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1. Design Methodology <br />The precipitation runoff (peak flow and volume) is estimated using the Soil Conservation <br />Service (SCS) 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 <br />the watershed, sedimentology information, spillway information, etc. <br />The precipitation amounts for the New Horizon Mine for the design storm durations were <br />obtained from the NOAA Precipitation Frequency Atlas Precipitation - Frequency Atlas of the <br />Western States Volume III - Colorado, 1973, and are listed below: <br />10 -year, 24-hour <br />- 2.0 inches <br />25 -year, 24-hour <br />- 2.4 inches <br />100 -year, 24-hour <br />- 3.0 inches <br />The runoff curve number (CN) is a factor relating the amount of rainfall to the amount of runoff <br />for a given area. The soil erodibility factor ("k" factor) is a factor of a soils susceptibility to <br />erosion and infiltration capacity. These factors are dependent upon the infiltration characteristics <br />of the soil types and the type and amount of vegetation. Tables 2.05.3(3)-1, 2.05.3(3) -IA, and <br />2.05.3(3)-2 in list the values of CN and "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 <br />average. The undisturbed area curve number and "k" factor information is based on the soils and <br />vegetation information found within Sections 2.04.9 and 2.04.10, respectively, of the New <br />Horizon mine permit application. <br />The sediment storage is calculated using the Modified Universal Soil Loss Equation contained <br />within the SEDCAD computer program. The spillways are designed using the spillway routing <br />subroutine program in the SEDCAD computer program and applicable engineering procedures <br />as described in the text of each sedimentation pond's design report. All sediment ponds are <br />designed and located to capture surface runoff from upstream disturbance areas and to comply <br />with all applicable State and Federal regulations. <br />Page 2.05.3(3)-8 October 2015 (TR -69) <br />