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

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• 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. Prior to construction, detailed site topography relative to designed sediment pond embankment elevations and locations, excavation depths, and outlet structure drainage slopes shall be verified. During construction of the sediment pond, analyses shall be performed to verify appropriate materials for use in pond embankments, placement and compaction specifications, and overall stability of pond embankment slopes. 6.0 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. 6.1 Chandler Creek Floodplain The Federal Emergency Management Agency (FEMA) has established approximate 1% annual • chance of inundation floodplain (100 -year floodplain) boundaries for Chandler Creek adjacent to the Northfield Project site. The FEMA Flood Insurance Rate Map (FIRM), Map Number 08043C0643E, that includes the Chandler Creek floodplain is included in Appendix C and the floodplain boundary is illustrated on the Site Drainage Map. The established floodplain for Chandler Creek adjacent to the project site is designated by FEMA as Zone A, approximate floodplain. Floodplains with this designation are based on the best available data and base flood elevations have not been established. Activities within designated floodplains are controlled to minimize the potential for increased flood hazards. Construction of the access road, a small portion of the facility site, and part of the sediment pond are planned to occur within the established Chandler Creek floodplain. These activities and all excavation or fill within the floodplain are proposed to be temporary and floodplain elevations will be returned to approximate pre -mine conditions during reclamation. Although Northfield Project activities are not anticipated to significantly affect Chandler Creek flood flows or potential flood hazards, no construction will be performed within the floodplain boundary without first obtaining a Floodplain Development Permit. 6.2 Access Road Crossing The site access road will originate from Fremont County Road 79 and will cross Chandler Creek. For design of the creek crossing, a detailed hydrologic study of the Chandler Creek basin was not performed. As a guide to the crossing design, the Fremont County Road 11A bridge crossing downstream of the site was used. According to discussions with Fremont County personnel, the bridge has been in existence for decades and there is no knowledge of previous flow impediment. Based on collected bridge parameter data, and a wager surface elevation at 75% of the bridge height, maximum flow beneath the bridge was estimated to be approximately 3,500 cfs (as presented in the Northfield Project Drainage and Sediment . Control Plan prepared by Applegate Group, Inc., 2007)