FLOOD00005

• County: Statewide
• Title: Precipitation Records and Flood-Producing Storms in Cheyenne Wyoming
• Date: 10/23/2003
• Prepared For: Cheyenne
• Floodplain - Doc Type: Educational/Technical/Reference Information

Results

Dimensionless hyetographs for nine storms identified as major flood-
producing storms in Druse and others (1986) and the storm of August 1, 1985,
are plotted in figure 6. The 10-percent and 90-percent limit curves for
thunderstorms in Wyoming, as described by Hasfurther and Tyrrell (1984) are
plotted on the same figure. l'attern indicies were calculated for the 10
storms and are presented in table 5.

The dimensionless hyetograph for the August 1 storm lies below the
other curves in figure 6, and the pattern index for the August 1 storm is
smaller than the pattern index for each of the other nine storms (table 2).
The mean pattern index for the nine storms from 1904 through 1973 is 0.63
with a standard deviation of 0.064. The storm of August 1 has a pattern
index of only 0.40, three standard deviations below the mean, and is an
outlier. The August 1 storm started with a smaller dimensionless rainfall
intensity, and increased to greater intensity during the latter part of the
storm, than did other historical flood-producing storms for which records
were available. A storm having these characteristics would tend to have a
larger peak runoff than a storm of equal magnitude, duration, and maximum
precipitation intensity but higher pattern index.

Table 5.--Storm characteristics for 10 flood-producing storms

Length of Total precipi- l'attern
Date of storm tation index
storm (minutes) (inches) (dimensionless)
May 20, 1904 105 1.05 0.67
July 18, 1918 50 .80 .76
August 14, 1918 30 .57 .61
June 14, 1926 60 2.50 .65
June 10-11, 1935 25 1.30 .64
August 26, 1946 35 1.45 .65
June 14, 1955 265 2.40 .60
July 19, 1973 775 3.27 .52
September 8, 1973 85 2.70 .57
August 1, 1985 210 6.06 .40

41