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<br />2.3 Principal Flooding Problems <br /> <br />A comprehensive history of flooding in the City is presented in Floods in Boulder County <br />by Sherry Oaks. Briefly, occasional flooding has been recorded in Fourmile Canyon <br />Creek, with property damage reported in 1916, 1941, and 1951. No large floods are <br />reported on Wonderland Creek <br /> <br />2.4 Flood Mitigation <br /> <br />Flood mitigation efforts for Fourmile Canyon Creek and Wonderland Creek within the <br />City include channel improvements, floodproofing, and replacing existing road crossings <br />of the creeks with larger crossings. <br /> <br />3.0 ENGINEERING METHODS <br /> <br />3.1 Hydrologic Analysis <br /> <br />Fourmile Canyon Creek and Wonderland Creek hydrology was first studied in 1987 in a <br />Phase "B" Major Drainageway Planning report (Greenhome & O'Mara, 1987). Flood <br />peaks along the Creeks for the 10-, 50-, and 100-year events were identified using <br />hydrographs developed at design points using the Colorado Unit Hydrograph Procedure <br />(CUHP) Version B. CUHP generated hydrographs were routed with HEC-l computer <br />model using the Modified PuIs method. Flood flows which spill from Fourmile Canyon <br />Creek at Broadway to Wonderland Creek were estimated, and these estimates were added <br />to Wonderland Creek below Broadway (flood peaks in Fourmile Canyon Creek, <br />however, were not reduced). Flood peak discharges for the two Creeks from the 1987 <br />Phase "B" report are summarized in Table 1, and the regulatory spill discharge estimate <br />for the 1 OO-year event is shown on Table 2. <br /> <br />Flood peak estimates and hydrographs along the two Creeks were updated under the <br />direction of the City and UDFCD. The CUHP input parameters were checked for <br />reasonableness and corrections were made as needed. Flood peaks for 10-, 50-, and 100- <br />year events were defined using hydrographs generated by CUHPF (UDFCD, 1998b, <br />2000) and routed using UDSWM (UDFCD, 1998a, 2000). <br /> <br />Table 1. Regulatory Flood Peak Discharges (cfs) <br /> <br />Fourmile Canyon Creek <br />Location <br /> <br />1.1 Mi. Upstream <br />of Broadway! <br />Broadway <br />28th Street <br />SH 119 <br />Confluence <br /> <br />10-yr <br /> <br />730 <br /> <br />830 <br />1000 <br />1050 <br />920 <br /> <br />Recurrence Interval <br />50-yr 100-yr 500-yr2 <br /> <br />2385 3325 7500 <br /> <br />2530 <br />2850 <br />2805 <br />2890 <br /> <br />3520 <br />3920 <br />3930 <br />3960 <br /> <br />7600 <br />8200 <br />7600 <br />8900 <br /> <br />-7- <br />