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5 Sediment Management Alternatives and Opportunities
Pages 88-102

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From page 88...
... In addition, questions have been raised recently about the potential for sediment management along the Missouri to affect coastal wetlands and marine water quality as far away as the Mississippi delta of Louisiana and the hypoxic zone in the northern Gulf of Mexico. Some of the proposed sediment management activities are technically feasible, while others are more complex, uncertain, expensive, and as yet are subjects only of preliminary discussion and analysis.
From page 89...
... MISSOURI RIVER SEDIMENT REINTRODUCTION ALTERNATIVES Emergent Sandbar and Shallow Water Habitat Projects The Corps' Emergent Sandbar Habitat and Shallow Water Habitat projects have implications for sediment loadings and transport and therefore for channel morphology and habitat maintenance. Sediment in transient storage during its passage along the river channels and floodplains of the Missouri River valley has value for habitat formation and has both positive and negative influences on infrastructure.
From page 90...
... . That report describes how, according to current habitat construction plans, approximately 15,000 acres of land that accumulated in the decades following Bank Stabilization and Navigation Project implementation would be excavated, and approximately 34 million tons (MT)
From page 91...
... Removing Bank Stabilization Structures Downstream of Sioux City, Iowa, the Missouri River is channelized and controlled by levees, revetments, jetties, wing dikes, and other river training and control structures of the BSNP (USACE, 2009b)
From page 92...
... Since such a program of releasing the river from its lateral constraints would happen gradually over many decades, and would be accompanied by both sediment removal from the floodplain and sequestration of sediment on the floodplain, the net effect on the downstream annual sediment flux would not be a large percentage change. Over coming decades, however, the sediment locked in storage by Bank Stabilization and Navigation Project structures would be released gradually back into the river.
From page 93...
... There are no river restoration projects in the United States that have removed channel controls on the scale of the Missouri River, and such a program would have to evolve gradually in the context of regional development so as to minimize possible impacts and disruptions to other sectors. Several more limited and local strategies, applied singly or in combination, could be used to move the channel toward a more complex and dynamic configuration, with more secondary channels, sandbars, and shallow water habitat: Removal of revetments and removal or reshaping of dikes would allow the river to erode its banks, resulting in widening of the channel.
From page 94...
... Given the simplicity of the channel and the amount of existing data on sediment characteristics, hydraulics, and channel geometry within the lower Missouri River, such an analysis would be quite straightforward. Bypassing Sediment Around Mainstem Dams Bypassing of sediment around major dams has been suggested as a means of ameliorating some of the undesirable effects of the massive reduction of sediment flux down the Missouri River since the 1930s.
From page 95...
... . The method is performed in two reservoirs within the Missouri River basin: Guernsey Reservoir on the North Platte River, and Spencer Dam on the Niobrara River.
From page 96...
... . The decision on the most appropriate method for moving sediment past a dam and making the dam and its reservoir sediment neutral for the river depends on economic analyses of costs and benefits, the acceptability of the method and its results to stakeholders, and on laws and institutional agreements for Missouri River operations.
From page 97...
... Louis, it would constitute only a roughly 10 percent increase in the total sediment flux into the Mississippi from the Missouri. This amount is considerably smaller than the 34 MT/yr expected to be returned to the river by Corps projects for shallow water habitat, although at this point the habitat construction projects are slated to be conducted over a 15-year period.
From page 98...
... MISSOURI RIVER SEDIMENT MANAGEMENT AND LOUISIANA WETLAND BUILDING Since closure of the Missouri River mainstem dams and the construction of bank stabilization projects under the BSNP, Louisiana's coastal wetlands have experienced substantial erosion and losses. Louisiana has lost 1,900 square miles of coastal wetlands since the 1930s (Barras et al., 2003)
From page 99...
... : • crustal downwarping; • sea level rise; • natural consolidation of soils; • reduced sediment delivery from the Mississippi River because of stabilization of the river banks; • construction of flood control structures along the mainstem Mississippi River in Louisiana that prevent flooding of wetland by sediment-laden waters and conveyance of river sediments to the delta front and over the edge of the continental shelf; • wetland edge erosion by storms that are likely exacerbated by the larger open water fetch in ever-enlarging distributary bays (e.g., Atchafalaya and Barataria) ; • navigation and pipeline canals cut through the wetlands by the oil and gas industry; and • offshore disposal of dredged materials by the Corps of Engineers.
From page 100...
... These values are maxima because the sand supply would be diminished by being incorporated into bars and the floodplain, and a portion of the washload could be lost to the vegetated floodplain. Approximately 3.6 MT/yr of sand are dredged from the bed of the middle Mississippi River between the Missouri and Ohio river confluences, and the Mississippi River main channel in that stretch is narrow and meandering, with scattered chutes and backwater channels (Pinter et al., 2004)
From page 101...
... As described in Chapter 2, prominent features of the pre-regulation mainstem Missouri River were a high sediment load traveling in suspension and along its bed, and highly turbid conditions. These Emergent Sandbar Habitat and Shallow Water Habitat projects are reintroducing some sediment into the Missouri River, and are gradually reintroducing channel mobility and hydraulic connections between the main channel and its floodplain that support new habitat formation.
From page 102...
... However, there is little potential in the near future for any strategy described in this chapter to reestablish volumes of downstream sediment delivery that approach preregulation sediment volumes delivered to Louisiana. The Corps of Engineers Missouri River habitat construction projects could release enough sediment to increase the supply to the Mississippi delta by 10-20 percent for at least the next 15 years (depending on the trapping efficiency of the Mississippi floodplain)


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