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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5. DESIGN AND CONSTRUCTION CONSIDERATIONS

a. Engineering Analysis: The following analyses were performed for use in
developing all alternatives. The results were used as input to the design of the selected plan.

(1) Flow-Through Chute Analysis. A hydraulic analysis of each pilot channel
alternative and an ultimate condition chute channel were performed using HEC-2 for the Langdon
Bend site. These analyses evaluated the potential flow in 10-, 25-, and 50-foot wide pilot
channels, with their respective spoil material berms, for various Missouri River discharges. Split
flow through the 2oo-foot wide ultimate condition chute channel, which was determined by a trial
and error procedures, was also estimated for various Missouri River discharges. Details of the
flow-through chute analysis are included in Appendix B.

The results of the HEC-2 flow-through chute analysis indicate that the best pilot
channel alternative would be trapezoidal in section with a 10-foot bottom width and 2H
(horizontal) to I V (vertical) side slopes. This geometry would require less excavation than the
25- or 50-foot wide pilot channels. The estimated ultimate chute would be a trapezoidal channel
with a 2oo-foot bottom width and 3H to I V side slopes.

(2) Floodplain Analysis. A hydraulic analysis of the Missouri River main channel
and floodplain at Langdon Bend was performed to determine the impacts on the loo-year flood
profile due to the proposed improvements. A HEC-2 model was run for existing floodplain
conditions and for alternative conditions with 10-, 25-, and 50-foot wide pilot channels with their
associated spoil material berms. The proposed backwater/wetland complex was also modeled in
order to determine loo-year flood profile impacts due to the proposed excavated wetland area
which would be connected to the Missouri River near the southern end of the site.

The results of the HEC-2 analysis indicate that there is no negative impact to the
Missouri River IOO-year floodplain/floodway caused by the IO-foot wide and 25-foot wide pilot
channel excavation/spoil operations. The other alternatives would have a small negative impact
on the lOO-year floodplain as compared to the existing conditions. The wetland excavation in
combination with the disposal mound would cause a maximum lOO-year water surface increase
of about 0.6 feet as computed by the HEC-2 model. Details of the floodplain analysis are
included in Appendix C. There would be no significant adverse impacts to the navigation channel
expected as a result of constructing any of the alternatives.

b. Design: Design activities included alternate pilot channel configurations and
appurtenance details, ultimate chute channel determination, and backwater/wetland complex layout
and grading. A discussion of each activity follows.

(1) Pilot Channels. Pilot channels through the Langdon Bend right overbank
were designed to hydraulically reconnect the overbank area to the Missouri River main channel.
The pilot channels were configured to carry diverted river flows through the floodplain and then

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