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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Q = q T
s s

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where T is the channel top width; then the following expression for the

equilibrium slope of a reach may be derived;

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S
eq

S .
eX1st

Q sup
Qexist

2
) b - [0.6 (2/3 b + c)]

(5.3)

where

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is the

is the equilibrium slope, S .
eq ex~st
supply of sediment to the reach, Q . t
eX1S
band c are defined by Equation 5.1.

S

is the existing slope,

Q
sup
is the existing transport

capacity of the reach, and

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Since hydraulic conditions change with flow magnitude, the equilibrium

slope can vary with discharge. For this reason it is important to choose the

discharge level appropriately. The discharge from the 2-year flood was chosen

as the representative or dominant discharge. This discharge occurs frequently

and has a greater long-term effect on channel plan form than higher but less

frequent discharges such as the lOa-year flood. However, the longer, less

frequent discharges can cause large changes in a short period of time. These

changes cannot be ignored, but are best evaluated by a detailed sediment

routing methodology.

Since Fossil Creek receives water from tributaries as it flows

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downstream, the sediment supply increases. This sediment loading must be

accounted for or the sediment supply will be underestimated. Additional supply

of sediment was added in for Burns Tributary, Lang Gulch, Mail Creek, and

Stanton creek at the appropriate locations. Similarly Smith Creek's sediment

load was adjed to Lang Gulch at the confluence of the two. The sediment

loading from Burns Tributary, Lang Gulch, Mail Creek, and Stanton Creek was

calculated from known hydraulics. The loading due to Mail Creek was estimated

from discharge, slope, and drainage area.

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5.4.2 Results for Equilibrium Slope Analysis

Tables 22 and 23 present the results of the equilibrium slope analysis

for existing and developed conditions, respectively. The following reaches

are all experiencing significant aggradation; Fossil Creek from Taft Hill

Road to Shields Street, Burns Tributary above Shields Street; smith Creek bet-

ween Shields Street and Lang Gulch, and Lang Gulch between Smith Creek and the

Colorado & Southern Railroad Crossing No.2. Of these, Lang Gulch has the

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