Flood mitigation along the Mississippi by Emma Clack https://padlet.com/emma519/jxy50bfvi0xd en-us 2019-02-12 13:25:47 UTC 2019-02-12 14:12:47 UTC hello@padlet.com Levees emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330312012 A levee is a large earthen embankment that is used to contain the Mississippi River. They are largely just mounds of dirt covered in sod. The Mississippi has 3,500 miles of levees running its banks averaging almost 25 feet in height. There's nothing complicated about a levee. The main difficulty, really, is getting enough dirt to build these huge artificial hills. When a hole breaks in a levee, it's called a crevasse. Some get famous enough to have names.]]> 2019-02-12 13:33:18 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330312012 Weirs emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330312555 A weir is like a dam that is designed to be topped. That is to say, it's a structure that's built under the surface of the river's water to change the flow characteristics of the water. Bendway weirs are a popular concept for reducing the erosion and meandering problems noted above. Weirs angled upstream appear to cut down on the amount of erosion on the outer bank of the river by minimizing the secondary currents of the river spinning outward. Over 190 bendway weirs have been built in the Mississippi since 1989.]]> 2019-02-12 13:34:33 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330312555 Dredging emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330312908 Since 1930, the Army Corps of Engineers has been tasked with dredging the river to maintain a nine-foot deep navigation channel. That action takes place upriver using a boat operated by the Corps' St. Paul district called The Goetz Dredge. "Under optimum conditions, the dredge can pump as much as 1,300 cubic yards per hour as far as 1,650 feet and up to 800 feet inland," the Corps says. "Booster pumps are sometimes used in combination with the Goetz to pump material up to approximately 10,000 feet."]]> 2019-02-12 13:35:26 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330312908 Revetments emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330313302 Revetments are a way of strengthening the outer bank of a river to keep it from eroding. Over 360 miles of the river in the New Orleans district alone have been revetted. A popular kind of revetment on the Mississippi is basically a massive "concrete mat." In the video above, you can see one being built. In essence, thin concrete blocks held together by metal are created on land, loaded onto a ship, and dropped into the river.]]> 2019-02-12 13:36:20 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330313302 The Old River Control Structure emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330313791 The Mississippi really wants to flow down a different path than the one that it currently does. Rather than its current route, gravity is driving it to move down the Atchafalaya river bed to the Gulf of Mexico. Humans, having built cities and businesses with the current configuration, don't want this to happen. This is the main narrative of McPhee's outstanding New Yorker story. In essence, we block the river's natural path and divert the water into the Mississippi. That occurs at a former Mississippi tributary called Old River. "Rabalais gestured across the lock toward what seemed to be a pair of placid lakes separated by a trapezoidal earth dam a hundred feet high," McPhee writes. "It weighed five million tons, and it had stopped Old River. It had cut Old River in two. The severed ends were sitting there filling up with weeds. Where the Atchafalaya had entrapped the Mississippi, bigmouth bass were now in charge. The navigation lock had been dug beside this monument. The big dam, like the lock, was fitted into the mainline levee of the Mississippi. In Rabalais's pickup, we drove on the top of the dam, and drifted as wed through Old River country. On this day, he said, the water on the Mississippi side was eighteen feet above sea level, while the water on the Atchafalaya side was five feet above sea level." If the structure that keeps the Mississippi from becoming the Atchafalaya fails, it would be one of the largest catastrophes in American history. ]]> 2019-02-12 13:37:19 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330313791 The Flood Control Plan emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330324936 The flood control plan is designed to control the "project flood." It is a flood larger than the record flood of 1927. At Cairo, the project flood is estimated at 2,360,000 cubic feet per second (cfs). The project flood is 11 percent greater than the flood of 1927 at the mouth of the Arkansas River and 29 percent greater at the latitude of Red River Landing, amounting to 3,030,000 cfs at that location, about 60 miles below Natchez. 

Description of Plan

The four major elements of the Mississippi River and Tributaries Project are: levees for containing flood flows; floodways for the passage of excess flows past critical reaches of the Mississippi; channel improvement and stabilization for stabilizing the channel in order to provide an efficient navigation alignment, increase the flood-carrying capacity of the river, and for protection of the levees system; and tributary basin improvements for major drainage and for flood control, such as dams and reservoirs, pumping plants, auxillary channels, and the like.
]]> 2019-02-12 13:59:09 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330324936 Floodways emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330326053 Cairo to New Madrid, Mo., the east bank bluffs and the levee as originally built on the west bank left only a narrow channel through which the river could flow at flood stage. To protect communities along the Mississippi and Ohio rivers and to reduce the flood heights to which the controlling levees on the Missouri side would otherwise be subjected, the project provides for a setback levee about 5 miles west of the riverfront levee through this reach. The strip between this setback levee and the levee adjacent to the river forms what is known as the Birds Point-New Madrid Floodway, operated only at extremely high stages. Water enters the floodway through lower levee sections or "fuse plugs" in the old front levee below Cairo and reenters the main river just above New Madrid. The floodway was operated in 1937 and was of material aid in reducing flood heights at and above Cairo. 

At the latitude of Red River Landing, the project flood is estimated at 3,000,000 cfs. The project provides for dividing this great quantity of water, with 1,500,000 cfs of the flow continuing down the main river channel, the remaining 1,500,000 cfs being diverted to the Atchafalaya River via the Morganza and West Atchafalaya floodways, and the Old River Control structures. 

Of the 1,500,000 cfs flowing down the main channel below Morganza Floodway, 250,000 cfs are to be diverted to Lake Pontchartrain and the Gulf through the Bonnet Carre' Spillway, located about 25 miles above New Orleans. The remaining 1,250,000 cfs will continue down the river to the Gulf. 

That portion of the flow diverted from the main channel near Old River is carried by the Atchafalaya River, the Morganza Floodway, and the West Atchafalaya Floodway. The Morganza and the West Atchafalaya floodways follow down on opposite sides of the Atchafalaya River until the end of the levee system along the Atchafalaya River is reached; there they merge into a single broad floodway that passes the flow to the Gulf through two outlets, Wax Lake and Berwick Bay. In major floods, the Morganza would be the first of these two floodways to be used, with water entering it through a control structure just above Morganza. 

]]> 2019-02-12 14:01:02 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330326053 Channel improvement and stabilisation emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330326696 Stabilization and protection of the riverbanks are important to the flood control and navigation plan, serving to protect flood control features and to insure the desired alignment of the river's navigation channel. This is accomplished by:

Cutoffs | Shortening the river and reduce flood heights.
Revetment | Controlling the river's meandering.
Dikes | Directing the flow.
Improvement Dredging | Realigning the channel.

]]> 2019-02-12 14:02:10 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330326696 Dams emma519 https://padlet.com/emma519/jxy50bfvi0xd/wish/330332377 The locks and dam located here are part of a much larger system of 29 locks and dams on the Upper Mississippi River. This series of locks and dams operated by the U.S. Army Corps of Engineers maintains a nine-foot channel on the Mississippi from St. Paul, MN to St. Louis, MO.]]> 2019-02-12 14:11:36 UTC https://padlet.com/emma519/jxy50bfvi0xd/wish/330332377