FLOOD00918

• County: Larimer
• Community: Fort Collins
• Title: Precipitaion Frequency Study
• Date: 7/16/1998
• Prepared For: Fort Collins
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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C.

of these data sets on the resulting statistical analyses are discussed in later sections. The data used for

the qualitative precipitation study is discussed in Section IV of this report.

OTHER STUDIES:
At the request of WRC, the NOAA/NWS Office of Hydrology performed a preliminary statistical
analysis of the Fort Collins gage data as well as several gages in the Fort Collins region. The purpose
of this request was to determine the potential impact that the revised NOAA study could have on the
currently published depth-duration-frequency values contained in NOAA Atlas 2. NWS provided a
preliminary statistical analyses for the gages in the Fort CoIlins Region using L-Moments. The L-
Moment data was determined in two ways. The fIrst approach used the four rain gage stations closest
to Fort Collins as a region. The second approach used all of the stations within 39-4 I N x 104-
105.36W as a region. The results provided by the Office of Hydrology are contained in Appendix B.
The results suggest that the additional period of record from 1969 to 1994 did not have a signifIcant
impact on the resulting 24 hour rainfall depths. For the longer duration values, the predicted depths
decreased slightly in comparison to NOAA Atlas 2 for the Fort Collins gage. The reader is reminded
that the extreme rainfall event of July 1997 is not included in the analyses and cautioned by WRC and
the NWS to consider these analyses as preliminary only.

A study titled "Probability of Extreme 24-Hour Precipitation Events in Fort Collins" was prepared by

the Colorado Climatology Office (November 1976). This study evaluated the statistics for rare events

that are typically used for the design of small dams. This study also evaluated the potential impact that

inclusion of a 10 inch event in the record would have on the resulting statistics. Based on the results

of this study, inclusion of the large (10") event resulted in signifIcant impacts on the results.

III. METHODS OF ANALYSIS

A.

STATISTICAL METHODS:

The purpose of this evaluation is to determine the relative impact of the additional record on the rainfall
statistics. Analyses by WRC and OIi Sveinsson at Colorado State University attempted to use the same
criteria for related analyses to achieve a comparable result to one another and determine the
signifIcance of the L-moment probability distribution used by Oli Sveinsson. The analysis done by
WRC was based solely on the rainfall data from the Fort CoIlins precipitation gage (NWS Gage 05-

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