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• The input consists of precipitation amount (inches), watershed area (acres), time of <br />concentration (hours), runoff curve number (CN), representative particle-size distributions ofthe <br />soil types in 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 - Freguencv Atlas of the <br />Western States Volume 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 />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 <br />characteristics of the soil types and the type and amount of vegetation. Tables 2.05.3(3)-1 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 <br />and 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 />With regards to Pond 007, it is currently over-designed for its watershed area: Originally, the <br />watershed area was calculated to be approximately 514 acres (see Map 2.05.3(3)-2-2), and the <br />pond was built in 1995 using that value. Pond 008 was permitted in 1999 and built in 2000 in <br />the western portion of the Pond 007 watershed. The Pond 008 drainage area directly subtracts <br />from the original Pond 007 watershed, so the Pond 007 watershed has been reduced to about <br />423 acres. Because of this significant reduction in area (17.7%), new run-off calculations for <br />• Pond 007 were not done. <br />(Revised duy zoos) 2.05.3(3)-14 <br />