FLOOD03181

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Plan
• Date: 8/1/1983
• Prepared For: Greeley, CWCB
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydraulic and cross-section data prepared by the U.S. Army Corps of

Engineers (COE) were obtained and utilized during the preparation of this

report.

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I I. STUDY AREA DESCRIPTION

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Drainage Basin Characteristics

The Cache la Poudre River originates near the Continental Divide in Rocky

Mountain National Park and flows through Larimer and Weld Counties before

joining the South Platte River approximately five stream miles east of the

Greeley corporate limits.

The drainage basin consists of heavily forested mountain areas in the

western portion of the basin and rolling prairie in the eastern portion. The

drainage area of the Cache la Poudre River is 1,890 square miles at the

confluence with the South Platte River. The climate of the basin ranges from

alpine in the headwaters to semiarid at the mouth.

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Mapping

Topographic mapping (1" = 200' with four-foot contour intervals) was

obtained from Falcon Air Maps and was based on aerial photography made in

1973. Aerial photographs made in 1982 were obtained and enlarged to 1" = 200'

scale. The topographic mapping was updated using the new aerial photographs

(October 1982) to locate all new buildings and other physical features.

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Acknowledgements

The City of Greeley assembled a Floodplain Panel in 1982 to study the

problems and assess future uses of the Cache la Poudre floodplain. The

Floodplain Panel provided valuable input during the course of this study

regarding community concerns about existing problems and future planning for

the study area.

We wish to acknowledge the following individuals for their help and

guidance throughout this study:

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Study Area

The study area consists of the floodplain of the Cache laPoudre River

within the City. The river enters the corporate limits at approximately 16th

Avenue and flows in a southeasterly direction about 2.5 miles to Ash Avenue

where it leaves the corporate limits (see Figure 1). The river has a slope of

7.3 feet per mile in the study area. The Cache la Poudre River is crossed by

6th, 8th, 11th and Ash Avenues, 5th and 8th Streets, and the Union Pacific

Railroad. A control structure located at the City wastewater treatment plant

is used to divert water to the Great Western Sugar Company and also is used as

a crossing structure for sewer lines connecting the two sides of the

wastewater treatment plant.

The floodplain is heavily developed throughout most of the study area

with residential, commercial and industrial developments. The river is chan-

nelized with flood control levees present in much of the study area. The

Union Pacific Railroad tracks act as a barrier to flood flows. The tracks are

several feet higher than the adjacent land creating a large ponded area during

large flows. The Colorado and Southern Railroad embankment contains flood

flows south of the river and west of the Union Pacific Railroad.

Immediately upstream of the study reach is an area undergoing gravel

mining. Numerous active and abandoned gravel pits are located in this area.

Also present in the floodplain are remnants of the old river channel and other

evidence of channel migration.

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Mr. Sam Sasaki

Community Development Director
for the City of Greeley and
Floodplain Panel member

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Mr. Tom Cowan

Floodplain Panel member

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Mr. John Pacheco

Floodplain Panel member

Mr. Tom Romero

Floodplain Panel member

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Mr. Bob Stoll

Floodplain Panel member

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Mr. Gerry McRae

Floodplain Panel member

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We also wish to thank Mr. Larry Lang and Mr. Randy Seaholm of the

Colorado Water Conservation Board for providing technical assistance and

Mr. Harold Law and Ms. Ann Jamison for additional assistance.

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