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

aselines used for horizontal control on the detailed studied streams were
obtained from these same maps.
Much of the overbank areas are covered with extremely dense vegetation. The
roughness coefficients for bridges and culverts were assumed to be equivalent to
the channel Manning's "n" value.
The dimensions of backwater-producing structures were identified through field
investigations and construction plans obtained from the Arizona Department of
Transportation. The backwater effects of a structure that was assumed to fail
during high-magnitude flow events were not included ill the hydraulic analysis.
This structure is an earthen flow-diversion dike along the Verde River which was
constructed for irrigation purposes.
Starting water-surface elevations for the tributary streams, Deception Wash, Bitter
Creek, and Bitter Creek-South Fork, were computed at normal depth at a cross
section located within the 1-percent annual chance floodplain limits of the Verde
River.
The hydraulic models of tributary streams to the Verde River at the Town of
Clarkdale determined that flow is in a supercritical mode within these channels;
however, backwater computations (subcritical flow models) were utilized to
compute the flow profiles. Critical depth was assumed at nearly every cross
section for these tributary studies.
Minguez Wash, Mescal Gulch, and North Fork Mescal Gulch which were studied
by approximate methods, utilized the Flood Hazard Boundary Map (U.S.
Department of Housing and Urban Development, 1975) aid in delineating flood
hazard areas. Additionally, a HEC-2 analysis was used to compute the Minguez
Wash floodplain area. These maps were checked in the field to ensure their
validity.
In order to simulate the character of stream channels and their adjacent overbanks
for the City of Cottonwood, cross sections were compiled utilizing topographic
maps of the stream channels specified for study by detailed methods (Cooper
Aerial Surveys, Topographic Maps, Town of Cottonwood, Yavapai County,
Arizona, Scale 1:2,400, Contour Interval 2 feet, Tucson, Arizona, 1978; Kenny
Aerial Mapping, 1983, 1984).
Some discrepancies were noted between portions of the topographic mapping
utilized for Silver Springs Gulch in the 1981 FIS of the City of Cottonwood, and
the restudy, which is based on current mapping. The mapping was field verified
for accuracy and supersedes information presented in the previous study.
An unauthorized excavation of approximately 1,100 feet at Silver Springs Gulch
occurred in the summer of 1985, This excavation was brought to the attention of
the study contractor by a letter from the Planning and Zoning Administrator for
the City of Cottonwood on September 5, 1985, and occurred beginning
approximately 1,000 feet west of 6 th Street, south of U.S. Highway Bypass 89A.
A site visit was subsequently performed by the study contractor on September 27,
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