FLOOD10328 - Laserfiche WebLink

Home Browse Search

I I Floodplain Information Report K:\35600\004\FLOODPLAIN INFORMATION REPORTdoc 28 June 1998 Floodplain Information Report K:\35600\004\FLOODPLAIN INFORMA nON REPORT doc 29 June 998 I I I I I I The 500-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, Coloradoo Just because a person is living above the 100-year flood boundary does not mean that his is completely safe from floodingo The 500-year flood event can also be used for regulating high- risk developments within the floodplain such as the storage and manufacture of toxic or explosive materialso 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 5.4 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 statuteso 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 occupationo Therefore, all floodplain regulations are based upon the 100-year floodo 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 HoB. 1041, Colorado's land use bill. 500- 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 1130 In addition, 20 control markers have been set in the field at strategic locations by the mapping contractor at the request of Boulder Count Yo The additional Boulder County control markers are brass caps set in concreteo The locations of all benchmarks and control markers are shown on the topographic maps prepared for this studyo 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, Inco Technical Addendum available for review at the Colorado Water Conservation Board and the Public Works Department, Boulder Countyo I 5.3 100-Year Flood Event 5.7 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 mapo Horizontal and Vertical Control Monument I I I 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 sue of the more frequent floods as design criteria may be considered. 502 10- and 50- Year Flood Events Flood crest elevations for the 10-,50-, 100-, and 500-year floods may be found in four separate locations in this report. The Flood Frequency-Elevation and Discharge Data table, Table 6, lists these elevations at each cross section within the study reach. The flooded 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 500-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 floodso 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 reacho I I The particular uses for the various flood events in addition to those stated above are as follows: These various flood events have an average occurrence of once in the number of years indicatedo For example, the 100-year period, and has a one percent change of being equaled or exceeded in any given year. 506 Flood Elevations I The 10-, 50-, 100-, and 500-year flood events were used as the flood frequencies for this floodplain analysiso Thus, the data developed in this report will be compatible not only for regulation purposes and HoB. 1041 designation, but also for Federal Insurance Administration flood insurance rate studieso The area delineated on the flooded area plates at the 100-year floodplain meets the requirements of HoB. 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. I As stated above, the 100-year frequency flood has been selected by the State of flood event to be used for floodplain delineation and regulation. I 5.1 Flood Fre and Dischar e Colorado as the I I SECTION 5 - INTERPRETATION AND USE OF REPORT DATA North St. Vrain & St. Floodplain Update Vrain Creeks Town of Lyons 5.5 North St. Vrain & St. Floodplain Update Flooded Areas Vrain Creeks Town of Lyons