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Average Recurrence Interval, <br />Years <br />Peak Discharge from <br />Thunderstorm, CFS <br />Attenuated Peak Discharge, <br />CFS <br />2 <br />910 <br />Unknown <br />5 <br />1,390 <br />Unknown <br />10 <br />1,750 <br />Unknown <br />25 <br />3,510 <br />Unknown <br />50 <br />4,030 <br />Unknown <br />100 <br />7,690 <br />5,850 <br />200 <br />10,090 <br />Unknown <br />500 <br />12,030 <br />Unknown <br />IX. 100 -YEAR BASE FLOOD <br />Table 4- Reported Flood Discharge Peak Rates <br />Report in Support of the Recertification of the <br />University of Colorado 's Flood Control Levee <br />Page 13 <br />IX.1. Hydrology <br />The current flood hydrology was developed by HDR, the consultant for Boulder. It was <br />submitted in 2007 to FEMA for review. The 100 -year flood for South Boulder Creek used to <br />generate the flood plain is based on a thunderstorm which generates an estimated flow at US <br />Highway 36 of 7,690 cubic feet per second (CFS). This compares with earlier estimates for the <br />100 -year flood that range from 5,000 CFS (US Army Corps of Engineers in 1969) to 9,340 CFS <br />(TEA in 2000), and to the prior Regulatory Flood of 6,200 CFS. The flood hydrograph is <br />"attenuated" as it passes through the flood plain; a portion of the flow is stored on the flood <br />plain, with the result that the reported effective flow at Highway 36 is reduced down to 5,850 <br />CFS. <br />The table below shows how the flood discharge for South Boulder Creek at US Highway 36 <br />varies for various storm events. The data are taken from the HDR Climatology Hydrology <br />Report that was submitted to FEMA for review in 2007. <br />Several flood hydrographs were provided by DHI, Boulder's hydraulic consultant, for use in this <br />analysis. The hydrograph locations are shown on Figure 25. LRE obtained tabular versions of <br />the hydrographs which all contained nine hours worth of data. The hydrograph for the 100 -year <br />flood at US Highway 36 is shown on Figure 26. We estimated the remainder of the declining <br />limb of the main hydrograph (13.7 million cubic feet) (MCF) using the reported volume of the <br />whole flood (121.6 MCF), and our computed volume for the nine -hour hydrograph that was <br />provided (107.9 MCF). We estimate that the main flood has a total duration of 14 hours. <br />Only a portion of the whole flood actually flows against the Levee. This is because a significant <br />portion of the flow is conveyed in the channel of South Boulder Creek, and on the flood plain <br />immediately adjacent to the creek. The portion of the flood that is reported to flow against the <br />© Leonard Rice Engineers, Inc. June 2009 — 207FIP05 <br />