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i <br /> 4 <br /> An inflow design flood hydrograph using a probable maximum thunderstorm <br /> 1-hour rainfall was prepared for proposed tailing pond No. 4. <br /> An inflow design flood hydrograph using less than a probable maximum <br /> thunderstorm 1-hour rainfall was prepared for abandoned tailings ponds <br /> No. 1 , No. 2, and No. 3. <br /> The design peak flood discharges, determined from the design hydrographs, <br /> were used to size diversion ditches that will divert surface runoff around <br /> the tailing ponds and to size culverts that will pass diverted runoff <br /> under State Highway 110. <br /> The design of the decant system for the proposed pond No. 4 was evaluated <br /> ,i <br /> to determine its capacity for passing storm water. <br /> 100-year design flood calculations, based on peak snowmelt runoff, were prepared <br /> i <br /> to estimate a design flood stage for the Animas River along the proposed i <br /> • I <br /> new alignment for State Highway 110. <br /> :SIGN FLOOD HYDROGRAPHS <br /> MESA guidelines recommend that both the probable maximum thunderstorm i <br /> � I <br /> hour duration and the probable maximum 6-hour general-type storm rainfall <br /> ;nflow design floods be prepared for tailing impoundments. However, for <br /> � 'Iis report, only the probable maximum thunderstorm design flood was considered. <br /> Tt is believed that it will be the more critical of the two types of inflow <br /> dg o Y <br /> esi n floods, because of the small size of the subject drainage <br /> very <br /> ,,teas (less than one square mile). <br /> The procedure used to compute the inflow design floods for the ungauged <br /> drainages at the site is based on that given in the Bureau of Reclamation' s <br /> 'Design of Small Dams". From the above reference, the probable maximum ' <br /> thunderstorm 1-hour point rainfall for the Silverton area is 8.5 inches.' <br /> i <br /> I �i ;spy <br /> 19 ";,i <br />