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Please Share! High stage on the deltaic Mississippi River throughout the summer has implications for water management. Hurricane season officially begins on 1 June, when the average river stage in New Orleans is about 3. On 1 June , river stage was at 5 meters For example, Hurricane Katrina drove the river stage up by about 3.
The levees all along the deltaic Mississippi have been improved since Hurricane Katrina. Because peak flood season in the deltaic Mississippi River and peak hurricane season do not generally coincide, however, the river levees are not designed to accommodate storm surge on a flooded river. Levee heights vary, but the lowest levees are at about 6.
Luckily, the storm impacts were not as great as initially forecasted, levees on the deltaic Mississippi River did not overtop, and a Katrina-scale disaster was averted. High stage also means that the levees are at greater risk of failing. As river water piles up on levees, the added weight increases the susceptibility of the underlying soil to seepage and formation of sand boils that may undermine the integrity of levees over time.
Even when levees do not overtop or fail, prolonged flooding has impacts on industry. Navigation on the winding deltaic Mississippi River is always complicated and requires different ship captains with localized knowledge in different stretches of the river even in the best circumstances. However, higher stage means swifter flow, making the river much harder for ships to navigate. There are more shipping accidents, sometimes resulting in deaths, when the river is high.
Stronger currents also mean that ships at anchorage can be unmoored and drag anchors. More space is required between anchored vessels, resulting in fewer spots for anchorage, and some ports become entirely unusable in high water. Tugboats also push smaller loads at higher discharge, further resulting in reduced shipping. Downstream trips are faster, but upstream trips are slower and require more fuel. And dredging efforts cannot keep pace with siltation at the mouth of the river, limiting the size of ship that can come up the river.
Amid the challenges of prolonged flooding is the silver lining that higher flows on the deltaic Mississippi River could contribute to land building. Coastal wetlands protect New Orleans and all of southern Louisiana from the impacts of hurricanes. An often-cited statistic is that Louisiana loses the equivalent of a football field of wetland every hour, or Optimized sediment diversions would occur only during high-stage, sediment-laden discharges to maximize the amount of river sediment entering receiving basins in the coastal marshes while minimizing the introduction of fresh water into saltwater ecosystems.
The state of Louisiana is currently designing two large sediment diversions , each with a flow conveyance capacity of about 2, cubic meters 74, cubic feet per second, to be located roughly 50 kilometers downstream of New Orleans Figure 2. Ultimately, the land-building potential of these projects will depend on sea level rise; however, numerical modeling suggests they could produce 20—60 square kilometers of new marshland, or the equivalent area of the modern Wax Lake Delta , over 5 decades of operation.
There are caveats to this approach, however: As we have learned from spillway operations, altering salinity in the estuaries surrounding the delta may negatively affect habitat for species such as oysters, brown shrimp, blue crab, and bottlenose dolphins.
In addition, a wide range of stakeholders managing or making a living from coastal resources would likely be affected by sediment diversions, creating many legal and economic constraints that may require selective use of diversions and limit potential land building.
The future of the deltaic Mississippi River remains uncertain, in part because it is surrounded by many changing systems. Over the past century, the upstream network that delivers water and sediment to the deltaic Mississippi River has been highly engineered.
Dams control flooding and reduce sediment supply, whereas increased channelization of the network changes flow patterns and travel times. The watershed has also seen extensive urban and agricultural development, which lead to faster and greater-volume deliveries of water to the Mississippi network.
Climate change will also continue to have multiple impacts. On the downstream end, the number of hurricanes overall and the number of very intense hurricanes are predicted to increase [ Kossin et al. Further, relative sea level rise will drive the terminus of the river upstream. The only real certainty is that past recipes for managing the river through hard-structure engineering will not be adequate given all the stressors on the system.
The design of flood management infrastructure has typically relied on historical precedent, such as flood frequency records. Going forward, these designs must account for the potential of future conditions looking significantly different from those of the past. Efforts to modernize planning are helped by the fact that researchers have made great strides in improving abilities to predict future conditions through the development of high-quality community hydrologic and climate numerical models [e.
Ambitious physical laboratory models are also providing opportunities to test the effects of proposed experimental river management projects such as those related to sediment diversions. Ultimately, the only real certainty is that past recipes for managing the river through hard-structure engineering will not be adequate given all the stressors on the system.
After large floods, there are often discussions about relocating people away from floodplains and about changing zoning laws, but as the amount of time since a disaster increases, the sense of urgency for such changes dwindles. The urgency is now here to stay. Creative, although possibly unpopular, solutions beyond infrastructure are also required to manage the deltaic Mississippi River. The most effective options for the long term are nature-based solutions that leverage ecosystem functions, such as fostering vegetation growth to dissipate storm surge [ Barbier et al.
Tough decisions, including abandoning some areas where people live, may be part of the answer as well, and these discussions are already occurring. Solutions must not put the entire burden of change on marginalized socioeconomic communities, however, as has occurred with other development projects in the region, such as the siting of new industrial plants. An equitable solution that relies on sound science should be the priority.
Recent analysis shows that the average time between spillway openings has decreased from an average of every 7 years to every 3. Levee failures in Arkansas and Missouri caused predictions to change and the stage height that would trigger the opening of the Morganza Floodway has not been reached during this river flood. The Corps began closing the structure on July 22nd. Particularly as hurricane season starts, communities in and around south Louisiana are hoping that the river recedes quickly.
Furthermore, in addition to water coming down from one-third of the continental United States and two Canadian Provinces, communities across coastal Louisiana also face a future with water coming up from the Gulf of Mexico inching closer every day.
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