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The precipitation runoff (peak flow and volume) is estimated using the Soil Conservation Service (SCS) <br />• triangular hydrograph techniques. <br />Design of Small Dams (U.S. Dept. of Interior, 1977). <br />Computations necessary for this runoff estimating technique have been computerized by various <br />individuals and agencies. The SEDCAD program was used for the design of Pond 007. <br />The input consists of precipitation amount (inches), watershed area (acres), time of concentration <br />(hours), runoff curve number (CN), representative particle-size distributions of the soil types in the <br />watershed, sedimentology information, spillway information, etc. <br />The precipitation amounts forthe New Horizon Mine forthe design storm durations were obtained from <br />the NOAA Precipitation Frequency Atlas Precipitation - FreauencyAtlas of the Western States Volume <br />III -Colorado, 1973, and are listed below: <br />10-year, 24-hour - 2.0 inches <br />25-year, 24-hour - 2.4 inches <br />100-year, 24-hour - 3.0 inches <br />• <br />The runoff curve number (CN) is a factor relating the amount of rainfall to fhe amount of runoff for a <br />given area. The soil erodibility factor ("k" factor) is a factor of a soils susceptibility to erosion and <br />infiltration capacity. These factors are dependent upon the infiltration characteristics of the soil types <br />and the type and amount of vegetation. Tables 2.05.3(3)-1 and 2.05.3(3)-2 in list the values of CN and <br />"k" for different soils types at the New Horizon Mine. <br />The curve number (CN) and "k"factor for each of the ponds is determined as a weighted average. The <br />undisturbed area curve number and "k" factor information is based on the soils and vegetation <br />information found within Sections 2.04.9 and 2.04.10, respectively, of the New Horizon mine permit <br />application. <br />The sediment storage is calculated using the Modified Universal Soil Loss Equation contained within <br />the SEDCAD computer program. The spillways are designed using the spillway routing subroutine <br />program in the SEDCAD computer program and applicable engineering procedures as described in <br />the text of each sedimentation pond's design report. <br />• (Revised ~/zoos) 2.05.3(3) - 9 <br />