FLOOD09485

• Designation Number: 206
• County: Larimer
• Community: Fort Collins
• Stream Name: Fossil Creek Drainage Basin
• Basin: South Platte
• Title: Master Drainageway Planning - Fossil Creek Drainage Basin, Fort Collins, Larimer County, Colorado
• Date: 8/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.5 Hydraulic Analysis

The analyses of hydraulic characteristics of Fossil Creek, Stanton Creek,

Lang Gulch, Smith Creek and Burns Tributary were conducted to determine water-

surface elevations for peak flows based on existing and developed conditions

for 2-, 5-, 10-, 50-, 100- and 500-year recurrence interval floods. The

water-surface elevations were computed using the U.S. Army Corps of Engineers'

HEQ-2 (ref. 20). Digitized cross sections were based on 1977 photogrammetry.

Additional cross sections along Stanton Creek were taken from City topographic

maps. Bridge and culvert geometries were field measured. Elevations at the

center lines of channel crossings and channels were obtained from the

topographic mapping and used to determine channel inverts and low chord eleva-

tions. Roughness coefficients were estimated from field reconnaissance and

review of photographs taken during the reconnaissance. A summary of the esti-

mated roughness coefficients has been presented in a separate document titled,

"Documentation for Selection of Manning's Roughness Coefficients for Fossil

Creek Basin," (ref. 18). The roughness coefficients range from 0.03 to 0.04

along the channel and from 0.035 to 0.05 along the overbanks.

The storm water is mostly confined to the channel, overbanks and ponded

areas behind embankments. Along Lang Gulch where it crosses Trilby Road

(County Road 34), approximately 35 cfs flows along the east side of the C&S

Railroad #3 and returns to Lang Gulch at the downstream C&S Railroad crossing

#2 during the 100-year flood for developed conditions.

Hydraulics of bridges and culverts were calculated separately as a check

on the HEC-2 calculations. Bridge and culvert capacities are listed in Table

9. Debris blockage Aas not considered to b~ significant because an inspection

of the flood plain showed limited sources of large debris. There is capacity

for the 100-year flow at Fossil Creek and Highway 287, Fossil Creek and the

C&S Railroad, Lang Gulch and C&S Railroad #1 and Lang Gulch and C&S Railroad

#2. The remaining crossings pond water, and of those the following major

crossings are overtopped; Fossil Creek at Timberline Road (County Road 11),

at Trilby Road (County Road 34), at Lemay Avenue (County Road 13), and at

Shields Street; Stanton Creek at County Road 32 and at Lemay Avenue (County

Road 13); and Lang Gulch at Trilby Road (COunty Road 34), at C&S Railroad #3

and at Shields Street.

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Cross-section location, channel bottom elevation, bridge and road loca-

tions and water surface profiles for the 10-, 50-, 100- and 500-year floods

existing and developed conditions for Fossil Creek, Stanton Creek, Lang Gulch,

Smith Creek and Burns Tributary are presented in Plates 1 through 74. Channel

stationing, channel elevation, discharges and corresponding water-surface ele-

vations for each cross section are presented in Tables 10 through 19. The

discharges represent the 2-, 5-, 10-, 50-, 100- and 500-year floods for

existing and developed conditions.