Mats, Concrete, Blocks, and Rocks: The Lowdown on Riprap

July 1, 2002

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-in. flexible revetment is needed. The mat is supplied in 99-ft.-long rolls that measure 6-9 ft. 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 yd.3 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.”

For Hard Armor, Caltrans Has Specs

In California, the Department of Public Transportation (Caltrans) sets specifications for using hard armor. Rock used for slope protection should exhibit stability, durability, and consistency. It should meet the specifications of the Army Corps of Engineers or Caltrans, with the supplier having no rejected loads due to any inferior materials. The other specification, of course, is availability. To be a good selection, the material always should be locally available when it’s needed.

If the project engineer decides to use rock rather than other riprap alternatives, a good rock for stabilization projects is either crushed stone or angular rock. Angular or irregular edges offer a greater chance of the rock later providing interlocking qualities. Once the rocks settle and interlock, soil can be trapped and actually help stabilize the structure. Round stones or cobblestones are not recommended for erosion control. Round stones allow a higher degree of slippage when wet and tend to wash out on slopes greater than 1:2, allowing degradation of the entire project.

Coordinating public agencies in California specify that riprap meet Section 72 of the Caltrans Standard Specifications. The standards set limits for size or gradation, apparent specific gravity, and absorption and durability index. The appropriate riprap materials show test results that meet these standards. Caltrans identifies 11 different sizes: 8-ton, 4-ton, 2-ton, 1-ton, _-ton, Light, and No. 1, No. 2, or No. 3 backing. Additionally, two specific placement methods are specified, depending on size and placement mechanism. Basically one method requires setting individual stones (usually large boulders), while the other allows for dumping or dozing the material.

Alternatives to Riprap

As with many other commodities, the affordability of rock is based on availability of rock. For many states, a rock quarry is inaccessible, making the use of rock on projects cost-prohibitive. Some of those same states might have rock available, but it is of poor quality and cannot be considered for use as hard-armor revetment.

For this and other reasons having to do with performance, aesthetics, and life cycle cost benefit, other forms of hard armor serve as alternatives in today’s arsenal of engineered revetment solutions. Two such alternatives, ArmorFlex articulating concrete block mats (ACBs) and A-Jacks armor units, have gained widespread acceptance within the engineering community. Armortec, headquartered in Bowling Green, KY, markets both products.

ArmorFlex fabricated mattress is installed by lifting the system into position by means of a spreader bar. Thousands of square feet of product can be placed in a short time. Popular ACB uses are open-flow channels, shoreline protection, dam overtopping, and pipeline protection as well as drainage ditches and reservoir slope protection. The mats can be seeded and vegetated to provide a pleasing “green” hard-armor solution that many property owners, state and federal agencies, and engineers favor. Equally important, ArmorFlex needs no maintenance, which characteristically has been the Achilles heel of rock riprap.

A-Jacks provide an alternative to riprap where streambank stabilization is needed, especially for urban stormwater runoff and other high velocity flows. A-Jacks units have a two-part component design that interlocks into a highly permeable matrix and serves to stabilize the toe of streams; bioengineering techniques can be used on the remaining slope. A-Jacks also are used on coastal breakwaters and jetties instead of rock as wave attack protection.

Rock Riprap in Action: Projects Across the US

Once the environmental engineers have decided which types of reinforcement to use, the job of purchasing, moving, and placement begins. In the case of Kinkaid Lake in Illinois, moving tons of rock to the eroding beach was only one of the challenges, reports Scott Martin, the district conservationist with the Natural Resources Conservation Service (NRCS) there. After trying vegetative plantings, only to have beavers chew them down, Martin says the NRCS decided to use rock riprap to build up a berm around the area.

The US Forest Service and NRCS, along with the Kinkaid Lake Conservancy District, own the land. The managing agencies teamed up to provide various grants and funded the project. Recommendations for rocks weighing between 50 and 150 lb. each came from a streambank specialist working with the NRCS. The challenge not only was to transport the rocks but also to find the equipment to place them, relates Martin.

“Other alternatives, such as the baskets, were more expensive,” he points out. “You have to put them in place and then they are still filled with rock.”

The project team decided to use quarried rock that came out with irregular shapes and provided for the desired effect of interlocking once the pieces settled in. In addition, says Martin, they used a 12-in. piece of mat material under the rocks to prevent them from sinking into the sand and moving to the water’s edge. The rocks were floated out to the beach area along with a trackhoe to help move the rocks into place. The riprap was placed 20 ft. from land to create a moat affect.

“The land slopes are loess soils and stand up vertically,” Martin describes. “The pounding action of the waves eats at the toe of the undercut. Eventually the slope will fall in and create about a two-to-one slope area, and on the inside there it creates a little ecosystem of its own. There’s already a lot of frogs in there.”

The project at Kinkaid Lake used approximately 1.3 tons of riprap per foot and stabilized more than 1,500 ft. of shoreline. The project was funded by an $80,000 grant from the Conservation 2000 Funds, which is administered by the Illinois Department of Natural Resources. The Kinkaid Reed’s Creek Conservancy District and the Kinkaid Area Watershed Project donated an additional $4,000 to the project. The project was sponsored by the Jackson County Soil and Water Conservation District and the Shawnee Resource Conservation and Development Area. The US Department of Agriculture and NRCA provided the technical assistance.

Idaho’s Rural Use of Riprap

In rural areas, including parts of Idaho, rock materials are readily available from quarries, farms, and construction sites. In the changing landscape of the rural and urban interface, used concrete continually is being torn out and recycled in landscapes. Rock is used, often with willow and cattail plantings, to stabilize streambanks and create wildlife habitat ponds.

As part of the Middle Snake River Nutrient Management Plan in Twin Falls, ID, the University of Idaho and Agricultural Research Service (ARS) teamed up with the Twin Falls and North Side Canal Companies to reduce erosion sediment inflow into the Snake River. As part of a five-year plan, the Twin Falls Canal Company identified several areas that were severely eroding.

The two major drains identified for stabilization were the LQ and LS drains. Both of these drains are major return flow streams to the Snake River. Between the two, they return flows from over 7,000 irrigated ac. in the Magic Valley area back to the Snake River. Moderate-to-steep slopes on highly erosive silt loam soils characterize the drainage areas for these two streams.

The two drains have a long history of being major contributors to sediment and phosphorus in the river. The LQ drain alone deposited 9,300 tons of sediment to the Snake in 1977, according to a study by the University of Idaho and ARS. The study also determined that the LS drain discharged 12,000 tons of sediment during the irrigation season in 1980. Although a project in the 1980s to reduce the load by on-farm and large pond systems was successful, the practices quickly were abandoned after the end of the official project. The result was the return to preproject levels of sediment being deposited to the Snake River. By the 1995 irrigation season, some 9,250 tons of sediment were being returned to the river.

“Right now that number is probably about half,” states Brian Olmstead, Twin Falls Canal Company manager. “We’ve already removed a lot of that.”

A pond, known as Brennan pond, was dug about four years ago as part of the overall project. Now a mature pond, it presents large cattail, willows, and other native grasses along the shores. The tons of riprap used to stabilize the edges are only slightly visible. The overflow area, made up of various sizes of rocks, is intact and provides a smooth transition for water to spill over to the lower land area.

The Malone pond project on the lower, overflow land is a 17-ac. project that includes an 8-ac. sediment-settling pond. As part of building and reinforcing the pond, Olmstead says approximately 300 tons of riprap will be layered this year.

“In the overflow area we use alternating layers of rock and gravel,” he says, describing the rocky spillways. “It also keeps the rodents and small animals out. But we’ve never had a rock overflow fail.”

After the addition of biological plantings and some time, the Malone pond will mature into wildlife habitat for waterfowl. Olmstead says that up the Snake River Canyon another 10 mi. or so, in a place known as Pigeon Cove, the plan is to restore a former commercial fishpond for sport fishing.

Although the Brennan and Malone ponds use riprap in conjunction with biological remediation, there are times when it is necessary to use riprap alone. Olmstead refers to the Milner Dam in Murtaugh, ID. The dam, built in the early 1900s and rebuilt in 1992, must be inspected and renewed each year by the Federal Energy Regulatory Commission and the Canal Company.

“Milner Dam is a good example of an earth-filled dam,” Olmstead notes. “And out there you wouldn’t want to have anything biological with roots growing in because each year we have to draw the water down to inspect it.”

The Trend Toward “Soft” Techniques

Although there are a few examples of times when hard armor is used exclusively, such as the dam or in some urban flood remediation projects in the Southwest, many researchers maintain that “softer” is better. The Idaho Fish and Game natural resource biologist, Chuck Warren, defers to a study and literature review by David A. Schmetterling, Christopher G. Clancy, and Troy M. Brandt of Montana Fish, Wildlife, and Parks. Brandt is a fisheries biologist for Water Consulting in Hamilton, MT. The study examined the effects of riprap on stream salmonids in the Western US.

The authors are adamant in their belief that riprapping streambanks is responsible for fish habitat loss and degradation. They point to woody vegetation and dense root systems as a more viable means for sustaining bank integrity. By increasing bank roughness, they say, the stream energy is dissipated during peak flows. An alternative to hard armor in low-gradient streams might be using sod mats, while higher-gradient streams are dependent upon the woody vegetation and root systems.

In essence, the use of more natural material, such as trees and rootwads, provides a good alternative and is proving to be of greater benefit to the ecosystem than rock riprap is. Streams, being the dynamic and living systems that they are, need a continual sloughing of organic matter to keep them healthy and alive. The “soft” techniques, as they are referred to, aim to slow the erosion rather than stop it completely. These techniques are, however, not as researched and proven as riprap and therefore are more risky to landowners. To some degree they might be more labor-intensive and expensive and take longer to provide the stabilization protection desired. 

Whether to use riprap alone, to use it with vegetation, or to use only soft techniques might depend entirely upon where it’s being used and its intended use in the future. The study does acknowledge that there are times that large, angular rocks were introduced on severely degraded rivers and habitat quality was seen to improve because “large substrates can allow for fish utilization of interstices.” Additionally, the fish specialists stress that the impacts of riprap on stream geomorphology, riparian conditions, and large, woody debris are areas that need more assessment and study.

One of the most important conclusions of the study is that property developers should be discouraged from using floodplain properties and that development of management plans that incorporate the “unpredictability of rivers and streams is essential where development already exists.”

In conclusion, the three summed up the entire dilemma by saying, “Resource managers will continue to struggle with the paradox of allowing natural fluvial processes and protecting private property and public infrastructure from those same processes.”

About the Author

Linda Robinson

Linda Robinson is a journalist specializing in agriculture and land-use planning.