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, --~~ . \ y /i :+ ' '-'j'#t.~-,: on Exhibits 1, 2, and 3 and broken down with the Fletcher distribution into twelve 5-minute, twelve IS-minute, and twelve 30-minute amounts. The SWMM program ~as then executed for nine events. These values ~ere used to construct the discharge-frequency curves shown on Exhibit 4. The State of Colorado, in cooperation with the U.S. Geological Survey, has published a Technical Manual entitled, ''Manual for Estimating Flood Characteristics of Natural Flow Streams in Colorado." The m.anual contains a set of regression equations for use in calculating peak discharges. These equations are based On the following two parameters; contributing drainage area and basin slope. The results of these equations are shown on Exhibit 5. These equations are to be used only as a guide and are not necessarily representative of conditions in the North Fork Republican River basin. The United States Geological Survey has prepared a report entitled, ''Magnitude and Frequency of Floods in Nebraska," for the State of Nebraska in cooperation 1Vith the Nebraska Department of Roads. This report includes many stations outside of Nebraska located in border states such as Colorado, Wyoming, South Dakota, Iowa, Missouri, and Kansas, as well as rivers within the State of Nebraska itself. The discharges calculated in this report are based on regression equations. These equations incorporHte the following three parameters; contributing drainage area, main stream slope, and the National Weather Service TP-40 twenty-four hour, 50-year rainfall amount. The results are shown on Exhibit 6. The historic streamflow data was analyzed utilizing "Water Resources Council Bulletin No. 17, Flood Flow Frequency." This program calculates standard deviation, adopted skew, and mean discharge as well as the peak discharges for various rE,currence intervals. The Colorado-Nebraska State line gage records (USGS 8230, 45 years) were used in this calculation. A discharge-frequency curve was plotted and is shown on chart 3. An Upper Republican River Basin Regional study was made based On sta- tion records from thirteen gages in Colorado and Nebraska. A revised mean and standard deviation were calculated for the North Fork basin and used to obtain peak discharges for the 500, 100, and 10 year frequency floods. The results were plotted and aTe shown on chart 4. 7. Results and Comments. A summary of the results obtained from each technique is shown on chart 5. This area of Colorado is subject to high intensity, short duration, relatively small areal coverage, cloudburst thunderstorms. The North Fork Republican River has experienced very fe1V of these types of thunderstorms, while adjacent basins have experienced extremely intense thunderstorms. By their very nature, it is almost impossible to analyze these rainfalls 3