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into the natural channel of Unnamed Gulch B. Characteristics of the quarry's internal drainage network would be no different than those ' described under Alternative I. Alternative III A final alternative means of controlling storm runoff from the tributary area above and within the proposed quarry expansion is to utilize the quarry as a detention area. Under this option, water would be allowed ' to flow into the operating area via controlled drop structures. The water would initially be routed through sedimentation ponds and allowed to overflow into the natural channel downstream. In the later stages of t mining, the runoff would be trapped in the pit and no means of drainage would be provided. Due to the geologic conditions in the area, it is unlikely that any stormwater would be permanently detained in the pit; most likely, runoff would percolate into the ground or evaporate within ' a short period following a storm. If the pit did start to fill up with stormwater, a controlled overflow section or alternative means of ' draining the structure into the natural channel below the mined area could be provided. POST-MINING CONDITION 1 Gravel mining of the proposed quarry expansion is expected to continue for 50 years or more. The pit which remains when mining is finished may be useful as a flood control structure as indicated above. Flood waters ' captured in the pit would evaporate and/or seep into the surrounding formations; the pit is not expected to fill up and overflow. If the pit ' did fill, overflow could be released to the natural channel of Unnamed Gulch A without any adverse im acts downstream p . Some alternative post-mining uses for the existing Morrison puarry were identified in the Thorne report (197?) and the Leonard Rice Consulting Water Engineers report (1975). These uses include: sanitary landfill; storage for recreational vehicles; parking terminal for mass trans- portation into Denver; overnight "cliff-dweller" campground; stadium; ' open space. Amore detailed analysis of post-mining drainage consid- erations may be necessary prior to selection of one of these alterna- rives. Should a post-mining land use be decided upon while the quarry is still operating, modification to the drainage plan can be made at that time to facilitate implementation of the final land use plan. ' OT??ER POTENTIAL CONCERNS ' The preceding sections have dealt primarily with a description of the surface runoff characteristics of the basins to be affected by the proposed Morrison Ouarry expansion, and recommendations for means to control surface flows in a manner which will mitigate adverse quantita- ' tive impacts downstream of the mining operations. Several additional concerns not previously mentioned are outlined below: 5