FLOOD08913

• Designation Number: 203
• County: Larimer
• Community: Fort Collins
• Stream Name: Dry Creek
• Basin: South Platte
• Title: Major Drainageway Planning - Dry Creek, Fort Collins, Larimer County, Colorado
• Date: 4/1/1980
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

Since the hydrograph shown in Figure 3 is the result of a storm
distributed over the entire Dry Creek basin, the storm intensity
used in the analysis is not so great as that used for a localized
storm event that could occur over the lower basin. Therefore, the
secondary peak from local runoff in Figure 3 may be less than the
actual IOO-year peak for the basin below the Larimer-Weld Canal.

HYDRAULICS

Procedure

A thunderstorm was modeled for this lower basin
and lO-year flood peaks from localized runoff.
analysis are shown in Table V.

to verify the 100-year
The results of this

A hydraulic analysis was completed along the 6.s-mile study reach to
determine water surface profiles for the 10-year, 50-year, 100-year
and 50 a-year flood events. These water surface elevations were computed
using the HEe-II computer model for backwater analysis (Reference S).
Water surface profiles for the 10-year, 50-year, lOa-year and SOO-year
events are presented on Sheets P-l through P-S. The lO-year, 50-year,
lOO-year and SOO-year frequency wa'ter surface elevation nt each of the cross
sections is tabulated in Table VI.

Table V

Alta Vista

650

520
160

Cross sections used in this study were developed using topographic
mapping with a two-foot contour interval. At crossing structures
and in other areas critical to the hydraulic analysis, cross sections
were supplemented with field survey information. The geometry of all
crossing structures was measured in the field, and photographs or
sketches of each structure are included in the Technical Addendum to
this report (Reference 7).

FREQUENCY DISCHARGES
(WITH CHANNEL DIVERSION)

lOa-Year
Location Discharge (cfs)

10-Ye;:u:
DisCharqe (cfs)

Confluence with poudre River 1,400

~lannings' roughness coefficients ("n" values) were determined through
field inspection. A photographic sUIr.mary, by reaches, of the Dry
Creek flood plain was prepared, and the "n" values used for ch",nncl
and overbank areas were presented to the Technical Committee for
review prior to completion of the hydraulic analysis. The summary
of roughness coefficients is included in the Technical Addendum to
this report (Reference 7). ~lannings "n" values varied greatly throughout
the study reach. Ranges of "n" villucs used in this study were the
following:

^ more thorough description of the hydrology prepilred for this report
is presented in the TeChnical Addendum (Reference 7).

Channel S",,,tiol\s

.03 to .05

Ovcrbanlc Areas

.015 to .10

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