RipRap for Banks, Culverts and Lakeshore in New Hampshire

Bank work has a season here, and it is not the season the damage happens in. The damage happens in the thaw, when runoff and ice take a bite out of a bank or blow the ground out below a culvert. The repair waits for low water, which is usually somewhere between late June and the first hard frost.

That gap is the awkward part. What fails in April gets looked at in July.

RipRap is the stone for that repair. Four inches to twelve inches and up, large and heavy, chosen because moving water cannot lift it.

Reading the water before the stone arrives

Go back to the spot in low water and look at what the high water did.

Where is the scour? A culvert outlet usually washes a hole directly below the pipe and undercuts the bank on one side of it. A stream bank scours at the toe first, and the slope above it slides later because the bottom went.

How high did it get? Debris caught in brush, a silt line on a tree trunk, a flattened band of grass. Armor to a little above that line, not to the water level you are standing at in August.

Where is the water coming from? If a ditch upstream is delivering more water than it used to, stone at the outlet treats the symptom and the next storm finds a new place to cut.

What goes under it, and why that part is not optional

Heavy stone on bare soil fails from underneath. Water gets between the pieces, works the soil out through the gaps, and the stone settles into the hole it made.

The layer sequence, from the ground up:

1. Shape the slope. Cut the toe into firm ground so the bottom course has something to sit against.
2. Filter fabric, or a graded filter layer of smaller stone, over the whole area and up the bank.
3. The stone, placed thick enough that pieces bear on each other rather than on the fabric alone.
4. Carry the armor past the scoured area at both ends and above the high-water line.

The toe is the part people skimp. It is also the part the water attacks first.

Where riprap goes once the water is down

Culvert outlets. An apron of stone below the pipe takes the energy out of the discharge before it reaches soil. Size the apron to the pipe and the fall, and extend it further than looks necessary.

Stream and river banks. The classic spring-runoff repair, and the reason the North Country and the hill towns order it.

Lakeshore banks. Wind and boat wake wear a bank at the waterline, and the loss is worst at high water. Note that work in or near the water can need a permit from the town or the state, so make that call before you book stone.

Ditch lines. A lined channel on a steep camp road resists scour where a grassed ditch has stopped holding.

Construction entrances. Heavy stone at the point where equipment leaves a site onto a paved road. It holds up where smaller aggregate gets pressed into the mud in a week.

The one place it does not go: under a paver patio

People ask, because heavy stone sounds like a strong base. It is not one.

Four to twelve inch rock leaves large voids that nothing fine fills, so a patio built over it settles unevenly the first winter and keeps going. A paver base wants a graded, compacted layer of crusher run topped with a bedding course of stone dust.

Same logic for a drive or a walk. Riprap protects ground that water crosses. It does not carry anything you plan to stand on.

Yardage for 45 feet of stream bank and a dry creek line

Hypothetical jobs. RipRap runs about 3,100 lb per cubic yard on this site's published figures, near enough to 1.55 tons per yard, so the tonnages below are approximate. Other lengths and thicknesses go through the rip rap calculator.

Stream bank, 45 ft long, 8 ft of slope face, 18 inches thick

• 45 x 8 = 360 sq ft of face
• 360 x 18 / 12 = 540 cu ft
• 540 / 27 = 20 cu yd
• 20 x 1.55 = 31 tons

Eighteen inches is not arbitrary. Armor wants to be thicker than the largest stone in it, so a load running to twelve inches and up needs a foot and a half to give you two courses that bear on each other.

Dry creek line, 30 ft by 4 ft, 12 inches

• 30 x 4 = 120 sq ft
• 120 x 12 / 12 = 120 cu ft
• 120 / 27 = 4.44 cu yd
• 4.44 x 1.55 = 6.9 tons

Measure the slope face, not the plan view, on anything steep. A bank that is 8 feet of slope covers less than 8 feet of ground, and using the ground measurement leaves you short.

What the first spring runoff tells you

Walk it after the thaw. A well-set job looks the same as it did in October, with maybe a little settlement where the pieces found their seats.

What to look for: stone gone from the toe, a gully forming at either end of the armored section, or soil showing through between pieces. The first two mean the armor did not reach far enough. The third usually means the filter layer was missed or torn during placement.

A few stones reset in one summer saves rebuilding the section in three. That is most of the upkeep there is. Otherwise heavy stone sits where it was put and gets on with it.

Set the quantity and the delivery date on this page. Delivery is arranged at checkout by ZIP code, and heavy stone needs a dump spot with firm ground and clear height, as close to the bank as a truck can safely get.