FLOOD04472

• County: Statewide
• Community: Nemaha County Nebraska
• Stream Name: Missouri River Miles 532.4-528.9
• Basin: Statewide
• Title: Langdon Bend Habitat Restoration/Preservation Project Volume 1
• Date: 6/1/1995
• Prepared For: US Army Corps of Engineers
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Project

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preserved. A portion of the site would be kept in an agricultural land use with a percentage of
the crop left standing for use by wildlife, The acreage that would be left fallow would eventually
revert to forest through natural successional patterns. Presumably, these areas would first be
colonized by willow and cottonwood and would eventually be succeeded by more upland species
as the site evolves as described for Alternative C.

7. EVALUATION OF ALTERNATIVES

This section evaluates the proposed alternatives with respect to the project goals and
objectives at Langdon Bend. Table 6 provides a comparative rating of the alternatives and Table
7 presents a comparison of construction and average O&M costs for each alternative. Detailed
evaluation criteria tables used to compile Table 6 are included in Appendix D.

Enhancement of aquatic and terrestrial habitats for the entire project site would result from
the proposed project. Alternatives A and B include excavation and would result in short-term
adverse effects on benthos habitat due to disturbance; however, no long-term significant adverse
effects would be expected with any of the alternatives.

a. Alternative A - Flow-through Chute: Excavating a pilot channel along the
approximate alignment of the historic chute would provide a hydraulic connection with the
Missouri River. This channel would allow the flow of water to erode accreted soil along the
banks of the chute, eventually obtaining an ultimate channel width of 200 feet. Hydraulic
excavation of the chute channel may take place somewhat slowly, however, one or more high
stage events could accelerate the process, Grade control structures would be incorporated into
the design to prevent excessive bed scour and channel degradation. While 11.3 acres of wetlands
and 0.4 acres of riparian forest would be impacted to construct this alternative (IO-foot pilot
channel), this impact is offset by the 48 acres of chute/wetlands that would be created when the
chute assumes it ultimate width.

Alternative A offers the best opponunity to approximate the 1923 pre-control cross-
sectional configuration of the site. The inlet and outlet elevations have been designed to provide
a reliable flushing of the chute; therefore, this alternative should have a longer project life than
Alternative B. Monitoring will still be required to verify development of the chute channel,
development of the desired habitat features, to monitor any change to these features, and to
evaluate the overall success of the project.

Three pilot channel widths (lO-feet, 25-feet, and 50-feet) were evaluated for the pilot
channel excavation. The pilot channel alternatives were evaluated to detennine their impacts upon
the Ioo-year floodplainlfloodway, Hydraulic modeling efforts determined that the lO-foot wide
and 25-foot wide pilot channels would have no adverse impacts on the floodplainlfloodway. The
50-foot wide pilot channel and spoil berm would have a small negative impact. Details of the
floodplain analysis can be found in Appendix B. Excavation of a 10-foot wide pilot channel will
generate approximately 195,000 yards of material, while excavating a 25-foot wide channel would

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