1.9 BMP C122 - Nets and Blankets
Erosion control nets and blankets are intended to prevent erosion and hold seed and mulch in place on steep slopes and in channels so that vegetation can become well established. In addition, some nets and blankets can be used to permanently reinforce turf to protect conveyance ditches during high flows. Nets (commonly called matting) are strands of material woven into an open, but high-tensile strength net (for example, coconut fiber matting). Blankets are strands of material that are not tightly woven, but instead form a layer of interlocking fibers, typically held together by a biodegradable or photodegradable netting (for example, excelsior or straw blankets). They generally have lower tensile strength than nets, but cover the ground more completely. Coir (coconut fiber) fabric comes as both nets and blankets.
Erosion control netting and blankets shall be made of natural plant fibers unaltered by synthetic materials.
Erosion control nets and blankets should be used:
To aid permanent vegetated stabilization of slopes 2H:1V or greater and with more than 10 feet of vertical relief.
For conveyance ditches and swales (highly recommended). The application of appropriate netting or blanket to conveyance ditches and swales can protect bare soil from channelized runoff while vegetation is established. Nets and blankets also can capture a great deal of sediment due to their open, porous structure. Synthetic nets and blankets can be used to permanently stabilize channels and may provide a cost-effective, environmentally preferable alternative to riprap. 100 percent synthetic blankets manufactured for use in conveyance ditches may be easily reused as temporary conveyance ditch liners.
Disadvantages of blankets include:
On slopes steeper than 2.5:1, blanket installers may need to be roped and harnessed for safety;
Advantages of blankets include:
Can be installed without mobilizing special equipment;
Can be installed by anyone with minimal training;
Can be installed in stages or phases as the project progresses;
Seed and fertilizer can be hand-placed by the installers as they progress down the slope;
Can be installed in any weather;
There are numerous types of blankets that can be designed with various parameters in mind. Those parameters include: fiber blend, mesh strength, longevity, biodegradability, cost, and availability.
1.9.3 Design and Installation Specifications
See Figure 3 - 3: Nets and Blankets – Slope Installation, and Figure 3 - 4: Nets and Blankets – Channel Installation, for typical orientation and installation of blankets used in channels and as slope protection.
Note: These are typical only; all blankets must be installed per manufacturer’s installation instructions.
Installation is critical to the effectiveness of these products. If good ground contact is not achieved, runoff can concentrate under the product, resulting in significant erosion.
Installation of Blankets on Slopes:
Complete final grade and track walk up and down the slope.
Slope surface shall be free of rocks, clods, sticks and grass. Nets/blankets shall have good contact with the soil.
Apply permanent seeding per BMP C120 - Temporary and Permanent Seeding or mulching per BMP C121 - Mulching before placing net/blanket.
Dig a small trench, approximately 12 inches wide by 6 inches deep along the top of the slope. Cover with soil to secure.
Install the leading edge of the blanket into the small trench, backfill the trench with soil and staple approximately every 18 inches. Staples are metal. ”U”-shaped, and a minimum of 6 inches long. Longer staples are used in sandy soils. Biodegradable stakes are also available and should be used where applicable.
Roll the blanket slowly down the slope as the installer walks backwards. The blanket rests against the installer’s legs. Staples are installed as the blanket is unrolled. It is critical that the proper staple pattern in used for the blanket being installed. The blanket should not be allowed to roll down the slope on its own as this stretches the blanket, making it impossible to maintain soil contact. In addition, no one should be allowed to walk on the blanket after it is in place.
If the blanket is not long enough to cover the entire slope length, the trailing edge of the upper blanket should overlap the leading edge of the lower blanket and be stapled. On steeper slopes, this overlap should be installed in a small trench, covered with soil and stapled.
With the variety of products available, it is impossible to cover all the details of appropriate use and installation. Therefore, it is critical that the design engineer consults the manufacturer's information and that a site visit takes place in order to ensure that the product specified is appropriate.
Jute matting must be used in conjunction with mulch (BMP C121 - Mulching). Excelsior, woven straw blankets, and coir (coconut fiber) blankets may be installed without mulch. There are many other types of erosion control nets and blankets on the market that may be appropriate in certain circumstances.
In general, most nets (e.g., jute matting) require mulch in order to prevent erosion because they have a fairly open structure. Blankets typically do not require mulch because they usually provide complete protection of the surface.
Extremely steep, unstable, wet, or rocky slopes are often appropriate candidates for use of synthetic blankets, as are riverbanks, beaches, and other high-energy environments. If synthetic blankets are used, the soil should be hydromulched first.
100 percent biodegradable blankets are available for use in sensitive areas. These organic blankets are usually held together with a paper or fiber mesh and stitching which may last up to a year.
Most netting used with blankets is photodegradable, meaning it will break down under sunlight (not UV stabilized). However, this process can take months or years even under bright sun. Once vegetation is established, sunlight does not reach the mesh. It is not uncommon to find non-degraded netting still in place several years after installation. This can be a problem if maintenance requires the use of mowers or ditch cleaning equipment. In addition, birds and small animals can become trapped in the netting.
Good contact with the ground must be maintained, and erosion must not occur beneath the net or blanket.
Repair or staple any areas of the net or blanket that are damaged or not in close contact with the ground.
If erosion occurs due to poorly controlled stormwater and surface water, fix the problem and protect the eroded area.
Figure 3 - 3: Nets and Blankets – Slope Installation
Figure 3 - 4: Nets and Blankets – Channel Installation