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Inputs include precipitation (inches), watershed area (acres), time of concentration (hours), runoff <br />• curve number, representative particle size distribution of soils in the area, sedimentology <br />information, spillway dimensions, etc. The drainage area for the watershed, as well as, slope lengths <br />and distances for time of concentration estimates were measured from permit topographic maps (see <br />Map 2.05.3 -M4). <br />Precipitation and Storm Type: The precipitation amounts for the design storms were obtained from <br />Frequency Atlas of the Western United States — Volume 11 — Colorado (National Oceanic <br />Atmospheric Administration [NOAA], 1973). A standard Soil Conservation Service (SCS) Type II <br />storm was utilized for each analysis. <br />• 10 -year, 24 -hour event: 1.7 inches <br />• 25 -year, 24 -hour event: 2.1 inches <br />SCS Curve Number and "k" Factor: The approved curve number (CN) values used for the SEDCAD <br />demonstration are presented on Table 2.05.3 -E2 -1 in Exhibit 2.05.3 -E2. The "k" Factor values were <br />based on approved soil survey information adapted from the Seneca II Mine Permit (Permit C -80- <br />003) and National Resources Conservation Service soil interpretations. The curve numbers and "k" <br />factors were then "weighted" for each sub - watershed as presented in the Hydrology Input Parameter <br />tables found at the back of Addendum 2.05.3- E2.A -1. <br />• Sedimentology: To determine the sediment loading to Pond 002, the average annual "R" method <br />was used. The sediment storage volume required is determined based on the ratio of the RUSLE <br />annual R factor to the calculated R storm value and annual sediment yield to the storm sediment <br />yield. The number of years of sediment storage (Y) and the average annual R factor are input values. <br />The design analysis used an annual R value of 10. The value is representative of watersheds <br />dominated by undisturbed and/or successfully reclaimed and revegetated landscapes in northwestern <br />Colorado (Figure 8A.1, Isolines of R factor for Western U.S. [from Design Hydrology and <br />Sedimentology for Small Catchments, Haan et. al, 1993]) — approximately 94% of the contributing <br />watershed is either undisturbed or successfully reclaimed and revegetated. Previous SEDCAD <br />evaluations of reclaimed conditions at the Seneca II Mine, approved by CDRMS, used this value for <br />reclaimed conditions. <br />A sensitivity analysis was conducted by varying the number of storage years (Y) which allowed the <br />design to maximize storage capacity and maintain compliance with the applicable rules and <br />regulations. The number of storage years was increased in the sedimentology sub - routine until the <br />model results indicated the pond discharge exceeded the water quality criterion (0.5 milliliters per <br />liter [ml/1]), or until there was approximately 1.5 to 2 feet of elevation difference between the top of <br />the top of sediment storage and combination spillway invert elevation). This elevation difference is <br />within the range recommended by Warner et al (1998). The SEDCAD4 model results presented <br />• herewith reflect a (Y) value that allows for the maximum sediment storage while retaining capacity <br />to store the design storm runoff and maintain compliance with applicable rules and regulations. <br />2.05.3 -E2.A3 12/09 <br />