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' <br /> travel overland into the quarry pit. There, it could be detained in <br /> sedimentation ponds and released at a controlled rate into the natural <br />' channel of Unnamed Culch A. Thls drainage system would, therefore, <br /> provide a flood control benefit: peak flows in Unnamed Gulch B would be <br /> limited to a maximum of 100 cfs as compared to the historic 100-year <br /> rate of 130 cfs. Peak flows in Unnamed Gulch A would be dependent upon <br /> final design of the sedimentation ponds and their flow attenuation <br /> characteristics; structures should be designed with sufficient capacity <br /> to ensure that the historic 100-year peak flow rate of 200 cfs will not <br />' be exceeded. <br />t An internal drainage system is necessary to control runoff within the <br />mining area and reduce erosion potential. As the configuration of the <br /> quarry expansion will be changing constantly, it is not possible to <br /> develop a rigid drainage plan for the operating area itself; design and <br />' revision of [his system must be closely tied to [he actual mining <br /> operation, with recognition given to [he following general design <br /> considerations: 1) Slopes of drainage ditches and channels should be <br /> kept moderate, or control drops or channel protection provided to reduce <br />' erosion potential; 2) Culverts should be provided under constructed <br /> roadways giving consideration to natural drainageways and adequate <br /> design flows; 3) Riprap protection should be provided in areas which <br />' would otherwise be susceptible to erosion. <br /> Throughout the earlier stages of mining, drainage from the mining area <br />' should be routed into one or more ponds to allow settlement of sediment <br /> prior to release into the natural channel(s) downstream. Runoff from <br /> the open cuts of the mining operation will invariably result in some <br /> erosion. This eroded material, unless stopped, will travel downstream <br />' in the runoff water as suspended sediment. Construction of sedimenta- <br /> tion ponds will allow this material to settle out prior [o entering the <br /> lower reaches of the Unnamed Gulches. These ponds should be located <br /> near the lower edge or corner of the proposed operating area, and <br /> drainage ditches and/or flood control berms should be provided to ensure <br /> that all runoff from the area is routed through the ponds. If an outlet <br />' to the ponds is provided, it should allow controlled release of the <br /> runoff to the natural channel(s) below. <br />' In the final stages of mining, all runoff from precipitation falling on <br />the mining area will be trapped in the quarry pit. Since there will be <br />no drainage out of the disturbed area, sedimentation ponds will no <br />' longer be necessary. <br />Alternative II <br />1 <br />1 <br /> <br /> <br /> <br />Alternative II for handling runoff is similar to Alternative I. The <br />major difference is that the proposed channel to be used to convey flows <br />around the operating area and into the natural channel(s) below the <br />quarry would be located within the mining area rather than outside it. <br />One or more controlled drop structures would be used to lower runoff <br />water to a bench on the upstream quarry face at approximately the 6655' <br />level. This 80' wide bench would be graded in such a manner as to serve <br />the same purpose as the diversion channel decribed above, conveying the <br />100-year peak flow around the southwest edge of the operating area and <br />4 <br />