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6.0 i OTHER STUDIES A Flood Plain Information Report was prepared in January of 1969 for Left Hand Creek by the U.S. Army Corps of Engineers, Omaha District (Reference 3). The lOa-year flood elevations plotted in the Flood Plain Information Report do not agree with those used in this study. This is due to channel im- provements implemented along the study reach in the interim, which confine water to the channel during low frequency events. Previously, large floods were allowed to spread out over the presently developed flood plain. The improvements generally consist of a 20 foot wide channel, 10 feet deep, with 2:1 side slopes whic~ will convey the lOa-year event. The SOO-year flood will he conveyed in part along Missouri Avenue and in the yards o~ the adjacent properties. In June of 1972 the Soil Conseryation Service completed a Flood Insurance Study encompassing the unincorporated areas of Boulder County in which Left ijand Creek was studied in detail (Reference 18). Water-surface profiles and flood plains for Left Hand Creek from the Sotl Conservation Service study do not agree with this study for the same reasons stated above. A Flood Platn Information Report was completed in June of 1972, also by the iu.s. Army Corps of Engineers, Omaha District of- fice, for Lqwer St. Vrain Creek (Reference 4). The lOa-year water-surfaqe elevations computed for this study are generally within 0.5 ~oot of those plotted in the St. Vrain Creek Flood Plain Information Report. , Two previous~ hydrologic investigations were conducted for the City of Longmont on Spring Gulch. The first investigation was completed in~ June 1971 by the U. S. Army Corps of Engineers, Omaha District office. The lOa-year discharge computed for this study a~ 9th Avenue was 3200 (cfs) (Reference 12). A second study' completed in 1973 by Bruns, Incorporated (Ref- erencell), ~ave a lOa-year discharge of 2117 (cfs) at the Oligarchy Ditch crossing of Spring Gulch. This discharge, however, was' based on the assumption that certain detention facilities wpuld be constructed upstream. These improvements have neither, been constructed nor funded. Both the U.S. Army Corps of Engineers and Bruns, Incorporated were contacted to I verify assumptions made in their runoff calculations. Based on existing ~onditions the discharges computed for this study agree with t~ose computed by the U.S. Army Corps of Engineers. No published, reports or information are available regarding possible flooding on Dry Creek or Loomiller Basin pertaining to flood profiles and flood plain limits referred to in this study. 29 ~~