2010-05-07_PERMIT FILE - C2006085 (7)

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• 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 (luring 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 (NRCS - formally Soil Conservation Service) Type 1, Type IA, Type Il, 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 a user - 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 1.00 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 A total rainfall depth is associated with each Design Storm used. Total rainfall depths were taken from regional NOAH Atlas Maps. The following rainfall depths were used for the Northfield Project: 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 floodplain and backwater effects. Since the SEDCAD4 model uses a simplified routing technique and does not account for floodplain storage, use of the 241 -point distribution would have resulted in an over - design of run -off from the design