2010-05-07_PERMIT FILE - C2009087A (28)

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Peabody Sage Creek Mine — Drainage and Sediment Control Plan 4.4.3 Turf Reinforcement Mat Design • Turf Reinforcement Mats (TRMs) allow vegetation to be used in areas where flow conditions exceed the limits of natural vegetation (e.g., limiting velocities greater than 5 fps). They can be used for steeper slopes (1.511:1V, or flatter) and under medium to high flow conditions. In select situations SCCC may use TRMs as an alternative channel lining. Typical matting construction consists of a three- dimensional corrugated matrix (with either coconut/straw or polypropylene fiber) that will anchor and reinforce roots and stems for long -term stability. In addition, the TRM deflects flowing water away from the soil surface and increases the surface roughness. TRM products that SCCC may use at the Peabody Sage Creek Mine are manufactured by the following companies (this is not an all inclusive list; if other products are available they may be utilized if they meet the minimum design characteristics): • American Excelsior Company • Colbond Geosynthetics • Greenfix America • North American Green • Propex Geosynthetics • Channel stability was analyzed using the erodible channel sub - routine. The designs assume the TRM is installed over bare soil. The channel roughness coefficient (i.e., Manning's n) and the limiting velocity values were hand entered. Examples of manufacturer's published performance values for select TRM products under bare soil conditions are summarized in Table 2.05.3 -E2 -9, TRM Performance Guide. Manufacturer's performance specifications for these products are found in Appendix 2.05.3 -E2.G, Attachment 2.05.3- E2.G -3, Turf Reinforcement Mat Design and Installation Information. To calculate the greatest potential instability (i.e., the highest velocity) SCCC selected a roughness coefficient value of 0.023, which is less than any of the manufacturer's published values. SCCC selected 9.0 feet per second as the limiting velocity value. Channel capacity was analyzed using the vegetated channel sub - routine. The channel roughness coefficient is automatically calculated by the model. The limiting velocity values were hand entered (manufacturer's published limiting velocity values for select TRM products are summarized in Table 2.05.3 -E2 -9. The designs assume a mixed grass community 11 — 24" tall, which corresponds to a "B" retardance class, and results in a higher surface water elevation. The postmine channels at Peabody Sage Creek Mine lined with TRMs will be either three (3) or ten (10) feet wide. TRM installation will be in accordance with recommendations developed by the Erosion Control Technical Council (ECTC, 2006) or the individual manufacturer's recommendations. 2.05.3 -E2.19