9.2 Rolled Erosion Control Products (RECPs): Temporary ECBs

9.2 Rolled Erosion Control Products (RECPs): Temporary ECBs

Quick Reference: Rolled Erosion Control Products (RECPs) encompass temporary Erosion Control Blankets (ECBs) and permanent Turf Reinforcement Mats (TRMs). Standardized by the Erosion Control Technology Council (ECTC), temporary ECBs are categorized into Types 1 through 4, ranging from Type 1 (ultra-short term, 3 months) to Type 4 (long-term 100% coconut coir, 36 months). Slope installations must always unroll vertically DOWN the slope (parallel to runoff flow)—never horizontally! The crest of the blanket must be anchored in an excavated top-of-slope terminal anchor trench at least 6 inches wide by 6 inches deep. To protect wildlife from entrapment in plastic netting, environmentally sensitive areas mandate 100% biodegradable woven organic nettings (such as jute or leno-weave coir) rather than photodegradable polypropylene.

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RECP Industry Standards & Classification Framework

The engineering performance, material specifications, and testing protocols for Rolled Erosion Control Products are governed collaboratively by the Erosion Control Technology Council (ECTC) and the American Society for Testing and Materials (ASTM). Specifically, ASTM D6459 (Standard Test Method for Determination of Rolled Erosion Control Product Performance in Protecting Hillslopes from Rainfall-Induced Erosion) evaluates blankets under simulated 2:1 and 1:1 slopes subjected to rainfall intensities of 2 to 6 inches per hour.

Temporary ECBs are designed to provide immediate soil cover during seed germination and vegetative root establishment, progressively degrading as mature vegetation assumes full hydraulic protection. ECTC classifies temporary blankets into four distinct tiers based on material composition, netting structure, and functional longevity:

Type 1 (≤ 3 mos) ──► Type 2 (≤ 12 mos) ──► Type 3 (≤ 24 mos) ──► Type 4 (≤ 36 mos)
Single-Net Straw     Double-Net Straw      70% Straw / 30% Coir  100% Coconut Coir

ECTC Classification Tiers for Temporary ECBs

1. Type 1: Ultra-Short Term ECB (Longevity: Up to 3 Months)
   • Matrix: 100% agricultural straw or rapidly degrading wood excelsior fibers ($0.5\text{ lbs/yd}^2$).
   • Netting: Single lightweight photodegradable synthetic or organic net stitched to the top face.
   • Applications: Flat to gentle slopes ($\le 4:1$) and low-energy conveyances where fast-germinating temporary cover crops (such as annual ryegrass or cereal rye) establish within 30 to 60 days.
2. Type 2: Short-Term ECB (Longevity: Up to 12 Months)
   • Matrix: 100% clean agricultural straw ($0.5\text{ lbs/yd}^2$) or curled wood excelsior fibers ($0.73\text{ lbs/yd}^2$).
   • Netting: Available in single-net (top face only) or double-net (matrix sandwiched between top and bottom nettings). Double-net configurations provide significantly higher tensile stability, resisting tearing under heavier rainfall impact.
   • Applications: Standard moderate slopes ($3:1\text{ to }2:1$) where perennial turf grasses require a full single growing season to achieve 70% vegetative density.
3. Type 3: Extended Term ECB (Longevity: Up to 24 Months)
   • Matrix: Composite blend consisting of 70% agricultural straw and 30% coconut coir fibers ($0.5\text{ lbs/yd}^2$).
   • Netting: Double-net configuration utilizing UV-stabilized polypropylene netting or heavy organic netting stitched with UV-resistant thread.
   • Applications: Severe cut-and-fill slopes ($2:1\text{ to }1.5:1$), low-flow ditch inverts, and arid or semi-arid climatic regions where sparse rainfall and cold winters prolong seed dormancy, requiring two full seasons for vegetative establishment.
4. Type 4: Long-Term ECB (Longevity: Up to 36 Months)
   • Matrix: 100% coconut coir fibers ($0.5\text{ to }0.6\text{ lbs/yd}^2$). Coconut fibers contain high concentrations of natural lignin (over 40%), making them exceptionally resistant to biological microbial decay, moisture rot, and photodegradation.
   • Netting: Double heavy-duty UV-stabilized polypropylene netting or 100% biodegradable woven coir twine.
   • Applications: High-stress slopes ($1.5:1\text{ to }1:1$), roadside cut faces, and channel banks subjected to extended inundation where slow-growing native woody vegetation, shrubs, or arid grasses require up to three years of structural soil cover.

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Photodegradable vs. 100% Biodegradable Netting: Wildlife Hazards

A critical environmental dilemma in modern erosion control practice centers on the selection of netting material. Historically, manufacturers utilized extruded plastic polypropylene netting treated with UV inhibitors to regulate degradation rates.

The "Green-Net Hazard" and Wildlife Entrapment

Standard plastic netting consists of rigid, extruded polypropylene filaments welded into fixed rectangular or square apertures (typically $0.5\text{ in} \times 0.5\text{ in}$ to $0.75\text{ in} \times 0.75\text{ in}$). Under field conditions, several ecological problems emerge:

1. Wildlife Entrapment: Ground-dwelling fauna—including snakes (such as black racers, rat snakes, and threatened rattlesnakes), box turtles, salamanders, migratory songbirds, and small mammals—forage across blankets. When a snake attempts to crawl through the fixed plastic grid, its head and neck pass through, but its wider midbody becomes wedged. Because snake scales point backward, the animal cannot back out. As it struggles, the plastic filament slices into the scales and flesh, causing constriction, gangrene, dehydration, and mortality.
2. Slow Photodegradation in Shaded micro-environments: Plastic netting relies on direct solar UV-B radiation to break polymer chains (photodegradation). However, as seeded grasses germinate and develop a lush canopy, the netting becomes shaded from sunlight. In shaded, moist microclimates, so-called "photodegradable" plastic netting can persist structurally for 5 to 10+ years, persisting as an invisible underground wildlife snare and fouling maintenance mowing equipment.
3. Microplastic Contamination: As plastic netting eventually fragments, it disintegrates into millions of secondary microplastic particles that enter aquatic ecosystems, accumulating in stream sediments.

100% Biodegradable Organic Alternatives

To safeguard endangered species and sensitive riparian corridors, natural resource agencies (US Fish and Wildlife Service, state DOTs, and state wildlife departments) frequently prohibit plastic-netted blankets, mandating 100% Biodegradable RECPs:

• Woven Jute Yarn: Open-mesh woven textiles made from natural jute fibers. Jute fibers absorb water (increasing weight and soil conformity) and decompose into harmless organic soil matter within 12 to 24 months.
• Coir Twine Netting: Woven from spun coconut husks, providing high tensile strength and longevity without synthetic residues.
• Leno-Weave Netting: A specialized mechanical weave pattern where two warp (longitudinal) threads twist around each weft (transverse) thread. This allows the mesh openings to shift and expand dynamically under animal pressure, allowing trapped snakes or turtles to push through unharmed without becoming tangled or constricted.

ECTC Temporary Erosion Control Blanket (ECB) Specification Matrix

Slope Installation Engineering Specifications

Even the highest quality Type 4 coconut blanket will fail catastrophically if basic installation standards are compromised. The following engineering specifications represent industry best practice under CPESC standards:

1. Seedbed Preparation

Prior to unrolling blankets, the receiving slope must undergo rough grading followed by fine raking. All large clods, tree roots, sharp rocks, and construction debris greater than 1.5 inches (38 mm) in diameter must be removed. The surface must be loose, friable, and track-walked or raked smooth. All seed, lime, and fertilizer amendments must be applied directly to the soil BEFORE blanket rollout so that seeds rest directly at the soil-blanket interface.

2. Top-of-Slope Terminal Anchor Trench

Runoff originating from upland plateaus or highway shoulders must not be allowed to slip behind the top edge of the blanket.

• Location: The terminal anchor trench must be excavated 2 to 3 feet back from the crest (shoulder break) of the slope on planar ground.
• Dimensions: The trench must be excavated to a minimum depth of 6 inches (150 mm) and minimum width of 6 inches (150 mm) (frequently specified as $6\text{ in} \times 12\text{ in}$ in high-runoff regions).
• Anchoring Sequence: The top of the blanket roll is laid into the open trench, pinned along the bottom of the trench with staples spaced 12 inches (300 mm) on center, backfilled with excavated earth, and compacted flush with surrounding grade. The remaining roll length is then folded over the compacted soil and deployed down the slope.

3. Direction of Roll Deployment

The Cardinal Slope Rule: Rolled erosion control blankets must ALWAYS be unrolled vertically DOWN the slope face (parallel to the direction of overland water flow). Blankets must NEVER be unrolled horizontally across the slope contour!

Deploying blankets horizontally across the slope creates continuous horizontal seams perpendicular to runoff flow. Runoff cascades down the face, hits the horizontal edge, penetrates beneath the seam, pulls out staples, and cuts severe rills behind the blanket. When rolled vertically down the slope, runoff glides smoothly along the long axis of the blanket fibers.

4. Overlap Dimensions (Shingle Effect)

• Longitudinal (Side-to-Side) Overlaps: Consecutive rolls deployed down the slope face must overlap by a minimum of 3 to 4 inches (75 to 100 mm). Overlaps must follow a shingle-lap fashion, where the edge receiving prevailing wind or cross-slope sheet flow lies on top.
• Transverse (End-to-End) Overlaps: When a roll ends mid-slope and a new roll begins, the uphill roll must overlap the downhill roll by a minimum of 6 to 8 inches (150 to 200 mm) in a shingle pattern (top roll over bottom roll). This ensures that downward-flowing sheet runoff steps over the seam rather than catching the lip and peeling the lower blanket upward.

5. Stapling Patterns, Hardware & Densities

Blankets must be pinned firmly to the subgrade using approved anchoring devices:

• Wire Staples: Minimum 11-gauge wire, U-shaped staples measuring 6 to 8 inches long by 1 inch wide. In soft, sandy, or loose soils, extended staples measuring 8 to 12 inches must be specified to prevent pullout.
• Bio-Stakes: In environmentally sensitive areas or around maintenance mower paths, wooden stakes (hardwood pegs) or 100% biodegradable starch-based polymer pins are specified.
• Staple Density: Staple density increases proportionally with slope steepness:
  • Gentle slopes ($4:1$ or flatter): 0.8 to 1.0 staple per square yard.
  • Moderate slopes ($3:1\text{ to }2:1$): 1.2 to 1.5 staples per square yard.
  • Steep slopes ($1.5:1\text{ to }1:1$): 2.0 to 2.5+ staples per square yard.
  • Seams and overlap edges: Staples spaced every 18 to 24 inches along seams, and every 12 inches along anchor trenches.

6. The "Tenting" Phenomenon and Intimate Ground Contact

The Tenting Failure Mode: Tenting occurs when a blanket bridges across surface depressions, rock clods, ruts, or abrupt grade breaks because it was stretched too tightly during installation or applied over a poorly prepared, uneven seedbed.

When tenting occurs, the blanket hovers 1 to 3 inches above the mineral soil surface. Overland runoff naturally channels into these subterranean voids. Because the blanket does not touch the soil, it provides zero frictional resistance to the runoff. Runoff accelerates, initiating severe sheet and rill erosion beneath the blanket (sub-canopy piping). The blanket appears completely intact from an inspection vehicle, while feet of soil are silently washed away underneath. To prevent tenting, workers must loosely unroll blankets without stretching, walking the mat into micro-depressions to achieve 100% intimate, continuous ground contact.