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July 2011 » Features » PROGRESSIVE ENGINEERING
Erosion Control Technology Council develops a new specification for hydraulically applied erosion control products.
Laurie Honnigford
The Erosion Control Technology Council defines a hydraulically applied erosion control product — HECP — as "a manufactured, temporary, degradable, pre-packaged, fibrous material that is mixed with water and hydraulically applied as a slurry designed to reduce soil erosion and assist in the establishment and growth of vegetation."

Recent natural disasters have reminded us that Mother Nature is one tough lady. She also knows best how to protect herself. Few could dispute that when it comes to holding soil in place, vegetation does the job better than almost any other method.

However, the very nature of construction and building changes that equation and can leave soil surfaces bare and subject to soil erosion. Exposed topsoil at construction sites easily can be washed or blown away if nothing is holding it down. Further, large paved surfaces can collect massive amounts of water and send it rushing downhill rather than allowing the water to infiltrate over a broad expanse of the ground where it lands. The rushing water gathers sediment and can scour downhill lands. Creative entrepreneurs have come up with numerous solutions for helping Mother Nature until she can do the job on her own. One such solution is hydraulically applied erosion control products (HECPs).

HECPs have become an important tool for controlling soil erosion and establishing vegetation. The first applications consisted of basic mulches and water mixed with seed and sprayed on soil surfaces. Next, color was added to the slurry to trace the areas where the seed had been placed. The next advancement was the use of sprayed mulch and seed with rolled erosion control blankets placed over them. In the 1990s, engineers and contractors were looking for ways to control erosion on slopes that equipment and workers could not access to install rolled erosion control blankets. Since then, engineers and manufacturers have been improving the component formulations and the base fibers to enhance the effectiveness of these products.

What has evolved is a full array of product types appropriate for a variety of applications and situations. HECPs now comprise a broad range of products. The Erosion Control Technology Council (ECTC) defines an HECP as "a manufactured, temporary, degradable, pre-packaged, fibrous material that is mixed with water and hydraulically applied as a slurry designed to reduce soil erosion and assist in the establishment and growth of vegetation."

Because of increased use of HECPs, the ECTC has developed a specification and guidance document to assist users in selecting, designing, and installing HECPs. This specification is the product of consensus among leading manufacturers of HECPs in the industry, and is designed to assist engineers, designers, hydroseeding contractors, and other end-users of these products with their proper use and installation.

ECTC divided HECPs into five classifications based on their "functional longevity" — the length of time needed for the HECP to work in order for vegetation to be established. ECTC defines functional longevity to be "the estimated time period that a material can be anticipated to provide erosion or sediment control." Once vegetation is established, the job of the HECP is complete and vegetation will do the job of holding soil in place. The functional longevity of a product varies within each micro-climate depending on soil microbial activity, moisture, and sunlight.

The five categories of HECP functional longevity are (times listed are general in nature):

  • Ultra short term:≤ 1 month
  • Short term: ≤ 2 months
  • Moderate term: ≤ 3 months
  • Extended term: ≤ 6 months
  • Long term: ≤ 12 months
The Texas Transportation Institute test beds are sediment-control tests, 30 feet long, 5 feet wide, and 0.7 feet deep. An erosion control product manufacturer can elect to test on a 2:1 or 3:1 slope gradient.

To the untrained eye, the materials exiting the hydraulic mulching equipment appear commonplace. However, contained within these slurries are a growing family of refined fiber matrices, tackifiers, super-absorbents, flocculating agents, man-made fibers, plant biostimulants, and other performance-enhancing additives. In addition to providing different functional longevities, the higher-performance products can be used on steeper and longer slopes.

Table 1 lists the properties for each category. The driving force behind the development of this standard was to move away from compositional-based categories and move to performance-based categories that are shaped by functional longevity, severity and length of slope that can be protected, and the ability to establish vegetation. It is important for users to note that the information presented in the table is a general guideline only. HECP manufacturers can provide specific guidance and information for application rates, instructions, gradients, maximum continuous slope lengths, and other site-specific recommendations.

HECP test methods
ASTM is working to develop a large-scale test method for HECPs. In the interim, there are several methods being used to generate coverage factors (C-Factors) and other values. Each test has merits and limitations, so users of the ECTC specification should evaluate the method used to derive the C-Factors.

Sediment retention fiber rolls or other slope-interruption devices can be installed perpendicular to the water flow to reduce slope length in accordance with Table 1.

As the state-of-the-practice continues to evolve, it is likely that the industry will settle on one method in order to have an equitable comparison of values. Until that occurs, users need to understand the basic methods, the advantages they have, and the limitations that occur. Information presented below is general in nature, and users would be well served to contact the laboratory to get a better understanding of variations in testing. ECTC will continue to develop this standard and refine it as more standardized tests for HECPs become available from ASTM.

Currently, there is not an ASTM-approved HECP large-scale test that will provide C-Factor data. The rolled erosion control products standard ASTM D6459 has been modified and used to test HECPs for large-scale testing. C-Factors have been generated by the modified ASTM D6459 and the test has been used by AASHTO's National Transportation Product Evaluation Program (NTPEP) for HECPs.

The ASTM D6459 method is characterized by a 40-foot slope; 3:1 gradient; and 2-, 4-, and 6-inch-per-hour rainfall events. The 40-foot slope allows the testing agency to see the cumulative effects of erosion over a long slope length. This method currently is tested outdoors where variations in weather can affect the values generated. Test laboratories have created barriers to limit the impact of weather (i.e., wind, humidity, et cetera) on the test, but these variations still are introduced into the test.

There are several other large-scale test methods that have been used by the California Department of Transportation (Caltrans) and others to develop C-Factors. While test methods, parameters, soil types, and rainfall intensities may differ from lab to lab, it is important to remember that the results can be used to compare the relative performance of a particular product to that of the bare soil type used at the lab. The Caltrans method has been conducted at San Diego State University. This method is characterized by a plot approximately 9.8 feet wide by 34.8 feet long, and the test bed can be raised to provide a gradient up to 2:1 slope. The soil on the plot is 12 inches thick placed on a porous, open-grid system for drainage. The slope can be changed in the laboratory to test various gradients to see how products can work. The testing at San Diego State University is conducted inside and, as such, weather factors do not impact results.

Utah State University also has a method for testing HECPs. The Utah State method is characterized by a 4-foot-wide by 19.5-foot-long slope, as great as a 1:5 to 1:0 gradient, and rain intensity variable over a wide range. Most of the HECP testing is conducted on a 2.5:1 slope with a 5-inch-per-hour rain event. The soil on the plot is 12 inches deep with drainage under the soil. Researchers measure the water that comes off the bed and the water that drains through the bed and report them separately for each plot. Some argue that the drainage layer takes the testing one step away from actual field conditions. However, given the limited plot size and the rainfall intensity, the drainage layer allows the test to continue through higher intensities before failure. The Utah State testing is done indoors and, as such, weather does not impact test results. The shorter slope length doesn't give the end user the longer length to evaluate the cumulative effects of erosion. Some contend that the rain drops are much larger than typically found during rain events; others in the industry counter that larger rain drops make the test more severe.

Both the Texas Transportation Institute (TTI) and Colorado State University have hydraulic laboratories and conduct large-scale testing of erosion control products, which also may develop C-Factors. TTI's test beds are sediment-control tests, 30 feet long, 5 feet wide, and 0.7 feet deep. The frames are steel and plywood with porous bottoms. A layer of filter fabric controls the loss of material from the beds over the bottom grid. Two types of soil are used: a loamy sand and a plastic clay. An erosion control product manufacturer can elect to test on a 2:1 or 3:1 slope gradient. The rainfall simulation reproduces the more damaging precipitation drop sizes found in more intense storms.

Installation
As with any construction product, proper installation is key to a successful project. HECPs are installed using hydroseeding equipment with either jet or mechanical agitation, referring to the method employed to mix the materials and create the slurry. Certain slurries may not be mixed in jet-agitated machines because of slurry thickness; check with manufacturers.

Jet-agitation machines generally are made of plastic and most commonly come in 150- to 900-gallon tank capacities. Mixing occurs by recirculating the water and slurry through the pump and forcing the material back into the tank through a series of jets. The force of the material re-entering the tank causes turbulence in the tank that blends the materials into a homogenous mixture.

Mechanical agitation machines are characterized by steel tanks with large internal paddles. These paddles physically mix the material into the slurry. Machines range in size from 300- to 4,000-gallon units in both trailer- and truck-mounted forms. The machines come in various engine and pump configurations. Engine selection is determined by overall equipment size and can range from small gas engines to large diesels. Pumps also vary between centrifugal, rotary gear pumps as well as pumps specifically designed for sprigging.

In addition to the fibers that make up the core material of the HECP, there are many different performance-enhancing additives. Some additives are intended to enhance erosion control, while others enhance vegetation establishment. These additives can be blended with the core fibers during manufacturing or can be added directly to the hydroseeding machine. Following is a list of common performance-enhancing additives:

  • Fertilizer — macro and micro-nutrients critical to plant establishment.
  • Tackifiers — organic or inorganic binders that bond fiber to fiber and fiber to soil, improving the erosion control performance of the applied fiber mulch.
  • Soil amendments — pH neutralization for maximum nutrient uptake by the newly established plant material.
  • Bio-stimulants — plant hormones, cytokinins, auxins, and amino acids that improve cell structure, root mass, and nutrient uptake of the newly established plant material.
  • Super-absorbents — organic or inorganic polymers that absorb water, holding 100 to 400 times their weight, slowly releasing moisture to the seed and newly established plant material.
  • Reinforcing fibers — organic or inorganic fibers that improve the mechanical entanglement of the HECP's core fibers.
  • Marker dyes — dyes used to help meter the application of seed and fertilizer or to improve the color of the fiber mulch.
  • Flocculation agents — organic or inorganic linear polymers that stabilize soil by utilizing soil particle charges to cause clay platelets to clump together forming a floc, stabilizing the exposed soil.

The application rate of material is manufacturer specific. Once the HECP is applied, the HECP will achieve maximum performance after a sufficient curing period, which varies based on site-specific conditions. The HECP forms a protective layer that controls erosion and allows for enhanced seed germination and accelerated plant growth.

Table 1: Performance chart for standard hydraulic erosion control products (HECPs)
Hydraulic Erosion Control1
Type2 Term Functional Longevity3 Typical Application Rates lb./acre (kg/ha) Typical Maximum Slope Gradient (H:V) Minimum Uninterrupted Slope Length (ft.) Maximum C Factor4,5 (3:1 test) Maximum Vegetation Establishment6
1
Ultra Short Term
1 mo.
1,500 – 2,500 (1,700 – 2,800)
≤ 4:1
20
0.75
150%
2
Short Term
2 mo.
2,000 – 3,000 (2,250 – 3,400)
≤ 3:1
25
0.5
150%
3
Moderate Term
3 mo.
2,000 – 3,500 (2,250 – 3,900)
≤ 2:1
50
0.15
200%
4
Extended Term
6 mo.
2,500 – 4,000 (2,800 – 4,500)
≤ 1:1
75
0.1
300%
5
Long Term
12 mo.
3,000 – 4,500 (3,400 – 5,100)
≤ 0.5:1
100
0.02
400%
1. This table is for general guidelines only. Refer to manufacturer for application rates, instructions, gradients, maximum continuous slope lengths, and other site-specific recommendations.

2. These categories are independent of rolled erosion control products (RECPs) categories, despite the identical names.

3. A manufacturer's estimated time period, based upon field observations, that a material can be anticipated to provide erosion control as influenced by its composition and site-specific conditions.

4. "C" Factor calculated as ratio of soil loss from HECP protected slope (tested at specified or greater gradient, h:v) to ratio of soil loss from unprotected (control) plot based on large-scale testing.

5. Acceptable large-scale test methods may include ASTM D 6459 or other independent testing deemed acceptable by the engineer.

6. Minimum vegetation establishment is calculated as outlined in ASTM D 7322 as a percentage by dividing the plant mass per area of the protected plot by the plant mass per area of the control plot.

If it is necessary to reduce slope length in accordance with Table 1, install sediment retention fiber rolls (SRFRs) or other slope-interruption devices perpendicular to the water flow. Proper installation guidelines for SRFRs are available on the ECTC website (www.ectc.org).

HECPs offer designers and contractors one more tool to help them control soil erosion. The goal of ECTC's specification is to offer guidance on product selection and installation.

Brad Braden, Southeast regional manager for Mat Inc., is an ECTC directing member and former chairperson of the HECP technical subcommittee. He can be contacted at bbmatinc@bellsouth.net. Laurie Honnigford is executive director of ECTC. She can be contacted at laurie@ectc.org. Andy Iturriria, P.E., is engineering and sales manager for Terra Novo Inc. He can be contacted at andy@terranovo.com. Sam Morris, vice president of sales and manufacturing for Phoenix Paper Products, serves as the vice chairman of ASTM D18-25-01 and is a board member and HECP subcommittee chairman for the ECTC. He can be contacted at smorris@greenchoice.com.

A copy of ECTC's HECP specification can be downloaded at www.ectc.org/specifications.asp#hecp the website also contains more information about HECPs.


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