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velocities and 100 -year water - surface elevations are shown at each cross - section for the given <br />runoff and slope. Channel slopes were input for maximum and minimum proposed slopes for <br />each channel. No velocities exceeded 5 feet -per- second for the 100 -year flows. Worksheets and <br />results for the " Haestad F1owMaster" software analysis are included in the attachments. <br />7.4 Stormwater Routinz and Retention <br />The required retention pond volume was calculated using the Haestad Pond Pack software <br />utilizing SCS methods. DRMS requires complete retention of the 10 -year runoff, with the ability <br />to temporarily detain and properly pass the 100 -year flows downstream. Given the location of <br />the adjacent Uravan disposal area and the results of some onsite soil samples, it was decided to <br />provide complete retention of the 100 -year event as an extra factor of safety. Analysis of the <br />proposed retention pond indicates it is large enough and capable of collecting and storing the <br />100 -year event with no discharge downstream. Additional volume is available to provide storage <br />of sediment from initial storms without necessitating removal. Worksheets and results of the <br />stormwater routing and retention calculations are included in the attachments, but a summary <br />follows: <br />ONSITE BASIN TOP OF POND ELEV 100 -YR WATER- SURFACE <br />Basin 30 (mine yard area) Top of pond: 5713.00 5711.01 <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, containment berm, and retention pond) are properly sized and capable of <br />managing stormwater for the site in compliance with DRMS criteria. <br />8.0 Stormwater Management Plans <br />8.1 Best Management Practices <br />BMPs for the site include the following: <br />• Divert offsite runoff around stockpile areas <br />• Retention of onsite surface runoff <br />• Secondary containment of above ground fuel tank <br />• Proper storage of petroleum products <br />• Good site housekeeping <br />The above ground diesel fuel tank should be locked when personnel are not present at the site. <br />Secondary containment at the site includes a lined retention basin capable of containing at least <br />110 percent of the volume of the tank. Periodic inspections of the containment area are to ensure <br />that precipitation is pumped from the containment area as needed. <br />ESWMP -6 <br />