Mats, Concrete, Blocks, and Rocks:
The Lowdown on Riprap – Part 1

Options for preventing erosion in channels and streambanks

Soil

Since the beginning of time, people have found ways to use rocks as tools. The first rock tools probably were used for hunting. The use of rocks to help reinforce ditches probably was not seen until irrigation agriculture had been practiced quite some time. Slowly people began to find many uses for rock: building a dam to help back up water or a berm to divert and direct the flow. But when did they begin to realize that the cutting action of water against bare soil or sand was a significant phenomenon? When did the first rock go up along a stream to reinforce it and prevent it from crumbling?

Although nobody knows for certain when the use of riprap–rock or pieces of concrete for channel stabilization–first began, University of Idaho Water Engineer Howard Neibling credits the Army Corps of Engineers with its earliest use in the United States. In the early part of the century, the corps began building log-and-rock jetties on the Missouri River in an attempt to slow the velocity of the river.

The Missouri River might not have been the first river to be riprapped because it took a very long time for people to expand their encroachment onto riverbanks and bluffs. In some areas farmers actually depended on flooding to replenish the soil fertility. But it’s certain that anytime hydrologic action–waves pounding on beach sand, floodwaters rushing a river from a storm–meets with bare soil and sand, erosion is imminent. At times, water on soil has been an advantage: Miners used the forces of water long ago to sluice for gold–albeit at the mountain’s expense–but the practice helped open up the entire Western US. In some Middle Eastern countries water has been used in controlled and precise ways to create some of the most sophisticated canal systems in the world.

As knowledge increased, the use of riprap to help stabilize streams and beaches grew into an area of expertise for engineers to study. Sheer stress, wave action, force, and slope each took its own place in the equation. Poured concrete came onto the scene to replace rocks on some projects, or at least to stabilize them more permanently. The semiliquid mass could seep into places that rocks left void. Rocks, in various sizes and shapes, are still the primary choice for most projects. In 1800s Europe, placing rocks in baskets helped add stability during floods. And finally, in more recent years, engineers and private manufacturers have developed alternatives to rocks and concrete. These techniques are generally referred to as “hard armoring” a bank.

The Gabion Basket

Maccaferri Inc. developed the grandfather to the modern-day gabion basket in Italy in the 1800s. The basket, which is basically zinc-coated wire made into double-twisted hexagonal mesh, got its start as protection along the River Reno. The company has since developed many protective products, including the Reno mattress, which takes its name from its historical beginnings. The double-twist design that Maccaferri incorporates into its products came later, around the turn of the century.

“The Maccaferri Company came to the United States in the 1950s,” explains Doug Kolz of Maccaferri’s Western region. “But we’ve been working on river stabilization for over 100 years all over the world.”

Maccaferri’s standard gabion mats typically are used for channel protection where 12- to 18-inch flexible revetment is needed. The mat is supplied in 99-foot-long rolls that measure 6–9 feet wide. As an alternative to riprap, the mats provide flexibility and reduce the thickness of protection required.

“With the traditional Reno mat it’s 6 to 9 inches thick, but with the high flow applications, such as the flash flooding in the Southwest, we developed a standard gabion mat into a 99-foot mat that comes 9 or 6 feet wide by 1 or 1.5 feet deep,” Kolz states.

He stresses that the gabion mat is not the same as a Reno mat. The Reno mat is designed with 6- x 8-cm openings, while the gabion has 8- x 10-cm mesh. Hence, larger stones are used with gabion rather than the Reno mat.

Confining natural stone or rock and preventing it from moving is the main goal of stabilization, Kolz continues. In the Southwest, Maccaferri has concentrated on projects for flood control. In one unusual operation, the company used approximately 68,000 cubic yards  of material in the Skunk Creek project in Arizona to control flash-flood waters in that area. Maybe it was the 270 days to completion of the project that made it such a milestone operation.

“The problem that we have in the Southwest is the arid climate and flash flooding,” he notes. “But then add the steep gradient, and when a storm event happens, they’re in trouble.”

Storm events in the Southwest will move almost any unprotected rock no matter how or where it was laid down. But confining the rock materials prevents them from moving and becoming harmful to people or dwellings.

In an effort to meet the demands of the environmental engineering community, Maccaferri has been working on development of a newer soil-bioengineering mat that would allow and enable deep-rooted plant communities to survive a major storm event. Kolz emphasizes that without a good base to begin with, any major storm event still can rip up any riprap–with or without vegetation. But with sound soil bioengineering techniques, a base can be developed that will withstand sheer stress. He concludes, “You want to develop a skeleton to integrate the plants on the surface so that even if you have an event, it leaves the base skeleton intact.”

 

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