Feb 02, 2022

Vegetating Extreme Slopes for Erosion Control

While one solution cannot fix all problems, evaluating slopes, looking at available options & putting a plan in place are key steps in vegetation

Cover image, erosion control

Two common questions regarding slope projects are: "Do I have to use riprap on this slope?" and "Can I vegetate the slope instead of installing a block wall?" Most of the time, the answers are a resounding “yes” and “yes.” So, how do you know where and when to use one solution over another? In this article, we will look at evaluating the slope and what options are available, and then, how to put a plan in place, understanding that no one solution can fix all issues.

For the purpose of this article, it is important to clarify the definition of an extreme slope. When evaluating a steep slope, I consider any incline more than 20 feet tall that exceeds a 1:1 slope to be extreme. Additionally, an incline that is 200 feet tall with a 1.5:1 slope would also be considered extreme. The true definition of an extreme slope tends to be subjective, and depends on the site conditions, soil composition, and access to the area in question.

Evaluating the Slope

When evaluating any slope, one needs to look at a few key components. First, consider the global stability of the slope. What does the toe of the incline look like? Is there water seeping through the soil? Are there hydraulic pressure issues? Before considering vegetation, it is crucial to address any global stabilization or drainage issues. Establishing sustainable vegetation on a slope with global stability and/or drainage issues will prove to be a wasted effort.

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I have been on many job sites throughout my career where an extreme slope with beautifully established vegetation ultimately failed (let go and slid down the incline) due to hydraulic pressure underneath the vegetation. For this reason, it is advisable to have a geotechnical engineer look at and evaluate the slope in question to identify these types of issues.

If you have established that no global or drainage issues are present, the next critical step is to evaluate access to the top and bottom of the slope, as minimal access can shrink your solution options. Once you have determined accessibility, you can then start looking at sustainable vegetation solutions (erosion control options) in your "toolbox."

What Options Are Available?

Industry professional Pete Hanrahan used to say, "If the only tool in your toolbox is a hammer, everything starts to look like a nail." That is funny, but also very true. In the past, when considering erosion control options for slopes, the recommendation was riprap, riprap and more riprap. Then rolled erosion control blankets (RECBs) came on the scene, and it became riprap, riprap and now maybe use a RECB.  Since the introduction of RECBs, the industry has continually evolved and created many more tools for an erosion control toolbox. Here are a few of the more common tools:

  • Turf reinforcement mats (TRM)
  • High-performance turf reinforcement mat (HPTRM)
  • Hydraulic erosion control product (HECP) — mulch categories include stabilized mulch matrix (SMM), bonded fiber matrix (BFM) and flexible growth media (FGM)
  • Cellular confinement system (CCS)

When considering how to deal with extreme slopes that have a long run, the best practice is to install slope interruption devices designed to slow the velocity of storm water. Weakening the water flow will allow the best chance to establish sustainable vegetation. One example of a slope interruption device is a straw wattle or compost sock. These are placed along a slope's contour and spaced out per manufacturer recommendations. If you are considering the use of HECP mulches, the HECP manufacturers will have their own specific recommendations for slope interrupters and spacing requirements based on the type of slope and how long the run is.

Considering the global stability of the slope and examining the toe of the incline are two steps to take in the beginning of the process.
Considering the global stability of the slope and examining the toe of the incline are two steps to take in the beginning of the process.

When looking to add topsoil to an extreme slope, consider using a CCS, also known as geocells. These systems are flexible soil stabilization systems that come in different depths, sizes and anchoring options. For extreme slopes, using a pipe deadman anchoring system is one option. This system incorporates a dug trench situated behind the crest of the upper slope and a pipe placed within the trench that has tendons/straps securely attached. The pipe is then buried, and the tendons are run through the entire length of the geocells. The tendons need to be evenly spaced down the entire slope for equal weight distribution of the topsoil. Once the geocells are properly installed and inspected, it is time to infill with topsoil and seed. It is a good idea to use a RECB or use an HECP over the geocell system to contain topsoil on an extreme slope while vegetation is established.

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When considering the topsoil quality, there are various options available. It is highly advisable to always perform a soil test to check that the physical properties, such as water retention and filtration, permeability and aeration provide a suitable environment for establishing vegetation. There may be a situation in which the soils on the slope are not suitable for plant growth. In this case, you can add additional materials to improve the soil quality, known as soil amendments. These added materials can help with germination for sustainable vegetation. Talk to an erosion control specialist and/or manufacturer’s representative to help choose the right options for your soil condition.

Putting a Plan in Place

Enlisting an erosion control specialist or a manufacturer’s representative to visit your site at the beginning of any new project  is recommended. It is also advisable to reach out to the manufacturers you plan to work with. An erosion control specialist or manufacturer’s representative can help assess the site conditions and select the best option for the project. They can work directly with the manufacturer to provide design assistance. Most manufacturers either have design software available to use or can assist you with the design. When requesting assistance, the manufacturers will want to know the height, grade percentage and length of the slope. Often, a soil sample will also be requested. Once the results of the soil test come back, the manufacturer can help make recommendations for any soil amendments to ensure sustainable vegetation can be established. Manufacturers will also provide design details according to their specifications and designs.

Once the project is ready for installation, you should once again request your erosion control specialist or a manufacturer’s representative to be on-site to answer any questions that may come up. This important step can ensure proper installation of the products and provide smoother execution overall.

About the author

Mike Everhart is Eastern erosion control & geo-product specialist for EJ Prescott. Everhart can be reached at mike.everhart@ejprescott.com.

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