YAVAPAI COUNTY, ARIZONA Federal Emergency ... - FEMA Region 9

The boundaries for the approximate study along Beaver Creek School Wash and
Rimrock Creek in the were delineated using 2-foot contour mapping created from
aerial photography flown at a scale of 1 inch = 100 feet provided by Cooper
Aerial and collected in 1998. HEC-RAS and WSP modeling was done by
Claycomb/Rockwell Associates, Inc., in 1999.
The boundaries for the approximate study of 4.5 miles along Yarber Wash in the
Town of Dewey-Humboldt were delineated using 2-ft contour mapping created
from aerial photography flown and provided by Southwest Mapping
Technologies, Inc., and collected in 2005. HEC-RAS modeling was done by
Project Engineering Consultants, Ltd., in 2005.
The boundaries for the approximate study along Oak Wash, Cherry Hills Wash,
Rio Mesa Wash, Christina Draw, and Pipe Creek were delineated using 4-foot
contour mapping created from aerial photography flown at a scale of 1” = 200 feet
and provided by Analytical Surveys, Inc., and collected in 1992. HEC-RAS
modeling was done by McLaughlin Kmetty Engineers, Ltd., in 1995.
4.2 Floodways
Encroachment on floodplains, 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 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 this concept, the area of the 1-percent annual chance 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 1-percent annual chance flood can be carried without substantial increases in
flood heights. Minimum Federal standards limit such increases to one 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 for selected cross sections (see Table 9, "Floodway Data," shown in
Volume 2). In cases where the floodway and 1-percent annual chance floodplain
boundaries are either close together or collinear, only the floodway boundary is
shown.
In the case of shallow flooding in Yavapai County, the unpredictable flowpaths
have made determination of a floodway meeting FEMA criteria impossible;
therefore no floodway is presented in these areas (FEMA, 1999).
The hydraulic modeling of Deception Wash, Bitter Creek, and Bitter Creek South
Fork in the Town of Clarkdale indicated that floodway encroachments would not
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