RipRap for Virginia Culvert Outlets, Ditch Lines and Tidal Banks

The orders arrive in two waves. One comes in late summer as the tropical storm and nor'easter season gets close and people look at a bank that lost ground last year. The other comes the week after a storm, when a culvert has scoured a hole at its outlet and the ditch beside the lane has cut back into the shoulder. RipRap is the material for both, but the first wave gets the better job, because the ground is dry enough to shape properly and the work is not being done around standing water.

Booking the stone before the late-summer storm season

Timing decides how much of this you can build yourself.

• Late spring through midsummer is the workable window on the coast and along the tidal rivers. Ground is firm, water levels are predictable, and there is time to shape a bank before the storm season.
• Autumn is the awkward stretch near the Chesapeake Bay and the tidal creeks, when tropical systems and nor'easters push water up over the very ground you are trying to protect.
• Winter is generally workable in Hampton Roads and the low country, where hard freezes are short. In the Blue Ridge, the western valleys and the coalfield hollows, frozen ground and snow stop work for longer stretches.
• After any long wet spell, saturated Piedmont clay will not hold a machine or a cut face, so a ditch job waits for the ground to give back.

Before any of this: work in or along a stream, a tidal shoreline or a public ditch is frequently regulated, and a culvert at a road entrance sits under a separate set of rules again. Check with your locality, and with the relevant state or federal authority for work below the tide line or in a waterway, before stone is ordered.

Cutting the toe and laying the bed while the ground is still firm

Stone tipped loose down a bank slides. Stone placed into a prepared face stays.

1. Shape the slope back to something the stone can rest on, and take out the loose or slumped material first.
2. Cut a toe trench at the bottom. The toe is what holds the whole face up, and a run of armor without one unravels from the base when water undercuts it.
3. Lay a filter layer over the shaped face, either a geotextile or a bedding stone, so the soil underneath cannot wash out through the gaps.
4. Place the stone from the toe upward, working larger pieces into contact with each other rather than spreading an even carpet.
5. Carry the armor past the point where water leaves it, upslope and to each side, because a storm attacks the ends of a lined section first.

Culvert outlets, ditch lines and where a foundation drain discharges

Size here is 4 inches to 12 inches and up, large heavy stone rather than anything a compactor touches.

• A culvert outlet is the classic job. Water leaves a pipe at speed, and the apron of stone at the exit takes that energy before it reaches soil. Build the apron wider than the pipe and carry it far enough downstream that flow has spread before it hits bare ground.
• A ditch line beside a lane or a lot takes armor on the sections that scour, usually the steeper runs and the outside of bends. Lining the whole ditch is rarely necessary and rarely where the money should go.
• The discharge end of a French drain along a foundation is the one people forget. The trench itself takes a washed stone such as #57 Stone; this material belongs where the pipe daylights, so the water leaving it does not cut a channel across the garden.
• A tidal or pond edge is armored against wave action and storm tide rather than against flow, and the stone size and the toe detail both matter more there.

Where a regraded low spot needs armor and where it needs fill

A hollow that ponds after every storm is not usually a stone problem.

Sizing a 30 foot ditch line and a 10 foot outlet apron

Take the face you are covering as a length by a width, use the layer thickness as the depth, and divide by 27 for cubic yards. A common planning assumption is a layer at least as thick as the largest stone, so 12 inches is a reasonable starting point for 4 to 12 inch material.

• Example, a ditch run 30 ft long with 6 ft of wetted face, at 12 inches: 30 x 6 = 180 square feet. 180 x 1.0 = 180 cubic feet. 180 / 27 = 6.7 cubic yards.
• Example, a culvert outlet apron 10 ft long and 8 ft wide, at 12 inches: 80 square feet x 1.0 = 80 cubic feet. 80 / 27 = 3.0 cubic yards.
• Example, a toe trench 30 ft long, 2 ft wide and 18 inches deep: 60 square feet x 1.5 = 90 cubic feet. 90 / 27 = 3.3 cubic yards.

This site publishes roughly 3,100 lb per cubic yard for rip rap and armor stone, about 1.55 tons per yard, and that weight is approximate: the ditch run above is 6.7 x 1.55 = roughly 10.4 tons, with the culvert outlet apron about 4.7 tons and the toe trench about 5.1 tons.

Measure the sloping face rather than its footprint on plan, because a 3 to 1 bank is appreciably longer up the slope than it is across the map. The rip rap calculator converts cubic yards into tons for this material. Where an engineer or a permit specifies a class and a thickness, that specification governs the order.

Set your quantity and a delivery date once the face is cut and the filter layer is on site. Delivery is arranged at checkout from your ZIP code, and with stone this heavy it is worth walking the access route first: the closer the tip goes to the bank, the less of it you move twice.

What the first big rain shows, and the checks after it

The first real storm after placement is the inspection.

• Walk the ends of the lined section. Scour at the top or bottom edge means the armor stopped short of where the water goes.
• Look at the toe. Stone that has settled slightly is normal; a gap opening under the bottom course is not.
• Check for soil coming through the face. That points at the filter layer rather than at the stone.
• Clear debris from a culvert apron. A blocked outlet sends water around the armor instead of through it.