FLOOD10704 ‎(2)‎

• County: Delta
• Community: Delta
• Title: FIS - Delta
• Date: 8/19/1991
• Prepared For: Delta
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

4.1 Floodplain Boundaries

To provide a national standard without regional discrimination, the
1 percent annual chance (lOO-year) flood has been adopted by FEMA
as the base flood for floodplain management purposes. The
0.2 percent annual chance (SOO-year) flood is employed to indicate
additional areas of flood risk in the community. For each stream
studied by detailed methods, the 100- and 500-year floodplain
boundaries have been delineated using the flood elevations
determined at each cross section. 8etween cross se.ctions, the
boundaries were interpolated using topographic maps at scales of
1:2,400, 1:600, and 1:1,200, with contour intervals of 2, 1, and 2
feet, respectively (References 12, 13, and 14).

The 100- and SOO-year floodplain boundaries are shown on the Flood
Insurance Rate Map (Exhibit 2). On this map, the lOO-year
floodplain boundary corresponds to the boundary of the areas of
special flood hazards (Zone AE); and the SOO-year floodplain
boundary corresponds to the boundary of areas of moderate flood
hazards. In cases where the 100- and SOa-year floodplain
boundaries are close together, only the IOO-year floodplain
boundary has been shown. Small areas within the floodplain
boundaries may lie above the flood elevations but cannot be shown
due to limitations of the map scale and/or lack of detailed
topographic data.

4.2 Floodways

Encroachment on floodplains, such as structures and fi 11, reduces
flood-carrying capacity, increases' flood heights and velocities,
and increases flood hazards in areas beyond the encroachment
itself. One aspect of floodplain management involves balancing the
economic gain from floodplain development against the resulting
increase in flood hazard. For purposes of the NFIP, a floodway is
used as a tool to assist local communities in this aspect of
floodplain management. Under thi s concept, the area of the 100-
year floodplain is divided into a floodway and a floodway fringe.
The floodway is the channel of a stream, plus any adjacent
floodplain areas, that must be kept free of encroachment so that
the IOO-year flood can be carried without substantial increases in
flood heights. Minimum Federal standards limit such increases to
1.0 foot, provided that hazardous velocities are not produced. The
floodways in this study are presented to local agencies as minimum
standards that can be adopted directly or that can be used as a
basis for additional floodway studies.

The floodways presented in this study were computed for certain
stream segments on the basis of equal conveyance reduction from
each side of the floodplain. Floodway widths were computed at cross
sections. Between cross sections, the floodway boundaries were
interpolated. The results of the floodway computations are
tabulated at selected cross sections (Table 2). In cases where the
floodway and lOO-year floodplain boundaries are either close
together or collinear, only the floodway boundary has been shown.

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