FLOOD05212

• County: Larimer
• Community: Fort Collins
• Stream Name: Boxelder Creek, Spring Creek, Dry Creek
• Basin: South Platte
• Title: Flood Control for Boxelder Creek at Fort Collins
• Date: 2/1/1993
• Prepared For: Fort Collins
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

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EXISTING CONDITIOSS HYDRAULIC ANALYSIS

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The following is a summary of the detailed existing conditions hydraulic analysis
for Dry Creek presented as Appendix C.

<Appendix C). These areas will be evaluated in more detail during the feasibility
phase of the study.

The purpose of the hydraulic analysis was to identify the existing conditions flood
problems using as much existing infonnatioo as possible and to develop a feasible
design alternative 10 reduce the flood haurd from Dry Creek for the City of Fort
Collins, Colorado. The sludy was conducted on Dry Creek from its confluence with
the Cache La Poudre River to a location approximately 6.5 miles northwest of the
confluence where it crosses North Shields Street.

Data obtained from a previous study indicates that during a IOO-year peak
discharge on Dry Creek about a 50-year peak discharge occurssimullancousIy on the
Cache La Poudre River. A sensitivity analysis of water surface profiles on Dry Creek
indicated that the only economic subareas affected hy coincident flooding wOll]d be
subareas land 2 which account fOfaoout I percent of the expected annual damages.

FLOOD DAMAGEANALYSIS

The Dry Creek study reach is located in the northeast section of the City of Fort
Collins. Typically, the overbanks are wide and flat. In general, the channel is well
defined except for L'le reach from 1.IJe airport upstream (0 the Larimer and Weld
Canal where little or no channel exists. Even in areas of a well defined channel, the
capacity is limited by undersized culverts.

Land use information was collected for the 500-year flood plain during a fleld
survey conducted in June 1990. All values used in the analysis are based on July
1990price levels. Included in the analysis are flood damages to structures and their.
contents. These darn~ges were estimated using the Corps' flood damage model.
Damage$ to eMernal components including utililies, streets, and cleanup costs, were
ba$ed on a percentage of the flood damages computed by the model.

The floodplain for the Dry Creek basin is extremely wide in the lower portion
of the basin even though the magnitude of the lOO-year discharge is relatively small
for this size basin. This is due to ",ide, flat overbanks. The depth of flooding in the
ovetbanks is seldom greater than two feet and in some areas there is divided flow
where the water intheoverbanks is not hydmulical1y connected to the channel. As
ageneraJ ru]e,there isnodefinable channel Ul)wnstn.:am of the Larimer and Weld
Canal untillhe area of the airport isreaehed.

STIJDY AREAS
The SOO.year flood plain was subdh'ided into 17 subareas to facilitate data

Ulj;;..IIiL.04iiOH llll,j ..muYI;:;. Ti,=:.c: ~UUd.l""" d.Ie :;;""Wll Oil p;al=) 1: <u'u 3. TII=
subareas begin at the Dry Creek confluence with the Cache La Poudre River, as
shown on plate 2,and cod near North Shields Street, as shown on plate 3. This
subdivision was necessary because land use information is organized on a subarc.1
basis, and the Corps' flood damage model aggregates estimated damages by subarea.
The seleclion ofsllbarcas is also an important component of the incremental analysis
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u. ,,'^""-' ........",,, ..,,~..,....,..... . uw... ~', ,...r Y';:;'~ .orm...anvr. p..;po=, ..",.: s...'::, AI""

A cursory analysis of overlapping flood plains betwccn the Cache La Poudre
River and Dry Creek resulted infivearea.s being affocted as shown on I'latcs 5 and (;

was divided into three reaches as in the Gingery study. Reach A consists of subareas

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