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7.2 Design Storms <br />DRMS requires analysis using 10 -year and 100 -year, 24 -hour storm events. The National <br />Oceanic and Atmospheric Administration (NOAA) Atlas 2, Volume III isopluvials were used in <br />conjunction with site locations (latitude and longitude) to determine precipitation amounts for the <br />given storms. Copies of these charts are included in the attachments. <br />7.3 RunoMischarze Calculations <br />Peak flows were determined for the 100 -year storm in all basins to determine required capacities <br />for the diversion channels. SCS Unit Hydrograph Methods were applied utilizing "Haestad Pond <br />Pack" software. Worksheets and results produced by the software are included in the <br />attachments. Primary input requirements are: runoff curve number (CN), precipitation, and time <br />of concentration. NOTE: The two offsite basins are labeled "OFF 10" and "OFF 20 ", and the <br />one onsite basin is labeled "30 ", or "ONSITE 30 ". <br />Curve numbers were taken from tables available from several 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 is considered " Pinyon/Juniper" <br />with either poor or fair stands or a weighted combination of both. The larger offsite basins have <br />areas with varying concentrations of pinyon/juniper stands and are undisturbed; therefore, they <br />received weighted combinations of CN values (varying from 58 to 75). The onsite basins are <br />mainly disturbed areas void of vegetation and were therefore assigned a CN of 75 for "poor" <br />stands in Type `B" soils. <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 storage area. <br />Results of the 100 -year peak flows are summarized here for each basin: <br />BASIN RUNOFF DESCRIPTION <br />Offsite 10 Q100 = 1.40 cfs Undisturbed pinyon/juniper stands <br />Offsite 20 Q100 = 2.38 cfs Undisturbed pinyon/juniper stands <br />Onsite 30 Q100 = 3.84 cfs Disturbed, mine yard area, and stockpile areas <br />Capacities of the diversion channels were analyzed using Manning's Equation for improved <br />cross - sections at locations shown on the attached sheets. Channel analysis shows that the <br />improved diversion channels will be capable of transporting the 100 -year flows generated by <br />their respective basins around the site generally within the bottom 12" of the channel. Actual <br />ESWMP -5 <br />