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Pond 015 Postmining SEDCAD Modeling <br />This sediment control plan update addresses the current conditions of the watershed tributary to <br />Pond 015 to confirm its capability to contain without discharging the 100 year, 24 hour storm for <br />its current tributary watershed. The system collects mainly the runoff for existing Ditch C-29 <br />and an adjacent berm, comprising a tributary area of 2.74 acres, into a below grade sediment <br />control pond excavated at the west end of this tributary area. The tributary area is conservatively <br />assumed to be at curve number of 83 although portions of the watershed are undisturbed land <br />with a lower curve number. <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 015 Area hereafter referred to simply as "Map 1 ". <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 estimating annual sediment accumulation for this <br />location has been identified as 2.0 inches. The 100 year, 24 hour precipitation event for storm <br />containment analysis for this location has been identified as 3.0 inches. <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 />Other System Components <br />As shown on Map 1, one existing ditch, Ditch C29, routes the entire tributary area to the pond. <br />The ditch is modeled a trapezoidal erodible channel of cobbles and shingles, with a 4 -foot <br />bottom and 1H:1V sideslopes, and with an average gradient of 2.0% as determined from field <br />surveys, as depicted on Map 1. Several cross sections are also shown on Map 1. The actual side <br />slopes and bottom width may vary in the field, and the ditches may have a flatter side slope or a <br />wider bottom in some locations. Some reaches may be cut in rock. As such, the actual ditch <br />Attachment 2.05.3(3)-33 Revision Date: 1/08/2016 <br />Page 2 Revision No.: TR -70 <br />