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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Development of Alternatives
A number of different alternatives were developed to reduce flooding
problems. These included:

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and bridge replacement costs are an additional $0.3 to $2.2 million, dependent

upon which bridge alternatives are selected. The total improvement costs

range from $2.4 to $4.3 million dollars.

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1. Maintaining the existing condition.

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Hydraulic analysis was performed for these alternatives to determine the

feasibility of each alternative. Improving the bridges does not, by itself,

solve flooding problems. Dredging the channel to increase flow area is also

not practical or effective. The remaining alternatives were developed

further, costs for the alternatives were estimated and other factors were con-

sidered. Several improvements are common to the improvement alternatives.

The bridges at 8th Street and 5th Street must be enlarged or replaced. The

6th Avenue bridge must be enlarged, replaced, removed, or bypassed during

flooding. The Great Western Sugar Company diversion structure at the City

wastewater treatment plant causes flooding problems due to the constriction on

the channel (as evidenced during the 1983 flood) and should be replaced with a

non-constrictive structure.

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Future Study Recommendations

The mapping used for this study was made based on aerial photography

completed in 1973. Improvements made between 1973 and 1982 were added to the

mapping for this study, but topographic mapping should be updated for the

channel area prior to final design. Considerable excess excavated material

will be available following construction of the proposed improvements and

potential markets for this material should be located in order to realize

significant cost savings, The 8th Street bridge is on Colorado Highway 263

and the Colorado Department of Highways should be requested to replace the

structure. Transportation needs of the 6th Avenue area should be evaluated to

determine which bridge alternatives are feasible.

The City should enter the National Flood Insurance program to allow

proper management of the floodplain area and to allow the solicitation of

Federal aid in regards to flood problems.

2. Raising the existing levees.

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3.

Channelizing the river to contain the 100-year flood.
ferent channel sections were evaluated.)

(Several dif-

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4. Improving the bridges at 8th Street, 5th Street, and 6th Avenue.

5. Dredging the channel to increase flow area.

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6. Combinations of improvements.

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Recommended Alternative

The alternative improvement plans were presented to the Floodplain Panel

for review and selection of a recommended alternative. A decision was made to

utilize a combination channel for the majority of the study reach and a trape-

zoidal channel where limited right-of-way was available. The combination

channel consists of a narrow, deep, low-flow channel to continue normal stream

flows with a wide overbank section to convey larger flows. A trapezoidal

channel has sloping sides and a flat bottom and is utilized downstream of 8th

Street and at the Union Pacific Railroad. The proposed improvements will con-

tain the 100-year flood within the channel and allow development to the edge

of the proposed channel. The cost of the channel improvements is $2.1 million

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