2008-03-03_PERMIT FILE - C2006085A ‎(4)‎

• Permit No: C2006085A
• IBM Index Class Name: Permit File
• Doc Date: 3/3/2008
• Doc Name: Drainage and Sediment Control Plan
• Section_Exhibit Name: Exhibit V
• Media Type: D
• Archive: Yes

• NRCS Distribution Resolution
• Subwatershed Area
• Time of Concentration
• Muskingum Routing
• Curve Number
' • Unit Hydrograph Response Shape
Hydrologic calculation input values are explained in following sections.
2.1 Storm Type
The selected Storm Type is a rainfall distribution that approximates the intensity of the storm at
different time periods during the event. The rainfall distribution is an important factor used to model
the run-off hydrograph. SEDCAD4 allows the user to choose between the Natural Resources
Conservation Service (MRCS -formally Soil Conservation Service) Type I, Type IA, Type II, or
Type III rainfall distributions. These distributions represent typical types of rainfall events in
different parts of the United States. If site-specific information is available, SEDCAD4 also allows a
user-defined distribution or auser-defined storm event to be entered. The NRCS Type II storm
distribution, typical for rainfall events in the Western United States, was used for estimating the
storm types associated with the Northfield Project.
2.2 Design Storm
The Design Storm is described by frequency (recurrence interval in years) and storm duration
(hours). For the Northfield Project, a storm frequency of 10 years and duration of 24 hours was used
for the design and evaluation of the northern facility ditches and the sediment pond treatment
capacity. A storm frequency of 100 years and duration of 24 hours was used for the design and
( evaluation of the diversion ditches upgradient and downgradient of the mine waste stockpile on the
southern end of the facility. A storm frequency of 25 years and duration of 24 hours was used to
' design the sediment pond emergency spillway.
2.3 Rainfall Depth
i A total rainfall depth is associated with each Design Storm used. Total rainfall depths were taken
from regional NOAA Atlas Maps. The following rainfall depths were used for the Northfield
' Project:

n

10-year, 24-hour storm event = 2.5 in.
25-year, 24-hour storm event = 2.9 in.
100-year, 24-hour storm event = 3.7 in.
The applicable NOAA Atlas Maps are included in Appendix A.
2.4 NRCS Distribution Resolution
SEDCAD4 provides two options for fitting a NRCS rainfall distribution, namely the 49-point and the
241-point distributions. The 49-point distribution is traditionally used in textbooks and previous
versions of SEDCAD. The only difference between these two distributions is that for the 49-point
distribution, a linear interpolation between 0.5-hour values is applied and for the 241-point
distribution a curve fitting technique is used. Using the 241-point distribution will result in a higher
peak flow than using the 49-point distribution and is typically used when temporary storage effects
can be modeled. The higher predicted peak flow of the 241-point fit is accounted for by modeling
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