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RULE 2 PERMITS <br /> Estimated Tributary Areas Acres <br />Basin Premining Postmining Differences <br />Goodspring Creek 25,600 25,600 0 <br />Tributaries to <br />Goodspring Creek: <br />Streeter Gulch 1,424 1,186 -238 <br />Gulch "A" 419 648 +229 <br />Prospect Gulch and 905 881 - 24 <br />Other Drainages <br />Taylor Creek 3,977 3,977 0 <br />Tributaries to <br />Taylor Creek: <br />Taylor tributary 1,379 1,403 +24 <br />All of the mining area drainages have been designed in accordance with the Colorado Mined Land <br />Reclamation Board's regulations. Particular attention has been given to the design of the permanent <br />channels draining Streeter Fill, the Gulch A mine area and the unnamed tributary to Taylor Creek. <br />When designing channels constructed in erodible material, several criteria must be met. The channel <br />must have adequate capacity to carry the maximum flow (in this case the runoff from the 100-year, 24- <br />hour precipitation event) and it must have adequate stability to resist the erosive action of flowing water <br />under all flow conditions. There are two main design procedures for insuring the stability of erodible <br />channels. One procedure is based on a limiting velocity concept and the other on a limiting tractive force <br />concept. Table 2.04.7-24 shows allowable velocities and tractive force values for several kinds of <br />channels. This table is taken from Lane (1955) and is based on the work of Forier and Scobey (1926). <br />When using the limiting velocity concept, one sizes the channel so that it has adequate capacity and so <br />that the average velocity does not exceed the permissible velocity for any given material. It is estimated <br />this maximum allowable non-erosive velocity in the Colowyo mine area is approximately 6 feet per <br />second. <br />When using the limiting tractive force concept, a channel with adequate capacity will not erode if its <br />average shear stress (given by the equation, shear stress is equal to the unit weight of the fluid times the <br />hydraulic radius of the channel times the slope of the channel in ft/ft) is equal to or less than the <br />permissible tractive force for any given material. It is estimated this maximum allowable tractive force is <br />approximately 0.7 lbs. per square foot at the Colowyo Mine site. <br />It can be seen that the allowable velocities and tractive forces for erodible channels are quite small, thus <br />requiring very wide shallow channels for the slopes at the Colowyo Mine. This type of channel would <br />appear unnatural and becomes unacceptable as rill erosion and gullying would develop in the channel <br />bottoms during the low flows associated with precipitation events less than the given design storm. On the <br />other hand, if the channel is protected from erosion, the allowable velocities can be increased resulting in <br />a deeper, narrower and more natural looking channel. An effective and permanent form of protection is <br />rock riprap lining of channels in those sections with excessive velocities or tractive forces. <br />An examination of the premining and postmining stream profiles show that the stream gradients, while at <br />somewhat different elevations, are very similar (see Map 33). The original stream channels in steep slopes <br />had scatterings of rock, boulders and debris. Frequently these channels were into bedrock, providing a <br />stable channel. It should be kept in mind that the original streams had developed over centuries to reach <br />this state of dynamic equilibrium. The mining operation necessarily removes the bedrock in its natural <br />Rule 2 Permits 2.04.7-36 Revision Date: 6/23/08 <br />Revision No.: MR-91