After a storm, walk the bank once the water drops. Displaced stone at the toe, fresh soil showing between stones or exposed fabric are the three things to look for, and pushing stone back into place with a bar beats rebuilding a face later. Use RipRap for the armor itself, and place the bedding order alongside it so both arrive together.

Common questions

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