Laserfiche WebLink
NOV C-94-037 <br />West Pit Sediment Control Practices <br />Background <br />CDMG rules are not specific as to how the perimeter sediment <br />control structures (ditches and ponds) should be designed. <br />Rule 4.05.5 states that "The sedimentation storage capacity of <br />practices in and downstream from the disturbed area shall reflect <br />the degree to which successful mining and reclamation techniques <br />are applied to reduce erosion and control sediment. Sediment <br />control measures consist of the utilization of proper mining and <br />reclamation methods and sediment control practices, singly or in <br />combination. Sediment control methods include but are not limited <br />to: <br />... (c) retaining sediment within disturbed areas, ... <br />(f) Using ... dugout ponds, and other measures that reduce <br />overland flow velocity, reduce runoff volume, or trap sediment;..." <br />Regarding how sedimentation ponds should be designed, Rule <br />4.05.6(3)(a) states "... In determining the runoff volume, the <br />characteristics of the mine site and reclamation procedures and on- <br />site sediment control practices shall be considered." <br />In summary, sedimentation pond design methodologies are not <br />specified but clearly, the design must reflect upstream mining and <br />reclamation practices. Nowhere in the regulations are operators <br />required to ignore mining and reclamation practices in modeling in <br />favor of modeling based upon the search for a "worst case <br />scenario." <br />Historically, Colowyo has always chosen to design ponds using <br />modeling assumptions that included flow calculations and sediment <br />yield from upstream areas consisting of a combination of <br />revegetated mining areas, undisturbed areas and the compensatory <br />effect of surface water interception by the pit in conjunction with <br />adjacent disturbed areas (Permit page 2.04.7-87). By utilizing <br />this methodology, we believe our as-built sedimentation ponds truly <br />represent the intent of the regulations and reflect upstream mining <br />and reclamation practices while providing appropriate sediment <br />control. In addition, the structures were intuitively designed to <br />reflect long term revegetated conditions of the minesite and as <br />such, have some theoretical long term utility with respect to <br />controlling runoff and sediment from reclaimed areas. In addition <br />to meeting the regulatory requirements, all Colowyo sedimentation <br />ponds have maintained complete compliance with NPDES discharge <br />standards. <br />Colowyo's designs for the West Pit Pond were similarly modeled <br />and initially submitted to the Division in August, 1991. Between <br />the initial submittal date and the approval date of January 14, <br />1993, Division personnel changed four times, resulting in four <br />changes to modeling assumptions required of Colowyo. <br />