FLOOD04412

• Designation Number: 245
• County: Garfield
• Community: Rifle
• Basin: Colorado Mainstem
• Title: Flood Insurance Study - Rifle, Colorado
• Date: 1/1/1986
• Designation Date: 1/1/1986
• Floodplain - Doc Type: Floodplain Report/Masterplan

cross section. Between cross sections, the boundaries were inter-
polated using topographic maps at a scale of 1:6,000, with a contour
interval of S feet (Reference 11). Flood plain boundaries for
Helmer and Ramsey Gulches were delineated using the above referenced
topographic maps.

Flood boundaries for the Colorado River were originally delineated
using topographic maps at a scale of 1:24,000, with contour intervals
of 20 or 40 feet (Reference 12). Those boundaries have been revised
for this Flood Insurance study using topographic maps at a scale
of 1:6,000, with a contour interval of S feet (Reference 11).

The 100- and SOO-year flood plain boundaries are shown on the Flood
Boundary and Floodway Map (Exhibit 2). In cases where the 100-
and SOO-year flood plain boundaries are close together, only the
100-year flood plain boundary has been shown. Small areas within
the flood plain 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.

Approximate 100-year flood plain boundaries were delineated using
topographic maps at a scale of 1:6,000, with a contour interval of
S feet (Reference 11).

For the streams studied by approximate methods, only the 100-year
flood plain boundary is shown.

4.2 Floodways

Encroachment on flood plains, such as structures and fill, reduces
flood-carrying capacity, increases flood heights and velocities,
and increases flood hazards in areas beyond the encroachment itself.
One aspect of flood plain management involves balancing the economic
gain from flood plain 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 flood plain
management. Under this concept, the area of the 100-year flood
plain is divided into a floodway and a floodway fringe. The floodway
is the channel of a stream, plus any adjacent flood plain areas,
that must be kept free of encroachment so that the 100-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
flooe~ay studies.

The floodways presented in this study were computed on the basis
of equal conveyance reduction from each side of the flood plain.
The results of these computations are tabulated at selected cross
sections for each stream segment for which a floodway is computed
(Table 2).

A floodway is not appropriate on an alluvial fan. For this reason
no floodways are presented for Helmer and Ramsey Gulches.

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