2008-12-04_PERMIT FILE - C1994082A ‎(4)‎

• Permit No: C1994082A
• IBM Index Class Name: Permit File
• Doc Date: 12/4/2008
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Tab 13 Attachment 13-12
• Media Type: D
• Archive: Yes

~~ -
Febntary 1999 Y'oart ttiline * 13 12 Drainage and Sediment Control Plan •3
structure and/or subwatershed are presented in the SEDCAD4 printouts.
2.8 CURVE NUMBER, CN
The run-off curve number is a factor relating the amount of rainfall to the amount of run-off for a given area.
Curve numbers were assumed based on site-specific soils and vegetation information (see Tab 9, Soil Baseline
Study [Exhibit 9-1, Soils Type and Soils Salvage Map]; and Tab 10, Vegetation [Exhibit 10-1, PremJrur~g
Yegetatron Map]). Where no vegetation or soils data were available, average conditions were assumed. Where
multiple conditions e4isted within the watershed, anarea-weighted curve number was calculated. A surnrnaiy of the
curve numbers used to estimate runoff for drainage and sediment control features is presented in Table 13-12.1,
Yoast Mine Curve Numbers. The values in this table were originally determined during discussions
between SCC and Division representatives as part of the south mine area expansion in 1999.
Recent experience has shown that curve number values for undisturbed areas were not inclusive of
all vegetation communities and cover conditions. Consequently, the table has been expanded to
include the values found in Table 13-1. Variations between the two tables are identified in the
footnotes of Table 13-12.1.
3.0 SEDIMENTATION CALCULATIONS
The sediment calculations used to model the Yoast 1Vline site were performed utilizing SEDCAD4. The
SEDCAD4 computer model calculates annual sediment yield using the Revised Universal Soil Loss
Equation (RUBLE). The input parameters used in the sedimentation analyses are presented in the following
sections.
3.1 RAI1~iFALL FACTOR, R
A rainfall factor of 30 was used for the original sediment pond designs at the Yoast l~Line site. This value is
suitable for bare or disturbed land that is easily eroded. In 2006, SCC initiated updates to pond storage
capacity demonstrations to account for the postmine conditions which generate significantly less sediment
than disturbed lands. The updated demonstrations utilize a rainfall factor of 10 which is representative of
conditions in northwestern Colorado (Renard, et a1.,1997).
3.2 SOIL ERODIBILITY FACTOR, K
The soil erodibility factors for the subject site were determined based on soil survey information as
presented in Tab 9, Soil Baseline Study (Nyenhuis, 1994). The values are shown on Table 13-12.2, Soil
Erodibility Factor.
3.3 LENGTH-SLOPE FACTOR, LS
Input parameters to determine the LS factor were obtained from permit maps of the watershed areas.
Values measured from the permit maps are presented in the attached SEDCAD4 results.
3.4 CONTROL FACTOR, C
The control factor is used to account for vegetative or mulch cover. The values utilized are shown on Table
13-12.3, Control Factor and were adapted from Renard et al. (1997) as presented in Warner et al.
(1998). These values were agreed upon during discussions between SCC and Division
representatives as part of the south mine area expansion in 1999.
Montgomery Watran Mining Group * 16f S. Union Boulevarrr, Suite 460 * Lakewood, Colorado 80228 * [phone]
x_37 Revised 08/08