In nature, soil, if left undisturbed, will erode at the same rate as soil profiles are being formed from parent material. The fear that man will, indeed, leave a desert in his foot steps is a very real one. Photo by George Schmok.

When soil erosion is so severe that plants and animals can no longer exist, the land is said to have desertified. Desertification, which is as old as civilization itself and which must be regarded as the world’s number one problem, has received increasing attention since the United Nations held a conference devoted entirely to this topic in Nairobi, Kenya in 1977. From this conference we learned that desertification annually costs the world 16 billion dollars in lost agricultural production, and could reduce the arable land by one-third by the year 2000, at which time world food needs will have doubled.

Rill erosion and gullies at a housing development on a surface that was not properly prepared. Photo by GS.

Slope erosion by water is usually classified in terms of increasing magnitude: sheet erosion that on the smallest scale results primarily from rainfall or sprinkler water impact and is relatively uniform over the surface. Rill erosion occurs where runoff has concentrated and gained enough force to detach soil particles. Rill patterns develop due to topographic variations that affect flow patterns of runoff. The depth of the rills may be several inches, representing considerable loss of soil over the entire slope. Continued erosion generates larger features in the slope called gullies. Steep slopes may also exhibit surface instability in the from of shallow sloughs.

This is an example of a steep slope application of North American Green SC150 Straw/ coconut blanket. This is but one of the many kinds of short term erosion control/slope stabilization products currently available. Photo courtesy of North American Green.

Erosion is accelerated in urbanized watersheds due to man’s changes to the environment. These changes upset the system’s delicate balance and set into motion processes that are difficult to reverse. Erosion is not only a problem in urbanized areas. A 1986 Worldwatch study on soil erosion reports that soil erosion now threatens the productivity of almost half of the world’s farmland. As demand for food climbs, the world is beginning to mine its soils, converting a renewable resource into a nonrenewable one. An estimated 25 billion tons of topsoil is, in effect, disappearing each year.

Proper slope management using woody type plant material with a complex root system. Photo by GS.

Fortunately, a wide variety of erosion control approaches and methods are available today to help mitigate these impacts. From an environmental standpoint, it is always preferable to use the restoration of vegetation as a primary approach. However, when the changes have resulted in high velocity concentrated flow, some sort of structural method may some sort of structural method may have to be used. A variety of structural methods utilize both natural and manufactured products, and can often be combined with vegetation to provide a more visually attractive and environmentally compatible result.

Another form of slope stabilization in the development stage. Soil Seal tackifier is used to stabilize the slope until vegetation is established. Photo by GS.

Permanent vegetative stabilization should be long-lived and require little maintenance. A diverse plant assemblage that reflects the environmental characteristics of the site will perform best whether maintained or not; the diversity enhances the success of the vegetation by reflecting natural processes, providing overall stability and displacing alien weeds. A complex root system encourages the formation of a water-stable soil structure. A varied canopy and ground cover complex provides a multi-layered protection of the ground from wind and raindrop impact. The selection of plant types should be based on specific site conditions and level of maintenance foreseen.

Once the soil is stabilized, vegetation is established that will make the hill water stable. Photo by GS.

A positive form of site alteration is to place six-inch silt fences on large, steep cut slopes. Plant development is often poor on these sites because the absence of soil and vegetation allows runoff and erosion to minimize regrowth of vegetation. The silt fences will slow runoff and trap sediment, allowing plants to develop behind them.

An unnamed channel at the perimeter of a new housing development before Maccaferri Gabions are installed. Photo courtesy of Maccaferri Gabions.

Slope length and gradient are major factors affecting erosion. Although flattened slopes expose more surface area to be eroded, their erosiveness is lessened and both the construction of erosion protection measures and the long-term effectiveness of revegetation is enhanced.

The same channel in the previous photo after gabion erosion control structures are installed. Photo courtesy of Maccaferri Gabions.

The roots of vegetation may not penetrate highly compacted fill soils of low moisture content. Whenever a fill volume is not designed to support a structure and settlement is not a concern, the degree of compaction on the outer one to three feet of soil should be reduced. Even a reduction from 90 to 85 percent in the outer one to two feet of a non-structural fill makes a difference. The deeper root penetration which results, provides soil reinforcement and protection for the soil layer, and a means of dewatering the fill mass by evapotranspiration.

Polymers, such as Terra-Sorb, pictured here, help absorb and hold moisture up to 100 times its own weight in water. Photo courtesy of Industrial Services International, Inc.

For more severe conditions, gravel or cobbles can provide long-term protection against erosion and allow infiltration of rainfall for vegetative growth. Erodible soil on steep slopes may also be stabilized with building blocks (block pavers) or other structural material, some of which have openings allowing vegetative growth. Severe conditions may also be treated with an erosion control reinforcement matting, or blanket, which is used with vegetation to provide a higher degree of protection than vegetation alone.

Enkamat, a product of Akzo Geomatrix Systems is used on this slope for permanent stabilization. These types of products cover a wide range of applications that would previously have been dealt with through much costlier methods. Photo courtesy of Akzo Geomatrix Systems.

Where concentrated flow velocities are too high for non-structural methods, a more substantial treatment is needed. Some methods available for protection of slopes subject to high-velocity flow are riprap, gabion mattresses, or interlocking concrete blocks, all underlain by filter fabric. Riprap protection consists of a blanket of large rocks with sufficient size and weight to withstand anticipated water velocities. Gabbion mattresses are, relatively thin (six to eight inches) wire mattresses filled rock. Interlocking, concrete blocks are relatively thin precast concrete blocks that interlock into a continuous mat. As with gabions and riprap, this system is both flexible and permeable.

Coastline erosion is a problem around developed areas and can threaten homes if not held in check.

According to John Luigi of Maccaferri Gabions, “Gabion structures can be used in a variety of erosion control applications. They can be used to form monolithic protection which can withstand earth thrust, eliminate underground water and be unaffected by frost heave and ice action. They can also be used as the most technical and economical solutions to problems such as channel lining, river training, retaining structures, marina and seashore protection, bridges and culvert protection, water front restoration and reclamation projects and many applications in landscape construction. Gabion structures are flexible, permeable and durable. Their efficiency increases instead of decreases with age. Silt will collect within the void and vegetation will grow, further solidifying the structure.”

Placing Coquina rock directly on 8-ounce Phillips Fibers Corporation Supac .Some of the boulders weigh up to 5,000 pounds. Photo courtesy of Phillips Fibers Corp.

Another method of erosion control are panel, or geomatrix systems which can be anchored into the earth and filled with rock to prevent soil erosion. Armater, a product put out by Geomatrix Systems, a division of Akzo, is a three dimensional, semi-rigid geomatrix made of non-woven, polyester fabric. The hexagons that make up Armater’s honeycomb design are four inches deep and have eight inch sides. After it has been placed on a surface, it can be filled with native soils, sand, pea gravel, or other aggregate. Surface water flow is impeded and slowed by the raised edges of the hexagons. Other systems produced by Geomatrix Systems work on the same principle, with different structural design. These product applications are wide and may be used in a variety of situations. (See related story, Erosion Control Products: Making The Right Choice)

A section of the revetment. While presenting a formidable barrier to the sea, it permits water to drain through the geotextile, leaving precious sand. Photo courtesy of Phillips Fibers Corp.