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The runoff curve number (CN) is a factor relating the amount of rainfall to the runoff based on the landuse, <br />vegetation and hydrological soil type. The areas covered by the ponds are disturbed areas. The landuse and <br />vegetation type are chosen as "Newly Graded Area" in the SEDCAD program. The hydrological soil type is <br />classified as "C" based on the information from "Map 2.04.9 -1 New Horizon North Mine NRCS Soils Map" and <br />"Soil Survey of San Miguel Area, Colorado ". The curve number 91 was selected from SEDCAD curve number <br />table. <br />Sediment <br />The sediment data required for pond design are the particle size distribution and total sediment yield during a <br />runoff event. <br />The most important sediment data required to design a sedimentation pond to meet effluent limitation is the <br />particle size distribution of the sediment influent. With no site specific particle size distribution data available, the <br />best estimation of particle size distribution is documentation of previous and nearby mining operations. <br />Unfortunately, for the New Horizon North Mine, previous and nearby data is not readily available. <br />Hence the particle size distribution is based on the site specific soil textural class and physical properties. Table 1 <br />of soil textural class and its size distribution was developed based on the "Map 2.04.9 -1 New Horizon North Mine <br />NRCS Soils Map" and nearby soil survey report mentioned above. The particle size distribution of the "Loam -clay <br />loam" was used for calculations. Additionally, the sediment yield for the design storm event is calculated using <br />the Modified Universal Soil Loss Equation contained in the SEDCAD program. <br />Drainage Network Overview <br />Ponds and collection ditches make up the drainage network for the site. Figure 1 shows an overview of the pond <br />and ditch locations throughout the site. <br />Effluent Limitations <br />The solids effluent quality limitation is an average daily value of 35 mg /I of total suspended solids (TSS) for 30 <br />consecutive days and a maximum of 70 mg /I TSS for any one day. <br />The settleable solids concentration limitation is 0.5 ml /I during any discharge or overflow resulting from a <br />precipitation event less than or equal to the 10 -year, 24 -hour precipitation event. Any discharge or overflow <br />resulting from a precipitation event greater than the 10 -year 24 -hour precipitation event is not subject to this solids <br />effluent quality limitation. <br />All NHN sedimentation ponds are gated and manually controlled by valves. For precipitation events less than or <br />equal to the 10 -year 24 -hour event, discharge is released when the water quality meets the effluent limitation. <br />Storage Volume Requirement <br />The design criteria of the three ponds abide by Rule 4.05.9(2)(b). The ponds are designed with a combination of <br />a principal and an emergency spillway. Additionally, there is no out -flow through the emergency spillway during <br />the passage of runoff for the 10 -year 24 -hour precipitation event. The principal spillways of three ponds are gated <br />and manually operated. The principal spillways discharge after water quality requirements are tested. The <br />designed capacity of the sedimentation ponds should be able to contain the runoff from a 10 -year 24 -hour event. <br />2 <br />