FLOOD10070

• County: Larimer
• Community: Fort Collins
• Basin: South Platte
• Title: Master Drainage Plan Update Colorado State University
• Date: 1/1/1998
• Prepared For: Fort Collins
• Prepared By: Ayres Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SUMMARY AND CONCLUSIONS

The rainfall event of June 28, 1997 was of an extreme magnitude that overwhelmed
drainage facilities both on and off campus. The overtopping of the irrigation canals west of
campus resulted from over eight inches of rain causing approximately 1,900 cfs of water to
enter campus near Elizabeth and Shields Streets.

On campus rainfall of 5.30 inches in six hours compares to 153 percent of 3.46 inches in six
hours for the 100-year event. The rainfall on campus was the largest event in over 100-
years of records that have been kept of rainfall on campus. The rainfall west of campus
was 8,51 inches or 246 percent of the 1 OO-year event.

The overtopping of the canals resulted in 1900 cfs of flow and 265,5 ac-ft of overtopping
compared with 200 cfs and 19.3 ac-ft of flow estimated for a 100-year rainfall. This is nearly
ten times the peak flow and over ten times the volume that was estimated for a 100-year
event.

All of the runoff from campus, and west of campus, overwhelmed the campus drainage
system. This caused major damage to fourteen buildings, and minor damage to an
additional ten buildings on campus.

The SWMM model previously developed for the campus master drainage plan adequately
estimated the high-water elevations experienced. Using the SWMM model proposed
master plan improvements were simulated. Although these improvements would have
reduced the high-water elevations that were experienced, the reduction would not have
significantly reduced the damages.

The enlargement of proposed detention facilities in the athletic area were explored, but the
extreme volume of the 1997 event would not be contained in the largest pond that could be
constructed on the site,

As a resuit, the best way to avoid major damage from another event of this magnitude
would be protection of buildings, and exploration of cooperation with the City to develop a
comprehensive plan for control of storm water to the west, in conjunction with improvements
of drainage facilities on Campus.

The following recommendations are made with regard to planning drainage improvements
on the CSU Campus:

. Determine the level of protection that is desired for campus facilities. This may be
variable depending upon the facility. For example, the library and heating plant may
require a 500-year level of protection, while other ciassroom facilities may only need
100-year protection,

. Provide building protection for critical facilities such as the library, heating plant, steam
tunnels, and student center,

. Continue with the construction of the Phase I improvements on campus as
recommended in the 1996 report.

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Ayres Associates