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<br />" <br /> <br />respective drainage areas are less than 200 square miles. The , <br />6-hour rainfall amounts were distributed in accordance with letter <br />SHDED-XH, dated 15 February 1974, subject: Maximum Six-lIour <br />Rainfall Distribution of the Standard Project and Probable Haxi- <br />mum Storms. Standard project rainfall values for each basin under <br />investigation in this report are presented as hyetographs on <br />exhibits 2, 3, and 4. <br /> <br />5. 'Standard Project Floods. Standard project floods for <br />Anderson and King Arroyos were developed by applying standard <br />project excess rainfall to each basin's unit hydrograph and <br />are shown in exhibits 2 and 3. The SPF for Anderson Arroyo has <br />a peak of 30,000 c.f.s. and the SPF for King Arroyo has a peak <br />of 18,000 c.f.s. The standard project flood for the Arkansas <br />River at La Junta was developed in the same manner except that <br />flood routings to La Junta of several tributary flows were required. <br />An isohyeta1 storm pattern was prepared and centered critically <br />above La Junta and the average basin rainfall for each major tribu- <br />tary was then computed. The resultant flood 'hydrographs were then <br />routed to La Junta and combined to yield the SPF and has a peak <br />discharge value of 212,000 c.f.s. as shown in exhibit 4. <br /> <br />c <br /> <br />6. Flood Routings. Flood routings of tributary flows were <br />required for development of the standard project flood at La Junta. <br />Flows from F~untain Creek, St. Charles River, Chico Creek, lIuerfano <br />River, Kramer Creek, Apishapa River, and Timpas Creek were routed <br />to La Junta using lIodified PuIs routing method. This method was <br />selected because of the considerable storage available in the <br />Arkansas River flood plain. Storage-outflow curves at selected <br />cross sections from Fountain Creek to the St. Charles River were <br />developed by HEC-2 backwater computations. Cross-section data <br />for this reach were obtained from digitized sections from 2-foot <br />contour interval mapping. Storage-outflow curves for the reach <br />from the St. Charles River to La Junta were obtained from normal <br />depth computations at selected cross sections. These sections <br />were developed from field surveys and 7y,-minute USGS quadrangle <br />maps. The IffiC-1 computer program was utilized for these flood <br />routings. Calibration of the HEC-1 model is discussed in para- <br />graph 30 of the main report. <br /> <br />7. Discharge Frequencies. The discharge-frequency curve <br />for the Arkansas River at La Junta was computed in accordance <br />with Bulletin {!17 entitled "Cuidelines for Determining Flood <br />F1ct~ Frequency" by the Water Resources Council. A continuous <br />stream flow record is available at La Junta from 1912 to the <br />present time. The annual peak discharge data were adjusted <br />for operation of Pueblo Reservoir to reflect present conditions. <br />Downstream channel capacities and irrigation demands were taken <br />into consideration when adjusting the data. Also, expected <br /> <br />5 <br />