2010-05-07_PERMIT FILE - C2009087 (34)

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4.0 SEDCAD Demonstration The hydrology and sedimentology of the watershed were analyzed using the SEDCAD4 computer program (Schwab, 1998). Model methodology and assumptions are presented in Exhibit 2.05.3 -E2, Drainage and Sediment Control Plan. Addendum 2.05.3- E2.D -1 and 2.05.3- E2.D -2 of this appendix present the modeling demonstrations for the 10 -year, 24 -hour and 25 -year, 24 -hour design storm events, respectively. The pond configuration in the model is based on the values contained in the Seneca II Mine as -built report prepared as part of TR -37 (see Attachment 2.05.3- E2.D -2). Inputs include precipitation (inches), watershed area (acres), time of concentration (hours), runoff curve number, representative particle size distribution. of soils in the area, sedimentology information, spillway dimensions, etc. The drainage area for the watershed, as well as, slope lengths and distances for time of concentration estimates were measured from permit topographic maps (see Map 2.05.3 M4). Precipitation and Storm Type: The precipitation amounts for the design storms were obtained from Frequency Atlas of the Western United States — Volume 11 — Colorado (National Oceanic Atmospheric Administration [NOAA], 1973). A standard Soil Conservation Service (SCS) Type II storm was utilized for each analysis. • 10 -year, 24 -hour event: 1.7 inches • 25 -year, 24 -hour event: 2.1 inches SCS Curve Number and "k" Factor: The approved curve number (CN) values used for the SEDCAD demonstration are presented on Table 2.05.3 -E2 -1 in Exhibit 2.05.3 -E2. The "k" Factor values were based on approved soil survey information adapted from the Seneca II Mine Permit (Permit C -80- 003) and National Resources Conservation Service soil interpretations. The curve numbers and "k" factors were then "weighted" for each sub - watershed as presented in the Hydrology Input Parameter tables found at the back of Addendum 2.05.3- E2.D -1. Sedimentolosv: To determine the sediment loading to PECOCO Pond, the average annual "R" method was used. The sediment storage volume required is determined based on the ratio of the RUSLE annual R factor to the calculated R storm value and annual sediment yield to the storm sediment yield. The number of years of sediment storage (Y) and the average annual R factor are input values. The design analysis used an annual R value of 10. The value is representative of watersheds dominated by undisturbed and/or successfully reclaimed and revegetated landscapes in northwestern Colorado (Figure 8A.1, Isolines of R factor for Western U.S. [from Design Hydrology and Sedimentology for Small Catchments, Haan et. al, 1993]) — approximately 98% of the contributing watershed is either undisturbed or successfully reclaimed and revegetated. Previous SEDCAD evaluations of reclaimed conditions at the Seneca II Mine, approved by CDRMS, used this value for reclaimed conditions. 2.05.3 -E2.D3 12/09