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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highest aggradation potential. Since the equilibrium slope analysis is long

term, it may be some time before this aggradation manifests itself as a

problem. This is especially true of the first three reaches, which are pri-

marily broad grassy swales. Aggradation within these reaches is primarily

caused by high resistance to flow due to vegetation in relation to shallow

flow depths. Dredging a low-flow channel or ditch through these reaches would

probably stop any aggradation. Severe aggradation in Lang Gulch is due mainly

to the sediment load from Smith Creek. Dredging or clearing Smith Creek would

aggravate the problem on Lang Gulch. Dredging of deposited sediments within

the aggrading portion of Lang Gulch may be necessary to prevent the stream

from becoming braided or meandering. Likewise, dredging the upstream portion

of Burns Tributary would result in increased aggradation in the lower portion

of Burns Tributary.

The only reach experiencing problem degradation is Stanton Creek from

Fossil Creek to just below County Road 32. Here, the flow is confined to a

deep channel and high velocities develop. Several erosion control structures

could effectively control this erosion.

Further erosion problems occur mainly below culvert outlets where local

scour due to the culvert, combined with general degradation in the downstream

reach, may lead to large undesirable scour holes just downstream of the

culvert. Particular instances of this are Fossil Creek at Shields Street,

Fossil Creek at Fossil Ridge Drive, and Fossil Creek at the colorado &

Southern Railroad crossing. Additional problems of a similar nature occur on

Lang Gulch at Colorado & Southern Railroad Crossing No.4, and possibly below

Trilby Road (County Rvad 34). Scour problems below culverts can be corrected

by use of an energy-dissipating device or riprap at the culvert outlet.

Fossil Creek near Portner Reservoir is slightly aggrading, which causes

it to meander. Due to this meandering as previously noted, the stream is

attacking the embankment which impounds Portner Reservoir. From field obser-

vations, considerable erosion of this embankment is apparent and it should

probably have some form of protection, such as gabions, installed.

Downstream of Fossil Creek Drive all the way to Fossil Creek Reservoir,

Fossil Creek is naturally aggrading, however, restrictive culverts at County

Roads 13 and 34 have caused extreme change from the natural condition.

Considerable ponding, even at moderate flows, occurs upstream of both

culverts. This results in deposition of nearly all sediment being transported

to these points and subsequent clear-water discharge downstream of the

culverts, which leads to aggradation upstream and severe degradation

downstream of the culverts. Remedial measures include resizing and replacing

these culverts and/or channel protection, such as riprap or concrete lining,

downstream of the culverts.

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