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INTERPRETATIO~ AND "SE OF REPORT DATA Flood Fr~quency and Diachar~e Thelo-,SC-, 100-and500-ye"r flood eVcnts Io'ere used as the flood frequenctesforthisflood-plainanalysis, Thus the data developed in this report will bc compatible not only for regulation purposes, and H.B. 1041 designations, but also for Federal Insurance Administration flood insurance rate lltudies. The various flood events have an average occurrence of once in the number of years as indicated. For example, the 100-year flood occurs, en thc average, once in a IOO-year period, and ha5 a 1 percent chance of being cq~lcd Or cxcccdcd in any givcn year. lOo-year Flood eVent The lOa-year flood event may also be used for engineering design purposes where s lower risk of failure than the lO~ or 50-year flood is desired. Howevcr, tho most important use of the lOa-year flood event lies in flood-plain designation snd land use regulation as setforrhint he State statutes. The State of Colorado considers the 100-year fr"quency flood as the flood event to be used in designing and protecting structures and dwellings for human occupation. Therefore, all flood-plain regulstions are based upon the lOO-year flood. Also, the area which would be inundated by the lOO-year flood may be delineated as an area of Statc interest as Set forthinH.B.I041--theState'slandusebill. SaC-year Flood event The SaC-year flood event ia useful in waking the public aware that floods larger than the lOa-year flood Can and do occur. Just becau5e a person is living above the lOO-year flood boundary docs not oean that he is c0mpletely safe fr~m fl~oding. The SaC-year flood event ~3n also be used for r"-gulating highrbk developlIlentS within the flood plain such as nuclear po,",er plants. or the storage or manufa~ture of toxic or explosive materials. 15 RE:\OURCE: CON\UlT~NT\ INC Table 5 li~t~ the discharges and water-surface elevations for the 10-, 50-, laC- and SOD-year flood events at sell'cted referen~e points. Flooded Areas The laO-year frequency flood has be"-n selected by the State of Colo- rsdo as the flood event to be u~ed for flood-plain delineation and regula- tion. Thus the FLOOded Area plates shm,' only the boundary of the 100-year flood plain. Since the base map for the Flooded Area plates is a 2-foot contour map, the outlines of the other flood events (500-year) can be readily plotted using the information in Table 5. I I I I I t I The area delineated On the Flooded Area plates as the lOa-year flood plain ~eets the requirements of H.B. 1041 as an srea of State interest. Also upon official approval of this report by the Colorado Water Conserva- tion Board the area outlined by the lOa-year flood boundary will be desig- nated a flOOd-plain area and may be regulated accordingly by the local officials. Flood Elevations Flood water surface elevations for the 10-, SO-, 100- and 500-year floods maybe found in four separate locations in this report. The flood frequency-elevation and diach,.rge data in Table 5 lists the5e elevstion~ at selected reference points. The Flooded Area Plates 2 through 10 givc the plan view of the flooded area on a contour base wap and the high water elevations for the 100-year flood can be interpolated from this. The Hood Profile plates, Plates 5 through 11, show the streambed elevation and the high water elevations for all four frequency floods. Also the Cross Section plates, Plates 14 to 17 show a graphical representation of the high water elevations at typical valley croas sections throughout the study reach. The Flo"d Profil"" ""'y he n~erl In areas where controvcrsy <lriscu ovcr the lOo-year flood boulldsry '''' tbe Flooded Area plates. Sinc" the Flood Profile plates give the elevations and distance Or stationing fro~ s kno~~ point, the high water elevations can be surveyed on the ground to alleviate " RE:\OURCE: CON\UlT~NT\ INC