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2010-05-07_PERMIT FILE - C2009087 (34)
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2010-05-07_PERMIT FILE - C2009087 (34)
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Last modified
8/24/2016 4:11:38 PM
Creation date
6/3/2010 10:38:32 AM
Metadata
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Template:
DRMS Permit Index
Permit No
C2009087
IBM Index Class Name
Permit File
Doc Date
5/7/2010
Doc Name
PECOCO Pond Design Report
Section_Exhibit Name
Exhibit 2.05.3-E2 Appendix 2.05.3-E2.D
Media Type
D
Archive
No
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4.0 SEDCAD Demonstration <br />The hydrology and sedimentology of the watershed were analyzed using the SEDCAD4 computer <br />program (Schwab, 1998). Model methodology and assumptions are presented in Exhibit 2.05.3 -E2, <br />Drainage and Sediment Control Plan. Addendum 2.05.3- E2.D -1 and 2.05.3- E2.D -2 of this appendix <br />present the modeling demonstrations for the 10 -year, 24 -hour and 25 -year, 24 -hour design storm <br />events, respectively. The pond configuration in the model is based on the values contained in the <br />Seneca II Mine as -built report prepared as part of TR -37 (see Attachment 2.05.3- E2.D -2). <br />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.D -1. <br />Sedimentolosv: To determine the sediment loading to PECOCO Pond, the average annual "R" <br />method was used. The sediment storage volume required is determined based on the ratio of the <br />RUSLE annual R factor to the calculated R storm value and annual sediment yield to the storm <br />sediment yield. The number of years of sediment storage (Y) and the average annual R factor are <br />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 98% 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 />2.05.3 -E2.D3 12/09 <br />
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