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Pond 013 Postmining SEDCAD Modeling <br />This sediment control plan addresses the condition of the watershed tributary to Pond 013 which <br />is assumed to be completely backfilled to the postmining topographic surface but only recently <br />reclaimed and retopsoiled. As such the entire area will be in a "topsoiled and seeded" condition. <br />The system collects all runoff from the 66.96 acres into an existing below grade sediment control <br />pond (Pond 013) near the northwest corner of this area. Two SEDCAD models accompany this <br />text discussion. <br />Except for a small (0.76) acre band of undisturbed land immediately adjacent to the C9 Ditch, <br />and another 1.14 acre undisturbed area immediately south of the pond, the entire watershed is <br />represented as "Topsoiled and Seeded" land. <br />In addition to the rainfall runoff inflow from the above described areas, there may also be inflow <br />collected from the trench drain in the reclaimed area as well as inflow from various other sources <br />such as Tuttle Draw and CCC irrigation ditch as described in detail later in this summary. <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, Postmining Drainage Analysis, Pond 013 Area, hereafter referred to as Map 1. <br />Hydrologic Methodology <br />The SCS method has been used to determine runoff volumes and runoff flow rates. The runoff <br />is a factor relating the amount of rainfall to the amount of runoff for a given area. The best <br />reference for application of this method to coal mine applications is Barfield, Warner, & Haan <br />(198 1) in "Applied Hydrology and Sedimentology for Disturbed Areas ", Oklahoma Technical <br />Press, Stillwater, OK. Tables are included in this reference, as well as several widely accepted <br />NRCS (formerly SCS) publications, present the appropriate CN's (s) for specific land uses and <br />conditions. <br />Although the volume of runoff can be calculated manually using the SCS method, for this <br />evaluation the SEDCAD computer model, developed and marketed by Civil Software Design, <br />was used to make the runoff calculations. In addition to its wide acceptance for use in coal mine <br />permitting work, it also provides a consistent tabular output of all design assumptions and all <br />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 25 year, 24 hour precipitation event produces 2.4 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)-32 Revision Date: 3/09/2016 <br />Page 2 Revision No.: TR -70 <br />