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a <br /> Ali <br /> This value, as required by MESA, was used to determine the inflow design <br /> flood for proposed tailing pond No. 4. This conservative approach was <br /> i <br /> taken because the embankments around proposed pond No. 4 will not be designed <br /> to impound probable maximum storm water; therefore, probable maximum precip- <br /> itation could result in failure of the structure due to overtopping. <br /> The inflow design floods for abandoned tailing ponds No. 1 , No. 2 <br /> and No. 3 were prepared for less than 100% of the probable maximum precip- <br /> itation (i.e. 2.3 inches). It is assumed that the surface of these ponds <br /> will be graded to preclude storm water impoundment and that failure would <br /> cause minor loss of structure with little damage of property and project <br /> operation. <br /> or the tailing ponds are designated as A, B, C <br /> The drainage areas f 9 P +i l + <br /> and D (refer to Figure 1 ). Triangular, 15 minute incremental runoff hydro- ! <br /> graphs were constructed to determine the total runoff hydrograph and peak <br /> discharge for a thunderstorm of 1-hour duration for each drainage. Drainage <br /> lag time (L) and time of concentration (Tc) of direct runoff from each j <br /> drainage was determined by the SCS method, where Tc = 0.6 L. Tables and <br /> formulas from the "Design of Small Dams" were used to determine the direct ! j <br /> runoff curve number (CN) and to calculate peak incremental runoff, time k <br /> from start of rise to peak rate of incremental runoff, and the time base <br /> for the incremental hydrographs. Refer to Figures II-2, II-3, II-4, and <br /> i <br /> II-5 for the design runoff hydrographs. <br /> � I <br /> DIVERSION DITCH AND CULVERT SPECIFICATIONS <br /> Diversion Ditches <br /> The design peak flood discharge values taken from the total runoff <br /> hydrographs for drainages A, B, C and D were used to establish design i <br /> fi t <br /> 20 <br /> A <br />