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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3005). A period of 1900-1969 was used to allemptto replicate the analysis done by NOAA Atlas 2.
This will provide a baseline for the comparison of WRC's longer precipitation record analyses. The
period from 1900-1996 and 1997 was used for the daily analyses to evaluate the impact of the longer
period of record. and specifically the July 1997 rainfall event, on the current NOAA Atlas 2 rainfall
values. Similarly the period from 1948-1996 and 1997 was used for the hourly analyses. Analyses
provided by Oli Sveinsson at Colorado State University use similar data for the "at-site" hourly
analyses. The regional hourly analyses provided by Oli Sveinsson incorporates the Boulder 2 and
Longmont 6NW gages with the Fort Collins gage.

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1. 24 Hour Duration Analysis:
Since the purpose of the analysis was to detennine the relative impact of the additional record
on the statistical results, the analysis was perfonned with two distinct periods of record. It was
assumed that NOAA Atlas was based on the period of record through 1969. The first task to
undertake is to allemptto replicate the NOAA Atlas 2 results. This will present a bases on
which to compare the different analyses by WRC with NOAA Atlas 2. The available data for
the Fort Collins gage includes daily data for the entire period and 24 hour data for only the
period from 1948 to 1997. Therefore, this comparative analysis was perfonned with the daily
data since the 24 hour data was not available for an equivalent period of record as was used
in the development of NOAA Atlas 2. The final results were converted to 24 hour results
using typical I day to 24 hour correlation factors provide by NOAA as shown in Figure 2 and
from annual maximum to partial duration series shown in figure 3. The results of these
analyses are presented in table I and figures 4 and 5. The supporting I-Day analyses are
presented in Appendix C. The 24 Hour analyses are in Appendix E.

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To accomplish the comparison between NOAA Atlas 2 and WRC's analyses, WRC used both
Log Pearson III in accordance with the procedures outlined in Bulletin 17B (1981) and Fisher-
Tippet distributions for approximately the same period of record, as stated earlier. To perfonn
the LP III analyses, a program developed by the USGS was used (PEAKFQL, Version 2.3).
For the Fisher-Tippet analyses. a spreadsheet was developed. The difference with NOAA
Atlas 2 is that it is based on regional analyses done using a Fisher-Tippet Type Idistribution,
developed by Gumbel in 1958 (NOAA Atlas 2,1973). The comparison of these analyses are
presented in Table I and Figures 4 and 5. The supporting analyses are contained in Appendix
C.

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