FLOOD01227

• Designation Number: 208
• County: Mesa
• Community: Grand Junction
• Title: FIS - Grand Junction, Mesa County, Colorado
• Date: 7/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

the flood boundaries may lie above the flood elevations and, there-
fore, not be subject to flooding, owing to limitations of the

4.2 Floodways

Encroachment on flood plains, such as artificial fill, reduces
the flood-carrying capacity, increases the flood heights of streams,
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 National Flood Insurance
Program, the concept of 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 lOa-year flood 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 in order that the lOa-year flood may be carried
without substantial increases in flood heights. Minimum standards
of the Federal Emergency Management Agency limit such increases
in flood heights to 1. 0 foot, provided that hazardous velocities
are not produced. The floodways in this report are presented
to local agencies as minimum standards that can be adopted or
that can be used as a basis for additional studies.

For Indian Wash, floodways were not computed for areas of 100-
year sheet flaw diversions. However, the complete filling of
these sheet flow areas will increase floodflows downstream, thereby
increasing the lOa-year flood hazard. For any channel modification
or fill in the sheet flaw areas, a detailed review of the improve-
ment should be undertaken to determine the effects downstream.

Floodways were not computed for Leach Creek and Horizon Drive
Channel due to shallow flooding in some areas and retention of
all flooding intervals in the stream channels for the remaining
ar.eas.. Howev_er:, ,l)Qencroacl:JIIl~r1tsbould be, all,9'</",<!_"ithin the
stream channel, as it would obstruct flow and cause an increase
in the water-surface elevations.

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 were tabulated at selected cross
sections for each stream segment for which a floodway was computed
(Table 3).

As shown on the Flood Boundary and Floodway Map (Exhibit 2), the
floodway widths were determined at cross sections, between cross
sections, the boundaries were interpolated. In cases where the
boundaries of the floodway and the lOa-year flood are either close
together or collinear, only the floodway boundary has been shown.

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