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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />INTERPRETATION AND USE OF REPORT DATA <br /> <br />Starti ng water surface el evati ons for Boul der Creek were computed <br />assuming that the Arapahoe Road bridge at Boulder was blocked with all <br />flow passi ng over the roadway as wei r flow. Starti ng water surface <br />elevations on Fourmile Creek were obtained by assuming that the Colorado <br />Highway 119 bridge was blocked for the 50-, 100-, and 500-year floods, <br />but open for the 10-year flood. Flow over the roadway for the 50-, 100-, <br />and 500-year floods was determined by weir computations. Flow through <br />the bridge for the 10-year flood was determined by pipe flow computations. <br />Bridges in this study were analyzed using a blockage criteria <br />dependent upon bri dge constructi on and water depth. Concrete and steel <br />bridges were assumed unobstructed until the upstream water surface <br />elevation reached the bridge "low steel" elevation at which time the <br />bri dge was assumed fully obstructed. Wooden bri dge decks were assumed <br />destroyed due to debris. This type of bridge was assumed unobstructed at <br />all discharges with wingwalls and abutments in place, but the deck <br />removed. Wooden footbridges on both streams and two wooden car bridges <br />on Boulder Creek were not included in the hydraulic model since removal <br />of the bridge deck left only a natural channel without abutments or <br />wi ngwall s. Head losses at fully obstructed bri dges were determined by <br />weir computations. Unobstructed bridge losses were computed by using the <br />normal bridge routine in HEC-2. <br /> <br />Flood Frequency and Discharge <br />The 10-, 50-, 100-, and 500-year flood events were used as the flood <br />frequencies for this floodplain analysis. Thus, the data developed in <br />this report will be compatible not only for regulation purposes and H.B. <br />1041 designations, but also for Federal Emergency Management Agency flood <br />insurance rate studies. <br />These various flood events have an average occurrence of once in the <br />number of years as indicated. For example, the 100-year flood occurs, on <br />the average, once in a 100-year peri od and has a one-percent chance of <br />being equaled or exceeded in any given year. <br />The particular uses for the various flood events in addition to those <br />stated above are as follows: <br />10-Year and 50-Year Events Information regarding these lower <br />frequency floods is especi ally useful for future engi neeri ng studi es and <br />land use planning purposes related to minor road systems, minor channel <br />improvements, the location of parks and recreational facil ities, <br />agri cultural 1 ands, and appurtenant structures. For structures and uses <br />of this type on the smaller tributaries and in areas where the high risk <br />of structural failure is economically feasible and the hazard to life and <br />property nonexistent, the use of the lower frequency floods may be <br />considered. <br />100- Year Event - The 100-year flood event may al so be used for <br />engineering design purposes where a lower risk of failure than the 10-, <br />or 50-year flood is desired. However, the most important use of the <br />100-year flood event lies in the floodplain designation and land use <br /> <br />7 <br />