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<br />IV. FLOODPLAIN MANAGEMENT CONCEPTS
<br />
<br />sOa-year flood event can also be used for regulating high risk
<br />developments withtn the floodplain such as nuclear po""er plants,
<br />or the storage or manufacture of toxic or explosive materials.
<br />
<br />A. Frequency and Discharge
<br />The 10-, 50- 100-, and sOO-year flood events are used as the
<br />flood frequencies for this floodplain analysis. Thus the data
<br />developed in thh report will be compatible not only for
<br />regulation purposes, and H.B. 1041 designation but also for
<br />Feder'll Insurance Adminhtration flood insurance rate studies.
<br />
<br />10~Year and 50-Year Flood Events
<br />
<br />B. Flood Elevations
<br />Flood crest elevations for the 10-, 50-, 100-, and 500~year
<br />floods as determined at each cross 6ection may be found in the
<br />Flood Frequency-Elevation and Discharge Data table, Tables
<br />11-13. The Cross Section plate6, Plates 28 through 30, show a
<br />graphical representation of the high water elevations at typical
<br />valley cross sections throughout the study reach. Water surface
<br />elevations at the cross sections were used to prepare the flood
<br />profiles, plates 15 through 27, which show the streambed
<br />elevation in relation to high water elevations for the 10-year
<br />and IOO-year frequency floods. For clairty, the 50-year and
<br />SOO-year floods were omitted from the profile.
<br />The Flood Profiles may be used in areas where .;;ontroversy arises
<br />over the 100-year flood boundary shown on the Flooded Area
<br />plates. Since rhe Flood Profile plates give the water surface
<br />elevation Bt a specific point on the referen.;;e line, the high
<br />water elevations can be surveyed on the ground to alleviate any
<br />discrepancies on the base map. A number of reference monuments
<br />on the same datum as the hase maps are in place along each study
<br />reach and .;;an be used for this purpose. The elevation, field
<br />identification, locativn and description of rhese monuments are
<br />described in Table 4-6.
<br />
<br />These various flood events have an average occurren.;;e of .;>nce in
<br />the number of years aa indicated. For example, the 100.year
<br />flood occurs, on the average, once 1n a 100 year period, and has
<br />a one percent chance of being equaled or exceeded in any given
<br />year.
<br />
<br />Th", particular uses for the various flood events in addition to
<br />those stated above are as follows:
<br />
<br />Information regarding these lower frequency floods is especially
<br />useful for future engineering studies and land use planning
<br />purposes related to minor road systems, minor channel
<br />improvements, the lo.;;ation of parks snd recreational facilities,
<br />agricultural lands, and appurtenant structures. For structures
<br />and uses vf this type on the smaller tributaries or in areas
<br />where the high risk ,)f structural failure is economically
<br />feasible, and the ha:;::ard to life and property nonexistent, the
<br />use vf the lower frequency floods may be .;;onsidered.
<br />lOO-Year Flood f.vent
<br />
<br />C. Flooded Areas
<br />
<br />Th", Flooded Area maps, plates 3 through i4, show the boundary of
<br />the 100-year and 500~year floodplains. Th", floodplain boundaries
<br />wt:re "lolt",'; [rvm tll", floo"! I"rvflles by determining the channel
<br />stationing of flood contours at the same interval as the
<br />topographic maps. Flood .;;ontours, sho",'n as wiggly lines, extend
<br />perpendiculsr to the direction of flow and intersect the ground
<br />at the edge of the floodplain.
<br />
<br />......~ '''''_''n"~ "'~_" ~H~_" _~.. ~l-~ ...~ ,.~n" ,,~~ ~~~'~n~~'~~ ~n~'~_
<br />u,,, LVV :I"'" 'L""" "."". '''":1 " "" ,",,_ ""...." 'V' """"."""""'", ,-,,,,,.,,..
<br />purposes where a lo"u~r risk of failure than the 10- or 50- year
<br />flood is desired. However, the most impvrtant use of the
<br />100-year flood event lies in floodplain cnanagement and land use
<br />planning as set forth i~ the state statutes. The St8te of
<br />Colorado considers the 100~year frequency flood as the flood
<br />event to be used in designing and protecting structures and
<br />dwellings for human occupation. Therefore, all floodplain
<br />regulations are based upon the 100~year flood.
<br />jOO-Y~~r Flvvo Ev~nt
<br />
<br />Upon official approval of this report
<br />Conservation Board, the area outlined
<br />boundary may be regulated accordingly by
<br />
<br />by
<br />by
<br />oh,
<br />
<br />the Colorado Water
<br />the 100-y~ar flood
<br />local officials.
<br />
<br />D,
<br />
<br />Flood Ill"u",,,'ce
<br />
<br />The SOD-year flood event
<br />that floods larger than
<br />Just because a person is
<br />does not t:lean that he
<br />
<br />is useful in making the public aware
<br />the 100-year flood can and do occur.
<br />living above the lOa-year flood boundary
<br />is cvmpletely :;afe frot!' flooding. Th..
<br />
<br />The National Flood Insurance Progrllm is a Federal progratll that
<br />enables property owners to ouy flood insurance at a reasonable,
<br />subsidized cost. In return, comt!'unities are required to carry
<br />out floodplain management measures ro protect lives and new
<br />construction from future fiooding.
<br />
<br />- I S -
<br />
<br />~16-
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