PERMFILE69474

• Permit No: C1994082A
• IBM Index Class Name: Permit File
• Doc Date: 9/27/2007
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Tab 13 Attachment 13-12
• Media Type: D
• Archive: Yes

1999 Yaart Mdne * 13-Y2 Drainare and Sediment Control P/an r2
choose between the Natural Resources Consetvarion Service (MRCS - foanally SCS) type I, type IA, type II, type III,
ox user defined stomp unit mass curves. The type II distribution was input as an estimation of the area stoma. •
2.2 STORM DURATION
A stomp duration of 24 hoots was used for the design and evaluation of the Yoast Mine sedimentation ponds and
ditches.
2.3 RETURN PERIOD/PRECIPITATION
A preapitarion amount is required for the appropriate retain period. The following retain periods and precipitation
events were used for the Yoas[ Mine:
10-yeaz, 24hour storm event 1.7 in
25-yeaz, 24hour storm event 2.1 in
2.4 Hydrograph Response Shape
A unit hydrograph shape is chosen fox each drainage azea or sub-area modeled to predict the runoff response. The
hydrograph responses available in the SEDCAD4 model aze slow, medium, and fast A slow response corresponds to
a forested azea or an azea with a number of obstructions. 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 intemal convolution increment is 0.05 hoots, and values are saved at the user-specified interval of 0.1 hours or
greater. A convolution increment of 0.1 was spedfied 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 runoff is assumed. As
exp]ained in the SEDCAD4 Useis Manual (Schwab and Wamer,1998), the time of concentration restriction of 0.125 •
hoots is somewhat mandated due to a combination of array size restrictions, minimum internal convolution interval,
and the user speafied time increment for saving convoluted values.
2.5 DRAINAGE BASIN AREA
The drainage basin areas were determined by digitizing directly from scaled drawings. The watersheds were located
based on azea topography and separated by conditions representing various stages of disturbance.
2.6 TIME OF CONCENTRATION, T~
The time of concentration was calcuated using the NRCS upland method (NAGS, 1986). The hydraulic lengths,
drainage heights and associated slopes were determined from scaled drawings. The values calculated for each
structure are shown on a subwatershed basis in the SEDCAD4 printouts. Overland flow conditions were assumed
for a maximum of 400 feet
2.7 MUSKINGUM ROUTING PARAMETERS, K, X
The Musldngwn Routing Parameters were also calculated using the NRCS upland method. The hydraulic lengths,
drainage heights and associated slopes were detemvned from scaled drawings. The values calculated between each
stmcture and/or subwatershed aze shown in the SEDCAD4 printouts.
2.8 CURVE NUMBER, CN
The inn-off curve number is a factor relating the amount of rainfall to the amount of inn-off for
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Montgomery Wahon Mining Group * 765 S. Union Boulevare7, Suite 460 * Lakemaad, Calomda 80228 * [phone]