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

FeGrua 1999 ~ Yoart Mine * 13-12 Drainage axd Sediment Control Plax •2
ry
2.1 PRECIPITATION DISTRIBUTION
A precipitation distribution is input to model the run-off hydrograph. SEDCAD4 allows the user to choose
between the Natural Resources Conservation Service (MRCS -formally Soil Conservation Service [SCS]) type I, type
IA, type II, type III, or user defined storm unit mass curses. The type II distribution was input as an estimation of the
area storms.
2.2 STORM DURATION
A storm duration of 24 hours was used for the design and evaluation of the Yoast Mine sedimentation ponds and
ditches.
2.3 RETURN PERIOD/PRECIPITATION
A precpitation amount is required for the appropriate return period. The following return periods and precipitation
events were used for the Yoast Mine:
10-year, 24-hour storm event 1.7 inches
25-year, 24-hour storm event 2.1 inches
100-year, 2~hour storm event 2.5 inches
2.4 HYDROGRAPH RESPONSE SHAPE
A unit hydrograph shape is chosen for each drainage area or sub-area modeled to predict the run-off response. The
hydrograph responses available in the SEDCAD4 model are slow, medium, and fast A slow response corresponds to
a forested area or an area with a number of obstmctions. A fast response corresponds to an un-vegetated or poorly
protected area. A medium response corresponds to average conditions and was used for the Yoast site.
The 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 0.125 hours bypasses the unit hydrograph technique and instantaneous run-off is assumed. As
explained in the SEDCAD4 Design Manual and Users Guide (Warner et al, 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 specified time increment for saving convoluted values.
2.5 DRAINAGE BASIN AREA
The drainage basin 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. Ponds O12,
013 and 014 design watersheds are shown on Exhibit 13-12.1, YoastMine Watersheds. Posmiining channel
watersheds, as well as, watersheds for the pond postmine permanent feature demonstrations are shown on
Exhibit 13-12.1A, Permanent Channel and Pond llt/atetsheds
2.6 TIME OF CONCENTRATION, T~
The 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 are presented on a subwatershed basis in the SEDCAD4 printouts: C-verland flow conditions were assumed
for a maximum of 400 feet.
2.7 MUSKINGUM ROUTING PARAMETERS, K, X
The Muskingum Routing Parameters were also calculated using the NRCS upland method. The hydraulic lengths,
drainage heights and associated slopes were determined from permit drawings. The values calculated between each
Maxtgomery lY/atrox Mining Grorsp * 165 S. Union Boulevarr~ Suite 460 * Ltke~voo~ Colorado 80228 * [phone]
TR-37 Revised 02/08