(No) Controlling Old Man River
Last month, I indicated that this posting would focus on two floodways connecting the Mississippi and Atchafalaya Rivers. The Old River Control Structures are 45 miles upriver from Baton Rouge, where the Mississippi and Atchafalaya once converged, while the Morganza Floodway is 10 miles downriver.
The Atchafalaya River begins at the terminus of the Red River near the top of the toe in Louisiana’s boot, about ten miles west of the Mississippi. It proceeds mostly southerly for 142 miles and empties into the Gulf of Mexico below Morgan City, while the Mississippi meanders 315 miles to the southeast past Baton Rouge and New Orleans. The Atchafalaya’s path to the Gulf is more than twice as steep as that of the Mississippi.
All rivers carry sediment due to rainfall runoff and to the river scouring its banks and bed. Some sediment reaches the river’s mouth, some is deposited when the river overtops its banks, and some is redeposited along its banks and bed. In the latter case, sediment buildup causes rivers to change course because water flow follows gravity. After all, it is the law. As the nation’s largest drainage basin, the Mississippi River obviously carries a lot of sediment.
By the 1800s, sediment had created an oxbow in the Mississippi called Turnbull’s Bend, causing it to meander west, where it was joined by the Red River. The two rivers flowed south, forming the source of the Atchafalaya, before the Mississippi meandered back eastward. In 1831, Captain Henry Miller Shreve, the founder of my hometown, dug a channel between the northern and southern sections of Turnbull’s Bend, thereby straightening the Mississippi and diverting the Red River to the Atchafalaya River. The upper section of the Bend silted in, while the lower section continues to connect the Atchafalaya and Mississippi Rivers and is known as the “Old River.” Except during periods of high water on the Red River, the Old River flowed west, diverting water from the Mississippi to the Atchafalaya.
Over time, it became apparent that the laws of nature would dictate that the Mississippi change its course to the Gulf of Mexico by diverting its flow to the Atchafalaya, with devastating consequences to the ports of Baton Rouge and New Orleans, as well as the communities in the Atchafalaya’s floodplain. To contain the Mississippi in its current channel and to provide flood control during periods of high water, the Mississippi River Commission recommended constructing several structures on the Old River, and Congress authorized the project in September, 1954.
Old River Control Structures. Today, five structures comprise the complex, but the original authorization included only three. Constructed between 1955 and 1959, the original complex includes a lock and dam, the low sill structure, and the over bank structure.
The lock and dam were constructed on the Old River (the southern arm of Turnbull’s Bend) and connect the Atchafalaya, Mississippi, and Red Rivers. The lock and dam both allow commercial and recreational navigation between the three rivers and preserve the Mississippi River channel in its southeasterly direction to Baton Rouge and New Orleans. Also, it limits the Red River’s flow solely to the Atchafalaya.
The low sill and over bank structures are upriver from the lock and dam and were constructed in a “dry hole” just west of the Mississippi River channel. Both structures contain a series of gates that can be closed or opened, allowing water to flow through the structure. After their construction, a channel was dug on each side of the low sill structure connecting the Mississippi River to the Red/Atchafalaya system. Its 11 gates are intended to remain open, thereby making the Atchafalaya a distributary of the Mississippi. The over bank structure lies just to the north on “dry” ground and only operates when the Mississippi flows over its banks. Engineers employ such creative naming conventions! The low sill structure measures almost 600 feet in length, while the over bank structure measures almost 3,400 feet, consisting of 73 spillway bays and gates. The gates are operated by a crane that traverses the structure.
A fourth structure, the auxiliary structure with six flood gates, was constructed between 1981 and 1986, and is located just southeast of the low sill structure. Like that structure, an inflow channel connecting the auxiliary structure to the Mississippi had to be dug after the structure’s completion. Its outflow channel connects to the channel used for the low sill structure. The auxiliary structure was necessitated after flooding in 1973 almost undermined the old sill structure. The fifth structure is a 192-megawatt hydroelectric power station that began operating in 1985.
The Old River Control Structures divert water from the Mississippi River to the Atchafalaya River. During normal periods, the low sill and lock and dam structures maintain the two rivers’ flow at rates similar to that before their construction. Of the combined upriver flow of the Mississippi and Red Rivers, 70% is channeled to the Mississippi and 30% is directed to the Atchafalaya. In “high water” periods, the over bank and auxiliary structures can divert an additional 620,000 cubic feet per second (cfs) of water to the Atchafalaya (over 260 million gallons per minute). Remember from last month that by the time the Mississippi empties into the Gulf of Mexico, its normal rate of flow is 500,000 cfs.
Morganza Floodway. Ten miles downriver, the Morganza Floodway lies on the western bank of the Mississippi River, at a bend in the river where the levee was prone to failure. This location did not require excavation of an inflow channel, like the low sill structure on Old River. Completed between 1949 and 1954, Morganza consists of a 3,906-foot-long spillway containing 125 gates. Each gate is just over 28 feet wide and can be opened (raised) by two cranes that move across the top of the spillway on railroad tracks. When its gates are opened, the spillway submerges a floodplain stretching to the eastern bank of the Atchafalaya, about 5 miles west, and 20 miles to the southwest. Prior to construction, the Army Corps of Engineers purchased land and easements in the floodplain, which is used for agricultural and recreational purposes when not inundated. Morganza’s gates have been opened only twice -- 42 gates in 1973 and 17 gates in 2011. At full capacity, the spillway can divert up to 600,000 cfs of flow from the Mississippi.
Conundrum. The Old River Control Structures, the Morganza Floodway, and the Bonnet Carre Spillway (last month’s posting) can divert up to 1,450 cfs of flow from the Mississippi River to prevent flooding in downriver urban areas. If all of the gates at the three floodways are fully opened, the Mississippi’s flow rate above Old River would be four times its normal flow rate at New Orleans. The floodways’ activation is based on measurements of the river’s elevation and flowage rate. However, the relationship between the two measurements has changed since 1950 – essentially, higher river elevations are accompanying lesser flow rates – resulting in some uncertainty as to when and how many flood gates to open.
Music. This month’s music repeats that from last month – sorta. Last month’s music has been covered by numerous musicians, including these versions recommended by subscribers. One name synonymous with Louisiana music is Neville. Thanks to Dan for recommending Aaron Neville’s version of "Louisiana 1927".
And thanks to Cal for recommending Kenny Wayne Shepherd’s version of “Backwater Blues.” If you think his version mimics Bessie Smith’s version, you’ve not listened to it all. And how sweet that Kenny Wayne and I share Shreveport as our hometown. Finally, Kenny Wayne’s repertoire includes several songs written by my high school pal, Buddy Flett.
Lagniappe: My initial posting did not have a lagniappe. Because my comment section does not recognize hyperlinks, here is a link to Carlos Guitarlos performing “ Damn Atchafalaya” as recommended by Eliot in my blog’s comment section. Thanks for this lagniappe, Eliot!