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CITY OF EVERETT STORMWATER MANAGEMENT MANUAL <br />example, coconut fiber matting). Blankets are strands of material that are not tightly woven, but instead <br />form a layer of interlocking fibers, typically held together by a biodegradable or photodegradable netting <br />(for example, excelsior or straw blankets). They generally have lower tensile strength than nets, but cover <br />the ground more completely. Coir (coconut fiber) fabric comes as both nets and blankets. <br />Conditions of Use <br />Erosion control nets and blankets should be used: <br />1. To aid permanent vegetated stabilization of slopes 211:1 V or greater and with more than 10 feet of <br />vertical relief <br />2. For drainage ditches and swales (highly recommended). The application of appropriate netting or <br />blanket to drainage ditches and swales can protect bare soil from channelized runoff while <br />vegetation is established. Nets and blankets also can capture a great deal of sediment due to their <br />open, porous structure. Synthetic nets and blankets can be used to permanently stabilize channels <br />and may provide a cost-effective, environmentally preferable alternative to riprap. One -hundred <br />percent synthetic blankets manufactured for use in ditches may be easily reused as temporary <br />ditch liners. <br />Disadvantages of blankets include: <br />1. Surface preparation is required <br />2. On slopes steeper than 2.5H:1 V, blanket installers may need to be roped and harnessed for safety <br />3. They cost at least $4,000 to $6,000 per acre installed. <br />Advantages of blankets include: <br />1. Can be installed without mobilizing special equipment <br />2. Can be installed by anyone with minimal training <br />3. Can be installed in stages or phases as the project progresses <br />4. Seed and fertilizer can be hand -placed by the installers as they progress down the slope <br />5. Can be installed in any weather <br />6. There are numerous types of blankets that can be designed with various parameters in mind. <br />Those parameters include: fiber blend, mesh strength, longevity, biodegradability, cost, and <br />availability. <br />Design and Installation Specifications <br />1. See DCSS Drawing #202 for typical orientation and installation of blankets used in channels and <br />as slope protection. Note: These are only typical examples; all blankets must be installed per <br />manufacturer's installation instructions. <br />2. Installation is critical to the effectiveness of these products. If good ground contact is not <br />achieved, runoff can concentrate under the product, resulting in significant erosion. <br />3. Installation of Blankets on Slopes: <br />a. Complete final grade and track walk up and down the slope. <br />b. Install hydromulch with seed and fertilizer. <br />c. Dig a small trench, approximately 12 inches wide by 6 inches deep along the top of the <br />slope. <br />Volume ll — Construction Stormwater Pollution Prevention <br />3-16 <br />