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<br />SE CTION I <br /> <br />I <br />':' <br />j <br />" <br /> <br />HYDROLOGICAL ANALYSIS OF THE DRAINAGE BASIN <br /> <br />..' <br /> <br />."-) <br /> <br />The drainage basin tributary to the Idaho Springs Storage Reservoir <br />covers an area of approximately 3210 acres or about 5.01 square miles. The <br />basin extends about 3. 5 miles in length from the north side of Mount Evans, <br />northeasterly to the reservoir. The tributary basin is located within Township <br />5 South, Ranges 73 and 74 west of the 6th P. M. and entirely within the Arapaho <br />National Forest. Elevations of the basin range from a high of about 13,600 <br />feet above mean sea level to the normal reservoir water surface elevation of <br />10,617 feet. Sheet No. 1 in the envelope in the appendix of this report indicates <br />the boundary of the drainage basin and its location relative to other topographic <br />features in the surrounding area. In the upper reaches of the basin there are <br />numerous small tributaries to the main channel of Chicago Creek and several <br />small impoundments of water including the familiar Chicago Lake. The upper <br />basin is mostly above the timberline with timber covering only the lower one- <br />fourth of the basin area. The upper reaches of the basin bave extremely steep <br />slopes with numerous sheer granite faces and large rock formations. <br /> <br />,,1 <br />'; <br /> <br />The hydrological analysis and the determination of the amount runoff <br />from the basin was calculated in accordance wHh the requirements of the <br />Colorado State Division of Water Resources. These requirements are essential- <br />ly the procedures outlined in the Bureau of Reclamation's water resources techni- <br />cal publication Design of Small Dams. Procedures for estimating various magni- <br />tudes of flood flows from both thunderstorm-type and general type storms are <br />outlined. The following flood peak flows and volume of flows were determined by <br />the above procedures for this drainage basin. <br /> <br />Flood from Design Thunderstorm Rainfall <br /> <br /> Peak Flow Volume of Flow <br />Flood Magnitude (sec. -ft. ) (acre-ft. ) <br />Maximum probable flood 35,851 3,113 <br />Flood less than maximum <br />probable, Assumption A 21,400 1,849 <br />Flood less than maximum <br />probable, Assumption B 5,600 510 <br /> <br />- 5 - <br />