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The flood elevation differences"FHFs, flood insurance zones, and base flood elevations for each flooding source studied in detail in the community are summarized in Table 3. S,4 Flood Insurance Rate Map Descrip~ion The Flood Insurance Rate Map for the of the City of Longmont, Colo- rado, is, for insurance purposes; the principal product of the Flood Insurance Study, This mapicontains the official deline- ation of flood insurance zones a~d base flood elevations. Base flood elevation lines show the locations of the expected whole- foot water-surface elevation of the base (lOO-year) flood. The base flood elevations and zone numbers are used by insurance agents, in conjunction with structure elevations and charac-teristics, to assign actuarial insurance rates to structures and contents insured under the NFIP. 6.0 OTHER STUDIES A Flood Plain Information report was prepared in January 1969 for Left- hand Creek by the COE, Omaha District ;(Reference 7). The 100-year flood elevations plotted in that report do not agree with those used in this study. The difference is due to channel improvements implemented along the study reach in the interim that confine water to the channel during low-frequency events. previously, large floods were allowed to spread out over the presently developed floodi plain. The improvements generally consist of a 20-foot-wide channel, 10 feet deep, with 2:1 side slopes that will convey the 100-year flood. The SOO-year flood will be con- veyed in part along Missouri Avenue and in the yards of the adjacent properties, In June 1972, the SCS completed a Flood Insurance Study encompassing the unincorporated areas of Boulder County in which Lefthand Creek was studied in detail (Reference 22). Water-surface profiles and flood plains for Lefthand Creek from the SCS study do not agree with this study for the same reasons stated: above, A Flood Plain information report was completed in June 1972, also by the COE, Omaha District, for Lower St. Vrain Creek (Reference 8). The 100- year water-surface elevations computed! for this study are generally with- in O.S foot of those plotted in the St~ Vrain Creek Flood Plain Informa- tion repor t. Two previous hydrologic investigations were conducted for the City of Longmont on Spring Gulch. The first investigation was completed in June 1971 by the COE, Omaha District, The ~OO-year flood discharge computed for this study at 9th Avenue was 3,200, cfs (Reference 16), A second study completed in 1973 by Bruns, Inc, (Reference lS), gave a 100-year flood discharge of 2,117 cfs at the Ol:igarchy Ditch crossing of Spring Gulch, This discharge, however, was based on the assumption that certain detention facilities would be constructed upstream. These improvements 22