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<br />. <br /> <br />. <br /> <br />. <br /> <br />., <br />-' <br /> <br />III - HYDROLOGY AND HYDRAULICS <br /> <br />3-01. HvdroloEv-Interior DrainaEe. Interior drainage plans for five affected <br />aress were formulated in accordance with Draft EC 1110-2-247 dated 23 <br />September 1983. These five areas are shown on Plate 1 Appendix A. Since these <br />areas are urban in nature, the Colorado Urban Hydrograph Procedure was used to <br />develop unit hydrographs and run off hydrographs. Drainage area sizes and <br />values of "L" and "Lea" were measured from the drainage area map. Values for <br />percentage of impervious cover for each area were taken from aerial <br />photographs and field inspection. Using this information and the unit <br />hydrograph procedure from the Denver Manual, unit hydrographs for each area <br />were computed and are tabulated in Tables 1 and 2 of Appendix A. <br /> <br />3-02. Discharge-Freauency Analysis. Discharge-frequency relationships for <br />each affected interior area were computed using rainfall-run off procedures. <br />Rainfall amounts for the 5-, 10-, 25-, 50- and 100-year frequencies were <br />obtained from NOAA At las 2, "Precipitation-Frequency Atlas of the Western <br />United States ," Volume 3 - Colorado. The rainfall is presented in Tables 3 <br />through 32 in Appendix A. After subtracting appropriate infiltration losses, <br />the rainfall excess was applied to the unit hydrographs to yie Id the run off <br />hydrographs. The peak flows were plotted On log-probability paper and a curve <br />was drawn through the points. To obtain the final frequency curve, the curve <br />was adjusted for expected probability based on a rainfall record length of 40 <br />years. The curves are shown on Plates 2 through 5 of Appendix A. <br /> <br />3-03. Coincident Flow. In order to route flows through the interior drainage <br />facilities, it was first necessary to determine a coincident flow in Fountain <br />Creek in order to rate the outlet structures. During the Phase I stage, a <br />study was conducted to determine if a correlation could be found between peak <br />flows or average daily flows in Fountain Creek and maximum daily rainfall at <br />Pueblo. From that study, it was deduced that large rainfalls in Pueblo have <br />not occurred during high stages in Fountain Creek. Therefore, to determine <br />coincident flow, an SPF storm was centered over Pueblo. The SPF for each <br />interior area and the concurrent flow in Fountain Creek at Pueblo were then <br />computed. It was determined that the small interior areas run off before <br />Fountain Creek has any significant flow. Therefore, all interior drainage <br />facilities are sized without any coincident flow in Fountain Creek. <br />Furthermore, the normal flows in Fountain Creek are too small to have any <br />effect on the interior drainage facilities. It would take a flow of 5,000 cfs <br />or greater to block the interior drainage structures. This flow is exceeded <br />0.01% of the time as can be seen on Plate 6 of Appendix A. <br /> <br />3-04. Area-Capacitv Data. Area and capacity relationships to elevation were <br />computed for Areas H, I, and K2 from 1 inch = 200 feet scale orthophoto maps <br />with 2-foot contour intervals. Areas G and Kl do not have ponding. The area- <br />capacity relationships were used to route the SPF and frequency floods through <br />the ponding areas using the Modified PuIs Procedure. Area and capacity curves <br />are shown on Plates 7, 8, and 9 of Appendix A. <br /> <br />3-05. PondinE Area I. Under existing conditions, run off from Drainage Area <br />I collects at a bend in Joplin Avenue. This area currently drains through an <br />existing levee by a 54-inch pipe equipped with a flap gate at its outlet. The <br />SPF and 100-year floods currently pond to maximum elevations of 4,651.7 and <br /> <br />III-1 <br />