2016-05-03_PERMIT FILE - C1981008 (8)

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Pond 016 Postmining SEDCAD Modeling This proposed sediment control plan addresses the future condition of the watershed tributary to Pond 016 which includes lands in "topsoiled and seeded" condition and 3+ years of reclamation. The system collects all runoff from the 120.65 acres into a below grade sediment control pond to be constructed in the southwest corner of this area. This area is currently draining to Pond 013 via the Tailwater Ditch along the north boundary, the C5 ditch along the south boundary, the postmining channel the west boundary below the Tailwater Ditch, and direct overland flow into Pond 016 from internal areas within the watershed. The design considerations used for this evaluation are described in the following paragraphs. The watershed boundaries, associated drainage areas, and runoff s are shown on Map 1, Postmining Drainage Analysis, Pond 016 Area, hereafter referred to as "Map V. Pond design details are provided on Map 2, Pond 016 Plan View and Design Details. Hydrologic Methodology The SCS method has been used to determine runoff volumes and runoff flow rates. The runoff is a factor relating the amount of rainfall to the amount of runoff for a given area. The best reference for application of this method to coal mine applications is Barfield, Warner, & Haan (198 1) in "Applied Hydrology and Sedimentology for Disturbed Areas", Oklahoma Technical Press, Stillwater, OK. Tables are included in this reference, as well as several widely accepted NRCS (formerly SCS) publications, present the appropriate CN's (s) for specific land uses and conditions. Although the volume of runoff can be calculated manually using the SCS method, for this evaluation the SEDCAD computer model, developed and marketed by Civil Software Design, was used to make the runoff calculations. In addition to its wide acceptance for use in coal mine permitting work, it also provides a consistent tabular output of all design assumptions and all calculated results. Location Specific Hydrologic Parameters The 10 year, 24 hour precipitation event for permitting efforts for this location has been identified as 2.0 inches. The 25 year, 24 hour precipitation event produces 2.4 inches of rain. The 100 year, 24 hour precipitation event is 3.0 inches. Site Specific Hydrologic Parameters Curve numbers were designated on a land use basis as determined from field observations and are presented on Table 2.05.3(3)-1 A. Other System Components Along the northern boundary, existing Tailwater Ditch routes the northern and easternmost parts of the affected area runoff to the postmining channel below the Tailwater Ditch, then southerly toward the pond, as shown on Map 1. The Tailwater Ditch is modeled as vegetative lined trapezoidal ditch with a 1 to 2 -foot bottom, 1 H:1 V to 2H:1 V sideslopes, and with gradient varying between 0.3% to 4.6%. as determined from field surveys, and as depicted on Map 1. Attachment 2.05.3(3)-29 Revision Date: 1/08/2016 Page 2 Revision No.: TR -70