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<br />5.4 Flood Insurance Rate Map Description <br /> <br />The Flood Insurance Rate Map for Boulder is, for insurance <br />purposes, the principal product of the Flood Insurance Study. <br />This map contains the official de~ineation of flood insurance <br />zones and base flood elevations. Base flood elevation lines show <br />the locations of the expected whole-foot water-surface elevation <br />of the base (lOO-year) flood. The base flood elevations and zone <br />numbers are used by insurance agents, 1n conjunction with <br />structure elevations and characteristics, to assign actuarial <br />insurance rates to structures and contents insured under the NFIP. <br /> <br />6.0 OTHER STUDIES <br /> <br />To ensure the reliability of the results of this study, the flood <br />insurance study flows were compared with flows developed in other <br />studies in the area. A number of floodplain hydrology studies in the <br />north metro/Boulder area were examined for use in this report. Reports <br />used for comparison purposes were selected based on having drainage <br />areas of less than 10 square miles and the absence of upstream flow <br />regulation (flood-storage reservoirs). <br /> <br />A chart was developed to compare the flows for this study with flows <br />developed in three studies, all of which were prepared for the UDFCD by <br />Wright-McLaughl in Engineers (Reference 4, 15, and 16). The Lena Gulch <br />flows (Reference 16) are based on future (developed) condi t ions. The <br />other studies are based on existing conditions, with proposed channel <br />improvements throughout Boulder. All three studies are based on <br />previous rainfall data and unit hydrograph procedures. <br /> <br />An approximate upper and lower boundary was drawn based on the above <br />studies. Flows developed in this area are expected to plot close to the <br />upper Limit (the final flows will be somewhat lower due to more precise <br />stream routing). The flows for Bear Canyon, Skunk, Wonderland, Twomile, <br />and Goose Creeks all plot very well on the chart. The Elmers Twomi le <br />Creek flow plots on the high side. The basin is completdy urbanized <br />and channelized; therefore, the flow seems reasonable. The Fourmile <br />Canyon Creek flow plots very low on the chart. The Fourmi le Canyon <br />Creek basin, from the mouth to the canyon narrows, is 30vOOO feet and <br />drains only 1,800 acres. This ~ivcs an average width of 2,600 feet, for <br />a basin width-to-Iength ratio of 1: 11. This unusual situation provides <br />for pxtensive attenuation of penk flows. If the proposed channelization <br />of Fourmile Canyon Creek is constructed in the future, the peak flows <br />would be comparable to those shown in a 1969 UDFCD Study (Reference 4). <br />The drainage area values used for the upstream basin of Fourmile Canyon <br />Creek are comparable to those in the UDFCD. <br /> <br />32 <br />