FLOOD09522

• County: Logan
• Community: Sterling, Atwood
• Basin: South Platte
• Title: Pawnee Creek Flood Mitigation, City of Sterling, Town of Atwood and Logan County
• Date: 7/1/1999
• Prepared For: Logan County
• Prepared By: URS Greiner Woodward Clyde
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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SECTIONTWO

Alternatives Considered

Photographs of the Highway 6 and UPRR bridges and Pawnee Creek upstream and downstream
of the bridges are shown in Exhibits 5 and 6.

Riverside Cemetery Area Improvements

Culverts, dikes, floodwalls, flood channel and drop structure would be sized to handle 5,000 cfs
of floodwater (projected 1 DO-year flood event) and would include the follO\\ing project features:

· Construction of a I ,500-foot-long by 50-foot-wide by 5-foot-high berm to the north and east
of the Riverside Cemetery

· Lowering a 300-foot section and raising an 800-foot section ofCR 24

· Construction of a 500-foot-long by 75-foot-wide channel with a concrete floodwall north of
the cemetery; the floodwall would be sized to allow 3 feet of freeboard, which means the top
of the floodwall would be 3 feet higher than the elevation of the I DO-year flood event.

· Building and installation of a closure structure for Highway 6 that is sized to accommodate
3 feet of freeboard. The closure structure would include manholes that, when opened, would
allow the installation of vertical I-beams which would serve as tracks for the placement of
wooden planks ("stop logs") to contain and transport the floodwater across Highway 6. This
structure includes a stop log structure upstream and dO\\TIstream of the actual crossing point.
The temporary closure structure would be erected only when flooding in the vicinity of
Riverside Cemetery was predicted.

· Construction of eleven 20-foot by 4-foot box culverts under the UPRR embankment

· Installation of a siphon under the new flood channel for the Sterling No. I Ditch

· Construction of a 7,000-foot-Iong by 250-foot-wide flood channel from the UPRR
embankment to the South Platte River

· Modification of two private drives and one minor county road by creating gradual depression
in road grade within the constructed channel, hardening the road surface within the
constructed channel, and installation of a low-flow culvert within each road grade to convey
low flows across the county road and two driveways

· Construction of a riprap-protected drop structure where the constructed flood flow channel
discharges to the South Platte River

Photographs of location of the flood channel and the UPRR embankment at the Riverside
Cemetery location are provided in Exhibits 7 and 8. Design features of the drop structure would
be the same as discussed for Alternative 2.

Alternative 3 . Pawnee Creek Overflow Cutoff

This alternative was identified as Alternative 4 in the applicant's Flood Hazard Mitigation
Feasibility Study (ICON 1998). This alternative has the same features as the Riverside Cemetery

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