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Pond 018 Sediment Control System <br />This proposed sediment control plan revision seeks to isolate and separate the active New <br />Horizon 2 Mine shop area away from the rest of the mine's sediment control system. The plan <br />will collect all runoff from the 20.86 acres into a below grade sediment control pond to be <br />excavated near the south end of this area. This area is currently draining to Pond 007 via the C18 <br />ditch. Pond 018 will be designed under the requirements of Colorado DRMS Rule 4.05.9 <br />(2)(e)(ii) and operated as a zero discharge facility for stormwater runoff up through the 100 year, <br />24 hour storm. <br />The 20.86 acres of still active lands in the southeast edge of the New Horizons Mine includes <br />5.64 acres of recently reclaimed land (1 to 2 years revegetation growth), 6.61 acres of a topsoil <br />storage pile, 0.59 acres of a mine office complex that occupies parts of a former residence, and <br />finally the remaining 8.02 acres consists of active yard, shops and access road areas. <br />The design considerations used for this evaluation are described in the following paragraphs. <br />The watershed boundaries, associated drainage areas, and runoff curve numbers are shown on <br />Maps 1 and 2. Pond design details are provided on Map 3. <br />Hydrologic Methodology <br />The SCS Curve Number method has been used to determine runoff volumes and runoff flow <br />rates. The runoff curve number is a factor relating the amount of rainfall to the amount of runoff <br />for a given area. The best reference for application of this method to coal mine applications is <br />Barfield, Warner, & Haan (198 1) in "Applied Hydrology and Sedimentology for Disturbed <br />Areas", Oklahoma Technical Press, Stillwater, OK. Tables are included in this reference, as <br />well as several widely accepted NRCS (formerly SCS) publications, present the appropriate CN's <br />(curve numbers) for specific land uses and conditions. <br />Although the volume of runoff can be calculated manually using the SCS Curve Number <br />method, for this evaluation the SEDCAD computer model, developed and marketed by Civil <br />Software Design, was used to make the runoff calculations. In addition to its wide acceptance <br />for use in coal mine permitting work, it also provides a consistent tabular output of all design <br />assumptions and all calculated results. <br />Location Specific Hydrologic Parameters <br />The 10 year, 24 hour precipitation event for permitting efforts for this location has been <br />identified as 2.0 inches. The 100 year, 24 hour precipitation event produces 3 0 inches of rain. <br />Site Specific Hydrologic Parameters <br />Curve numbers were designated on a land use basis as determined from field observations and <br />are presented on Table 2.05.3(3) -IA. <br />Attachment 2.05.3(3)-28 Revision Date: 11/24/15 <br />Page 1 Revision No.: TR -69 <br />