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<br />any discrepancies on the base map. To aid in the establishment of flood
<br />elevations, Larimer County has established permanent bcnch~arks in the
<br />
<br />field. Table 1 lists these benchmarks and related information.
<br />
<br />Theloca-
<br />
<br />tions of the benchmarks are appropriately labeled On Plates 2 through 10.
<br />
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<br />Encroachment on flood plains, such as artificial fill, reduces the
<br />flood-carrying capacity and increases flood heights, thus increasing flood
<br />hazards in areaS beyond the encroachment itself. One aspect of flood-plain
<br />management involves balancing the economic gain from flood-plain develop-
<br />ment against the resulting increase in flood hazard. For purposes of the
<br />National Flood Insurance Program, the concept of a flaod~ay is used as a
<br />tool to assist local cowaunities in this aspect of flood-plain manaKement.
<br />Under this concept, the area of the 100-year flood is divided into s
<br />flood~ay end a floodway fringe. The floodway is the channel of a stream,
<br />plus anv adja~ent flood-plain areas, that must be kept free of encroach~ent
<br />in order that the lOO-year flood be carried without substantial increases
<br />in flood heiehts. A~ minimum standards, the Federal Insurance Ad~inistra-
<br />tion limits such increases in flood heights to 1.0 foot, provided that
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<br />hazardo,," velocities are not produced.
<br />
<br />Larimer County has an ordinance
<br />
<br />~hich limits the increase in flood heights to 0,5 foot above the pre-
<br />floadway elevation for the plains portion of the county; therefore, a
<br />floodway hsving nO mor~ th~n ~ O.~-f~ot ~~rcharg~ has be~n ~elineat~d in
<br />this study.
<br />
<br />The floodway in this study was computed on the basis of equal convey-
<br />ance reduction for each side of the flood plain. The results of these
<br />computations are tabulated at selected cross sections for each stream
<br />segment for "hich a floodway is computed (Table 6).
<br />
<br />The area between the flood~ay and the boundary of the loo-year flood
<br />is tcrmed thc flood'..aj' fringc. Thc flood~ay frin!;e thus enCOlnpasses the
<br />portion of the flood plain that could be completely obstructed without
<br />increa~ing the water-surface elevation of the lOO-year flood more than O.S
<br />fcot at any point. Typical relationships b.,twccn the. floodway and th~
<br />floodway fringe and their significance to flood-plain development are sho~~
<br />in Fig. 1.
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<br />RE:\OURCE: CON\UlT~NT\ INC
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