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attachments. Primary input requirements are: runoff curve number (CN), precipitation, and time <br />of concentration. NOTE: The three offsite basins are labeled "Offsite 10 ", " Offsite 10A ", and <br />"Offsite 20 ", and the one onsite basin is labeled "Onsite 30 ". Offsite basin 10 flows generally to <br />the south around the west side of the waste rock in the large natural channel. It contains over <br />200 acres, mostly on top (Monogram Mesa), but also includes a smaller 3.6 acre subarea (basin <br />10 A) which lies just north of the mine area. This small basin is included independently in the <br />analysis because it drains into the north side of the site where flows are diverted to the west, by <br />the waste rock stockpile, into the large open drainage. A cross - section of this interface is <br />included in the attachments. <br />Curve numbers were taken from tables available from many sources based on surface soil types <br />and vegetation. Surface soils at the site are considered "Hydrologic Soils Group B" (soils having <br />a moderate infiltration rate when thoroughly wetted and consisting chiefly of moderately deep to <br />deep, moderately well to well drained soils with moderately fine to moderately coarse texture) <br />given the soils report provided by NRCS. Surface vegetation on the hillsides and around the <br />mine area is considered "pinyon/juniper" with either poor or fair stands or a weighted <br />combination of both. The large offsite basin has mainly flat sandy areas of sagebrush with grass <br />understory and smaller areas with varying concentrations of pinyon/juniper stands which are all <br />undisturbed; therefore, it received weighted combinations of CN values (varying from 58 to 75). <br />The onsite basin is mainly disturbed areas void of vegetation (partially considered "impervious, <br />dirt" for purposes of determining CN) with other areas containing various stands of <br />pinyon/juniper and was therefore assigned a weighted CN varying from 58 to 72. <br />Times of concentration were computed for each basin utilizing widely accepted formulas for <br />computation of sheet -flow, shallow concentrated flow, and channel -flow from the headwaters to <br />the receiving discharge point or storage area. <br />Results of the 100 -year peak flows are summarized here for each basin: <br />BASIN <br />Offsite 10 <br />Offsite l0A <br />Offsite 20 <br />Onsite 30 <br />RUNOFF <br />Qioo = 53.04 cfs <br />Qioo = 3.91 cfs <br />Qioo 9.46cfs <br />Qioo = 4.53 cfs <br />DESCRIPTION <br />Undisturbed sage /grass & pinyon/juniper stands <br />Undisturbed pinyon/juniper stands <br />Undisturbed pinyon/juniper stands, imp. soil <br />Disturbed, mine yard area, and stockpile areas <br />Capacities of the existing and proposed diversion channels were analyzed using Manning's <br />Equation via Haestad "FlowMaster" software for the surveyed cross - sections (existing channels) <br />and proposed cross - sections (improved channels) at the locations shown on attached detail sheets <br />in the back of this report. Analysis shows that the diversion channels are, or will be, capable of <br />transporting the 100 -year flows generated by their respective basins around the site without <br />overtopping the banks of the channels. Calculated velocities and 100 -year water - surface <br />elevations are shown at each cross - section. Worksheets and results for the software analysis are <br />included in the attachments. <br />ESWMP -6 <br />