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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Mark Forest
Alan Leak

WRC Nevada, Inc.
WRC Engineering, Inc.

II. PREVIOUS STUDIES AND AVAILABLE DATA

A. NOAA ATLAS 2:

As indicated above, NOAA Atlas 2 is the most recently published source for depth-duration-frequency
data for most of the State of Colorado. NOAA Atlas 2 contains a series of isopleth maps showing 2-.
5-. 10-.25-, 50- and 100-year, 6 and 24 hour depths. To develop data for a point of interest, the user
interpolates from the maps the 12 depths described above. These values are corrected through a best
fit plot of the data on a log-normal plot provided in the Atlas. A series of equations and nomographs
are provided for estimating the depths associated with each of the other durations. Based on the
procedures in NOAA Atlas 2, WRC utilized NOAA Atlas 2 to develop depth-duration-frequency curves
for the location that is near the center of the City of Fort Collins. The resulting curves and depth
estimates are shown on Figure I. The resulting values are consistent with the values currently used
by the City of Fort Collins for design purposes.

B.

A V AILABLE DATA:
The local precipitation gage with the greatest period of record is the gage at Fort Collins (NWS Gage
05-3005). This gage has been in continuous operation since 1889, but only the period from 1900 to
the present has been recorded in the NCDC digital database for daily rainfall totals. The portion of
the record between 1900 and 1948 has not all been digitized for the hourly rainfall events. Therefore,
the available data for the Fort Collins gage includes 1,2,3,6, 12,24.48 and 72 hour data from 1948
to 1995 and daily precipitation data from 1900 to 1994. Daily precipitation data for 1994 to 1997 was
obtained from the Colorado Climate Center. The remaining hourly data was provided to WRC by the
NOAA/NWS Office of Hydrology. The analysis for NOAA Atlas 2 was based on hourly data prior
to 1948 that was digitized by NOAA for that study. However, the digitized data set that was used for
NOAA Atlas 2 is not available.

The difference between the I day data and the 24 hour data is that the I day data is the total for that
Callander day. If the rainfall event occurred over the span of two Calender days, the resulting
maximum 24 hour depth could be greater than the depth recorded for either Calender day. The impact

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