FLOOD09640

• County: Weld
• Basin: South Platte
• Title: Supplementing Flood Frequency Analysis in Eastern Colorado using Rainfall Modeling
• Date: 8/2/1999
• Prepared For: CWCB
• Floodplain - Doc Type: Flood Documentation Report

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Figure 14 - Maximum discharges at various locations throughout the Palmer Divide, Front Range foothills, and
eastern Colorado. (Source: Jarrett, in review).

The gage was in place at Owl Creek tributary for eleven years. If even one I 00(+ )-year event
occurs in such a short systematic record, it may adversely affect the statistics by suggesting such
events aremore frequent. Such anomalous large entries in the systematic record may be deemed
as high outliers if, in the judgement of the analyst, are not indicative of the statistics of the site
(IACWD, 1982).

The models were calibrated first against the frequency curve using the original data set. Using
the same method implemented at Geary Creek tributary, the models could not be calibrated
within reason. In addition, the models could not reproduce the 100-year flood using parameters
that were physically reasonable. This provides some physically based evidence that the
frequency curve may be in error.

The top two systematic peaks were then eliminated as high outliers according to Bulletin 17B.
The envelope value of 11,000 cfs and return period of 5,000 years was used to extend the record.
Figure 15 shows the resulting frequency curve.

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