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1 - 3 - following completion of the mining operation. During mining, ' maintenance will be relatively easy and erosion will b.=_ a problem. Following mining, facilities will have to be pro- , vided, depending upon the after use of the area, to safely allow passage of floodwaters as near to historic conditions ' as possible. Field Investigations ' In addition to the various field investigations and obser- vations contained in the Thorne report, two site inspection ' trips were made to specifically observe the drainage related aspects for preparation of this report. One trip was made during the late fall of 1974 to observe dry weather conditions; ' the second was made during the snowmelt period in the winter of 1975 to observe existing drainage patterns. 1 HISTORIC CONDITION ' Th C l i i l f e oo ey Morr son quarry s ocated near the mouth o I Strain Gulch in the foothills of the Colorado East Range, west ' of Highway 8 near Morrison, Colorado. The tributary drainage area is approximately 0.7 square miles. Strain Gulch generally ' has a base flow only during the spring and early summer snow- melt runoff periods. It is a dry gulch during the summer and fall periods, except for runoff from the high intense, short duration thunderstorm events characteristic of this area. A ' detailed description of the basin and it's hydrologic charac- teristics is contained in the Thorne report of 1972. ' Bittinger & Wheeler separately have estimated the .100-year flood runoff to be between 210 and 253 cfs. The higher of these ' values has been used to size the drainage facilities described in this report. Under historic conditions, the 100-year flood runoff will flow to a depth of a minimum of 3 feet in St:rair. ' Gulch immediately above the operating area. Due to the steep- ness of the canyon, flows of this magnitude will have a high ' ~1 Leonard Rice Consult:np Wa!e~ E~Gineers.lec y