Erosion Control Blankets

Erosion Control Blankets (ECBs) are a type of rolled erosion control product consisting of flexible nets or mats, manufactured from both natural and synthetic materials, which can be brought to a site, rolled out, and fastened down on a slope. ECBs are constructed from a variety of degradable fibers that are woven, glued or structurally bound with nettings or meshes. ECBs are typically manufactured of fibers such as straw, wood, excelsior, coconut, polypropylene, or a combination thereof, and stitched or glued to or between geosynthetic netting or woven natural fiber netting (see Figures 1 and 2). Blanket effectiveness, durability, and longevity are strongly dependent on netting and fiber components; thus, ECBs span a very broad application range. Some blankets have a seed-impregnated, recycled cellulose medium incorporated into their base layer.

The purpose of ECBs is to provide short-term protection to a slope or streambank against surficial erosion, rainfall erosion and to promote vegetation establishment. They are not intended for permanent erosion protection or resisting severe scour or high shear stresses.

Useful for Erosion Processes:

	Toe erosion with upper bank failure
	Scour of middle and upper banks by currents
	Local scour
	Erosion of local lenses or layers of noncohesive sediment
	Erosion by overbank runoff
	General bed degradation
	Headcutting
	Piping
	Erosion by navigation waves
	Erosion by wind waves
	Erosion by ice and debris gouging
	General bank instability or susceptibility to mass slope failure

Spatial Application:

	Instream
	Toe
	Midbank
	Top of Bank

Hydrologic / Geomorphic Setting

	Resistive
	Redirective
	Continuous
	Discontinuous
	Outer Bend
	Inner Bend
	Incision
	Lateral Migration
	Aggradation

Conditions Where Practice Applies:

ECBs are used on slopes and streambanks where plant materials, such as seed, transplants, etc., need protection from the force of flowing water, or where performance thresholds of the selected blanket (or fabric) are not exceeded during design flows. As a very general rule and conservative guideline, shear stresses greater than 4.9 – 9.8 kg/m² (1 – 2 lbs/ft² ) require non-degradable synthetic fabrics (Washington State, 2003). The properties and typical applications of different types of rolled erosion control products (RECPs) are shown in Table 1. ECBs are useful where mulch must be anchored, but where such methods as crimping or tackifying are neither feasible nor adequate. These conditions apply on steep slopes, generally steeper than 1V:3H, and slopes with highly erosive soils. ECBs are appropriate for critical slopes adjacent to sensitive areas such as streams and wetlands where planting is likely to be slow in providing adequate protective cover.

TABLE 1: Performance Classification of Rolled Erosion Control Products (RECPs)
(adapted from Austin and Driver, 1995)

Required Performance Characteristics	Suitable Types of RECPs	Properties and Typical Applications
LOW-VELOCITY DEGRADABLE	• Single-net, organic fiber ECBs • Biodegradable natural fiber and photodegradable geosynthetic nets	• One to two-season longevity • Limited capacity to resist damage and erosion under severe conditions • Slopes of moderate grade, length and runoff; channels and streambanks where potential for damage during installation is minimal
HIGH-VELOCITY DEGRADABLE	• Double-net ECBs or high strength nettings and/or increased quantities of organic fibers • Dense, open-weave geotextiles or meshes (e.g., coir, jute, polypropylene)	• Similar to low-velocity degradables in installation and function, but designed for more severe site conditions • Heightened durability and longevity (1 – 5 years) • Steeper slopes & high velocity channel linings where natural, unreinforced vegetation is expected to provide permanent stabilization
LONG-TERM NON-DEGRADABLE	• High strength geosynthetic mats and cellular containment e.g., Turf Reinforcement Mats (TRMs), Geocellular Confinement Systems (GCSs).	• Provide immediate, high performance erosion protection followed by permanent reinforcement of established vegetation • Steep slopes with very erodible soils; channel linings subjected to relatively high, long-duration flows.

Complexity:

Low. Technique is relatively simple and straightforward to implement. Erosion control blankets are pre-manufactured and supplied in rolls that are simply unrolled on a slope and staked in place with pins and staples following in a pattern dictated by the slope gradient and type of soil (see Figures 3 and 4). Blanket manufacturers usually provide recommended attachment patterns and procedures.


Figure 3. Erosion control blanket being rolled out on a slope prior to attachment	Figure 4. Schematic illustration of blanket installation methods

Design Guidelines / Typical Drawings:

An erosion control blanket should be selected for the appropriate soil, site, and expected hydraulic loading conditions. Degradable blankets (see Table 1) provide erosion protection for approximately one to five years; they include biodegradable products made from natural fibers and photodegradable synthetic products. Non-degradable blankets are typically made from synthetic materials and are resistant to decay for at least ten years after installation. In order of increasing strength or resiliency, degradable blanket types include straw, jute, coir, and a few types of synthetic netting. Straw and jute are excellent for uplands but are generally not resilient enough for streambanks and floodplains (Washington State, 2003). Coir, and to a lesser extent, photodegradable synthetic fabrics and blankets, are the most applicable for streambank stabilization purposes.

Other factors that can affect ECB selection include cost, risk of failure, and available fabric or blanket-roll dimensions. Proper installation and attachment are also critical; otherwise, rills and small gullies may develop beneath the blanket, or the blanket may be torn away by flowing water.

The ground surface must be smoothed and seeded prior to ECB placement. The blanket or fabric should be rolled out and down the slope with adequate overlap. The upstream rolls or strips should overlap downstream strips as shown in Figure 5 and 6 for a slope and channel respectively. In addition to point attachments with pins, pegs, or staples, blankets should also be secured in place by means of an anchor trench at the top of a streambank. Blanket manufacturers normally recommend an attachment pattern and minimum number of attachment points based on the slope gradient and soil type.

Erosion Control Blankets Slope Installation Typical Drawing
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Figure 5. Typical drawing of an erosion control blanket installed on a slope or streambank.

Erosion Control Blankets Channel Installation Typical Drawing
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Figure 6. Typical drawing of an erosion control blanket installed in a channel

7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS

Erosion control blankets provide environmental benefits by assisting revegetation efforts. Mulching with erosion control blankets increases the germination and survival rates for grasses and legumes, and promotes the establishment of vegetation on streambanks, which benefits riparian and instream habitat.

8. HYDRAULIC LOADING

Erosion control blankets are not designed or intended for severe hydraulic loading conditions. The blankets slightly increase the allowable velocity limits of vegetation when used alone, as shown in Figure 7.

Figure 7. Allowable velocity for vegetation and other types of streambank cover. (Thiesen, 1992)

A website maintained by the Texas DOT (1999), www.dot.state.tx.us/insdtdot/orgchart/cmd/erosion/sect1a.htm (pdf) compares soil loss from different fabrics and blankets under a range of flows (with specified shear stress values) based on data collected in an outdoor flume. This source also provides comparative data on vegetation growth in different fabrics or blankets. Limiting shear velocity and shear stress data for channel lining systems including erosion control blankets have been reported by Chen and Cotton (1988). Limiting or allowable values for a variety of ground covers have been compiled by Fischenich (2001). The values tabulated in Table 2 were taken from a compilation by Washington State (2003).

TABLE 2: Limiting Shear Stress and Velocities for Erosion Control Blankets and Fabrics
(after Washington State, 2003)

Erosion control blankets can be used in conjunction or combination with a number of other techniques. They can be used for mid-slope protection on relatively gentle banks (< 2:1) that are protected by hard armor or toe-structures (see Techniques: Vegetated Riprap or Longitudinal Stone Toe). They can be used with fascines, and live stakes can be inserted through the blankets (see Techniques: Live Staking and Live Fascines). The blankets can be run into trenches and fascines staked in place over the blanket as shown in Figures 8 and 9. This is an excellent way to insure that the blankets are firmly attached to the soil and to prevent the development of rills underneath the blanket (Gray and Sotir, 1996). Erosion control blankets can be used with soil bioengineering techniques, such as grass plug planting, willow staking, fascines, etc.

Erosion control blankets provide immediate cover and protection. They facilitate vegetation establishment by moderating soil temperature, maintaining soil moisture, preventing desiccation, and protecting seeds from seed-eaters. Even if herbaceous vegetation does not grow, the blankets will provide protective cover for at least one season.

11. LIMITATIONS

Erosion control blankets only provide temporary protection; turf reinforcement mats must be used for long-term applications. ECBs are not designed to withstand severe scour and high tractive stresses and must be placed on relatively smooth, prepared slopes with gradients no steeper than 1:1. Rills and small gullies can form under a blanket if it is not firmly attached to the ground as shown in Figures 10 and 11.

Erosion control blankets are generally manufactured products that are typically produced in the form of mats of organic, biodegradable mulch such as straw, curled wood fiber (excelsior), coconut fiber or a combination thereof, evenly distributed on or between photo-degradable polypropylene or biodegradable natural fiber netting. The netting and mulch material are stitched to ensure integrity and the blankets are provided in rolls for ease of handling and installation. ECBs are normally installed with hand labor, and the main tools or equipment required consist of hammers, stapling devices, and shovels for trenching.

13. CONSTRUCTION / INSTALLATION

The placement of erosion control blankets is relatively straightforward; however, attention to particular details is important for successful installation. These details include surface preparation, fabric or blanket orientation, staking protocol, trenching, overlapping, and transitions. These considerations, which are reviewed briefly below, are discussed in greater detail by Washington State (2003) and Caltrans (2003).

Surface Preparation: Bank should be graded back preferably to a slope gradient no steeper than 1V:1½H and smoothed so that it is free of any projections or depressions.

Fabric Orientation: Fabrics or blankets have been placed in a variety of configurations, including rolling lengths parallel, perpendicular, or at an angle to stream flow. An orientation perpendicular to stream flow is normally recommended (refer to Figures 5 and 6). The most easily constructed, cost-effective option should be selected.

Staking Protocol: Blanket manufacturers normally provide recommended staking patterns and densities (stakes/unit area) which are dictated by slope gradient, soil type, and estimated hydraulic loading conditions. Stakes may consist of pegs, pins, or staples. Live stakes can be uniformly interspersed with inert fasteners but normally should be limited to no more than one-third of the total number of fastening devices used to secure a blanket.

Trenching: Trenching to secure fabric edges (refer to Figure 5) is a fundamental element of blanket installation on difficult sites. It is especially critical to bury leading (upstream) edges to prevent lifting and tearing. Trenches should be at least 15 cm (6 in) deep, then backfilled with common fill or topsoil, compacted and seeded. Blanket edges parallel to flow may also be trenched in a variety of ways for maximum erosion protection. The conjunctive use of live fascines staked down in the trench (refer to Figures 8 and 9) can provide secure and effective attachment.

Fabric Overlap: Overlapping of fabric edges is another important requirement. Upstream and upslope edges should always be lapped, or "shingled" over downstream and downslope edges (refer to Figures 5 and 6).

Transitions: Transitions between adjacent bank treatments, treated and untreated areas, or between fabric edges and existing infrastructure, such as bridges and culverts constitute potential weak points. Successful treatment of these zones may require special types of blanket orientation, staking, trenching, and construction sequencing.

Blanket effectiveness, durability, and longevity are strongly dependent on netting configuration (1- or 2-layer) and fiber components (straw, coconut fibers, wood shavings, or combinations thereof); thus, erosion control blankets span a broad range of types of composition and manufacturing processes. The costs of ECBs reflect this broad range. Non-degradable fabrics and blankets are often viewed as less desirable on streambanks because of the persistence of synthetic components (fibers and netting). In addition, non-degradable blankets can be more expensive and harder to work with than degradable fabrics. Nevertheless, they may be a cost-effective substitute for "hard" armor protection on upper portions of a streambank, and they are generally compatible with plantings. An example of an effective type of non-degradable composite blanket is composed of a three-dimensional synthetic matrix integrated with non-woven coir (Washington State, 2003). This ECB is a relatively cost-effective ($3.60 - $5.00 per m² ($3 to $5 per yd² )), high performance product that has been reported to work well on streambanks. Unit costs shown in Table 3 were compiled by Washington State (2003) for a variety of fabrics.

TABLE 3: Range of costs for fabrics and blankets used in streambank protection projects.
(after Washington State, 2003)

Basic monitoring consists of periodic visual inspections to determine blanket integrity and attachment performance. Rill development beneath the blanket or edge lifting are evidence of inadequate attachment. Additional staking and trenching can be employed to correct defects. Recently placed blankets may be replaced, but once vegetation becomes established, blanket replacement is not a reasonable option.

Design and/or installation errors are the most common reasons for failure. Design errors include selection of the wrong blanket type for the soil, site, and hydrologic loading conditions. Installation errors include poor surface preparation, wrong orientation, inadequate attachment, improper overlap, and failure to adequately consider transitions.

Erosion control blankets have been widely used to promote vegetative establishment on streambanks and waterways with perennial flow. They have also been used along channels and grassy swales that are used to handle and convey intermittent runoff. Examples of these applications are shown in Figures 12 to 15.

Allowable hydraulic loading should be investigated for erosion control blankets and fabrics that are used in conjunction with live fascines, i.e., when the blankets are run into the fascine trenches and the fascines staked down on the blanket (refer to Figures 8 and 9).

Austin, D. N. and Driver, T. (1995). Classifying rolled erosion control products, Erosion Control, Vol. 2, No. 1, pp. 48-53.

Caltrans (2003). Geotextiles, Mats, Plastic Covers and Erosion Control Blankets. Construction Site Best Management Practices Manual, Section 3, pp.3-7

Chen Y. H. & Cotton, G. K. (1988). Design of roadside channels with flexible linings. Hydraulic Engineering Circular No. 15. Publication No. FHWA-IP-87-7 (pdf)

Fischenich, J. C. (2001). Stability thresholds for stream restoration materials. EMRRP Technical Notes Collection (ERDC-TN-EMRRP-SR-29), U.S. Army Engineering Research and Development Center, Vicksburg, MS. (pdf)

Gray, D. H. & Sotir, R. (1996). Biotechnical and Soil Bioengineering Slope Stabilization. John Wiley and Sons, New York, N. Y.

Thiesen, M. (1992) Evaluation of Biotechnical Composites Under High Velocity and Shear Conditions. In Proceedings of Conference XXIII, International Erosion Control Association, 1992, Reno, Nevada.

Washington Dept of Fish & Wildlife (2003). Integrated Streambank Protection Guidelines, published in co-operation with Washington Dept. of Transportation and Washington Dept. of Ecology, June 2003. (Chapter 6 pdf) (Appendix L pdf) (Appendix H pdf) http://www.wa.gov/wdfw/hab/ahg/ispgdoc.htm (April 2003)

Fabric Type	Limiting Velocity m/sec (ft/sec)	Limiting Shear Stress kg/m² (lbs/ft² )
Degradable, non-woven Coir	1.8 (6)	2.4 (0.5)
Degradable, 700 g/sq.m Woven coir	4.3 (14)	4.9 - 9.8 (1 - 2)
Non-degradable, 2-D Synthetic blanket	6.1 (20)	26.9 (5.5)
Non-degradable, 2-D Biaxial grid	-	N/A
Non-degradable, 3-D Synthetic matrix	7.6 (25)	48.8 (10)
Non-degradable, 3-D Synthetic/Coir	N/A	10.7 - 39.1 (2.2 - 8)

Fabric Material	Unit Cost (per m² (per yd² ))
Woven coir fabric	$2.40 - $3.60 ($2.00 - $3.00)
Non-woven coir	$1.20 - $2.40 ($1.00 - $2.00)
Non-woven synthetic fabric	$0.60 - $0.84 ($0.50 - $0.70)
Biodegradable geotextile	$3.36 - $3.60 ($2.80 - $3.00)


Figure 1. ECB with straw fibers placed between degradable synthetic netting	Figure 2. ECB with coconut fibers placed between degradable synthetic netting


Figure 8. ECB (coir fabric) run into shallow fascine trench dug on contour	Figure 9. Fascine staked in place over ECB (coir fabric) in trench.


Figure 10. Rills and small gullies that developed beneath ECB	Figure 11. Rilling under excelsior blanket as result of inadequate attachment


Figure 12. Waterway protected by riprap toe and ECB placed on graded 2:1 slope	Figure 13. Narrow channel protected by ECB placed on graded bank


Figure 14. Streambank protected by straw-filled ECB	Figure 15. Sides of intermittent channel protected by straw-filled ECB