2007-10-30_PERMIT FILE - C1982057

• Permit No: C1982057
• IBM Index Class Name: Permit File
• Doc Date: 10/30/2007
• Doc Name: Drainge and Sediment Control Plan II-W South Extension
• Section_Exhibit Name: Tab 13 Attachment 13-5B
• Media Type: D
• Archive: No

January 2002 Seneca II-W South Expansion * Drainage and Sediment Control Plan • Page 3
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 SEDCAD User's Manual (Schwab and Warner,
1987), 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 scaled 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~
The time of concentration was calculated using the SCS upland method (Soil Conservation Service,
1986). The hydraulic lengths, drainage heights and associated slopes were determined from scaled
permit drawings. The values calculated for each structure are shown on a subwatershed basis in the
SEDCAD printouts. Overland 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 SCS upland method. The hydraulic
lengths, drainage heights and associated slopes were determined from scaled permit drawings. The
values calculated between each structure and/or subwatershed are shown in the SEDCAD 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 supplied vegetation and soils information. Where
no vegetation or soils data was available, average conditions were assumed. Where multiple conditions
existed within the watershed, a weighted average curve number was assigned. A summary of the curve
numbers used for this evaluation is presented in Table 1.
TABLE 1
SENECA II-W (South Expansion Area) CURVE NUMBERS
Area
Condition
Undisturbed
Cover T e
As en/Mountain Shrub Hydrologic
Condition
Good >70% Hydrologic
Soil Grou
B Curve
Number
30
Good (>70% C 41
Good >70% D 48
Sa ebrush Good >70% C 47
Good >70% D 55
Mesic Draina a Fair 30%<x<70% C 81
Herbaceous Good >70% D 85
Herbaceous Good (>70%) C 74
Disturbed Pit &Pre-Stripped
Spoil
Partial) Reclaimed Bare
Bare/Rough
Bare/rou h C
C
C 91
86
86
Haul Road Bare C 91
Reclaimed Good (>70%) C 71
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TR-59 Revised 05/07