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• <br />I. `~ <br />BACKGROUND <br />CIIRYE NIIMBER JLfBTIFICATION <br />Prior to developing the current SEDCAD Model of the Colowyc <br />Mine, discussions were held with Berhan Keffelew, previous <br />Reclamation Specialist and hydrology contact, MLRD. The current <br />model evolved from those discussions, utilizing the postmining <br />topography and undisturbed areas encompassed by the watershed <br />boundaries, as the worst case scenario with the maximum acreage <br />contributing runoff to the sediment ponds. This meant that there <br />would be two distinct types of land conditions taken into account <br />in the SEDCAD model, namely, the fully reclaimed grassland or <br />range, brush, weed, grass mix and the natural, undisturbed, semi- <br />arid rangeland. <br />It was agreed that the mining process does create areas in <br />which the runoff is increased (topsoil stripping for example), but <br />runoff from these limited acreage sites is most likely to be <br />intercepted and detained by the mine pit where it evaporates, <br />infiltrates, or is otherwise lost to the surface drainage system. <br />Additionally, it was agreed that during the process of mining, the <br />mine pit serves as a major sediment and runoff control structure, <br />intercepting runoff from a great portion of the watershed. By <br />rights, the quantity of runoff and sediment contained in the pit <br />should be taken into account in sediment pond sizing, but since the <br />mine pit and affected acreage is continually changing, no accurate <br />model could be developed, or accepted by MLRD, to reflect this <br />dynamic condition. Hence, the arrival at the current model. <br />Curve Number <br />As per the SEDCAD Version 3 Training Guide, page 5-15, <br />"The SCS Curve Number designates the runoff potential of <br />an area. Curve numbers depend on land use, treatment, <br />hydrologic soil group, and antecedent moisture <br />condition". <br />Additionally, as per Rule 4.05.6(3)(aj, <br />"In determining the runoff volume, the characteristics of <br />the mine site and reclamation procedures, and on site <br />sediment control practices shall be considered". <br />A discussion of each of these factors affecting the Curve <br />Number, follows. <br />• As per Colowyo's mine permit, page 2.04.3-5, Existing <br />-' Land Use and Land Use Classifications Under Local Law, the <br />lands within the permit and adjacent areas have been <br />