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' I , <br /> � <br /> ' The re-develo ment h dro ra h for <br /> p I p y g p the 25 year, 1 hour storm event shows a runoff <br /> � volume of 0.51 Acre - feet and the post-development hydrograph for the 25 year, 1 hour � <br /> storm (see hydrographs in Appendix B) shows a runoff volume of 0.72 Acre —feet. The <br /> following calculations show that the difference of the runoff volumes for the pre- and post- <br />� � ; <br /> development storm events are accommodated in the proposed stormwater facility: <br /> � , <br /> � <br /> !0.72 Ac-ft - 0.51 Ac-ft = 0.21 Ac-ft =9,148 cf (For this project the pre/post volume <br /> ;difference is greater than the half inch volume.) <br /> � <br />��I � The proposed stormwater facility will have a top of bank at elevation 91.5 feet with <br />; ;four to one side slopes to the bottom of the facility at elevation 84.5 feet. The <br /> � , volume difference is contained below an elevation of 86.5 feet as shown by the <br /> average end area method (calculation) below: <br /> ' � <br />� � ;Facility Area at 84.5 ft = 3,909 sf <br /> i r Facility Area at 86.5 ft = 6,154 sf <br /> i <br />�� Facility Volume between EI. 84.5 and 86.5 = <br />��� t (3,909 sf+ 6,154 sfl x (86.5— 84.5 ft) = 10,063 cf <br /> 2 <br />�, � l'he volume difference is recovered b e c <br /> � <br /> y p r olation in less than 36 hours as can be seen in <br />' the mounding analysis in Appendix A. For example, the mounding analysis shows that <br />� , after olne hour 22,139 cubic feet of runoff have been recovered by percolation, which is <br /> greater than required 9,148 cubic feet. <br />�' � � <br /> � <br /> � ; <br />'� � <br /> � <br /> ' <br /> � CadM/data/99/6001-99/6001-99drainage <br /> I <br /> I <br />