FLOOD00626

• County: Larimer
• Community: Fort Collins
• Title: An Analysis of Rainfall for the July 28, 1997 Flood in Fort Collins
• Date: 2/1/1998
• Prepared For: State of Colorado
• Prepared By: CSU
• Floodplain - Doc Type: Flood Documentation Report

Hourly precipitation data clearly showed that there were four primary rainfall episodes
(Table 3). Large accumulations of rain had already fallen over areas just west and northwest
of Fort Collins hours before the major flood-producing storm even began to develop.

Table 3.
Periods of significant rai nfall in the Fort Colli ns, Colorado, area
July 27-28, 1997 (based on recording rain gauge data).

Date Time
1) Sunday, July 27,1997 1600 . 2200 MDT
2) July 27-28, 1997 2200 . 0700 MDT
3) Monday, July 28, 1997 0700 - 1200 MDT
41 Julv 28 1997 1700 - 2300 MDT

Once the primary rainfall periods were identified, the next step was to determine rainfall
patterns for each period. To accomplish this, all rainfa1l reports were examined to determine if
measurements had been made for intervals that coincided with any or all of these four periods
(see Table 4). Although more than 300 rainfall reports had been received, relatively few
reports were sufficiently detailed and thorough to define rainfa1l precisely for each specific rain
episode. However, all but 33 of the reports were useful in determining rainfa1l amounts for
one or more of the various accumulation periods.

Table 4.
Rainfall Reports for Eastern Larimer County for Specific Time Periods

Rainfall Highly Reliable Helpful
f!!jgQ C" A" and "8" aualitv reoor1s I (.C. aualitvl I2!i!
1) Sunday evening ffT 2 69
2) Overnight 41 1 42
3) Monday morning 70 2 72
Periods 1-3 combined 97 5 102
4) Monday evening 110 16 126
Total 2-day Rainfall 209 42 251

Rainfa1l totals for each rain episode were summed and tabulated. Rough hand-plotted maps
were drawn using approximate measurement locations. Computer-generated maps were also
produced with precise locations. These formed the basis of an estimation procedure to make
use of the many incomplete rainfa1l reports.

Using the initial set of analyzed rainfall maps for eastern Larimer County, the location of each
site which had good quality measurements of rainfall totals for all or most of each storm
period were placed on each map. Linear interpolation was then used to produce an initial
estimate for each point for each rain episode. In many cases, very confident estimates were
possible since the density of measurement sites with data was quite high, especially in the
immediate Fort Collins area. The rainfall patterns for each period were also fairly systematic
and predictable. For example, for the initial rain period late afternoon on July 27, rainfall in
excess of 0.50 inches was limited to areas northwest, west and south of Fort Collins. Across
most of the city, only relatively light rain fell, mostly between 0.10 and 0.50 inches. Most

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