2012-06-08_PERMIT FILE - C2009087A (12)

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Peabody Sage Creek Mine — Drainage and Sediment Control Plan The minimum design configurations for the vegetated channels are summarized on Table 2.05.3 -E2 -5, Operational Runoff Ditch Design Schedule and Table 2.05.3 -E2 -6, Postmine Channel Design Schedule. 4.2.3 Rock Riprap Design Prior to 2007, all unstable channel reaches (e.g., flow velocities greater than 5fps) were armored with rock riprap. Riprap size was determined using the SEDCAD riprap lined channel sub - routine, utilizing either Simons /OSM (Simons, et al., 1982) or PADER (Pennsylvania department of environmental resources, Stover, 1990) method. The minimum channel designs for the rock riprap lined channels are summarized on Table 2.05.3 -E2 -5 and Table 2.05.3 -E2 -6. Given the uncertainty in determining riprap D50 for steep slopes with the PADER Method, PSCM adds an additional 3- inches to the model recommended D50 size. 4.2.4 Turf Reinforcement Mat Design Turf Reinforcement Mats (TRM) 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 (1.5H:1 V, or flatter) and under medium to high flow conditions. In select situations, PSCM may use TRM 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 PSCM 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 -3, Turf Reinforcement Mat Design and Installation Information. To calculate the greatest potential instability (i.e., the highest velocity) PSCM selected a roughness coefficient of 0.030, which is conservative TR -01 2.05.3 -E2.19 Revised 04/12 f