RipRap in Maryland: Armor Stone for Tidal Banks and Culvert Outlets

The ground a Maryland bank is made of decides how fast it disappears. A tidal creek edge in Calvert or Talbot County is unconsolidated sand, silt and clay with nothing holding it together but the roots on top, so every wake and every storm tide takes a little more of it. Inland, a Piedmont stream bank on silty loam gives way in slabs after a summer thunderstorm rather than grain by grain. Both problems have the same answer: cover the face with stone heavy enough that moving water cannot lift it.

Order RipRap when moving water is taking the ground

Choose it if any of these describe the site:

• A tidal shoreline or tidal river edge losing bank to wave action and storm tides.
• A stream or creek bank being scoured on the outside of a bend, worst after heavy rain and spring runoff.
• A culvert or pipe outlet cutting a hole in the ground where the discharge lands.
• A steep slope or embankment where runoff is sheeting off bare soil and starting a gully.
• A ditch or spillway where fast flow is carrying the channel away.

The size runs from 4 inches to 12 inches and up, and it arrives as a graded heap rather than one uniform stone. That spread is useful. Larger pieces do the holding, smaller ones wedge into the gaps and lock the face together, and the weight of the individual stones is the entire working principle.

Skip it for a flagstone walkway or a pole barn pad

Two jobs come up often enough on this site to be worth naming directly.

• Example, a flagstone walkway. The stone here is 4 inches to 12 inches and up, with irregular faces and no fines. It cannot be walked on, cannot be screeded and cannot be swept into a joint. A walking route wants Walkway Gravel over a compacted layer, with stone dust in the joints between flags.
• Example, a pole barn or equipment pad. Armor stone will not compact and will not carry a slab or a machine evenly, because the load lands on whichever three stones happen to be proud. A pad wants CR-6 built in lifts and rolled, or Crusher Run where a finer compacting blend suits.
• Example, a single-car gravel driveway. Not the surface, but there is a genuine role here: armoring the ditch and the culvert ends where the drive crosses a swale. That use is covered below.

The dividing line is whether moving water is the problem. Where it is, weight wins. Where the problem is load or foot traffic, a blend that locks up wins.

Tidal shoreline, inland bank and the culvert under a gravel drive

Work at the edge of tidal water in Maryland carries rules that go well beyond materials. Within 1,000 feet of tidal water, Critical Area requirements limit lot coverage and shoreline work generally needs approval before it begins. Settle the permissions for your property and water body before the stone is ordered.

Armor stone is a finished layer, not a base course

Nothing goes on top of riprap and riprap goes on top of very little. It is the outermost layer, and what sits underneath it decides whether it lasts.

• On fine-grained ground, which covers most Maryland banks, a filter layer goes between the soil and the armor. That is either a geotextile filter fabric or a graded stone layer, and its job is to hold the soil in while letting water out.
• Without it, water moving through the gaps carries fines away from behind the stones, voids open up, and the armor drops into them one rock at a time. The face looks fine for a season and then sags.
• Key the bottom of the layer into the toe of the bank rather than starting it at ground level. A layer that ends on the surface gets undercut from below on the first high tide that reaches it.
• Plan on the layer being at least one stone thick. Where pieces run to 12 inches that means roughly a foot of depth, and a single scattered course of big rock is decoration rather than protection.

Sizing 40 feet of tidal creek bank and a pair of culvert aprons

Measure the sloping face, not the ground plan, then multiply by the layer thickness.

• Example, a tidal creek bank, 40 feet long with a face 6 feet down the slope, at 12 inches thick: 40 x 6 = 240 square feet of face. 12 inches is 1 foot, so 240 cubic feet, divided by 27 = 8.9 cubic yards.
• Example, culvert aprons at both ends of a drive crossing, each 10 feet long by 8 feet wide at 12 inches: 10 x 8 = 80 square feet, so 80 cubic feet each, divided by 27 = 3.0 cubic yards per end, and 5.9 cubic yards for the pair.
• Round up on both. Placing heavy stone by hand is slow work, and running out 8 feet from the end of a bank leaves an unprotected edge that the next storm will find.

At this site's published weight for riprap and armor stone, about 1.55 tons per cubic yard and approximate, the creek bank face comes to roughly 13.8 tons and the pair of culvert aprons to about 9.1 tons. Take those cubic yards through the rip rap calculator to check the tonnage before you order.

Measure the face, work out the yardage, then set a tonnage and a delivery date on this page. Where a bank is long or the water's edge is hard to reach, send the measurements and a photograph through contact and the layer thickness can be talked through before anything is booked.