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

approximately 1,600 feet downstream of the former basin and has a 1-percent
annual chance stillwater elevation of 3,492.1 feet (Cooper Arial Surveys, 1978).
For some of the studies in the City of Prescott, flood-frequency data were
developed from discharge-frequency relationships of historic floods and
hydrologic study analyses performed by tile USACE (USACE, October 1973).
Various modifying factors were applied in developing the discharge frequency
curves to allow for the effects of existing floodplain developments.
Flood hydrographs were developed using unit hydrograph procedures. The unit
hydrograph was derived from synthetic S graphs as determined from
reconstitution in the basin or a similar graph.
Peak discharge values for Willow Creek Tributary and Willow Creek Reservoir
Tributary were determined utilizing the NRCS Method. This method is
recommended for drainage basin parameters such as drainage basin area, slope,
vegetation type and cover density, and hydrologic soil groups to determine the
rainfall-runoff relationship and the peak discharge for each recurrence interval.
On Willow Creek, the discharges have been reduced at several points upstream of
the mouth in accordance with the NRCS TR-20 model prepared by Henningson,
Durham & Richardson, Inc.
Discharges for the Agua Fria River, Navajo Drive Wash, and Lynx Creek in the
Town of Prescott Valley were developed using the computer program Project
Formulation Hydrology (FEMA, 1964). This NRCS methodology uses basin
physical characteristics, soil classification, cover conditions, and precipitation
amount as parameters for developing runoff.
A hydrologic analysis for North Navajo Drive Wash was done by adjusting the
discharge values of previously studied Navajo Drive Wash (FEMA, 1982, revised
1990), by applying the drainage area ratio. The peak 1-percent annual chance
discharge varies from 190 cubic feet per second (cfs) upstream to 740 cfs
downstream. For this study only the 1-percent annual chance flood was
calculated.
The upper Agua Fria River watershed basin was divided into subwatersheds, with
attention given to homogeneity of soil type and cover conditions. Times of
concentrations were computed using Manning's equation to develop the velocity
component. Drainage areas were measured using USGS topographic maps (U.S.
Department of the Interior, 1965, et cetera; U.S. Department of the Interior, 1947
et cetera; U.S. Department of the Interior, 1970).
Soil classification and cover conditions were determined through field surveys
and through the General Soil Map for Yavapai County (U.S. Department of
Agriculture, 1978). Antecedent moisture condition 2 was used for the soil
moisture content.
Tile National Oceanic and Atmospheric Administration Atlas 2 (USACE,
February 1977) was used to select rainfall amounts for the 10-, 2-, and 1-percent
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