Natchez to St. Francisville
279.6 - 279 RBD Morganza Spillway Entrance
There is nothing to see from the river, but paddlers can imagine 600,000 cfs flowing through the wooded banks around this bend in catastrophic flood waters. The US Army Corps of Engineers added this overflow valve in the 1950s to divert water from the Baton Rouge New Orleans stretch of river, whose levees can only handle up to 1.5 million cfs (cubic feet per second). To protect the Louisiana Delta during high water, the Army Engineers allow up to 620,000 cfs to pass through the Old River Control Structures. If more the river rises to above 2.2 million cfs the Morganza Floodway is opened.
Since its completion in 1954, the Morganza Spillway has been opened twice (in 1973 and 2011), and considered for opening during four other major floods of the Lower Mississippi River. The gates opened for the first time on April 17, 1973, in order to lower the water level of the Mississippi River and relieve pressure on the Old River Control Structure. The Corps of Engineers opened 42 of the 125 steel gates, allowing about half of its maximum designed flowrate to pass from the Mississippi into the Atchafalaya Basin. The spillway received minor scouring and slight damage to the stilling basin, and substantial flooding occurred in the Atchafalaya Basin. After the 1973 flood, the structure was restored to its original condition.
The Morganza Spillway was opened for the second time during the 2011 flood as waters rose higher and higher up the levees protecting New Orleans and its citizens, who scurried like mice up and down St. Charles underneath the looming presence of the cold brown water and the freighters floating by on it. A big pile of boulder-sized rocks was placed here recently (in 2014), probably as a means to better secure this useful drainage flood tool from eroding the riverbank in future flood years.
The second opening of the Morganza Spillway began with the lifting of a single floodgate on May 14, 2011. Diversion of 125,000 cubic feet per second (3,500 m3/s) of water from the Mississippi River to the Atchafalaya Basin was planned during this event, with the structure operating at about 21% of its capacity. This diversion was deemed necessary to protect levees and prevent major flooding in Baton Rouge and New Orleans, with the tradeoff of creating possibly severe flooding in the Atchafalaya Basin.
By May 18, 2011, a total of 17 gates (the largest number for the 2011 event) had been opened by the Corps of Engineers. The Corps estimated the flow rate at 114,000 cu ft/s (3,200 m3/s). However, on May 25, new estimates from the U.S. Geologic Survey (USGS) described a much higher rate of 172,000 cu ft/s (4,900 m3/s), resulting in the closure of 3 bays by May 26, and additional closures by May 29, bringing the total to 11 bays with an estimated diversion rate of 120,000 cu ft/s (3,400 m3/s). The Corps continued to evaluate the flow and close additional bays as appropriate. By June 6, the number of open gates had been reduced to seven, and by June 8, only two gates were still open. All bays were closed on July 7, 2011.
The Corps had estimated that it would take opening one-fourth of the spillway’s 125 bays — or 31 bays — to control the flow of the river through Baton Rouge in response to a forecast crest of 45 feet (14 m) anticipated on Tuesday, May 17, 2011, which must remain below 1,500,000 cu ft/s (42,000 m3/s) of water per second through Baton Rouge to ensure the integrity of the levee system. Since Morganza never operated above 172,000 cu ft/s (4,900 m3/s), the flooding in the Atchafalaya Basin was considerably lower than had been anticipated during the initial estimates of 300,000 cu ft/s (8,500 m3/s). By May 29, the Corps had also opened 330 of the 350 bays of the Bonnet Carré Spillway located near New Orleans.
(Written with information from Wikipedia)