2020-03-24_PERMIT FILE - C1981019A (5)

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RULE 2 PERMITS When using the limiting tractive force concept, a channel with adequate capacity will not erode if its average shear stress (given by the equation, shear stress is equal to the unit weight of the fluid times the hydraulic radius of the channel times the slope of the channel in ft/ft) is equal to or less than the permissible tractive force for any given material. It is estimated this maximum allowable tractive force is approximately 0.7 lbs. per square foot at the Colowyo Mine site. It can be seen that the allowable velocities and tractive forces for erodible channels are quite small, thus requiring very wide shallow channels for the slopes at the Colowyo Mine.This type of channel would appear unnatural and becomes unacceptable as rill erosion and gullying would develop in the channel bottoms during the low flows associated with precipitation events less than the given design storm. On the other hand, if the channel is protected from erosion, the allowable velocities can be increased resulting in a deeper, narrower and more natural looking channel. An effective and permanent form of protection is rock riprap lining of channels in those sections with excessive velocities or tractive forces. An examination of the premining and postmining stream profiles show that the stream gradients, while at somewhat different elevations,are very similar(see Map 33).The original stream channels in steep slopes had scatterings of rock, boulders and debris. Frequently these channels were into bedrock, providing a stable channel. It should be kept in mind that the original streams had developed over centuries to reach this state of dynamic equilibrium. The mining operation necessarily removes the bedrock in its natural state, and it is replaced in an unconsolidated condition. While it is impossible for the mining and reclamation operation to reconstruct the stabilizing affects of these drainages in bedrock, it is possible and desirable to recreate the stabilizing affects of the rock protection. This method has been elected to armor and stabilize the permanent stream channels constructed or to be constructed at the Colowyo Mine site. A summary of these channels is presented below,and the location of the areas to receive rock riprap protection are shown on Map 12- Hydrology, South. Length of Drainage Channels — Feet Destination Unprotected Rock Protected Total Southwest Tributary to Streeter 0 3,200 3,200 South East Tributary to Streeter 2,500 4,000 6,500 Streeter Gulch 5,700 13,000 18,700 Section 11 Ditch 0 1,000 1,000 Prospect 1,400 2,200 3,600 Final East Pit 1,200 4,600 5,800 Section 16 Fill 0 900 900 South Connection 0 1,200 1,200 West Pit 2,700 14,300 17,000 _ Taylor Tributary 2.500 11.500 14,000 Rule 2 Permits 2.04-39 Revision Date: 12/20/19 Revision No.: TR-135