2008-01-16_PERMIT FILE - C1994082A ‎(6)‎

• Permit No: C1994082A
• IBM Index Class Name: Permit File
• Doc Date: 1/16/2008
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Tab 13 Attachment 13-12
• Media Type: D
• Archive: Yes

1'~duuru'~ lJl!/ YoartMirJe*13-12 Drainage andSerk'mentControlPlan •2
choose bertveen the Natural Resources Conservation Service (NRCS -formally Soil Conservation Service [SCS])
tv~pe I, q~pe Ir1, type II, type III, or user defined storm unit mass curves. The type II distribution was input as an
estunation of the area stornis.
2.2 STORM DURATION
11 stone dtuation of 24 hours was used for the design and evaluation of the Yoast Mine sedimentation ponds and
ditches.
2.3 RETURN PERIOD/PRECIPITATION
~1 precipitation sunount is required for the appropriate return period. The following return periods and precipitation
events were used for the Yoast Mine:
10-year, 24-hour stone event 1.7 inches
25-year, 24-hour storm event 2.1 inches
100-year, 24-hour storm event 2.5 inches
2.4 HYDROGRAPH RESPONSE SHAPE
;\ unit h~-drolnaph shape is chosen for each drainage area or sub-area modeled to predict the run-off response. The
h~•clrograph responses available ul dze SEDCAD4 model are slow, medium, and fast. A slow response corresponds to
a forested area or an area with a number of obstructions. A fast response corresponds to an un-vegetated or poorly
protected uea. A medium response corresponds to average conditions and was used for the Yoast site.
T11e internal convolution increment is 0.05 hours, and values are saved at the user-specified interval of 0.1 hours or
greater. A convolution increment of 0.1 was specified for the subject site. It should be noted that a time of
concentration less than O.LS hours bypasses the unit hydrograph technique and instantaneous run-off is assumed. As
esplauied in the SEDCAD4 User's Manual (Schwab and blamer, 1998), the time of concentration restriction of 0.125
hours is somewhat mandated due to a combination of array size restrictions, minimum internal convolution interval,
and the user spccifiecl time increment for saving convoluted values.
2.5 llRAINAGE BASIN AREA
1'he drainage basu7 areas were determined by digitizing directly from approved permit drawings. The watersheds
were located based on area topography and separated by conditions representing various stages of disturbance.
2.6 TIME OF CONCENTRATION, T~
711e time of concentration was calculated using the NRCS upland method (SCS, 1986). The hydraulic lengths,
drainage heights and associated slopes were determined from permit drawings. The values calculated for each
structure ate peesenteci on a subwatershed basis u~ the SEDCAD4 printouts. Overland Ilow conditions were
;issumecl for a maximtun of 400 feet.
2.7 MUSKINGUM ROUTING PARAMETERS, K, X
"111e 1\luskingum Routuig Paruneters were also calculated using the NRCS upland method. The hydraulic lengths,
drauiage heights and associated slopes were determined from permit drawings. The values calculated between each
structure and/or subwatershed are presented in the SEDCAD4 printouts.
2.8 CURVE NUMBER, CN
"I'l;c run-<,ff cur~•e. number is a factor relating the amount of rainfall to the amount of run-off for
!11ou1~niur~• I t`'al.cnn Alining Group * JGS S. Union Barlerrarrl S~iite 4G0 * L.nkemoo~l Colorado 80228 * [phone]
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1'P.-36 Revised 08/07