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

Precipitation in the study area is produced by general winter, general summer, and
local storms. General winter storms normally occur over the southwestern United
States during the cooler months of November through March as extratropical
cyclones move inland from the Pacific Ocean, spreading light to moderate
precipitation over large areas for several days. Orographic effects are usually
quite pronounced with the mountain receiving greater precipitation than the lower
elevations. Much of the precipitation in the higher elevations also falls as snow
during this type of storm.
At times, however, warm heavy rain can fall on top of a ripe snow-pack creating
conditions favorable for heavy runoff. General summer storms normally occur
between July and October and may be associated with a tropical storm. These
storms usually consist of general steady or intermittent rain over large areas, with
moderate to heavy thunderstorms often embedded. Local storms are defined as
rainstorms of high to very high intensity, occurring over small areas for short
durations. They are most common during the summer months, but can occur at
any time of the year (USACE, 1976). Mean annual precipitation ranges from
approximately 10 inches in the desert to over 27 inches in the mountains. The
mean annual snow, sleet, and hail total for the mountains ranges between 40 and
50 inches. Crown King station recorded a monthly extreme of 116.1 inches in
January 1941 (Sellers and Hill, 1973).
Development of the floodplain varies. Most of the land along the study streams is
either agricultural or undeveloped. However, many areas have mobile homes,
houses and/or commercial structures as well as public utilities, roads, and bridges
located in the floodplain.
2.3 Principal Flood Problems
Flooding occurs as a result of discharges in excess of channel capacities, and in
some cases from backwater from restrictive bridges. The following descriptions
contain information on the principal flooding problems and historical flooding.
For the smaller drainage area streams, local storms, predominately during the late
summer months, cause the most severe flooding. With the exception of Pacific
Tropical Storm Norma in September 1970, major flooding in the larger basins
occurs from the general winter storms with snowmelt.
Flooding records date back to 1890 for the Hassayampa River and 1891 for the
Verde River. Six of the detailed study streams were correlated with information
obtained from the USGS stream gages.
The stream name, date, discharge, and estimated recurrence interval for some of
the larger peak discharges recorded are shown in Table 4, "Historic Discharges".
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