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<br />Hvdraulic Analvsis <br /> <br />The new hydrographs were then routed through the Cottonwood Creek RE,gi onal <br />Detention Pond (as designed by Muller Engineering, Inc.), using a computer model <br />of the Modified PULS routing method. The results of this routing follow in <br />Appendix G. <br /> <br />The water surface e 1 evat ions in the pond, as determi ned by the Mod i fi ed PULS <br />routing analyses were then used as the starting water surface elevations in the <br />regional detention pond for input to the U.S. Army Corps of Engineer's HEC.2 <br />computer program. This program was util ized in calculating the water surface <br />profiles. <br /> <br />Floodplain water surface profiles were analyzed for the 10-, 50-, 100., and 500- <br />year recurrence interval storm events. In addition, a floodway analysis of the <br />100-year storm event was analyzed. The criteria used in determining the 100- <br />year floodway are specified in the National Flood Insuranc'~ Program regulations, <br />44 CFR Ch. 1 (10-1-86 Edition), Part 60, Section 60,3(d)(2) which states that <br />" . . . the regul atory floodway must be des i gned to carry the waters of the base <br />flood, without increasing the water surface elevation of that flood more than <br />one foot at any point." In order to achieve a maximum one foot rise in the 100. <br />year water surface elevation, a 110 foot floodway, centered on the channel, was <br />chosen. thi s allows for a gradually i ncreas i ng flooclway depth, with a maxi mum <br />depth of approximately 0,8 feet. Floodplain and floodway calculations are <br />presented in Appendix I. <br /> <br />Cross'sections used as input to the MPULS routing and HEC-2 programs for the pond <br />and channel are taken from a survey of as-built conditions at the site dated 4- <br />20.88, by Greenhorne and 0' Mara. The cross. sect ions surveyed matched closely <br />to the design drawings, so no as-built changes are reflected on the drawings. <br />Plan and profile sheets illustrating the newly calculated floodplains and <br />floodway are included in the back pocket of this report. <br />