Keyword Site Search

Stands Up to an El Niño Winter

by Greg Northcutt

After receiving 210% the normal seasonal rainfall during the winter of 1997/98, soil loss and rilling on this untreated slope were extreme, as was the cleanup required.

Landscape Architects stymied by the challenge of establishing vegetation to control erosion on steep or rough slopes, have an able ally in a type of erosion control product called a bonded fiber matrix. As demonstrated in Southern California during the El Niño winter of 1997-98, this relatively new advance in erosion technology can keep disturbed soil in place despite a season of unusually heavy rainfall. This thick slurry of wood fibers, tackifier and a bonding agent cures to form a thin, three-dimensional porous cover that retains its form and strength when re-wetted. It protects seed and soil from the erosive forces of rain and wind better and longer than hydromulch. Also, it provides a practical alternative for controlling erosion on surfaces too rough or too steep for labor crews to install erosion control blankets properly and safely or where mechanical seeding equipment can't be used.

Usually packaged in a 50-lb. bag, a bonded fiber matrix is mixed in the tank of a hydraulic seeder. When sprayed on disturbed soil, the wood fibers, usually about 1/8 to 1/2 in. long, and soil particles form a matrix. This matrix extends to a depth of about 1/4 to 3/8 in. in the soil and is held together by a tackifier and a special bonding agent, which give the matrix its tensile strength. The bonding agent is the key to the durability and effectiveness of the product in controlling erosion. It ties the long-chain molecules of the bonding compound together. As with paint, this allows the bonded fiber matrix to mix and flow easily when wet and, when dry, to remain strong and insoluble to rain and stormwater runoff. The material retains water and reduces evaporation to enhance seed germination. Growing plants penetrate the material through the small voids in the matrix.

"A bonded fiber matrix can remain effective for an extended period of time, (two growing seasons in semi-arid regions,) until vegetation can be established to provide permanent erosion control," says Bill Agnew, Reveg Environmental Consulting, Lehi, Utah. He is a Certified Professional in Erosion and Sediment Control. This designation identifies individuals who have met rigorous experience and education requirements and have demonstrated expertise in erosion and sediment control planning. "This type of product applies nicely to provide a uniform cover over the soil," he says. "Although lacking the tensile strength of erosion control blankets, lab tests show it can control erosion equally as well or better than single and double netted straw or excelsior erosion control blankets. It can be a good choice when steep slopes or rocks, stumps and other debris prevent the use of erosion control blankets."

Benefits and Limitations

The severe rains caused underrilling with this blanket, resulting in significant soil loss and accumulation at the bottom of the slope.

A bonded fiber matrix conforms to bumps, dips and irregularities in the slope surface as it is sprayed on to maintain intimate contact with the soil, Agnew notes. This prevents tenting, which leaves part of the surface unprotected and prone to erosion. By contrast, properly installing an erosion control blanket to prevent tenting on rough, uneven surfaces is a much more difficult and time-consuming job.

Other advantages of a bonded fiber matrix:

It can be applied on most slopes, including near-vertical surfaces.

It retains moisture and reduces evaporation to enhance plant growth.

Typically, it can control erosion for one growing season in humid climates and two or three years in arid conditions.

It has no netting, which can trap or injure wildlife, and itis non-toxic to fish.

It is biodegradable and decomposes into carbon dioxide, which plants use for photosynthesis, and water.

It eliminates the time and expense of labor crews needed to install erosion control blankets.

It also eliminates the possible loss of seed and fertilizer from foot traffic when installing erosion control blankets.

A bonded fiber matrix is designed to control erosion temporarily until vegetation can become established to provide permanent erosion control. It does not control erosion caused by massive slope failure, surface peeling and frost heaving. This material is not recommended for long-term or permanent erosion control without vegetation. Also, it is not suited for controlling erosion in channels or for preventing erosion from concentrated overland flows of water.

"In terms of price, bonded fiber matrix products are highly competitive with low-to-medium cost erosion control blankets," says Earl Dahlin, P.E., Fiber Marketing International, Renton, Wash. "Depending on type and amount of netting, installed cost of a straw blanket can range from about 75 cents per sq. yd. to about $1.15 per sq. yd. Material and labor costs to install a bonded fiber matrix vary with the rate of application. Typically, a 3,000-lb. per acre treatment costs about 65 cents per sq. yd., while a 4,000-lb. per acre application costs around 90 cents per sq. yd."

How It Is Applied

The Bonded Fibre Matrix "controlled erosion well" under these circumstances, according to Scott Johannes. While there was some rilling, soil loss was minor and no new earthwork was required before development proceeded.

Normally, a bonded fiber matrix is applied at the rate of about 3,000 to 4,100 lb. per acre. The rougher the surface, the more surface area there is to be protected and therefore more product is needed for proper coverage. In humid climates, the bonded fiber matrix material can be mixed and applied with seed, fertilizer and other additives in a one-step operation, Agnew notes. However, he recommends a two-step process when applying the product in arid conditions. This helps ensure good soil-seed contact and prevents seed from remaining suspended in the cured bonded fiber matrix, which could desiccate the seed.

"In arid parts of the country, 200 to 300 lb. of bonded fiber matrix is often applied with the seed and any other additives as a tracer to show the treated areas," Agnew says. "Then, the bonded fiber matrix is applied by itself over the seeding. However, whether or not you apply the bonded fiber matrix in a one-step or a two-step process, it's important that you shoot the product from two opposite directions. This prevents shadowing, in which stones, tree limbs and other obstructions block the spray, leaving an untreated area of soil behind them."

One bonded fiber matrix manufacturer requires spray coverage be at least 1/8-in. thick over the entire surface area, while allowing occasional voids of 1 mm. When the product is applied on 1.5:1 or steeper slopes or cut or fill slopes 20 ft. tall or higher, the slopes must be stabilized as the faces are exposed, as practical. Also, slopes with lateral seeps must be dry before being treated with the product. Depending on air temperatures and humidity, the bonded fiber matrix usually cures in about 4 to 24 hours, generally 4 to 8 hours. Normally, the product can also set up to protect a new seeding even when applied at near-freezing temperatures.

The Costs of Failure

This slope from Bonita Canyon in Irvine, Calif. was treated with grass seed and EcoAegis Bonded Fiber Matrix in October 1997. As this July 2000 photo shows, the only treatment required after the massive rains in 97/98 was installation of groundcover plants. At the bottom of the slope new work is being done during road development.

The winter of 1997-98 gave Landscape Architects and specifiers in Southern California the chance to see how a bonded fiber matrix product can perform in the face of unusually heavy rains generated by El Niño. The stakes were high. Sediment washed from construction sites can pollute streams and lakes, harming the quality of drinking water. It can build up in rivers and reservoirs to impede boat traffic and reduce storage capacity. Sediment can also settle to the bottom of streams and lakes destroying fish habitat. Polluting water with sediment can also have some direct financial consequences.

Any developer or other property owner in the state must have a permit from the California Water Quality Control Board to disturb five or more acres of land. That permit requires the owner to control erosion and to keep any disturbed sediment on site. Otherwise, the owner faces some hefty fines-$10,000 per day of violation. If sediment from a construction site pollutes a stream for three days, the maximum fine would be $30,000, or $10 per gallon for all polluted water in excess of 1,000 gal. that leaves a site. These fines are in addition to any penalties local authorities assess for allowing sediment to wash or blow off site.

Field Results

At Serenata, in Irvine, Calif., the slopes treated with the bonded fiber matrix showed no erosion after the rains, but the untreated flat areas and roadways required significant labor to control run off and rework to bring the areas back to building standards.

In the fall of 1997, many developers in Southern California heeded the forecasts and took steps to protect bare soils at construction sites from El Niño. Their efforts proved prudent as a series of storms, spawned by El Niño, dumped heavier-than- normal rain throughout the region that winter. Successful erosion control practices included several bonded fiber matrix projects.

In the untreated sections at Serenata, even flat areas suffered significant erosion and had to be regraded.

• Site 1

One of these projects was installed at Serenata, a residential development in Carlsbad. In November 1997, about two acres of disturbed, very erosive soils were treated with EcoAegis, a bonded fiber matrix, to protect them from erosion over the winter until construction resumed in the spring. The 2:1 or slightly steeper slopes varied in length from about 100 to 150 ft.

The bonded fiber matrix was applied at the rate of 3,500 to 4,000 lb. per acre with 50 lb. per acre of Plantago insilarus seed. The plantago was used as a temporary measure to bolster the protection provided by bonded fiber matrix until the plants with their soil-holding roots died out after about three months.

The slopes treated with the bonded fiber matrix held up well throughout the rainy winter as evidenced by little, if any, sediment at the bottom of the slopes as spring weather returned. By contrast, nearby untreated slopes eroded causing such problems as deep rills, failed silt fences and mud slides, which had to be repaired.

• Site 2

Mike Sullivan, a Landscape Architect with SLA Studio Land, Costa Mesa, Calif., prepared an erosion control plan to defend about 10 acres of bare slopes against El Niño at the Bonita Canyon residential development project in Newport Beach, Calif. "The cost of using erosion control blankets to protect the entire site was too expensive," Sullivan reports. "So we did a cost comparison of other alternatives and used a combination of erosion control materials."

His plan was based on a soil engineer's drawings that showed areas of three distinct types of soil on the mostly 2:1 slopes. Sullivan specified a straw-coconut erosion control blanket to protect soils considered highly erodible; EcoAegis, the bonded fiber matrix, to control the erodible soils; and a hydraulically-applied gypsum-based product to treat slopes considered least likely to erode.

To control erosion on the erodible soils, a 20-lb. per acre mixture of two annual grasses - California brome and small fescue - plus 200-lb. per acre of starter fertilizer and 500-lb. per acre of EcoFibre a wood-fiber mulch were applied in October 1997. Then, EcoAegis, a bonded fiber matrix, was sprayed over the seeding at the rate of 3,400 lb. per acre.

The following February, the hydraulic seeding contractor re-applied the gypsum cementitious binder to about five previously treated acres and re-sprayed the bonded fiber matrix on about 1.5 of the acres originally treated with that product. By April, the area where the erosion control blankets were installed in the fall had experienced erosion and required some remedial repair. Parts of the area treated with the gypsum product had also eroded. Meanwhile, the slopes sprayed with the bonded fiber matrix showed little erosion and the annual grasses had become established.

• Site 3

At Serenata, in Irvine, California, the slopes treated with the bonded fiber matrix showed no erosion after the rains, but the untreated flat areas and roadways required significant labor to control run off and rework to bring the areas back to building standards. The Bonded Fiber Matrix controlled erosion well.

A bonded fiber matrix also controlled erosion successfully during the El Niño winter of 1997-98 at a large industrial site in Moorpark, Calif. About 10 acres of slopes had been exposed by grading and excavation work at the 50-acre SDI site. The clay and sandy loam slopes were as steep as 2:1. In November 1997, 3,500 lb. per acre of EcoAegis were sprayed on these cut slopes along with native grass and broadleaf seed. The slopes survived the winter of severe rainstorms with very little erosion, and required no repairs in the spring. The seed applied during treatment germinated successfully and no further revegetation efforts were necessary.

A Way to Meet Tougher Requirements

The owners of this industrial site in Moorpark, Calif. wanted the perimeter returned to as natural a condition as possible after excavation. In the fall of 1997, these cut slopes were hydroseeded with EcoAegis and native grass and broadleaf seed, prior to the rains expected during the El Niño event.

The use of bonded fiber matrix and other erosion control products is expected to increase now that the U.S. Environmental Protection Agency has issued the final Phase II rule of the National Pollutant Discharge Elimination System (NPDES). Under the Phase I rule, issued in 1990, municipalities with populations of more than 100,000 must have stormwater management programs that include controlling erosion and sediment at construction sites that disturb more than 5 acres of land. The Phase II rule, which becomes effective in March, 2003, extends this and other requirements to municipalities with populations of 10,000 to 100,000 and to construction activities that disturb more than one acre. As many as 6,000 or so communities throughout the country could be affected.

As the three California projects demonstrate, a bonded fiber matrix can be an effective tool for controlling erosion on slopes under very difficult conditions. This type of product can reduce stormwater runoff, save soil and seed and help boost seed germination rates. It is more effective than a standard hydraulic application of seed and mulch. It offers similar, if not better, performance to some types of erosion control blankets. What's more, a bonded fiber matrix can be applied much faster and easier and without the special site preparation and labor required to install erosion control blankets.

The treatment protected the slopes from the subsequent severe rains, including one episode where 7 inches of rain was recorded in one hour. Soil erosion was minimal, and germination of the native plants excellent. No further work was required on this site.

For Landscape Architects and specifiers facing steep, rough-surfaced slopes, where access by labor crews or equipment is restricted or when protection from erosion is needed in a hurry, a bonded fiber matrix can offer a cost-effective option for controlling erosion. LASN

Search Site by Story Keywords

Related Stories

June 26, 2019, 11:55 am PDT

Website problems, report a bug.
Copyright © 2019 Landscape Communications Inc.
Privacy Policy