Cliff Notes began as short plot summaries of classic novels and stories. Now they are cribs about complex topics and spin. Corps Speak is the US Army Corps of Engineers (Corps) clever spin that evades responsibility for higher and longer Mississippi River floods (a complex topic). This story began when Congress put the Corps in charge of flood control (1928 Flood Control Act) to prevent another 1927 flood disaster.
The Corps developed the Mississippi River and Tributaries Project (MRTP) master plan in response. It included levees, spillways, cutoffs, reservoirs, and other river projects. It was designed to carry the Corps’ maximum theoretical flood (Project Flood) safely to the Gulf. The original design was based on the Mississippi’s flow capacity in the 1930s. It was updated in 1950 based on flow capacity then. That flow capacity no longer exists. The 2011 flood showed that the Mississippi’s flow capacity has decreased since 1950. And that the MRTP levees won’t contain the river. The Corps’ 2019 flow line study confirms this. It predicts hundreds of miles of levees will overtop in a flood smaller than the Project Flood. How did this happen? A Congressional authorization in 1954 gave the Corps another mandate: prevent the Mississippi River from changing course down the Atchafalaya River. The Corps built the Old River Control Complex (ORCC) about 40 miles above Baton Rouge to do this. ORCC controls the river’s flow and diverts 23% to the straighter, swifter, shorter Atchafalaya to discharge to the Gulf below Morgan City, LA. It keeps the remaining 77% in the longer, slower, meandering main channel to discharge below New Orleans. The Corps knew that diverting part of the flow would concentrate sediments in the Mississippi’s main channel and that this could reduce its flow capacity. So engineers and scientists carefully designed and sited ORCC to minimize this. They were successful. The first ORCC structure began operating in 1963. Other structures were added later. There were no increases in river stages indicating a decrease in flow capacity until after 1990. (When channel capacity decreases, stages rise. The river must get higher to carry the same flow.) In 1982, the Federal Energy Regulatory Commission (FERC) authorized a privately owned power plant (Sidney A. Murray Hydroelectric Plant) to be sited just upstream of ORCC. The plant had nothing to do with flood control. But the Corps agreed to integrate it into ORCC operations anyway. It began operating in 1990. The Corps diverts flow containing only small, light sediments through the plant turbines to the Atchafalaya in place of a mixed flow with larger, heavier sediments it had diverted through ORCC.
This concentrates large, heavy sediments downstream of ORCC, clogs the channel, creates a bottleneck, and reduces flow capacity. The company says it’s not responsible for the sediments and refuses to dredge them. The Corps hasn’t dredged them either — even though it’s clearly responsible for flood control. The clog of sediments grew, the bottleneck got smaller, the Mississippi’s carrying capacity decreased — and floods got higher. The 2011 flood provided dramatic proof. It was not only the highest flood ever, it was higher than the Corps expected based on historic flows. This caused a near disaster. The Corps nearly waited too long to open the Morganza Spillway gates because the stage was higher than expected for the flow in the water control manual that triggered the opening. Fortunately, the Colonel in charge trusted his eyes, not the manual, opened the gates, and saved Baton Rouge. The manual was wrong because it too was based on flow capacity that no longer existed. To quantify lost flow capacity, look at stages at Tarbert Landing (just above ORCC). Corps rating curves show the stage was 7.2’ higher in 2015 vs pre-1990 readings. (Stage increases upstream are not as high due to the river’s slope. But longer floods upstream are about the same.) In 2016-17, upstream landowners near ORCC testified to the Mississippi River Commission (MRC) about higher, longer, more damaging floods inside the levees. MRC Generals blamed more rain. They didn’t tell landowners about the bottleneck. Mississippi’s Secretary of State also testified about the state’s flooded school lands. The Generals didn’t tell him either. In late 2017, LSU’s Dr. Xu reported a large sediment deposit below ORCC and major reductions in channel width and depth based on 2013 Corps measurements. He said the resulting bottleneck could cause the river to change course in a big flood. Flood duration (days above flood stage) increased after the record 2011 flood. Floods at Natchez were 3 times longer in the 5 years after 2015 vs the 5 prior years. The 2019 flood was above flood stage 154 days at Vicksburg, blocking the Yazoo’s discharge — and causing the longest backwater flood ever. Major floods occurred more often too after 1990. From 1927 to 1990, there was one major flood (57’ and higher) at Natchez in 63 years. From 1950 to 1990, there were none in 40 years. From 1990 to 2020, there were 5: one every 6 years. There was one every 2.5 years from 2010 to 2020. It’s clear that the river has lost flow capacity. And that this causes more frequent, higher, and longer floods. And that the Corps has failed at its job of flood control. And that it is unable to act on its own. So what to do now? Shut down and remove the power plant. Operate ORCC to divert flow and sediments as designed. Dredge Mudberg to restore the Mississippi’s carrying capacity. Build a diversion structure to take the top off of big floods. Urge Congress to order and fund these acts. Pray for time to make the changes before there’s a big flood.
This column was authored by Kelly Williams, Sr., Chairman of Bigger Pie Forum.
This post is a paid advertisement.
1 comment:
All of the efforts of the USACE are temporary. The Mighty Mississippi River is the most complicated cut & fill scenario imaginable, and it is constantly changing. At some point, it will overwhelm the ORCC and the river will take the shortest path to the Gulf of Mexico. That will leave a modest-sized flume ditch flowing through New Orleans. Baton Rouge will expand westward, and Lafayette will expand eastward. Together they will be the next major port city on the Gulf. The only question is whether or not I will live to see it.
Post a Comment