PERMFILE112595

• Permit No: C1994082A
• IBM Index Class Name: Permit File
• Doc Date: 2/28/2006
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Tab 13 Attachment 13-12
• Media Type: D
• Archive: Yes

Febrsmp' 7999 Yaart dune * 13-I? Drainage and Sediment Control Plan ?
choose between the Natura] Resources Consen-ation Sen~ice (NAGS -formally SOS) tpe I, type L'.,
type II, tpe III, or user defined storm unit mass cun'es. The type II distribution was input as an
estimation of the area storms.
2.2 Storm Duration
.1 storm duration of Z~ hours was used for the design and evaluation of the Yoast bone sedimentation
ponds and ditches.
2.3 Return Period/Precipitation
~1 precipitation amount is required for the appropriate return period. The following return periods and
precipitation events were used for the Yoast Mine:
10-yeaz, 24-hour storm event 1.7 in
2~-}-ear, 2d-hour storm event 2,1 in
2.4 Hydrograph Response Shape
A unit hydrograph shape is chosen for each drainage area or sub-area modeled to predict the run-ofE
response. The hydrograph responses available in the SEDC.~D~ model are slosg medium, and fast. ~
slow response corresponds to a forested area or an azea 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 0.0~ hours, and values are saved at the user-specified inten-al of
0.1 hours or greater. ~ 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 b}passes the unit hydrograph technique and
instantaneous mn-off is assumed. :1s explained in the SEDCAD~+ User's Manual (Schwab and \~'amer,
1998), the time of concenaation restriction of 0.125 hours is some~chat mandated due to a
combination of away size restrictions, minimum internal convolution inten-al, and the user specified
time increment for saving convoluted values.
2.5 Drainage Basin Acea
The drainage basin areas were determined b}' digitizing direcdy from scaled drawings. The watersheds
were located based on area topography and separated by conditions representing various stages of
disturbance.
2.6 Time of Concentration, T~
The rime of concentration was calculated using the NRCS upland method (MRCS, 1986). The
hydrauhc lengths, drainage heights and associated slopes were determined from scaled dratings. The
values calculated Eor each structure are shown on a subwatershed basis in the SEDCAD-i printouts.
Overland flow conditions were assumed for a maxirnum of X00 feet.
2.7 Muskingum Routing Parameters, K, X
The bluslungum Routing Parameters were also calculated using the NRCS upland method. The
hydrauhc lengths, drainage heights and associated slopes were deterrined from scaled drawings. The
values calculated between each structure and/or subwatershed are shown in the SEDCAD4 printouts.
2.8 Curve Number, CN •
The run-ofE cuca°e number is a factor relating the amount of rainfall to the amount of run-off for a
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