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Results of the 100 -year peak flows are summarized here for each basin: <br />BASIN RUNOFF DESCRIPTION <br />Offsite 10 Q100 = 2.44 cfs <br />Offsite 20 Q100 = 8.56 cfs <br />Onsite 30 Q100 = 3.18 cfs <br />Onsite 40 Q100 = 4.42 cfs <br />7.4 Stormwater Routing and Retention <br />ONSITE BASIN <br />8.0 Stormwater Management Plans <br />8.1 Best Management Practices <br />BMPs for the site include the following: <br />Undisturbed pinyon/juniper stands <br />Undisturbed pinyon/juniper stands <br />Disturbed, stockpile areas <br />Disturbed, mine yard 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 <br />Sheets in the back of this report. Analysis shows that the diversion channels are, or will be, <br />capable of transporting the 100 -year flows generated by their respective basins around the site <br />without overtopping the banks of the channel. Calculated velocities and 100 -year water - surface <br />elevations are shown at each cross - section. The proposed culvert was sized for the calculated <br />runoff produced to operate within allowable headwater and tailwater conditions using "Haestad <br />Culvertmaster" software. Worksheets and results for the software analysis are included in the <br />attachments. <br />The retention pond volume was calculated using the Haestad Pond Pack software utilizing SCS <br />methods. DRMS requires complete retention of the 10 -year runoff, with the ability to properly <br />pass the 100 -year flows downstream. Analysis of the proposed retention pond indicates it will <br />exceed this standard by being large enough to collect and completely store onsite runoff from the <br />100 -year event with no discharge downstream. Worksheets and results of the stormwater routing <br />and retention calculations are included in the attachments, but a summary follows: <br />TOP OF POND ELEV 100 -YR WATER- SURFACE <br />Pond 50 (Basins 30 & 40) Top of bank: 6420.00 6419.03 <br />7.5 Recommended Modifications to Existing BMP Structures <br />Given the information provided in this report, analysis has shown that the proposed site BMP's <br />(diversion channels, culverts, containment berms, and retention pond) are properly sized and <br />capable of managing stormwater for the site in a manner meeting or exceeding DRMS criteria. <br />ESWMP -6 <br />