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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak discharge- <br />frequency relationships for each flooding source studied in detail <br />affecting the community. <br /> <br />Peak discharges for the varlous recurrence intervals for the South <br />Arkansas River and poncha Creek were taken from data prepared by <br />Water Resources Consultants, Inc. (References 1 and 2). <br /> <br />Peak discharges for the various recurrence intervals for Chalk and <br />Cottonwood Creeks were taken from data prepared by the SCS (Refer- <br />ences 4 and 5). <br /> <br />. <br /> <br />Peak discharge-drainage area relationships for the South Arkansas <br />River and Poncha, Chalk, and Cottonwood Creeks are shown in Table 1. <br /> <br />3.2 <br /> <br />Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried out to provide estimates of the eleva- <br />tions of floods of the selected recurrence intervals. <br /> <br />Water-surface elevations for the 10-, 50-, 100-, and sOO-Year flood <br />recurrence intervals from the South Arkansas River and poncha Creek <br />were computed using the U.S. Army Corps of Engineers HEC-2 water-surface <br />profiles program (Reference 8). <br /> <br />Water-surface elevations for the 10-, 50-, 100-, and sOO-year flood <br />recurrence intervals for Chalk and Cottonwood Creeks were computed <br />using the SCS WPS-2 computer program (Reference 9). <br /> <br />Approximately 0.4 mile downstream of County Road 361 on Cottonwood <br />Creek, 100-year flood flows overtop the south bank and flow in older, <br />poorly defined channels. These flows return to the main channel <br />in the Town of Buena Vista. This 100-year flooding is shallow, <br />with flow depth averaging less than 2 feet. The 100-year water-surface <br />elevations for these flows were computed using the SCS WSP-2 computer <br />program (Reference 9). <br /> <br />Cross section data and bridge and culvert geometry were obtained <br />from field surveys, topographic maps, and aerial photographs (Re- <br />ferences 1,2,4, and 5). <br /> <br />Roughness coefficients (Manning's Un") were determined using photo- <br />graphs and field inspection. Roughness values ranged from 0.032 <br />to 0.110. <br /> <br />Starting water-surface elevations for the South Arkansas River, <br />poncha Creek, and Cottonwood Creek were determined using the slope-area <br />method. <br /> <br />7 <br />