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II II I I I I II I I I I I II I I I I I II Hydraulic Analysis The detailed hydraulic analysis prepared as part of the Flood Insurance Study was used wherever possible. The water surface profiles for the 10-, 50-, 100-, and SOO-year events that were computed using the U.S. Army Corps of Engineers' HEC-1I step-backwater computer program were used to determine water surface elevations for the various flood frequencies. For the most part, the channel geometry within the study reach remained unchanged after the June flood. However, cross-sections at selected locations were surveyed in December 1983 in order to update cross-section information contained in the Flood Insurance Study. In particular, the locations and height of the levees already in place were check ed. Low lying areas where structures are adjacent to the stream channel were identified as the potential problem areas and were analyzed in greater detail. In general, these areas shown on the map as Areas 1 through 5 are the same areas that have experienced problems in the past. The historic channel capacity was determined at these locations either by USin9 the existing computer runs or by normal depth calculations. Next, the expected depth of innundation for various frequency events was calculated and, finally, a determination was made as to whether temporary or permanent flood protection measures should be recommended. Rating curves which relate flow depths, or stage, to a particular discharge were developed for both the Plateau and Buzzard Creek bridges in Collbran.. A flood frequency can be estimated from the Discharge Table. Practical application of the rating curves is discussed in the Stream Monitoring Section. RECOMMENDED IMPROVEMENTS Area 1 (on map) - There are two structures in the 100-year floodplain in this area. The channel has a capacity of approximately 1600 cfs or the 22-year event. However, even during the 100-year event flow depths near the structures should only be about one foot. A temporary flood control levee, probably sandbags, is recommended when there are threatening high water conditions. Area 2 - On the downstream side of the Main Street bridge the north bank is much higher than the south bank. Consequently. flows in excess of 1400 cfs (the 14-year event) would escape down Park Street if it were not for the rock levee already in place there. Approximately 10 structures could be affected by overbank flooding here, therefore it is recommended that the levee be retained as a permanent flood control structure. It's present height (approximately 3 feet above the channel bank) should be adequate to contain the 100-year event with one foot of freeboard. If possible the levee should be re-built to conform 3