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-5- <br />Physical Features The tributary drainage area was estimated to be 106 square <br />miles. The approximate limit of the drainage area is shown on the map of <br />figure 1. The surface area of the lake was estimated to be 280 acres from the <br />USGS 7.5 minute quadrangle. In order to develop a stage capacity curve for <br />the surcharge volume above the natural level of the lake, the surface area was <br />assumed to increase 5% for each 1 foot increase in stage. The rating curve <br />for the outflow channel was developed from the formula representing a broad <br />crested weir. The outflow channel was assumed to be 80 feet wide and 5 feet <br />deep. <br />Precicitation Estimates Rainfall amounts for the depth - duration curve in a <br />24 -hour period were determined from the NOAA Atlas 2, published by the <br />National Weather Service. Adjustment was also made for aerial extent in <br />accordance with procedures outlined in the NQAA Atlas 2. The adjusted 24 -hour <br />value was about 3 inches. <br />Runoff Parameters Based on the soil -cover complex of the drainage basin, an <br />initial abstraction of 0. 94 inches and a minimum infiltration rate of 0. 2 <br />inches/hour was used. The lag time was determined to be 2. 75 hours. 7 <br />Unit Hy=raph A synthetic unit hydrograph was computed using the general <br />storm dimensionless S -graph data for the Rocky Mountains. The volume of the <br />unit hydrograph represents 1 inch of runoff from the entire drainage basin <br />with a peak flow of about 22,000 cfs. <br />psi <br />The inflow hydrograph for the 100 -year event resulted in a peak flow of 23,702 <br />cfs and a total volume of 6,155 acre feet. For this flow the peak stage of <br />the lake reached a level 7.58 feet above the bottom of the outlet channel or <br />2.58 feet overbank. <br />Because of overbank flow, various percentages of the 100-year flood were then <br />investigated. The results indicate the current capacity of the outflow <br />channel is capable of passing about 45% of the predicted 100 -year flood, at <br />bank -full depth The resulting peak discharge was determined to be 2,643 <br />cfs. The average channel velocities would be about 7 fps. <br />