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

1985. It was determined from the site visit that the excavation, although
significant, represented only a localized modification and lowering of the channel
area and did not contain an effective element to collect runoff at the upstream end
or release it at the downstream end. Therefore, the influence of this excavation on
floodplain characteristics was ignored, thus yielding more conservative results.
The dimensions of backwater producing structures were identified through field
investigation and construction plans obtained from the U.S. Department of the
Interior, National Park Service (U.S. Department of the Interior, 1966). The
backwater effect of a structure that was assumed to fail during high magnitude
flow events was not included in the hydraulic analysis. An example is the Verde
River crossing to the Dead Horse Campgrounds. This roadway was constructed
by the State Parks Commission in order to provide access to the campgrounds.
The roadway is presently impassable as previous floodflows have washed out the
roadway surface. Contacts with the State Parks Department have determined that
this roadway be reconstructed at channel grade in view of the probability of future
flows inundating this structure.
Starting water-surface elevations for Del Monte Wash, Silver Springs Gulch, and
Oak Wash were computed at normal depth at a cross section located within the 1-
percent annual chance floodplain limits of the Verde River. For Railroad Wash,
the starting water-surface elevation was computed at normal depth (using the
slope/area method) at a cross section located at the confluence with the
Cottonwood Ditch.
The hydraulic models of the streams being studied in the City of Cottonwood
determined that flow was 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
study streams.
As mentioned in Section 3.1, Railroad Wash has a detention/retention basin which
is located approximately 1,600 feet downstream of the runway at Cottonwood
Airport. To determine the depth of ponding in the basin during the 1-percent
annual chance flood, a flood routing was performed using a modified PULS
routing procedure (Arizona Highway Department, 1968, revised 1969). The
floodplain upstream of the detention/retention basin was determined using
approximate methods.
As described in Section 2.3 of this report, several flooding problems have resulted
in the past from the inadequate capacity of the structure at the East Main Street
crossing of Del Monte Wash to convey large-magnitude flows and large objects
such as trees and cars, from obstructing the flow through the culvert. The results
of hydraulic studies have determined that no flow in excess of the 10-percent
annual chance event could be passed beneath the roadway under unobstructed
flow conditions. The inadequate capacity of this structure has resulted in the most
significant flooding problems for the City of Cottonwood, as areas downstream of
this structure represent the most highly developed portion of the town.
Floodwaters which escape the Del Monte Wash stream channels as a result of
backwater effects at East Main Street and flow onto the adjacent floodplains
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