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

I

I

5.3.3 Stream Bank Inventory

A field investigation was undertaken to categorize, reach by reach, the

bank stability and failure potential for each stream in the study area. The

results are presented in Table 21. Figure 27, a map of the study area, iden-

tified the reaches discussed in Table 21.

Worth mentioning in discussing the study results is the fact that a rela-

tively large-magnitude flood last occurred in the Fossil Creek drainage basin

in 1974. Several banks which failed in the past have become revegetated and

appear fairly stable. However, the passing of a large-magnitude flood will

likely reveal these banks to be unstable to some degree.

Also of special interest is an escarpment formed by a meander bed of

Fossil Creek cutting into a hillside located in Reach 7. The height of this

bank ranges from 20 to 25 feet in height. Although the bank appears fairly

stable, any failure which may occur will be of a very large magnitude and

could have significant economic impacts, particulary if future development

occurs in the vicinity of the escarpment.

Reaches which have major bank instability problems include, as numbered

on Figure 27, Reaches 4, 6, 7, 10, and 15 on Fossil Creek and Reach 17 on

Burns Tributary. Reaches with less severe, though still significant, instabi-

lity problems include Reaches 3 and 9 on Fossil Creek and Reaches 20, 21, and

23 on Lang Gulch.

I

I

I

I

I

I

I

I

I

I

5.4 Equilibrium Slope Analysis

For Fossil Creek, above Fossil Creek Reservoir, and its major tribu-

taries, long-term aggradation and degradation potentials were calculated using

an equilibrium slope analysis. The equilibrium slope of a channel reach is

the slope at which the capacity of the channel to transport sediment is

exactly equal to the supply of sediment coming into the reach. If the

equilibrium slope is steeper than the existing slope then the channel will

tend to aggrade. If the equilibrium slope is flatter than the existing slope

then the channel will tend to degrade. The aggradation or degradation that

will occur will not be uniform, rather the change in slope should be thought

of as pivoting around a downstream control point such as a culvert, geologic

or other grade control. Due to the frequent occurrence of controls of this

type in the Fossil Creek system this method of analysis is especially appli-

cable.

I

I

I

I

I

I

I

43

5.4.1 Method for Equilibrium Slope Analysis

The equilibrium slope analysis is simplified by the application of a

sediment transport relation of the form;

b c
q =aV Y
s

(5.1 )

where ~ is the unit sediment discharge in cfs, a, b, and c are empiri-
cally determined coefficients, V is average channel velocity, and Y is
average depth.
The coefficients a, b, and c are determined fram a regression analy-
sis. Values of V and Yare generated by assuming that the wide channel
form of Manning's equation

Y =

qn
1.49 S 1/2

(5.2)

) 3/5

where V is the velocity, S is the friction slope assumed equal to bed

slope, Y is depth, q is water discharge per unit width, and n is

Manning's roughness parameter; adequately describes the hydraulics of the

system. A range of slopes is determined from available topographic data,

while a range of q's for the system is determined from multiple HEQ-2 runs.

For Fossil Creek and its tributaries, slopes range from approximately 0.0001

foot per foot to 0.03 foot per foot, while unit discharges range from 0.5 to

50 cfs per foot.

Values of q were determined using a Meyer-Peter, Muller and modified
s

Einstein procedure explained in the Technical Addendum. Since sediment size

distributions, and general channel geometry and hydraulics are relatively

similar in the main stem and tributaries, only one sediment transport

regression was derived for the whole study system. This gave quite reasonable

results when applied in the equilibrium slope analysis.

Using Equations 5.1 and 5.2 and the definition of equilibrium;

QS(in) = Qs(out)

where Q (' ) is the incoming sediment to a reach and
s J.n
ment leaving the reach, and the fact that

QS( out)

is the sedi-