Erosion control blankets (ECBs) and turf reinforcement mats (TRMs) are also referred to as rolled erosion control products (RECPs). They are made from a variety of materials, such as wood fiber, straw fiber, coconut fiber (coir) and polypropylene. Pictured here is a Core of Engineers levee project near New Orleans using high-performance TRMs made of high-tensile poly threads woven into a 3-D mat.
All Photos courtesy of Tensar Intl. Corp.

Controlling soil erosion and storm water runoff is an issue that has steadily received more attention due to a number of factors. These include litigations and fines for poor erosion control plans, landowners' desires to leave greener footprints, and technological advances such as soil bioengineering using erosion control blankets (ECBs) and turf replacement mats (TRMs).

For years, these rolled erosion control products (RECPs) have been favorite tools of landscape professionals when called in to protect a slope from runoff. ECBs and TRMs are relied on to provide erosion protection during the period of time it takes for plants to grow and stabilize a slope.

Material installation can be the weak point of a project. Even if the best materials are used, it is very important to have secure staking, adequate fabric overlap, key trenching, skilled construction supervision and aggressive revegetation plans.

The Tools
One major distinction of erosion control blankets is whether they are degradable or not. The degradable versions can be biodegradable or photodegradable, which depend on sunlight to decompose. Biodegradable blankets are made of a number of materials, which then decompose at different rates. Wood fiber blankets are credited with being the original ECBs, as pioneered by American Excelsior in the 1960s. They have a functional longevity ranging from 12 to 36 months depending on a region's climatic condition, as is the case with all ECBs.

Straw blankets contain straw fibers stitched to netting. They have a maximum functional longevity of 12 months. Blankets made from coconut fibers have a functional longevity of up to five years. And there are blankets that have a straw and coconut fiber blend: usually 70 percent straw fibers and 30 percent coconut fibers by weight. One thing to consider about this type is that even though it might be given a functional longevity of 24 months, the straw in it is generally rated as having a functional longevity of a year so only 30 percent of the blanket remains functional for the second 12 months of the product's life.

All of the options have advantages and disadvantages that must be weighed based on the nature of a given project. For instance, there are some blankets that are designed for rapid degrading: under two months. Considerations that might not be readily apparent include how wildlife-friendly the product is and how easy it is to add plants to it without compromising erosion control performance.

Nondegradable blankets are available for special purposes such as very steep slopes where reinforcing capabilities are needed permanently because the resulting vegetation alone will not be able to hold up to the force of the water flow. These types of blankets can be made of polypropylene and are referred to as turf reinforcement mats (TRMs).

Most of the time, the blankets and mats do not contain seed because matching the proper seed mixture to a site is a very important component of successful erosion control. Local landscapers are the best judges of which seeds to use in any given situation. There are however "flower carpets" that have seed embedded in them and, depending on the seed mix, could provide a less demanding alternative.

All ECBs are designed to be used within certain slope classes and as such are given a maximum slope rating.

This slope in the Southeast U.S. was stabilized with ECBs made from coir with a 3-D poly netting. As with most RECPs, the blankets were not pre-seeded. In most instances, local landscapers are relied on to choose the seed mix. These seeds were planted in the spring and this picture is from late summer. ECBs vary in price from about 50 cents to $2 per square yard. TRMs range from about $3 to $8 per square yard.

Installation
To prepare the site, the area of installation should be graded, shaped and properly compacted to remove any existing rills. It also should be free of obstructions such as tree roots, projecting stones, and other foreign objects.

In most cases, the next step is to prepare the seedbed since it is usually recommended that seeding be done prior to installation of the blankets. However, for TRMs and open weave matting, seeding is often done after installation.

Applying lime and mixed grade fertilizer is suggested prior to seeding. The bed should be level and smooth so that the blankets are in direct contact with the soil.

Straw or hay mulch may be added after seeding. All check slots and other areas disturbed during the installation process should be re-seeded.

ECBs and TRMs are, for the most part, easy to unroll. If more than one width is required, you can overlap or abut the edges together depending on the manufacturers' guidelines.

Trenching may be required. One manufacturer recommends that the blankets be trenched at the head of the slope if the blanket cannot be extended three feet over the slope crest or if overland flow is anticipated from upslope areas.

Secure staking is key. Steel wire staples. U-shaped metal staples (11-gauge minimum, six to 12 inches long) or wooden stakes at least 12 inches long can be used to anchor the blankets to the ground. A minimum distance of two inches from the edge of the blanket to the center of the staples or stakes is recommended. Follow each product's guidelines as to how to place, how many to place, and where to place the staples.

Check slots should be placed at 25 to 40 foot intervals by installing a row of staples four inches apart along the entire width of the slope. A second row should be placed four inches below in a staggered pattern.

You can also use biodegradable anchoring staples. They are often comprised of a plastic that is made from plant sugars and oils. They will degrade completely in eight to 24 months.

Blankets and mats should be rolled out flat, even and smooth without stretching the material. It is recommended that they be installed vertically on the slope, however on short slopes, it may be more practical to install horizontally.

Added Info
To help secure the success of a soil bioengineering project that incorporates ECBs or TRMs, the revegetation plans should be aggressive enough to ensure that plant roots and shoots will sufficiently replace the structural function of the fabrics as they weaken.

An extra benefit of biodegradable blankets and mats is that as they decompose, they provide nutrients to the soil. Conversely, some feel that as photodegradable ECB's decompose, they add harmful elements to the soil.

ECBs do not necessarily have to be removed before mowing. For the initial mowing, it's recommended that the blades be set at the highest level.

Installing erosion control blankets and mats are an important service that landscapers can provide. Before you can recommend their use, it is valuable to know how to identify situations that call for their use by learning about different types of: slopes, slope activities, sources of instability and erosion processes. The International Erosion Control Association offers various training courses on these subjects, which can be accessed from their website.