2008-10-07_PERMIT FILE - C1981047 (4)

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~ -,~-s~. 1675 cfs. The culvert is sized for 1,149 cfs therefore being 526 cfs under designed. Prior to the spring runoff in 1985 Blue Ribbon designed and installed a dip section over the existing culvert. This dip section is designed to safely pass the 526 cfs under design flow mentioned above. The dip section was constructed using Tensar GM-1 Fabric to stabilize the upstream and downstream slopes. This fabric functions like riprap or gab'ons to protect the road embankment form eroding. The overtopping of the dip se ion will likely be a gradual event. The water level upstream of the dip sect' n will slowly increase until it starts to overtop. The water will then fl w over the smooth compacted gravel surface and on down the stream with very 1' tle turbulence being induced by the dip section. During 1985 the stream low did overtop the dip section. During this event the dip section functio ed as designed with little or no erosion occuring. • The only disadvantage of a dip section is be closed during peak runoff periods. According to document the dip section would be overtopped from a 16 hours to 22 hours or a total of 6 hours during a event. The design of the dip section and the flog that the c ossing would the hydr graph of the permit ~proxim ely hydrograph time 20 ye r 24 hour peak runoff calculations are presented on Exhibit W. The design and construction are ce ified by a professional engineer on the exhibit. The No. 1 Pond was originally bu' t in 1977-78 when the bench and access road were constructed. The emban ent along Hubbard Creek was constructed for material removed from the pond sit This pond now serves for storage of water rights in Blue Ribbon Reservoi No. 1 decreed in Case No. W-0351. A 4 inch schedule 40 pipe was installe from Hubbard Creek into the pond in March 1983, capable of conveying the cfs decreed to Blue Ribbon Ditch. This pond also serves as sediment contr for sub areas 2, 4, 5 and 7 see E_xhibit~H. Calcula- • lions for the size and loc/a~ions of discharge structures are given in Appendix 1-B of the Slue Ribbon TQdhnical Revision Request for Mine Bench Stabilization. During the mine bench ,stabilization project of 1984-85 the embankment was raised 106 (Rev. 3/21/86)