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North St. Vrain & Sl Vrain Creeks Floodplain Update Town of Lyons North St. Vrain & St. Vrain Creeks Floodplain Update Town of Lyons SECTION 5 - INTERPRETATION AND USE OF REPORT DATA 5.5 Flooded Areas 5.1 As stated above, the 100-year frequency flood has been selected by the State of Colorado as the flood event to be used for floodplain delineation and regulation. Flood Frequency and Dischar~e The 10-, 50-, 100-, and SOD-year flood events were used as the flood frequencies for this floodplain analysis. Thus, the data developed in this report will be compatible no.t ~nly for regulation purposes and H.B. 1041 designation, but also for Federal Insurance AdrmnIstrahon flood insurance rate studies. The area delineated on the flooded area plates at the 100-year floodplain meets the requirements of H.B. 1041 as an area of state interest. Also, upon official approval of this report by the Colorado Water Conservation Board, the water surface elevations and the area delineated on the flooded area maps as the 100-year flood boundary will be designated as Floodplain Area and may bE: regulated accordingly by local governmental entities. These various flood events have an average occurrence of once in the number of years indicated. For example, the 100-year period, and has a one percent change of being equaled or exceeded in any given year. 5.6 Flood Elevations Flood crest elevations for the 10-, 50-, 100-, and SOD-year floods may be found in four separate locations in this renort. The Flood Freauencv-Elevation and Discharp-p Data tahlp. Tahlp 6. lists these elevations at 'each cross section within the study reach. The f1o~oded area maps (plates 13 through 19) give the plan view of the flooded area on a contour base map and the high water elevations for the 100-year and SOD-year floods can be interpolated from this. The flood profiles (Figures 1 through 6) show the streambed elevation and the high water elevations for all four floods. The cross sections in the Technical Appendix show a graphical representation of the high water elevations at typical canyon and valley cross sections throughout the study reach. The flood profiles may be used in areas where controversy arises over the 100-year flood boundary on the flooded area plates. Since the flood profile plates give the elevations and stationing, or distance, from a known point, the high water elevations can be surveyed on the ground to alleviate any discrepancies on the base map. The particular uses for the various flood events in addition to those stated above are as follows: 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 l~nds, a~d appurtenant structures. For structures and uses of this type on the smaller tnbutanes and In areas where the high risk of structural failure is economically feasible and the hazard to life and property nonexistent, the sue of the more frequent floods as design criteria may be considered. 5.3 100-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 regulations are based upon the 100-year flood. Also, the area which would be inundated by the 100-year flood may be delineated as an area of state interest as set forth In H.B. 1041, Colorado's land use bill. 5.7 Horizontal and Vertical Control Monument 5.4 SOD-Year Flood Event In order to locate points on the ground relating to information given in this report, 2 benchmarks set by federal agencies have been identified in the study area. These benchmarks are tied to United States Coast and Geodetic Survey lines 112 and 113. In addition, 20 control markers have been set in the field at strategic locations by the mapping contractor at the request of Boulder County. The additional Boulder County control markers are brass caps set in concrete. The locations of all benchmarks and control markers are shown on the topographic maps prepared for this study. The designation, elevations, and general location of the benchmarks and control markers are listed in Table 7. Additional survey notes and monument descriptions are included in the 1987 Engineering Professionals, Inc. Technical Addendum available for review at the Colorado Water Conservation Board and the Public Works Department, Boulder County. The SOD-year flood event is useful in making the public aware that floods larger than the 100- year event can and do occur as evident in 1997 in the City of Fort Collins, Colorado. Just because a person is living above the 100-year flood boundary does not mean that hIS IS completely safe from flooding. The SOD-year flood event can also be USE:d for regulating high- risk developments within the floodplain such as the storage and manufacture of tOXIC or explosive materials. To resolve elevation differences in previous floodplain studies and provide the basis for future elevation certifications, BRw investigated the established benchmarks located within and near the Town of Lyons corporate limits. Six existing benchmarks were located that were set by Floodplain Information Report 28 K:\356001004\FLOODPLAIN INFORMATION REPORT.doc June 1998 Floodplain Information Report 29 K:\356001004\FLOODPLAIN INFORMATION REPORTdoc June 1998