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

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5.0 SEDIMENT POND The Northfield Project site sediment pond was designed to effectively treat the run-off from the 10- year, 24-hour storm event. The pond was designed utilizing the SEDCAD4 computer model. The input parameters and results are shown on the SEDCAD4 printout located in the Appendix B. Sediment pond SEDCAD4 results are summarized in Table 3. The location of the pond and its corresponding subwatersheds are shown on the Site Drainage Map. Details for the sediment pond are located on the Sediment Pond Details sheet attached. A simplistic approach was taken in determining disturbed area contributing to run-off. All areas which could possibly be disturbed during the life of the project were considered disturbed for the sediment pond design. These areas included the acreage below the run-on diversion ditches and above the run-off collection ditches. Additionally, no sediment control barriers were used in the sedimentology design of the pond. This approach resulted in a conservative design for the pond sediment storage design. The sediment pond contains a principal and emergency spillway. The principal spillway has a 3-foot tall perforated riser. The riser will be enclosed with a trash rack screen to prevent clogging and pass the 10-year, 24-hour storm event without use of the emergency spillway. The principal spillway will also allow for approximately 0.19 ac-ft of dead pool storage (not counting unused sediment storage volume) to treat mine dewatering flows. The dead pool storage will allow for approximately 1.5 days of dewatering flow retention. The emergency spillway consists of a trapezoidal cha~mel with 3H:1V side slopes. The spillway crest and outlet slope will be armored. The emergency spillway is designed to safely pass the peak flow from the 25-year, 24-hour storm event while maintaining 1-foot freeboard from the top of the pond embankment. The spillway is conservatively designed assuming the principal spillway is clogged during the storm event. The sediment pond will have a volume of less than 20 ac-ft as measured from the invert of the emergency spillway to the bottom of the pond and the pond embankment will be less than 10 feet in vertical height from the crest to the original ground surface. The embankments will be constructed with 3H:1 V slopes and a top width of 15 feet. The embankment crest and outer slopes will be vegetated with an approved native grass seed mix. During final design of the sediment pond, analyses shall be performed to verify stability of pond embankment slopes for construction. 6.0 CHANDLER CREEK CROSSING ' The site access road will originate from Fremont County Road 79 and will cross Chandler Creek. The headwaters of the Chandler Creek drainage basin are approximately 10,000 feet in elevation and can receive substantial amounts of snow, potentially generating early spring run-off flows. The ' drainage basin is also over 11,000 acres in size. It would be anticipated that significant flows are periodically produced by localized, high intensity rainfall events within the basin. 1 r ~ '