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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Improved Flood Control Levees

This alternative requires raising the existing levees and widening the

channel to conta~n the 100-year flood. The levee must be at least three feet

above the 100-year water surface to satisfy FEMA criteria. An upstream sedi-

ment trap is required to reduce the sediment supply to the study reach.

Improved bridges at 8th Street, 5th street, and 6th Avenue are required.

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backwater effect can be seen in Figure 2 and created considerable damage

during the 1983 flood, The Flood Insurance Program provides flood insurance

and allows controlled improvements to be made within the floodplain. Entry

into the National Flood Insurance Program is essential to properly manage

floodplain areas.

Hydraulic analyses were performed for each improvement alternative uti-

lizing the U.s. Army Corps of Engineers' HEC-2 water surface profile computer

model. The hydraulic information was used to determine residual flooding and

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Improved River Crossings at 8th Street, 5th Street, and 6th Avenue

This alternative considers the effect of enlarging the existing bridges

at 8th Street, 5th Street, and 6th Avenue. Channel improvements are not

included. An upstream sediment trap is necessary.

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to size the improvements.

A tabulation of advantages and disadvantages of each alternative is given

in the following paragraphs.

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Trapezoidal Channel

A trapezoidal channel (a channel with sloping sides and an essentially

flat bottom) with either a 50- or 100-year capacity would be constructed

following the present channel alignment. The existing channel would be

enlarged. Improved bridges are required at 8th Street, 5th Street, and 6th

Avenue. An upstream sediment trap is required.

1. Maintain the existing condition.

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Advantages

Disadvantages

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No capital improvement
costs.

Prohibits development/
redevelopment of area within
floodway.

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Does not eliminate flooding
problems .

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Combination Channel

This channel section utilizes a narrow, deep, low-flow channel (5-year

capacity) and a wide overbank section to convey the 100-year flow. The chan-

nel would follow the existing alignment. Improved bridges are required at 8th

Street, 5th Street, and 6th Avenue. An upstream sediment trap is not

required.

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Requires continued dredging.

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2.

Improved flood control levees.

Advantages

Disadvantages

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Requires limited land
acquisition.

Causes increased flood ele-
vations and velocities which
may cause bank instability.

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Dredging to Increase Flow Area
This alternative requires dredging the existing channel to lower the
channel bottom to increase flow area. Improved bridges are required at 8th
Street, 5th Street, and 6th Avenue. An upstream sediment trap is required.

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Allows development of
property adjacent to the
river.

Creates possibility of potentially
more damaging floods if levees
are breached.

Does not allow multiple uses
of river area.

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Several improvements are common to all alternatives, namely (1) reloca-

tion of the Great Western Sugar Company diversion structure at the City

wastewater treatment plant, and (2) floodplain management by entering the

National Flood Insurance Program. The Great Western Sugar Company diversion

structure at present creates considerable backwater during large flows. This

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Requires continued dredging.

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