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. curves were developed by adjusting the curve at Nepest8 hy the ratin of their respective standard project floods to the Srl' at Nepl'stn. The Nepesta site was chosen because of its central location in thin reach of the Arkansas River. This procedure yielded consistent re- sults of flows in the Arkansas River from Pueblo to Las Animas. Also, a reasonable relationship between floods of designated fre- quency and the standard project flood was maintained throughout the study reach. . . DEVELOPMENT OF DESIGN FLOODS 30. FLOOD ROUTING CRITERIA. Flood routings were required for development of the standard project flood at several locations alonr, the Arkansas River. Routings were performed by the Modified PuIs method. Storage-outflow curves at selected cross sections from Fountain Creek to the St. Charles River were developed by HEC-2 backwater computations. Cross section data for this reach were.ob- tained from digitized sections from 2-foot contour interval mappinr. Storage-outflow curves for the reach from the St. Charles River to Las Animas were obtained from normal denth computations at selected cross sections. These sections were developed from field surveys and 7-l/2-minute USGS quadrangle maps. The HEC-I computer program was utilized for these flood routings. The HEC-I model was cali- brated for the .reach from Pueblo to below Chico Creek. Flood hydro- graph data were not available for the reach to Las Animas. Plates 10 and 11 show two 1921 flood hydrographs that were reproduced by the HEC-l model. The reproduced peaks are ahout 10 percent low which is probably due to unrecorded tributary inflows missinr. from the model. . . . 31. UNIT HYIlROGRAPIIS. Unit hydrorraphs were developed by syn- thetic methods because of lack of rainfall-discharge data to sub- stantiate natural unit hydrographs. The synthetic unit hydrorraphs were determined by use of the following two formulas: ' . qp a 12,650 (Sst+ 0.00109)1.08 " . 0.074 ['~:~.)0.3J 0.669 u II . . These formulas were developed by regression analyses during the studies of the Purgatoire River at Trinidad, Colorado. The results of those regression analyses are presented as Plates 12 and 13. Plotted on these plates are qp and tp for Willow Creek above Heron Reservoir, near Park View, New Mexico. Values of qp and tp for .. 12 .