More Than Just a Pile of Rocks...
There’s really nothing particularly lovely or beautiful about a pile of rocks or concrete. However, if you step back and imagine these somewhat mundane materials being strategically positioned to shore up riverbanks, support lakebeds or displace energy from overflowing creeks, then you might be able to see the beauty that lies within them. These materials can be crafted into systems that provide strong, lasting protection against erosion. These systems, depending on how they’re engineered, constructed or vegetated, can even be beautiful.
We’re talking hard armor here— riprap, gabions and retaining walls. These are the sentinels we turn to for defense against one of nature’s strongest forces, moving water; yes, even the mighty force of the ocean itself. Any area with erosion issues needing rock-solid protection may benefit from the use of one, or a combination of, any of the hard armor solutions.
It’s not hard to imagine cavemen building the very first riprap revetments, piling up rocks to keep water at bay or to create small reservoirs. Also called rubble shot rock and rock armor, riprap is the most basic, and usually the cheapest, means of erosion control available.
Simply speaking, to build a riprap wall, rocks are placed so that they naturally interlock, creating a structure. To a lay person, riprap may just look like a pile of stones that have been randomly dumped someplace. However, when built correctly, a riprap barrier is actually a carefully engineered system.
Riprap works well to protect rivers, stream banks and coastlines from erosion caused by water and ice. It can be used to shore up bridge abutments, pilings and other structures against damage and overflow, particularly in floodprone streams, rivers and channels.
“When many concrete channel linings were designed and installed, they were thought of as a permanent solution,” said Jay Fee, vice president of Erosion and Retaining Wall Structures, Inc. in Lewisville, Texas, “but in most cases they were not. This is due to the inevitable cracking and separating of the lining.” Fortunately, alternative solutions like riprap take advantage of rock to create a more natural durable system that can shift with the earth and water movement.
For added reinforcement, riprap is often stacked atop geotextile fabric or mats. This helps maintain separation between the rocks and the underlying soil, so the rocks won’t eventually sink in.
Depending on the terrain, rock placement can be done with a skid loader or crane. Sometimes a steel plate is used to tamp the rocks into place. You can self-adjust riprap to small amounts of earth movement. It’s relatively easy to install, and can be repaired simply by placing additional rock where needed.
The key to building a successful riprap barrier is to vary the layers and sizes of rock. You want them to fit together so they absorb, rather than deflect, the impact of water. Sharp-angled rocks are better suited for interlocking than smooth, rounded rocks.
Rock that’s native to an area is less expensive and better-suited to the local environment. It looks more natural against the existing landscape, and should be aesthetically pleasing.
Everything from broken limestone to rebar-free concrete rubble can be used as riprap.
However, certain climates call for different kinds or rock. In the freeze/thaw cycles of the northeast, for instance, granite is favored because it has proven its ability to withstand those harsh conditions.
Riprap structures are effective only when constructed correctly. When they fail, it’s often due to “flanking” (water overtopping and undermining the revetment). If improperly sized stones are used, currents or large waves can easily displace them. The Environmental Protection Agency doesn’t recommend this for use on slopes steeper than 2:1. Riprap can actually increase erosion when it’s used improperly.
It also isn’t the ideal choice for high-velocity water flows; the swift-moving water can just wash the rocks away. For those kinds of applications, harder-armored solutions, such as gabion baskets or articulated concrete walls, work better.
The word gabion is derived from the Italian word “gabbione,” meaning “big cage.” That’s a very good description; to create a gabion structure, rocks or broken concrete chunks are caged inside individual wire mesh baskets. These baskets are usually made of galvanized or polyvinyl chloride-coated steel.
Once the rocks are put into place inside the gabion, the lid is closed, and is attached to the next one, and the next, and so forth, creating a gabion wall. Together, a series of interlocking stone-filled baskets can form a flexible, permeable structure that can be used for a large variety of applications, such as bridge abutments, channel linings, streambank stabilization projects and many others.
One gabion success story is a project along the C&O Canal in Washington County, Maryland. This area continuously experienced flooding, which caused a deep section of the canal to ‘overtop’ (a term meaning water overflowing and compromising a bank). As Alan Dinges, manager at The Maccaferri Group, Williamsport, Maryland, reports, “The solution was to install 120 feet of gabions 12 to 15 feet deep on a section of the canal that was prone to flooding.”
“The bottom-most gabions were capped with concrete as additional protection against wave action,” said Dinges. “Now if the canal floods, the gabions will hold, protecting our waterway and its environment.” Because gabions allow free drainage, they work well in flood-abatement applications.
Groundwater can drain freely without the pressure buildup that might cause other walls or supports to buckle or fail.
Gabions are flexible. This makes them ideal for use on unstable land that’s prone to shifting and sliding, something that would topple a stiff structure such as a poured-concrete retaining wall. Gabions can withstand strong seismic waves or vibrations that would crumble riprap structures.
In some areas, gabions might be the only practical choice, particularly in remote sites that are offlimits or inaccessible to heavy machinery. They’re usually an economical choice as well. The unfilled baskets are lightweight, making them less expensive to transport. They don’t have to be custommade; baskets can be purchased prefabricated and assembled onsite. Most areas will have plenty of rocks around, and it doesn’t take highly skilled labor to put them into baskets.
“The beauty of gabions is that you can incorporate them into so many different applications and use them in conjunction with other methods,” says Joe Schweighofer, operations manager for ERWS, Inc., an erosion control company based in Euless, Texas. “We did a project along the Chickamauga Creek in Chattanooga, Tennessee, that incorporated a couple of different methods. With gabions, you can easily mix and match methods to get the results you need.”
Gabions can be stacked nearly vertical on side slopes, and because of their wire caging, they have the strength to stand up to high-velocity flow conditions. Another advantage is that they can be vegetated.
“Many people don’t realize that you can vegetate gabions any way you desire,” says Colin Glass, national sales and technical manager for Terra Aqua Gabions, Inc., Fort Smith, Arkansas. “Because so much of the gabion structure is void space, it can easily be backfilled with soil and planted. The vegetation and its roots will intertwine with the rock and wire, which further reinforces the structure.”
Glass continues, “After a number of years, a well-designed gabion structure should be well vegetated, with lots of roots adding extra strength to the structure. This also meets the objective to return the area to its natural environment. What better way to do that than with vegetation?” The good news is that gabion maintenance is minimal. The structures just need to be modified from time to time, to identify points of wear. If basket failure is found, it’s relatively easy to mend.
When gabion structures do fail, it can be because the baskets weren’t adequately filled, allowing for rocks to shift in place. This can cause abrasion to the basket wires, causing an opening. Baskets can also be damaged by corrosion and debris floating in the water.
When riprap or gabions aren’t enough, it’s time to bring in the really hard stuff. These solutions include mechanically stabilized earth (MSE), poured concrete, articulated concrete blocks, along with some others.
As mentioned, a lot of hard armor, such as poured concrete, isn’t flexible; it will crack or break if there is any earth movement. Gabions are flexible, but aren’t always right for every application. A fast-moving stream or the continual pounding of the ocean waves is going to need something sturdier, that will stay in place, to stop erosion.
When flexibility combined with even harder armoring is desired, articulated concrete block (ACB) systems may be the answer. Generally, these are individual concrete blocks, interconnected by cables, ropes, geotextiles, or geogrids. Some systems don’t use cables; instead they hook together using male and female tabs. Steve Kole of Erosion Prevention Products in Houston, Texas, says, “This interconnection allows them to move, ungulate, but stay connected.”
“Most of the time, concrete blocks are laid out to form long mattresses atop a prepared and graded base and filter fabric,” says Kole. “The concrete mats are usually put into place with heavy equipment and spreader bars. When placed side by side, they can cover thousands of square feet.”
Many concrete mat systems also offer open cells that can be filled with growth medium, so that the individual blocks can be seeded and vegetated. “Concrete block isn’t that aesthetically pleasing,” says Kole. “But, in approximately 18 months, the grass or vegetation will grow over the blocks so you can’t see them any longer.” ACB is a hard-armored solution that is also green.
However, all revetment systems have their advantages and disadvantages. “Interlocking hardarmor systems can be vulnerable both at the top and bottom areas when flood waters rise,” says Dr. Steven Abt, professor emeritus of civil and environmental engineering at Colorado State University.
“The toe, which is down where the river water is scouring, is the first vulnerable area. The concrete mat needs to be securely anchored a couple of feet below the bed level.”
The other area of vulnerability is the very top of an ACB wall. “When flood waters recede, water flows back into the river. If water causes gullies at the top of the bank, behind the interlocking concrete mat, the mat can cave in. And because the ACB mat is interconnected, the cave-in can take out the entire mat. It should be protected by placing it in a graded bank.”
“ACBs are skins of armor that can be laid on a slope or other area needing erosion protection,” says Kole. “They can be used where you would normally use riprap. The pricing is competitive with 18" of stone, if the stone is located nearby. But on all these jobs, you have to be aware of your rock-hauling costs. If riprap has to be hauled in from a great distance, it logically will be more expensive.”
“Rock also tends to catch flotsam, paper and other debris in the water,” Kole continues. “After awhile, that accumulated trash tends not to look very attractive.”
He says that you can see the attractive results in their Bayou project, which was 19-plus miles long and took more than ten years to complete. “The U.S. Army Corp of Engineers called us in because the residents who lived along this bayou put up such a stink about not wanting to see ugly concrete slopes in their backyards.”
Nor did they want riprap; apparently, when you call for riprap in Texas, you don’t get stone, you get broken-up concrete. “Because the residents wanted a natural look, the Corps went with articulating concrete block. Everyone is happy with the end results,” Kole said.
Whether your projects call for riprap, gabion or concrete block, one thing you can believe is that these rock-solid structures can do the job. If chosen carefully and constructed properly, they will provide long-lasting erosion control from one of nature’s strongest forces.