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

Revised discharge-drainage area relationships for the Verde River are shown
below.
TABLE 5 – SUMMARY OF DISCHARGES
FLOODING SOURCE
AND LOCATION
DRAINAGE
AREA
(sq. miles)
PEAK DISCHARGES (cfs)
10-PERCENT 2-PERCENT 1-PERCENT 0.2-PERCENT
Verde River
At USGS gage
no. 09504000 3,124 22,750 55,100 75,100 136,700
At US Route 89A bridge 3,247 23,900 58,200 79,600 149,700
Below confluence with
Oak Creek 3,776 28,700 72,100 100,000 193,900
Below confluence with
West Beaver Creek 4,287 33,500 86,300 121,200 241,000
Below confluence with
West Clear Creek 4,619 36,800 96,000 135,600 273,900
At USGS gage
no. 09506000 4,645 37,000 96,800 136,700 276,500
For the approximate Zone A studies, 1%-annual-chance discharges were taken
from section 3.1 of this FIS report. In cases where effective discharges were not
available, discharges developed by Yavapai County using USGS regional
regression equations were used (Reference 3).
e. Hydraulic Analysis
For the Verde River, water surface elevations and floodway limits were
computed using georeferenced modeling data within the USACE HEC-GeoRAS
computer software (Reference 4). Both upstream and downstream study limits
are bound by the Prescott National Forest, while the downstream limit is also
bound by the Coconino National Forest. No future developmental impacts are
anticipated in these areas.
The downstream boundary condition for the Verde River was set to utilize the
normal depth method and a slope of 0.004734 ft/ft, based on a 2,000-foot
sample reach. No known water surface elevation for this location was available
from the previous study.
The floodway was prepared using standard method 1 and method 4 in HEC-
GeoRAS. These two methods reflect the same encroachments at each cross
section, with the exception of a customized adjustment applied at the 10th Street
Bridge that results from poor alignment between the approach roadway and the
bridge structure.
Irrigation canals and diversion structures were modeled assuming they are at
capacity and cannot divert or convey flow. Additional modeling considerations
were made for the Interstate 17 embankment and bridges (Reference 1).
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