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<br />STORMWATER MANAGEMENT PLAN ADDITIONS TO EXISTING PERI\1IT <br />Please refer to the 1997 permit amendment as approved to review basic stormw~Rer <br />management criteria for the site. Basically, all overland surface water induced ley <br />meteoric water flow is directed into sediment ponds designed to contains and dissipate <br />the volume of a minimum 25 year event. Experience (200 year event, April, 1999, <br />9 inches rain over tour days), has shown that existing sediment ponds built on site since <br />1997 have proven to be of sufficient capacity to contain and dissipate even an unsually <br />large storm event. <br />Watershed data for the amended acreage is as follows: Total Watershed Acreag:: 175 <br />Highest Elevation in added watershed area (Fremont Peak) ............................ 7233' <br />Lowest elevation in added watershed area (Sediment Pond) ........................... 5600' <br />Longitudinal distance between highest and lowest point ............................... 6200' <br />Average slope of watershed ................................................................. 26% <br />Time of Concentration ....................................................................... .3 hr. <br />Cn Number used for stormwater calculations ............................................ 89 <br />10 year SCS 1124hr 100 year SCS II 24 hr <br />Peak Discharge Rate 80 cfs 224 cfs <br />Discharge Volume Projection 13 acre feet 35.6 acre feet <br />Referring to the stormwater Management Plan Map, it is apparent that part of the <br />watershed described above is already within the containment of sediment control <br />structures built in 1997 (app. 20%). The remaining azea will be routed to one large or <br />two smaller sediment ponds constructed east of the main roadway in the central .area of <br />the amended affected land. All watershed acreage impacted by mining or crossing <br />mining lands will be routed to this new sediment pond construction. Details of sediment <br />ponds construction including spillways, valved standpipe for emergency release. anti- <br />seep collars, use of geotextile membranes under appropriately sized rip-rap systems, and <br />rip-rap sizing will follow 1997 construction details, which have proven to function <br />successfully. An appropriately rip-rapped spillway will be installed in the final :;ediment <br />pond to effectively handle a 250 cfs discharge. This includes a minimum ]0 wide <br />spillway constructed with geotexitile membrane overlain with graded rip-rap systems to <br />