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

Problems created by large magnitude floodflow events on Del Monte Wash in the
City of Cottonwood are a result of inadequate culvert capacity at the main stream
crossing.
An earthen levee exists on the north bank of Willow Creek between Willow
Creek Road and Lorraine Drive in the City of Prescott. This levee was
reconstructed at several sections in 1987, in order to provide protection from 1-
percent annual chance floodflows for the Sandretto Hills Development, which is
located along the north side of Willow Creek (FEMA, 1982, revised 1990).
An organization was formed in 1978 in the City of Sedona area to predict
flooding of Oak Creek and warn residents living in the vicinity so that evacuation
of floodprone areas could be possible. The main purpose of the organization was
protect life. Several small dikes and riprapped embankments have been
constructed by private landowners along Oak Creek to protect their property from
inundation and erosion during floods.
There is a levee on the right bank (looking downstream) of Chino Valley Stream
downstream of U.S. Route 89. Because the levee does not meet the requirements
of Section 65.10 of the NFIP regulations, two flooding situations were evaluated
for the 1-percent annual chance floodplain and floodway: flooding due to the
levee failing and holding. Assuming the levee fails during the 1-percent annual
chance flood, Chino Valley Stream will converge with Santa Cruz Wash,
approximately 4,900 feet upstream of Old U.S. Route 89. Assuming the levee
holds during the 1-percent annual chance flood, Chino Valley Stream will
continue north along a different path for approximately 5,200 feet and converge
with Santa Cruz Wash just upstream of Old U.S. Route 89. The floodway for
Chino Valley Stream was determined assuming the levee fails during the 1-
percent annual chance flood. Because Chino Valley Stream will follow a different
path if the levee holds, a floodway was also determined for that reach.
The Towns of Chino Valley and Prescott Valley, and the Cities of Cottonwood,
Prescott and Sedona have passed floodplain regulations or zoning ordinances
designed to delineate areas of flood hazards and to guide and regulate
development within flood hazard areas so as to restrict future flood damage and
flood hazards.
3.0 ENGINEERING METHODS
For the flooding sources studied in detail in the county, standard hydrologic and
hydraulic study methods were used to determine the flood hazard data required for this
FIS. Flood events of a magnitude which are expected to be equaled or exceeded once on
the average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been
selected as having special significance for floodplain management and for flood
insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods,
have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded
during any year. Although the recurrence interval represents the long term average period
between floods of a specific magnitude, rare floods could occur at short intervals or even
within the same year. The risk of experiencing a rare flood increases when periods
greater than 1 year are considered. For example, the risk of having a flood which equals
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