FLOOD10401

• County: El Paso
• Community: Colorado Springs
• Stream Name: Fountain Creek
• Basin: Arkansas
• Title: Fountain Creek Drainage Basin Planning Study - Volume 1
• Date: 7/1/1994
• Prepared For: Colorado Springs
• Prepared By: Muller Engineering Company, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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incorporating 33 years of continuous data, historical data (Le. estimated
information from outside of the gage period of record) was not used. An
estimated Q100 discharge of 1,810 cfs was calculated at the Manitou Gage. This
value was then compared to the flow given when the estimated 1935 flood of 5,000
cfs was entered into the analysis as an historical flow. The inclusion of the
1935 flood as an historical flow increases the 10- and 100-year flow by almost
72 percent (to 3,180 cfs for the 100-year event). Therefore, care must be taken
when using historical data with the Log-Pearson Type III procedure with an
understanding that with large estimated historical values, the magnitude of flood
flows can be greatly increased. Time and date of occurrences, locations, gage
flow data, source of estimate, and how the estimate was arrived at are some items
that must be analyzed in an effort to determine the validity of the historical
flow estimate. In this instance, inclusion of the 1935 flood estimate value was
accomplished because it was felt that accuracy was added to the statistical
analysis.

Studies by Ott and Nasseri indicate that 80 percent of the estimates of the 100-
year flood based on 20-years of record will be too high and that 45 percent of
the overestimates will exceed 30 percent. Table 4.6-2 lists a recommended length
of record in years required to estimate floods of various probabilities with a
95 percent confidence (Kinsley, Kohler, Paulus 1985).

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TABLE 4-6.2
ACCEPTABLE DESIGN PROBABILITY ERROR FOR STREAM GAGE RECORDS

Design Acceptable Error
Probabil itv 10% 25%
0.1 (la-year) 90 18
0.02 (50-year) 110 39
0.01 (100-year) 115 48

The Tejon and Security Gages were evaluated statistically in the same manner as
the Manitou Gage and the results of these analyses are presented in Table 4.6-3
and 4.6-4, respectively. It is significant to note that no estimated flow for
the 1935 flood is available for the Security Gage, however, the 1965 flood was
handled alternatively as a high outlier.

4.7 DESIGN STORM HYDROLOGIC ANALYSIS

HYdroloQic Parameters and Model;nq

The second of these hydrologic procedures is a design storm method which uses
rainfall to produce runoff utilizing knowledge of the basins physical
characteristics.

4.0-7