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I I I I II I II I I' I I I I I 1\ II I I I 4.3 Hydraulic Analysis SECTION 5 - INTERPRETATION AND USE OF REPORT DATA Analyses of the hydraulic characteristics of the stream in the study areas were carried out to provide estimates of the water surface elevations of floods of the selected recurrence intervals along each stream. The water surface elevations were computed through use of the Corps of Engineers' HEC-2 step backwater computer program. Cross sections for the backwater analyses of the North St. Vrain and the main stream of the St. Vrain below Lyons were obtained by digitizing cross sections obtained from the topographic maps of the study reach. Additional cross sections were field-surveyed at close intervals above and below bridges and culverts in order to account for possible backwater effects of these structures. The locations of the cross sections used in the flood profile computations are referenced on the flooded area maps (Plates 4 through 32) and the flood profiles (Plates 33 through 63). Flood elevations and discharges for the 10-, 50-, 100-, and 500-year events for each reference cross- section are listed in Table 5. Selected cross-sections from throughout the study reach are depicted, with the 10-, 50-, 100-, and 500-year water surface elevations, on Plates 68 and 69. 5.1 Flood Frequency and Discharge The 10-, 50-, 100-, and 500-year flood events were used as the flood frequencies for this floodplain analysis. Thus, the data developed in this report will be compatible not only for regulation purposes and H.B. 1041 designations, but also for Federal Insurance Administration flood insurance rate studies. These various flood events have an average occurence of once in the number of years indicated. For example, the 100-year flood occurs, on the average, once in a 100-year period, and has a one percent chance of being equalled or exceeded in any given year. It is interesting to note that, statistically, there is a 37 percent chance that the 100-year event will NEVER occur in the next 100 years. The particular uses for the various flood events in addition to those stated above are as follows: Channel roughness coefficients (Manning's Un") for the flood profile computations were assigned on the basis of field inspection of the channel and floodplain through the study reach. The range of Un" values for channel and floodplain are summarized below: 5.2 10- and 50-Year Flood Events Information regarding these more frequent floods is especially useful for future engineering studies and land use planning purposes related to minor road systems, minor channel improvements, the location of parks and recreational facilities, agricultural lands, and appurtenant structures. For structures and uses of this type on the smaller tributaries and in areas where the high risk of structural failure is economically feasible and the hazard to life and property nonexistent, the use of the more frequent floods as design criteria may be considered. TABLE 4 ROUGHNESS COEFFICIENTS linn Channel Floodplain .030 - .040 .040 - .065 A detailed description of the hydraulic analyses and Manning's Un" report is included in the Technical Addendum. 5.3 lOO-Year Flood Event The 100-year flood event may also be used for engineering design purposes where a lower risk of failure than the 10- or 50-year flood is desired. However, the most important use of the 100- year flood event lies in floodplain designation and land use regulation as set forth in the state statutes. The State of Colorado considers the 100-year frequency flood as the flood event to be used in designing and protecting structures and dwellings for human occupation. Therefore, all floodplain -17-