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Pond 018 Sediment Control System <br />This sediment control plan revision will isolate and separate the active New Horizon 2 Mine <br />shop area away from the rest of the mine's sediment control system. The plan will collect all <br />runoff from the 21.09 acres into a sediment control pond to be excavated near the south end of <br />this area. This area is currently draining to Pond 007 via the C18 ditch. Pond 018 will be <br />designed under the requirements of Colorado DRMS Rule 4.05.9 (2)(e)(ii) and operated as a zero <br />discharge facility for stormwater runoff up through the 100 year, 24 hour storm. <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 />Map 1. Pond design details are provided on Map 2. <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 />Area - The 21.09 acres of still active lands in the southeast edge of the New Horizons Mine <br />includes 5.64 acres of recently reclaimed land (1 to 2 years revegetation growth), 6.61 acres of a <br />topsoil storage pile, 0.31 acres of former farmstead and finally the remaining 8.53 acres consists <br />of active yard, shops and access road areas. <br />Rainfall- Section 2.05.3(3) shows the 10 year, 24 hour precipitation event is 2.0 inches and the <br />100 year, 24 hour precipitation event is 3.0 inches of rain. <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: 04/21/16 <br />Page 2 Revision No.: TR -72 <br />