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5.4 flood Insurance Rate Map Description ."; The Flood Insurance Rate Map for Boulder is, for insurance purposes, the principal product of the flood Insurance Study. This map contains the official delineation of flood insurance zones and base flood elevations. Base flood elevation lines show the locations of the expected whole-foot water-surface elevations of the base (lOa-year) flood. The base flood elevations and zone numbers are used by insurance agents, in conjunction with structural elevations and characteristics, to assign actuarial insurance rates to structures and contents insured under the NFIP. ,., 6.0 OTHER STUDIES To ensure the reliability of the results of this study, the flood Insurance study flows were compared with flows developed in other studies in the area. A number of floodplain hydrology studies in the north metro/Boulder area were examined for use in this report. Reports used for comparison purposes were selected based on having drainage areas of less than 10 square miles and the absence of upstream flow regulation (flood-storage reservoirs). A chart was developed to compare the flows for this study with flows developed in three studies, all of which were prepared for UDFCD by Wright-McLaughlin Engineers (References 4, l5, and 16). The Lena Gulch flows (Reference 16) are based on future (developed) conditions. The other studies are based on existing conditions, with proposed channel improvements throughout Boulder. All three studies are based on previous rainfall data and unit hydrograph procedures. ,---,. An approximate upper and lower boundary was drawn based on the above studies. flows developed in this area are expected to plot close to the upper limit (the final flows will be somewhat lower due to more precise stream routing). The flows for Bear Canyon, Skunk, Wonderland, Twomile, and Goose Creeks all plot very well on the chart. The EImers Twomile Creek flow plots on the high side. The basin is completely urbanized and channel ized; therefore, the flow seems reasonable. The Fourmi le Canyon Creek flow plots very Iowan the chart. The Fourmile Canyon Creek basin, from the mouth to the canyon narrows, is 30,000 feet and drains only 1,800 acres. This gives an average width of 2,600 feet, for a basin width-to-length ratio of l: l1. This unusual situation provides for extensive attenuation of peak flows. If the proposed channelization of fourmile Canyon Creek is constructed in the future, the peak flows would be comparable to those shown in a 1969 UDfCD Study (Reference 4). The drainage area values used for the upstream basin of fourmile Canyon Creek are comparable to those in the UDfCD study. eJ_ 33